Biosynthesis of Sucrose in Plants

Level: GCSE foundations, A-level and introductory university extension. This lesson covers one pathway only — how a leaf builds sucrose. It does not cover what happens to sucrose afterwards: its breakdown is a separate lesson, Degradation of Sucrose in Plants, and the control of the pathway has its own page too.

Cut open a sugar beet root and roughly a fifth of what you are holding is sucrose — not starch, not glucose, but sucrose, dissolved in the sap of its cells. A field of sugar cane is the same story on a bigger scale: the plant spends its whole life making sucrose and parking it where it can be harvested. The question this lesson answers is a small one with a long answer: how does a leaf cell actually build that molecule, step by step, and why does it build sucrose rather than anything else?

First, what sucrose is — in one paragraph

Sucrose is a disaccharide: one glucose joined to one fructose by an α1→β2 glycosidic bond, running from carbon-1 of glucose to carbon-2 of fructose. Because both anomeric carbons are tied up in that bond, sucrose is a non-reducing sugar: it cannot react with proteins the way glucose can, which is exactly why a plant can ship it around its body without it damaging anything on the way. Table sugar is purified sucrose. The full story of disaccharides — which sugars reduce, which do not, and why it matters — is in the carbohydrate lesson, A Comprehensive Guide to Lipids (its title says lipids, its body teaches carbohydrates — the lesson itself explains the mismatch).

Where the carbon comes from

Sucrose synthesis starts in the chloroplast, but sucrose is not made there. The Calvin cycle fixes carbon dioxide into triose phosphate — glyceraldehyde-3-phosphate, a three-carbon sugar phosphate — in the stroma. That triose phosphate is the raw material, and it has to leave the chloroplast before anything else can happen. It crosses the inner membrane through a transporter that works as a strict swap: one triose phosphate out, one phosphate in. The swap matters, because the light reactions need that phosphate back to make ATP — if triose phosphate left without phosphate coming in, the chloroplast would run out of phosphate and photosynthesis would stall.

Figure 1. From sunlight to sucrose: the Calvin cycle makes triose phosphate in the chloroplast, the translocator swaps it for phosphate, five steps in the cytosol build sucrose, and the finished sucrose is stored in the vacuole or loaded into the phloem.

That export step is also the first decision point of the whole lesson, and it deserves its own section — because the leaf does not send all its triose phosphate out. Some of it stays behind.

The fork in the road: sucrose or starch?

Every triose phosphate the Calvin cycle makes faces the same fork. Exported to the cytosol, it becomes sucrose — the transport sugar, sent to the rest of the plant. Kept in the chloroplast, it becomes starch — the storage polymer, kept for later. The two products are made in different compartments, from the same starting material, and the leaf balances them through the day: more sucrose while the sun shines and sinks are calling for it, more starch stored as the light fades, so the plant has something to draw on at night. The phosphate swap is what links the two sides — every triose phosphate exported brings a phosphate home, and that phosphate is what the chloroplast needs to keep fixing carbon.

Figure 2. One fork in the road: triose phosphate exported to the cytosol becomes sucrose for transport; triose phosphate kept in the chloroplast becomes starch for storage. Phosphate returns to the chloroplast with every export.

Here is the fork as arithmetic, using clearly labelled practice figures rather than measurements from any real leaf. Suppose a leaf exports 100 triose phosphate molecules in a given period and sends 60% of them to sucrose synthesis while the rest stays in the chloroplast for starch. The sucrose share is 60 triose phosphates. Each sucrose molecule needs 4 triose phosphates — two hexose units of six carbons each — so the leaf can make 60 ÷ 4 = 15 sucrose molecules. The starch share is 40 triose phosphates, and each glucose unit of starch needs 2, so the starch gains 40 ÷ 2 = 20 glucose units. One starting material, two products, and the split between them is the leaf's answer to the question of what the plant needs right now.

