Biology (9700) | Photosynthesis as an energy transfer process

Cambridge International A Level Biology (9700) Study Notes

Chapter 13: Photosynthesis as an Energy Transfer Process

Hello future biologists! This chapter is the foundation of life on Earth. Photosynthesis is the incredible process by which plants turn light energy into chemical energy (food). Understanding this topic means understanding how energy flows through almost every food chain. Don't worry if the names of the stages seem long; we will break them down into simple, manageable steps!

Photosynthesis is the process by which light energy is captured by chloroplast pigments and converted into chemical energy, which is then used to synthesize complex organic molecules (like glucose) from simple inorganic ones (carbon dioxide and water).

The overall simplified equation is:

$$6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{Light energy, Chlorophyll}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$$

13.1 The Site of Photosynthesis: The Chloroplast

Photosynthesis occurs inside specialized organelles called chloroplasts. The structure of the chloroplast is perfectly adapted for capturing light and producing sugars.

Analogy: Think of the chloroplast as a tiny, specialised solar-powered factory.

Key Structures and Their Functions:

• Outer and Inner Membranes: Control the movement of substances into and out of the organelle.
• Stroma: This is the dense fluid (like the factory floor) inside the inner membrane. It is the site of the Light-Independent Stage (Calvin Cycle) where $\text{CO}_2$ is fixed to make sugars.
• Thylakoids: Flattened, sac-like membranes (like solar panels) that contain the photosynthetic pigments (chlorophylls). This is the site of the Light-Dependent Stage (LDS).
• Grana (singular: granum): Stacks of thylakoids. Stacking maximises the surface area available for light absorption and the components of the Electron Transport Chain (ETC).
• Thylakoid Space (Lumen): The space inside the thylakoid, essential for accumulating protons ($\text{H}^+$) to drive $\text{ATP}$ synthesis.
• 70S Ribosomes and small circular DNA: Allow the chloroplast to synthesize some of its own proteins (linking back to Topic 1: Cell Structure).

Key Takeaway: The light-dependent reactions happen on the thylakoid membranes; the light-independent reactions happen in the stroma.

13.2 Capturing Light Energy: Pigments and Spectra

To capture sunlight, chloroplasts use various pigments.

A. Chloroplast Pigments

These pigments are grouped into two main types, housed within the thylakoid membranes:

1. Chlorophylls ($\text{a}$ and $\text{b}$): The primary pigments. Chlorophyll a is the main reactant in the conversion of light energy to chemical energy.
2. Accessory Pigments ($\text{Carotene}$ and $\text{Xanthophyll}$): These broaden the range of light wavelengths absorbed and protect the chlorophylls from damage. They pass the absorbed energy on to chlorophyll a.

Did you know? Plants look green because chlorophyll absorbs mainly red and blue light, reflecting the green wavelengths back to our eyes.

B. Absorption and Action Spectra

You need to be able to interpret the graphs for these concepts:

• Absorption Spectrum: A graph showing the amount of light absorbed by a specific pigment (e.g., chlorophyll $\text{a}$) at different wavelengths (colours). The peaks are typically in the blue-violet and red regions.
• Action Spectrum: A graph showing the overall rate of photosynthesis at different wavelengths. The action spectrum closely matches the absorption spectrum of all pigments combined, proving that the pigments absorbed are indeed used for the reaction.

C. Separating Pigments using Chromatography

Chromatography is a technique used to separate and identify pigments based on their solubility.

1. The pigment extract (e.g., from a leaf) is spotted onto chromatography paper.
2. The paper is placed in a solvent.
3. The solvent moves up the paper, carrying the pigments. More soluble pigments travel further.
4. Each separated pigment band is identified by calculating its $R_f$ value.

$$R_f = \frac{\text{Distance travelled by pigment spot}}{\text{Distance travelled by solvent front}}$$

Memory Trick: Think $R_f$ = Relative front movement. This value is a constant for a given pigment and solvent, helping you identify which band is $\text{chlorophyll a}$, $\text{b}$, $\text{carotene}$, or $\text{xanthophyll}$.

Key Takeaway: Absorption spectrum shows *what* light is absorbed; action spectrum shows *how effectively* that light drives photosynthesis.

