Photosynthesis

Photosynthesis is the process by which plants make food. Without it, life on Earth would struggle to survive since it forms the basis of most food chains and provides the oxygen that we breathe. It happens in two, fairly complex, stages which you (unfortunately) need to know all about: the light-dependent and light-independent reactions.

Overall reaction

Photosynthesis combines carbon dioxide with water to form glucose and oxygen. The glucose is used in respiration to produce energy while the oxygen is released from the plant through the stomata.

Chloroplast structure

Photosynthesis takes in the chloroplasts of plant cells. Chloroplasts contain fluid-filled sacs called thylakoids. Thylakoids are stacked up like pancakes to form structures which we call grana. Each granum is connected together by pieces of thylakoid membrane called lamellae. The gel-like substance which surrounds the thylakoids is called the stroma.

The thylakoids within the chloroplast provide a large surface area to allow as much light to be absorbed as possible. Within the thylakoid membrane are photosystems which consist of a pigment molecules attached to proteins. The pigment is what gives plants their colour and includes chlorophyll a, chlorophyll b and carotene. Plants contain two photosystems, called photosystem I and photosystem II. PSI absorbs light at a wavelength of 700 nm while PSII absorbs light at a wavelength of 680 nm.

The overall process of photosynthesis can be split into two stages: the first one stage is known as the light-dependent reaction and (unsurprisingly) requires light to get going. The second is called the light-independent reaction which doesn’t need light but does need the products that were generated in the first stage.

Light-Dependent Reaction (LDR)

The LDR takes place in the thylakoid membranes of the chloroplasts. It takes place in the following stages:

1. Light energy is absorbed by PSII (even though PSII is involved before PSI, it was discovered afterwards - hence the confusing naming system). Light excites electrons within PSII and causes them to move into a higher energy state. The electrons are passed onto a series of electron carriers within the electron transport chain to PSI.

2. The electrons which have been lost from PSII need to be replaced. This happens through the photolysis of water - light energy causes a water molecule to split apart and release hydrogen ions, electrons and oxygen. The electrons from water replace the electrons lost from PSII.

3. As the electrons move along the electron transport chain, they move from high to low energy. The energy lost by the electrons is used to pump hydrogen ions from the stroma into the thylakoids. This generates a proton gradient across the thylakoid membrane.

4. Protons flow down their concentration gradient through ATP synthase. The energy from the movement of protons is used to phosphorylate ADP to ATP (photophosphorylation) in a process called chemiosmosis.

5. Light is absorbed by PSI causing another electron to become excited and be passed along the rest of the electron transport chain.

6. The electron is passed onto NADP to form reduced NADP (NADPH). NADPH is an electron carrier which transfers electrons from one molecule to another.

7. The ATP and reduced NADP move into the stroma for the next stage of photosynthesis, the light independent reaction.

This process is known as non-cyclic photophosphorylation. There is another process called cyclic photophosphorylation, in which electrons repeatedly cycle through PSI. Electrons leave PSI but instead of being accepted by NADP they flow back down the chain to the first electron acceptor. This means that ATP is produced but no NADPH and may happen when NAPDH is in plentiful supply. Cyclic photophosphorylation is more common in plants with especially high ATP needs and may prevent excess light damaging photosynthetic proteins.

The Light-Independent Reactions (aka the Calvin Cycle)

The Calvin Cycle takes place in the stroma of the chloroplast and uses the products of the LDR (ATP and reduced NADP) to form glucose. The reactions which take place can be divided into three main stages: carbon fixation, reduction and regeneration.

Carbon fixation

• Carbon dioxide is ‘fixed’ by adding it to a 5-carbon molecule called ribulose bisphosphate (RuBP), forming a 6-carbon molecule. This reaction is catalysed by an enzyme called Rubisco.

• The 6C molecule is unstable and immediately breaks down to form two 3-carbon compounds called glycerate-3-phosphate (GP).

Reduction

• An isomerisation reaction occurs which converts GP into a different 3-carbon compound called glyceraldehyde-3-phosphate (GALP). GALP is also known as triose phosphate (TP). This reaction requires energy so ATP (from the light-dependent reaction) is hydrolysed into ADP.

• This reaction also requires electrons from the electron carrier reduced NADP (also from the LDR). Reduced NADP transfers electrons to GP, reducing it to GALP.

• Some GALP is converted into organic molecules, such as glucose, but some will be used to regenerate RuBP. For every 6 molecules of GALP, 1 is used to produce organic molecules whereas 5 will be used for RuBP regeneration.

Regeneration

• GALP is converted back into RuBP - this process requires energy which is generated by ATP hydrolysis.

• The cycle is completed and another round of carbon fixation can take place.

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