Rubisco is a protein widely present in nature.

It allows the carboxylation of CO2 at the beginning of the Calvin cycle.

The two types of subunits are encoded in different parts of the cell:

– The genes that lead to the synthesis of small subunits are in the nucleus of the cell.

– The genes that lead to the synthesis of large subunits are still found in the DNA remains

(of the cyanobacterium) present in the plastids.

Control over the amount of rubisco in the chloroplasts occurs through very high proteolysis


By doing this, the plant cell always has rubisco available.

It is the enzyme that allows an inorganic molecule in the gaseous state (CO2) to bind to a

intermediate, triggering organicisation, therefore allowing all the formation of sugars.

(I’ll talk about this later)

Rubisco activation


Downstream process of photosynthesis.

Synthesis of sucrose in the cytosol

All parts of photosynthetic tissues have a specific function in carrying out the

process. These metabolic pathways are inevitably connected to each other.

Trioses are the net product of the Calvin cycle and, in turn, are necessary to make

subsequent reactions occur.

Compartmentalization in the organelles of eukaryotic cells allows for synchronization and

monitor the progress of these processes, some of which may also be

potentially competing.

This connection between biochemical processes is also seen in the synthesis of sucrose in the


When we balanced the Calvin cycle, we came to the end of 6 triose phosphates. From

these, 1 constitutes the net product of the Calvin cycle and will continue in the synthesis of

sucrose, while the other five will be reused to return to ribulose.

To synthesize a molecule of sucrose, we need 4 triose phosphates


The triose phosphates exit from the stroma of the chloroplast towards the cytosol, thanks to the activity of a

transporter that works in antiport (brings a molecule out of the stroma). This

transporter works in a ratio of 1:1 (for each triose that leaves the cytoplasm it must

re-entry of an inorganic phosphate from the cytoplasm into the chloroplast). These transporters are

the same ones used by ATP synthase.

The balance change has consequences.

From the triose phosphates of the reactions lead to the formation of fructose 1,6 diphosphate (ad form

ring, cyclic molecule. In carbons 1 and 6 there are phosphate groups).

The reactions that follow are shifts of phosphate ions or modifications from sugar to

sugar: we move on to fructose 6 phosphate. Detach the phosphate group at position 1. Part of

it undergoes a series of changes: first it becomes glucose 6 phosphate, then glucose 1

phosphate. This glucose reacts with UDP, forming the UDP-glucose complex.

Subsequently, sucrose is obtained with the elimination of UDP and the union between glucose and

fructose through the enzyme sucrose phosphate phosphatase (it detaches the phosphate group from the

sucrose phosphate and sucrose is obtained. The phosphate then binds to UDP. The process becomes


The position of the phosphates causes one enzyme to act rather than another


After the two phases defined as photosynthesis have taken place, we arrive at the synthesis of 3 intermediates, i

triose phosphates, are two distinct molecules in equilibrium (we are in the stroma and these must

be moved into the chlorophyll parenchyma). The synthesis of sucrose is a strategy that

plants adopt to make sugar transportable.

Obviously they must be transported to all living cells of the plant.

We know that the splitting of glucose releases a lot of energy, and should be used

immediately, so it is not suitable for transport. A compromise is found: a

disaccharide, with fructose and glucose.

The synthesis route begins with the transport of trioses, which causes the trioses to exit into the cytoplasm, they come

transformed into fructose 1,6 diphosphate, then a phosphate is removed and becomes

fructose 6 phosphate, and the subsequent rations of glucose 6 phosphate and glucose 1 phosphate (in

balance between them, so they can be transformed both forwards and backwards). Glucose

1-phosphate reacts with a UDP to become UDP-glucose. Subsequently we obtain the

sucrose with the elimination of UDP and the union between glucose and fructose through the enzyme

sucrose phosphate phosphatase (detaches the phosphate group from sucrose phosphate and we obtain the

sucrose. The phosphate then binds to UDP. The process becomes cyclical.)

Control of sucrose synthesis

Being in a key position of all metabolic pathways, the synthesis of sucrose is

controlled by:

– Anticipation mechanisms: one of them is the availability of raw material (needed

carbon dioxide. The more there is available, the more sugars are produced. CO2

enters the plant through the stomata).

– Feedback mechanisms: all as a consequence of each other. They can be

finely modulated (a mechanism, in a more or less direct way, has an influence on

another, but it doesn’t work like a switch). (they are the three nodes)

Some are a consequence of others, others exclude still others.

