O Level Biology Topical Revision Notes PDF

O Level Biology Topical Revision Notes is a comprehensive guide based on the Topical O LEVEL...

O Level Biology Topical Revision Notes is a comprehensive guide based on the Topical O LEVEL REVISION latest syllabus. It is written to provide candidates sitting for the O Level Biology examination with thorough revision material. Important concepts are presented in simple and concise points for easier reference. Relevant examples and diagrams NOTES are incorporated into the notes to facilitate the understanding of important concepts. TOPICAL REVISION NOTES O Level Topical Revision Notes Series: Mathematics Additional Mathematics BIOLOGY Physics C Chemistry Biology M Y Science Physics CM Science Chemistry MY Science Biology Lye Ai Fern BSc (Hons), PGDE CY CMY BIOLOGY K Includes ü Comprehensive Revision Notes ü Effective Study Guide ISBN 978 981 288 018 5 BIOLOGY Lye Ai Fern BSc (Hons), PGDE SHINGLEE PUBLISHERS PTE LTD 120 Hillview Avenue #05-06/07 Kewalram Hillview Singapore 669594 Tel: 6760 1388 Fax: 6762 5684 e-mail: [email protected] http://www.shinglee.com.sg All rights reserved. No part of this publication may be reproduced in any form or stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission in writing of the Publishers. First Published 2016 ISBN 978 981 288 018 5 Printed in Singapore  PREFACE O Level Biology Topical Revision Notes has been written in accordance with the latest syllabus issued by the Ministry of Education, Singapore. This book is divided into 16 topics, each covering a topic as laid out in the syllabus. Important concepts are highlighted in each topic, with relevant examples and diagrams to help students better understand the concepts. We believe this book will be of great help to teachers teaching the subject and students preparing for their O Level Biology examination. Preface iii  CONTENTS Topic 1 Cell Structure and Organisation 1 Topic 2 Movement of Substances 6 Topic 3 Biological Molecules 11 Topic 4 Nutrition in Humans 20 Topic 5 Nutrition in Plants 27 Topic 6 Transport in Flowering Plants 32 Topic 7 Transport in Humans 39 Topic 8 Respiration in Humans 51 Topic 9 Excretion in Humans 58 Topic 10 Homeostasis 65 Topic 11 Co-ordination and Response in Humans 70 Topic 12 Reproduction 82 Topic 13 Cell Division 95 Topic 14 Molecular Genetics 106 Topic 15 Inheritance 112 Topic 16 Organisms and their Environment 124 iv Contents  TOPIC Cell Structure and Organisation 1 Objectives Candidates should be able to: (a) identify cell structures (including organelles) of typical plant and animal cells from diagrams, photomicrographs and as seen under the light microscope using prepared slides and fresh material treated with an appropriate temporary staining technique: chloroplasts cell surface membrane cell wall cytoplasm cell vacuoles (large, sap-filled in plant cells, small, temporary in animal cells) nucleus (b) identify the following membrane systems and organelles from diagrams and electron micrographs: endoplasmic reticulum mitochondria Golgi body ribosomes (c) state the functions of the membrane systems and organelles identified above (d) compare the structure of typical animal and plant cells (e) state, in simple terms, the relationship between cell function and cell structure for the following: absorption – root hair cells conduction and support – xylem vessels transport of oxygen – red blood cells (f) differentiate cell, tissue, organ and organ system 1.1 Animal cell 1. The following is a diagram of a generalised animal cell as seen under an electron microscope: centriole cell surface cytoplasm membrane rough endoplasmic reticulum nucleus nucleolus vesicle smooth endoplasmic reticulum vacuole free ribosomes mitochondrion Golgi apparatus A generalised animal cell Cell Structure and Organisation 1  2. The cell surface membrane or plasma membrane is a partially permeable membrane surrounding the cytoplasm of the cell. It controls substances entering or leaving the cell. 3. The cytoplasm is the gel-like matrix embedded with organelles. It is the site of most cellular activities. 4. The cell vacuoles are small fluid-filled spaces bound by a membrane. In animal cells they store water and food substances. They are usually not permanent. 5. The nucleus is an organelle surrounded by an envelope called the nuclear envelope. It contains darker bodies called nucleoli (singular: nucleolus) and thread-like structures called chromatin which are made of DNA. The nucleus controls cellular activities such as growth, repair, and cell division. 6. The endoplasmic reticulum (ER) is a network of membranes forming tubes and flattened spaces. There are two types of ER: (a) The smooth endoplasmic reticulum (SER) does not have ribosomes attached to it. It synthesises fats and steroids such as sex hormones. It also contains enzymes that detoxify drugs and poisons. (b) The rough endoplasmic reticulum (RER) is studded with ribosomes. Ribosomes in the cell can either be free ribosomes (i.e. they lie freely in the cytoplasm) or be attached to the membrane of the RER. Ribosomes synthesise proteins. 