B4

Photosynthesis is represented by the equation:

carbon dioxide + water -→light-→ glucose + oxygen

6CO2 + 6H2O -→light-→ C6H12O6 + 6O2

Photosynthesis uses energy to change carbon dioxide and water into glucose and oxygen

It takes place in chloroplasts in green plant cells - they contain pigments like chlorophyll that absorb light

Energy is transferred to the chloroplasts from the environment by light

Photosynthesis is endothermic which means energy is transferred from the environment in the process

Temperature -

Photosynthesis increases with more heat

Usually if the temperature is the limiting factor it’s because it’s too low and the enzymes needed for photosynthesis work more slowly at low temperatures. If the plant gets too hot, the enzymes it needs for photosynthesis and its other reactions will be damaged. This happens at about 45 degrees

Light intensity -

Photosynthesis increases with more light

Light provides the energy needed for photosynthesis. As the light level is raised, the rate of photosynthesis increases steadily but only up to a certain point. Beyond that it won’t make any difference as light increases the rate will no longer increase. This is because it will be either the temperature or the carbon dioxide level which is now the limiting factor not light

Carbon dioxide concentration -

Carbon dioxide is one of the raw materials needed for photosynthesis. The amount of CO2 will only increase the rate of photosynthesis up to a certain point. This will show that CO2 is no longer the limiting factor.

Amount of chlorophyll -

The amount of chlorophyll in a plant can be affected by disease or environmental stress such as lack of nutrients. These factors can cause chloroplasts to become damaged or to not make enough chlorophyll. This means the rate of photosynthesis is reduced because they can’t absorb as much light

These factors interact and any one of them may be the factor that limits photosynthesis

The rate of photosynthesis is affected by light intensity and temperature.

At the start both of the lines show that as the light intensity increases, the rate of photosynthesis increases steadily. Then the lines level off when light is no longer the limiting factor. The line at 25 degrees levels off at a higher point than the one at 15 degrees showing that temperature must have been a limiting factor at 15 degrees

The rate of photosynthesis is affected by light intensity and CO2 concentration

Both the lines level off when light is no longer the limiting factor. The line at the higher CO2 concentration of 0.4 percent levels off at a higher point than the one at 0.04 percent. This means CO2 concentration must have been a limiting factor at 0.04 percent CO2. The limiting factor here isn’t temperature because it is the same for both lines

Distance and light intensity are inversely proportional to each other. Light intensity decreases in proportion to the square of the distance. This is called the inverse square law:

light intensity is proportional to 1

                                        distance^2

The inverse square law means that if you halve the distance the light intensity will be four times greater and if you third the distance, the light intensity will be four times smaller and if you treble the distance, the light intensity will be nine times smaller

Limiting factors are important in the economics of enhancing the conditions in greenhouses to gain the maximum rate of photosynthesis while still maintaining profit

The most common way to artificially create the ideal environment for plants is to grow them in a greenhouse. Greenhouses help to trap the suns heat and make sure that the temperature doesn’t become limiting. In the winter a farmer or gardener might use a heater as well to keep the temperature at the ideal level. In summer it could get too hot so they might use shades and ventilation

Light is always needed for photosynthesis so commercial farmers often supply artificial light after the sun goes down to give their plants more quality photosynthesis time

Farmers and gardeners can also increase the level of carbon dioxide in the greenhouse (by using a paraffin heater to heat the greenhouse. As the paraffin burns it makes CO2 as a by product)

Keeping plants enclosed in a greenhouse also makes it easier to keep them free from pests and diseases. The farmer can add fertilisers to the soil as well to provide all the minerals needed for healthy growth

Sorting all this out costs money but if the farmer can keep the conditions just right for photosynthesis the plants will grow much faster and a decent crop can be harvested much more often which can then be sold. It is important that a farmer supplies just the right amount of heat/light… enough to make the plants grow well but not more than the plants need as this would be a waste of money

