Plants, algae, and cyanobacteria are the only organisms capable of performing photosynthesis .

These organisms use light to manufacture their own food, and are called autotrophs (“self-feeders”).

Other organisms, such as animals, fungi, and most other bacteria rely on the sugars produced by the autotrophs for their energy and are termed heterotrophs (“other feeders”),

The importance of photosynthesis

• Capture sunlight’s energy.
•   Store the energy in solar radiation as high-energy electrons in the carbon-carbon bonds of carbohydrate molecules.
• Essential to the global carbon cycle
•  Photosynthesis powers Earth’s ecosystems.

Different pigments respond to different wavelengths of visible light. Chlorophyll, which is present in chloroplast in plants is  the primary pigment used in photosynthesis, Chlorophyll reflects green light and absorbs red and blue light most strongly. Refer to chapter 1.5 to know/ recollect about the chloroplast structure Chlorophyll A is the major pigment used in photosynthesis There are also several types of chlorophyll and numerous other pigments that respond to light, including red, brown, and blue pigments. These  pigments may help channel light energy to chlorophyll A or protect the cell from photo-damage.  

What Are the Steps of Photosynthesis?

Photosynthesis takes place in two stages namely : 

1. the light-dependent reactions and
2. The light independent reaction called the Calvin cycle.

"Photosynthesis " by E Laurent, via Wikimedia Commons is licensed under CC BY-SA 4.0

The light-dependent reactions These reactions take place at the thylakoid membrane, Chlorophyll absorbs energy from sunlight and then converts it into chemical energy with the use of water. The light-dependent reactions release oxygen from the hydrolysis of water as a byproduct. When light energy reaches the pigment molecules, it energizes the electrons within them, These electrons are shunted to an electron transport chain in the thylakoid membrane. Every step in the electron transport chain then brings each electron to a lower energy state and harnesses its energy by producing ATP and NADPH. Meanwhile, each chlorophyll molecule replaces its lost electron with an electron from water; this process essentially splits water molecules to produce oxygen Water (H₂O) is oxidized, and oxygen (O₂) is released. The electrons that freed from the water are transferred to ATP and NADPH. The light independent reaction or the dark reaction or  the Calvin cycle  These reactions takes place outside the thylakoid in the stroma,. In these reactions, the energy from ATP and NADPH is used to fix carbon dioxide (CO₂). hence this process is also known as carbon fixation. Energy from the ATP and NADPH molecules generated by the light reactions drives a chemical pathway during which the carbon in atmospheric carbon dioxide is used to build  a three-carbon sugar called glyceraldehyde-3-phosphate (GA3P). GA3P is then built in to a   wide variety of other sugars (such as glucose) and organic molecules. Many of these interconversions occur outside the chloroplast, following the transport of G3P from the stroma. The products of these reactions are then transported to other parts of the cell, including the mitochondria, where they are broken down to make more energy carrier molecules to satisfy the metabolic demands of the cell. In plants, some sugar molecules are stored as sucrose or starch. The carriers that move energy from the light-dependent reactions to the Calvin cycle reactions can be thought of as “full” because they bring energy.

After the energy is released, the “empty” energy carriers return to the light-dependent reactions to obtain more energy.

Video references

• Watch the video from Khan academy to understand the process of Photosynthesis 
• Watch the video Travel deep inside a leaf from California academy of sciences for an exciting virtual tour inside the plant leaf and to know understand the process of photosynthesis
• Watch the video from Khan academy to know the factors affecting photosynthesis 

Test your Understanding

"Prokaryotic Cell" by LadyofHats is in Public Domain

Eukaryotic Cells by Camosun College is licensed under CC BY 4.0 

• The [pb_glossary id="258"]cell wall[/pb_glossary] :  is located outside the cell membrane.  The cell wall gives shape , support and protect the  cell. It consists mainly of cellulose and may also contain lignin , which makes it more rigid. It prevents the cell from absorbing too much water and bursting. It also keeps large, damaging molecules out of the cell.
• The large [pb_glossary id="264"]central vacuole[/pb_glossary] is surrounded by its own membrane and contains water and dissolved substances. Its primary role is to maintain pressure against the inside of the cell wall, giving the cell shape and helping to support the plant.

• Plastids are membrane-bound organelles with their own DNA. Examples are chloroplasts and chromoplasts. Chloroplasts contain the green pigment chlorophyll and carry out photosynthesis. [pb_glossary id="267"]Chromoplasts[/pb_glossary] make and store other pigments. They give flower petals their bright colors.

"Animal cell " by OpenStax,via Wiki media commons is licensed under CC BY 4.0

Cell Membrane or the Plasma membrane

"Cell membrane " by Openstax is licensed under CC BY 4.0

1. Transmembrane proteins : these span the entire plasma membrane and are found in all types of biological membranes
2. Integral monotypic proteins : these are permanently attached to the membrane from only one side.

The Fluid Mosaic Model 

• The plasma  membrane comprises a phospholipid bilayer
• The integral membrane proteins are embedded in the phospholipid bilayer,
• Some of these proteins extend all through the bilayer ( Integral proteins), and some only partially across it ( peripheral proteins). These membrane proteins act as transport proteins and receptors proteins.
• The proteins and lipids of the membrane move around the membrane. Such movement causes a constant change in the "mosaic pattern" of the plasma membrane.

To further understand  the Fluid Mosaic Model  watch the video at http://www.youtube.com/watch?v=Qqsf_UJcfBc

Test your understanding about the structure of plasma membrane

Extensions of the Plasma Membrane

"Cilia on Bronchiolar epithelium" by Charles Daghlian via Wikimedia Commons is in the Public Domain, CC0

"Microvilli" by OpenStax CNX is licensed under CC BY 4.0

Cytoplasm

The Cytoskeleton

 They  help to maintain the shape of the cell, hold certain organelles in specific positions, allows cytoplasm and vesicles to move within the cell, and enables unicellular organisms to move independently.

"Cytoskeleton" by OpenStax is licensed under CC BY 4.0

[pb_glossary id="379"]Microfilaments [/pb_glossary] : These are the thinnest of the cytoskeletal fibers . They help in moving cellular components, for example, during cell division. They also maintain the structure of microvilli. These components are also common in muscle cells and are responsible for muscle cell contraction.

Test your understanding

Organelles/ cell structures specific for plant cell

"plant cell" by OpenStax is licensed under CC BY 4.0

The cell wall is a rigid covering that protects the cell,

It provides structural support, and gives shape to the cell.

Fungal and protistan cells also have cell walls.

The plant cell wall is mainly composed of an organic constituent called [pb_glossary id="337"]cellulose[/pb_glossary], a polysaccharide comprised of glucose units.

While the chief component of prokaryotic cell walls is peptidoglycan,

 Cellulose is a long chain polymer of β-glucose molecules connected by a 1-4 linkage. 