13.3 The Light-Dependent Stage (LDS)

This stage converts light energy into chemical energy in the form of $\text{ATP}$ and $\text{reduced NADP}$ (a hydrogen carrier).

Location: Thylakoid membranes (Grana).

Inputs: Light energy, $\text{H}_2\text{O}$, $\text{ADP}$, $\text{NADP}$.

Outputs: $\text{ATP}$, $\text{Reduced NADP}$, $\text{O}_2$ (as a waste product).

The LDS involves two main pathways for producing $\text{ATP}$, known as Photophosphorylation (literally: adding phosphate using light).

A. Non-Cyclic Photophosphorylation (The Primary Route)

This process produces both $\text{ATP}$ and $\text{reduced NADP}$ and releases oxygen. It uses both photosystems.

1. Photoactivation: Light energy is absorbed by pigments in Photosystem II (PSII). The energy excites electrons in the chlorophyll molecules to a higher energy level.
2. Electron Transport Chain (ETC): These energetic electrons are passed along a chain of electron carriers embedded in the thylakoid membrane. As they move, they release energy. (Details of specific carriers are not expected.)
3. Photolysis of Water: PSII replaces the lost electrons by splitting water using the oxygen-evolving complex. This process is called photolysis:

   $$\text{H}_2\text{O} \xrightarrow{\text{light}} 2\text{H}^+ + 2\text{e}^- + \frac{1}{2}\text{O}_2$$

   The $\text{O}_2$ is released into the atmosphere.

4. Chemiosmosis and ATP Synthesis: The energy released by the ETC is used to pump the $\text{H}^+$ ions ($\text{protons}$) from the $\text{stroma}$ into the $\text{thylakoid space}$. This creates a high concentration of protons in the thylakoid space. The protons flow back to the stroma through ATP synthase (facilitated diffusion), which uses the energy of the gradient to phosphorylate $\text{ADP}$ to form $\text{ATP}$.
5. Photosystem I (PSI): Electrons from the first ETC eventually reach PSI. Light absorption also excites electrons in $\text{PSI}$.
6. Reduction of NADP: These higher-energy electrons, along with $\text{H}^+$ ions from the stroma, are used to reduce $\text{NADP}$ to reduced NADP. This reaction is catalysed by the enzyme $\text{NADP}$ reductase.

Memory Aid: $\text{Non-Cyclic}$ has the letter 'N' like $\text{NADP}$ (it produces $\text{NADP}$), and starts the oxygen evolution ($\text{O}_2$).

B. Cyclic Photophosphorylation (The Backup Route)

This route is simpler, only involving $\text{PSI}$. It only produces $\text{ATP}$, not $\text{reduced NADP}$ or $\text{O}_2$.

• Only Photosystem I (PSI) is involved.
• Light energy causes photoactivation of chlorophyll in $\text{PSI}$.
• The energetic electrons are passed down a short $\text{ETC}$, releasing energy used to pump protons ($\text{H}^+$) across the thylakoid membrane.
• $\text{ATP}$ is synthesised by $\text{ATP synthase}$ (chemiosmosis).
• The electrons then return directly to PSI (hence "cyclic").

This occurs when the cell needs extra $\text{ATP}$ for the Calvin cycle but doesn't need to produce more $\text{reduced NADP}$.

Key Takeaway: $\text{LDS}$ converts light into chemical energy ($\text{ATP}$ and $\text{reduced NADP}$) and requires water.

13.4 The Light-Independent Stage (The Calvin Cycle)

This stage uses the chemical energy ($\text{ATP}$ and $\text{reduced NADP}$) produced by the $\text{LDS}$ to fix $\text{CO}_2$ and produce sugars.

Location: Stroma.

Inputs: $\text{CO}_2$, $\text{ATP}$, $\text{reduced NADP}$, $\text{RuBP}$.

Outputs: $\text{Triose phosphate (TP)}$ (used to make glucose/other organics) and regeneration of $\text{RuBP}$.

The Three Main Stages of the Calvin Cycle:

1. Fixation of Carbon Dioxide

   • Carbon dioxide ($\text{CO}_2$) enters the stroma.
   • The enzyme Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyses the reaction between $\text{CO}_2$ and Ribulose bisphosphate (RuBP), which is a 5-Carbon (5C) compound.
   • This unstable 6C intermediate immediately splits to form two molecules of Glycerate 3-phosphate (GP), a 3C compound.

