Photosynthesis

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Photosynthesis is the process by which light energy is converted to chemical energy, with the basic equation:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

The glucose molecules produced are used to make ATP during respiration.

Pigments and light absorption

For photosynthesis to begin, light energy must be captured by pigments known as chlorophyll a and chlorophyll b.

Different pigments absorb light of different wavelengths.
Different species have various combinations of pigments, giving them different colored leaves.
Having several pigments allows plants to use more of the spectrum of visible light for photosynthesis.

Light-dependent reaction

This stage occurs in the thylakoid membranes of the chloroplasts. Key steps include:

Excitation of Electrons:
- Chlorophyll in photosystem II (PSII) absorbs light energy, boosting the energy of a pair of electrons, raising them to a higher energy level (excited state).
- Electrons are taken up by an electron carrier. The chlorophyll is oxidised, while the electron carrier is reduced.
Electron Transport Chain:
- Electrons pass along electron carriers in a series of oxidation-reduction reactions.
- Energy is released at each stage as the electrons move to carriers of slightly lower energy levels.
- This energy pumps H⁺ ions from the stroma into the thylakoid lumen, creating a concentration gradient.
Chemiosmosis and ATP Production:
- H⁺ ions flow back into the stroma through ATP synthase, driving the phosphorylation of ADP into ATP.
Photosystem I (PSI):
- Electrons from PSII reach PSI, where they are re-excited by light.
- These high-energy electrons are used to reduce NADP to form reduced NADP (NADPH).
Photolysis of Water:
- Electrons lost from chlorophyll are replaced through the splitting of water (photolysis):
  H₂O → 2H⁺ + 2e⁻ + ½O₂
- Oxygen is either used in respiration or released as a by-product.

Light-independent reactions

The light-independent reaction occurs in the stroma of chloroplasts and does not require light directly. It relies on the products of the light-dependent reaction (ATP and NADPH). This stage is also known as the Calvin Cycle.

Carbon Fixation:
- Carbon dioxide diffuses into the leaf through stomata and into the stroma.
- It combines with ribulose bisphosphate (RuBP), a 5-carbon compound, in a reaction catalyzed by the enzyme rubisco.
- This forms two molecules of glycerate-3-phosphate (GP).
Reduction of GP:
- ATP and reduced NADP are used to convert GP into triose phosphate (TP).
- NADP and ADP + Pi return to the light-dependent reaction for regeneration.
Regeneration of RuBP:
- Most TP is used to regenerate RuBP to keep the cycle running.
- After six turns of the cycle, one molecule of glucose is produced.

Limiting factors

Photosynthesis can be limited by:

Light Intensity: Required for light-dependent reactions.
Carbon Dioxide Concentration: Required for the Calvin Cycle.
Temperature: Enzymes involved in photosynthesis operate best at their optimum temperature.

Limiting Factor Principle:

The rate of photosynthesis is controlled by the factor in shortest supply.
For example, in greenhouses, increasing CO₂ concentration or temperature can boost the rate of photosynthesis.
Day length can be extended using artificial lighting.

Definitions

Adenosine triphosphate (ATP): an activated nucleotide found in all living cells that acts as an energy carrier. The hydrolysis of ATP leads to the formation of adenosine diphosphate (ADP) and inorganic phosphate, with the release of energy.

Calvin cycle: a biochemical pathway that forms part of the light-independent reaction of photosynthesis, during which carbon dioxide is reduced to form carbohydrate (CO2 + RuBP → GP → TP → Glucose or RuBP)

Electron carrier molecule: a chain of carrier molecules along which electrons pass, releasing energy in the form of ATP as they do so.

Glycerate-3-phosphate: a three carbon sugar in the Calvin cycle. Two molecules are produced from ribulose bisphosphate + CO2, used to produce triose phosphate.

Granum: a stack of thylakoids in a chloroplast that resembles a pile of coins. This is the site of the light-dependent reaction of photosynthesis.

Light-dependent reactions: stage of photosynthesis in which light energy is required to produce ATP and reduced NADP.

Light-independent reactions: stage of photosynthesis which does not require light energy directly, but does need the products of the light dependent reaction to reduce CO2 and form carbohydrate.

Limiting factor: a variable that limits the rate of a chemical reaction.

Mesophyll: tissue found between the two layers of epidermis in a plant leaf comprising an upper layer of palisade cells and a lower layer of spongy cells.

NADP (nicotinamide adenine dinucleotide phosphate): a molecule that carries electrons produced in the light-dependent reaction of photosynthesis.

Oxidation: chemical reaction involving the loss of electrons.

Oxidation-reduction: a chemical reaction in which electrons are transferred from one substance to another substance. The substance losing electrons is oxidised and the substance gaining the electrons is reduced.

Photolysis: light energy splits water molecules, yielding electrons, hydrogen ions and oxygen (light-dependent reaction).

Photophosphorylation: the production of ATP using the energy in light

Producer: an organism that synthesises organic molecules from simple inorganic ones such as carbon dioxide and water. Most producers are photosynthetic and form the first trophic level in a food chain. See also consumer.

Reduction: chemical process involving the gain of electrons.

Ribulose bisphosphate: a five carbon sugar in the Calvin cycle. Produced from triose phosphate, combines with CO2.

Stroma: matrix of a chloroplast where the light-independent reaction of photosynthesis takes place.

Thylakoid: series of flattened membranous sacs in a chloroplast that contain chlorophyll and the associated molecules needed for the light-dependent reaction of photosynthesis.

Triose phosphate: three carbon sugar in the Calvin cycle. Produced from glycerate-3-phosphate, used to produce glucose or ribulose bisphosphate.

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