Chapter 11: Respiration and Gas Exchange PDF

# Chapter 11: Respiration and Gas Exchange ## Respiration and Gas Exchange In this chapter you will: - Find out how the body uses energy, and how aerobic and anaerobic respiration supply this energy. - Learn equations for aerobic and anaerobic respiration. - Describe how gas exchange happens in t...

# Chapter 11: Respiration and Gas Exchange ## Respiration and Gas Exchange In this chapter you will: - Find out how the body uses energy, and how aerobic and anaerobic respiration supply this energy. - Learn equations for aerobic and anaerobic respiration. - Describe how gas exchange happens in the lungs, and the features of gas exchange surfaces. - Investigate the differences in composition between inspired and expired air. - Explain how breathing happens. - Explain how activity affects breathing rate. ## 11.1 Respiration ### Using energy Every living cell needs energy. In humans, our cells need energy for: - Contracting muscles, so that we can move parts of the body. - Making protein molecules (protein synthesis) by linking together amino acids into long chains. - Cell division, so that we can repair damaged tissues and can grow. - Active transport, so that we can move substances across cell membranes up their concentration gradients. - Growth, by building new cells, which can then divide to form new cells. - Transmitting nerve impulses, so that we can transfer information quickly from one part of the body to another. - Producing heat inside the body, to keep the body temperature constant, even if the environment is cold. All of this energy comes from the food that we eat. The food is digested - that is, broken down into smaller molecules - which are absorbed from the small intestine into the blood. The blood transports the nutrients to all the cells in the body. The cells take up the nutrients that they need. The main nutrient used to provide energy in cells is glucose. Glucose contains a lot of chemical energy. In order to make use of this energy, cells have to break down the glucose molecules and release the energy from them. They do this in a series of metabolic reactions called respiration. Like all metabolic reactions, respiration involves the action of enzymes. ### Aerobic Respiration Most of the time, our cells release energy from glucose by combining it with oxygen. This is called aerobic respiration. This happens in a series of small steps, each one controlled by enzymes. Most of the steps in aerobic respiration take place inside mitochondria. We can summarise the reactions of aerobic respiration as an equation: $glucose + oxygen \rightarrow carbon \space dioxide + water$ The balanced equation is: $C_6H_{12}O_6 + ​6O_2 \rightarrow 6CO_2 + 6H_2O $ ### Anaerobic Respiration It is possible to release energy from glucose without using oxygen. It is not as efficient a process as aerobic respiration, and not much energy is released per glucose molecule. This is called anaerobic respiration ('an' means without). Anaerobic respiration happens in the cytoplasm of a cell, not in mitochondria. Yeast, a single-celled fungus, often respires anaerobically. It breaks down glucose to alcohol: $ \space glucose \rightarrow alcohol + ​carbon \space dioxide $ $C_6H_{12}O_6 \rightarrow 2C_2H_5OH + ​2CO_2$ You can see from the equation that carbon dioxide is made in this process. Plants can also respire anaerobically like this, but only for short periods of time. Some of the cells in your body, particularly muscle cells, can also respire anaerobically for a short time. But they do not do this in the same way as yeast. They make lactic acid instead of alcohol, and no carbon dioxide is produced. This happens when you do vigorous exercise, and your lungs and heart cannot supply oxygen to your muscles as quickly as they are using it. The muscle cells are able to release at least some energy from glucose without using oxygen, just to keep them going until oxygen is available again. $glucose \rightarrow lactic \space acid$ Table 11.1 compares aerobic and anaerobic respiration: | Respiration Type | Description | |---|---| | Aerobic respiration | - Involves chemical reactions in cells that break down glucose to release energy. <br> - Uses oxygen. <br> - No alcohol or lactic acid is made. <br> - A large amount of energy is released from each molecule of glucose.<br> - Carbon dioxide is made. | |Anaerobic respiration | - Involves chemical reactions in cells that break down glucose to release energy. <br> - Does not use oxygen. <br> - Alcohol (in yeast and plants) or lactic acid (in animals) is made. <br> - Much less energy is released from each molecule of glucose.<br> - Carbon dioxide is made by yeast and plants, but not by animals. | Table 11.1: A comparison of aerobic and anaerobic respiration. ## 11.2 Gas Exchange in Humans ### Gas Exchange Surfaces If you look back at the aerobic respiration equation near the beginning of this chapter, you will see that two substances are needed: they are glucose and oxygen. The way in which cells obtain glucose is described in Chapters 6 and 7. Animals get sugars such as glucose from carbohydrates which they eat. Plants make theirs by photosynthesis. Oxygen is obtained in a different way. Animals and plants get their oxygen directly from their surroundings - either from the air for terrestrial (land-living) organisms or from oxygen dissolved in water for aquatic (water-living) ones. If you look again at the aerobic respiration equation, you can see that carbon dioxide is made. This is a waste product, and it must be removed from the organism. In organisms, there are special areas where the oxygen enters and carbon dioxide leaves. One gas is entering, and