"Chloroplast structure" by OpenStaxis licensed under CC BY 4.0

Watch the Video on Chloroplast structure and Function.

https://youtu.be/c8wA_0pfBu8?si=1GKC_2sBdEhbEgHm

The Central Vacuole

The central vacuole is an unique and essential component of plant cell and occupies most of the space of the cell. The central vacuole plays a key role in regulating the concentration of water in the cell against the changing environmental conditions. The the liquid inside the central vacuole provides turgor pressure, which is the outward pressure caused by the fluid inside the cell. The wilting of a plant when we forget to water it for few days is because of shrinking of the central vacuole caused due to the movement of water out of the central vacuole ,as the water concentration in the soil becomes lower than the water concentration in the plant. This loss of support to the cell walls of plant cells results in the wilted appearance of the plant. On the contrary when the  central vacuole holds more water, the cell gets larger without having to invest a lot of energy in synthesizing new cytoplasm. The  fluid inside the central vacuole has a very bitter taste, which discourages consumption by insects and animals. The central vacuole also functions to store proteins in developing seed cells. In plant cells, the digestive processes take place in vacuoles.

Plasmodesmata

Plasmodesmata(singular = plasmodesma) are junctions between plant cells,

Neighboring plant cells cannot touch one another because they are separated by the cell walls The Plasmodesmata are numerous channels that pass between the cell walls of adjacent plant cells, They serve to connect the cytoplasm , signal molecules and  transport the nutrients  from one plant cell to another.   Organelles/ cell structures specific for Animal cell

Lysosomes

 The lysosomes are the cell’s “garbage disposal units ” .

The lysosomes contain enzymes that  aid the breakdown of proteins, polysaccharides, lipids, nucleic acids, and even worn-out organelles. These enzymes are active at a much lower pH than that of the cytoplasm. Therefore, the pH within lysosomes is more acidic than the pH of the cytoplasm. Many reactions that take place in the cytoplasm could not occur at a low pH, this is the advantage of compartmentalizing the eukaryotic cell into organelles .

"lysosome " by Open stax is licensed under CC BY 4.0

Extracellular Matrix of Animal Cells

Most animal cells release materials into the extracellular space. The primary components of the extra cellular matrix  are glycoproteins and the protein collagen. Collectively, these materials are called the extracellular matrix . Their function  include :

• Hold the cells together to form a tissue
• Allows the cells within the tissue to communicate with each other.

Test your understanding of the plant cell structure.

Test your understanding of the cell organelles

Molecules in Cell

"Molecules in cell" by Openstax is licensed under CC BY 4.0

Significant Functional Groups in molecules of the cell

"Functional groups " by Openstax is licensed under CC BY 4.0

Macromolecules

 Carbohydrates

These are abundant biomolecule on earth and are abundant in terrestrial ecosystem. They are  primarily combination of carbon and water. They have the empirical formula (CH₂O)_n, where n is the number of repeated units. These molecules are regarded as “hydrated” carbon atom chains in which water molecules attach to each carbon atom, leading to the term “[pb_glossary id="844"]carbohydrates.”[/pb_glossary]

Functions of carbohydrates include;

• They are food sources.

• These molecules parts of nucleic acids  that store and transmit genetic information (i.e., DNA and RNA).

• They  impart strength to various structural components of organisms (e.g., cellulose and chitin), and

• They are the primary source of energy storage as starch and glycogen.

Monosaccharides, or simple sugars. 

"Monosaccharides " by Openstax is licensed under CC BY 4.0

Monosaccharides with four or more carbon atoms are typically more stable when they adopt cyclic, or ring, structures. These ring structures result from a chemical reaction between functional groups ,carbonyl group and a relatively distant hydroxyl group. Glucose, for example, forms a six-membered ring

"Monosaccharides " by Openstax is licensed under CC BY 4.0

Disaccharides

Two monosaccharide molecules f may chemically link together by a covalent bond called glycosidic bond  to form a disaccharide. Glycosidic bonds form between hydroxyl groups of the two saccharide molecules,

  $$\text{monosaccharide—OH}+\text{HO—monosaccharide}⟶\underset{\text{disaccharide}}{\underbracket{\text{monosaccharide—O—monosaccharide}}}$$ 

Examples of  disaccharides are

[pb_glossary id="848"]Maltose[/pb_glossary] : the grain sugar maltose, made of two glucose molecules; [pb_glossary id="849"]Lactose[/pb_glossary]: the milk sugar lactose, made of a galactose and a glucose molecule; and t [pb_glossary id="850"]Sucrose [/pb_glossary]: the table sugar sucrose, made of a glucose and a fructose molecule

."Common disaccharides" by Openstax is licensed under CC BY 4.0

Polysaccharides

Polysaccharides are non sweet carbohydrates . They are also called glycans. They are large polymers composed of hundreds of monosaccharide monomers, linked  together by glycosidic bonds..  They are not soluble in water.

Polysaccharides are very diverse in their structure. The most biologically important polysaccharides are starch, glycogen, and cellulose.

[pb_glossary id="337"]Cellulose[/pb_glossary] : consists of a linear chain of glucose molecules and is a common structural component of cell walls in plants and other organisms. [pb_glossary id="856"]Glycogen[/pb_glossary]  :   branched polymers; glycogen is the primary energy-storage molecule in animals and bacteria, [pb_glossary id="857"]Starch [/pb_glossary]:  branched polymer Plants primarily store energy in starch. The orientation of the glycosidic linkages in these three polymers is different as well and, as a consequence, linear and branched macromolecules have different properties.

Other structural polysaccharides.

Examples of these types of structural polysaccharides are N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM) found in bacterial cell wall peptidoglycan. Polymers of NAG form chitin, which is found in fungal cell walls and in the exoskeleton of insects.

"Polysccharides " by Openstax is licensed under CC BY 4.0

Test your Understanding

[h5p id="109"]

I.Movement of substances across the cell membrane

"Simple Diffusion across Plasma membrane" is licensed under CC BY 3.0

"Facilitated transport" by Mariana Ruiz Villareal (modified ) is licensed under CC BY-NC 4.0

"Facilitated transport" by Mariana Ruiz Villareal (modified ) is licensed under CC BY-NC 4.0

Osmosis :  Osmosis is the diffusion of water across a semipermeable membrane. This diffusion depends on the concentration gradient, or the amount of water on each side of the membrane. The amount of water is inversely proportional to the concentration of solutes i.e., the higher the concentration of water, the lower the concentration of solutes, and vice versa.  Due to the presence of aquaporins water can move readily across most membranes but the membrane limits the diffusion of solutes in the water.

"Osmosis" by OpenStax is licensed under CC BY 4.0

Active Transport:  This mechanism of transport requires energy, usually in the form of adenosine triphosphate (ATP) and transports substances against concentration gradient that is, if the concentration of the substance inside the cell is greater than its concentration in the extracellular fluid (and vice versa).  Active transport mechanisms move both small-molecular weight materials, such as ions, through the membrane as well as larger molecules.

"Active Transport Proteins" by Connectivid-D via Wikimedia Commons is licensed under CC BY-SA 4.0

1. The plasma membrane of the cell invaginates, forming a pocket around the target particle.
2. The pocket pinches off, with the particle of transport being contained in a newly created intracellular vesicle formed from the plasma membrane.

