10.2 Plants, Growth & Photosynthesis
Key Takeaways
- Plants need light, water, air (carbon dioxide), mineral nutrients, space and a suitable temperature to survive and grow - remove any one and growth suffers
- Roots anchor the plant and absorb water and minerals, the stem supports the plant and transports water and food, leaves make food by photosynthesis, and flowers make seeds for reproduction
- Photosynthesis happens in the chloroplasts of leaves: carbon dioxide plus water, using light energy, becomes glucose plus oxygen - and almost all life on Earth depends on it
- In a fair plant-growth test, change only the variable you are investigating, keep everything else identical across all pots, and repeat with several plants to trust the result
- Phototropism is a shoot growing towards light: unequal light makes the shaded side of the shoot elongate more, bending the tip towards the source - a survival response, not a decision
What Plants Need to Survive
The resources needed for the survival of living things is a named topic on ICAS Paper D, and plants make the perfect case study because their needs are so easy to test. A healthy plant requires six things:
- Light - the energy source for making food
- Water - absorbed by the roots; used in photosynthesis and to keep cells firm
- Air - specifically carbon dioxide, the raw material for photosynthesis
- Mineral nutrients - such as nitrogen, phosphorus and potassium, absorbed from the soil in tiny amounts
- Space - room for roots to spread and leaves to catch light without being shaded
- Suitable temperature - warmth for seeds to sprout and for the plant's chemistry to run
Notice what is missing from that list: soil. Soil is not strictly a need - it is simply the usual delivery service for water, minerals and support. Plants grown in water with dissolved nutrients, a technique called hydroponics, thrive with no soil at all. That is a classic ICAS reasoning point: if a question shows plants growing without soil, do not conclude the experiment is impossible; check whether water, minerals, light and air are still being supplied.
The Parts of a Plant and Their Jobs
| Part | Job |
|---|---|
| Roots | Anchor the plant in the ground and absorb water and mineral nutrients; root hairs give a huge surface area for absorption |
| Stem | Holds the leaves up to the light and transports water and minerals up from the roots and food from the leaves to the rest of the plant |
| Leaves | The food factories: broad and flat to catch maximum light, and packed with chloroplasts for photosynthesis |
| Flowers | The reproductive parts: they make seeds, often with help from pollinators such as bees |
The shape of each part matches its job - a flat leaf for catching light, a branching root for reaching water. When ICAS shows an unfamiliar plant, reason from structure to function the same way: a cactus with a thick, swollen stem is storing water, and spines instead of broad leaves means losing less water in a dry environment.
Photosynthesis: The Recipe That Feeds the World
Photosynthesis is the process by which green plants make their own food. Learn it as a recipe:
carbon dioxide + water --(light energy, trapped by chlorophyll)--> glucose + oxygen
The ingredients enter through different doors. Carbon dioxide diffuses into the leaf from the air through tiny pores called stomata, mostly on the underside. Water travels up from the roots through the stem. The energy comes from light, trapped by the green pigment chlorophyll inside the chloroplasts. The products are glucose - a sugar the plant uses for energy and to build new material - and oxygen, which is released back into the air as a by-product.
Two facts earn marks again and again. First, photosynthesis happens only in the green parts of the plant and only when light is available - not at night, and not in roots. Second, almost all life on Earth depends on it. Every food chain begins with a producer making glucose by photosynthesis, and the oxygen you are breathing right now was made by a plant or by ocean algae.
Germination and Fair Tests
A seed is a baby plant in a protective coat with a packed lunch. Germination is the moment the seed springs to life: it absorbs water, the coat splits, a root pushes down and a shoot pushes up. Seeds need only three things to germinate - water, oxygen and a suitable temperature. They do not need light or soil at this stage, because the seed carries its own food store. Light becomes essential only once the shoot is above ground and must start photosynthesising. Confusing germination needs with adult-plant needs is one of the most common ICAS traps in this whole topic.
Plant growth experiments are ICAS's favourite fair-test context, because the variables are so concrete. Suppose you want to test whether seedlings grow better in bright light or shade. The fair-test rules apply exactly:
- Change only the independent variable - here, the amount of light.
- Keep every controlled variable identical: same type and number of seeds, same pot size, same soil, same amount of water, same temperature.
- Measure the dependent variable the same way each time: seedling height in centimetres every two days, for example.
- Repeat with several pots in each condition, not just one plant, so a single weak seed cannot fool you.
If a stimulus shows one pot watered daily in the sun and another watered weekly in the shade, the conclusion is worthless - two variables changed at once, so you cannot say which one caused the difference. Spotting that flaw is often the entire question.
Worked Stimulus: Bending Towards the Light
A student places a potted seedling on its side near a window. Over the next few days the shoot bends upward and towards the glass, while the root bends downward. What is going on?
This is phototropism - a tropism is a growth response to a stimulus, and photo means light. The shoot shows positive phototropism, growing towards the light, because light falling unevenly on the shoot causes the shaded side to elongate more than the lit side, bending the tip towards the source. The root shows the opposite behaviour, growing downwards away from the light (and with gravity). The reasoning you should voice in the exam is about survival: a shoot that reaches the light can photosynthesise more, and a root that grows down finds water and anchors the plant. The plant is not choosing or thinking - it is an automatic growth response built into its cells. When an ICAS question shows a plant bending in a time-lapse diagram, look for the direction of the stimulus and ask which response improves the plant's access to what it needs.
A class plants bean seeds in four setups: (1) dry cotton wool in the light, (2) moist cotton wool in the light, (3) moist cotton wool in a dark cupboard, and (4) moist cotton wool in a refrigerator. Only setups 2 and 3 germinate. What is the best conclusion?
A student wants to find out whether bean seedlings grow taller with more light. Which design is the fairest test?
In a time-lapse video, a seedling's shoot bends steadily towards a lamp placed to one side, while its root grows straight down. Which explanation is best?