the other leaving, so these are surfaces for gas exchange. **The gas exchange surfaces** have to be permeable, so that oxygen and carbon dioxide can move easily through them. They have other characteristics which help the process to be quick and efficient: - They are thin to allow gases to diffuse across them quickly. - They are close to an efficient transport system to take gases to and from the exchange surface. - They have a large surface area, so that a lot of gas can diffuse across at the same time. - They have a good supply of oxygen. ## The Human Breathing System Figure 11.3 shows the structures which are involved in gas exchange in a human. The most obvious are the two lungs. Each lung is filled with many tiny air spaces called air sacs or alveoli. It is here that oxygen diffuses into the blood, so the surface of the alveoli is the gas exchange surface. Because they are full of air spaces, lungs feel very light and spongy to touch. The lungs are supplied with air through the windpipe or trachea. - Larynx - Trachea - Cartilage supporting trachea - Bronchus - Bronchiole - Alveoli (air sacs) - Pleural membranes - Pleural fluid - Diaphragm ## The Nose and Mouth Air can enter the body through either the nose or mouth. Hairs in the nose trap dust particles in the air. Inside the nose are some thin bones which are covered with a thin layer of cells. Some of these cells, called goblet cells, make a liquid containing water and mucus (Figure 11.4). The water in this liquid evaporates into the air in the nose and moistens it. - Mucus released from goblet cell - Cilia beating - Ciliated cell - Goblet cell - Nucleus - Basement membrane ## The Trachea From the nose or mouth, the air then passes into the windpipe or ***trachea***. Just below the epiglottis is the voice box or ***larynx***. This contains the vocal cords. The vocal cords can be tightened by muscles, so that they make sounds when air passes over them. The trachea has rings of cartilage around it. As you breathe in and out, the pressure of the air in the trachea increases and decreases. The cartilage helps to prevent the trachea collapsing at times when the air pressure inside is lower than the pressure of the air outside it. ## The Bronchi The trachea goes down through the neck and into the ***thorax***. The ***thorax*** is the upper part of your body from the neck down to the bottom of the ribs and diaphragm. In the thorax, the trachea divides into two. The two branches are called the right and left bronchi (singular: bronchus). One bronchus goes to each lung and then branches out into smaller tubes called ***bronchioles***. ## Alveoli There are many tiny air sacs or alveoli at the end of each bronchiole (Figure 11.5). This is where gas exchange takes place. - Bronchiole - Air - Blood vessels bring blood without much oxygen from the pulmonary arteries. - Alveolus - Blood vessels return oxygenated blood to the pulmonary veins. ## Breathing Movements To make air move in and out of the lungs, you must change the volume of your thorax. First, you make it large so that air is sucked in. Then you make it smaller again so the air is squeezed out. This is called **breathing**. Muscles in two parts of the body help you to breathe. Some of them, called the **intercostal muscles**, are between the ribs (Figure 11.8). The others are in the **diaphragm**. The diaphragm is a large sheet of muscle and elastic tissue which stretches across your body, underneath the lungs and heart. - Backbone - Sternum - External intercostal muscle - Internal intercostal muscle - Rib - Cartilage at ends of ribs, allowing them to swing up and down ### Breathing in (inspiration) When breathing in, the muscles of the diaphragm contract. This pulls the diaphragm downwards, which increases the volume in the thorax (Figure 11.9). At the same time, the external intercostal muscles contract. This pulls the rib cage upwards and outwards. This also increases the volume of the thorax. - The diaphragm is lowered. - The rib cage is raised. - The volume of the thorax increases. So air is drawn into the lungs. ### Breathing out (expiration) - The diaphragm springs up. - The rib cage is lowered. -The volume of the thorax decreases. So air is forced out of the lungs. ## Control of breathing rate The rate at which your breathing muscles work - and therefore your breathing rate - is controlled by the brain. The brain constantly monitors the pH of the blood that flows through it. If there is a lot of carbon dioxide or lactic acid in the blood, this causes the pH to fall. When the brain senses this, it sends nerve impulses to the diaphragm and the intercostal muscles, stimulating them to contract harder and more often. The result is a faster breathing rate and deeper breaths. ## Summary Organisms need energy for muscle contraction, protein synthesis, cell division, active transport, growth, transmitting nerve impulses, and maintaining a constant body temperature. **Respiration** is a metabolic reaction that takes place in all living cells and releases energy from glucose and other nutrient molecules. Like all metabolic reactions, respiration is controlled by enzymes. In yeast, an increase in temperature causes an increase in the rate of respiration, but at temperatures above about 40 °C, enzymes are denatured, and so the rate decreases. **Aerobic respiration** happens in mitochondria. Oxygen is combined with glucose, releasing a lot of energy and producing carbon dioxide and water. **Anaerobic respiration** happens in the cytoplasm. Glucose is broken down without using oxygen, releasing a small amount of energy. In humans, lactic acid is produced. In yeast, carbon dioxide and ethanol are produced. The place where