The five steps in the cytosol

Once triose phosphate reaches the cytosol, five steps turn it into sucrose. Learn them as a chain, because each step hands its product to the next.

Step 1 — joining. Two triose phosphates join to make fructose-1,6-bisphosphate, catalysed by aldolase. One of the two is first converted to its twin, dihydroxyacetone phosphate, so the pair fits together into a six-carbon sugar.

Step 2 — trimming. Fructose-1,6-bisphosphatase (FBPase) removes one phosphate, giving fructose-6-phosphate. This is the first committed step towards sucrose: once the phosphate is gone, the molecule is on the sucrose route.

Step 3 — shuffling. Some fructose-6-phosphate is converted to glucose-6-phosphate and then to glucose-1-phosphate, by phosphoglucomutase. The cell now holds the two hexose phosphates it needs: fructose-6-phosphate for one half of sucrose, glucose-1-phosphate for the other.

Step 4 — arming. Glucose-1-phosphate reacts with UTP to give UDP-glucose and pyrophosphate, catalysed by UDP-glucose pyrophosphorylase (UGPase). The pyrophosphate is immediately split in two, which pulls the reaction forward — the cell spends energy here to make sure UDP-glucose keeps being made. UDP-glucose is glucose with a handle attached: the UDP group is what lets the next enzyme grab it and join it to the other half.

Step 5 — joining and finishing. Sucrose-phosphate synthase (SPS, EC 2.4.1.14) joins UDP-glucose to fructose-6-phosphate, making sucrose-6-phosphate and releasing UDP. Then sucrose-phosphate phosphatase (SPP, EC 3.1.3.24) removes the phosphate, leaving free sucrose. SPS is the enzyme that gives the pathway its name and its speed; SPP is the enzyme that makes the whole route run one way, because throwing the phosphate away is not something the cell can easily undo.

Figure 3. The two enzymes that build sucrose: SPS (EC 2.4.1.14) joins UDP-glucose to fructose-6-phosphate, making sucrose-6-phosphate, and SPP (EC 3.1.3.24) removes the phosphate to give sucrose. UDP is recharged to UTP with ATP, and UGPase remakes UDP-glucose.

Notice what the chain costs. The cytosol spends one ATP equivalent per sucrose, recharging UDP back to UTP. That is a small bill — the bigger one was already paid upstream, in the Calvin cycle, and it is worth counting, because it shows how expensive a sugar molecule really is.

Worked example: counting the cost of one sucrose

Start from carbon dioxide and count forward. One sucrose molecule has 12 carbons, so the Calvin cycle must fix 12 carbon dioxide molecules to supply them. Those 12 carbons arrive as 4 triose phosphates, each carrying 3 carbons. The Calvin cycle spends 9 ATP and 6 NADPH for every net triose phosphate it exports, so 4 triose phosphates cost 4 × 9 = 36 ATP and 4 × 6 = 24 NADPH. Then the cytosol spends 1 more ATP equivalent recharging UDP to UTP. Total for one sucrose molecule: 36 ATP and 24 NADPH from the light reactions, plus 1 ATP in the cytosol. A molecule of table sugar carries the price of roughly 37 ATP — which is why a plant does not make it lightly, and why it stores and ships it carefully rather than wasting it.

Why sucrose, and not glucose, travels

Glucose is the fuel every cell burns, so it seems odd that the plant does not simply ship glucose around its body. It ships sucrose instead, for three reasons that all come back to the bond in the first paragraph.

First, sucrose is non-reducing. Glucose has a free anomeric carbon that reacts with amino groups on proteins — the same chemistry that browns food and damages proteins in a diabetic's blood. A plant that pumped glucose through its phloem all day would be pickling its own proteins. Sucrose's locked bond makes it chemically quiet.

Second, sucrose is very soluble and stable. It dissolves easily in sap at high concentration, and it survives the journey without breaking down — unlike glucose, which cells would grab and burn the moment they met it.