Practical

Canadian pondweed can be used to measure the effect of light intensity on the rate of photosynthesis. The rate at which the pondweed produces oxygen corresponds to the rate at which it is photosynthesising - the faster the rate of rate of oxygen production the faster the rate of photosynthesis

A source of white light is placed at a specific distance from the pondweed

The pondweed is left to photosynthesise for a set amount of time. As it photosynthesises the oxygen released will collect in the capillary tube

At the end of the experiment the syringe is used to draw the gas bubble in the tube up alongside a ruler and the length of the gas bubble is measured. This is proportional to the volume of oxygen produced

For this experiment any variables that could affect the results should be controlled (the temperature and the time the pondweed is left to photosynthesise)

The experiment is repeated twice with the light source at the same distance and mean volume of of oxygen produced is calculated

Then the whole experiment is repeated with the light source at different distances from the pondweed

The glucose produced in photosynthesis may be

Used for respiration - this transfers energy from glucose which enables the plants to convert the rest of the glucose into various other useful substances

Converted into insoluble starch for storage - glucose is turned into starch and stored in roots, stems and leaves, ready for use when photosynthesis is not happening like in the winter. Starch is insoluble which makes it much better for storing than glucose - a cell with lots of glucose in would draw in loads of water and swell up

Used to produce fat or oil for storage - Glucose is turned into lipids for storing in seeds

Used to produce cellulose which strengthens the cell wall - Glucose is turned into cellulose for making strong plant cell walls

Used to produce amino acids for protein synthesis - Glucose is combined with nitrate ions (absorbed from the soil) to make amino acids which are then made into proteins

To produce proteins, plants also use nitrate ions that are absorbed from the soil

Respiration involves many reactions. These are really important reactions as respiration transfers the energy that the cell needs for all living processes

Respiration is the process of transferring energy from the breakdown of glucose and it goes on in every cell in your body continuously

Respiration is exothermic - it transfers energy to the environment

The energy transferred supplies all the energy needed for living processes

Respiration in cells can take place aerobically (using oxygen) or anaerobically (without oxygen) to transfer energy

Aerobic respiration

Needs lots of oxygen

Produces carbon dioxide and water

Transfers more energy

Anaerobic respiration

Used when not enough oxygen

Produces lactic acid

Doesn’t transfer as much energy because glucose isn’t fully oxidised because it doesn’t combine with oxygen

Organisms need energy for:

chemical reactions to build larger molecules

movement

keeping warm

Aerobic respiration is represented by the equation:

glucose + oxygen → carbon dioxide + water

C6H12O6 + O2 → CO2 + H2O

Anaerobic respiration in muscles is represented by the equation:

glucose → lactic acid

As the oxidation of glucose is incomplete in anaerobic respiration much less energy is transferred than in aerobic respiration

Anaerobic respiration in plant and yeast cells is represented by the equation:

glucose → ethanol + carbon dioxide

Anaerobic respiration in yeast cells is called fermentation and has economic importance in the manufacture of bread and alcoholic drinks

During exercise the human body reacts to the increased demand for energy

The heart rate, breathing rate and breath volume increase during exercise to supply the muscles with more oxygenated blood

If insufficient oxygen is supplied anaerobic respiration takes place in muscles. The incomplete oxidation of glucose causes a build up of lactic acid and creates an oxygen debt. During long periods of vigorous activity muscles become fatigued and stop contracting efficiently

Blood flowing through the muscles transports the lactic acid to the liver where it is converted back into glucose. Oxygen debt is the amount of extra oxygen the body needs after exercise to react with the accumulated lactic acid and remove it from the cells

Metabolism is the sum of all the reactions in a cell or the body

The energy transferred by respiration in cells is used by the organism for the continual enzyme controlled processes of metabolism that synthesise new molecules

Metabolism includes:

conversion of glucose to starch, glycogen and cellulose

The formation of lipid molecules from a molecule of glycerol and three molecules of fatty acids

The use of glucose and nitrate ions to form amino acids which in turn are used to synthesis proteins

Respiration

Breakdown of excess proteins to form urea for excretion