The Reactions: Part II

Summary:

1. Light Dependent Reactions

Stage 1: Capturing light energy

Stage 2: Synthesizing ATP and NADPH

2. Light Independent Reactions (Dark)

Stage 3: The Calvin Cycle

2. Light Independent Reactions

• All reactions of photosynthesis not directly dependent upon light are known as the light independent reactions.

• They occur whether there is light present or not.

• They only depend on the presence of the energy carriers ATP and NADPH made during the light dependent reactions.

2. Light Independent Reactions

Stage 3: The Calvin Cycle

Where: stroma (liquid in chlorplast)

Requirements: CO2 , ATP and NADPH

• also called the Calvin Cycle • This is where the sugars are manufactured • The ATP and the H+ are used to bind CO2 molecules to a 5

carbon molecule (biphosphate) making an unstable 6 carbon molecule which breaks down to 2 3-Carbon molecules.

• These 3C molecules will be joined to for a simple sugar (C6H12O6)

• The most important sugar for consumer organisms is the sugar form of glucose

• Some of the glucose produced will join to form sucrose (much used table sugar).

• The energy of the sun has now been packaged into a useable chemical form.

Purpose of Calvin cycle

• It is a series of reactions involving many steps

• To take the energy from ATP, NADPH and energized electrons and add it to CO2 to make glucose or sugar.

Glucose molecule

Cycle

• A path which ends where it began

• In the Calvin cycle, some of the starting materials are regenerated in the process

• Three CO2 molecules combine with three 5-carbon molecules (a.k.a. ribulose bisphosphate or RuBP) to form three intermediate 6-carbon molecules. This reaction is catalyzed by RUBISCO.

• The three intermediate molecules quickly split into six 3-carbon phosphoglycerate (PGA) molecules.

• Each 3-carbon molecule is reduced to bisphosphoglycerate (BPG) when it receives an additional phosphate from the oxidation of an ATP to ADP. The ATP was supplied by the light reaction.

• The six BPG’s receive a pair of electrons from NADH (from light reaction) and are reduced to a six 3-carbon sugar (a.k.a. G3P).

• Five of the sugars’s are recycled. One goes on to form a glucose (C6H12O6) molecule.

• Using three molecules of ATP, the five recycled 3-carbon sugars’s are converted to three 5-carbon RBP’s.

• The RBP’s can now accept another three CO2’s and the cycle repeats.

• Each glucose molecule needs 2 of the 3-carbon sugars to make it so two complete turns of the cycle are required to complete it.

• Organisms, in the kingdom Animalia, require energy to survive

• Herbivorous animals, ie. Deer, get this energy as glucose when they eat plant material

• Excess glucose in animals is converted to lipid molecules and stored in fat deposits.

• The glucose the animal has stored is converted to transportable ATP by cellular respiration.

1. Carbon fixation

Glucose

Factors that Affect the Rate of Photosynthesis

1) Physical factors such as temperature (the lower the temperature, the slower the photosynthesis)

2) The availability of light is a factor that limits the rate of photosynthesis (light decreases, the rate of photosynthesis decreases)

Other ways…..

• There are two other methods that some plants use to make glucose

C4

CAM

The C4 & CAM plants

The Calvin cycle and the “C4 Plants”

In certain climates, the sun is too abundant and the plants never run out of energy for photosynthesis. However, these climates are also very dry and hot. Here the CO2 that limits the amount of photosynthesis that plants can do.-

- Think of it as the availability and the loss of water (H2O)

When the plant photosynthesises in the sun, the CO2 must enter the leaves through the stomatas (little holes under the leaves) but when these holes are open, H2O is lost and the plant dehydrates

If you close the stomatas, CO2 cannot enter so this temporally stops or slows photosynthesis. In C4 plants, the stomatas are only partially opened during the day

These plants have an enzyme that can transform the CO2 into a 4-carbon molecule. These molecules are stored in the different cells and the CO2 can be released when the plant needs to.

Examples of C4 plants are corn, sugar canes

Crassulacean Acid Metabolism (CAM)

The ultimate prevention of CO2 loss is found in desert plants like the cactus in these plants, the stomatas are only opened at night

The plants fixes the CO2 into 4-carbon molecules during the night and transfer the carbon to the Calvin cycle during the day

In the day, the stomatas are completely closed and there is no water loss !!