Biology - Bioenergetics

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 - this means energy is transferred from the environment in the process. The word equation for photosynthesis is:

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

Making cellulose - glucose is converted into cellulose for making strong plant cell walls.

Making amino acids - glucose is combined with nitrate ions (absorbed from the soil) to make amino acids, which are then made into proteins.

Stored as oils or fats - glucose is turned into lipids (fats & oils) for storing in seeds.

Stored as starch - glucose is turned into starch and stored in roots, stems and leaves, ready for use when photosynthesis isn't 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.

The rate of photosynthesis is affected by the intesnity of light, concentration of CO2, and temperature. Any of these three factors can become the limiting factor - this just means that it's stopping photosynthesis from happening any faster. These factors have a combined effect on the rate of photosynthesis, but which factor is limiting at the particular time depends on the environmental conditions:

- at night it's pretty obvious that light is the limiting factors

- in winter it's often the temperature

- if it's warm enough and bright enough, the amount of CO2 is usually limiting

Chorophyll can also be a limiting factor of photosynthesis. The amount of chlorophyll in a plant can be affected by disease (e.g. infection with the tobacco mosaic virus) or environmental stress, such as lack of nutrients. These factors can cause chloroplasts to become damaged or to not make enough chlorphyll. This means the rate of photosynthesis is reduced because they can't absorb as much 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 intensity increases, the rate will no longer increase. This is because it'll be either the temperature or the CO2 level which is now the limiting factor, not light. In the lab you can change the light intensity by moving a lamp closer to or further away from your plant.

CO2 is one of the raw materials needed for photosynthesis. The amount of CO2 will only increase the rate of photosynthesis up to a point. After this the graph flattens out - as the amount of CO2 increases, the rate no longer increases. This shows that CO2 is no longer the limiting factor. As long as light and CO2 are in plentiful supply then the factor limiting photosynthesis must be temperature.

Usually, if the temperature is the limiting factor it's because it's too low - the enzymes needed for photosynthesis work more slowly at low temperatures. But 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 celsius (which is pretty hot for outdoors, although greenhouses can get that hot if you're not careful).

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's photosynthesising - the faster the rate of oxygen production, the faster the rate of photosynthesis. In the experiment 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 (e.g. the temperature and time the pondweed is left to photosynthesise). The experiment is repeated twice with the light source at the same distance and the mean volume of oxygen produced is calculated. Then the whole experiment is repeated twice with the light source at different distances from the pondweed. The apparatus can be altered to measure the effect of temperature or oxygen on photosynthesis (e.g. the test tube of pondweed can be put into a water bath at a set temperature, or a measured amount of sodium hydrogencarbonate can be dissolved in the water - which gives off oxygen). The experiment can then be repeated with different temperature of water / concentrations of sodium hydrogencarbonate.

In the pondweed experiment, when the lamp is moved away from the pondweed, the amount of light that reaches the pondweed decreases. You can say that the as distance increases, the light intensity decreases. In other words, distance and light intensity are inversely proportional to each other. However, light intensity decreases in proportion to the square of the distance. This is called the inverse square law and is written out, as shown below. The inverse square law means that you if halve the distance, the light intensity will be four times greater, and if you third the distance, the light intensity will be nine times greater. If you double the distance, the light intensity will be four times smaller and if you treble the distance, the light intensity will be nine times smaller.

The most common way to create the ideal environment for plants is to grow them in a greenhouse. Greenhouses help to trap the sun's heat and make sure that the temperature doesn't become limiting. In 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 to cool things down. 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 & gardeners can also increase the level of carbon dioxide in the greenhouse (e.g. by using a paraffin heater to heat the greenhouse. As the paraffin burns, it makes carbon dioxide as a by-product). Keeping plants enclosed in a greenhouse also makes it easier to keep them free from pests and disease. 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's important that a farmer supplies just the right amount of heat, light, etc. - enough to make the plants grow well, but not more than the plants needs, as this would just be wasting money.

Respiration involves many reactions. These are really important reactions, as respiration transfers the energy that the cells need to do just about everything - this energy is used for all living processes. Respiration is not breathing in and out, as you might think. Respiration is the process of transferring energy from the breakdown of glucose (sugar) - and it goes on in every cell in your body continuously. It happens in plants too. All living things respire. It's how they transfer energy from their food to their cells. Respiration is exothermic - it transfers energy to the environment. Here are three examples of how organisms use the energy transferred by respiration:

- to build up larger molecules from smaller ones (like proteins from amino acids)

- in animals it's used to allow the muscles to contract (so they can move about)

- in mammals and birds the energy is used to keep their body temperature steady in colder surroundings (unlike other animals, mammals and birds keep their bodies constantly warm)

NOTE: RESPIRATION is the process of TRANSFERRING ENERGY FROM GLUCOSE, which goes on IN EVERY CELL.