1. The  the inward-facing surface of the plasma membrane is coated with a protein called clathrin,
2. The coated portion of the membrane then extends from the body of the cell and surrounds the particle, eventually enclosing it.
3. The clathrin  then disengages from the membrane
4. The vesicle then merges with a lysosome and enclosed substance is broken down
5.  Nutrients from the degradation of the vesicular contents are extracted,
6. The newly formed endosome merges with the plasma membrane and releases its contents into the extracellular fluid. The endosomal membrane again becomes part of the plasma membrane.

"Exocytosis" by OpenStax is licensed under CC BY 4.0

II. Cell division

IIII .DNA Replication and Protein synthesis

DNA Replication

Protein Synthesis

1. dividing
2. develops organelles that perform specific tasks.   These adaptations lead to the development of eukaryotic cells.

"Stomata" by CNX OpenStax, is licensed under CC BY 4.0

Stomatal Opening and Closure

Stomatal Opening

When turgor pressure increases in guard cells, the cells swell but the thickened inner walls  of the guard cells near the stoma cannot expand and as a result they curve to accommodate the expanding outer walls. This curving of the guard cells opens the stoma

What is the role of light in the opening of stomata?

1. blue/ultraviolet-A absorbing cryptochromes (CRY1/2) and phototropins (PHOT1/2),
2. red/far-red absorbing phytochromes (PhyA-E), and
3. ultraviolet-B absorbing photoreceptor UVRESISTANCE LOCUS 8

"Guard cells" by  Jen Valenzuela is licensed under CC BY-NC 4.0

Stomatal Closure

 When water is low, roots synthesize abscisic acid (ABA),

Xylem conducts ABA  to the leaves.

In the leaves  abscisic acid causes calcium channels to open.

Calcium (Ca²⁺) opens anion channels and as a result malate, chloride, and nitrate exit the cell.

The membrane potential decreases as anions leave the cell and is called depolarization

Potassium exits the cell in response to this depolarization.

The loss of these solutes in the cytosol results in water leaving the cell and a decrease in turgor pressure.

The guard cells regain their original shape, and the stoma closes

"Guard cells" by June Kwak is in the Public Domain, CC0

Stomatal transpiration accounts for between 50-97%of total transpiration.

Cuticular transpiration

 Lenticular  transpiration

•  water potential,
• evapotranspiration, and
• stomatal regulation 

Water Transport from Roots to Shoots

Water potential is a measure of the potential energy in water. It is based on potential water movement between two systems. It is the difference in potential energy  at atmospheric pressure and ambient temperature  between any given water sample and pure water .

Water potential can be positive or negative,

The solute potential (Ψ_s)

Pressure potential (Ψ_p)

"Water potential" by Open Stax is licensed under CC BY 4.0

Movement of Water and Minerals in the Xylem

The water on the surface of mesophyll cells of the leaf  saturates the cellulose microfibrils of the primary cell wall. The leaf contains many large intercellular air spaces for the exchange of  gases.o When the wet cell wall is exposed to this leaf internal air space the water on the surface of the cells evaporates into the air spaces, This decreases the thin film on the surface of the mesophyll cells and creates a tension  on the water in the mesophyll cells thereby increasing the pull on the water in the xylem vessels. The xylem vessels and tracheids are structurally adapted to cope with large changes in pressure. Rings in the vessels maintain their tubular shape. Small perforations between vessel elements reduce the number and size of gas bubbles that can form via a process called cavitation. The formation of gas bubbles in xylem interrupts the continuous  flow of xylem sap and the the water from base to the top of the plant. This break is termed as embolism. The taller the tree, the greater the tension forces needed to pull water, and the more cavitation events. In larger trees, the resulting embolisms can plug xylem vessels, making them non-functional.

Transportation of Photosynthates

The products of photosynthesis are called photosynthates, which are usually in the form of simple sugars such as sucrose. Structures that produce photosynthates for the growing plant are referred to assources. The sugars produced in sources like leaves are delivered to the growing parts of plants through Phloem by a process called Translocation These sugars are delivered to required places such as roots, young shoots, and developing seeds. These structures are  called sinks. Seeds, tubers, and bulbs can be either a source or a sink, depending on the plant’s stage of development and the season.

Translocation: Transport from Source to Sink

The mesophyll cells of photosynthesizing leaves produce sugars /photosynthates such as sucrose. The phloem then   translocate these to structures where they are used or stored. The photosynthates move through cytoplasmic channels called plasmodesmata that connect the mesophyll cells and reach phloem sieve-tube elements (STEs) in the vascular bundles. From the mesophyll cells, the photosynthates are loaded into the phloem STEs through active transport. This is coupled to the uptake of sucrose with a carrier protein called the sucrose-H⁺ symporter.

Phloem STEs have reduced cytoplasmic contents.

They are  connected by a sieve plate with pores This facilitates pressure-driven bulk flow, or translocation, of phloem sap. The energy for STEs are produced through the metabolic activities of companion cells  associated with STEs

"Pholem" by Open Stax is licensed under CC BY 4.0 

[h5p id="46"]

Passive Transport Interactive questions by Charles Molnar and Jane Gair is licensed under CC BY 4.0 

"Plant Growth and Development" is in the Public Domain, CC0

• Plants do not have specialized organs for gas exchange.
• Each part of the plant takes care of its own gas exchange.
• The rate of respiration is relatively much slower in plants compared to animals.
• The liquid transport system of plants is not involved in gas transport
• The parenchymal cells of leaves, stems and roots are loosely packed with air spaces . The gases diffuse through thee intercellular air spaces.
• Oxygen and carbon dioxide pass through the cell wall and plasma membrane by diffusion. Membrane channels called aquaporin also help in the diffusion of carbon dioxide.

Gaseous exchange in the leaves

 Could fossil leaves tell us about past levels of CO₂ in the atmosphere?

In a study reported in  Nature, 411:287, 17 May 2001) by Gregory Retallack  it was said that the fossil leaves of the ginkgo and its relatives showed high stomatal indices  late in the Permian period (275–290 million years ago) and again in the Pleistocene epoch (1–8 million years ago). Geological evidence show that both these periods are known for low levels of atmospheric CO₂ and ice ages. On the contrary  during the Cretaceous period, a time of high CO₂ levels and warm climate , stomatal indices were  reported to be low.

These studies support the role of  carbon dioxide as a greenhouse gas and its contribution in global warming.

