0610 · 12.1Respiration

Respiration and energy uses

0:00You are sitting still, doing nothing more energetic than watching this video. Yet your cells are still using energy. They are making proteins, moving substances across membranes, and keeping essential processes running. By the end of this video, you will understand what respiration supplies energy for, and how a yeast investigation reveals why temperature affects its rate. First, separate respiration from breathing. Breathing moves air into and out of your lungs, while respiration involves chemical reactions inside living cells that release energy from nutrient molecules.

0:31Respiration happens in all living cells, including plant cells and the cells of tiny organisms such as yeast. Breathing supports respiration in humans, but the two processes are not the same thing. Think of food molecules as containing a store of chemical energy. Simply having food available does not mean a cell can immediately use all that energy. Respiration releases energy that can be transferred to processes taking place in the cell.

0:56Notice the word releases, because respiration does not create energy. One important use is muscle contraction. When you lift your school bag, muscle cells use energy to contract and pull on your bones. Even while you sit still, muscles involved in breathing continue working. When muscles work harder, their energy requirements increase, so they need a greater rate of respiration. Cells also need energy for protein synthesis, which means making proteins by joining amino acids together.

1:24Proteins include enzymes, as well as materials used to build and repair cells. Cell division requires energy too, because a cell must prepare its contents and separate into new cells. These activities help explain why growth requires energy, rather than just a supply of building materials. Imagine constructing a house. Having bricks on the site is not enough, because work is needed to assemble them into a building. Similarly, an organism needs both materials and energy to grow.

1:51Protein synthesis and cell division are important parts of growth, but each is also a useful energy requirement to identify separately. Another use is active transport. This is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration, using energy from respiration. It is like moving something against the direction in which it would naturally spread. For example, root hair cells can use active transport to absorb mineral ions from a dilute soil solution.

2:20Energy is also needed for the passage of nerve impulses. Nerve cells must maintain the conditions that allow electrical signals to travel, including differences in ion concentrations across their membranes. Finally, humans and other mammals use energy released by respiration to help maintain a constant body temperature. Heat released during respiration helps replace heat lost to the surroundings, and shivering increases muscular activity when we are cold.

Investigating yeast respiration

2:44Now let us investigate respiration using yeast. Yeast is a single celled fungus, so each yeast cell carries out processes that require energy. When supplied with sugar under suitable conditions, yeast releases carbon dioxide during respiration. We can use the gas released over a measured time as an indicator of its respiration rate. A rate tells us how quickly something happens, not simply how much happens altogether. To compare respiration rates, we could count carbon dioxide bubbles released in a fixed time.

3:14Alternatively, we could measure the volume of gas collected in that time. Measuring volume is usually a better comparison because bubbles are not necessarily all the same size. Set up a tube containing a measured amount of yeast and a measured volume of sugar solution. Place it in a water bath, with a thermometer checking the bath temperature. Connect the tube to a gas syringe through a bung and delivery tube, so released gas can be collected.

3:38The syringe plunger must move freely, and the connections must not leak. The water bath helps keep the yeast mixture at a controlled temperature. Allow the mixture to reach that temperature before beginning the timed measurement. Then record the volume of gas collected over a fixed period. Repeat the investigation at several temperatures, using fresh, equal portions of the same yeast and sugar mixture each time. Temperature is the independent variable, meaning the factor we deliberately change.

4:04The dependent variable is the gas volume collected per unit time, which indicates respiration rate. Keep the yeast mass, sugar concentration, solution volume and measuring time the same. Otherwise, an increase in gas could be caused by more yeast or more available sugar, rather than by temperature. Repeat measurements at each temperature and calculate a mean to reduce the influence of random variation. Keep other conditions, such as the acidity of the mixture, consistent too.

4:31Use the same apparatus and allow the same time for temperature equilibration. Wear eye protection and handle heated water carefully, because a fair investigation must also be a safe one.

Temperature and enzyme activity

4:41What pattern should we expect? At low temperatures, respiration is relatively slow because the reactions are controlled by enzymes. Enzymes are proteins that act as biological catalysts, speeding up chemical reactions. As temperature rises towards the optimum, particles move faster and there are more successful collisions between enzymes and their substrates, so respiration becomes faster. The optimum temperature is the temperature at which the respiration rate is highest under those conditions.

5:08Above the optimum, high temperatures cause enzymes to denature, meaning their shape changes. Their active sites no longer fit their substrates properly, so fewer reactions can take place. The respiration rate therefore falls, even though the temperature is still rising. Cooling and excessive heating have different effects. At a low temperature, enzymes generally work slowly, but they are not necessarily denatured. Warming the yeast towards its optimum can therefore increase the rate again.

5:35Heating beyond the optimum can damage enzyme structure, so cooling afterwards does not necessarily restore the original activity.

Comparing rates and exam answers

5:43Let us work through a practical comparison. Suppose you are asked to investigate whether yeast respires faster at twenty degrees Celsius or thirty degrees Celsius. First, prepare equal yeast samples with the same sugar concentration and solution volume. Put them in separate water baths at the chosen temperatures, because temperature must be the only deliberate difference. Next, allow both mixtures to reach their bath temperatures, then collect gas from each for the same measured time.

6:08Compare the gas volumes, using repeated measurements and their means rather than relying on a single trial. The sample producing more gas per unit time has the greater measured respiration rate. However, testing only two temperatures would not establish the optimum, because the peak might lie elsewhere. If different collection times were unavoidable, compare rates rather than raw volumes. Divide each gas volume by its collection time to find the gas volume released per minute.

6:35This puts both measurements on the same basis. A larger total volume alone does not prove faster respiration if that sample was measured for longer. Now consider an exam style question using the graph on screen. It asks you to describe the effect of temperature on respiration in yeast and explain the decrease above the optimum. For describe, state the pattern shown by the results. For explain, connect that pattern to the enzymes controlling respiration.

7:00The rate of respiration increases as temperature rises towards the optimum. The rate reaches a maximum at the optimum temperature. Above the optimum, the rate decreases. High temperatures denature the enzymes controlling respiration, changing their active sites so that substrates no longer fit. For a question asking you to state uses of energy in living organisms, name the processes directly. Energy is used for muscle contraction, protein synthesis and cell division.

7:27It is also used for active transport and growth. Further uses include the passage of nerve impulses and the maintenance of a constant body temperature. The main mistakes all come from losing track of what is being measured. More bubbles suggest faster gas release, but different bubble sizes can make counting less reliable than measuring volume. A warmer mixture does not always respire faster, because temperatures above the optimum denature enzymes.

7:51And respiration is neither breathing nor energy creation. It is a cellular process that releases energy. Let us bring everything together. Living cells release energy through respiration to support movement, building, growth, transport, nerve impulses and temperature maintenance. In yeast, carbon dioxide released per unit time provides an indicator of respiration rate. A fair temperature investigation controls the other variables and reveals an enzyme controlled pattern, with an increasing rate towards an optimum, followed by a decrease as enzymes denature.