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Photosynthesis II: The Carbon Reactions (Videos Collection Playlist)

Writer: Scense Me
Scense Me
2 hours ago
18 min read

Photosynthesis II: The Carbon Reactions is a series of 20 short, narrated and animated videos on how the chloroplast turns carbon dioxide (CO₂) into sugar. It picks up where Photosynthesis I: Introduction and the Light Reactions left off, following the adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH) made by the light reactions into the stroma, through every step of the Calvin–Benson cycle and the switches that turn it on in the light, and on to starch and sucrose. Every chemical is named in full, the carbon is counted at every step, and common textbook misconceptions are corrected along the way.


The series is written for undergraduate plant science, agriculture and biology students, and for anyone curious about how plants build their food. Each video opens with a different crop, from padi and keladi to durian and keledek. Each runs between 2:38 and 3:32 minutes, and the whole series takes about 62 minutes. Watch them in order, starting with Video 1. English subtitles are available on every video.



What you will learn

  • Why the “dark reactions” are better called the carbon reactions

  • The three phases of the Calvin–Benson cycle: carboxylation, reduction and regeneration

  • How Rubisco (ribulose 1,5-bisphosphate carboxylase/oxygenase) works, and why it is slow but abundant

  • How the cycle rebuilds ribulose 1,5-bisphosphate (RuBP), and what one triose phosphate costs in ATP and NADPH

  • Why carbon fixation takes time to start when light returns (photosynthetic induction)

  • How light switches the cycle on: Rubisco activase, the ferredoxin–thioredoxin system, stromal pH and magnesium, and CP12

  • Where the carbon goes: starch in the chloroplast and sucrose in the cytosol

  • How starch is built, broken down at night and balanced against sucrose export


All 20 videos at a glance

  1. Video 1: From Light Reactions to Carbon Reactions (3:22)

  2. Video 2: The Three Phases of the Calvin–Benson Cycle (3:06)

  3. Video 3: Rubisco, the Enzyme that Fixes Carbon Dioxide (3:27)

  4. Video 4: Carboxylation, One Carbon Joins Five (3:32)

  5. Video 5: Reduction, Turning 3-PGA into Triose Phosphate (3:23)

  6. Video 6: The Triose Phosphate Pool, G3P and DHAP (3:18)

  7. Video 7: Regeneration I, Building Six- and Four-Carbon Skeletons (3:00)

  8. Video 8: Regeneration II, Returning to RuBP (3:08)

  9. Video 9: The Carbon and Energy Balance, What One Triose Phosphate Costs (3:12)

  10. Video 10: Why the Cycle Takes Time to Start, Photosynthetic Induction (3:01)

  11. Video 11: How Rubisco Becomes Active, Carbamylation and Rubisco Activase (3:13)

  12. Video 12: The Ferredoxin–Thioredoxin Switch, How Light Turns Enzymes On (3:05)

  13. Video 13: Light Raises Stromal pH and Magnesium in the Chloroplast (2:54)

  14. Video 14: CP12, a Protein Complex That Helps Regulate the Cycle (2:54)

  15. Video 15: Two Destinations, Starch and Sucrose (2:38)

  16. Video 16: Starch, Amylose, Amylopectin and Granules (2:42)

  17. Video 17: How Plants Synthesise Starch (2:58)

  18. Video 18: How Transitory Starch Is Mobilised at Night (2:56)

  19. Video 19: Sucrose Synthesis in the Cytosol (3:05)

  20. Video 20: Who Gets the Carbon? Regulation and Recap (3:14)


Part 1: The cycle at a glance


Video 1: From Light Reactions to Carbon Reactions

The first video of Photosynthesis II moves from the light reactions to the carbon reactions. The light reactions made ATP (adenosine triphosphate) and NADPH (reduced nicotinamide adenine dinucleotide phosphate), but neither is rice starch. In the stroma of the chloroplast, the Calvin–Benson cycle, also called the photosynthetic carbon reduction cycle, uses ATP for energy and NADPH for electrons to turn carbon dioxide (CO₂) into organic carbon. We map its three phases (carboxylation, reduction and regeneration) and explain why "carbon reactions" is a better name than "dark reactions": no step of the cycle absorbs light directly, but the cycle depends on ATP and NADPH from illuminated chloroplasts, and light switches on several of its enzymes.

