Photosynthesis (2.9, 8.3) Flashcards
What is photosynthesis?
The ability of plants to capture light energy and turn it into chemical energy. It requires a photosynthetic pigment called chlorophyll. In plants, photosynthesis occurs in the chloroplast.
Carbon dioxide + water -> glucose + oxygen
6CO₂ + 6H₂O -> C₆H₁₂O₆ + 6O₂
Give the two steps of photosynthesis.
Light dependent reactions.
Light independent reactions (the Calvin cycle).
Define photophosphorylation
The process of using light energy from photosynthesis to convert ADP to ATP. It occurs in both cyclic and noncyclic light dependent reactions.
Describe the light dependent reactions of photosynthesis.
Light dependent reactions occur on the thylakoid membrane and may be cyclic or non-cyclic.
Describe non-cyclic photophosphorylation.
The photolysis of water generates O₂ and electrons. These electrons enter Photosystem II and are charged by light energy. These high energy electrons are released and pass along an electron transport chain producing ATP via chemiosmosis. The electrons lose energy as they pass through the electron transport chain and enter Photosystem I de-energised. The electrons are charged again by light energy and reduce NADP to NADPH + H+.
What does noncyclic photophosphorylation allow for?
Noncyclic photophosphorylation produces NADPH + H+ carriers and ATP so allows for the synthesis of organic molecules in light INDEPENDENT reactions.
Describe cyclic photophosphorylation.
Electrons from the chloroplast in photosystem I are excited by light energy. The high energy electrons are released and pass along an electron transport chain producing ATP via chemiosmosis. They then return to Photosystem I de-energised.
Cyclic photophosphorylation does not produce NADPH + H+ carriers which is needed for light independent reactions so it cannot be used to make organic molecules.
Compare cyclic and noncyclic photophosphorylation.
Noncyclic photophosphorylation uses both Photosystem I and II and produces both ATP and NADPH + H+ molecules used in the Calvin cycle of light independent reactions. Whereas cyclic photophosphorylation only uses Photosystem I and produces just ATP so does not have the ability to synthesise organic molecules.
Describe chemiosmosis in terms of phosphorylation.
As the electrons pass through the electron transport chain they lose energy. This energy is used to pump H+ ions from the stroma (the cell liquid of the chloroplasts) into the thylakoid. The buildup of H+ ions inside the thylakoid creates an electrochemical gradient (also called a proton motive force). The H+ ions return to the stroma with the facilitation of the ATP synthase enzyme which uses the potential energy from the proton motive force to convert ADP and Pi into ATP. This process is called chemiosmosis.
Describe the light independent reactions of photosynthesis.
The light independent reactions use the chemical energy derived from light dependent reactions to form organic molecules. It occurs in the stroma (the fluid inside the chloroplast).
It involves three steps:
1) Carbon fixation
2) Reduction
3) Regeneration of RuBP
Describe the steps of the light independent reaction.
*Note: a single cycle involves three sub-cycles.
1) Carbon fixation
CO₂ is attached to 5C RuBP (carboxylation). This reaction is catalysed by the enzyme Rubsico. An unstable 6C compound is formed which immediately breaks down into x2 3C GP molecules. (In one cycle 6 GP are produced).
2) Reduction
Each GP molecule is phosphorylated by ATP and then reduced by NADPH + H+ (from the light dependent reactions) forming a 3C TP molecule. (In one cycle 6 TP molecules are produced).
3) Regeneration of RuBP
Of the 6 TP molecules produced, 1 can be used to form half of a sugar molecule, i.e. two cycles are needed to produce a sugar molecule. The remaining TP molecules are recombined to regenerate stocks of RuBP using ATP. With RuBP regenerated the cycle can repeat many times and be used to construct a chain of sugars for the plant.
Give a brief summary of the Calvin Cycle
RuBP is carboxylated by CO₂ to form an unstable 6C compound which immediately breaks down into molecules of glycerate-3-phosphate (GP). The GP is converted by ATP and NADPH + H+ into molecules of triose phosphate (TP). TP can be used to form organic molecules or can be recombined by ATP to regenerate stocks of RuBP.
Define anabolic reactions.
The set of metabolic pathways that construct molecules from smaller units. These reactions require energy.
Define catabolic reactions.
The set of metabolic pathways that breaks down molecules into smaller units. These reactions release energy.
What can the rate of photosynthesis be measured by?
1) CO₂ uptake
Measure the change in pH of a plant in water. An increase in the uptake of CO₂ = increase in pH.
2) Measuring O₂ production
In an enclosed space, submerge a plant in water and connect it to a sealed gas syringe. O₂ will bubble out and change the water level in the syringe.
3) Measuring biomass (indirect)
- Dehydrate a plant to remove its water mass and measure its change in weight over time.
- Stain a plant with iodine and quantify the level of starch with a colorimeter. (Glucose is stored as starch. Iodine turns purple in the presence of starch).
