Revise Photosynthesis as an energy transfer process for Cambridge 9700 (13.1)

At its heart, photosynthesis is a remarkable biochemical reaction. It takes simple inorganic compounds, energises them with light, and produces complex organic molecules. This is an endothermic process, meaning it requires an input of energy (light) to proceed. The products have more chemical energy stored in their bonds than the reactants.

6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ + 6O₂

Let's break this down:

• Reactants: Carbon dioxide (CO₂) from the atmosphere and water (H₂O) from the soil are the raw materials. Light energy is the essential energy input.
• Products: Glucose (C₆H₁₂O₆) is the primary organic molecule produced, storing the captured chemical energy. Oxygen (O₂) is released as a vital by-product.

In eukaryotic cells (plants and algae), photosynthesis occurs within specialised organelles called chloroplasts. Their structure is perfectly adapted for this function:

• Thylakoids: A system of interconnected membrane-bound sacs. They are often stacked into structures called grana (singular: granum). The thylakoid membranes contain the photosynthetic pigments and are the site of the light-dependent reactions.
• Stroma: A gel-like matrix that fills the chloroplast, surrounding the grana. It contains enzymes, ribosomes, and chloroplast DNA. The stroma is the site of the light-independent reactions (Calvin cycle).

Pigments are substances that absorb specific wavelengths of light. The main pigments in chloroplasts are arranged in light-harvesting clusters called photosystems within the thylakoid membranes.

• Chlorophylls: These are the primary pigments. Chlorophyll a is found at the reaction centre of photosystems and is essential for the energy conversion. Chlorophyll b is an accessory pigment that absorbs light at different wavelengths and passes the energy to chlorophyll a.
• Carotenoids: These are accessory pigments, such as carotene (orange) and xanthophyll (yellow). They absorb wavelengths of light that chlorophylls do not, broadening the spectrum of light that can be used for photosynthesis. They also protect chlorophyll from photodamage.

Absorption and Action Spectra

An absorption spectrum is a graph that shows the percentage of light absorbed by a pigment at different wavelengths. Chlorophylls, for instance, absorb light most strongly in the red and blue-violet regions of the spectrum and reflect green light, which is why plants appear green.

An action spectrum is a graph that shows the rate of photosynthesis at different wavelengths of light. The action spectrum for photosynthesis closely matches the combined absorption spectra of the chloroplast pigments, providing strong evidence that the light absorbed by these pigments is used to drive the process.

1. The Light-Dependent Stage

This stage occurs in the thylakoid membranes and requires light. Light energy is used to produce ATP and reduced NADP.

1. Photoactivation: Light energy is absorbed by pigments in the photosystems, exciting electrons to a higher energy level.
2. Photolysis of Water: To replace the electrons lost from chlorophyll, water molecules are split using light energy. This reaction produces electrons, protons (H⁺), and oxygen gas (O₂).
3. Photophosphorylation: The excited electrons pass along an electron transport chain, releasing energy. This energy is used to pump protons across the thylakoid membrane, creating a proton gradient. As protons flow back down their concentration gradient through an enzyme called ATP synthase, ATP is generated. This process is called chemiosmosis. The electrons, along with protons, are used to reduce NADP to reduced NADP.
4. The Light-Independent Stage (Calvin Cycle)

This stage occurs in the stroma and does not directly require light, but it depends on the products of the light-dependent stage (ATP and reduced NADP).

1. Carbon Fixation: A molecule of CO₂ combines with a 5-carbon compound called ribulose bisphosphate (RuBP). This reaction is catalysed by the enzyme RuBisCO. The resulting 6-carbon compound is unstable and immediately splits into two molecules of a 3-carbon compound, glycerate-3-phosphate (GP).
2. Reduction: Each molecule of GP is converted into another 3-carbon compound, triose phosphate (TP). This step requires energy from ATP and reducing power from reduced NADP.
3. Regeneration: For every 6 molecules of TP produced, one is used to synthesise organic molecules like glucose, while the other five are used to regenerate the starting RuBP, a process that requires more ATP.

The conversion of light energy to chemical energy in glucose is fundamental for several reasons:

1. Basis of Food Webs: The chemical energy stored in glucose provides the initial energy source for the plant itself and forms the base of nearly all food chains. This energy flows through ecosystems as organisms consume one another.
2. Oxygen Production: The oxygen released as a by-product is essential for aerobic respiration in most living organisms, allowing for the efficient release of energy from food.
3. Carbon Cycling: Photosynthesis removes carbon dioxide from the atmosphere and 'fixes' it into organic molecules, playing a critical role in regulating Earth's climate and the global carbon cycle.
4. Formation of Biomass: The organic molecules synthesised are used to build the plant's structure (e.g., cellulose, proteins, lipids), contributing to the total biomass of ecosystems.