Chapters

  • 0:00 A field powered by sunlight

  • 0:48 Where the next reactions happen

  • 1:19 Two distinct contributions

  • 2:05 Three parts of one cycle

  • 2:35 You may have learnt: the dark reactions


Video 2: The Three Phases of the Calvin–Benson Cycle

Why does keladi need a cycle to fill its corms with carbon? The Calvin–Benson cycle does not start with glucose and end with a pile of sugar. It continually takes in carbon dioxide (CO₂), produces useful organic carbon and restores the molecule that accepts the next CO₂. We follow its three phases: carboxylation, where the enzyme Rubisco joins CO₂ to ribulose 1,5-bisphosphate (RuBP) to give two molecules of 3-phosphoglycerate (3-PGA); reduction, where ATP (adenosine triphosphate) and NADPH (reduced nicotinamide adenine dinucleotide phosphate) turn 3-PGA into triose phosphate, especially glyceraldehyde 3-phosphate (G3P); and regeneration, where sugars with 4, 5, 6 and 7 carbons are rearranged and ATP restores RuBP. Finally we count the carbon for every three CO₂ fixed: 3 + 15 = 18 carbons in, 18 − 15 = 3 carbons net gain, one triose phosphate's worth.

Chapters

  • 0:00 Why keladi needs a cycle

  • 0:35 Phase one: carboxylation

  • 1:10 Phase two: reduction

  • 1:47 Phase three: regeneration

  • 2:20 The important net result


Video 3: Rubisco, the Enzyme that Fixes Carbon Dioxide

A pisang plant needs a continuous supply of fixed carbon to build its broad leaves, pseudostems and fruits. A key enzyme behind that supply is Rubisco, ribulose 1,5-bisphosphate carboxylase/oxygenase. In land plants it is an L8S8 assembly of eight large subunits, which carry the catalytic sites, and eight small subunits. The large subunits are encoded in the chloroplast genome and the small subunits in the nuclear genome. Each catalytic site binds ribulose 1,5-bisphosphate (RuBP), adds carbon dioxide (CO₂) and releases two molecules of 3-phosphoglycerate. It is slow: only about 1 to 6 CO₂ per catalytic site per second in land plants. So many sites work side by side, but more Rubisco alone does not guarantee a bigger yield. Rubisco can also react with oxygen (O₂), which leads to photorespiration in Photosynthesis III.

Chapters

  • 0:00 The enzyme behind a plantation

  • 0:47 An unusually abundant protein

  • 1:25 What one catalytic site does

  • 2:08 Why so much Rubisco

  • 2:45 Two jobs, one name


Part 2: Carboxylation, reduction and regeneration, step by step


Video 4: Carboxylation, One Carbon Joins Five

Kacang soya seeds are built from carbon fixed in the leaves. This video follows one carboxylation at the Rubisco active site. Ribulose 1,5-bisphosphate (RuBP) has five carbons and two phosphate groups. In the active site it briefly becomes an enediol, and carbon dioxide (CO₂) adds at carbon 2. The result is not a stable six-carbon sugar but a short-lived six-carbon intermediate. Hydration and cleavage then give two molecules of 3-phosphoglycerate (3-PGA): 5 + 1 = 3 + 3. 3-PGA is the first stable product of CO₂ fixation in C3 plants. Three carboxylations use 3 RuBP (3 × 5 = 15 carbons) and 3 CO₂ (3 × 1 = 3 carbons) to make 6 × 3-PGA (6 × 3 = 18 carbons).

Chapters

  • 0:00 Begin with five carbons

  • 0:38 You may have learnt: a stable six-carbon sugar?

