Photosynthesis - Marine Science (9693) - Cambridge A-Level

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Marine Science A Level (9693): Chapter 7.1 Photosynthesis

Hello future marine scientist! This chapter dives into the single most important chemical process on Earth: photosynthesis. In the ocean, this process, carried out primarily by tiny phytoplankton, macroalgae, and marine plants, forms the absolute base of almost every food web and profoundly influences global climate control. Mastering this topic is key to understanding energy flow (Topic 7) and ecological dynamics in the marine environment.

Section 1: Light in the Water Column

For photosynthesis to happen, marine producers need light. However, the ocean acts like a giant, selective filter.

1.1 Understanding Light Properties (7.1.1, 7.1.2)

Light, or white light, is actually composed of a spectrum of colours (Red, Orange, Yellow, Green, Blue, Indigo, Violet), each having a different wavelength.

Wavelength: The distance between successive crests of a wave. Different colours correspond to different wavelengths (e.g., red has a long wavelength, blue has a short wavelength).
Intensity: A measure of the power or brightness of light (how much light is shining on a surface).
Penetration: How far light can travel into the water column.

1.2 Light Penetration and Depth (7.1.3)

As light enters the water, it is absorbed or scattered. The wavelength determines how deep the light can penetrate:

The longer the wavelength, the more quickly the light is absorbed by water molecules.

Red light (long wavelength) is absorbed very quickly, usually disappearing within the top few metres of the water column.
Blue/Green light (short wavelengths) penetrates the deepest. This is why the deep ocean often looks blue or greenish-blue—these are the wavelengths that remain after others have been absorbed.

Key Takeaway: Producers in deep waters must be adapted to capture the blue light that makes it down to them, as red light is unavailable. This adaptation relies on special pigments (discussed below).

Section 2: The Photosynthesis Overview

Photosynthesis is the process by which producers (like phytoplankton and macroalgae) convert inorganic substances ($CO_2$ and $H_2O$) into energy-rich organic substances (glucose), fixing carbon into the food chain (7.1.4).

2.1 The Overall Equation (7.1.4)

The process can be summarised by the following balanced chemical equation:

$6CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2$

(Carbon Dioxide + Water $\xrightarrow{\text{light}}$ Glucose + Oxygen)

Important Note: This process is essential for generating biomass (the total mass of living material) which provides energy for subsequent trophic levels in the food chain (7.1.6).

2.2 The Two Stages of Photosynthesis (7.1.5)

Photosynthesis is not a single reaction but a two-stage process occurring within the chloroplast:

The Light-Dependent Stage (LD): Requires light energy.
The Light-Independent Stage (LI) / Calvin Cycle: Does not directly require light.

Energy is captured in the first stage and then transferred to the second stage using two key energy-carrying molecules: ATP (Adenosine triphosphate) and reduced NADP (Nicotinamide adenine dinucleotide phosphate) (7.1.6).

Analogy: Think of the Light-Dependent stage as the "solar farm" that captures light and generates usable energy currency (ATP and reduced NADP). The Light-Independent stage is the "factory" that uses that currency to build the final product (glucose).

Section 3: The Chloroplast and Pigments

Photosynthesis occurs in chloroplasts, organelles found in producers. Their structure is highly adapted to ensure the two stages occur efficiently (7.1.7, 7.1.8).

3.1 Chloroplast Structure (7.1.7)

A typical chloroplast has the following features:

Outer and Inner Membranes: Control the entry and exit of substances.
Stroma: The fluid interior (like the cytoplasm of the chloroplast). This is the site of the Light-Independent Stage.
Thylakoids: Flattened sacs or discs. The membranes of these sacs are where the pigments are located.
Thylakoid Space: The internal space within the thylakoid.
Grana (singular: Granum): Stacks of thylakoids. This is the site of the Light-Dependent Stage.

3.2 The Role of Photosynthetic Pigments (7.1.9, 7.1.10)

Pigments are molecules that absorb specific wavelengths of light. They are located within the thylakoid membrane in the grana.

The primary pigment is Chlorophyll a, which absorbs primarily red and blue light (and reflects green light, hence the typical green colour of many plants).

Accessory Pigments (7.1.9, 7.1.10):

Marine producers, especially those living deeper, use accessory pigments to capture the wavelengths that chlorophyll a misses, particularly the blue-green light that penetrates deep water.

Examples: Xanthophylls (yellow/brown) and Phycobilins (red/blue).
Marine Adaptation (7.1.10): Deep-sea algae (like some red algae) contain high levels of phycobilins, allowing them to capture the blue-green light that travels furthest. This explains why macroalgae colour changes with depth (from green near the surface, to brown, to red in deeper water).

Quick Review: The LD stage happens in the Grana (where pigments are), and the LI stage happens in the Stroma (the fluid).

Section 4: The Photosynthesis Mechanism Step-by-Step

We simplify the two stages, focusing only on the inputs, outputs, and energy transfer (as per syllabus 7.1.13 and 7.1.14).