Gaseous Exchange in Roots and Stems

Main Components of the Digestive System

1. Mouth: Digestion of food  begins in the mouth. The food is chewed and mixed with saliva in the mouth so as to break it down into a form that can be easily swallowed and digested.
2. Esophagus: It is the muscular tube that connects the mouth to the stomach. The food  moves from the mouth through  esophagus by a wave-like muscle contraction process called [pb_glossary id="910"] peristalsis[/pb_glossary]  .
3. Stomach: The food is further broken down in the stomach  using stomach acids and enzymes.  The food is now turned into a semi-liquid form called[pb_glossary id="912"] chyme[/pb_glossary].The stomach links the esophagus to the duodenum. The empty stomach is only about the size of  fist, but can stretch to hold as much as 4 liters of food and fluid,  and then return to its resting size when empty. There are four main regions in the stomach: the cardia, fundus, body, and pylorus.
   • The cardia (or cardiac region) : This is the point where the esophagus connects to the stomach and through which food passes into the stomach.
   • Fundus : is located inferior to the diaphragm, above and to the left of the cardia. It  is the dome-shaped .
   • Body : This is  the main part of the stomach and is located below the fundus.
   • Pylorus : This is  funnel-shaped. The  pylorus connects the stomach to the duodenum. The wider end of the funnel is called  the pyloric antrum, This  connects to the body of the stomach. The narrower end is called the pyloric canal, which connects to the duodenum. The pyloric sphincter is located at the bottom of the stomach, between the stomach and the duodenum. It controls stomach emptying. In the absence of food, the stomach deflates inward, and its mucosa and submucosa fall into a large fold called a ruga.
4. Small Intestine: The small intestine includes the the duodenum, jejunum, and ileum. Most of the digestion and absorption of nutrients occurs in the small intestine. It is the longest part of the alimentary canal, the small intestine is about 3.05 meters (10 feet) long in a living person. The diameter of the small intestine is about 2.54 cm (1 in).The folds and projections of the lining of the small intestine gives  an enormous surface area of about  200 m² 
5. Large Intestine: The large intestine runs from the appendix to the anus and includes the cecum, colon, and rectum. The water and salts from the material that has not been digested are absorbed in the large intestine. The large intestine  is responsible for forming and expelling feces. The large intestine frames the small intestine on three sides and has a  diameter more than twice of the small intestine, about 3 inches. The ileocecal valve, located at the opening between the ileum and the large intestine. It controls the flow of chyme from the small intestine to the large intestine. The cecum is a sac-like structure that is suspended inferior to the ileocecal valve. It is about 6 cm (2.4 in) long. It  receives the contents of the ileum, and continues the absorption of water and salts. Attached to the cecum is the appendix (or vermiform appendix) which is a winding tube . The 7.6-cm (3-in) long appendix contains lymphoid tissue, suggesting an immunologic function. The cecum blends seamlessly with the colon. On the right side of the abdomen, the food residue initially passes through the ascending colon. The colon becomes the transverse colon at the inferior surface of the liver, where it bends to form the right colic flexure (hepatic flexure).The final third of the transverse colon marks the start of the area known as the hindgut. After passing through the transverse colon, food waste reaches the left side of the abdomen, where it reaches the left colic flexure (splenic flexure), where the colon angles abruptly just inferior to the spleen. From there, food residue goes through the descending colon, which runs down the left side of the posterior abdominal wall. It becomes the s-shaped sigmoid colon after entering the pelvis .Food residue leaving the sigmoid colon enters the rectum in the pelvis, near the third sacral vertebra. The final 20.3 cm (8 in) of the alimentary canal, the rectum extends anterior to the sacrum and coccyx. 
6. Anus: This is the  final part of the digestive tract. Anus expels feces  from the body. The anal canal is located in the perineum and is  completely outside of the abdominopelvic cavity. This 3.8–5 cm (1.5–2 in) long structure opens to the exterior of the body at the anus. The anal canal has two sphincters namely  the internal anal sphincter  and the external sphincter .The internal sphincter is made of smooth muscle, and its contractions are involuntary. The external anal sphincter is made of skeletal muscle, which is under voluntary control.  Both the sphincters usually remain closed except when defecating.

Accessory Organs of the digestive system

1. Liver: The bile is produced in the liver  which helps digest fats.
2. Gallbladder: The bile produced in the liver is stored and concentrated in the Gall bladder from where it is released  into the small intestine.
3. Pancreas: Produces digestive enzymes and bicarbonate, which are released into the small intestine to aid in digestion.

Phases of Digestion

1. Cephalic Phase: Triggered by the sight, smell, or thought of food, stimulating saliva and digestive juices.
2. Gastric Phase: Begins when food enters the stomach, stimulating the release of gas
3. Intestinal Phase: Starts when chyme enters the small intestine, triggering the release of digestive enzyme and bile

• Inspiration is the process that causes air to enter the lungs, and
• Expiration is the process that causes air to leave the lungs  

• To provides oxygen to body tissues for cellular respiration,
• Removal of  the waste product carbon dioxide,
• To maintain acid–base balance .

• for adults is 10–20 breaths per minute .
• For Children less  than one year is  30–60 breaths per minute,
• For children by the age is usually 18–30 .
• By adolescence, the respiratory rate is  12–18 breaths per minute (Much similar to adults ).
• The normal respiratory rates  decreases from birth to adolescence and often increase slightly over the age of sixty-five.

The Respiratory System

Nose and Nasal Cavity

Pharynx (Throat)

Larynx (Voice Box)

Trachea (Windpipe)

Bronchi and Bronchioles

Lungs

Alveoli

1. Heart: A muscular organ that pumps blood throughout the body.
2. Arteries: Blood vessels that carry oxygenated blood away from the heart.
3. Veins: Blood vessels that carry deoxygenated blood back to the heart.
4. Capillaries: These tiny blood vessels permit the exchange of oxygen, carbon dioxide, and other substances between blood and tissues.

• Oxygen and Nutrient Transport: Delivers oxygen and nutrients to cells.
• Waste Removal: Removes carbon dioxide and other metabolic wastes from cells.
• Hormone Distribution: Transports hormones to target organs and tissues.
• Temperature Regulation: Helps maintain body temperature by distributing heat.

The Heart

• The heart is a muscular organ that pumps blood through the three divisions of the circulatory system namely
  • the coronary (vessels that serve the heart),
  • pulmonary (heart and lungs), and
  • systemic (systems of the body)

• Right Atrium: Receives deoxygenated blood from the body.

• Right Ventricle: Pumps deoxygenated blood to the lungs.

• Left Atrium: Receives oxygenated blood from the lungs.
•  The left atrium then receives the oxygen-rich blood from the lungs through the pulmonary veins. This blood passes through the bicuspid valve or mitral valve (the atrioventricular valve on the left side of the heart) to the left ventricle

Blood Vessels

1. There are three types of blood vessels: the arteries, veins, and capillaries.
2. Arteries 

• • Tunica Intima: The innermost layer, consisting of endothelial cells and elastic fibers.
  • Tunica Media: The middle layer, made up of smooth muscle cells and elastic fibers, which helps control the diameter of the artery.
  • Tunica Externa: The outermost layer, composed of elastic fibers and collagen, providing structure and support.

• • • Elastic Arteries: These are closer to the heart (e.g., the aorta) and contain more elastic fibers to handle the high pressure of blood pumped from the heart.
    • Muscular Arteries: These are further from the heart and contain more smooth muscle tissue, allowing them to regulate blood flow by constricting or dilating.

1. Aorta: The largest artery in the body, originating from the left ventricle of the heart and branching into smaller arteries.
2. Coronary Arteries: Supply blood to the heart muscle itself.
3. Carotid Arteries: Supply blood to the brain, neck, and face.
4. Femoral Arteries: Supply blood to the lower limbs.