  • 1:26 Why two products appear

  • 2:07 Now perform it three times

  • 2:47 A forward pointer


Video 5: Reduction, Turning 3-PGA into Triose Phosphate

Kacang tanah seeds, rich in oil and protein, are built from carbon fixed in the leaves. Rubisco has made six molecules of 3-phosphoglycerate (3-PGA), but they are not yet sugars. Two enzymes turn them into triose phosphate. First, 3-phosphoglycerate kinase uses ATP (adenosine triphosphate) to make 1,3-bisphosphoglycerate, and ATP becomes ADP (adenosine diphosphate). Then NADP-dependent glyceraldehyde-3-phosphate dehydrogenase (NADP-GAPDH) uses NADPH (reduced nicotinamide adenine dinucleotide phosphate) to reduce it to glyceraldehyde 3-phosphate (G3P), releasing inorganic phosphate. For three CO₂ fixed, six 3-PGA need 6 ATP and 6 NADPH, and the carbon stays at 6 × 3 = 18 throughout. The cycle does not release free glucose: its immediate reduced products are triose phosphates.

Chapters

  • 0:00 Carbon is fixed but not yet a sugar

  • 0:43 First enzyme: phosphorylation

  • 1:15 Second enzyme: reduction

  • 1:58 Six lanes, one budget

  • 2:34 You may have learnt: glucose leaves the cycle?


Video 6: The Triose Phosphate Pool, G3P and DHAP

Tomato fruit grow on sugars made from carbon fixed in the leaves. The Calvin–Benson cycle makes a small pool of three-carbon compounds: glyceraldehyde 3-phosphate (G3P), an aldehyde, and dihydroxyacetone phosphate (DHAP), a ketone. Each has three carbons and one phosphate group. The enzyme triose phosphate isomerase converts them back and forth, rearranging the same atoms without making or losing carbon. In the regeneration phase, aldolase joins DHAP to other sugar phosphates and G3P supplies transketolase reactions, so the pool is not a one-way pipe. For three CO₂ fixed, 6 triose phosphate equivalents hold 6 × 3 = 18 carbons. Five of them (5 × 3 = 15 carbons) rebuild 3 ribulose 1,5-bisphosphate (RuBP), leaving 18 − 15 = 3 carbons as net product, though the newly fixed carbons become mixed over repeated turns. Some triose phosphate carbon makes starch in the chloroplast; some is exported through a transporter in the inner envelope membrane to make sucrose in the cytosol.

Chapters

  • 0:00 Two three-carbon partners

  • 0:43 A fast reversible reaction

  • 1:20 Why both forms are needed

  • 1:58 Which carbon is the net gain

  • 2:39 Where the carbon can go


Video 7: Regeneration I, Building Six- and Four-Carbon Skeletons

Bendi keeps fixing carbon only while ribulose 1,5-bisphosphate (RuBP) is rebuilt. Five triose phosphate equivalents (5 × 3 = 15 carbons) must be rearranged into three five-carbon precursors (3 × 5 = 15 carbons). First, aldolase joins glyceraldehyde 3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP) into fructose 1,6-bisphosphate: 3 + 3 = 6 carbons. This makes a new carbon–carbon bond but fixes no new carbon dioxide. Then fructose 1,6-bisphosphatase (FBPase) removes one phosphate, giving fructose 6-phosphate, still six carbons. Next, transketolase moves a two-carbon fragment from fructose 6-phosphate to another G3P, giving erythrose 4-phosphate and xylulose 5-phosphate: 6 + 3 = 4 + 5, so no carbon is lost. The seven-carbon branch and the return to RuBP follow in Video 8.

