Plants are autotrophs – they use light energy to synthesise complex organic molecules from inorganic molecules.
Photosynthesis takes place in organelles called chloroplasts.
Structure of chloroplasts:
- Disc shaped and 2-10 μm long
- Surrounded by an envelope (double membrane)
- Small intermembrane space between inner and outer membranes
- Outer membrane permeable to small ions
- Inner membrane less permeable and embedded with transport proteins. It’s folded into lamellae, which are stacked into grana. Between the grana there are intergranal lamellae.
- The grana contain thylakoids where the light-dependent stage occurs.
- The stroma is the fluid matrix where the light independent stage occurs.
Adaptations of chloroplasts:
- The transport proteins in the inner membrane control of substances between the cytoplasm and stroma.
- The grana create a large surface area
- The stroma contains many enzymes to catalyse the reactions in the light independent stage.
- The grana are surrounded by the stroma so products of light dependent stage can easily pass into the stroma.
- Chloroplasts have their own DNA and ribosomes so can synthesise their own proteins.
- The photosynthetic pigments are arranged into photosystems, which are embedded in the grana by proteins.
- The chlorophylls contain a porphyrin group. When this magnesium ion is hit by light a pair of electrons are excites.
- Chlorophyll a is found at the primary pigment reaction centre.
- There are 2 types of chlorophyll a – P680 in photosystem II and P700 in photosystem I (where the number indicates the wavelength of light in nm the molecule absorbs best).
- The accessory pigments are chlorophyll b and the carotenoids (including carotene and xanthophyll). They absorb different wavelengths of light to chlorophyll a and pass this energy associated with that light to chlorophyll a.
Light-dependent stage (Photophosphorylation):
- Cyclic photophosphorylation:
- Only involves PSI (P700)
- A photon hits chlorophyll a causing it to lose an excited pair of electrons.
- The excited pair of electrons are passed to an electron acceptor then back to the chlorophyll molecule they were lost from.
- Some ATP is produced.
- Non-cyclic photophosphorylation:
- Light strikes PSII, exciting a pair of electrons.
- These electrons pass along a chain of electron carriers.
- The energy produced pumps protons (from the photolysis of water in PSII: 2H2O → 4H+ + 4e– + O2) out of the thylakoids membrane into the thylakoids space. As the protons accumulate a proton gradient is created, and so protons flow back into the thylakoids membrane down this gradient via channels associated with ATP synthase. (This is chemiosmosis)
- Light also strikes PSI which too loses a pair of electrons.
- These electrons join with the protons from photolysis to reduce NADP to reduced NADP.
- The electrons from photolysed water replace those from PSII and those from PSII replace those from PSI.
- The overall products are ATP, reduced NADP and oxygen
Light-dependent stage (The Calvin Cycle):
- CO2 diffuses into the stroma
- It combines with ribulose bisphosphate (RuBP) (a carbon dioxide acceptor). This is catalysed by the enzyme rubisco. The RuBP is carboxylated.
- The CO2 now fixed in the products of this reaction: 2 molecules of glycerate-3-phosphate (GP).
- GP is reduced and phosphorylated to triose phosphate (TP) using ATP and reduced NADP.
- 5/6 TP molecules recycled by phosphorylation (using ATP) to 3 RuBP in the pentose shunt reactions.
- TP can be used to make hexose sugars or glycerol, GP can be used to make amino acids and fatty acids.
The factors affecting the rate of photosynthesis are called limiting factors. They are light intensity, CO2 concentration and temperature.
- Light intensity: this affects the light-dependent stage. If an increase occurs more ATP and NADP can be produced, so more GP can be turned into TP and more TP can be recycled to RuBP. If a decrease occurs the light dependent stage stops, so there is no more ATP and NADP, so GP accumulates, TP levels fall which means no RuBP is formed so RuBP levels also fall, reducing the fixation of CO2 and GP formation.
- CO2 concentration: this affects the light-independent stage. If light intensity isn’t limiting, an increase will mean more CO2 fixation in the Calvin cycle. But as the stomata open transpiration may occur more rapidly causing the stomata to close, and as CO2 levels fall, RuBP accumulates, so levels of GP and TP fall.
- Temperature: this affects the light-independent stage as this stage is a series of enzyme controlled reactions. An increase will initially increase the rate of photosynthesis but above 25°C rubisco oxygenates more than it carboxylates, so photorespiration will exceed photosynthesis, dissipating and wasting reduced NADP and ATP. High temperatures may also damage proteins, or cause stomata to close due to transpiration, both of which will reduce the rate of photosynthesis.