Photosynthesis

• Photosynthesis is a physiological process used by plants, algae and some types of bacteria to convert light energy from sunlight into chemical energy.

• Organisms can use this chemical energy to synthesise large organic molecules, which form the building blocks of living cells, from simple inorganic molecules such as water and carbon dioxide - this is autotrophic nutrition.

• Organisms that photosynthesise are called "photoautotrophs" because they use light as the energy source for autotrophic nutrition.

• These organisms are also described as producers because they are at the beginning (first trophic level) of a food chain and provide energy and organic molecules to other, non-photosynthetic, organisms.

• A photon is a particle of light; each photon contains an amount ( a quantum) of energy.

• The main product of photosynthesis is a monosaccharide sugar, which can be converted to disaccharides for transport and then to starch for storage.

• Photosynthesis is an example of carbon fixation- the process by which carbon dioxide is converted into sugars. The carbon for synthesizing all types of organic molecules is provided by carbon fixation.

• Carbon fixation is endothermic, and so needs energy. Carbon fixation also needs electrons; the addition of electrons is a reduction reaction.

• Carbon fixation helps regulate the concentration of carbon dioxide in the atmosphere and oceans.

• Most forms of life on Earth rely directly or indirectly on photosynthesis.

• Plants and other organisms that photosynthesize also respire.

• During respiration, they oxidize the organic molecules that they have previously synthesized and stored, releasing chemical energy.

• Non-photosynthetic organisms such as fungi, animals, many protists, and many types of bacteria are described as heterotrophs.

• They obtain energy by digesting complex organic molecules of food to 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 oxidized to produce carbon dioxide and water.

• Respiration releases chemical energy (it's exothermic) that can drive the organism's metabolism. 

• 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 respiration removes oxygen from the atmosphere and adds carbon dioxide.

• Photosynthesis removes carbon dioxide from the atmosphere and adds oxygen.

• Plants respire all the time. However, they only photosynthesize 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 stores used up by respiration.

• When photosynthesis and respiration proceed at the same rate, so that 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.

• Chloroplasts are the organelles within plant cells where photosynthesis takes place. Algae have bacteria, but photosynthetic bacteria do not.

• Most plant chloroplasts are disc-shaped and around 2-10um long.

• Each is surrounded by a double membrane, the envelope, with an intermembrane space of width 10-20nm between the inner and outer membrane. The outer membrane is highly permeable.

• There are two distinct regions, visible on electron micrographs, inside a chloroplast: the fluid-filled matrix called the stroma, and the grana that consists of stacks of thylakoid membranes.

• The first stage if photosynthesis, the light-dependent stage, takes place in the grana.

• Chloroplasts have three distinct membranes -outer, inner and thylakoid, giving three separate internal compartments - the intermembrane space, stroma, and the thylakoid space.

• The thylakoid within a granum may be connected to thylakoids within another granum by intergranal lamellae (aka intergranal thylakoids).

• The thylakoid membrane of each chloroplast is less permeable and is folded into flattened disc-like sacs called thylakoids that form stacks. Each stack of thylakoids is called a granum.

With many granan in every chloroplast and with many chloroplasts in each photosynthetic cell, there is a large surface area for:

• Distribution of the photosystems that contain the photosynthetic pigments that trap light energy.
• The electron carriers and ATP synthase enzymes needed to convert light energy into ATP.

Proteins embedded in the thylakoid membranes hold the photosystems in place.

The grana are surrounded by the stroma, so the products of the light-dependent stage can easily pass into the stroma to be used in the light-independent stage.

• The stroma is the fluid-filled matrix.

• It contains the enzymes needed to catalyze the reactions of the light-independent stage of photosynthesis, as well as starch grains, oil droplets and small ribosomes similar to those found in prokaryotes.

• The loop of DNA contains genes that code for some of the proteins needed for photosynthesis.

• These proteins are assembled at the chloroplast ribosomes.

• Within the thylakoid membranes, each chloroplast are funnel-shaped structures called photosystems.

• These photosystems contain photosynthetic pigments.

• Each photosynthetic pigment absorbs light of a particular wavelength and reflects other wavelengths of light.

• Each pigment appears, to our eyes and brain, the colour of the wavelength of light it's reflecting.

• The energy associated with the wavelength of light it is reflecting.