7. All proteins made in the RER depart in membrane-bound vesicles to the Golgi apparatus. 8. The Golgi apparatus resembles a stack of flattened disc-shaped spaces surrounded by membranes. It stores, sorts and modifies substances made by the ER, and packages them in vesicles to be secreted out of the cell. 9. The mitochondria (singular: mitochondrion) are small elongated organelles with folded inner membranes. Aerobic respiration takes place in the mitochondria. Aerobic respiration is the process where energy is extracted from food substances in the presence of oxygen. This energy is used by the cell to perform cellular activities such as growth and cell division. 10. The centrioles are a pair of barrel-shaped structures at right angles to each other. They play a role in cell division. Centrioles are usually absent in plants. 2 TOPIC 1 1.2 Plant cell 1. The following is a diagram of a generalised plant cell as seen under an electron microscope: Golgi apparatus free ribosome vesicle chloroplast smooth endoplasmic reticulum nucleolus nucleus rough endoplasmic reticulum large central vacuole tonoplast cell surface membrane mitochondrion cell wall cytoplasm A generalised plant cell 2. The plant cell contains most of the structures present in an animal cell, with a few differences: (a) Instead of many small vacuoles, plant cells have a large central vacuole filled with cell sap, surrounded by a membrane called the tonoplast. Cell sap is mainly made up of water, with dissolved amino acids and mineral salts. Besides storage, the vacuole also takes in waste products and water. (b) Presence of a cellulose cell wall – The cell wall is non-living and fully permeable. It protects the cell from injury and gives the cell its shape. (c) Presence of chloroplasts – Chloroplasts are oval membrane-bound organelles filled with chlorophyll. They are the sites of photosynthesis, which is the process by which plants make food. (d) Centrioles are absent. Note: The structures visible under a light microscope would be: cell membrane, cytoplasm, nucleus, vacuoles, cell wall and chloroplasts. Cell Structure and Organisation 3  1.3 Adaptation of cells to their functions 1. Red blood cells deliver oxygen to the body tissues via the blood. Adaptations to this function include: (a) Red blood cells contain haemoglobin, an oxygen-carrying protein. (b) Red blood cells have no nucleus, so they have a flattened biconcave shape with a dumbbell-shaped cross section. This enables them to have a higher surface area to volume ratio for faster diffusion of oxygen. It also allows the cell to be more flexible when squeezing through blood capillaries. dumbbell-shaped cross section Cross-section of a red blood cell 2. Xylem vessels are elongated hollow tubes that are made of xylem cells linked end to end. Xylem cells are dead at maturity. They conduct water and mineral salts from the roots to the leaves of the plant. They also play a role in mechanical support. Adaptations to these functions include: (a) Absence of protoplasm and cross-walls which could impede water flow through the lumen (internal cavity) (b) Deposition of lignin on the cell walls which strengthens vessel walls, providing support lumen lumen lumen lumen lignin Xylem vessels 4 TOPIC 1  3. Root hair cells are cells which extend into the soil to absorb water and mineral salts. An adaptation to this function is a long and narrow structure called the root hair, which extends into the soil to absorb water. This increases the surface area to volume ratio of the cell, resulting in faster absorption. soil particles cellulose cell wall root hair vacuole nucleus A root hair cell 1.4 Organisation of a multicellular organism 1. The cell is the most basic unit of a living organism that can be classified as living. 2. A group of cells of the same type that are found near each other and carry out the same function comprises a tissue. 3. An organ is made up of different tissues working together to perform a specific function or a group of functions within an organism. An organ has a distinct shape which allows it to carry out its function well. 4. A group of functionally-related organs form an organ system. 5. Many organ systems working together make up a multicellular organism. Cell Structure and Organisation 5  TOPIC Movement of Substances 2 Objectives Candidates should be able to: (a) define diffusion and describe its role in nutrient uptake and gaseous exchange in plants and humans (b) define osmosis and describe the effects of osmosis on plant and animal tissues (c) define active transport and discuss its importance as an energy-consuming process by which substances are transported against a concentration gradient, as in ion uptake by root hairs and uptake of glucose by cells in the villi 2.1 Diffusion 1. Diffusion is the net (overall) movement of molecules from a region of higher concentration to a region of lower concentration down a concentration gradient. Concentration refers to the number of particles per unit volume. 