Third, one sucrose molecule carries 12 carbons in a single package. Shipping carbon two hexoses at a time is more efficient than shipping it one at a time, and the receiving tissue can split the package into glucose and fructose the moment it arrives — which is what sucrose synthase and invertase do at the sink.

Sucrose synthase: the reversible enzyme that mostly works backwards

There is one more enzyme in this story. Sucrose synthase (SS, EC 2.4.1.13) catalyses a reversible reaction: sucrose + UDP ↔ UDP-glucose + fructose. It is not a "backup path" for making sucrose, as if the leaf kept it in reserve for building. In a test tube it can build sucrose. In a living plant it almost never does. In sink tissues — growing roots, young leaves, developing seeds — sucrose synthase runs in the cleavage direction, splitting incoming sucrose and handing the UDP-glucose straight to cellulose synthesis for new cell walls and to starch synthesis for storage. It is not a spare builder; it is the unloading crew. The synthesis route is SPS plus SPP, and that is the only route this lesson teaches, because it is the only one the leaf uses.

Source to sink: where the sucrose goes

A mature leaf that makes more sucrose than it uses is a source. A root tip, a young leaf, a fruit or a seed that needs more than it makes is a sink. Companion cells load sucrose into the phloem's sieve tubes, water follows by osmosis, and the pressure this builds pushes sap from source to sink — the pressure-flow hypothesis, in one sentence. At the sink, sucrose is unloaded and split: sucrose synthase gives UDP-glucose for walls and starch, invertase gives glucose and fructose for energy and growth.

Figure 4. Source to sink: a mature leaf makes sucrose and loads it into the phloem; sap flows to growing shoot tips, roots and fruits or seeds. At the sink, sucrose synthase splits sucrose into UDP-glucose and fructose for building cell walls and starch.

The same leaf can change sides. A young leaf still growing is a sink — it imports sucrose. Once it is fully expanded and photosynthesizing hard, it becomes a source. The labels describe what a part is doing, not what it is.

Five confusions to separate

SPS is not SPP, and neither is SS. SPS joins UDP-glucose to fructose-6-phosphate. SPP removes the phosphate from sucrose-6-phosphate. SS splits sucrose at the sink. Three enzymes, three jobs, three different EC numbers — and the exam question that catches people out is the one that asks which one does which.

Sucrose is not glucose. Glucose is a single six-carbon sugar and the fuel cells burn. Sucrose is two sugars joined, the package the plant ships. A question about transport is a question about sucrose; a question about respiration is a question about glucose.

Synthesis is not degradation. This lesson builds sucrose. Breaking it down — invertase, sucrose synthase in the cleavage direction, and what the hexoses become — is the next lesson, Degradation of Sucrose in Plants.

Source is not sink. The words describe a role, not a place. A leaf can be a sink while it grows and a source once it is mature.

Triose phosphate is not hexose phosphate. Triose phosphate has 3 carbons and comes from the Calvin cycle; hexose phosphate has 6 carbons and is made in the cytosol by joining two triose phosphates. Counting carbons is the fastest way to check any step in this pathway.

Five mistakes that sound like plant biochemistry

1. "Sucrose is made in the chloroplast." It is not. Triose phosphate is made there; sucrose is made in the cytosol, after export.

2. "Sucrose synthase builds sucrose in leaves." In the living plant it mainly splits sucrose in sinks. SPS and SPP build it.

3. "The plant ships glucose around its body." It ships sucrose. Glucose is too reactive to travel.

4. "Starch and sucrose are made in the same place." They are made from the same starting material in different compartments — starch in the chloroplast, sucrose in the cytosol.

5. "Sucrose is a reducing sugar because it is made of two reducing sugars." Its parts are reducing, but the finished molecule is not — both anomeric carbons are locked in the bond.

How to answer an exam question on this topic

Use the sequence carbon source → compartment → steps → enzymes → destination. Name where the carbon comes from, say which compartment each step happens in, list the steps with their enzymes, and finish with where the sucrose goes.

