Photosynthesis

Chloroplasts vary, but most are disc-shaped and approximately 2-10μm long. Each has a double membrane (called an envelope), consisting of the inner membrane and outer membrane.

Chloroplasts contain stacks of flattened membrane compartments.Each stack is called a granum (plural: grana) and each compartment is a thylakoid. Chloroplast has 3 distinct membranes so it also has 3 separate internal compartments: the intermembrane space, the stroma and the thylakoid space. Thylakoids in one granum may be connected to those in a different granum by a membrane called the intergranal lamellae.

With so many grana in each chloroplast and so many chloroplasts in one photosynthetic cell, there is a very large surface area which is useful in:
-The distribution of photosystems that contain the photosynthetic pigments that trap sunlight energy
-The electron carriers and ATP synthase enzymes needed to convert light energy into ATP.

The fluid surrounding the grana is called stroma. It contains enzymes needed to catalyse the reactions of the light-independent stage of photosynthesis as well as starch grains, oil droplets, small ribosomes and DNA. The DNA codes for proteins needed for photosynthesis and the ribosomes synthesise them. 

Starch grains can also be found in the stroma matrix, as well as DNA and ribosomes, which can be used to make proteins. The two stages of photosynthesis are the light-dependent reactions (the first or ‘light’ stage) and the light-independent reactions (the second or ‘dark’ stage). The first stage takes place in the grana, and the second stage in the stroma fluid of the chloroplast. 

Within the thylakoid membranes of each chloroplast are funnel-shaped structures called photosystems. These photosystems contain photosynthetic pigments. Each pigment absorbs light of a particular wavelength and reflects other wavelengths of light. Each pigment of the light it is reflecting.

The energy associated with the wavelengths of light captured is funnelled down to the primary pigment reaction centre, consisting of a type of chlorophyll at the base of the photosystem (p700 or p680). The primary pigment is chlorophyll a. The accessory pigments are chlorophyll b and carotenoids. There are 2 main types of photosystem, photosystem I and photosystem II.

Chlorophylls are a mixture of pigments. All have similar molecular structures consisting of a porphyrin group, in which there is a magnesium atom and a long hydrocarbon chain.

Chlorophyll a
There are 2 forms of chlorophyll a- both of them appear blue-green. They absorb red light but have different absorption peaks on the visible light spectrum:

• P680 is found in photosystem II and its peak of absorption is the light of wavelength 680nm
  
• P700 is found in photosystem and its peak of absorption is the light of wavelength 700nm

Chlorophyll a also absorbs some blue light of wavelength around 440nm

Chlorophyll b
Chlorophyll b absorbs light of wavelengths 400-500nm and around 640nm. It appears yellow/green. 

Accessory Pigments

• Carotenoids absorb blue light of wavelengths 400-500nm. They reflect yellow and orange light.
• Xanthophylls absorb blue and green light of wavelengths 375-550nm. They reflect yellow light.

1. Photon of light strikes PSII, its energy is channelled to the primary pigment reaction centre
2. The light energy excites a pair of electrons that leave the chlorophyll molecule from the primary pigment reaction centre and are captured by an electron carrier, which is a protein with an iron at its centre, embedded in the thylakoid membrane
3. These electrons are replaced by electrons derived from photolysis.
4. When the iron ion combines with the electron, it becomes reduced Fe2+. It can then donate the electron (and become reoxidised Fe3+) to the next electron carrier in the chain.
5. As the electrons pass along the electron carriers embedded in the thylakoid membrane, at each carrier, some energy that is associated with the electron is lost.
6. This energy is used to pump protons across the thylakoid membrane into the thylakoid space.
7. Eventually, the electrons are captured by another molecule of chlorophyll a in PSI. These electrons replace those lost from PSI due to excitation by light energy.
8. A protein-iron-sulfur complex (ferredoxin) accepts the electrons from PSI and passes them to NADP in the stroma
9. As protons accumulate in the thylakoid space, a proton gradient forms across the membrane.
10. Protons diffuse down their conc grad through special channels in the membrane associated with ATP synthase enzymes and as they do so, the flow of protons causes ADP and inorganic phosphate to join and create ATP.
11. As protons pass through the channel they are accepted, along with electrons, by NADP which becomes reduced. The reduction of NADP is catalysed by the enzyme NADP reductase. 

