2.Plant Physiology
2.1 Photosynthesis
Photosynthesis
Dr V Malathi
Photosynthesis is an essential biological process to all life forms on earth that captures energy originating in space ( from sunlight) and convert it into chemical compounds ( like carbohydrates) that powers the metabolism of every organism.
Global carbon cycle involves the The building and breaking of carbon-based material. It involves the building up of complex organic molecules through photosynthesis from carbon dioxide and then back to carbon dioxide through the process of respiration.
The energy from coal and petroleum products which we use today represents the energy captured from sunlight by photosynthesis around 200 million years ago. These fossil fuels are the ancient remains of once-living organisms, and they provide best example of the carbon cycle .
The carbon cycle would not be possible without photosynthesis, because this process accounts for the “building” portion of the cycle
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.
The Process
Photosynthesis is a multi-step proces
It requires sunlight, carbon dioxide, and water as substrates . The products of the process include Oxygen and Glyceraldehyde-3-phosphate ( GA3P).
GA3P can subsequently converted into glucose, sucrose or other sugars , which the living things need for their energy,
on Earth.
All photosynthetic cells contain special pigments that absorb light energy.
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.
- the light-dependent reactions and
- 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.
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