Model paragraph: Sucrose is made in the cytosol of leaf cells from triose phosphate exported from the chloroplast by the triose phosphate/phosphate translocator. Two triose phosphates join to form fructose-1,6-bisphosphate, which FBPase converts to fructose-6-phosphate; some of this is converted to glucose-1-phosphate, which UGPase joins to UTP to make UDP-glucose. Sucrose-phosphate synthase then joins UDP-glucose to fructose-6-phosphate, forming sucrose-6-phosphate, and sucrose-phosphate phosphatase removes the phosphate to give sucrose. The sucrose is loaded into the phloem and transported to sinks such as roots and fruits, where sucrose synthase splits it for growth. That paragraph names the source, the compartments, the steps, the enzymes and the destination, which is what the marks are for.

Frequently asked questions

Why does the plant bother making sucrose at all — why not ship triose phosphate?
Triose phosphate is a reactive intermediate with a phosphate attached; it is not stable enough to travel, and the plant needs the phosphate back in the chloroplast. Sucrose is stable, soluble and non-reducing, so it can sit in sap at high concentration without reacting with anything.

Is sucrose the same as table sugar?
Yes. Table sugar is purified sucrose, usually from sugar cane or sugar beet. The sucrose in your tea and the sucrose in a leaf are the same molecule.

Does sucrose synthesis happen at night?
Not from photosynthesis — there is no triose phosphate coming out of the chloroplast in the dark. At night the leaf breaks down the starch it stored during the day and makes sucrose from that instead, so the sinks are still fed.

What is the difference between sucrose synthase and invertase?
Both split sucrose, but sucrose synthase gives UDP-glucose + fructose and is reversible, while invertase gives glucose + fructose and is not. Sucrose synthase feeds building work; invertase feeds energy and sweetness.

Why do sugar beet roots hold so much sucrose?
Because their storage cells park sucrose in large vacuoles, and the plant keeps loading more in through the phloem all season. The vacuole is a warehouse with no other job.

Can a plant make sucrose without light?
It can make it from stored starch, as above, but it cannot fix new carbon without light. Every sucrose molecule ultimately traces back to carbon dioxide fixed in the light.

Continue learning

• A Comprehensive Guide to Lipids — the carbohydrate lesson: disaccharides, reducing and non-reducing sugars, and the classification sucrose belongs to.
• Unraveling the Sweet Mysteries of Carbohydrates — homo- and heteropolysaccharides, including the starch that shares the branch point with sucrose.
• Degradation of Sucrose in Plants — the next lesson: how sucrose is split at the sink, by invertase and sucrose synthase.
• Starch biosynthesis — the other side of the fork: how triose phosphate kept in the chloroplast becomes starch.
• Starch and sucrose breakdown — compare the enzymes that release usable sugars from storage and transport carbohydrates.
• Bioenergetics in Living Organisms — where the 36 ATP per sucrose comes from: the energy rules, ATP and redox behind the Calvin cycle's bill.

Sources and further reading

These notes were written from the sources below and from the site's own linked lessons. The worked examples use clearly labelled practice figures, not measurements from any real leaf.

• OpenStax, Biology 2e, Chapter 8: Photosynthesis — the Calvin cycle, triose phosphate export and carbon partitioning.
• OpenStax, Biology 2e, Chapter 3: Biological Macromolecules — carbohydrates, glycosidic bonds and reducing sugars.
• Khan Academy, Biology: Photosynthesis — the Calvin cycle and the products of carbon fixation.
• Biology LibreTexts, Plant Biochemistry — sucrose synthesis, sucrose-phosphate synthase and sucrose synthase.
• BBC Bitesize, Biology: Photosynthesis — carbon fixation and the uses of glucose in plants.
• Taiz, L. and Zeiger, E., Plant Physiology and Development — sucrose synthesis, the triose phosphate translocator and source–sink transport.

