Lecture 12

Photosynthesis II

LIF1017-9-2011Friday Dr. Jonaki Sen

ATP formation in the chloroplast

Photolysis of water releases oxygen, hydrogen ions and electrons which are picked up by an acceptor which transfers them to a electron transport system. The hydrogen ions accumulate in the thylakoid lumen and two oxygen atoms combine to form molecular oxygen which diffuses out. The electron transport chain transports the electrons and at the same time shunts hydrogen ions into the thylakoid lumen.

How is the ATP synthesized?

The concentration of hydrogen ions slowly builds up in the thylakoid compartment and this sets up a electrochemical gradient of hydrogen ions across the thylakoid membrane. The hydrogen ions move through a ATP synthase molecule which is a transporter of hydrogen ions. The movement of hydrogen ions down their electrochemical gradient drives the synthesis of ATP.

The chemiosmotic model of ATP synthesis

The light-independent reactionsCalvin-Benson cycleThe “synthesis” part of photosynthesis are called light independent reactions which just require energy from ATP, hydrogen ions and electrons from NADPH and carbons and oxygen from carbon dioxide. These reactions proceed in a cyclic manner.

Carbon dioxide diffuses into a leaf cell and an enzyme attaches the carbon from it to RuBP (ribulose bisphosphate) that has a five carbon backbone. This forms an unstable intermediate which splits into 2 molecules of phosphoglycerate (PGA) which is a three carbon containing stable compound. The carbon from carbon dioxide is transferred to a stable compound this is called “carbon fixation”

How do cells build glucose?

Each PGA molecule accepts a phosphate from ATP and a hydrogen and electrons from NADPH giving rise to an intermediate call PGAL (phosphoglyceraldehyde).

To build one molecule of a six-carbon sugar phosphate. Six molecules of carbon from CO2 must be fixed and 12 molecule of PGAL produced. Most of the PGAL is rearrranged to become RuBP and can be reused to fix more carbon. But two molecules of PGAL combine to form one molecule of phosphorylated glucose.

Photosynthetic cells will convert the phosphorylated glucose to sucrose or starch during daylight hours.

Of all plant carbohydates sucrose is the most easily transported and starch is the most common storage form. Cells can also directly convert excess PGAL to starch. They briefly store starch in the stroma. After the sun sets they convert the starch in the stroma to sucrose and transport it as food to other cells in leaves, stem and root.

Ultimately the products and intermediates of photosynthesis end up as energy sources and building blocks of for all lipids, amino acids and other compounds.

C3 plants

Plants in which the first intermediate of photosynthesis is a three-carbon molecule e.g. PGA. In C3 plants photorespiration predominates during hot dry days and reduces the efficiency of photosynthesis.

The leaf cells exchange oxygen and carbon dioxide from the atmosphere through stomata (small openings in the leaf.)

The water loss also occurs through the stomata therefore on hot dry days the stomata are closed and oxygen builds up while there in a fall in carbon dioxide as it cannot enter.

At high oxygen levels a process called photorespiration occurs when. Rubisco is the enzyme that attaches the carbon of CO2 to RUBP. Rubisco can also attach oxygen to RUBP which results in the formation of one PGA and one glycolate.

CO2 or O2

Rubisco affixes O2 to RuBP

CO2 + H2O

One glycolate +

Only one PGA (not two)

Decreases CO2 uptake and fewer sugars can form

C3 Plants: with low CO2 / high O2

Photorespiration predominates

CALVIN-BENSON CYCLE

C4 plantsThere are other plants where the first intermediate of carbon fixing is a four carbon molecule oxaloacetate. These are called C4 plants.

They fix carbon twice in two kinds of cells. One produces oxaloactetate which is then transferred to the other cells and converted back into CO2 and enters the calvin-benson cycle. These plants are thus more efficient under hot dry conditions at making glucose.

Lower leaf surface

Bundle-sheath cell

CO2 moves through stoma into air-spaces in leaf

Upper leaf surface vein Mesophyll cell

Cross section of a leaf from a C4 plant

CO2

Oxaloacetate Carbon fixation in mesophyl cells

That Carbon is fixed again in bundl-sheath cells

CO2 level in leaf is enhanced no

photorespiration

C4 Plants: with low CO2 / high O2

Calvin-Benson cycle predominates

CALVIN-BENSON CYCLE

CAM plantsThe plants are dessert plants that do not fix carbon twice like C4 plants but they do it at different times. At night there is less chance of water loss so they open their stomata and take in CO2 fix it and store intermediates. This fixed CO2 is used later during the day for photosynthesis keeping the stomata closed. This way they grow slowly but they conserve water which is essential in the dessert.

CAM Plants: with low CO2 / high O2

Calvin-Benson cycle predominates

CO2 Stomata opens at night :CO2 uptake but no water loss

Stomata closed during day:CO2 in leaf used

CALVIN-BENSON CYCLE

Autotrophs, humans and the biosphere

Large increase in photosynthetic activity in the ocean during springtime.

Photosynthetic microrganisms mop up a large amount of CO2 that we humans are sending into the atmosphere.

We are sending a lot of CO2 into the atmosphere which is causing global warming and will cause rise of seawater and flooding very soon.

Important points to remember:

1) The events of the light-dependent and light-independent phases of photosynthesis.

2) The differences between photosystem I and II. Which one is more ancient ? What is the significance of coupling photosystems I and II?

3) Differences between cyclic and non-cyclic electron transfer?

4) How did the earth’s atmosphere become rich is oxygen

4) Calvin-Benson cycle and the steps via which carbon from carbon-dioxide is fixed to make glucose.

5) What is photorespiration?

6)The differences between C3, C4 and CAM plants and how each is adapted to its environment.