Chapters

  • 0:00 Why a cycle must rebuild its acceptor

  • 0:42 Aldolase joins three and three

  • 1:11 FBPase removes a phosphate

  • 1:48 Transketolase redistributes carbon

  • 2:24 The ledger continues


Video 8: Regeneration II, Returning to RuBP

Terung leaves keep fixing carbon dioxide only if the Calvin–Benson cycle keeps rebuilding its five-carbon acceptor. Picking up from Video 7, we have erythrose 4-phosphate (4 carbons), xylulose 5-phosphate (5 carbons) and two more triose phosphates: 4 + 5 + 3 + 3 = 15 carbons. Aldolase joins erythrose 4-phosphate and dihydroxyacetone phosphate (DHAP) into sedoheptulose 1,7-bisphosphate (4 + 3 = 7). Sedoheptulose 1,7-bisphosphatase (SBPase) then removes one phosphate, giving sedoheptulose 7-phosphate, still seven carbons. A second transketolase reaction with glyceraldehyde 3-phosphate (G3P) gives ribose 5-phosphate and xylulose 5-phosphate (7 + 3 = 5 + 5). Epimerase and isomerase turn all three five-carbon sugars into ribulose 5-phosphate. Phosphoribulokinase (PRK) then uses 3 ATP (adenosine triphosphate) to make 3 ribulose 1,5-bisphosphate (RuBP), 3 × 5 = 15 carbons. Regeneration costs ATP but no NADPH, separately from the 6 ATP and 6 NADPH used in reduction.

Chapters

  • 0:00 Rejoin our unfinished diagram

  • 0:38 Make a seven-carbon intermediate

  • 1:09 Transfer two carbons again

  • 1:43 Change the forms, then add ATP

  • 2:27 Close the loop accurately


Video 9: The Carbon and Energy Balance, What One Triose Phosphate Costs

What does one net three-carbon product of the Calvin–Benson cycle cost? Starting from a betik (papaya, Carica papaya) tree, this video writes the complete steady-state carbon and energy budget. Three ribulose 1,5-bisphosphate (RuBP) molecules (3 × 5 = 15 carbons) plus three carbon dioxide (CO₂) molecules (3 × 1 = 3 carbons) give eighteen carbons, and Rubisco turns them into six molecules of 3-phosphoglycerate (3-PGA): 6 × 3 = 18 carbons. Reduction to six triose phosphates uses 6 adenosine triphosphate (ATP) and 6 reduced nicotinamide adenine dinucleotide phosphate (NADPH) but adds no carbon. Five triose phosphate equivalents (15 carbons) plus 3 more ATP regenerate three RuBP, leaving 18 − 15 = 3 carbons as one net triose phosphate. Per 3 CO₂: 9 ATP and 6 NADPH. Per 1 CO₂: 9 ÷ 3 = 3 ATP and 6 ÷ 3 = 2 NADPH. This is the idealised biochemical requirement, not the photon requirement of a real crop canopy.

Chapters

  • 0:00 What the papaya plant invests

  • 0:37 Start with eighteen carbons

  • 1:10 Reduction does not change the count

  • 1:41 Regeneration uses five trioses

  • 2:18 Write the complete budget


Part 3: How light switches the cycle on


Video 10: Why the Cycle Takes Time to Start, Photosynthetic Induction

When light reaches a kopi (Arabica coffee, Coffea arabica) leaf after darkness, carbon dioxide (CO₂) fixation does not jump to its daytime rate at once. The leaf goes through an induction period, in which the rate of carbon assimilation rises over time. This video explains three reasons. First, the supply chain: in darkness chloroplasts make little adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH), and even when the supply returns, the enzymes that use them may still be relatively inactive. Second, the pools: intermediates such as ribulose 1,5-bisphosphate (RuBP), 3-phosphoglycerate (3-PGA) and triose phosphates are low in the dark, so newly processed carbon must first refill them and regenerate enough RuBP. Third, enzyme activation: light changes stromal conditions, and Rubisco, thioredoxin-regulated enzymes, pH, magnesium and protein complexes respond at different speeds. A static drawing of the Calvin–Benson cycle hides this time dimension; the next four videos look at the switches behind it.

