Uses of Glucose from Photosynthesis

Glucose is the product. The plant has five things it can do with it — and you need to know all of them.

SPEC 4.4.1.3 • MODULE 4 • PAPER 1

Where Does All The Glucose Go?

You learned in lesson 4.01 that photosynthesis produces glucose. But plants don't just float in a sea of sugar. So what actually happens to it all?

The spec lists exactly five uses. Each one is examinable. Questions often ask you to list all five or to identify a specific use from a description.

Why This Matters

Glucose is the raw material from which the plant builds everything else: its structure (cellulose), its food store (starch), its proteins (with help from nitrate), its energy (via respiration), and its long-term reserves (fats/oils). One molecule, five fates.

What You'll Learn

Objective 1

State all five uses of glucose from photosynthesis, with the correct spec terminology for each.

Objective 2

Explain why each use matters to the plant — including why starch is chosen for storage (it's insoluble).

Objective 3

Explain why plants need nitrate ions from the soil (for amino acid & protein synthesis) — and recognise the synoptic link to Module 3 plant deficiency.

Objective 4

Recall the food tests (AT 8) for starch, glucose, and protein — the colour changes and reagents.

Recap — Glucose is the Product

From lessons 4.01 and 4.02:

Photosynthesis Equation

carbon dioxide + water → glucose + oxygen

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Glucose is the product — and the starting point for everything else the plant builds. The reactant CO₂ entered through stomata; the water came up the xylem; the energy came from light absorbed by chlorophyll. Now we follow the glucose.

The Five Uses of Glucose

From the spec: "The glucose produced in photosynthesis may be…" — five outcomes, each examinable.

Glucose at the centre, with five fates radiating outward. Note: protein synthesis needs both glucose-derived amino acids AND nitrate ions from the soil.

Use 1 — Respiration

What Happens

Some glucose is broken down in respiration — the reaction that releases the energy stored in glucose so the plant can use it for cell processes (growth, active transport, building proteins, repair).

Energy Vocabulary

Respiration releases or transfers energy. It does not "produce" energy or "make" energy. Energy was already stored in the glucose; respiration just unlocks it. Mark schemes are strict on this — the word "produces" can lose you the mark.

Forward Link — Lesson 4.04

You'll meet the respiration equation in detail next lesson. For now, just know: glucose + oxygen → CO₂ + water, with energy released. This is why plants also respire — even though they make glucose, they still need to release the energy from it.

Use 2 — Storage as Starch

Spec Wording

"Converted into insoluble starch for storage." The exact phrasing matters — questions often want this wording back.

Why Insoluble?

Insoluble means doesn't dissolve in water. This is essential because:

Stored starch doesn't affect water movement in/out of the cell

It can't easily leak away — stays where it's stored

The cell's water balance is unchanged

Where It's Stored

Starch is stored in:

Roots — e.g. potato tubers, carrots

Stems

Seeds — e.g. wheat, rice (early energy supply for germinating seedlings)

Synoptic Link — Module 2 Food Tests

To detect starch, you add iodine solution. If starch is present, the orange iodine turns blue-black. This test is examinable on its own (AT 8) and crops up in plant biology questions.

Photosynthesis Doesn't Make Starch

Mark schemes explicitly reject "starch is made in photosynthesis". The correct sequence is: photosynthesis makes glucose; excess glucose is then converted into starch for storage. Write "glucose converted to starch" or "excess glucose is stored as starch", not "starch made in photosynthesis".

Day–Night Cycling — Examinable Synoptic Link

This connects uses 1 (respiration) and 2 (starch):

By day: photosynthesis makes glucose → excess is converted to starch (stored)

By night: no light → no photosynthesis → starch stores are converted back to glucose → glucose used in respiration to release energy

This is why plants respire 24 hours a day but only photosynthesise in the light. Examiners reward students who explain plant behaviour at night using this conversion.

Use 3 — Fat or Oil for Storage

Spec Wording

"Used to produce fat or oil for storage." The spec accepts either "fat" OR "oil" — they're closely related and the difference is just whether they're solid (fat) or liquid (oil) at room temperature.

Why Bother — Aren't Starch & Oil Both Storage?