2. A concentration gradient is the difference in concentration between a region of higher concentration of a substance and a region of lower concentration of the substance. 3. When the concentration gradient is steeper, the rate of diffusion will be faster. 4. When a concentration gradient exists, diffusion will take place until the particles are evenly distributed throughout the region. 2.2 Diffusion in biological systems 1. Diffusion is an important mode of nutrient uptake and gaseous exchange in cells. 2. The cell surface membrane is a partially permeable membrane that allows gases such as oxygen and carbon dioxide to pass through freely but not some other substances. 3. In cells which undergo respiration, oxygen is continually being used up within the cell. This creates a concentration gradient where oxygen concentration is lower inside the cell than in the surroundings. Thus, dissolved oxygen diffuses into the cell. 6 TOPIC 2  4. Carbon dioxide and other waste products are generated by the cell. This sets up a concentration gradient where the concentration of these substances is higher within the cell than outside. Thus, the substances leave the cell by diffusion. 5. In unicellular organisms such as the amoeba, diffusion is an important mode of nutrient uptake. 2.3 Osmosis 1. Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane. 2. Water potential is a measure of the tendency of water molecules to move from one region to another. Since water is the solvent, forming the volume of a solution, it is not meaningful to think about the concentration of water, i.e. the number of water molecules per unit volume. 3. Water molecules that surround solutes causing them to dissolve are not able to move about freely as they are bound to the solutes. The more concentrated a solution is, the lower the number of freely moving water molecules present, hence the lower the water potential of the solution. As a result, a dilute solution has a higher water potential than a concentrated solution and pure water has the highest water potential. Example A U-tube filled with sucrose solutions of different concentrations was set up as shown in Fig. (a). After a few hours, it was observed that the water level in one arm of the U-tube had increased while the water level in the other arm had decreased as shown in Fig. (b). Describe and explain what had taken place in terms of the movement of the particles in the sucrose solutions. 10% sucrose 20% sucrose solution solution H 2O partially permeable membrane Fig. (a) Fig. (b) Movement of Substances 7  Answer: The 20% sucrose solution is more concentrated than the 10% sucrose solution. Hence, it has a lower water potential as compared to the 10% sucrose solution. The partially permeable membrane does not allow sucrose molecules to pass through as sucrose molecules are too big; it only allows water molecules to pass through. As a result, water will move through the partially permeable membrane by osmosis, from the arm with the 10% sucrose solution (higher water potential) to the arm with the 20% sucrose solution (lower water potential), until the water potentials of the sugar solutions in both arms are the same. The net movement of water molecules is from left to right, hence the right arm has a higher water level at the end of the experiment. 2.4 Osmosis in plant cells 1. Osmosis in living systems refers to the movement of water molecules across the partially permeable cell surface membrane. The cell wall is fully permeable. 2. As the large central vacuole occupies most of the space in a plant cell, the water potential of the cell sap is considered to be the water potential of the plant cell. 3. When a plant cell is immersed in a solution of higher water potential relative to its cell sap, water molecules enter the cell by osmosis. 4. The vacuole increases in size and the expanded cell contents exert pressure on the cell wall. 5. The cellulose cell wall of a plant cell is strong and rigid. 6. The cell wall exerts an opposing pressure on the cell contents, preventing the entry of more water. This prevents the cell from overexpanding and bursting. 7. At this point, the plant cell is very firm or turgid. Turgor pressure provides mechanical support for many non-woody plants. cell wall cell membrane A turgid plant cell 8 TOPIC 2  8. When a plant cell is immersed in a solution with a lower water potential relative to its cell sap, water diffuses out of the cell into the solution by osmosis. 9. The vacuole shrinks and the cell stops exerting pressure on the cell wall. The cell becomes limp or flaccid. If it is placed in a solution with a high water potential at this point, turgidity can be restored. 