The light energy has been converted into chemical energy in the form of ATP by photophosphorylation. ATP and reduced NADP are now in the stroma ready for the light-independent stage of photosynthesis.

This uses only PSI (P700). As light strikes PSI, a pair of electrons in the chlorophyll molecule at the reaction centre gain energy and become excited. They escape from the chlorophyll and pass to an electron carrier system and then pass back to PSI.

During the passage of electrons along the electron carriers, a small amount of ATP is generated. However, no photolysis of water occurs, so no protons or oxygen are produced. No reduced NADP is generated.

Chloroplasts in guard cells contain only PSI. They produce only ATP which actively brings potassium ions into the cells, lowering the water potential so that water flows in by osmosis. This causes the guard cells to swell and open the stroma.

-Occurs in the grana (thylakoids) of chloroplasts and involves the photosystems. It involves the direct use of light energy. This stage consists of:
1. Light harvesting at the photosystems
2. Photolysis of water
3. Formation of reduced NADP
4. Photophosphorylation- the production of ATP in the presence of light
-Oxygen is also produced in the light-dependent stage

Two Types of photosystem

• In photosystem I (PSI), the pigment at the primary reaction centre is a type of chlorophyll a, which has a peak absorption of red light of wavelength 700nm (P700)
• In photosystem II (PSII), the pigment at the primary reaction centre is alsoa type of chlorophyll a, but this has a peak absorption of red light of wavelength 680nm (P680).

In PSII, there is an enzyme that, in the presence of light, splits water molecules into protons (hydrogen ions), oxygen and electrons. The splitting of water this way is called photolysis:

2H2O -> 4H+ +2O + 2e-

Some of the oxygen produced during photolysis is used by plants cells for aerobic respiration, but during periods of high light intensity the rate of photosynthesis is greater than the rate of respiration in the plant, so much of the oxygen by-product will diffuse out of the leaves, through stomata into the surrounding atmosphere.

Water:

• Is the source of protons that will be used in photophosphorylation
• Donates electrons to chlorophyll to replace those lost when light strikes chlorophyll
  
• Is the source of the by-product, oxygen.
• Keeps plant cells turgid, enabling them to function.

Photophosphorylation is the generation of ATP from ADP and inorganic phosphate, in the presence of light.

There are 2 types of photophosphorylation:

• Non-cyclic photophosphorylation involves PSI and PSII. It produces ATP, oxygen and reduced NADP
• Cyclic photophosphorylation involves only PSI. It produces ATP but in smaller quantities than are made by non-cyclic photophosphorylation

When there are a number of factors which contribute towards a process and control its efficiency, the factor which is working at the least favourable level will be the limiting factor of the process. In photosynthesis, the limiting factor is the quality or condition of the environment which is least favourable (lowest) and therefore acts as a constraint on the rate of photosynthesis.
For example:

• on a warm night, the light intensity is the limiting factor (the temperature is sufficient but there is no light). Light provides the energy required to power the first stage of photosynthesis and produce ATP and reduced NADP needed for the next stage. 
• on a cold, frosty morning, the temperature is the limiting factor (the temperature impairs the photosynthetic rate) 
• on a normal, sunny day the carbon dioxide concentration in the atmosphere may be the limiting factor The maxima tend to be around 30oC (temperature), 0.2% - 0.5% carbon dioxide concentration and bright sunlight during the day. However, the levels of carbon dioxide in the atmosphere and in aquatic habitats are usually high enough that it is usually not a limiting factor. 

The light-independent stage of photosynthesis is the second and final set of reactions. It is named so because the reactions involved do not need light to occur, and so technically can take place without light.

