Photosynthesis

Photosynthesis is an endothermic reaction: plants take in light energy (transferred from the surroundings) to make glucose. It happens in chloroplasts, which contain the green pigment chlorophyll.

The glucose made is then used to: build cellulose and other molecules for growth, be stored as starch, be converted to fats/oils for storage, make amino acids (with nitrate ions) for proteins, and be used in respiration to release energy.

Watch out: in the balanced symbol equation you need 6 in front of CO₂, H₂O and O₂. Light is written above the arrow — it is not a reactant that gets used up as an atom.

Products of photosynthesis

Limiting factors

The rate of photosynthesis is controlled by three factors. Whichever is in shortest supply is the limiting factor — the one holding the rate back:

• Light intensity — more light → faster rate, until another factor limits.
• Carbon dioxide concentration — usually the main limiting factor on a bright day.
• Temperature — raises the rate up to an optimum, but too high denatures the enzymes and the rate falls.

Inverse-square law (light): light intensity ∝ 1 ÷ distance². If a lamp is moved twice as far away, the intensity falls to a quarter (1 ÷ 2² = 1/4), not a half.

Your turn — inverse-square law

Leaf structure

A leaf is a specialised organ for photosynthesis and gas exchange. Its cells and tissues are adapted for the job:

• Palisade mesophyll — column-shaped cells packed with chloroplasts near the top surface to absorb the most light.
• Spongy mesophyll — loosely packed with air spaces so gases (CO₂, O₂) diffuse quickly.
• Stomata — pores (mostly on the underside) where gases enter/leave and water vapour escapes.
• Guard cells — open and close the stomata; they close in dry conditions to reduce water loss.

Which tissue does the job?

Tap a role, then tap the leaf part that does it.

Xylem & transpiration

Xylem vessels are hollow, dead tubes strengthened with lignin. They carry water and dissolved mineral ions in one direction only — up from the roots to the leaves.

The loss of water vapour from the leaves (through the stomata) is called transpiration. It pulls a continuous stream of water up the plant — the transpiration stream.

Transpiration happens faster when:

• Temperature is higher — water evaporates faster.
• Light intensity is higher — stomata open wider.
• Air movement (wind) is greater — removes the humid air around the leaf.
• Humidity is lower — a steeper water-vapour gradient out of the leaf.

Higher tier: we can measure the rate of water uptake with a potometer. Rate = distance the bubble moves ÷ time.

Your turn — transpiration rate

Phloem & root hair cells

Phloem is made of living cells joined end to end. It carries dissolved sugars (mainly sucrose) from the leaves to where they are needed for growth or storage. This two-way movement is called translocation.

Root hair cells are adapted for absorbing water and minerals: a long, thin projection gives a large surface area; water enters by osmosis and mineral ions by active transport (against the gradient, so they need energy from respiration — hence many mitochondria).

How do minerals get in?

Plant hormones & tropisms

Plants respond to their environment by growing. A growth response to a stimulus coming from one direction is called a tropism. The hormone auxin controls it:

• Phototropism — growth in response to light. Shoots are positively phototropic (grow towards light).
• Gravitropism (geotropism) — growth in response to gravity. Roots are positively gravitropic (grow downwards); shoots are negatively gravitropic (grow up).

Commercial uses: auxins are used as weedkillers and rooting powders; gibberellins end seed dormancy and promote flowering; ethene is used to ripen fruit during transport/storage.

Match each clue to the term

Tap a clue on the left, then its matching term on the right.

Reading the graph

Your turn — rate of photosynthesis

Which way do roots grow?

The big ideas to know

You've covered all of Edexcel Topic 6 — plant structures and their functions. Press Finish to see your score.