Cambridge International A Level Biology (9700) Study Notes: Photosynthesis

Hello Biologists! Welcome to Topic 13: Photosynthesis. This is one of the most fundamental processes on Earth—it literally fuels almost all life! Understanding how plants convert light energy into chemical energy is key to understanding entire ecosystems and global carbon cycles. Don't worry if the reaction stages seem complex; we will break them down into simple, manageable steps.

Let's dive into the fascinating world of energy transfer in plants!

13.1 Photosynthesis as an Energy Transfer Process

The Big Picture: The Photosynthesis Equation

Photosynthesis is the process where light energy is used to convert carbon dioxide and water into complex organic molecules (sugars) and oxygen.

The overall simplified equation is:

\(6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light, chlorophyll}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2\)

The process occurs in two main stages, spatially separated within the chloroplast:

  1. Light-Dependent Stage (LDS): Requires light, produces ATP and reduced NADP.
  2. Light-Independent Stage (LIR or Calvin Cycle): Does not require light directly, uses ATP and reduced NADP to make glucose.

Chloroplast Structure and Function

Photosynthesis in eukaryotes takes place exclusively inside the chloroplasts. The structure is perfectly adapted for these two stages:

Key Structures and Their Roles:
  • Thylakoids: Flattened sacs/discs that contain the photosynthetic pigments (chlorophyll).
  • Grana (singular: granum): Stacks of thylakoids. The large surface area provides space for the light-absorbing pigments and electron transport chains.
  • Thylakoid Space (Lumen): The internal space within the thylakoid membranes where protons ($\text{H}^+$) build up, essential for ATP synthesis.
  • Stroma: The fluid-filled space surrounding the grana. It contains enzymes (like Rubisco) necessary for the Light-Independent Stage (Calvin Cycle).
  • 70S Ribosomes and small circular DNA: Chloroplasts, like mitochondria, possess their own DNA and small ribosomes, supporting the endosymbiotic theory.

Analogy: Think of the chloroplast as a green energy factory. The Grana/Thylakoids are the solar panels and power generators (LDS). The Stroma is the production floor where raw materials ($\text{CO}_2$) are turned into food using the generated energy (LIR).

Chloroplast Pigments and Light

Photosynthetic organisms use pigments to capture light energy. These pigments are embedded in the thylakoid membranes.

Major Pigments:

  • Chlorophyll a: The primary photosynthetic pigment.
  • Chlorophyll b, Carotenes, and Xanthophyll: Accessory pigments that extend the range of light wavelengths absorbed and transfer energy to chlorophyll a.
Understanding Light Spectra
  • Absorption Spectrum: A graph showing which wavelengths (colours) of light are absorbed by a specific pigment. Chlorophylls primarily absorb blue/violet and red light, reflecting green light (which is why leaves look green!).
  • Action Spectrum: A graph showing the relative rate of photosynthesis at different wavelengths of light. This usually mirrors the absorption spectrum of the overall pigments, showing peak photosynthesis rates in blue/red regions.

Quick Review: Pigment Separation (Chromatography)

We can separate and identify these pigments using chromatography. Identification involves calculating the R\(_f\) value (Retention Factor):

$$\text{R}_f = \frac{\text{Distance moved by pigment spot}}{\text{Distance moved by solvent front}}$$

The $\text{R}_f$ value is unique for a given pigment in a specific solvent, allowing identification.

The Light-Dependent Stage (LDS): Generating Energy Currency

The LDS occurs in the thylakoid membranes (grana). It converts light energy into chemical energy stored in ATP and reduced NADP (a hydrogen carrier).

Photophosphorylation: The Mechanism

Photophosphorylation is the process of generating ATP using light. There are two types:

1. Non-Cyclic Photophosphorylation (The Main Event)

This process involves both photosystems, generates ATP, reduced NADP, and releases Oxygen.

  1. Photoactivation: Light hits the pigment molecules in Photosystem II (PSII). The energy excites electrons to a higher energy level, causing them to leave the PSII reaction centre (chlorophyll).
  2. Photolysis of Water: To replace the lost electrons, water is split (photolysis) by the oxygen-evolving complex:
    $$\text{H}_2\text{O} \to 2\text{H}^+ + 2\text{e}^- + \frac{1}{2}\text{O}_2$$

    This is where the oxygen we breathe comes from!

  3. Electron Transport Chain (ETC) and Chemiosmosis: The excited electrons from PSII pass through an ETC.

    Key Concept: As the energetic electrons move down the ETC, they release energy. This energy is used to pump protons ($\text{H}^+$) from the stroma into the thylakoid space (lumen), creating a high concentration gradient.

  4. ATP Synthesis: Protons flow back down their concentration gradient, from the lumen into the stroma, via facilitated diffusion through the enzyme ATP synthase. This flow provides the energy (chemiosmosis) to convert ADP + $\text{P}_i$ into ATP.
  5. Photosystem I (PSI): Electrons reach PSI. Light energy hits PSI, re-exciting the electrons.
  6. NADPH Formation: The highly energetic electrons from PSI are then used, along with protons ($\text{H}^+$), to reduce the carrier molecule NADP in the stroma:
    $$\text{NADP}^+ + 2\text{e}^- + \text{H}^+ \to \text{Reduced NADP}$$
2. Cyclic Photophosphorylation (The Quick Fix)

This process involves only Photosystem I (PSI).

