Sciences - Co-ordinated (Double) (0654) | Plant nutrition

🌱 Welcome to Plant Nutrition (B6) Study Notes!

Hello future biologist! This chapter, Plant Nutrition (B6), is all about how plants make their own food. Without this process, known as photosynthesis, almost all life on Earth would eventually stop. Understanding it is crucial not only for your IGCSE exams but also for appreciating the fundamental biology supporting our planet. Don't worry, we will break down the complex steps into clear, easy-to-understand chunks!

Let's dive into how plants fuel themselves!

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B6.1 Photosynthesis: Making Plant Food

What is Photosynthesis? (Core Content)

Photosynthesis literally means "making things with light." It is the process by which plants use light energy to make their own food (carbohydrates) from simple raw materials.

The main purpose of photosynthesis is the synthesis (making) of carbohydrates (sugars), which the plant uses for energy, growth, and development.

The Essentials of Photosynthesis

• Raw Materials (Input): Carbon dioxide (\(\text{CO}_2\)) and water (\(\text{H}_2\text{O}\)).
• Energy Source: Light (usually sunlight).
• Catalyst/Pigment: Chlorophyll.
• Products (Output): Glucose (a carbohydrate) and oxygen (\(\text{O}_2\)).

Core Concept: Photosynthesis occurs primarily in the leaves, specifically inside tiny structures called chloroplasts. Chloroplasts contain the green pigment, chlorophyll.

Word Equation for Photosynthesis (Core):

carbon dioxide + water \(\rightarrow\) glucose + oxygen
(in the presence of light and chlorophyll)

The Role of Energy and the Balanced Equation (Supplement/Extended)

The chemical reactions of photosynthesis involve transferring light energy into stored chemical energy. This energy is then used to synthesize carbohydrates.

Chlorophyll's Key Job:

Chlorophyll is not just a green colour; it's an energy converter! It transfers energy from light into chemical energy. This chemical energy is what powers the assembly of glucose molecules.

The Balanced Symbol Equation (Supplement):

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

(Where \(\text{C}_6\text{H}_{12}\text{O}_6\) represents glucose)

Investigating the Requirements for Photosynthesis (Core Investigation)

You must understand the need for light, chlorophyll, and carbon dioxide, typically proven through practical investigations:

1. Testing for Starch: We check if photosynthesis has occurred by testing for the presence of starch (the storage carbohydrate). We use iodine solution, which turns blue-black if starch is present.
2. Need for Chlorophyll: Use a variegated (two-coloured) leaf. The green parts (with chlorophyll) test positive for starch, while the non-green parts test negative.
3. Need for Light: Cover a leaf with aluminium foil (a light-proof cap) for a few days. The covered part tests negative for starch.
4. Need for Carbon Dioxide: Place a plant in a sealed container with soda-lime (which absorbs \(\text{CO}_2\)). The leaves will test negative for starch.

Did you know? The oxygen released during photosynthesis is a waste product to the plant, but it is absolutely essential for aerobic respiration in animals (and plants themselves!).

Subsequent Use and Storage of Carbohydrates (Supplement/Extended)

Once glucose is made, the plant quickly processes it. Glucose is rarely stored as glucose itself because it is soluble and could affect the water potential of the cells, causing osmotic problems.

The carbohydrates are quickly converted for specific uses:

• Storage: Converted to starch (an insoluble molecule) for long-term energy storage, often found in leaves, roots, or seeds (like potatoes or rice).
• Structure: Converted to cellulose to build strong cell walls. Cellulose provides structural support.
• Energy Release: Used immediately in respiration (B12) to release energy (\(\text{ATP}\)) for metabolism, growth, and other life processes.
• Transport: Converted to sucrose (a soluble sugar) for transport throughout the plant in the phloem tissue (B8).
• Reproduction: Converted into nectar (a sweet, sugary liquid) to attract insects for pollination.

B6.1 Essential Mineral Ions (Supplement/Extended)

Plants need simple inorganic ions, absorbed from the soil, to build complex molecules necessary for survival. You must know the importance of two specific ions:

1. Nitrate Ions (N\(\text{O}_3^-\)):

   These are crucial for making amino acids. Amino acids are the building blocks of proteins (including enzymes!). If a plant lacks nitrates, its growth will be stunted and its leaves may turn yellow.

2. Magnesium Ions (\(\text{Mg}^{2+}\)):

   These are vital for making chlorophyll. If a plant is deficient in magnesium, it cannot produce enough chlorophyll, leading to chlorosis (yellowing of leaves), which severely limits photosynthesis.