• Oxygen and Nutrient Transport: Arteries carry oxygenated blood and nutrients to tissues and organs.
• Blood Pressure Regulation: The smooth muscle in the tunica media helps regulate blood pressure by adjusting the diameter of the arteries.
• Pulse: The expansion and contraction of elastic arteries with each heartbeat create the pulse that can be felt in various parts of the body.

• Systemic Circuit: Carries oxygenated blood from the heart to the body.
• Pulmonary Circuit: Carries deoxygenated blood from the heart to the lungs for oxygenation

Components of the Nervous System

1. Central Nervous System (CNS): Comprising the brain and spinal cord, the CNS processes information and coordinates activities.
2. Peripheral Nervous System (PNS): Consists of nerves that connect the CNS to the rest of the body. It includes:
   • Somatic Nervous System: Controls voluntary movements.
   • Autonomic Nervous System: Regulates involuntary functions and is further divided into:
     • Sympathetic Nervous System: Prepares the body for ‘fight or flight’ responses. The sympathetic nervous system prepares the body in response to stress by activating the organs and glands in the endocrine system.
     • Parasympathetic Nervous System : This functions opposite to the sympathetic system and allows the body to recover , for example clams the body by slowing the heart and breathing.

• Neurons: The basic units of the nervous system. The nervous system comprises about 100 billions of neurons. The function of neurons is to receive and transmit signals .Neurons transmit signals through electrochemical impulses.
• The structure of neurons include three majorts namely
  • a cell body or soma  : contains nucleus of the cell and keeps the cell alive.
  • Dendrite : This has a branching tree like structure . It collects information from other cells and sends it to soma. Some neurons have hundreds or even thousands of dendrite
  • axon: This is a long, segmented fibre . This tranmits information away from soma to other neurons or to muscles and glands.
    • Myelin sheath :  is a layer of fatty tissue surrounding the axon of a neuron that both acts as an insulator and allows faster transmission of the electrical signal.
    • Axons branch out toward their ends, and at the tip of each branch is a terminal button.
• Watch the interesting video from Gregl on neuron impulse 

• Synapses : These are junctions where neurons communicate with other neurons, muscles or glands through neurotransmitters.A synaptic connection between a neuron and a muscle cell is called a neuromuscular junction. Also called: neuronal junction. Synapses can be  chemical i.e. these synapses communicate using chemical messengers.
• Or can be electrical; in these synapses, ions flow directly between cells.
• At a chemical synapse, an action potential triggers the presynaptic neuron to release neurotransmitters. These molecules bind to receptors on the postsynaptic cell and make it more or less likely to fire an action potential.
• Upon binding with its receptor on a receiving cell, a neurotransmitter opens or closes ion channels. As a result the voltage across the membrane, or membrane potential, of the receiving cell may alter locally .
• Neurotransmitter molecules attach to receptor proteins on postsynaptic cells after diffusing across the synaptic cleft. Ion channels in the cell membrane open or close in response to postsynaptic receptor activation. Depending on the ions involved, this might be either hyperpolarizing, making the inside of the cell more negative, or depolarizing, making the inside of the cell more positive.
• The alteration may increase the target cell's propensity to fire its own action potential in specific situations. Here, the membrane potential change is referred to as an excitatory postsynaptic potential, or EPSP. In other situations, the alteration is known as an inhibitory post-synaptic potential, or IPSP, and it reduces the likelihood that the target cell will fire an action potential. An EPSP depolarizes a cell by increasing its internal potential and moving the membrane potential closer to the point at which an action potential would be fired. Occasionally, a single EPSP isn't very big , but it can sum together with other EPSPs to trigger an action potential.

Coordination Mechanism

1. Sensory Input: Sensory neurons detect changes in the environment and send signals to the CNS.
2. Integration: The CNS processes these signals and determines the appropriate response.
3. Motor output : Motor neurons carry signals from the CNS to effectors to excute the response.

Reflex Actions

Male Reproductive System

1. Testes: The testes are a pair of male gonads . The testes produce sperm and reproductive hormones ( Testosterone). Each testis is approximately 2.5 by 3.8 cm (1.5 by 1 inch) in size .It is  divided into wedge-shaped lobes by septa.  These lobes houses the coiled seminiferous tubules that produce sperm.
2. Epididymis: This is a narrow ,tightly coiled tube attached to the testicles that  stores sperm as they mature. The Sperm cells finish maturing in the epididymis and stored. 
3. Vas Deferens:  Vas deferens is a long , muscular tube that carries mature sperm from the epididymis to the urethra prior to ejaculation
4. Seminal Vesicles and : The seminal vesicles  are also called vesicular glands or seminal glands .These are a pair of convoluted glands that lie  behind the urinary bladder of males . They secrete seminal fluid that nourishes and transports sperm.
5. Prostate Gland :  The  prostrate gland is located  just below the bladder and in front of the rectum in men. It is the largest accessory gland of the male reproductive system. The prostatic fluid comprise about one-third volume of the ejaculate .The main constituents of the prostatic fluid include Zinc, citric acid, choline , acid phosphatase ( enzyme), seminin, plaminogen activator and Prostate Specific Antigen (PSA).PSA is an important blood marker of prostate size and proliferation and its concentration increases in benign prostate hypertrophy, inflammation and prostatic cancer.
6. Penis:  The penis is located at the front of the body at the base of pelvis .The scrotum containing the testes is situated beneath the penis.  The penis delivers sperm into the female reproductive tract. The penis drains urine from the urinary bladder and is a copulatory organ during intercourse.

Female Reproductive System

1. Ovaries:  The ovaries are a pair of egg shaped  female glands ,on each side of the uterus , that produce eggs (ova) and hormones like estrogen and progesterone. These hormones play an important role in the development of female traits, such as breast development, body shape, and body hair and  are also involved in the menstrual cycle, fertility, and pregnancy. Each of the egg produced by the ovary is located inside a structure called a follicle. The cells in the follicle protect the egg and help it mature.
2. Fallopian Tubes:  These extend from the upper corners of the uterus and connect the ovaries to the uterus . The ovary end of the tube has a fringelike structure that moves in waves. The motion sweeps eggs from the ovary into the tube. The fallopian tubes are the site of fertilization.
3. Uterus:  The uterus is a muscular organ shaped like an inverted pear. The uterus has a thick lining of tissues called the endometrium. The uterus houses and nourishes the developing fetus until birth. During pregnancy the uterus can expand and can accommodate the developing fetus The contractions of the muscular walls of the uterus push the baby through the cervix and out of the body during birth . 
4. Cervix: The lower part of the uterus that opens into the vagina. It is a muscular tunnel like organ. The cervix is about 4 cm long and 3 cm in diameter, and its size varies according to a woman's age and reproductive history.The sperm travel from the vagina up the cervical canal into the uterine cavity, then into the fallopian tubes to fertilize the egg. Fluids such as the menstrual blood, to pass from the uterus into the vagina through the cervix.  It also widens during the birth of a baby. The cervix has two main parts, the ectocervix  ( outer part of the cervix ) and the endocervix ( inner part of the cervix that forms a canal that connects the vagina to the uterus). The area between the endocervix and ectocervix  is called the squamocolumnar junction (also called the transformation zone). Most abnormal cell changes and cervical cancers begin in the squamous cells (thin, flat-looking cells under a microscope) of the squamocolumnar junction.
5. Vagina:  This is a tube-like structure about 9 centimeters (3.5 inches) long. The  vagina begins at the vulva  and extends upward to the uterus. It has muscular walls lined with mucous membranes. It Receives sperm during intercourse and serves as the birth canal.
6. Vulva :  The external female reproductive structures are referred to collectively as the vulva and include the labia , which are the “lips” of the vulva. The labia protect the vagina and urethra.  Both of vagina and urethra have openings in the vulva .

