The Importance of Photosynthesis
Photosynthesis = the process whereby light energy from the sun is transformed into chemical energy and used to synthesise large organic molecules from inorganic substances.
Photosynthesis transforms light energy into chemical potential energy that is then available to consumers and decomposers. It also releases oxygen from water into the atmosphere, so aerobes depend on it for respiration.
Autotrophs = organisms that can make their own food by using light energy or chemical energy and inorganic molecules such as carbon dioxide and water to synthesise complex organic molecules such as carbohydrates, lipids, proteins, neucleic acids and vitamins.
Heterotrophs = organisms that cannot make their own food, but that digest complex organic molecules into simpler soluble ones from which they synthesise complex organic molecules such as lipids, proteins and neucleic acids, or respire to gain energy.
Word equation for photosynthesis:
Carbon dioxide + water (+ light energy) --> glucose + oxygen
Structure and Function of Chloroplasts
- Surrounded by a double membrane - the outer membrane is permeable to many small ions, whilst the inner membrane is less permeable and has transport proteins embedded in it.
- The inner membrane is folded into lamellae; these are thin membranes that attach one granum to the other.
- Each granum is a stack of thylakoids, which are flattened membrane compartments.
- The stroma is a fluid-filled matrix that contains enzymes and fills the spaces in the chloroplast. In the stroma are ribosomes, starch grains, lipid droplets and DNA.
- Inner membrane - controls entry and exit of substances
- Starch grains - where glucose is stored as starch
- Ribosomes - assembles some of the proteins required for photosynthesis
- Stroma - contains necessary enzymes for the light-independent stage to take place
- Grana - provide a large surface area for the photosynthetic pigments, electron carriers and ATP synthase enzymes for the light-dependent stage
- Photosynthetic pigments - arranged into photosystems for maximum absorption of light energy
Photosynthetic pigments are substances that absorb certain wavelengths of light and reflect others. They are in thylakoid membranes, arranged into photosystems, held in place by proteins.
- Chlorophyll is a mixture of photosynthetic pigments
- There are 2 forms of chlorophyll a - P680 and P700 (yellow/green)
- P680 - found in photosystem II; peak absorption = 680nm
- P700 - found in photosystem I; peak absorption = 700nm
- Both are found at the centre of photosystems and are known as the primary pigment reaction centre
- Chlorophyll a absorbs blue light of wavelength 450nm
- Chlorophyll b absorbs blue/green light of wavelengths around 500nm and 640nm
- Carotenoids such as carotene (orange) and xanthophyll (yellow) reflect their colours of light and absorb blue light.
- They absorb light wavelengths that aren't well absorbed by chlorophylls and pass the energy associated with that light to the chlorophyll a at the base of the photosystem.
Light-Dependent Stage 1
The light-dependent stage takes place on the thylakoid membranes, in which photosystems I and II are embedded, along with the photosynthetic pigments. The pigments trap light energy so it can be converted to chemical energy in the form of ATP.
The role of water:
Photosystem II contains an enzyme that can split water molecules into H+ ions (protons), electrons and oxygen. This is called photolysis. Some of the oxygen is used by the plant in aerobic respiration, but a lot of it diffuses out of the leaves through the stomata.
Water is a source of:
- H+ ions - used in chemiosmosis to produce ATP. These proteins are then accepted by a coenzyme NADP which becomes reduced NADP.
- Electrons to replace those lost by the oxidised chlorophyll.
Water also keeps plant cells turgid, enabling them to function.
Light-Dependent Stage 2
Photophosphorylation = the making of ATP from ADP and Pi in the presence of light. There are 2 types - cyclic and non-cyclic.
Photons = particles of light
Chemiosmosis = the flow of H+ ions (protons) through ATP synthase enzymes. The force of this flow allows the production of ATP.
1. When a photon hits a chlorophyll molecule, the energy is transferred to 2 electrons and they become excited.
2. The electrons are captured by electron acceptors and passed along a series of electron carriers, during which energy is released.
3. Protons are pumped across the thylakoid membrane in to the thylakoid space, so a proton gradient is formed which the protons flow down, through channels associated with ATP synthase enzymes (chemiosmosis).
4. This produces a force that joins ADP and Pi to make ATP, and the kinetic energy from the flow is converted to chemical energy.
Light-Dependent Stage 3
- Only photosystem I (P700)
- The excitied electrons pass to an electron acceptor and back to the chlorophyll molecule from which they were lost.