4.1 Stage 1: The Light-Dependent (LD) Stage (7.1.13)

This stage occurs in the grana (thylakoid membranes). It requires light energy.

Photoactivation: Light energy is absorbed by the chlorophyll pigments, exciting electrons to a higher energy level. This process is called photoactivation.
Photolysis of Water: The excited electrons need replacing. This happens when water is split by light (photolysis):

Water $\rightarrow$ Protons ($H^+$) + Electrons ($e^-$) + Oxygen ($O_2$)
Energy Transfer: The high-energy electrons released during photoactivation transfer their energy, which is used to generate ATP and reduced NADP.

Outputs of LD Stage: ATP, reduced NADP, and Oxygen ($O_2$) (released as a waste product).

4.2 Stage 2: The Light-Independent (LI) Stage / Calvin Cycle (7.1.6, 7.1.14)

This stage occurs in the stroma. It does not require light directly, but depends entirely on the ATP and reduced NADP generated in the LD stage.

Carbon Fixation: Carbon dioxide ($CO_2$) from the environment is "fixed" (converted from inorganic to organic form). This is done using the enzyme Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase).
Synthesis: The fixed carbon uses the energy provided by ATP and the reducing power from reduced NADP (7.1.6) to synthesise organic molecules, eventually resulting in glucose ($C_6H_{12}O_6$).

Output of LI Stage: Glucose (used for biomass and respiration, as per AS content 3.2.6 and 3.2.7).

Did you know? Photosynthesis is the opposite of aerobic respiration. Glucose produced here is later broken down by the producer itself (and by consumers) in respiration to release usable energy (ATP).

Section 5: Investigating Photosynthesis Pigments (7.1.11, 7.1.12)

To understand which pigments are involved and what light they use, scientists perform two key types of analysis:

5.1 Absorption and Action Spectra (7.1.12)

Absorption Spectrum: A graph showing which wavelengths of light a specific pigment absorbs. (Chlorophyll *a* shows peaks in the blue and red regions).
Action Spectrum: A graph showing the overall rate of photosynthesis at different wavelengths of light.

Interpretation: The action spectrum should closely mirror the combined absorption spectra of all the pigments present. High absorption = high rate of photosynthesis.

5.2 Chromatography (7.1.11 PA)

Chromatography is a technique used to separate and identify different pigments found in a chloroplast sample (e.g., from an algal specimen).

Pigments are extracted and spotted onto chromatography paper.
The paper is placed in a solvent (mobile phase).
The solvent moves up the paper, carrying the pigments at different speeds depending on their solubility and size.
This separates the pigments into distinct coloured bands (e.g., chlorophylls, xanthophylls, carotenes).

$R_f$ Values: To identify unknown pigments, we calculate the Retention Factor ($R_f$), which is a ratio comparing how far the pigment travelled compared to the solvent front:

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

This calculated value can be compared to known values to identify the separated pigment.

Memory Aid: The Absorption spectrum shows what the pigment Absorbs. The Action spectrum shows the Actual rate of the process.

Section 6: Limiting Factors of Photosynthesis (7.1.15)

The rate of photosynthesis can only proceed as fast as the factor in shortest supply allows. This is the limiting factor (7.1.15). In marine environments, several factors can be limiting:

6.1 Key Limiting Factors

Photosynthesis relies on four main factors:

Light Intensity:
- Effect: Rate increases as light intensity increases, until another factor becomes limiting.
- Marine Relevance: In the surface layers (epipelagic zone), light intensity is usually high, but it drops dramatically with depth. Below the compensation point, light intensity is too low to sustain net production.
Wavelength of Light:
- Effect: Only certain wavelengths (blue and red) are most effective. If only green light is available, the rate will be low.
- Marine Relevance: In deep water, only blue light penetrates, limiting the rate of photosynthesis unless the producer has appropriate accessory pigments (7.1.10).
Carbon Dioxide Concentration ($CO_2$):
- Effect: An increase in $CO_2$ generally increases the rate of the Light-Independent stage, until saturated.
- Marine Relevance: $CO_2$ is relatively abundant in seawater but can sometimes limit growth in very productive areas or symbiotic relationships (like corals), where rapid uptake depletes local supplies.
Temperature:
- Effect: Photosynthesis involves many enzymes (especially Rubisco in the LI stage). Enzymes have optimal temperatures. Too low, and the rate slows; too high, and the enzymes denature.
- Marine Relevance: Producers in polar or deep-sea environments face low temperatures, slowing the process. Conversely, high sea surface temperatures (due to climate change) can cause stress, damaging the photosynthetic machinery (e.g., coral bleaching).

Key Takeaway: When studying experiments on photosynthesis (7.1.16 PA), remember that to test one factor (like light intensity), all other factors ($CO_2$, temperature, wavelength) must be kept constant (standardised) so that they don't become the limit.