Yes — and plants use both, but for different purposes:

Starch: short-to-medium-term energy reserve, easily mobilised

Oil/fat: long-term, energy-dense storage (gram for gram, oils store more energy than starch)

Where You Find Plant Oils

Plant oils are concentrated in seeds — think sunflower seeds, olives, peanuts, rapeseed. The high energy density lets the seedling survive on its food store while it grows leaves and starts to photosynthesise.

Use 4 — Cellulose for Cell Walls

Spec Wording

"Used to produce cellulose, which strengthens the cell wall." Note: the cell wall, not the cell membrane. This single-word mistake costs a lot of marks.

Cell Wall vs Cell Membrane

Cell wall: the rigid outer layer of plant cells, made of cellulose. Gives the plant its shape and strength.

Cell membrane: the thin selectively permeable layer made of lipids and proteins, controlling what enters/leaves the cell. Not made of cellulose.

Both are present in plant cells, but only the WALL is made of cellulose.

Synoptic Link — Module 2 Cell Structure

From Module 2: plant cells have a cell wall (made of cellulose) outside the cell membrane. The wall provides strength and rigidity, supports the plant against gravity, and stops cells bursting when water enters by osmosis. Without cellulose, plants would collapse.

Use 5 — Amino Acids for Protein Synthesis

Spec Wording

"Used to produce amino acids for protein synthesis." Then: "To produce proteins, plants also use nitrate ions that are absorbed from the soil."

The Two-Step Logic

Glucose + nitrate ions → amino acids (the plant builds amino acids using glucose carbon-skeletons plus nitrogen from nitrate)

Amino acids → proteins (linked together to make protein chains)

Why Glucose Alone Isn't Enough

Glucose contains carbon, hydrogen, and oxygen — but amino acids and proteins also contain nitrogen. Glucose has no nitrogen. So the plant has to get nitrogen from somewhere: it absorbs nitrate ions from the soil. This is why plants need both glucose AND nitrate to make proteins.

Why Nitrate Ions — and Why Specifically?

"Nitrogen" vs "Nitrate Ions"

The air is 78% nitrogen gas (N₂), but plants cannot use nitrogen gas directly — it's too unreactive. Plants absorb nitrate ions (NO₃⁻) from the soil. Mark schemes are strict: write "nitrate ions" or "nitrates", not just "nitrogen".

How Plants Absorb Nitrate

Nitrate ions enter the plant through the roots, dissolved in soil water. Because nitrate is usually at a higher concentration inside the root cell than in the soil, plants use active transport — they spend energy (from respiration) to absorb it against the concentration gradient.

Synoptic Link — Module 3 Plant Deficiency

Without enough nitrate, plants can't make enough protein. Effects include stunted growth and yellow older leaves. This is why farmers add nitrate-containing fertilisers to soil — to boost protein production and increase crop yield. Don't confuse this with magnesium: magnesium is needed for chlorophyll (deficiency causes yellowing too — chlorosis), whereas nitrate is needed for protein.

Food Tests — AT 8

The spec links this topic to AT 8 (qualitative reagent tests). Three tests you must know for plant biochemistry:

Three food tests in one place. Only Benedict's needs heating (≥60 °C). Benedict's positive result is a colour scale — green/yellow/orange/brick-red — depending on sugar concentration; mark schemes accept any of these.

Practice — PEE Worked Example

A farmer grows a crop in a field where the soil is deficient in nitrate ions. The plants show stunted growth.

Explain why a lack of nitrate ions in the soil leads to stunted growth.3 marks · AO2

P

Point: Without enough nitrate ions, the plant cannot make enough protein.

E

Evidence: Plants use glucose + nitrate ions to build amino acids, which then join together to form proteins.

E

Explain: Proteins are needed for growth and cell division (e.g. structural proteins, enzymes). Without enough protein, the plant cannot build new cells or grow new tissue, so growth is stunted.

⚠️

Mark-loss check: Writing "plants need nitrogen" would lose a mark — the spec requires "nitrate ions" specifically. Saying "without nitrate the plant dies" is also wrong — it's about growth being slowed, not death.

Three Examiner Alerts

Alert 1 — Nitrate Ions, Not Nitrogen

Plants absorb nitrate ions (NO₃⁻) from the soil — NOT nitrogen gas from the air. "Nitrogen" alone is too vague and often rejected by mark schemes. Always specify "nitrate ions" or "nitrates".