- No photolysis of water, or generation of reduced NADP, but small amounts of ATP are made
- Used in guard cells to bring in K+ ions, lowering the water potential and causing osmosis - the guard cells swell which opens the stomata.
- Involves both photosystems I and II (P700 and P680)
- An excited pair of electrons from PSII leave the chlorophyll molecule via the primary pigment reaction centre and are passed along a chain of electron carriers - the energy released in used to synthesise ATP.
- A pair of electrons has been lost from PSI; they, along with protons, join NADP to become reduced NADP.
- Electrons from oxidised PSII replace the electrons lost from PSI, and electrons from photolysed water replace those lost by the oxidised chlorophyll in PSII.
Light-Independent Stage 1
The light-independent stage takes place in the stroma. Light isn't directly used, but it is vital for this stage to take place. Carbon dioxide is the source of carbon for the production of all large organic molecules, which act as energy stores for all the carbon-based life forms.
The Calvin cycle:
1. CO2 diffuses into leaf through stomata, spongy mesophyll, palisade mesophyll, cellulose walls, cell surface membrane, cytoplasm and chloroplast envelope into the stroma.
2. CO2 combines with RuBP (5C), which is catalysed by the enzyme rubisco. RuBP becomes carboxylated - combined with CO2 so it now has a carboxyl group.
3. The product of the reaction is 2 molecules of GP (3C); the CO2 has now been fixed.
4. GP is reduced (gain in electrons) and phosphorylated to TP (3C).
5. 5/6 molecules of TP are recycled by phosphorylation using ATP to 3 molecules of RuBP.
Light-Independent Stage 2
How the products of the Calvin cycle are used:
- GP used to make amino acids and fatty acids
- Pairs of TP combine to form hexose sugars, e.g. glucose
- Some glucose isomerised to form fructose (another hexose sugar)
- Glucose and fructose combine to form sucrose (a disaccharide)
- Hexose sugars polymerised into carbohydrates (polysaccharides), e.g. cellulose and starch
- TP converted to glycerol which combines with fatty acids from GP to make lipids
Limiting Factors - Light Intensity
An increase in light intensity will alter the rate of the light-dependent reaction:
- More light energy available to excite more electrons
- Electrons take part in photophosphorylation, so increased light intensity = more ATP and more reduced NADP
- These are both used in the Calvin cycle as sources of energy to reduce GP to TP
- ATP is also used to phosphorylate 5/6 molecules of TP to regenerate RuBP
If there is no or very little light, then the light-dependent stage will stop, and consequently, so will the light-independent stage as it needs products from the LDS:
- GP can't be changed to TP - GP will accumilate and TP will fall
- Lower amount of RuBP, reducing fixation of CO2 and the formation of more GP
Light has 3 main effects:
- Causes stomata to open so that CO2 can enter the leaves
- Is trapped by chlorophyll where it excites electrons
- Splits water molecules (photolysis) to produce protons
The electrons and protons are used in photophosphorylation, producing ATP for the fixation of carbon dioxide.
Limiting Factors - CO2 Concentration
An increase in carbon dioxide concentration may lead to an increase in CO2 fixation:
- This will lead to more molecules of GP, and hence more molecules of TP, and more regeneration of RuBP.
- The number of stomata that open to allow gaseous exchange leads to increased transpiration, which may cause the plant to wilt. This leads to a stress response which will make the stomata close.
- This will reduce carbon dioxide uptake and reduce the rate of photosynthesis.
If CO2 concentration is reduced below 0.01%, then RuBP will accumulate. As a result, levels of GP and TP will fall.
Limiting Factors - Temperature
- Increasing temperature will at first increase the rate of photosynthesis.
- However, as temp rises above 25 degrees, the oxygenase activity of rubisco increases more than its carboxylase activity.
- This means that photorespiration exceeds photosynthesis.
- As a result, ATP and reduced NADP are wasted - this reduces the overall rate of photosynthesis.
- Very high temperatures may also damage proteins involved in photosynthesis.
- Increased temps cause an increase in water loss from leaves by transpiration - this may lead to closure of stomata and reduction in the rate of photosynthesis.
Between 0 and 25 degrees, the rate of photosynthesis approximately doubles for each 10 degree rise. Above 25 degrees, the rate of photosynthesis levels off and then falls as the enzymes work less efficiently, and as oxygen competes more successfully for the active site of rubisco that prevents it from accepting CO2.