20 Explained MCQs — answers hidden until opened

Open each question to reveal its answer and explanation. The interactive quiz below uses a different set of questions.

1. What two sugars join to make one sucrose molecule?

A. Glucose + fructose   B. Glucose + glucose   C. Glucose + galactose   D. Fructose + galactose
Answer: A. Sucrose is glucose joined to fructose. Two glucoses make maltose; glucose + galactose makes lactose.

2. What bond joins the glucose and the fructose in sucrose?

A. A β1→4 glycosidic bond   B. An α1→β2 glycosidic bond   C. An α1→4 glycosidic bond   D. An α1→6 glycosidic bond
Answer: B. The bond runs from carbon-1 of glucose to carbon-2 of fructose, written α1→β2. The α1→4 bond is the one in maltose and starch.

3. Why is sucrose a non-reducing sugar?

A. It has no oxygen atoms   B. Both anomeric carbons are locked in the glycosidic bond   C. It has no carbon atoms   D. It is too large a molecule to react
Answer: B. A sugar reduces only if it has a free anomeric carbon. In sucrose both anomeric carbons are tied up in the bond, so neither is free.

4. Where in a leaf cell is sucrose actually built?

A. In the vacuole   B. In the cytosol   C. In the mitochondrion   D. In the chloroplast stroma
Answer: B. The Calvin cycle runs in the chloroplast, but sucrose synthesis happens in the cytosol, after triose phosphate is exported.

5. What leaves the chloroplast to feed sucrose synthesis?

A. Starch   B. Glucose   C. Sucrose   D. Triose phosphate
Answer: D. Triose phosphate (glyceraldehyde-3-phosphate) is the Calvin cycle's export. Sucrose is made later, in the cytosol.

6. What does the triose phosphate / phosphate translocator exchange across the chloroplast membrane?

A. Sucrose out, glucose in   B. Oxygen out, carbon dioxide in   C. ATP out, ADP in   D. Triose phosphate out, phosphate in
Answer: D. The translocator is a strict antiport: one triose phosphate leaves only as one phosphate enters, which keeps phosphate available for the light reactions.

7. Which enzyme removes the phosphate from fructose-1,6-bisphosphate in the cytosol?

A. Sucrose-phosphate phosphatase (SPP)   B. Fructose-1,6-bisphosphatase (FBPase)   C. Sucrose-phosphate synthase (SPS)   D. Aldolase
Answer: B. FBPase hydrolyses fructose-1,6-bisphosphate to fructose-6-phosphate. Aldolase does the joining step before it; SPS and SPP act later.

8. Which enzyme makes UDP-glucose from glucose-1-phosphate and UTP?

A. UDP-glucose pyrophosphorylase (UGPase)   B. Sucrose-phosphate phosphatase (SPP)   C. Sucrose-phosphate synthase (SPS)   D. Sucrose synthase
Answer: A. UGPase joins glucose-1-phosphate to UTP, giving UDP-glucose and pyrophosphate. It is the step that arms glucose with its carrier.

9. Sucrose-phosphate synthase (SPS) joins which two molecules?

A. Glucose + fructose   B. UDP-glucose + fructose-6-phosphate   C. Sucrose + UDP   D. ADP-glucose + fructose
Answer: B. SPS transfers glucose from UDP-glucose onto fructose-6-phosphate, making sucrose-6-phosphate and releasing UDP.

10. What does sucrose-phosphate phosphatase (SPP) do?

A. Builds starch from glucose units   B. Splits sucrose into glucose and fructose   C. Removes the phosphate from sucrose-6-phosphate, giving sucrose   D. Joins UDP-glucose to fructose-6-phosphate
Answer: C. SPP hydrolyses sucrose-6-phosphate to sucrose + Pi. That single cut is what makes the whole route run one way.