• The energy associated with the wavelengths of light captured is funneled down to the primary pigment reaction center, consisting of a type of chlorophyll, at the base of the photosystem.

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

Chlorophyll a

There are 2 forms of chlorophyll a:

• Both of them appear blue/green. 
• Both are situated at the center of photosystems.
• Both absorb red light

They have different absorption peaks:

• P680 is found in photosystem II and its peak of absorption is light of wavelength 680nm.
• P700 is found in photosystem I and its peak absorption is light of wavelength 700nm.
• It also absorbs some blue light, of wavelength around 440nm.

Chlorophyll b - absorbs light of wavelengths 400-600nm and around 640nm. It appears yellow-green.

Carotenoids

• Absorb blue light of wavelengths 400-500nm.
• They reflect orange and yellow light.

Xanthophylls

• Absorb blue and green light of wavelengths 375-550nm.
• They reflect yellow light.

The light-independent stage of photosynthesis occurs in the grana (thylakoids) of chloroplasts and involves photosystems.

It involves the direct use of light energy.

This stage consists of:

• Light harvesting at the photosystems.

• Photolysis (separation by the action of light) of water. 

• Photophosphorylation - the production of ATP in the presence of light.

• The formation of reduced NADP.

Oxygen, the by-product of photosynthesis, is also produced in the light-independent stage.

Photosystem I

• The pigment at the primary reaction centre is a type of chlorophyll a.

• It has a peak absorption of red light of wavelength 700nm.

Photosystem II

• The pigment at the primary reaction center is a type of chlorophyll a.

• It has a peak absorption of red light of wavelength 680nm.

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

Some of the oxygen produced by photolysis is used py plant 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 the stomata, into the surrounding atmosphere.

Water:

• Is the source of protons (hydrogen ions) 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 two 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.

Both involve iron-containing proteins embedded in the thylakoid membranes that accept and donate electrons and form an electron transport system.

This only uses PSI. 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 PSI. They produce only ATP which actively brings potassium ions into the cells, lowering the water potential so that water follows by osmosis. This causes the guard cells to swell and open the stomata.

• When a photon of light strikes PSII, its energy is channelled to the primary pigment reaction centre.

• The light energy excites a pair of electrons inside the chlorophyll molecule.

• The energised electrons escape from the chlorophyll molecule and are captured by an electron carrier, which is a protein with iron at its centre, embedded in the thylakoid membrane.

• These electrons are replaced by electrons derived from photolysis.

• When this iron ion combines with an electron it becomes reduced (Fe2+). It can then donate the electron, becoming reoxidised (Fe+3), to the next electron carrier in the chain.

• As electrons are passed along a chain of electron carriers embedded in the thylakoid membrane, at each step of the some energy associated with the electrons is released.

• This energy is used to pump protons across the thylakoid membrane into the thylakoid space.

• 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.

• A protein-iron-sulfur complex called ferredoxin accepts the electrons from PSI and passes them to NADP in the stroma.

• As protons accumulate in the thylakoid space, a proton gradient forms across the membrane.

• Protons diffuse down their concentration gradient 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, forming ATP.

• As the 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.

• The light-independent stage of photosynthesis takes place in the stroma of chloroplasts.

• Although it does not directly use light energy, it uses the products of the light-dependent stage.

• If the plant is not illuminated, the light-independent stage soon ceases, because ATP and hydrogen are not available to reduce the carbon dioxide and synthesise large complex organic molecules.

• Carbon dioxide is the source of carbon for the production of all organic molecules found in all the carbon-based life forms on Earth. 

• These organic molecules may be used as structures (e.g. cell membranes, antigens, enzymes muscle proteins, cellulose cell walls) or act as energy stroes ( starch and glycogen).

• Carbon dioxide enters the leaf through the stomata and diffuses through the spongy mesophyll layer to the palisade layer, into the palisade cells, through their thin cellulose cell walls, and then through the chloroplast envelope in the stroma.

• The fixation of carbon dioxide in the stroma maintains a concentration gradient that aids this diffusion.

• Carbon dioxide that is a by-product of respiration in plant cells may also be used for this stage of photosynthesis.

• The series of reactions whereby carbon dioxide is converted to organic molecules is called the Calvin cycle.  