10. If more water leaves the cell, the vacuole and cytoplasm shrink to such an extent that the cell surface membrane pulls away from the cell wall. The phenomenon in which the cell surface membrane pulls away from the cell wall is called plasmolysis. This can be lethal if the cell is not quickly transferred to a solution with a higher water potential relative to its cell sap. cell wall cell membrane A plasmolysed plant cell 2.5 Osmosis in animal cells 1. When an animal cell is immersed in a solution with a higher water potential relative to its cytoplasm, water diffuses into the cell by osmosis. 2. The cell swells. As more water enters the cell, it swells to such an extent that it bursts. This is because it does not have a cell wall. This process is called cytolysis. An animal cell undergoing cyto cytolysis Movement of Substances 9  3. When an animal cell is immersed in a solution with a lower water potential, relative to its cytoplasm, water diffuses out of the cell by osmosis. 4. The cell shrinks and become dehydrated. In red blood cells, little spikes appear on the cell surface membrane, and the cell is said to have undergone crenation. The animal cell will die if it is not removed from the solution. A crenated red blood cell 2.6 Active transport 1. Active transport is the process in which energy is used to transport substances across a biological membrane against a concentration gradient. 2. The energy used for active transport is obtained through cellular respiration. 3. Uptake of dissolved mineral salts by root hair cells and glucose uptake by cells in the villi of the small intestine are examples of active transport. 10 TOPIC 2  TOPIC Biological Molecules 3 Objectives Candidates should be able to: (a) state the roles of water in living organisms (b) list the chemical elements which make up carbohydrates fats proteins (c) describe and carry out tests for starch (iodine in potassium iodide solution) reducing sugars (Benedict’s solution) protein (biuret test) fats (ethanol emulsion) (d) state that large molecules are synthesised from smaller basic units glycogen from glucose polypeptides and proteins from amino acids lipids such as fats from glycerol and fatty acids (e) explain enzyme action in terms of the ‘lock and key’ hypothesis (f) explain the mode of action of enzymes in terms of an active site, enzyme-substrate complex, lowering of activation energy and enzyme specificity (g) investigate and explain the effects of temperature and pH on the rate of enzyme catalysed reactions 3.1 Role of water in animals 1. About 70% of the human body consists of water. Water is found in cell cytoplasm, blood, digestive juices, tissue fluid, fluid in joints and contained within organs i.e. spinal cord, the brain, the eyes, gastrointestinal tract, etc. 2. Water moderates body temperature. It has a high specific heat capacity, which means that a lot of energy is required to raise the temperature of water by 1°C. Hence, water helps the cell resist changes in temperature. 3. It plays a role in evaporative cooling. Water is a component of sweat, which removes heat from the body when it evaporates. 4. Water is a reactant in certain chemical reactions in the body, such as the hydrolysis of food molecules during digestion. 5. Water is a component of body fluids with lubricative or protective properties such as lubricants in joints, coating the stomach lining, mucus in the oesophagus, and cervical mucus in the female reproductive system. Biological Molecules 11  6. Water is an extremely versatile solvent. More things dissolve in water than in any other solvent. Because of this property, (a) water is the medium in which chemical reactions take place in living organisms, and (b) water serves as a transportation medium. It transports water-soluble food products from the small intestine to other parts of the body and waste materials from cells to the excretory organs for removal. It transports hormones to the target organs or tissues. Blood is the main transport medium in the body. 3.2 Role of water in plants 1. Water is a key reactant in photosynthetic processes. 2. It provides physical support to the plant in the form of turgor pressure. 3. Water is required to transport dissolved mineral salts from the roots to other parts of the plant through xylem vessels. 4. Water is required to transport sugars made in the leaves to other parts of the plant. 3.3 Simple carbohydrates 1. Carbohydrates are organic molecules made up of carbon, hydrogen and oxygen with the general formula for most carbohydrates being C n H2nOn. 2. Carbohydrates are classified into 3 main groups: monosaccharides, disaccharides and polysaccharides depending on the number of basic sugar units they have. 3. Monosaccharides are the most basic unit of carbohydrates and are the simplest form of sugars. Common examples are glucose, fructose and galactose. 4. Disaccharides are formed when two monosaccharides undergo a condensation reaction. Common examples are maltose (formed by 2 glucose units), sucrose (1 glucose, 1 fructose) and lactose (1 galactose, 1 glucose). 