Alert 2 — Cell Wall, Not Cell Membrane

Cellulose strengthens the cell wall, not the cell membrane. They are different structures, and mark schemes will reject "cellulose makes the cell membrane".

Alert 3 — Energy is RELEASED, not Produced

Respiration releases or transfers energy from glucose. It does not "produce" or "make" energy. The energy was already stored in the glucose by photosynthesis.

Alert 4 — "Iodine" Must Be Qualified

Mark schemes reject "iodine" alone as the reagent name. You must write "iodine solution" or "iodine reagent". This single word costs students marks every year. Similarly: "Benedict's" needs "solution/reagent", and "Biuret" needs "reagent/solution".

Wrong vs Right

Five recurring mistakes on uses-of-glucose questions, each turned into a mark-scheme-friendly version.

✗ Wrong

"Plants take in nitrogen from the soil to make proteins"

✓ Right

"Plants absorb nitrate ions from the soil to make amino acids (and then proteins)"

✗ Wrong

"Cellulose makes the cell membrane stronger"

✓ Right

"Cellulose strengthens the cell wall"

✗ Wrong

"Respiration produces energy from glucose"

✓ Right

"Respiration releases energy from glucose"

✗ Wrong

"Glucose is stored directly in roots and seeds"

✓ Right

"Glucose is converted into insoluble starch for storage"

✗ Wrong

"Plants make proteins from glucose"

✓ Right

"Plants make proteins from amino acids — which need both glucose AND nitrate ions"

✗ Wrong

"Starch is made in photosynthesis"

✓ Right

"Glucose is made in photosynthesis — excess glucose is then converted into starch for storage"

✗ Wrong

"Add iodine to test for starch"

✓ Right

"Add iodine solution (or iodine reagent) to test for starch"

Three Takeaways

Five Fates of Glucose

Respiration (energy released); starch (insoluble storage); fat/oil (long-term storage, esp. seeds); cellulose (cell wall); amino acids → protein (with nitrate).

Nitrate Ions for Protein

Glucose has no nitrogen. Plants need nitrate ions (not nitrogen gas) from the soil, absorbed by active transport, to build amino acids.

Food Tests (AT 8)

Iodine → blue-black (starch). Benedict's + heat → brick-red (glucose). Biuret → purple (protein).

Spec Wording To Use

"Used for respiration", "converted into insoluble starch for storage", "produce fat or oil", "produce cellulose, which strengthens the cell wall", "produce amino acids for protein synthesis". Use the spec phrasing where you can — it always scores.

Lesson 4.03 Summary

The Five Uses (Spec 4.4.1.3)

1

Respiration — releases energy from glucose for cell processes

2

Converted into insoluble starch — storage in roots, stems, seeds; insoluble so doesn't affect water balance

3

Used to produce fat or oil — long-term, high-energy storage (especially in seeds)

4

Used to produce cellulose — strengthens the cell wall

5

Used to produce amino acids — joined to make proteins (plant also needs nitrate ions from soil)

Why Plants Need Nitrate Ions

Glucose contains only C, H, O — no nitrogen

Amino acids and proteins contain nitrogen

Plants absorb nitrate ions (NO₃⁻) from soil through roots (by active transport)

Nitrate deficiency → stunted growth (Module 3 synoptic link)

Food Tests (AT 8)

Starch: iodine solution → orange-brown to blue-black (no heat needed)

Glucose (reducing sugar): Benedict's solution + heat → blue to brick-red

Protein: Biuret reagent → blue to purple/lilac (no heat needed)

Spec Reference

Spec 4.4.1.3 Uses of glucose from photosynthesis — Paper 1. AT 8 covered through linked food tests.

Next: Lesson 4.04

You've covered the inputs (4.01), the rate (4.02), and now the products (4.03) of photosynthesis. Next, the partner reaction.

Coming Up — Aerobic & Anaerobic Respiration

Photosynthesis stores energy in glucose. Respiration releases it. Next lesson covers spec 4.4.2.1–4.4.2.3: aerobic respiration (with O₂), anaerobic respiration (without — and the differences between plants and animals), the response of the body to exercise, and metabolism. The combined synoptic link: photosynthesis produces what respiration uses.

Before Lesson 4.04

Test yourself: list all five uses of glucose from memory. State why plants need nitrate ions. Recall the colour change for each food test. If all three are solid, you're set for respiration.