Biology (0610) | Photosynthesis

Welcome to the exciting world of Plant Nutrition! This chapter focuses on Photosynthesis – the single most important chemical process on Earth. It's how plants act as tiny chefs, turning simple ingredients into food and oxygen, making life possible for almost everything else!

Don't worry if the chemistry seems tricky at first. We will break down the process, the ingredients, the factory (the leaf), and what controls the speed of the cooking!

✨ Section 1: The Magic Recipe – What is Photosynthesis?

What is Photosynthesis? (Core 6.1.1)

Photosynthesis is the process by which plants, using energy from light, synthesize (make) carbohydrates (sugars/starches) from simple raw materials: carbon dioxide and water.

Think of it like this: A plant is running a solar-powered factory. The Sun provides the power, and the plant uses basic ingredients (water and \(\text{CO}_2\)) to build its food (glucose).

The Ingredient List (Equations) (Core 6.1.2 & Supplement 6.1.10)

The process can be summarised using two key equations:

Word Equation (The Core version):

Carbon dioxide + Water \(\rightarrow\) Glucose + Oxygen (in the presence of light and chlorophyll)

Balanced Chemical Equation (The Supplement/Extended version):

$$ 6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 $$

The Key Player: Chlorophyll (Core 6.1.3 & 6.1.4)

Photosynthesis cannot happen without a green pigment called chlorophyll.

• It is found inside the chloroplasts (the specialised organelles in plant cells).
• Chlorophyll’s job is to capture the energy from sunlight.
• This light energy is then converted into chemical energy, which is used to synthesise (make) carbohydrates (glucose).

🌱 Section 2: What Happens to the Food? (Core 6.1.5 & 6.1.6)

Once the plant makes glucose, it has several uses. Glucose is a simple sugar, so plants convert it immediately into other forms for storage or transport.

Uses and Storage of Carbohydrates:

1. Starch: Glucose is quickly converted into starch for storage (especially in roots, seeds, and stems). Starch is insoluble, so it doesn't affect the water concentration inside the cell.
2. Respiration: Glucose is used immediately by the plant cells during respiration to release the energy (ATP) needed for all life processes (growth, transport, cell division).
3. Cellulose: Used to build cell walls, providing structure and support for the plant.
4. Sucrose: Used for transport. Glucose is converted into sucrose (a slightly larger sugar) before being moved from the leaves to other parts of the plant via the phloem.
5. Nectar: Glucose is converted into sugars that make up nectar, a sweet substance used to attract insects for pollination.

Did you know? The potato you eat is essentially a storage organ packed with starch made through photosynthesis!

The Importance of Mineral Ions

Plants need more than just \(\text{CO}_2\) and water. They absorb mineral ions from the soil (usually dissolved in water) to make other vital compounds:

• Nitrate Ions (\(\text{NO}_3^-\)): Essential for making amino acids. Amino acids are the building blocks of proteins (needed for growth, repair, and enzymes).
• Magnesium Ions (\(\text{Mg}^{2+}\)): Essential for making chlorophyll. Without magnesium, the leaves turn yellow (a condition called chlorosis) and the plant cannot capture enough light energy.

📜 Section 3: Investigating Photosynthesis (Core 6.1.7)

We need to prove that chlorophyll, light, and carbon dioxide are actually necessary for photosynthesis. We use experiments with controls to do this.

Step-by-Step Experiment: Testing for Starch (The basic indicator of photosynthesis)

1. De-starching: Keep the plant in the dark for 24-48 hours. This uses up all existing starch stores.
2. Treatment: Apply the required experimental condition (e.g., place a light-proof cover on a leaf for the 'need for light' test).
3. Boiling Water: Boil the leaf for 1 minute. This kills the cells and breaks down the cell membranes, making the cell walls permeable.
4. Hot Ethanol: Boil the leaf in hot ethanol (in a water bath!) for several minutes. This removes the green chlorophyll, leaving a white/pale leaf so colour changes can be seen clearly.
5. Rinsing: Dip the leaf back in water to soften it.
6. Iodine Solution: Add iodine solution.

Result: If starch is present, the leaf turns blue-black. If starch is absent, it remains brown/yellow.

The Three Key Requirements Experiments:

• Need for Light: Use a plant where one leaf has been partially covered with a light-proof material (like black paper) while the plant is placed in bright light. The uncovered part tests positive (blue-black); the covered part tests negative (brown).
• Need for Chlorophyll: Use a variegated leaf (a leaf that is partially green and partially white/yellow). Only the green parts (where chlorophyll is present) will test positive for starch.
• Need for Carbon Dioxide: Place a destarched plant in a sealed bell jar with a chemical that absorbs \(\text{CO}_2\) (like soda lime or potassium hydroxide). A control plant is placed in another bell jar with a chemical that does NOT absorb \(\text{CO}_2\). The plant without \(\text{CO}_2\) tests negative for starch.