  • Electrons leaving PSI are passed back to the ETC, rather than reducing NADP.
  • This recycling of electrons continues to pump protons, generating additional ATP.
  • It produces ATP only; no reduced NADP is made, and no water is split/oxygen released.
  • Why is it needed? The Light-Independent Stage requires more ATP than reduced NADP, so cyclic phosphorylation makes up the deficit.

Key Takeaway for LDS: The job of the light-dependent stage is to turn sunlight into ATP (energy) and reduced NADP (reducing power) for the next stage.

The Light-Independent Stage (LIR): The Calvin Cycle

The LIR takes place in the stroma of the chloroplast and uses the ATP and reduced NADP generated by the LDS to fix carbon dioxide and produce sugars.

Don't worry if all the intermediates seem complicated. The syllabus only requires you to outline the three main stages and name the key compounds (RuBP, GP, TP).

The Three Main Stages of the Calvin Cycle:
  1. Carbon Fixation
    • Carbon dioxide ($\text{CO}_2$) enters the stroma and combines with a 5-carbon compound called Ribulose Bisphosphate (RuBP).
    • This reaction is catalysed by the enzyme Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase).
    • The unstable 6-carbon intermediate immediately splits, forming two molecules of Glycerate 3-Phosphate (GP) (a 3-carbon compound).
  2. Reduction
    • The two molecules of GP are converted (reduced) into two molecules of Triose Phosphate (TP) (a 3-carbon sugar).
    • This reduction requires energy from ATP and hydrogen from reduced NADP (both supplied by the LDS).
    • TP is the first stable organic molecule formed that can be used by the plant.
  3. Regeneration
    • Most (5 out of 6) molecules of Triose Phosphate (TP) are used to regenerate the original 5-carbon acceptor molecule, RuBP.
    • This process requires additional energy supplied by ATP.
    • The cycle is now ready to fix more $\text{CO}_2$.

Did you know? Rubisco is thought to be the most abundant enzyme on Earth because it is needed in vast quantities to catalyse the fixation step in every plant!

Fate of the Calvin Cycle Products

Only 1 molecule of TP out of every 6 produced is used to create useful organic molecules. The rest is recycled.

  • TP (Triose Phosphate): The primary output molecule. It is used to synthesise carbohydrates (like glucose for respiration or starch for storage), lipids (like phospholipids for membranes), and other complex molecules.
  • GP (Glycerate 3-Phosphate): An intermediate that can be diverted to produce some amino acids.

Key Takeaway for LIR: The Calvin cycle uses the energy packets (ATP and reduced NADP) to permanently incorporate $\text{CO}_2$ into organic sugar molecules (TP).

13.2 Investigation of Limiting Factors

Photosynthesis is influenced by environmental conditions. The Principle of Limiting Factors states that the rate of a physiological process (like photosynthesis) is limited by the factor that is nearest its minimum required value.

The Three Main Limiting Factors:

  1. Light Intensity
  2. Carbon Dioxide Concentration ($\text{CO}_2$)
  3. Temperature

Effects of Changing Factors on Rate

When studying these factors graphically, remember that increasing a factor will increase the rate of photosynthesis only until another factor becomes limiting (the graph plateaus).

  • Light Intensity: Rate increases proportionally with light intensity until another factor (like $\text{CO}_2$ concentration or temperature) becomes limiting. The LDS is directly light-dependent.
  • $\text{CO}_2$ Concentration: $\text{CO}_2$ is the substrate for Rubisco in the LIR. Rate increases with $\text{CO}_2$ concentration until light intensity or temperature limits the rate.
  • Temperature: Photosynthesis is enzyme-catalysed (especially Rubisco in the LIR).

    At low temperatures, rate is slow due to low kinetic energy. Rate increases up to the optimum temperature.

    Above the optimum, enzymes (especially Rubisco) begin to denature, causing a sharp drop in the rate.

Common Mistake: Temperature mainly affects the enzyme-controlled LIR, while light intensity affects the LDS. The $\text{CO}_2$ concentration affects the LIR (carbon fixation).

Investigating the Rate of Photosynthesis (Practical Work)

Method 1: Using Whole Aquatic Plants (e.g., Pondweed)

This is a classic experiment used to determine the effects of $\text{CO}_2$, temperature, or light intensity.

  • Measurement: The rate of photosynthesis is measured by counting the number of oxygen bubbles produced per minute, or by collecting and measuring the volume of oxygen gas produced.
  • Varying Factors:
    Light Intensity: Change the distance of a lamp from the plant.
    Temperature: Place the apparatus in water baths of different temperatures.
    $\text{CO}_2$ Concentration: Use different concentrations of sodium hydrogen carbonate solution.
Method 2: Using Chloroplast Suspensions and Redox Indicators

This technique directly measures the rate of the Light-Dependent Stage.

  • Principle: The LDR generates reduced NADP. We can use artificial electron acceptors (redox indicators) that change colour when they are reduced.
  • Indicators: Commonly used indicators are DCPIP (Dichlorophenolindophenol) and Methylene Blue.
  • Procedure: Chloroplast suspension, indicator, and buffer are mixed. The time taken for the indicator to change colour (e.g., DCPIP changes from blue to colourless/clear when reduced) is measured.
  • Results Interpretation: A faster colour change means a faster rate of electron transfer, and thus a faster rate of the light-dependent stage.

You can use this method to investigate the effects of light intensity or the effects of different wavelengths (colours) of light using specific colour filters.

Key Takeaway for Limiting Factors: Always identify which factor is limiting the rate. If you increase the rate-limiting factor, the overall rate increases; if you increase a factor that is already in excess, the rate stays the same until the next factor becomes limiting.