5. A condensation reaction is a chemical reaction when two molecules combine together to form a single molecule with the elimination of a water molecule. 6. A disaccharide can be split into its component monosaccharides by undergoing hydrolysis in which a water molecule is added to the disaccharide to break it down into its component monosaccharides. Enzymes are usually required for this process. 12 TOPIC 3 3.4 Test for reducing sugars 1. The test for reducing sugars is known as the Benedict’s test. 2. The main reagent is Benedict’s solution which contains copper(II) sulfate. 3. Reducing sugars can reduce copper(II) ions in Benedict’s solution to copper(I) in the form of copper(I) oxide, a brick-red precipitate. 4. Reducing sugars are glucose, fructose, galactose, maltose and lactose. Sucrose is not a reducing sugar. 5. Procedure: Add 2 cm3 of Benedict’s solution to 2 cm3 of sample solution and mix the contents thoroughly. Heat the test tube in a boiling water bath for 5 minutes. If the sample is an insoluble solid, crush it or cut it into small pieces before adding 2 cm3 of water and 2 cm3 of Benedict’s solution. 6. The colour of the solution changes from green to orange to brick-red with increasing amounts of reducing sugars present. 3.5 Complex carbohydrates 1. Polysaccharides include starch, glycogen and cellulose. They are long chains of glucose molecules linked together in condensation reactions. Each chain may contain thousands of glucose molecules. 2. In starch, the glucose molecules are linked together in long straight chains or branched chains. It is a storage molecule in plants. A starch molecule 3. In glycogen, the glucose molecules are linked together in highly branched chains. It is a storage molecule in animals and fungi. A glycogen molecule Biological Molecules 13  4. In cellulose, the glucose molecules are linked in long straight chains. The linkage between the glucose molecules is not the same as that in starch. Cellulose is the tough material found in cell walls of plants. Cellulose is the fibre necessary in a healthy diet. A cellulose molecule 5. Glycogen and starch are the storage forms of glucose in animal and plant cells respectively. This is because (a) they are insoluble in water and do not affect water potential in cells, (b) they are too large to diffuse out of the cells and thus remain within the cells, (c) they have compact shapes, and (d) they can be easily hydrolysed into glucose for cellular respiration. 3.6 Test for starch 1. The test for starch is called the iodine test. Iodine is added to the sample and the colour change (if any) is observed. 2. Procedure: Add a few drops of iodine solution to the sample. If the sample contains starch, it will turn blue-black in colour. 3.7 Fats 1. Fats (lipids) are organic molecules made up of carbon, hydrogen and oxygen. There is no general formula for fats. The ratio of hydrogen to oxygen is much higher in fats than in carbohydrates, where the ratio of hydrogen to oxygen is 2 : 1. 2. Fats are made from two types of smaller molecules: glycerol and fatty acids. Each fat molecule contains a glycerol molecule and 3 fatty acids. Each fatty acid is linked to the glycerol backbone in a condensation reaction. fatty acid glycerol fatty acid fatty acid A fat molecule 3. When 3 water molecules are added to a fat molecule with the help of enzymes in a hydrolysis reaction, the fat molecule breaks down into fatty acids and glycerol. 14 TOPIC 3  4. Fats are storage molecules that can store a large amount of energy. 5. They are also an important component of cell membranes. 6. Fats are used to make steroids and certain hormones. 7. Fats are also used as insulating material to prevent the loss of body heat. 8. Fat is also a solvent for fat-soluble vitamins. 3.8 Test for fats 1. The test for fats is known as the ethanol emulsion test. 2. Ethanol is added to the sample to allow the fats present in it to dissolve. Water is then added to the ethanolic mixture. Since fats do not dissolve in water, they precipitate out of the solution to give a cloudy white emulsion. 3. Procedure: Add 2 cm3 of ethanol to the sample in a test tube and shake the contents thoroughly. Add 2 cm3 of water and mix the contents. If fats are present, a white emulsion will be observed. 3.9 Proteins 1. Proteins are complex organic molecules made up of carbon, hydrogen, oxygen and nitrogen. They may also contain sulfur. 2. In the form of enzymes, proteins participate in all cellular processes and are responsible for almost everything living organisms do. 3. There are tens of thousands of different proteins, each serving a different function and having a unique structure. 4. Proteins are made up of amino acids. 5. An amino acid is a molecule with the general structure: side R chain amino acidic (carboxyl) group group NH2 CH COOH An amino acid 6. There are about 20 different naturally-occurring amino acids which have different side chains (also known as R groups). 