In a cell there are lots of chemical reactions happening all the time, which are controlled by enzymes. Many of these reactions are linked together to form bigger reactions. 

               Enzyme              Enzyme               Enzyme

Reactant ProductProductProduct

In some of these reactions, larger molecules are made from smaller ones. For example:

- lots of small glucose molecules are joined together in reactions to form starch (a storage molecule in plants cells), glycogen (a storage molecule in animal cells) and cellulose (a component of plant cell walls).

- lipid molecules are each made from one molecule of glycerol and three fatty acids

- glucose is combined with nitrate ions to make amino acids, which are then made into proteins.

In other reactions, larger molecules are broken down into smaller ones. For example:

- glucose is broken down in respiration. Respiration transfers energy to power all the reactions in the body that make molecules.

- excess protein is broken down in a reaction to produce urea. Urea i then excreted in urine.

The sum (total) of all the reactions that happen in a cell or the body is called its metabolism.

Aerobic respiration is respiration using oxygen. It's the most efficient way to transfer energy from glucose. Aerobic respiration goes on all the time in plants and animals. Most of the reactions in aerobic respiration happen inside mitochondria. Here are the word and symbol equations for aerobic respiration.

When you do vigorous exercise and your body can't supply enough oxygen to your muscles, the start doing anaerobic respiration as well as aerobic respiration. "Anaerobic" just means "without oxygen. It's the incomplete breakdown of glucose, making lactic acid. Here's the word equation for anaerobic respiration in muscle cells:

Anaerobic respiration does not transfer nearly as much energy as aerobic respiration. This is because glucose isn't fully oxidised (because it doesn't combine with oxygen). So, anaerobic respiration is only useful in emergencies (e.g. during exercise when it allows you to keep on using your muscles for a while longer.

Plants and yeast cells can respire without oxygen, but they produce ethanol (alcohol) and carbon dioxide instead of lactic acid. Here is the word equation for anaerobic respiration in plants and yeast cells:

Anaerobic respiration in yeast cells is caled fermentation. In the food and drinks industry, fermentation is of great value because it's used to make bread and alcoholic drink (e.g. beer & wine). In bread-making, it's the carbon dioxide from fermentation that makes bread rise. In beer and wine-making, it's the fermentation process that produces alcohol.

rMuscles need energy from respiration to contract. When you exercise, some of your muscles contract more frequently than normal so you need more energy. This energy comes from increased respiration. The increase in respiration in your cells means you need to get more oxygen into them. Your breathing rate and breath volume increase to get more oxygen into the blood, and your heart rate increases to get this oxygenated blood around the body faster. This removes oxygen more quickly at the same time. When you do really vigorous exercise (like sprinting) your body can't supply oxygen to your muscles quickly enough, so they start respiring anaerobically. This is NOT the best way to transfer energy from glucose because lactic acid builds up in the muscles, which gets painful. Long periods of exercise also cause muscle fatigue - the muscles get tired and then stop contracting efficiently. 

REMEMBER LACTIC ACID IS FORMED FROM THE INCOMPLETE OXIDATION OF GLUCOSE.

After resorting to anaerobic respiration, when you stop exercising you'll have an "oxygen debt". An oxygen debt is the amount of extra oxygen your body needs to react with the build up of lactic acid and remove it from the cells. Oxygen reacts with the lactic acid to form harmless oxygen and water. In other words, you have to "repay" the oxygen that you didn't get to your muscles in time, because your lungs, heart and blood couldn't keep up with the demand earlier on. This means you have to keep breathing hard for a while after you stop, to get more oxygen into your blood, which is transported to the muscle cells. The pulse and breathing rate stay high whilst there are high levels of lactic acid and oxygen. Your body has another way of coping with high level of lactic acid - the blood that enters your muscles transports the lactic acid to the liver. In the liver, the lactic acid is converted back to glucose.

You can measure breathing rate by counting breaths, and heart rate by taking the pulse. For example, you could take your pulse after:

- sitting down for 5 minutes

- then after 5 minutes of gentle walking

- then again after 5 minutes of slow jogging

- then again after running for 5 minutes

Your pulse rate will increase the more intense the exercise is, as your body needs to get more oxygen to the muscles and take more carbon dioxide away from the muscles.