   This fixation reaction is the start of building organic molecules!

2. Reduction of GP

   • The $\text{GP}$ molecules are converted (reduced) into Triose phosphate (TP), which is a 3C sugar.
   • This reduction requires energy and hydrogen: $\text{ATP}$ provides the energy, and $\text{reduced NADP}$ provides the hydrogen.
   • For every six molecules of $\text{TP}$ produced, only one net molecule leaves the cycle to become the final product (e.g., glucose). The other five are needed for the next stage.

3. Regeneration of RuBP

   • The remaining five molecules of $\text{TP}$ are reorganised and converted back into Ribulose bisphosphate ($\text{RuBP}$) (5C).
   • This regeneration requires energy supplied by $\text{ATP}$.
   • The cycle can now continue, ready to fix more $\text{CO}_2$.

Fate of Calvin Cycle Intermediates (Products)

The Calvin cycle is not just about making glucose. The intermediate compounds are used to synthesise all the complex organic molecules a plant needs:

• Triose phosphate (TP): Used to make carbohydrates (like glucose, sucrose, starch, cellulose), lipids (fatty acids, glycerol), and amino acids (after conversion).
• Glycerate 3-phosphate (GP): Can be used to produce some amino acids and lipids.

Key Takeaway: The Calvin cycle fixes $\text{CO}_2$ using $\text{Rubisco}$ and converts the 3C molecule ($\text{GP}$) into the final sugar product ($\text{TP}$), regenerating the acceptor molecule ($\text{RuBP}$).

13.5 Limiting Factors of Photosynthesis

The rate of photosynthesis depends on several environmental factors. The concept of a limiting factor means that when a process depends on multiple factors, the rate is limited by the factor that is nearest its minimum required value.

A. Light Intensity

• Effect: At low light intensities, the rate of photosynthesis increases linearly with increasing light intensity because light is the limiting factor.
• Plateau: At high light intensities, the rate plateaus (stops increasing) because the $\text{LDS}$ is occurring at its maximum speed. $\text{CO}_2$ concentration or temperature then becomes the new limiting factor.

B. Carbon Dioxide Concentration

• Effect: At low $\text{CO}_2$ concentrations, the Calvin cycle cannot run quickly because there is not enough substrate ($\text{CO}_2$) for $\text{Rubisco}$ to fix. $\text{CO}_2$ is the limiting factor.
• Plateau: When $\text{CO}_2$ concentration is high, the rate plateaus, as the plant can no longer produce $\text{ATP}$ and $\text{reduced NADP}$ quickly enough (the $\text{LDS}$ is now limiting the rate).

C. Temperature

• Effect: Photosynthesis involves many enzyme-catalysed reactions (especially in the Calvin cycle, involving $\text{Rubisco}$). Therefore, temperature has a strong effect.
• Increase: Rate increases up to the optimum temperature (around $25-30^\circ\text{C}$).
• Decrease: Above the optimum, enzymes (like $\text{Rubisco}$) begin to denature, causing the rate to drop sharply.
• Interaction: Temperature only limits the rate if light intensity and $\text{CO}_2$ concentration are sufficient. If $\text{CO}_2$ is low, temperature will have a smaller effect on the overall rate, as the $\text{LIS}$ is already restricted.

D. Practical Investigation using Redox Indicators

We can measure the rate of photosynthesis (specifically the LDS) using redox indicators like DCPIP or Methylene Blue, in a suspension of isolated chloroplasts.

• DCPIP is a blue dye that acts as an electron acceptor.
• When DCPIP accepts electrons from the $\text{ETC}$ during the $\text{LDS}$, it becomes reduced and changes from blue to colourless.
• The faster the colour change (decolorisation), the faster the rate of the light-dependent reaction.
• This setup can be used to investigate the effect of varying light intensity or light wavelength on the rate.

Common Mistake to Avoid: Remember that temperature primarily affects the enzyme-controlled $\text{LIS}$ ($\text{Calvin Cycle}$), while light intensity affects the $\text{LDS}$ ($\text{Photophosphorylation}$).

Key Takeaway: The rate of photosynthesis is determined by whichever factor ($\text{Light}$, $\text{CO}_2$, or $\text{Temperature}$) is least available or farthest from its optimum.