oxygen enters an organism's body and carbon dioxide leaves is called the **gas exchange surface**. In terrestrial animals, these surfaces have a large surface area, a thin surface, a good blood supply, and good ventilation with air. In humans, the gas exchange surface is the ***alveoli*** in the lungs. Air moves to them through the trachea, bronchi, and bronchioles. **Inspired air** contains more oxygen and less carbon dioxide than ***expired air***. The **intercostal muscles and the diaphragm** cause breathing movements, which ventilate the lungs. When breathing in, the diaphragm and external intercostal muscles contract, which increases the volume of the thorax and therefore reduces the air pressure in the lungs. To breathe out, these muscles all relax. **Physical activity** causes the rate and depth of breathing to increase, in order to supply extra oxygen to muscles. The depth and rate of breathing is controlled by the brain, which detects a decrease in the pH of the blood when it contains more carbon dioxide or lactic acid than usual. During exercise, faster aerobic respiration by muscles produces more carbon dioxide, and therefore results in an increase in breathing depth and rate. During intense physical activity, muscles may use anaerobic respiration as well as aerobic respiration, to supply extra energy. After exercise has finished, the lactic acid produced is broken down in the liver by aerobic respiration, which requires extra oxygen. This is called an **oxygen debt.** ## Exam-style questions 1. What are the products of anaerobic respiration in human muscle cells? A. Carbon dioxide only B. Carbon dioxide and water C. Lactic acid only D. Lactic acid and water 2. What is the approximate concentration of carbon dioxide in expired air? A. 0.16% B. 0.4% C. 4% D. 16% 3. Which process in living organisms does __not__ use energy released in respiration? A. Cell division to make new cells for growth B. Diffusion of oxygen from the alveoli into the blood C. Maintaining a constant body temperature D. Passage of impulses along a nerve cell 4. This equation for anaerobic respiration in yeast is not balanced. $C_6H_{12}O_6 \rightarrow XC_2H_5OH + YCO_2 $ What numbers do X and Y represent, to balance the equation? XY A. 11 B. 22 C. 23 D. 32 5. Which statement describes what happens when the diaphragm muscles contract? A. The volume of the thorax decreases, so the pressure decreases, and air moves into the lungs. B. The volume of the thorax decreases, so the pressure increases, and air moves out of the lungs. C. The volume of the thorax increases, so the pressure decreases, and air moves into the lungs. D. The volume of the thorax increases, so the pressure increases, and air moves out of the lungs. ## Exercise 11.1: Focus Check your understanding of respiration by correcting someone else's mistakes. Here are some statements that a student made, about respiration. Each statement has a mistake in it. Rewrite each statement, correcting the mistakes. 1. Every cell uses energy to help it to respire. 2. Aerobic respiration produces energy by combining nutrient molecules, such as glucose, with oxygen. 3. Anaerobic respiration happens in mitochondria. 4. In human muscle, both aerobic respiration and anaerobic respiration produce carbon dioxide. 5. Anaerobic respiration releases much more energy from each glucose molecule than aerobic respiration does. ## Exercise 11.2: Practice - Practise constructing and completing a results table. - Apply your knowledge of respiration and photosynthesis to make a prediction and explanation. A student had a fish tank in which she kept tropical fish. She knew it was meant to be a good idea to keep living plants in the tank as well. She wanted to find out how the plants affected the concentration of carbon dioxide in the water. Figure 11.1 shows the apparatus that she set up. She used hydrogencarbonate indicator solution because it is yellow when it contains a large amount of carbon dioxide, orange with a small amount, and red when it contains no carbon dioxide at all. - A - B - C - D The student left all four tubes in a sunny place for 30 minutes. When she looked at the tubes again, she found the indicator had turned yellow in tube A, deep red in tube B, and stayed orange in tubes C and D. 2. Draw a results table in the space below and fill it in to show the student's results. 3. Explain the results in each tube. **Tip**: Remember that living organisms all respire all the time, and that plants also photosynthesise in the light. - Tube A - Tube B - Tube C - Tube D 4. Predict the results that would be obtained if all the tubes were left in the dark. 5. Discuss what these results and your predictions in 4 suggest about whether or not it is good to have living plants in a fish tank. ## Exercise 11.4 - Practise handling data provided in a table, including making a calculation. - Compare two sets of data. - Use information provided and your own understanding, to suggest an explanation for patterns in data. ### Focus Rat lungs have a similar structure to human lungs. Researchers measured the surface area of the alveoli in the lungs of female and male rats of different ages. They also measured the mass of each rat and calculated the number of square centimetres of alveolar surface area per gram of body mass. Their results are shown in Table 11.1. |Age / days | Ratio of alveolar surface area to body mass / cm² per gram | |---|---| |21 | Females: 21.6 <br> Males: 23.1 | |33 | Females: 15.4 <br> Males: 15.2 | |45| Females: 12.9 <br> Males: 12.1 | |60 | Females: 13.4 <br> Males: 10.9 | |95 | Females: 13.4 <br> Males: 9.4 | **Table 11.1**: Ratio of alveolar surface area to body mass in male and female rats.