4._Gaseous_exchange_(LM)[1].pdf

Unit 4: Gaseous Exchange (AQA Pages 60-61) Making Distinctions Cellular respiration is a chemical process that takes place inside cells. In respiration, glucose (a sugar) is broken down to release energy. The energy released during respiration is stored in the form of the molecule...

Unit 4: Gaseous Exchange (AQA Pages 60-61) Making Distinctions Cellular respiration is a chemical process that takes place inside cells. In respiration, glucose (a sugar) is broken down to release energy. The energy released during respiration is stored in the form of the molecule ATP. Breathing is a set of muscular movements that move air in and out of the lungs. It allows for ventilation of the lungs. Figure 1: The interaction between different organ-systems Gaseous exchange is the physical process by which gases move passively by diffusion across a surface. Ms Marika Pisani/Ms Lorraine Vella 1 Characteristics of Gaseous Exchange Surfaces Unicellular organisms and small multicellular organisms do not need specialized gaseous exchange surfaces to obtain oxygen and remove carbon dioxide. This is because: The organism has a large surface area over which exchange of gases takes place at an efficient rate. The distance between the surface and the centre of the organism is small. Figure 2: Gaseous exchange in unicellular organisms Large multicellular organisms have a small surface area to volume ratio. Diffusion of gases over the surface of the organism cannot supply all the cells with the oxygen they need and remove carbon dioxide at an efficient rate. Rather, these organisms need a specialised gaseous exchange surface. An efficient gaseous exchange surface has several adaptations, such as the following: a large surface area over which the gases can be exchanged. a membrane which is fully permeable to the gases involved. a thin membrane so that the distance through which the gases move is very short. a layer of moisture (since gases must be in solution before they can pass across a cell membrane) mechanisms which help maintain a high concentration gradient between the external environment and the internal environment, such as: Ms Marika Pisani/Ms Lorraine Vella 2  o a ventilation system which allows fresh air to reach the gaseous exchange surface. o a network of blood capillaries which allows for rapid movement of gases to and from the rest of the organism. Gaseous Exchange in Humans The system is made up of the following main structures: The human gas exchange system. Ms Marika Pisani/Ms Lorraine Vella 3 Structure Description Function Nasal The space behind the nose. As air passes through the nasal Cavity cavity, it is: warmed by the blood passing through the blood capillaries of the nasal cavity. moistened by mucus secreted by goblet cells. filtered by hairs (cilia)in the nose to remove bacteria and dust particles. Trachea A tube supported by rings of The trachea carries air from the (windpipe) cartilage to keep it open. nasal cavity to the lungs. At the top of the trachea is a piece if cartilage, the epiglottis, which closes the trachea during swallowing of food. Lung A spongy organ containing: Air passes down the trachea, a bronchus (plural: bronchi). along the bronchus and along In the thorax, the trachea the bronchioles to reach the divides into 2 branches called alveoli. the left and right bronchus. One bronchus goes to each lung and then branches out into smaller tubes called bronchioles. several bronchioles. At the end of each bronchiole are many tiny air sacs called alveoli. millions of air sacs called Gaseous exchange occurs in the alveoli (singular alveolus) alveoli. Ribcage A frame consisting of bone and It protects the lungs and is used cartilage. in ventilation. Intercostal muscles are present between the ribs. Ms Marika Pisani/Ms Lorraine Vella 4  Diaphragm A strong sheet of muscle and It protects the lungs and is used elastic tissue which stretches in ventilation. across the body, under the lungs. Figure 3: The human respiratory system The alveolus is an effective gaseous exchange surface since: The number of alveoli present in the lungs, and the shape of the alveolus provide a large surface area for diffusion. The wall of the alveolus is permeable to oxygen and carbon dioxide. The wall of the alveolus is one-cell thick. Therefore, there is a short distance between the air in the lungs and the blood capillaries. Moisture in the lungs allows gases to diffuse more easily. The concentration of oxygen in the alveolus is higher than the concentration of oxygen in the blood capillaries. As a result, oxygen diffuses from the alveolus into the blood. This high concentration gradient is maintained in two ways: Ms Marika Pisani/Ms Lorraine Vella 5  o The numerous blood capillaries surrounding the alveolus carry blood rich in carbon dioxide into the lungs and blood rich in oxygen away from the lungs. o Ventilation brings air rich in oxygen into the lungs. Figure 4: Gaseous exchange in the alveolus The following table explains how ventilation takes place. Inhaling (breathing in) Exhaling (breathing out) Movement of intercostal muscles contract intercostal