Photosynthesis
The chloroplast in photosynthesis
The chloroplasts are the organelles where the two stages of photosynthesis take place.
The granum (plural grana) is a stack of membranes which contains the photosynthetic pigments; this is where the light dependent stage takes place:
· The pigments are chemicals which absorb light energy and convert it to chemical energy.
· The principal pigment is chlorophyll a.
· Chlorophyll a absorbs light mainly in the red and blue regions.
· Accessory pigments such as chlorophyll b, xanthophyll and carotene absorb light from other regions of the spectrum and pass the energy onto chlorophyll a.
· The accessory pigments broaden the absorption spectrum so that more energy is available for photosynthesis.
The stroma is the fluid part of the chloroplast which contains the enzymes controlling the carbon fixation reactions.
The light dependent stage
This stage takes place in the granum.
Light energy is absorbed by chlorophyll and is used to regenerate adenosine tri-phosphate or ATP and split water. This is called the photolysis of water. Photolysis of water results in the release of oxygen, as a by-product, and the release of hydrogen. The hydrogen released from the water molecule is transferred to the hydrogen acceptor. NADP becomes reduced to form NADPH2.
The products of the light stage are ATP and NADPH2.
The hydrogen and ATP are used in the carbon fixation reactions:
· ATP provides the energy for the reactions.
· NADPH2 provides the hydrogen for the reduction of carbon dioxide to form carbohydrate
Carbon fixation (The Calvin Cycle) - the Light Independent Reaction
The carbon fixation stage occurs in the stroma and results in the production of glucose.
It is a result of an enzyme controlled sequence of reactions requiring ATP and hydrogen (NADPH2) from the light stage, and carbon dioxide. It involves the reduction of carbon dioxide, that is the addition of hydrogen, to form carbohydrate.
· CO2 is accepted by the 5C compound ribulose 1,5-biphosphate (RuBP) to form an unstable 6C compound.
· The 6C compound formed immediately splits into two molecules of a 3C compound called glycerate 3-phosphate (GP).
· Energy is used to convert GP into triose phosphate, a 3 carbon compound. It is at this point that the ATP and hydrogen produced in the light dependent stage are used.
· Triose phosphate doubles up to form glucose. Glucose may then be used in either respiration to provide energy, stored as starch, or used to synthesise cellulose for cell walls.
· The cycle is completed when the leftover GP molecules are met with a carbon acceptor and converted into RuBP, which is then joined with carbon dioxide to re-start the cycle.
All the major biological molecules in plants are derived from the photosynthetic process:
· Proteins
· Fats
· Carbohydrates
· Nucleic acids
The limiting factors of photosynthesis
There are three main limiting factors in photosynthesis:
· Lack of carbon dioxide. If there is no carbon dioxide available RuBP cannot be converted into GP. As a result the RuBP starts to build up and no more glucose will be produced.
· Low temperatures. These limit photosynthesis since the enzymes controlling the reactions are below their optimum temperature.
· Lack of light. In the absence of light neither the ATP or the NADPH2 will be produced and so the GP cannot be converted into glucose. This results in the GP building up and the RuBP being used up.
What is Photosynthesis - links to other sites
How does Chlorophyll Absorb Light Energy?
Biology of Plant - Photosynthesis (Interactive Exercises, Animations)- Registration is free
Photosynthesis - Information and Diagrams