Science - Combined (0653) | Plant nutrition

Hello Future Biologists! Welcome to Plant Nutrition (B6)

Have you ever wondered how a tiny seed grows into a huge tree, seemingly out of thin air? This chapter, Plant Nutrition, holds the secret!

We're going to dive into the most vital chemical reaction on Earth: photosynthesis. You’ll learn the ingredients plants need, the structures they use (the awesome parts of a leaf), and what controls how fast they can make their food. Understanding this is key to appreciating all life on our planet!

B6.1 Photosynthesis: The Ultimate Plant Recipe

What is Photosynthesis? (Core Content)

Photosynthesis is the process by which plants (and some algae) use light energy to convert simple raw materials (carbon dioxide and water) into complex food molecules (carbohydrates, specifically glucose). Oxygen is released as a waste product.

Think of a plant as a kitchen that runs entirely on solar power!

The Word Equation

The simplest way to remember the process is through the word equation:

Carbon Dioxide + Water → Glucose + Oxygen
(In the presence of Light and Chlorophyll)

The Role of Chlorophyll and Chloroplasts

• Chlorophyll is the green pigment found inside structures called chloroplasts, mainly located in the palisade mesophyll cells of the leaf.
• Chlorophyll's job is crucial: it traps light energy from the Sun.

The Chemical Detail (Supplement Content)

The Balanced Symbol Equation

For those aiming for top grades, you need to know the balanced chemical formula. Don't worry, it's just a precise way of writing the recipe!

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

The product, \(\text{C}_6\text{H}_{12}\text{O}_6\), is glucose, a type of sugar and the plant's immediate food source.

Energy Conversion

The amazing job that chlorophyll performs is an energy transfer:

• Chlorophyll absorbs Light Energy.
• It converts this Light Energy into Chemical Energy, which is stored in the bonds of the glucose molecules.
• This stored chemical energy is what all living things (including us!) rely on.

Did you know? Glucose is often quickly converted into starch for storage because starch is insoluble, meaning it doesn't affect the water potential of the plant cell.

Investigating the Requirements (Core Content)

We can test if the main ingredients (light, chlorophyll, and carbon dioxide) are truly necessary using a simple test for starch.

The Starch Test:
To test a leaf for starch, you must first:

1. Boil the leaf in water: This kills the plant cells, stopping all chemical reactions.
2. Boil the leaf in ethanol (alcohol): This removes the green chlorophyll (called *decolouring* the leaf) so you can see the colour change clearly.
3. Rinse the leaf: This softens the leaf so it doesn't break.
4. Add Iodine Solution:
   • If starch is present, the iodine turns from brown/orange to blue-black.
   • If starch is absent, the iodine remains brown/orange.

1. Is Chlorophyll Necessary?

Use a variegated leaf (a leaf with green and white patches).

• The green parts contain chlorophyll and test positive (blue-black) for starch.
• The white parts lack chlorophyll and test negative (brown/orange) for starch.
• Conclusion: Chlorophyll is needed for photosynthesis and starch production.

2. Is Light Necessary?

Take a plant that has been destarched (kept in the dark for 48 hours so all existing starch is used up). Cover a small part of one leaf with opaque foil.

• The covered part tests negative for starch.
• The uncovered part tests positive for starch.
• Conclusion: Light is needed for photosynthesis.

3. Is Carbon Dioxide (\(\text{CO}_2\)) Necessary?

Use two destarched potted plants. Place one (the control) in normal air. Place the other in a sealed container with sodium hydroxide solution, which absorbs all the \(\text{CO}_2\) in the air. Both are placed in light.

• The leaf exposed to air (with \(\text{CO}_2\)) tests positive for starch.
• The leaf exposed to sodium hydroxide (no \(\text{CO}_2\)) tests negative for starch.
• Conclusion: Carbon dioxide is needed for photosynthesis.

B6.2 The Rate of Photosynthesis (Limiting Factors)

Even if a plant has all the raw materials, the speed at which it can photosynthesise (the rate) is often limited by external factors. The factor that is in shortest supply is called the limiting factor.

Imagine photosynthesis as an assembly line. If one part of the line slows down, the whole process slows down.