Chapters

  • 0:00 Morning is not an instant switch

  • 0:42 One reason: the supply chain

  • 1:18 Another reason: rebuilding the pools

  • 1:53 Enzyme activity also changes

  • 2:27 The takeaway is dynamic


Video 11: How Rubisco Becomes Active, Carbamylation and Rubisco Activase

A kentang (potato, Solanum tuberosum) leaf may contain plenty of Rubisco yet fix carbon dioxide (CO₂) slowly if much of that Rubisco is inactive. This video separates Rubisco activation from carboxylation. First, an activator CO₂ reacts with the amino group of a particular lysine in the active site, forming a carbamate (carbamylation), and a magnesium ion (Mg²⁺) then binds to stabilise the catalytically competent enzyme. Only then does ribulose 1,5-bisphosphate (RuBP) bind and form an enediol, which reacts with a separate substrate CO₂ to give two molecules of 3-phosphoglycerate (3-PGA). Finally, we correct a common misconception: Rubisco activase does not fix carbon dioxide. It uses energy from adenosine triphosphate (ATP) to promote the release of tightly bound inhibitory sugar phosphates, and of RuBP from uncarbamylated sites, so that activation and catalysis can resume.

Chapters

  • 0:00 An enzyme needs to be ready

  • 0:37 Activator carbon dioxide

  • 1:15 Now the substrate reaction

  • 1:50 You may have learnt: activase fixes carbon?

  • 2:35 Two different uses of carbon dioxide


Video 12: The Ferredoxin–Thioredoxin Switch, How Light Turns Enzymes On

Photons do not power Calvin–Benson enzymes directly, yet light changes whether several of them are active. Using sawi (pak choy, Brassica rapa) as our crop, this video follows a relay of reducing power: reduced ferredoxin supplies electrons to ferredoxin–thioredoxin reductase (FTR), which reduces the small protein thioredoxin. Reduced thioredoxin then reduces particular regulatory disulphide bonds (S–S) on target enzymes to two sulphydryl groups (–SH), changing their activity. Four key targets are fructose 1,6-bisphosphatase (FBPase), sedoheptulose 1,7-bisphosphatase (SBPase), phosphoribulokinase (PRK) and nicotinamide adenine dinucleotide phosphate-dependent glyceraldehyde-3-phosphate dehydrogenase (NADP-GAPDH). In the dark, the supply of reduced ferredoxin falls, regulatory disulphides return towards their oxidised states and key carbon-cycle activities decrease.

Chapters

  • 0:00 Light reaches the leafy crop

  • 0:36 A relay of reducing power

  • 1:06 What changes on the target enzyme

  • 1:37 Name the four key targets

  • 2:19 What happens in the dark


Video 13: Light Raises Stromal pH and Magnesium in the Chloroplast

Light does more than supply adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH). Using kacang hijau (mung bean, Vigna radiata) as our crop, this video shows how illumination changes the chloroplast stroma itself. During photosynthetic electron transport, hydrogen ions (protons) accumulate in the thylakoid lumen, from water oxidation and through the electron transport chain, so stromal pH rises from about 7 in darkness to about 8 in light. A rise of 1 pH unit means 10 times fewer hydrogen ions (0.0000001 ÷ 0.00000001 = 10). Magnesium ions (Mg²⁺) move from the lumen towards the stroma, raising free stromal Mg²⁺ from about 2 to about 5 millimolar (illustrative textbook values). Rubisco, fructose 1,6-bisphosphatase (FBPase), sedoheptulose 1,7-bisphosphatase (SBPase) and phosphoribulokinase (PRK) need Mg²⁺ and work better near pH 8 than pH 7. The lumen's proton gradient supports ATP production, while the alkaline, magnesium-enriched stroma favours carbon-cycle activity.

