Plant structures and their functions

Welcome to the World of Plants!

Welcome to your study guide for Topic 6: Plant structures and their functions. This is a key part of your Paper 2 exam. We often take plants for granted, but they are the ultimate "solar-powered factories" that keep almost all life on Earth alive! In this chapter, we’ll look at how plants make food, how they move water around, and how they even "decide" which way to grow. Don't worry if it seems like a lot to take in—we’ll break it down leaf by leaf!

1. Photosynthesis: The Energy Producer

Plants and algae are producers. This means they make their own food and produce biomass (the material that makes up living things), which eventually feeds nearly every other organism on the planet.

What is Photosynthesis?

Photosynthesis is an endothermic reaction. This means it takes in energy from the surroundings (specifically, light energy from the sun). Plants use this energy to turn carbon dioxide and water into glucose and oxygen.

The Word Equation:
Carbon dioxide + water \(\rightarrow\) glucose + oxygen

The Symbol Equation:
\(6CO_{2} + 6H_{2}O \rightarrow C_{6}H_{12}O_{6} + 6O_{2}\)

Limiting Factors

A "limiting factor" is just something that stops the rate of photosynthesis from increasing. Imagine you are making sandwiches: if you have 100 slices of bread but only 2 slices of cheese, the cheese is your limiting factor. In plants, the three main limiting factors are:

1. Temperature: If it's too cold, enzymes work slowly. If it's too hot (usually above 45°C), the enzymes denature (lose their shape) and the reaction stops.
2. Light Intensity: More light provides more energy, up to a point.
3. Carbon Dioxide Concentration: Plants need \(CO_{2}\) as a raw material. If there isn't enough, they can't make glucose.

Quick Review: On a graph, if the line levels off (becomes flat), it means that specific factor is no longer the thing holding the plant back—something else has become the limiting factor!

2. The Maths of Light: The Inverse Square Law

As you move a light source further away from a plant, the light intensity doesn't just drop a little—it drops significantly. This is called the Inverse Square Law.

The Rule: Light intensity is inversely proportional to the square of the distance.

The Formula: \(Light \ Intensity \propto \frac{1}{d^{2}}\)

Example: If you double the distance (\(\times 2\)), the light intensity becomes 4 times smaller (\(1/2^{2} = 1/4\)). If you triple the distance (\(\times 3\)), the light intensity becomes 9 times smaller (\(1/3^{2} = 1/9\)).

3. Plant Plumbing: Roots, Xylem, and Phloem

Plants have a "vascular system" (a fancy word for plumbing) to move things around.

Root Hair Cells

These are the "drinking straws" of the plant. They are adapted to absorb water and minerals from the soil:
- Large Surface Area: They have long, hair-like extensions to soak up as much water as possible.
- Thin Walls: To make the distance for osmosis (water movement) very short.

Xylem vs. Phloem

It is very easy to mix these two up! Use these tricks to remember them:

Xylem (The Water Pipe):
- Made of lignified dead cells (lignin makes them very strong).
- Transports water and mineral ions UP from the roots to the leaves.
- Think: "Xylem carries Water" (X and W are near each other in the alphabet).

Phloem (The Food Delivery):
- Made of living cells.
- Transports sucrose (sugar) all around the plant. This process is called translocation.
- Think: "Phloem carries Phood (Food)".

Did you know? Translocation in the phloem requires energy, which is why phloem cells are living, whereas xylem is just a series of hollow, dead tubes!

4. Transpiration: The Upward Pull

Transpiration is the loss of water vapor from the leaves. This creates a "suction" that pulls water all the way up from the roots, like drinking through a straw. This movement is called the transpiration stream.

The Stomata

Stomata are tiny holes on the bottom of the leaf. They are controlled by guard cells.
- When the plant has plenty of water, guard cells swell and the stomata open to allow gas exchange.
- When the plant is short of water, the stomata close to stop the plant from wilting.

Factors affecting Transpiration

Think about what makes clothes dry faster on a washing line—it’s the same for plants!
1. Light Intensity: Brighter light opens stomata, increasing transpiration.
2. Air Movement (Wind): Higher wind blows water vapor away from the leaf, making more water evaporate.
3. Temperature: Warmer particles move faster and evaporate more quickly.

Key Takeaway: Transpiration is a passive process (no energy needed), while Translocation (in phloem) is an active process (needs energy).

5. Leaf Adaptations (Higher Tier/B Topics)

Leaves are perfectly designed for photosynthesis and gas exchange:

- Waxy Cuticle: A waterproof layer that stops water loss from the top of the leaf.
- Upper Epidermis: Transparent so light can pass through to the cells below.
- Palisade Mesophyll: Packed with chloroplasts and located near the top to catch the most light.
- Spongy Mesophyll: Has lots of air spaces to allow gases (\(CO_{2}\) and \(O_{2}\)) to diffuse easily.

Extreme Environments

Plants in deserts (xerophytes) have special tricks to survive:
- Thick waxy cuticles to stop water loss.
- Spikes instead of leaves to reduce surface area.
- Sunken stomata to trap moist air.

6. Plant Hormones: Growth and Control

Plants don't have nerves, but they use hormones to react to their environment. The most important hormone is auxin.

Tropisms

A tropism is a growth response to a stimulus.
1. Phototropism (Light): Plant shoots grow towards light. Auxin builds up on the shaded side, causing those cells to elongate (stretch), which bends the plant toward the light.
2. Gravitropism (Gravity): Roots grow downwards. In roots, a high concentration of auxin actually stops growth, causing the root to bend down into the soil.

Commercial Uses of Hormones

Humans have "hacked" plant hormones for farming:
- Auxins: Used as weedkillers (they make weeds grow so fast they die) and rooting powders (to help cuttings grow roots).
- Gibberellins: Used to start seed germination, make flowers/fruit grow bigger, and produce seedless fruit.
- Ethene: Used to control the ripening of fruit during transport so it arrives at the supermarket perfectly ripe.

Memory Aid: "Ethene makes it Ripe, Auxin makes it Stretch, Gibberellins make it Germinate!"

Final Quick Check!

Before your exam, make sure you can:
- State the photosynthesis equation.
- Explain why a graph of light intensity eventually goes flat.
- Describe the difference between Xylem and Phloem.
- Explain how a shoot bends toward the light using the word "elongation".
You've got this! Plants might be stationary, but their biology is moving fast!