Limiting Factors (Supplement Content)

1. Light Intensity

• Effect: As light intensity increases, the rate of photosynthesis increases linearly (in a straight line).
• Limit: Eventually, the rate stops increasing and levels off. This means the plant is working as fast as it can, and either \(\text{CO}_2\) or temperature has become the new limiting factor.

2. Carbon Dioxide Concentration

• Effect: As \(\text{CO}_2\) concentration increases, the rate increases.
• Limit: Since the atmosphere only contains about 0.04% \(\text{CO}_2\), this is often the limiting factor in natural environments. Just like light, the rate will level off when another factor (like temperature) takes over.

3. Temperature

Photosynthesis involves enzymes (see Chapter B5) that control the chemical reactions. Enzymes are affected by temperature.

• Effect: As temperature increases, the rate increases, reaching an optimum temperature (usually around 25°C - 35°C).
• Limit: If the temperature gets too high, the enzymes start to break down (denature), and the rate of photosynthesis drops rapidly.

Practical Application: Greenhouse growers manage all three factors (light, \(\text{CO}_2\), and temperature) to achieve the fastest possible growth rate for crops.

Gas Exchange in Aquatic Plants (Supplement Content)

We can investigate gas exchange (the balance between photosynthesis and respiration) in aquatic plants using a special tool: hydrogencarbonate indicator solution.

This indicator is brilliant because it changes colour depending on the concentration of carbon dioxide in the water:

• High \(\text{CO}_2\): Yellow
• Atmospheric \(\text{CO}_2\): Orange/Red
• Low \(\text{CO}_2\): Purple/Magenta

The Investigation: Light vs. Dark

1. In the Dark: Plants only respire. Respiration produces \(\text{CO}_2\).
   

   Indicator colour shifts towards Yellow (High \(\text{CO}_2\)).

2. In Bright Light: Plants photosynthesise rapidly (Rate of Photosynthesis > Rate of Respiration). They use up lots of \(\text{CO}_2\).
   

   Indicator colour shifts towards Purple (Low \(\text{CO}_2\)).

3. In Dim Light (Compensation Point): Photosynthesis rate equals Respiration rate. The net amount of \(\text{CO}_2\) remains the same.
   

   Indicator stays Orange/Red.

B6.2 Leaf Structure and Function (Core Content)

The leaf is the plant's main organ for photosynthesis. Its structure is perfectly adapted to capture sunlight, absorb \(\text{CO}_2\), and transport materials.

Key Structures and Their Roles

You must be able to identify these structures in diagrams and images and know what they do!

Outer Layers (Protection)

• Cuticle: A thin, waxy layer covering the upper and lower epidermis. It is waterproof, reducing uncontrolled water loss (transpiration).
• Upper/Lower Epidermis: Protective, transparent layers (no chloroplasts usually) that allow sunlight to pass straight through to the cells below.

Mesophyll Layers (Photosynthesis and Gas Exchange)

• Palisade Mesophyll: Located directly below the upper epidermis. These cells are packed with chloroplasts and are the main site of photosynthesis, as they get the most sunlight.
• Spongy Mesophyll: Located below the palisade layer. These cells have fewer chloroplasts and are irregularly shaped, creating large air spaces.
• Air Spaces: Allow carbon dioxide to quickly diffuse from the stomata to the photosynthesising cells, and oxygen to diffuse out.

Gas Exchange Apparatus

• Stomata (singular: stoma): Tiny pores, usually on the lower epidermis, that allow for gas exchange (\(\text{CO}_2\) in, \(\text{O}_2\) and water vapour out).
• Guard Cells: A pair of specialised cells surrounding each stoma. They control the opening and closing of the pore to regulate gas exchange and water loss.

Vascular Tissues (Transport)

These structures are grouped together to form the vascular bundles (veins) in the leaf:

• Xylem: Transports water and mineral ions from the roots up to the leaf (needed for photosynthesis).
• Phloem: Transports manufactured food (mainly sucrose and amino acids) away from the leaf to other parts of the plant (e.g., storage organs or growing points).

End of Chapter B6 Review

Great work! You have successfully mastered Plant Nutrition. Remember, every time you see a green plant, you are looking at a living solar-powered food factory! Keep practicing the equations and linking the leaf structure back to the job of photosynthesis.