muscles relax and Ribs and pull ribs upwards and pull ribs downwards and outwards inwards Movement of contracts and moves relaxes and returns to its diaphragm downwards original shape Ms Marika Pisani/Ms Lorraine Vella 6 Result The volume of the lungs The volume of the lungs increases. decreases. Refer to AQA The pressure in the lungs The pressure in the lungs Biology, page becomes lower than the becomes higher than the 60, Figure 2. external atmospheric external atmospheric pressure. pressure. Due to this difference in Due to this difference in pressure, air is forced into the pressure, air is forced out of lungs. the lungs. Ms Marika Pisani/Ms Lorraine Vella 7 Exercise and breathing rate All the cells in your body need oxygen for respiration and all this oxygen is supplied by the lungs and carried away to all parts of the body by the blood. During exercise the breathing rate increases because the heart needs to pump more blood around the body, so that it can supply all the cells with oxygen faster. The more intense the exercise, the faster the breathing rate. The Effects of Cigarette Smoking Cigarettes contain tobacco leaves. As these leaves burn, they release several different chemicals which are inhaled into the lungs and absorbed into the blood (which transports them around the body). Many of these chemicals, including the following, are linked to disease. Nicotine is a highly addictive substance. It also increases the heart rate. Certain chemicals stop the mechanism that removes mucus from the respiratory system. The mucus (together with the dirt and pathogens it traps) accumulates in the lungs. This causes coughing and increases the chances of infections. Tar further increases the chances of bronchitis (inflammation and infection of the bronchi). Tar is a carcinogen. It forms a sticky layer on the tissues present in the breathing system and increases the chances of cancer in the lungs, throat, and trachea, amongst others. The chemicals in tobacco smoke weaken the walls of the alveoli. As a result, during coughing, the alveoli burst. This condition is known as emphysema, and it is irreversible. The alveoli become unable to support the bronchioles. As a result, the bronchioles become narrower. Healthy Lungs Bronchitis Emphysema Figure 5: Examples of chronic obstructive pulmonary disease Ms Marika Pisani/Ms Lorraine Vella 8 Cigarette smoking affects the efficiency of the lungs since: Carbon monoxide (one of the chemicals produced by cigarette smoke) combines with red blood cells more easily than oxygen. Therefore, there is an oxygen shortage in blood. If emphysema develops, the alveoli start to burst, and the surface area of the lungs is reduced. Furthermore, emphysema narrows the bronchioles. This blockage traps air inside the lungs. The concentration gradient between the air in the alveoli and the blood is reduced. The rate of diffusion slows down. In the case of lung cancer, tumours can grow in or press on the airways of the lungs. This blockage traps air in the lungs. Lung cancer can also cause a build-up of fluid in the lungs. This fluid makes it difficult for the lungs to expand. Gaseous Exchange in Plants The leaf is an organ. It is responsible for several functions, including: production of food by photosynthesis diffusion of gases for respiration and photosynthesis (these processes require the use of oxygen and carbon dioxide respectively) food storage Figure 6: Generalised cross-section through the leaf Ms Marika Pisani/Ms Lorraine Vella 9 Structure Description Function Spongy The spongy mesophyll Spongy mesophyll cells carry Mesophyll consists of layers of loosely out photosynthesis. packed cells that do not The air spaces allow gases to have a particular shape. diffuse throughout the leaf. The spongy mesophyll cells Many spongy mesophyll cells contain chloroplasts. are in contact with the air The spongy mesophyll cells spaces, providing a large are separated by several air surface area for gas exchange spaces. to happen. The cell membrane of the spongy mesophyll cells is thin, permeable and moist. This allows for more efficient gaseous exchange. Stoma A stoma consists of a When the stoma opens, gases (plural: stomatal pore surrounded by can diffuse in and out of the stomata) two guard cells. leaf. Guard cells are specialised This ventilation allows the leaf (Figure 1 cells which open and close to maintain a high on Page the stomatal pore. concentration gradient 66) between the air spaces and the cells. The stomata close to prevent excessive water loss from the leaf. This helps maintain a moist environment inside the leaf. Ms Marika Pisani/Ms Lorraine Vella 10 Practice Questions Characteristics of Gaseous Exchange Gaseous Exchange in Plants Surfaces Sample Paper Section A question 4 AQA Biology page 23 questions 1 and 3 AQA Biology page 24 questions 4 and 5 Gaseous Exchange in Humans AQA Biology page 61 questions 1-3 AQA Biology page 70 question 4 Sample Paper Section A question 5 SEC Sept 2017 Paper 2B question 5 SEC May 2018 Paper 1 question 5 SEC Sept 2020 Paper 2A question 4 SEC May 2021 Paper 1 question 5 Ms Marika Pisani/Ms Lorraine Vella 11