However, the products (ATP and reduced NADP) of the light-dependent reactions are required for the light independent stage to take place, and so in practice, these reactions will not continue for very long without light, as the light-dependent stage will stop producing ATP and NADPH.

The light-independent reactions occur in the stroma of the chloroplast (fluid matrix surrounding the grana). It is also called the Calvin cycle. Whilst the products of the light-dependent stage (ATP and NADPH) are needed for this stage, carbon dioxide is also required. This comes from the atmosphere. Carbon dioxide diffuses through the stomata of leaves, and enters leaf cells and then the chloroplasts.

Carbon dioxide is also a by-product of respiration and this may be used for the Calvin cycle in photosynthesis. 

1. The carbon dioxide combines with a molecule called ribulose bisphosphate (RuBP), a five-carbon compound and a carbon dioxide acceptor. This reaction is catalysed by RuBisCO (ribulose bisphosphate carboxylase-oxygenase), an enzyme which adds a carboxyl group (COO-) to the RuBP molecule

2. This combination forms an unstable six-carbon compound, before splitting into the products of the reaction, which are two molecules of glycerate 3-phosphate (GP), a three-carbon compound, and at this stage the carbon dioxide is fixed. (Fixed = being changed from inorganic CO2 to an organic compound).

3. The GP molecules are reduced (using two hydrogen atoms donated by reduced NADP from the light-dependent stage) into triose phosphate (TP). Energy from ATP, also made during the light-dependent stage, is used at this stage at the rate of 2 ATP molecules for every C02 molecule that has been fixed during stage 2.

4. 10 out of every 12 molecules of TP are recycled via phosphorylation (using another ATP molecule from the light-dependent stage of photosynthesis) into 6 more molecules of RuBP. The remaining 2 molecules of TP are the product. 

As already mentioned, the products of the light-dependent stage, namely ATP and reduced NADP,are continuously needed for the Calvin cycle to run.

During the light-dependent stage, protons are pumped from the stroma and into the thylakoid spaces, so the concentration of free protons in the stroma decreases, raising the pH to around 8 (becomes more alkaline). This pH is ideal for RuBisCO. RuBisCO is also activated by the presence of extra ATP in the stroma.

In daylight, the concentration of magnesium ions increases in the stroma. These ions attach to the active site of RuBisCO, acting as cofactors to activate it.

The ferredoxin that is reduced by electrons in PSI activates the enzymes involved in the reactions of the Calvin cycle. 

Some TP molecules are used to synthesise organic compounds for example:

• Some glucose is converted into sucrose, some to starch and some to cellulose. 
• Some TP is used to synthesise amino acids, fatty acids, glycerol and carbs

The rest of the TP is recycled to regenerate the supply of RuBP. Five molecules of the three-carbon compound TP interact to form three molecules of five-carbon compound RuBP.

       Light provides the energy to power the light-dependent stage (the first stage). This is required to produce ATP and reduced NADP needed for the second stage. Light also causes the stomata to open so gaseous exchange can occur. When they are open, transpiration can happen, leading to more uptake of water. 

At a constant favourable temperature and constant suitable CO2 conc, light intensity is a limiting factor. When light intensity is low, rate of photosynthesis is low. As it increases, rate of photosynthesis increases.

At a certain point, even when light intensity increases, the rate of photosynthesis does not increase. Now another factor other than light intensity is the limiting factor. 

1. GP cannot be reduced to TP
2. TP levels fall and GP accumulates
3. If TP falls, RuBP cannot be regenerated 

The levels of carbon dioxide in the atmosphere and in aquatic habitats are high enough that carbon dioxide is not usually a limiting factor. 

If the concentration of carbon dioxide falls below 0.01%:

1. RuBP cannot accept it, and accumulates.
2. GP cannot be made
3. Therefore, TP cannot be made. 

The Calvin Cycle involves many enzyme-catalysed reactions and therefore is sensitive to temperature.

