Photosynthesis

Definition. Photosynthesis is the process by which plants make GLUCOSE (a carbohydrate) from carbon dioxide and water, using LIGHT energy absorbed by CHLOROPHYLL.

Word equation:

carbon dioxide + water → glucose + oxygen (in the presence of light energy and chlorophyll)

Balanced symbol equation:

$6\text{CO}_{2} + 6\text{H}_{2}\text{O} \xrightarrow{\text{light, chlorophyll}} \text{C}_{6}\text{H}_{12}\text{O}_{6} + 6\text{O}_{2}$

Where it happens. Inside CHLOROPLASTS, found mainly in palisade mesophyll cells of the leaf. Chloroplasts contain CHLOROPHYLL — the green pigment that absorbs light energy.

Inputs and outputs:

Input	Where from?
Carbon dioxide	Air (through stomata)
Water	Soil (taken up by roots, transported in xylem)
Light energy	The sun
Chlorophyll	Inside chloroplasts

Output	What happens to it?
Glucose	Used / stored / converted (see uses section)
Oxygen	Released through stomata (or used by the plant for respiration)

Worked qualitative. Why is photosynthesis crucial for life on Earth?

• It makes glucose — the base of nearly every food chain.
• It releases oxygen — the gas almost all aerobic organisms (including us) need to respire.
• It removes CO₂ from the atmosphere — slows the greenhouse effect.

Cambridge tip. Always include LIGHT and CHLOROPHYLL above the arrow in the equation. Mark schemes deduct for omitting either.

Why look at the leaf? The leaf is the main site of photosynthesis. Its structure is highly adapted to maximise the rate.

Adaptations and their reasons.

Feature	How it helps
Broad, flat shape	Large surface area to absorb light and CO₂.
Thin	Short diffusion path for gases (CO₂ in, O₂ out).
PALISADE mesophyll at top	Tightly packed cells full of chloroplasts — maximum light absorption.
Spongy mesophyll	Air spaces let CO₂ diffuse to all cells.
STOMATA (mostly underside)	Pores for gas exchange; can be opened/closed by guard cells.
Veins (xylem + phloem)	Bring water + minerals; remove sugars.
Waxy cuticle	Reduces water loss without blocking light.
Chlorophyll	Absorbs RED + BLUE light; reflects green (why leaves look green).

Worked qualitative. Why are most chloroplasts in the upper part of the leaf? Light comes from above. The palisade layer at the top has the most chloroplasts because it gets the most light → maximum photosynthesis. Lower layers (spongy mesophyll) have fewer chloroplasts but more air spaces — they specialise in CO₂ delivery.

A LIMITING FACTOR is the variable that, when in shortest supply, controls the rate of a process. As you increase a limiting factor, rate goes up — until something else becomes limiting.

1. Light intensity.

• More light → more energy for the light reaction → faster rate.
• Up to a saturation point: above it, light is no longer limiting (something else takes over).
• In darkness: rate = 0 (no light = no photosynthesis).

2. CO₂ concentration.

• More CO₂ → more raw material → faster rate.
• Up to a saturation point: above it, the enzymes can't keep up.
• Atmospheric CO₂ is around 0.04% — for a glasshouse plant, this is often THE limiting factor on a sunny day.

3. Temperature.

• Photosynthesis uses ENZYMES.
• 0-35°C: rate increases with temperature (more KE for collisions).
• Above ~40°C: enzymes start to DENATURE → rate falls SHARPLY.
• Optimum for most plants: ~25-35°C.

Reading limiting-factor graphs.

• A flat plateau means another factor is limiting.
• Increasing one factor while another is limiting: little or no effect.
• Look at the SHAPE: rising = that factor IS limiting; flat = it ISN'T.

Worked qualitative. A graph shows photosynthesis rate vs CO₂ concentration at two light intensities (low + high). The HIGH light line plateaus at a higher rate. Why?

• At low light, light is the limiting factor → adding more CO₂ doesn't help much.
• At high light, CO₂ becomes limiting → adding more CO₂ raises the rate further.

Worked qualitative. Why do greenhouse growers add EXTRA CO₂ to the air?

• To remove CO₂ as a limiting factor.
• Faster photosynthesis → faster growth → bigger crops.

Cambridge tip. Always state which factor is limiting AT WHICH POINT on the graph. "At low light, light is limiting; at high light, CO₂ becomes limiting" earns more marks than "rate increases".

Photosynthesis MAKES the glucose; the plant then USES it.

1. Respiration. The plant respires (day and night) — broken down to release energy for growth and other processes.

2. Stored as STARCH. Starch is insoluble, compact, doesn't affect water potential — perfect storage. Stored in:

• LEAVES (short term — used overnight).
• ROOTS (long term, e.g. potatoes).
• SEEDS (food for the embryo).

3. Cellulose. Many glucose units join to make cellulose — the long, strong molecule that builds CELL WALLS. Cellulose can't be broken down by the plant for energy — it's structural.

4. Amino acids and proteins. Glucose + nitrate (from soil) → amino acids → proteins. Proteins are needed for growth, enzymes, etc.

5. Lipids (fats and oils). Some glucose is converted to lipids — energy stores in seeds (e.g. sunflower oil).

6. Sucrose (transport sugar). Glucose is converted to SUCROSE for transport through the phloem (sap). Sucrose is the sugar moved around the plant.

Worked qualitative. Why is starch (not glucose) used as a storage molecule?

• Glucose is soluble — would affect water potential of cells.
• Starch is INSOLUBLE — no osmotic effect.
• Starch is COMPACT — packs more energy per unit volume.
• Starch is easily broken down to glucose when needed.

Cambridge tip. When asked "uses of glucose", give FOUR specific uses, not three. The fourth (proteins) is often overlooked but worth a separate mark.

1. Test a leaf for STARCH (proves photosynthesis happened).

Steps:

1. Boil leaf in water (kills cells, breaks down membranes).
2. Boil in ETHANOL (removes chlorophyll). Use a water bath — ethanol is flammable.
3. Rinse in water (softens leaf).
4. Add IODINE solution.
5. Result: parts of the leaf with starch turn BLUE-BLACK. Parts without stay yellow-brown.

2. Showing chlorophyll is needed (variegated leaf).

• Use a leaf with green AND white patches.
• Leaf has been in light → starch test.
• Result: GREEN patches → BLUE-BLACK (had chlorophyll, made starch). WHITE patches → no colour change (no chlorophyll, no starch).
• Conclusion: chlorophyll is required for photosynthesis.

3. Showing CO₂ is needed.

• Set up two destarched plants. One has soda lime (absorbs CO₂); one has sodium bicarbonate (releases CO₂).
• Both in light, several hours.
• Test leaves for starch.
• Result: only the bicarbonate plant shows starch.
• Conclusion: CO₂ is required.

4. Showing light is needed.

• Cover part of a destarched leaf with foil/black paper.
• Plant in light for several hours.
• Test the leaf for starch.
• Result: covered area = no starch. Uncovered = starch.
• Conclusion: light is required.

5. Counting bubbles from pondweed.

• Submerge pondweed (e.g. Elodea) in water.
• Place lamp at varied distances; count gas bubbles per minute.
• Result: closer lamp = more bubbles = faster photosynthesis.
• The bubbles are oxygen — proof photosynthesis is happening.

Why "destarch" first? A plant left in darkness for 24-48 hours uses up its starch (respiration continues). Without destarching, you can't tell whether starch came from your experiment or from before.

Cambridge tip. Always describe the CONTROL. "Compare a covered area to an uncovered area on the SAME leaf — same plant, same conditions, only variable is light."