11. Why does the SPS/SPP route run in one direction in the living cell?

A. The cell has an unlimited supply of ATP   B. SPS can only work in the dark   C. Sucrose cannot leave the cytosol   D. SPP's removal of the phosphate makes the overall pathway effectively irreversible
Answer: D. The SPS step itself is reversible, but SPP's hydrolysis throws the phosphate away, so the net pathway cannot run backwards in the cell.

12. What is sucrose synthase's main job in a living plant?

A. Splitting sucrose in sink tissues to feed growth   B. Loading sucrose into the phloem   C. Making sucrose in the leaves   D. Storing sucrose in vacuoles
Answer: A. Sucrose synthase can make sucrose in a test tube, but in the plant it mostly works the other way: it cleaves sucrose in sinks, giving UDP-glucose for cell walls and starch.

13. What is the main transport sugar in most plants?

A. Sucrose   B. Glucose   C. Starch   D. Fructose
Answer: A. Sucrose travels in the phloem to every part of the plant. Glucose is the fuel burned inside cells; starch never leaves the cell that made it.

14. Why is sucrose, rather than glucose, the sugar a plant ships around its body?

A. It is non-reducing and stable, so it does not react with proteins on the journey   B. It is the smallest sugar   C. It contains nitrogen   D. It does not dissolve in water
Answer: A. Glucose is a reducing sugar and would react with amino groups on proteins during transport. Sucrose's locked anomeric carbons make it chemically quiet.

15. What is a 'source' in source–sink transport?

A. A root that stores starch   B. A seed that is germinating   C. A part that makes more sucrose than it uses, such as a mature leaf   D. A fruit that is ripening
Answer: C. A source exports sucrose; a sink imports it. A mature leaf is the classic source, while roots, fruits and growing tips are sinks.

16. At a sink, sucrose synthase splits sucrose into which two products?

A. UDP-glucose + fructose   B. Glucose + glucose   C. Starch + water   D. ATP + ADP
Answer: A. Sucrose + UDP gives UDP-glucose + fructose. The UDP-glucose then feeds cellulose and starch synthesis in the growing tissue.

17. Practice figures, not real measurements: a leaf exports 100 triose phosphate molecules and sends 60% of them to sucrose synthesis. How many sucrose molecules can it make?

A. 15   B. 20   C. 30   D. 60
Answer: A. 60 triose phosphates go to sucrose, and each sucrose needs 4 of them (two hexose units of 6 carbons): 60 ÷ 4 = 15 sucrose molecules.

18. Practice figures: how many carbon dioxide molecules must the Calvin cycle fix to provide the carbon for one sucrose molecule?

A. 4   B. 6   C. 24   D. 12
Answer: D. Sucrose has 12 carbons, and each fixed CO2 contributes one carbon, so 12 CO2 are needed. They arrive as 4 triose phosphates.

19. Which statement about the sucrose–starch branch point is correct?

A. The branch point is in the mitochondrion   B. All triose phosphate becomes starch   C. Triose phosphate exported to the cytosol becomes sucrose; triose phosphate kept in the chloroplast becomes starch   D. Sucrose is made in the chloroplast and starch in the cytosol
Answer: C. The fork is the chloroplast membrane: export means sucrose in the cytosol, staying put means starch in the stroma.

20. Which claim stays inside the evidence of the practice figures in this lesson?

A. Every leaf makes exactly 15 sucrose molecules a day   B. Sugar beet roots always hold a fifth of their weight as sucrose   C. Sucrose synthesis is faster than starch synthesis in all plants   D. A leaf that sends 60 of 100 triose phosphates to sucrose synthesis can make 15 sucrose molecules
Answer: D. Only the claim that a leaf sending 60 of 100 triose phosphates to sucrose synthesis can make 15 sucrose molecules repeats the practice figures. The others turn one worked example into claims about all leaves and all plants.

Separate Interactive Quiz — 20 different questions

These quiz questions are different from the MCQs above. Select an answer for each question. You will not see whether an answer is correct until you submit all 20 questions. Use Reset to try again.

Course guide: Plant Biochemistry II course guide.