Photosynthesis

• Chloroplasts are small, flattened organelles found in plant cells.
• They have a double membrane called the chloroplast envelope.
• Thylakoids (fluid-filled sacs) are stacked up in the chloroplast into structures called grana (singular granum). The grana are linked together by bits of thylakoid membrane called lamellae (singular = lamella).
• Chloroplasts contain photosynthetic pigments (e.g chlorophyll a, chlorophyll b and carotene). These are coloured substances that absorb the light energy needed for photosynthesis. The pigments are found in thylakoid membranes - they're attached to proteins. The protein and pigment is called a photosystem.
• There are two photosystems used by plants to capture light energy. Photosystem 1 (or PS1) absorbs light best at a wavelength of 700 nm and Photosystem 2 (or PS2) absorbs light best at 680nm.
• Contained within the inner membrane of the chloroplast and surrounding the thylakoids is a gel-like substance called the stroma. It contains enzymes, sugars and organic acids.
• Carbohydrates produced by photosynthesis and not used straight away are stored as starch grains in the stroma.

1. The Light-Dependent Reaction

• As the name suggests, this reaction needs light energy.
• It takes place in the thylakoid membranes of the chloroplasts.
• Here, light energy is absorbed by photosynthetic pigments in the photosystems and converted to chemical energy.
• The light energy is used to add a phosphate group to ADP to form ATP, and to reduce NADP to form reduced NADP. ATP transfers energy and reduced NADP transfers hydrogen to the light-dependent reaction.
• During the process H2O is oxidised to O2.

2.The Light-Independent Reaction

• This is also called the Calvin Cycle and as the name suggests it doesn't use light energy directly. (But it does rely on the products of the light-dependent reaction.)
• It takes place in the stroma of the chloroplast.
• Here, the ATP and reduced NADP from the light-dependent reaction supply the energy and hydrogen to make glucose from CO2.

In the light-dependent reaction, the light energy absorbed by the photosystems is used for three things:

1. Making ATP from ADP and Pi. This reaction is called photophosphorylation.

2. Making reduced NADP from NADP.

3. Splitting water into protons (H+ ions), electrons and oxygen. This is called photolysis.

To understand the process you need to know that the photosystems (in the thylakoid membranes) are linked by electron carriers. Electron carriers are proteins that transfer electrons. The photosystems and electron carriers form an electron transport chain - a chain of proteins through which excited electrons flow.

• Light energy is absorbed by PS2.
• The light energy excites electrons in chlorophyll.
• The electrons move to a higher energy level (i.e they have more energy).
• These high-energy electrons move along the electron transport chain to PS1.

• As the excited electrons from chlorophyll leave PS2 to move along the electron transport chain, they must be replaced.
• Light energy splits water into protons (H+ ions) electrons and oxygen. (So the O2 in photosynthesis comes from water.)

• The excited electrons lose energy as they move along the electron transport chain.
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• This energy is used to transport protons into the thylakoid so that the thylakoid has a higher concentration of protons than the stroma. This forms a proton gradient across the membrane.
• Protons move down their concentration gradient, into the stroma, via an enzyme called ATP synthase. The energy from this movement combines ADP and Pi to form ATP.

• Light energy is absorbed by PS1, which excites the electrons again to an even higher energy level.
• Finally, the electrons are transferred to NADP, along with a proton (H+ ion) from the stroma, to form reduced NADP.

• The Calvin Cycle takes place in the stroma of the chloroplasts.
• It makes a molecule called triose phosphate from CO2 and ribulose biphosphate (a 5 carbon compound). Triose phosphate can be used to make glucose and other useful organic substances.
• There are a few steps in the cycle and it needs ATP and H+ ions to keep it going.
• The reactions are linked in a cycle, which means the starting compound, ribulose biphosphate, is regenerated.

• CO2 enters the leaf through the stomata and diffuses into the stroma of the chloroplast.
• Here, it's combined with ribulose biphosphate (RuBP), a 5-carbon compound. This gives an unstable 6-carbon compound, which quickly breaks down into two molecules of a 3-carbon compound called glycerate 3-phosphate (GP).
• Ribulose biphosphate carboxlyase (rubisco) catalyses the reaction between CO2 and ribulose biphosphate.

• Now ATP, from the light-dependent reaction, provides energy to turn the 3-carbon compound,GP, into a different 3- carbon compound called triose phosphate (TP).
• This  reaction also requires H+ ions, which come from reduced NADP (also from the light-dependent reaction). Reduced NADP is recycled to NADP.
• Triose phosphate is then converted into many useful organic compounds, e.g glucose.
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• Five out of every six molecules of TP produced in the cycle aren't used to make hexose sugars, but to regenerate RuBP.
• Regenerated RuBP uses the rest of the ATP produced by the light-dependent reaction.

The Calvin Cycle is the starting point for making all the organic substances a plant needs. Triose Phosphate (TP) and glycerate 3-phosphate (GP) molecules are used to make carbohydrates,lipids and proteins:

• Carbohydrates- hexose sugars (e.g glucose) are made by joining two triose phosphate molecules together and larger carbohydrates (e.g sucrose,starch,cellulose) are made by joining hexose sugars together in different ways.
• Lipids- these are made using glycerol,which is synthesised from triose phosphate, and fatty acids, which are synthesised from glycerate 3-phosphate.
• Proteins- some amino acids are made from glycerate 3-phosphate, which are joined together to make proteins.