Efficiency of Exchange and Transport in Organisms
Organisms need to transport substances in and out for a range of reasons. Oxygen, glucose and water are needed for respiration and to carry around substances, while urea, carbon dioxide and excess salts must be transported out. Urea is the waste product from breakdown of excess amino acids.
Exchange surfaces are required for substances to move into and out of parts of the body. They are usually thin to decrease diffusion distance, and have a large surface area for greater space for particles to diffuse through. The greater the surface area : volume ratio, the faster the rate of diffusion
The alveolus is adapted for gas exchange. Blood enters from the body with a higher CO2 concentration and a lower O2 concentration than the gases in the alveolus. Blood the capillary is continuously pumped through to maintain this concentration gradient. The walls of the alveolus and capillary are one-cell thick to reduce diffusion distance, while the shape of the alveolus increases the surface area. There is a net movement of CO2 out of the blood and of O2 into the blood. This oxygenated blood then travels back around the body. The lungs contain many alveoli to increase the rate of gas exchange
Fick’s Law (below) shows how different factors affect rates of diffusion. If the surface area is greater, then there is more space across which particles can move meaning in a given time there is greater movement. If distance is decreased, then there is a lower distance through which the particles must pass and therefore rate of movement is quicker. The steeper the concentration gradient, the faster the rate of diffusion as there is a greater amount of concentration change required to reach equilibrium.
The Circulatory System
| Structure in Blood | Function |
| Erythrocytes(Red Blood Cells) | Packed with haemoglobin with which oxygen binds, turning the cells bright red. No nucleus so there is more space for haemoglobin and biconcave shape allows for large SA:V ratio for movement of O2 |
| Phagocytes and Lymphocytes(White Blood Cells) | Remove foreign cells from inside the body. Lymphocytes produce antibodies that attach to antigens on foreign cells to destroy them. Phagocytes surround foreign cells and digest them |
| Plasma | Straw-coloured liquid which carries dissolved substances like glucose, carbon dioxide and urea |
| Platelets | Fragments of cells with no nuclei which clot the blood at site of injury |
| Blood Vessel | Adaptions |
| Arteries – carry blood from heart to body | Narrow tube with thick layer of elastic and muscle fibres to withstand pressure increases. They contract to allow smooth blood flow. Wave of stretching felt as a pulse. |
| Capillaries – carry blood through tissues and organs | Wall is once cell thick to allow faster diffusion if substances into and out of the capillary. Very narrow tube |
| Veins – carry blood from body to heart | Wide tube with a thin, flexible wall. Blood flows under low pressure, pushed along by muscles with valves preventing flow the wrong way |
There are four chambers in the heart. Blood enters the right atrium from the body through the vena cava, while blood from the lungs enters the left atrium through the pulmonary vein. The muscles around the walls contract when the chambers are filled, and blood enters the ventricles, and a contraction of muscles in the ventricle walls force blood out of the heart through the aorta or the pulmonary artery. Heart valves stop blood from flowing the wrong way by closing when parts of the heart contract.
Heart rate is the number of times the heart beats in a minute. Stroke volume is the volume of blood pushed into the aorta in each beat. Cardiac output is the volume of blood pushed into the aorta each minute, calculated using the following equation.
Cellular Respiration
Cellular Respiration is an exothermic reaction which occurs continuously in living organisms to release energy for metabolic processes.
Aerobic Respiration occurs in mitochondria: Glucose + Oxygen → Carbon Dioxide + Water
Circulation ensures that reactants reach cells through the bloodstream and waste products are carried away.
During exercise the muscles need more energy and the rate of aerobic respiration increases, meaning more oxygen needs to be carried around the body in a shorter time. During strenuous activity, anaerobic respiration starts in the cytoplasm where glucose is converted to lactic acid and energy is released in small quantities. This causes muscles to tire quickly.
Heart rate remains high after exercise so extra oxygen can replace oxygen lost in blood, and energy is required to remove lactic acid.
CORE PRACTICAL: Respiration Rates
A – Collect a tube containing soda lime, which absorbs CO2, held in place by cotton wool
B – Shake some small organisms into the tube
C – Insert bung and capillary tube, and set up a control tube
D – Place tubes into a rack in a water bath at a fixed temperature and wait for organisms to adjust
E – Pour coloured liquid into the ends of the capillary tubes and mark the position. After five minutes, remark and measure distance travelled
F – Repeat at different temperatures