Chapters

  • 0:00 A changing environment inside a leaf

  • 0:40 Where the hydrogen ions go

  • 1:13 Magnesium also redistributes

  • 1:47 Enzymes respond to both conditions

  • 2:19 Two compartments, two functions


Video 14: CP12, a Protein Complex That Helps Regulate the Cycle

Carbon fixation depends not only on which enzymes are present but on how they are regulated. Using nangka (jackfruit, Artocarpus heterophyllus) as our crop, this video introduces CP12, a small regulatory protein. In a dark, oxidising state, CP12 helps stabilise a complex with phosphoribulokinase (PRK) and a form of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), in which both enzymes are strongly inhibited. In the light, the ferredoxin–thioredoxin system changes regulatory disulphide bonds, favouring dissociation of the complex and releasing active PRK and GAPDH. GAPDH isoforms differ in their regulation, and some shift between oligomeric assemblies, so one cartoon does not fit every plant. We finish by bringing together the switches from Videos 11 to 14: Rubisco activation, thioredoxin, stromal pH and magnesium, and CP12 complexes.

Chapters

  • 0:00 One more light-sensitive control

  • 0:33 Meet the three partners

  • 1:07 Light changes the assembly

  • 1:43 Why the exact protein matters

  • 2:15 Bring the switches together


Part 4: Where the carbon goes, starch and sucrose


Video 15: Two Destinations, Starch and Sucrose

Source leaves must supply carbon for growth and fruit development, as in a rambutan (Nephelium lappaceum) tree. The Calvin–Benson cycle makes triose phosphates, and a leaf cannot leave all that carbon circulating. Inside the chloroplast, triose phosphate carbon contributes to starch, which stays in granules inside the plastid. Exported triose phosphates are used in the cytosol to make sucrose, which is soluble and can be loaded into the phloem for sink tissues. Transitory starch builds up in the leaf by day and is broken down at night to supply carbon for respiration and continued sucrose supply. A common misconception is corrected: intact starch granules do not travel in the phloem. Sucrose, made of glucose and fructose units, is the major long-distance transport sugar in many crops. The split between starch and sucrose depends on carbon assimilation, metabolic demand, inorganic phosphate and enzyme regulation.

Chapters

  • 0:00 Where the orchard gets its carbon

  • 0:31 Two main routes from the stroma

  • 1:04 Starch supports the night

  • 1:30 You may have learnt: starch travels in phloem?

  • 2:03 Partitioning is controlled


Video 16: Starch, Amylose, Amylopectin and Granules

A durian (Durio zibethinus) tree still needs carbon at night, so its source leaves store some daytime photosynthates as starch granules inside chloroplasts. Starch is a large polymer built from many glucose units. Amylose has most glucose units joined by α-1,4 (alpha-one-four) glycosidic bonds, giving a largely linear chain that can coil; it is not necessarily perfectly unbranched, but has far fewer branch points than amylopectin. Amylopectin has α-1,4 chains with α-1,6 (alpha-one-six) branch points, making a much more highly branched molecule with many chain ends. Amylose and amylopectin associate within insoluble starch granules, whose proportions and fine structure vary with species and tissue. Unlike soluble sucrose, a starch granule stays in its plastid. Transitory starch sits in chloroplasts of source leaves; storage starch accumulates in amyloplasts of sink tissues, which build their own starch from transported carbon, mainly sucrose.

Chapters

  • 0:00 A starch reserve in a leaf

  • 0:34 Amylose: mainly long chains

  • 1:01 Amylopectin: branches and clusters

  • 1:35 The granule is a packaged store

  • 2:03 Two locations, different time scales


Video 17: How Plants Synthesise Starch

A kelapa (coconut, Cocos nucifera) palm needs a steady supply of carbon for developing fruit and new tissues. Starch does not grow by free glucose molecules simply sticking to a chain. Instead, Calvin–Benson cycle carbon is converted to fructose 6-phosphate, glucose 6-phosphate and glucose 1-phosphate. ADP-glucose pyrophosphorylase (AGPase) then uses glucose 1-phosphate and adenosine triphosphate (ATP) to make adenosine diphosphate glucose (ADP-glucose) and pyrophosphate (PPi); the ATP-derived part becomes part of ADP-glucose, so ATP does not simply turn into free ADP. Starch synthases transfer glucose from ADP-glucose to the nonreducing end of a growing glucan chain and release ADP. Granule-bound starch synthase is especially associated with amylose, while soluble starch synthases extend chains used in amylopectin. A starch branching enzyme moves a segment of an α-1,4-linked chain to form an α-1,6 branch without adding any glucose (12 units before, 12 after).