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B6.1 Limiting Factors on the Rate of Photosynthesis (Supplement/Extended)

The rate at which a plant photosynthesizes can be affected by several external factors. The factor that is in shortest supply—the one slowing the process down—is called the limiting factor.

Analogy: Imagine a factory assembling cars. If you run out of tires, even if you have endless steel and paint, you can only assemble cars as quickly as you get new tires. The tires are the limiting factor.

The three main limiting factors are:

1. Light Intensity

• Effect: The brighter the light, the faster the rate of photosynthesis, up to a certain point.
• Limitation: If light is low, it limits the energy available for the chlorophyll to transfer.
• Why it levels off: Once light is plentiful, the plant is probably limited by carbon dioxide or temperature instead.

2. Carbon Dioxide Concentration (\(\text{CO}_2\))

• Effect: Increasing the \(\text{CO}_2\) concentration increases the rate of photosynthesis, up to a certain point.
• Limitation: \(\text{CO}_2\) is a raw material. If there isn't enough, the reactions slow down.
• Context: In greenhouses, farmers often pump extra \(\text{CO}_2\) in to maximize crop growth.

3. Temperature

• Effect: The rate increases with temperature up to an optimum point (usually around 25°C to 35°C), then decreases sharply.
• Limitation: Photosynthesis involves enzymes (B5). At low temperatures, enzymes work slowly. At very high temperatures, the enzymes start to denature (lose their shape and function), causing the rate to fall quickly.

Gas Exchange in Aquatic Plants Investigation (Supplement)

We can observe the balance between photosynthesis (using \(\text{CO}_2\)) and respiration (producing \(\text{CO}_2\)) using a solution called hydrogencarbonate indicator solution.

• How it works: This indicator changes colour based on the level of dissolved \(\text{CO}_2\).
  • Yellow: High \(\text{CO}_2\) concentration (Acidic)
  • Red/Orange: Medium/Normal \(\text{CO}_2\) concentration (Neutral)
  • Purple: Low \(\text{CO}_2\) concentration (Alkaline)
• In the Light: The aquatic plant photosynthesizes faster than it respires. It rapidly uses up \(\text{CO}_2\), lowering the concentration. The indicator turns purple.
• In the Dark: Photosynthesis stops, but respiration continues (releasing \(\text{CO}_2\)). The plant releases \(\text{CO}_2\), increasing the concentration. The indicator turns yellow.

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B6.2 Leaf Structure and Adaptations

Leaves are perfectly designed to maximize the capture of light and the intake of raw materials needed for photosynthesis.

Overall Adaptations (Core)

• Large Surface Area: This maximizes the absorption of sunlight.
• Thin Structure: This ensures a short distance for gases (\(\text{CO}_2\) and \(\text{O}_2\)) to diffuse into and out of the cells.

Key Leaf Structures (Core Identification)

You must be able to identify and know the function of the following structures in a leaf diagram:

Structure	Function & Adaptation (Supplement/Extended)
Cuticle	Waxy, protective layer (usually thin on the top, thicker on the bottom). Prevents excessive water loss (desiccation).
Epidermis (Upper and Lower)	Protective layer, usually transparent. Allows sunlight to reach the palisade layer easily.
Palisade Mesophyll	Cells tightly packed and located just below the upper epidermis. Contains the highest concentration of chloroplasts to capture maximum sunlight.
Spongy Mesophyll	Irregularly shaped cells with large air spaces. Allows rapid diffusion of gases (\(\text{CO}_2\), \(\text{O}_2\), water vapour) to and from the stomata.
Stomata (Stoma singular)	Tiny pores, usually on the lower epidermis, controlled by guard cells. Allow \(\text{CO}_2\) to enter and \(\text{O}_2\) and water vapour to exit the leaf.
Vascular Bundles (Veins)	Contains xylem and phloem. Xylem brings water and mineral ions (like Mg2+ and nitrate ions) to the photosynthetic cells. Phloem transports sugars (sucrose) away for use or storage (B8).

Don't worry if all the names seem intimidating. Focus on the 'Palisade = Photosynthesis' and 'Spongy = Space for Gas Exchange' connection!

Key Takeaway from Plant Nutrition:

Photosynthesis is the fundamental energy conversion process in biology. Plants capture light energy using chlorophyll, convert carbon dioxide and water into glucose and oxygen, and use that glucose for every aspect of their life, from growth (cellulose and proteins) to fueling their metabolism (respiration).