Investigating Gas Exchange (Core 6.1.9)

We can use Hydrogencarbonate Indicator Solution to investigate gas exchange (the balance between photosynthesis and respiration) in aquatic plants.

• The indicator solution changes colour depending on the concentration of \(\text{CO}_2\).
• High \(\text{CO}_2\) (Acidic pH): Yellow (Happens in the dark when only respiration occurs).
• Normal \(\text{CO}_2\): Orange/Red (Normal conditions).
• Low \(\text{CO}_2\) (Alkaline pH): Purple (Happens in bright light when photosynthesis uses up \(\text{CO}_2\) faster than respiration produces it).

🚦 Section 4: What Slows Down the Process? (Limiting Factors)

The rate of photosynthesis is how fast the plant produces glucose. This rate can be affected by environmental factors (Core 6.1.8).

The Concept of Limiting Factors (Supplement 6.1.11)

A limiting factor is the factor (light, \(\text{CO}_2\), or temperature) that is in the shortest supply, restricting the rate of the reaction, even if other factors are abundant.

Analogy: If you are baking a cake, even if you have tonnes of flour and sugar, if you only have one egg, the egg is the limiting factor for how many cakes you can bake.

The Three Main Limiting Factors:

1. Light Intensity:
   • If light intensity is low, the rate is low because chlorophyll isn't receiving enough energy.
   • Increasing light intensity increases the rate (up to a point).
   • If light intensity is high, something else (like \(\text{CO}_2\)) becomes the limiting factor.
2. Carbon Dioxide Concentration:
   • \(\text{CO}_2\) is a raw material. If its concentration is low, the rate is restricted.
   • Increasing \(\text{CO}_2\) concentration increases the rate (up to a point).
   • \(\text{CO}_2\) concentration is usually the main limiting factor in natural environments.
3. Temperature:
   • Photosynthesis involves enzymes (like all metabolic reactions).
   • Low temperatures slow down enzyme activity, reducing the rate.
   • Increasing temperature increases the rate (up to the optimum temperature, usually around 25-35°C).
   • If the temperature gets too high (above 45°C), the enzymes begin to denature, and the rate drops rapidly.

🌿 Section 5: The Green Factory – Leaf Structure (Core 6.2.1, 6.2.2 & 6.2.3)

The leaf is perfectly designed to capture light, absorb gases, and transport materials – making it the primary site for photosynthesis.

General Adaptations of Leaves (Core 6.2.1)

• Large Surface Area: Maximises the absorption of sunlight and carbon dioxide.
• Thin Structure: Ensures a short diffusion distance for gases (\(\text{CO}_2\) in, \(\text{O}_2\) out) and for water to reach the cells.

Leaf Structures and Functions (Core 6.2.2 & 6.2.3)

Below is a breakdown of the key structures you must identify in diagrams and know the function of:

Layer 1: Protection and Control

• Cuticle: A thin, waxy, waterproof layer on the top of the leaf.

  Adaptation: Reduces water loss by evaporation (a process called transpiration).

• Upper & Lower Epidermis: Thin, protective cell layers on the top and bottom.

  Adaptation: Transparent (no chloroplasts) to allow light to pass through to the palisade layer below.

• Stomata (singular: Stoma) & Guard Cells: Small pores, usually on the lower epidermis, surrounded by two guard cells.

  Adaptation: The stomata open to allow \(\text{CO}_2\) to diffuse into the leaf and \(\text{O}_2\) and water vapour to diffuse out. Guard cells control this opening.

Layer 2: Photosynthesis Headquarters

• Palisade Mesophyll: Column-shaped cells located right under the upper epidermis.

  Adaptation: Packed tightly and contain the highest concentration of chloroplasts. This is where most photosynthesis occurs, as they get the maximum amount of light.

• Spongy Mesophyll: Irregularly shaped cells below the palisade layer.

  Adaptation: Contains large air spaces. These air spaces allow for rapid diffusion of gases (\(\text{CO}_2\) and \(\text{O}_2\)) to and from the palisade cells and the stomata.

Layer 3: Transport System

• Vascular Bundles (Leaf Veins): Contains Xylem and Phloem.
  • Xylem: Transports water and mineral ions (like magnesium and nitrate) to the photosynthesising cells.
  • Phloem: Transports manufactured food (in the form of sucrose) away from the leaf to other parts of the plant.

  Adaptation: Provides a network to bring raw materials in and take products out efficiently.