Chapters

  • 0:00 You may have learnt: free glucose makes starch?

  • 0:38 From triose phosphate to glucose phosphate

  • 1:08 AGPase makes ADP-glucose

  • 1:46 Starch synthases extend chains

  • 2:21 Branching creates architecture


Video 18: How Transitory Starch Is Mobilised at Night

After sunset, the leaves of a mangga (mango, Mangifera indica) tree no longer receive enough light for the usual light reactions, yet roots, growing shoots and developing fruits still respire and need carbon. The plant draws on transitory starch made in chloroplasts during the day. First the granule must become accessible: glucan–water dikinase (GWD) adds phosphate groups to glucan chains at the granule surface, and related phosphatases later remove them, so starch does not turn into phosphate. Hydrolytic enzymes then do the cutting: β-amylase (beta-amylase) releases maltose (two glucose units) from suitable chain ends, while debranching enzymes deal with the α-1,6 (alpha-one-six) branch points of amylopectin and other reactions produce glucose. A large, insoluble polymer becomes smaller, transportable carbohydrates. Maltose and glucose leave the chloroplast through specific envelope transporters, while the granule stays inside. In the cytosol this carbon supports sucrose synthesis and respiration, so the phloem keeps carrying organic carbon even without current carbon fixation. Leaf starch is low at dawn and builds up by late afternoon, and plants pace its breakdown so the supply lasts through the dark period.

Chapters

  • 0:00 Night arrives, carbon is still needed

  • 0:31 The granule must become accessible

  • 1:09 Hydrolytic enzymes do the cutting

  • 1:41 Move the products out

  • 2:13 Daily storage is a timed strategy


Video 19: Sucrose Synthesis in the Cytosol

In a kacang buncis (common bean, Phaseolus vulgaris) crop, carbon fixed in green leaves must travel to developing pods and seeds, and a major transported carbohydrate is sucrose. Sucrose is synthesised mainly in the cytosol of source-leaf cells, outside the chloroplast. First, the triose phosphate/phosphate translocator (TPT), a carrier in the inner chloroplast envelope, exchanges one triose phosphate out for one inorganic phosphate (Pi) in, rather than acting as an open hole; this keeps phosphate available for photosynthesis. In the cytosol, triose phosphates are converted to fructose 6-phosphate, glucose 6-phosphate and glucose 1-phosphate. UDP-glucose pyrophosphorylase then makes uridine diphosphate glucose (UDP-glucose) and pyrophosphate (PPi) from glucose 1-phosphate and uridine triphosphate (UTP). UDP-glucose is the donor for sucrose, whereas adenosine diphosphate glucose (ADP-glucose) is the starch donor inside plastids. Sucrose 6F-phosphate synthase transfers the glucosyl unit from UDP-glucose to fructose 6-phosphate, giving sucrose 6F-phosphate and releasing UDP. A separate enzyme, sucrose 6F-phosphate phosphatase, then removes inorganic phosphate to make sucrose. Only the finished sucrose, without phosphate, is loaded into the phloem and carried to sinks such as pods and seeds, where it is respired, used for growth or converted into storage compounds.