The Process of Human Reproduction

1. Ovulation: Ovulation is the release of egg cells from the ovaries as part of the ovarian cycle. Women are born with about 1 million to 2 million eggs of which  only 300 to 400 are released  through ovulation during their lifetimes. Ovulation is a phase in the menstrual cycle of a women. On average, it happens on day 14 of a 28-day menstrual cycle. The process of ovulation begins when the hypothalamus of the  brain releases the  Gonado tropin releasing hormone  (GnRH). GnRH causes your pituitary gland of the brain to secrete Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH). FSH stimulates the maturation of the follicles in the ovary and at around the 14th day of the mensural cycle the LH surges causing the ovary to release the egg which is referred to as ovulation.. After ovulation, the hormone progesterone rises and prepares the uterus for pregnancy.
2. Fertilization: Fertilization is a complex process that refers to the process of union of the sperm and ova to form a zygote ( fertilized egg). The zygote which is a single cell, contains all the genetic information ( half from mother, half from father) needed to become a baby and initiates embryonic development. Fertilization marks the beginning of pregnancy. Of the millions of sperm released in to the vagina during ejaculation many are overcome by the acidity of the vagina ( pH 3.8). Further the thick  cervical mucus prevent their entry in to the uterus. In addition to all these ,  of the sperms that enter , thousands are destroyed by the phagocytic uterine leukocytes. The journey of the sperm is thought to be facilitated by uterine contractions and usually takes from 30 minutes to 2 hours. If the sperm do not encounter an oocyte immediately, they can survive in the uterine tubes for another 3–5 days. On the other hand  oocyte can survive independently for only approximately 24 hours following ovulation.

"Sperm and the Process of Fertilization" by Rice University is licensed under CC BY 4.0

Mendelian Genetics

Johann Gregor Mendel is regarded as “Father of modern genetics,"

•Genetics is the study of heredity.

•Mendel was born in Austria in 1822. He was a monk and he discovered the basic principles of heredity

• He conducted experiments on Pea plant in his monastery's garden.

•His experiments laid the foundation of modern Genetics

• The postulates put forth by Mendel form the basis of classical, or Mendelian, genetics.

    Pea plants - his primary model system to study a specific biological phenomenon to be applied to other systems. In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants to the local Natural History Society. He demonstrated that traits are transmitted faithfully from parents to offspring independently of other traits and in dominant and recessive patterns. In 1866, he published his work, Experiments in Plant Hybridization, in the proceedings of the Natural History Society of Brünn in 1866 The scientific world largely ignored Mendel's discoveries because they mistakenly thought that heredity involved combining parental features to give kids an intermediate physical appearance. During his lifetime, Mendel did not receive recognition for his outstanding contributions to science. It was not  until 1900 that his work was rediscovered, by three European botanists working independently namely, Hugo de Vries: From Holland , Carl Correns: From Germany ,Erich von Tschermak: From Austria .

Mendel's Experiments with Pisum sativum as Model System

Mendel carried out his experiments in  the garden pea, Pisum sativum, to study inheritance.

• Mendel performed hybridizations Experiments which involves mating two true-breeding individuals that have different traits.
• Pea, is naturally self-pollinating.
• Mendel pollinated the pea plants by manually transferring pollen from the anther of a mature pea plant of one variety to the stigma of a separate mature pea plant of the second variety.
• In plants, pollen carries the male gametes to the stigma, a sticky organ that traps pollen and allows the male gamete to move down the pistil to the female gametes (ova)
• To prevent the pea plant that was receiving pollen from self-fertilizing and confounding his results, Mendel removed all of the anthers from the plant’s flowers before they had a chance to mature.
• Plants used in first-generation crosses were called P0, or parental generation one, plants .
• Mendel collected the seeds belonging to the P0 plants that resulted from each cross and grew them the following season.
• These offspring were called the F1, or the first filial (filial = offspring, daughter or son), generation.
• Once Mendel examined the characteristics in the F1 generation of plants, he allowed them to self-fertilize naturally.
• He then collected and grew the seeds from the F1 plants to produce the F2, or second filial, generation.
• Mendel’s experiments extended beyond the F2 generation to the F3 and F4generations, and so on.
• Interestingly it was the ratio of characteristics in the P0−F1−F2 generations that were became the basis for Mendel’s postulates.

Why did Mendel choose pea plant for his experiments?

• This species naturally self-fertilizes, such that pollen encounters ova within individual flowers.
• The flower petals remain sealed tightly until after pollination, preventing pollination from other plants. The result is highly inbred, or “true-breeding,” pea plants and produce offspring that look like the parent.
• By carrying out his experiments with true-breeding pea plants, Mendel avoided the appearance of unexpected traits in offspring that might occur if the plants were not true breeding.
• The garden pea also grows to maturity within one season, meaning that several generations could be evaluated over a relatively short time.

• Large quantities of garden peas could be cultivated simultaneously, allowing Mendel to conclude that his results did not come about simply by chance.

Mendel reported the results of his crosses involving seven different characteristics, each with two contrasting traits .A trait is defined as a variation in the physical appearance of a heritable characteristic. The contrasting characteristics  studied by Mendel in  pea plant included plant height, seed texture, seed color, flower color, pea pod size, pea pod color, and flower position. For the characteristic of flower color, for example, the two contrasting traits were white versus violet.

"Mendel hybridization experiment" by Nancy Barrickman; Kathy Bell, DVM, MPH; and Chris Cowan, M.S. is licensed under CC BY 4.0

Mendel generated large numbers of F1 and F2 plants in order to fully examine each characteristic. His findings were consistent. Reginald Punnett, who developed a simple tool, now known as the Punnett Square, to predict the probability of genotypes and phenotypes from controlled crosses.

"Mendel hybridization experiment" by Nancy Barrickman; Kathy Bell, DVM, MPH; and Chris Cowan, M.S. is licensed under CC BY 4.0

Monohybrid Cross

• A monohybrid cross – is a cross between two homozygous individuals resulting in the hybrid of two individuals • It can be easily shown through a Punnett Square.

"Monohybrid cross" by Shelli Carter and Lumen Learning. is licensed under CC BY 4.0

•Based on this experimental findings Mendel concluded that the characteristics could be divided into expressed and latent traits.

•He called these, respectively, dominant and recessive traits.

