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• plant adaptations to water loss 9.5

It is affected by various environmental factors, including changes in temperature and the availability of raw materials required for photosynthesis.

Plants obtain carbon dioxide through open stomata but this also involves the loss of water by transpiration. However the loss of water vapour results in the transport of important ions and minerals from the soil to the leaves.

A balance therefore has to be maintained and this can be upset by changes in the environment of the plant.

Limiting factors

When one of the factors needed for a plant to photosynthesis is in short supply, it reduces the rate of photosynthesis and is therefore a limiting factor.

Factors that affect the rate of photosynthesis

• Light intensity - light is needed as an energy source. As light intensity increases, ATP and reduced NADP are produced at a higher rate.

• Carbon dioxide concentration - carbon dioxide is needed as a source of carbon, so if all other conditions are met, increasing the carbon dioxide concentration increases the rate of carbon fixation in the Calvin cycle and therefore the rate of TP production.

• Temperature - affects the rate of enzyme controlled reactions, as the temperature increases, the rate of enzyme activity increases until the point that the globular proteins denature. An increase in temperature increases the rate of enzyme controlled reactions in photosynthesis such as carbon fixation. The rate of photorespiration also increases above 25’c meaning higher photosynthetic rates may not be seen at a higher temperature even if enzymes are not actually denatured.

Stomata on plant leaves and other surfaces will close to avoid water loss by transpiration during dry spells when plants undergo water stress.

The closure of stomata stops the diffusion of carbon dioxide into the plant, reducing the rate of the light-independent reactions and eventually stopping photosynthesis.

Although water is required for photosynthesis it is never considered a limiting factors because for water potential to have become low enough to limit the rate of photosynthesis the plant will have already closed its stomata and ceased photosynthesis. Plants, except those with adaptations to drought conditions are unlikely to survive.

Law of limiting factors - state that the rate of a physiological process will be limited by the factor which is in shortest supply.

Investigating the factors that affect the rate of photosynthesis

Data loggers - are electronic devices that record data over time using sensors. Physical properties are recorded such as light intensity, temperature, pH (which can be used to measure co2 concentration), pressure and humidity. Readings can be displayed in graphical form or on a spreadsheet.

They are usually equipped with a microprocessor (which inputs digital data) and internal memory for data storage. Data loggers can usually interface with a computer using specialised software.

Readings are taken with high degrees of accuracy and can be taken over long periods of time.

They can be set to take many readings in a short period of time or used when there is a risk involved like extreme cold or heat.

The factors affecting the rate of photosynthesis can be investigating using live pondweed like Elodea.

The rate of photosynthesis can be estimated by

• calculating the rate of oxygen produced

• carbon dioxide used

• increase in dry mass of the plant

Sodium hydrogen carbonate would be used to provide carbon dioxide. The pond weed should be kept illuminated before use. The apparatus should be left to equilibrate for 10 minutes before readings are taken. The oxygen sensor may also need to be calibrated using the oxygen concentration of air (21%)

The software can be set to take readings at desired intervals for the required length of time.

The effect of reducing light intensity on the Calvin cycle

Reducing light intensity will reduce the rate of light-dependant stage of photosynthesis. This will reduce the quality of ATP and reduced NADP produced. ATP and reduced NADP are needed to convert GP to TP. The concentration of GP will increase and the concentration of TP will decrease.

As there will be less TP to regenerate RuBP, the concentration of RuBP will also decrease. The reverse will happen when the light intensity is increased.

The effect of Carbon dioxide concentration and temperature on the Calvin cycle

All the reactions making up the Calvin cycle are catalysed by enzymes. At lower temperature enzymes and substrate molecules will have less kinetic energy resulting in fewer successful collisions and a reduced rate of reaction. This means decreasing temperature results in lower concentrations of GP, TP and RuBP.

The same effect will be seen at higher temperature as enzymes will be denatured - this is irreversible.

As carbon dioxide is an essential substrate of the Calvin cycle, low concentrations will lead to reduced concentrations of GP (as there is less carbon dioxide to be fixed) and TP.

The concentration of RuBP will increase as it is still being formed from TP and not being used to fix carbon dioxide.

Artificial photosynthesis, a win-win solution.

The burning of fossil fuels and respiration is continually releasing huge quantities of carbon dioxide into the atmosphere. The overall concentration of carbon dioxide, a greenhouse gas, is increasingly leading to more heat from the Sun being trapped in the atmosphere. This is enhanced global warming and is causing the polar ice caps to melt, increasing sea levels and changing climate around the world.

Fossil fuels have a limited supply and will eventually run out, leading to fuel shortages and many parts of the world already suffer from this.

Therefore we have a surplus of carbon dioxide which needs removing and a shortage and fuel and food, both of which are forms of biomass produced by plants using carbon dioxide.

Photosynthesis would appear to offer a solution, it uses co2 and energy from sun to produce carbohydrates which can be used as both food and fuel.

The problem is that the process of photosynthesis is not particularly efficient so relying on plants is not the solution.

However artificial photosynthesis can be a possible solution, by improving on the natural process of photosynthesis carried out by plants, more CO2 could be removed from the atmosphere and more carbohydrates products could be produced which could help with fuel and food shortages.

Different types of photosynthesis

Most plants use the form of photosynthesis that is this - which is also known as C3 photosynthesis and it is most efficient in cool, wet climates with average sunshine values.

Plants that live in hot, arid climates like the desert which are exposed to intense sunlight use different types of photosynthesis. Plants which use C4 and crassulacean acid metabolism (CAM) types of photosynthesis use water more efficiently and can photosynthesis at faster rates at higher temperatures and light intensities. Plants are adapted to their different environments to photosynthesis as efficiently as possible.

C4 photosynthesis

Plants that undergo C4 photosynthesis are adapted to high temperatures and limited water supply. They are able to fix carbon dioxide more efficiently and so do not need to have their stomata open for as long as C3 plants, meaning that less water is lost by transpiration.

PEP carboxylase present in mesophyll cells, which first fixes carbon dioxide is not inhibited by oxygen (like RuBisCO), increasing the efficiency of fixation.

The 4 carbon molecules produced are transported to bundle sheaths, formed form tightly packed cells deeper inside the plant. These molecules are then decarboxylated and the carbon dioxide is then fixed by RuBisCO and enters the Calvin cycle. As RuBisCO is shielded from atmospheric oxygen, the waster of resources by photorespiration is reduced. Corn is an example of C4 plant.

CAM photosynthesis

Other plants use CAM photosynthesis and open their stomata at night, usually closing them during the day, again reducing water loss by transpiration.

Carbon dioxide is converted to an acid and stored during the night. During the day the acid is broken back down releasing carbon dioxide to RuBisCO.

During very dry spells stomata can remain closed night and day. The carbon dioxide released from respiration is used in photosynthesis and the oxygen released by photosynthesis is used for respiration. Cacti are CAM plants.