Chapters

  • 0:00 Carbon must reach the pods

  • 0:34 Export by exchange: the triose phosphate/phosphate translocator

  • 1:09 Activating glucose: UDP-glucose, not ADP-glucose

  • 1:52 Two separate enzymes make sucrose

  • 2:25 Sucrose leaves the leaf


Video 20: Who Gets the Carbon? Regulation and Recap

In a keledek (sweet potato, Ipomoea batatas) field, the leaves fix carbon dioxide, but that carbon does not reach the developing storage roots automatically. It may be used immediately, exported as sucrose, or stored temporarily as transitory starch, and the division, called carbon partitioning, changes with light, metabolism and the demands of the rest of the plant. The triose phosphate/phosphate translocator (TPT) in the inner chloroplast envelope exports triose phosphate while inorganic phosphate (Pi) enters in exchange; that phosphate is needed for continued reactions inside the chloroplast, so when carbon use, export and phosphate availability are out of balance, photosynthetic carbon assimilation can be affected. In the cytosol, fructose 2,6-bisphosphate is a regulatory signal, not a building block of sucrose: higher levels inhibit cytosolic fructose 1,6-bisphosphatase (FBPase), limiting the route towards hexose phosphates for sucrose. Under strong photosynthesis, a high triose phosphate to phosphate ratio often favours lower fructose 2,6-bisphosphate and more flow towards sucrose. Sucrose 6F-phosphate synthase (SPS) is promoted by glucose 6-phosphate, reduced by inorganic phosphate and controlled by reversible protein phosphorylation, while ADP-glucose pyrophosphorylase (AGPase) regulates starch synthesis in the chloroplast. There is no single fixed percentage of carbon that every crop sends to starch versus sucrose. The video ends with a recap of the whole series, from adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH) to Rubisco, reduction, regeneration of ribulose 1,5-bisphosphate (RuBP), light regulation, and starch or sucrose.

Chapters

  • 0:00 The central question is allocation

  • 0:28 A phosphate exchange connects compartments

  • 1:04 Fructose 2,6-bisphosphate regulates flow

  • 1:42 Sucrose synthesis has another control

  • 2:20 The whole journey and what comes next


Frequently asked questions

No step of the Calvin–Benson cycle absorbs light directly, but the cycle runs on ATP and NADPH from illuminated chloroplasts, and light switches on several of its enzymes. In darkness the cycle slows and stops. See Videos 1 and 12.

3-phosphoglycerate (3-PGA). Rubisco joins one carbon dioxide (CO₂) to ribulose 1,5-bisphosphate (RuBP, five carbons) through a short-lived six-carbon intermediate that splits into two molecules of 3-PGA: 5 + 1 = 3 + 3. See Video 4.

For every 3 CO₂ fixed, reduction uses 6 ATP and 6 NADPH and regeneration uses 3 more ATP: 6 + 3 = 9 ATP and 6 NADPH. Per CO₂, that is 9 ÷ 3 = 3 ATP and 6 ÷ 3 = 2 NADPH. These are idealised biochemical requirements, before photorespiration. See Video 9.

No. Its immediate products are triose phosphates, three-carbon sugar phosphates. Glucose units appear later, as the plant builds starch in the chloroplast and sucrose in the cytosol. See Videos 5 and 15.

Through several switches that work at different speeds: Rubisco activase and carbamylation make Rubisco active, the ferredoxin–thioredoxin system reduces regulatory disulphide bonds on key enzymes, the stroma becomes more alkaline (about pH 7 to 8) and richer in magnesium, and CP12 complexes break apart to release active enzymes. See Videos 10 to 14.

No. Starch granules stay inside plastids. At night, leaf starch is broken down to maltose and glucose, which leave the chloroplast and feed sucrose synthesis in the cytosol. Sucrose is the sugar loaded into the phloem. See Videos 15 and 18.


Where this series fits

Previous series: Photosynthesis I: Introduction and the Light Reactions, on how the chloroplast captures light and makes ATP and NADPH.

Next series: Photosynthesis III picks up the other reaction of Rubisco, with oxygen (O₂) instead of carbon dioxide, which leads to photorespiration.


Sources

  • Taiz L, Møller IM, Murphy A, Zeiger E (2023) Plant Physiology and Development, 7th edition. Oxford University Press. Chapter 10. (Main reference for the whole series.)

Diagrams are schematic and not to scale. Image credits for photographs and published figures are listed in each video’s YouTube description.

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