• Carbon dioxide combines with a carbon dioxide acceptor called ribulose bisphosphate (RuBP). This reaction is catalysed by the enzyme ribulose bisphosphate carboxylase-oxygenase (RuBisCO).

• RuBP, by accepting the carboxyl (COO-) group, becomes carboxylated, forming an unstable six carbon intermidiate compound that immediately breaks down.

• The product of this reaction is two molecules of glycerate-3-phosphate (GP). The carbon dioxide has now been fixed.

• GP is then reduced, using hydrogens from the NADPH made during the light-dependent stage, to triose phosphate (TP). Energy from ATP, also made during the light-dependent stage, is used at this stage at the rate of two molecules of ATP for every molecule of carbon dioxide fixed during stage 3.

• In 10 of every 12 TP molecules, the atoms are rearranged to regenerate six molecules of RuBP. This process requires phosphate groups. Chloroplasts contain only low levels of RuBP, as it continually being converted to GP, but is also continually being regenerated. The remaining 2 of the 12 molecules of TP are the product.

• The products of the light independent stage, ATP and NADPH, are contiuously needed for the Calvin cycle to run.

• During the light-dependent stage, hydrogen ions are pumped from the stroma into the thylakoid spaces, so the concentration of free protons falls raising the pH to around 8, which is the optimum for the enzyme 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 from PSI activates enzymes involved in the reactions of the Calvin cycle.

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

• Some glucose is converted to sucrose, some to starch and some to cellulose.

• Some TP is used to synthesise amino acids, fatty acids and glycerol.

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 the five-carbon compund RuBP.

The factors that affect the rate of photosynthesis operate simultaneously.

These factors include:

• The raw materials (carbon dioxide and water) 
• The energy source (light intensity)
• Availability of chlorophyll
• Availability of electron carriers
• Availability of relevant enzymes
• Temperature
• Turgidity of the cells

At any given moment, the rate of metabolic process that depends on a number of factors is limited by the factor that is present at its least favourable (lowest) level.

• Light provides the energy to power the first stage of photosynthesis and produce ATP and NADPH needed for the next stage.

• Light also causes stomata to open so that gaseous exchange can occur. When stomata are open transpiration also occurs, and this leads to uptake of water and its delivery to the leaves.

• At a constant favourable temperature and constant suitable carbon dioxide concentration, light intensity is the limiting factor. When light intensity is low, the rate of photosynthesis is low. As light intensity increases, the rate of photosynthesis increases.

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

• GP cannot be reduced to TP.

• TP levels fall and GP accumulates.

• If TP levels fall, RuBP cannot be regenerated.

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

The effect of changing the carbon dioxide concentration on the Calvin cycle

If the concentration falls bellow 0.01%

• RuBP cannot accept it, and accumulates.

• GP cannot be made.

• 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 

• From temperatures bellow 30*C, if plants have enough water and carbon dioxide and a sufficient light intensity, the rate of photosynthesis increases as temperature increases.

• At temperatures above 30*C, 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 a lack of TP, RuBP cannot be regenerated.

• At temperatures above 45*C, 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 sufficient water in the soil, then the transpiration stream has a cooling effect on the plant. The water passing up the xylem to leave also keeps the plant cells turgid so they can function. Turgid guard cells keep the stomata open for gaseous exchange.

If not enough water is available to the plant (water stress)

• The roots are unable to take up enough water to replace that lost via transpiration.

• Cells lose water and become plasmolysed.

• Plant roots produce abscisic acid that, when translocated to leaves, causes stomata to close, reducing gaseous exchange.

• Tissues become flaccid and leaves wilt.

• The rate of photosynthesis greatly reduces.

There are many ways to measure the rate of photosynthesis, including the rate of uptake of raw materials such as carbon dioxide, or the rate of production of the by-product oxygen. 

In each case, to measure the rate we need to calculate the quantity taken up or produced per unit of time.

In school laboratories, the rate of photosynthesis is often found by measuring the volume of oxygen produced per minute by an aquatic plant. There are limitstions with this method because:

• Some of the oxygen produced by the plant will be used for its respiration.

• There may be some dissolved nitrogen in the gas collected.

However, the same apparatus can be adapted and used to measure the effects of light intensity, temperature or carbon dioxide availability on the rate of photosynthesis.