The effects of changing temperature on the Calvin cycle are:

• From low temperatures to approx 25-30oC, if plants have enough water and carbon dioxide and a sufficient light intensity, the rate of photosynthesis increases as temperature increases.
• At temperatures above 30oC, for most plants, growth rates may reduce due to photorespiration: oxygen competes with carbon dioxide for the enzyme RuBisCO's active site. This reduces the amount of carbon dioxide being accepted by RuBP and subsequently reduces the quantity of GP and therefore of TP being produced, whilst initially causing an accumulation of RuBP. However, due to lack of TP, RuBP cannot be regenerated.
• At temperatures above 45oC, enzymes involved in photosynthesis may be denatured. This would reduce the concentrations of GP and TP, and eventually of RuBP as it could not be regenerated due to lack of TP. 

If a plant has access to enough water in the soil, then the transpiration stream has a cooling effect on the plant. The water passing up the xylem to leaves also keeps plant cells turgid so they can function. Turgid guard cells allow the stomata to stay open for gas exchange.

If insufficient water is available to the plant (water stress):

1. Roots are unable to take up enough water to replace that lost via transpiration
2. Cells lose water and become plasmolysed 
3. Roots produce abscisic acid that, when translocated to the leaves, causes stomata to close, reducing gaseous exchange.
4. Tissues become flaccid and the leaves wilt
5. The rate of photosynthesis is greatly reduced

-Physiological process used by algae, plants and some types of bacteria to convert sunlight into chemical energy. Organisms use this energy to synthesise large organic molecules from simple ones like carbon dioxide and water- this is called autotrophic nutrition

-Organisms that photosynthesise are called photoautotrophs because they use light as the energy source for autotrophic nutrition. They are producers because they are at the first level of the food chain.

General equation is:
6CO2 + 6H2O + energy from photons ------> C6H12O6 + 6H2O   (photon = particle of light)

The main product of photosynthesis is monosaccharide (glucose) sugar which can be converted to disaccharide for transport (sucrose) and polysaccharide for storage (starch). 

Photosynthesis is an example of carbon fixation- a process where carbon dioxide is converted into organic molecules and sugars. It is an endothermic reaction and requires an input of energy. It also needs electrons, the addition of electrons is a reduction reaction. It helps to regulate the concentration of CO2 in the atmosphere and oceans. 

Plants and other organisms that photosynthesise also respire. During respiration, they oxidise the organic molecules that they have previously synthesised by photosynthesis (glucose) and stored, releasing chemical energy. 

Non-photosynthetic organisms such as fungi, animals, many protoctists and many types of bacteria are described at heterotrophs. They obtain energy by digesting complex organic molecules of food o smaller molecules that they can use as respiratory substrates. They obtain energy from the products of digestion by respiration.

During respiration, glucose and other organic compounds are oxidised to produce carbon dioxide and water. Respiration releases chemical energy (it is exothermic) that can drive the organism's metabolism. 

glucose + oxygen ----> carbon dioxide + water + energy

• Both photosynthesis and aerobic respiration are important  in cycling carbon dioxide and oxygen in the atmosphere. The products of one process are the raw materials for the other process: aerobic
• The products of one process are the raw materials for the other process: aerobic respiration removes oxygen from the atmosphere and adds carbon dioxide, but photosynthesis does the other.

• Plants respire all the time. However, they only photosynthesise during daylight. Plants often compete with each other for light. The intensity of light has to be sufficient to allow photosynthesis at a rate that replenishes the carbohydrate used up in respiration.
• When photosynthesis and respiration proceed at the same rate, so there is no net gain or loss of carbohydrate, the plant is at its compensation point. The time a plant takes to reach its compensation point is called the compensation period. The compensation period for different plant species is different. Shade plants can use light of low intensity than sun plants.
• When exposed to light after being in darkness, shade plants reach their compensation point sooner (they have a shorter compensation period) than sun plants, which require a higher light intensity to achieve their optimum rate of photosynthesis.