Biology 3.2 - Photosynthesis

The structure of a chloroplast consists of:

• Two membranes.
• A fluid filled stroma that contains some of the products of photosynthesis, including lipid droplets and starch granules. 
• Circular DNA and 70s ribosomes that allow the chloroplast to synthesise some of its own proteins and self replicate. 
• Within the stroma there are thylakoids. A stack of thylakoids is called a granum.
• Within thylakoids photosynthetic pigments such as chlorophyll can be found. The arrangement of thylakoids therefore provides a large surface area increasing the efficiency of trapping light energy.

Chloroplasts need light and so are found in the stem and leaves of a plant. 

• LARGE SURFACE AREA - maximise absorption of light.
• THIN - to allow light to penetrate the leaf and reach the chloroplasts. 
• STOMATAL PORES - to allow carbon dioxide into the plant for photosynthesis.
• AIR SPACES IN SPONGY MESOPHYLL - this allows carbon dioxide to diffuse towards the photosynthesising cells.
• SPACES BETWEEN PALISADE CELLS - this allows carbon dioxide to diffuse into the palisade (photosynthesising) cells. 

• CUTICLE/EPIDERMIS ARE TRANSPARENT, CELLULOSE CELL WALLS ARE THIN - light can easily penetrate through to the mesophyll.
• PALISADE CELLS HAVE A LARGE VACUOLE - chloroplasts form a single layer at the periphery of each cell so they do not shade each other. 
• PALISADE CELLS ARE CYLINDRICAL, ELONGATED AND AT RIGHT ANGLES TO THE SURFACE OF THE LEAF -  leaves can accomodate a large number of palisade cells; light only passes through two epidermal cell walls and one palisade wall before reaching chloroplasts. If the cells were stacked horizontally, light would be absorbed by passing through many cell walls, preventing it reaching chloroplasts. 

• LARGE SURFACE AREA - to maximise the absorption of light.
• CHLOROPLASTS MOVE WITHIN PALISADE CELLS - they move towards the top of palisade cells on dull days to maximise the absorption of light, and visa versa. 
• CHLOROPLASTS ROTATE WITHIN PALISADE CELLS - to maximise the absorption of light.
• PIGMENTS IN THE THYLAKOIDS ARE IN A SINGLE LAYER AT THE SURFACE OF THE THYLAKOID MEMBRANE - to capture as much light as possible. 

Transducer - a chemical that changes energy from one form to another. 

Chloroplasts act as transducers as they convert light energy into chemical energy.

Chloroplasts contain two main classes of pigments:

• Chlorophylls - Chlorophyll a and chlorophyll B.
• Carotenoids - B-carotene and xanthophyll. 

The function of these pigments is to absorb light energy and begin its conversion into chemical energy. Different pigments trap different wavelengths, allowing a large number of wavelengths to be absorbed. 

This is a graph showing how much light is absorbed at different wavelengths. 

An action spectrum is a graph showing the rate of photosynthesis at different wavelengths. 

If the action spectrum is superimposed onto the absorption spectrum, it shows that the pigments responsible for absorbing light are used in photosynthesis. 

Where are photosystems found? Thylakoid membrane. 

They are comprised of:

• Antenna complex - a cluster of pigments held together by protein molecules found in the thylakoid membrane. These pigments transfer energy from light to chlorophyll a at the REACTION CENTRE. 
• Reaction centre - two molecules of chlorophyll a. When these two molecules absorb light an electron is excited and is therefore emitted. 

There are two types of reaction centre:

• PS1 = P700
• PS2 = P680 

SUMMARY: 

• Light = energy source.
• ATP, reduced NADP and oxygen are produced. 
• Occurs on the thylakoid membrane. 

• PS1 absorbs photons, which excite electrons in the chlorophyll molecules in the reaction centre.
• An electron is emitted and picked up by an electron acceptor. It is then passed down a series of electron carriers via redoz reactions. This releases enough energy to phosphorylate ADP into ATP.
• The electron is returned to PS1. 

• Sometimes electrons do not return to PS1 and so its chlorophyll gains a positive charge. The electrons are instead transferred to oxidised NADP in the stroma, and alongside protons from the photolysis of water, it is reduced.
• PS2 emitts electrons to neutralise the positive charge of PS1. The electrons have been excited by the absorption of photons, emitted and are again passed down a series of carriers, producing energy used to phosphorylate ADP into ATP. 
• This leaves the chlorophyll of PS2 with a positive chrage. This is neutralised by electrons emitted from the photolysis of water. 

This is non-cyclic phosphorylation. 

In the thylakoid spaces, water molecules absorb light, causing them to dissociate, producing hydrogen, oxygen and electrons. 

• The hydrogen ions are used to reduce NADP, helping to maintain the electrochemical gradient.
• The electrons are used to neutralise the positive charge of PS2.
• Oxygen diffuses out of the chloroplast and cell, through the stomata as a waste product. 