•Dominant traits are those that are inherited unchanged in a hybridization.

•Recessive traits become latent, or disappear, in the offspring of a hybridization.

•The recessive trait does, however, reappear in the progeny of the hybrid offspring.

•For example pea plants that are true-breeding for the dominant yellow phenotype are crossed with plants with the recessive green phenotype. This cross produces F1 heterozygotes with a yellow phenotype.

• Punnett square analysis can be used to predict the genotypes of the F2 generation.

• A self-cross of one of the Yy heterozygous offspring can be represented in a  Punnett square because each parent can donate one of two different alleles.
• Therefore, the offspring can potentially have one of four allele combinations: YY,Yy, yY, or yy .
• There are two ways to obtain the Yy genotype: a Y from the egg and a y from the sperm, or a y from the egg and a Y from the sperm ( Reciprocal cross)
• Both of these possibilities must be counted.
• The result of these heterozygous combinations are genotypically and phenotypically identical offsprings despite their dominant and recessive alleles deriving from different parents. They are grouped together.
• Because fertilization is a random event, we expect each combination to be equally likely and for the offspring to exhibit a ratio of YY:Yy:yy genotypes of 1:2:1 . ( genotypic ratio)
• Furthermore, because the YY and Yy offspring have yellow seeds and are phenotypically identical, applying the sum rule of probability, we expect the offspring to exhibit a phenotypic ratio of 3 yellow:1 green.
•  Mendel observed approximately this ratio in every F2 generation resulting from crosses for individual traits.
• Mendel validated these results by performing an F3 cross 
• In F3 he self-crossed the dominant- and recessive-expressing F2 plants. When he self-crossed the plants expressing green seeds, all of the offspring had green seeds, confirming that all green seeds had homozygous genotypes of yy.
• When he self-crossed the F2 plants expressing yellow seeds, he found that one-third of the plants bred true, and two-thirds of the plants segregated at a 3:1 ratio of yellow:green seeds.
• In this case, the true-breeding plants had homozygous (YY) genotypes, whereas the segregating plants corresponded to the heterozygous(Yy) genotype. When these plants self-fertilized, the outcome was just like the F1 self-fertilizing cross.

Based on these experimental results Mendel postulated the Laws of Heredity , which are popularly called as Mendel's Laws of Inheritance.

Law of dominance:

The law of dominance states that “In a cross between a pair of organisms with pure contrasting characteristics, only the dominant of the pair expresses itself phenotypically while the other remains hidden in the F1 generation”. The character that expresses in F1 is called Dominant character While that is hidden is called Recessive character" Test cross

"Mendel hybridization experiment" by Nancy Barrickman; Kathy Bell, DVM, MPH; and Chris Cowan, M.S. is licensed under CC BY 4.0

Back Cross

• Backcrossing is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent, • In order to achieve offspring with a genetic identity which is closer to that of the parent. • It is used in horticulture, animal breeding and in production of gene knockout organisms. "Back cross" by WikiLectures, project of the First Faculty of Medicine, Charles University is licensed under CC BY-SA 4.0

Law of Segregation

• This law is also known as Mendel's Law of purity of gametes • The law states that “each characteristic of an organism is controlled by two alleles. During gametes formation in meiosis I, the alleles from each gene will segregate from each other and each gamete will only carry one of the alleles”

• When a pair of alleles are brought together in the F1 generation, they remain together without mixing or contaminating each other and separate completely during the gametogenesis.
• Also called Law of purity of gametes because each gamete remains pure in itself i.e., having T gene for Tallness and t gene for dwarfness.

Dihybrid cross

Dihybrid cross is a cross between two different genes that differ in two observed traits A dihybrid cross is a cross between two true-breeding parents that express different traits for two characteristics. for example consider the characteristics of seed color and seed texture for two pea plants, one that has green, wrinkled seeds (yyrr) and another that has yellow, round seeds (YYRR). Because each parent is homozygous, the law of segregation indicates that the gametes for the green/wrinkled plant all are yr, and the gametes for the yellow/round plant are all YR. Therefore, the F1 generation of offspring all are YyRr i.e., each gamete receives either an R allele or an r allele along with either a Y allele or a y allele. The cross is based on Mendel’s Law of Independent Assortment which states that “When two or more characteristics are inherited, individual hereditary factors assort independently during gamete production and the inheritance of one trait does not affect the inheritance of another” The law of independent assortment states that a gamete into which an r allele sorted would be equally likely to contain either a Y allele or a y allele. Therefore when the F1 heterozygote is self-crossed : four equally likely gametes that can be formed lows: YR, Yr, yR, and yr. Arranging these gametes along the top and left of a 4 by 4 Punnett square gives us 16 equally likely genotypic combinations. From these genotypes, we infer a phenotypic ratio of 9 round/yellow:3 round/green:3 wrinkled/yellow:1 wrinkled/green. The 9:3:3:1 dihybrid phenotypic ratio can be divided into two 3:1 ratios due to separate assortment and dominance; these ratios are typical of any monohybrid cross that exhibits both dominant and recessive traits. In the aforementioned dihybrid cross, if we were to ignore seed color and simply take seed texture into account, we would anticipate that three quarters of the F2 generation progeny would be round and one quarter would be wrinkled. If we were to separate out solely the color of the seeds, we would predict that three-quarters of the F2 offspring would be yellow and the remaining one-quarter would be green. We can use the product rule because the sorting of alleles for texture and color is an independent occurrence. Consequently, it is anticipated that the proportion of round and yellow F2 offspring would be (3/4) Å~ (3/4) = 9/16, and the proportion of wrinkled and green offspring is expected to be (1/4) Å~ (1/4) = 1/16. These ratios are the same as what a Punnett square would yield. Because each of these genotypes has a dominant and a recessive phenotype, the product rule can also be used to determine the round, green, wrinkled, yellow offspring. Thus, the formula for calculating each proportion is  calculated as (3/4) Å~ (1/4) = 3/16. The law of independent assortment also indicates that a cross between yellow, wrinkled (YYrr) and green, round (yyRR) parents would yield the same F1 and F2 offspring as in the YYRR x yyrr cross.

"Mendel hybridization experiment" by Nancy Barrickman; Kathy Bell, DVM, MPH; and Chris Cowan, M.S. is licensed under CC BY 4.0

Co dominance

Codominance is a form of inheritance • In this case the alleles of a gene pair in a heterozygote are both expressed. • As a result, the phenotype of the offspring is a combination of the phenotype of both the parents. • Thus, the trait is neither dominant nor recessive

ABO Blood Group system as an example for Codominance

"Co Dominance " by DylanAudette, via Wikimedia Commons is in the Public Domain, CC0

Sickle Cell Anaemia as an example for Co dominance

Incomplete Dominance

"Incomplete Dominance " by Sciencia58, via Wikimedia Commons is in the Public Domain, CC0

Sex Determination in Mammals

The Y Chromosome

Genes of Y Chromosome

"Y Chromosomes" by Christinelmiller is licensed under CC BY-SA 4.0

MSY- Male-specific region of Y:

SRY gene:

PAR:

The AZF gene

ZFY gene

Functions of Y chromosome

Inheritance pattern of Y chromosome:

• The Y chromosome follows a unique pattern of inheritance that is known as the Y-linked inheritance
•  In this type of inheritance the genes on the Y chromosome transmit only from father to their male child only
• The inheritance pattern of the Y chromosome is widely useful for tracing the lineage of family.
•  Genes located on Y chromosomes only inherited to male individuals and hence diseases or traits associated with the Y chromosome are thus inherited only in the males.