The conveyor must always be in a 1:1 ratio, otherwise it slows down.

The consequence of possible imbalances in the conveyors can lead to a greater effect

amplified: if the imbalance is too prolonged, no more sucrose will be formed in the

cytosol but a polysaccharide, starch.

Summary: Everything is controlled by mechanisms.

– Anticipation: ability of the plant to regulate the entry and exit of CO2, plus

carbon dioxide enters the more photosynthesis can occur.

– Feedback: direct control of triose phosphate transporters, regulation of enzymes

cytosolics of sucrose synthesis, and a complete deviation of the synthesis pathway.

Control of the triose phosphate transporter

The ratio is 1:1. For each triose phosphate there is one phosphate.

To synthesize one molecule of sucrose you need 4, so the ratio will be


The four trioses combine to form 2 molecules of fructose 1,6 diphosphate.

These will be used to make the conveyor work: one of them will undergo a series of

reactions until it becomes sucrose.

Summary: for each triose that exits into the cytoplasm, an inorganic phosphate must return

in the chloroplast, balance is guaranteed by the fact that these phosphates arrive from the synthesis of

sucrose (I see where the Ps are taken in the photo = we see them when fructose 1,6-diphosphate

becomes fructose 6-phosphate, then one during the transformation of glucose 1-phosphate into

udp-glucose, and finally one is taken when the phosphate group is removed from sucrose


First feedback mode

Regulation of cytosolic enzymes of sucrose synthesis

These pathways are controlled by the relative concentrations of reactants and products, with interventions

of intermediates and enzymes that modify these positions of phosphate groups and therefore the

relative concentration of the different intermediates.

Cytosolic enzymes can be highly regulated, via strategies common to many steps

– These enzymes are controlled by the amount of reactants and products (more reactants

the more the reaction will occur)

Second feedback mode: (see below)

– Increased concentration of an intermediate similar to what we need, but not

the same, but it will still slow down production (e.g. fructose 6 phosphate ha

high concentration, fructose 6-phosphate 2 kinase activity is promoted

producing fructose 2,6-diphosphate

Second feedback mode

Regulation of sucrose phosphate kinase

The addition of phosphate groups and their removal is one strategy to control metabolism.

So the addition or removal of a phosphate group directly on the enzyme determines its

activity, this is ensured by the effect of another enzyme (sucrose phosphate synthase kinase).

It is not a total inhibition, but modulation of the activity of the enzyme, so it is slowed down or


3 feedback modes

Diversion of C from the calvin cycle towards starch production in the chloroplast

Longer-term modification.

Asynchrony of the triose transporter, therefore when the ratio is no longer 1:1

When the export of triose phosphates into the cytosol is low, the Calvin cycle intermediates

are diverted towards the synthesis of starch (storage polysaccharide).

The production is similar, see photo steps

The accumulation of starch has as a consequence (photo 2) the formation of real granules, which

coexist with the thylakoid system, but if it becomes continuous the thylakoid membranes

begin to degrade, this leads the chloroplast to become amyloplast, then a plastid with

accumulation function.

Negative control of starch synthesis operated by sucrose synthesis

If we have high sucrose synthesis, the release of phosphates increases, therefore they fall

quickly in the plastid causing triose to come out. The ratio between the triose and this decreases

inhibits the starch synthesis pathway.

In summary:

If there is already a lot of sucrose the cell tends to slow down the synthesis, but obviously it already has

ready intermediates makes them become accumulation.

However, if there is no sucrose, the cell uses intermediates to produce it.

Carboxylation and oxygenation catalyzed by Rubisco

Rubisco has a greater affinity for CO2. Oxygenation occurs at a rate of approximately 25%

with respect to carboxylation. This relationship has metabolic consequences, as seen.


It aims to reconstruct the second molecule of 3-phosphoglycerate which was not possible

form with oxygen.

The oxygenase action is probably an ancestral residue of bacteria that used C

inorganic for growth in anoxic environments. Then with the increase in O2 it was favored.

But all this leads to the loss of one molecule of carbon and therefore one less opportunity

synthesize sugars.

(Photo B)

We are in the stroma of the chloroplast the rubisco creates this 2-phosphoglycolate molecule, it passes

in the peroxisome via intermediates then passes into the mitochondrion where the loss of CO2 occurs,

it returns to the peroxisome and finally to the chloroplast where it will be transformed into a single molecule of


In the