• As the electrons pass through the electron transport chain the energy released can be used to pump protons from the stroma into the thylakoid space. 
• This transfer of protons, along with the release of hydrogen ions by the photolysis of water generates an electrochemical gradient since the thylakoid membrane is imbermeable.
• This is a source of potential energy. 
• Chemiosmosis occurs in which the protons diffuse out of the thylakoid space into the stroma through ATP syntetase. This makes energy available to synthesise ATP.
• Once the protons enter the stroma they are used to reduce NADP along with electrons from PS1. This helps to maintain the electrochemical gradient. 

What three factors maintain the proton gradient?

• The photolysis of water in the thylakoid space.
• The oxidation of NADP, removing protons from the stroma.
• Proton pump. 

The light independent reactions (CALVIN CYCLE) occur in the stroma.

ATP and reduced NADPH (NADPH + H+) are used from the light dependent stage: ATP acts as an energy source and NADPH is required to reduce carbon dioxide. 

• Ribulose bisphosphate combines with carbon dixoide to form an unstable 6C compound.
• The reaction is catalysed by rubisco.
• The 6C compound splits into two molecules of glycerate-3-phosphate. 
• This is reduced to triose phosphate. ATP and NADPH are required from the light-dependent stage as a source of energy and reducing power. 
• NADP is reformed.
• Some of the triose phosphate is converted into glucose phosphate, then starch by condensation.
• Most of the triose phosphate is used to reform ribulose bisphosphate via ribulose phosphate. ATP is required from the light-dependent stage. 

All of these processes begin with the triose phosphate from the Calvin Cycle.

• The first hexose made is fructose phosphate.
• This is converted to glucose and combined to make sucrose, for transport around the plant. 
• Some a-glucose is converted into starch for storage.
• Some B-glucose is made into cellulose for plant cell walls. 

• AcCoA can be synthesised from glycerate-3-phosphate, and converted into fatty acids. 
• Triose phosphate can be converted into glycerol.
• Fatty acids and glycerol can combine by hydrolysis to produce triglycerides. 

• Glycerate-3-phosphate can be converted into amino acids from rpotein synthesis.
• The amino group is derived from the ammonium ions, which in turn are made from nitrate ions taken in at the roots. 

Limiting factor - a factor that limits the rate of a physical process by being in short supply. If the limiting factor increases the rate of the process increases. 

There are four main limiting factors for photosynthesis:

• Carbon dioxide concentration
• Light intensity
• Water
• Temperature 

• As the carbon dioxide concentration increases, the rate of the light independent reactions increases. THe rate of photosynthesis increases: carbon dioxide concentration is a limiting factor.
• If the concentration increases above 0.5% it is no longer a limiting factor: the rate of photosynthesis remains constant.
• At 1% concentration the rate decreases as the stomata close to prevent carbon dioxide uptake. 
• Carbon dioxide is a limiting factor for most terrestrial plants as the carbon dioxide concentration of the air is 0.04%. 
• The photosynthesis of crop plants is most efficient at 0.1%. 
• Tomoatoes are most efficient at 0.5%. 

The rate of the slowest reaction in a sequence determines the rate of the process = this is the rate determining step. In this case, the light-independent reactions of photosynthesis, particularly the reaction catalysed by rubisco, is the rate-determining step. 

• In darkness, the light-dependent reactions can not occur and so the rate of photosynthesis is very low. No oxygen is evolved.
• As the light intensity increases the light-dependent reactions can occur with increasing efficiency. Therefore, the rate of photosynthesis increases. This shows light intensity is a limiting factor.
• At around 10,000 lux the rate of the light-dependent reactions remains constant: light intensity is no longer a limiting factor. 
• If the light intensity increases even further the rate of photosynthesis will decrease as the photosynthetic pigments will be bleached, so they cannot absorb light as efficiently.

Some species of plant have adapted so that their photosynthesis is more efficient at different light intensities.

Sun plant: Salvia

Shade plant: Lily of the Valley 

• As the light intensity decreases, the rate of the light-dependent reactions also decreases. Consequently, less ATP and NADPH are produced, causing the light-independent reactions to also decrease. Less carbon dioxide is required, so its rate of uptake decreases.
• At a particular light intensity, so little carbon dioxide is required for the light-independent reactions that respiration rpovides all that is required. 
• There is no gass exchange: all the oxygen needed for respiration is provided by photosynthesis.
• This is called the light compensation point. 
• The light compensation point occurs at a lower intensity for shade plants. 

Water is a limiting factor of photosynthesis, but since so many systems are affected, it is difficult to measure its affect on photosynthesis alone.

If water is scarce... cells may plasmolyse, guard cells may become flaccid and close stomatal pores and the plant may wilt. Less carbon dioxide can be taken in and so the rate of photosynthesis may decrease. 

Nitrogen - involved in the synthesis of protein, nucleic acids and chlorophylls. 

Symptoms of nitrogen deficiency include:

• Reduced growth of plant.
• Chlorosis - the yellowing of the leaves due to inadequate chlorophyll production. 

Magnesium - forms part of chlorophyll molecules. A symptom of magnesium deficiency is again chlorosis.