The X Chromosome

Diseases due to Changes in the structure or number of X chromosomes

Functions of X chromosome

Sex Determination

Sex Determination in Mammals-XX/XY System

XX-XOType

ZO-ZZ Type

ZW-ZZ type

"Avian sex determination" by CFCF via wikimedia commons is licensed under CC BY-SA 3.0

1. The wild-type version,  has Genotype : C⁺C⁺, is expressed as brown fur.
2. The chinchilla , genotype : has  c^chc^ch, is expressed as black-tipped white fur.
3. The Himalayan phenotype, has genotype :  c^hc^h, has black fur on the extremities and white fur elsewhere.
4.  The albino, or “colorless” phenotype,  genotype cc, is expressed as white fur.

• The A blood group blood RBCs express the Antigen A on their  surface ; Expressed by the alleles I^A I^A   and  I^A    i
• The B blood group blood RBCs express the Antigen B on their  surface ; Expressed by the alleles I^B I^B   and  I^B   i
• The A Bblood group blood RBCs express both the Antigen A and B on their  surface ; Expressed by the alleles   I^A I^B   
• The O blood group blood RBCs do not express any Antigen A on their  surface ; These possess the alleles   ii

" ABO blood system in humans " by Melissa Hardy is licensed under CC BY-NC 4.0

Genetic Recombination

1. Interchromosomal recombination : This occurs through independent assortment of alleles, which are on different but on homologous chromosomes.
2. Intrachromosomal recombination : Occurs through crossing over

Crossing over

Chiasma and Crossing Over

Chiasma Terminalization

Mechanism of Crossing Over

Linkage

• But if genes are situated in the same chromosome and if they are fairly close to each other ,they tend to be inherited together. This is called Linkage • The linked genes do not assort independently but tend to stay together in the same combination as they were in the parents.
• Genetic linkage describes the tendency of allele that are located close together on a chromosome to be inherited together during the meiosis phase of sexual reproduction. • Genes whose loci are nearer to each other are less likely to be separated onto different chromatids during chromosomal crossover. • Such genes are said to be genetically linked i.e., the nearer two genes are on a chromosome, lesser is the chance of a crossing occurring between them, and therefore they are more likely they are to be inherited together.
• Genetic linkage was first discovered by the British geneticists William Bateson, Edith Rebecca Saunders and Reginald Punnett shortly after Mendel's laws were rediscovered. • The study about genetic linkage was expanded by the work of Thomas Hunt Morgan. • Morgan observed that the amount of crossing over between linked genes differs • This observation led to the idea that crossover frequency might indicate the distance separating genes on the chromosome.

Chromosomes Theory of Linkage

Chemical Structure of DNA Subunits

"DNA Nucleotide" by Charles Molnar and Jane Gair is licensed under CC BY 4.0

"Purines and Pyrimidines" by Charles Molnar and Jane Gair is licensed under CC BY 4.0

"DNA" by Charles Molnar and Jane Gair is licensed under CC BY 4.0

The Structure of RNA

There is a second polymer of nucleotides in the cell ,  called ribonucleic acid, or RNA.

• a nitrogenous base- namely adenine,  guanine, uracil and cytosine (they do not contain thymine, which is instead replaced by uracil, symbolized by a “U.)”
• a five-carbon  Ribose sugar, and
• a phosphate group.

"Ribose and Deoxyribose" by Charles Molnar and Jane Gair is licensed under CC BY 4.0

 Unlike DNA which is double stranded , RNA exists as a single-stranded molecule.

DNA Replication

DNA Replication in Eukaryotes

Initiation

Elongation

Termination

"Ends of eukaryotic chromosomes" by Charles Molnar and Jane Gair is licensed under CC BY 4.0

DNA Replication in Prokaryotes

DNA Repair

Key Assumptions of the Hardy-Weinberg Equilibrium

1. Large Population Size: Genetic drift (random changes in allele frequencies) is minimal in large populations.
2. Random Mating: Individuals pair by chance, not according to genotype or phenotype.
3. No Mutation: No new alleles are introduced or altered.
4. No Migration: No new members enter or leave the population, keeping allele frequencies stable.
5. No Natural Selection: All genotypes have equal chances of surviving and reproducing.

The Hardy-Weinberg Equation

• p (frequency of the dominant allele, Y)
• q (frequency of the recessive allele, y)

• p² = frequency of individuals with the homozygous dominant genotype (YY)
• 2pq = frequency of individuals with the heterozygous genotype (Yy)
• q² = frequency of individuals with the homozygous recessive genotype (yy)

Example of the Hardy-Weinberg Principle

• p = 0.8
• q = 0.2

• p² = (0.8)² = 0.64 (64% are AA)
• 2pq = 2 * 0.8 * 0.2 = 0.32 (32% are Aa)
• q² = (0.2)² = 0.04 (4% are aa)

"Hardy-Weinberg Principle of Equilibrium" by Libre Texts Biology is licensed under CC BY 4.0

The Biotic and Abiotic factors of the Ecosystem

• Biotic Factors: These are the living parts of an ecosystem and include:
  • Producers:are living things that use photosynthesis to create their own food, such as plants and algae.
  • Herbivores (plant-eaters), carnivores (meat-eaters), omnivores (plant-and meat-eaters), and
  • Consumers: Organisms that eat other living things for energy, including herbivores (plant-eaters), carnivores (meat-eaters), omnivores (both plant and meat-eaters), and decomposers (organisms like fungi and bacteria that break down dead matter).
  • Decomposers  : These organisms break down dead materials and  returns nutrients to the soil.
• Abiotic Factors: These are the non-living elements of an ecosystem, which include:
  • Sunlight: The majority of ecosystems rely on sunlight as their main energy source
  • Water : is necessary for all living things.
  • Soil: Supports a variety of species and gives plants nutrition.
  • Climate and temperature  :have an impact on whether species can survive and flourish in a given ecosystem.

Energy flow and Nutrient cycling in an Ecosystem

Food Chain

• Producers: Algae or plants that use photosynthesis to create their own nourishment. (For instance, grass)
•  Primary consumers :Herbivores that consume producers are the  Primary consumers. (grasshopper, for instance)

• Test your understanding

• Secondary Consumers : Carnivores or omnivores  that consume herbivores are examples of secondary consumers. (For instance, frogs)

• Test your understanding

• Tertiary consumers : Carnivores that ingest secondary consumers are known as tertiary consumers. (for instance, a snake)
• Quaternary consumers :Top predators with few or no natural enemies are known as quaternary consumers. (For instance, hawk)

• Test your understanding

Food Web

Differences Between Food Chain and Food Web