7. Amino acids are combined in many different ways to form different protein molecules. Biological Molecules 15  8. Amino acids link up in a condensation reaction to form a polypeptide chain. The bonds between the amino acids are known as peptide bonds. 9. Proteins are made of one or more polypeptide chains twisted, folded and coiled into a unique 3-dimensional structure. 10. The bonds between the amino acids, peptide bonds, are strong but the bonds that hold the 3-dimensional coiled structures together are weak and can easily be broken by heat or by changes in pH. Examples of such bonds are hydrogen bonds, ionic interactions and van der Waals interactions. weak bonds polypeptide backbone A protein molecule 11. When these bonds are broken, the protein loses its 3-dimensional conformation. This process is called denaturation. Proteins can be denatured if they are heated or placed in an environment with unsuitable pH. Denaturation usually leads to loss of function as proteins require their 3-dimensional shape to function. Denaturation can also cause proteins to lose their solubility and precipitate out of the solution. 12. Many proteins are enzymes, which catalyse chemical reactions within our body. 13. Structural proteins found in muscle cells play a role in movement. 14. Other proteins take part in cell growth, repair and reproduction. 15. Antibodies are proteins in our body that help us fight diseases. 3.10 Test for proteins 1. The test for proteins is known as the biuret test. 2. The main reagents are sodium hydroxide and copper(II) sulfate. 3. Procedure: Add 1 cm3 of sodium hydroxide solution to 1 cm3 of sample solution in a test tube and mix thoroughly. Add a few drops of 1% copper(II) sulfate solution dropwise into the mixture, shaking after each drop. Allow the mixture to stand for 5 minutes. 4. If proteins are present, a violet colouration will be observed. 16 TOPIC 3 3.11 Enzymes 1. Enzymes are biological catalysts that speed up the rate of chemical reactions without being altered in the reaction. They are made of proteins. 2. Enzymes work by lowering the activation energy of a chemical reaction. Activation energy is the amount of energy needed for a reaction to take place. 3. Enzymes allow biochemical reactions to take place without drastic conditions such as high temperatures because less heat energy is required to start a reaction. 4. Enzymes can break down or build up biological molecules. 5. Enzymes are required in small amounts because they remain unchanged in the chemical reactions they catalyse and can be reused. 6. They are substrate-specific. Substrates are the reactants that an enzyme acts on. Each enzyme can only act on the particular substrate of the reaction they are supposed to catalyse. For example, amylase can only digest starch and not cellulose even though they are both polymers of glucose. 7. Therefore, each enzyme catalyses a different reaction. This is due to its unique 3-dimensional structure. 3.12 ‘Lock and key’ hypothesis 1. The ‘lock and key’ hypothesis relates enzyme specificity to the presence of active sites. An active site is the region on an enzyme molecule that the substrate binds to. It is usually a pocket or groove on the surface of the enzyme that is part of the enzyme’s unique 3-dimensional structure. 2. The shape of the active site conforms to the substrate. Only the correct substrate is able to fit into the active site. 3. The process begins when the substrate molecule binds to the active site of the enzyme to form an enzyme-substrate complex. 4. The reaction is then catalysed at the active sites to convert the substrate into product molecules. 5. The product molecules depart from the active site, leaving the enzyme free to catalyse another reaction. Biological Molecules 17  6. The diagram below illustrates the ‘lock and key’ hypothesis for a reaction in which an enzyme breaks down a substrate molecule into 2 product molecules: products substrate active site enzyme enzyme + substrate enzyme-substrate enzyme-products enzyme + products entering active site complex complex leaving active site Process of an enzyme-catalysed reaction 3.13 Effects of temperature on the rate of enzyme-catalysed reactions 1. The effects of temperature on the rate of enzyme-catalysed reactions is shown in the graph below: at optimum temperature rate of reaction 0 temperature Effect of temperature on the rate of reaction 2. At low temperatures, enzymes are inactive and the rate of reaction is very low. Substrate and enzyme molecules have little kinetic energy, hence the frequency of collision is low. In addition, most substrate molecules do not contain sufficient energy to overcome the activation energy required to start a reaction. 3. As temperature increases, the rate of enzyme activity increases. Enzyme activity doubles with every 10°C rise in temperature. This is because the reactants have higher levels of energy, and the substrate molecules are able to collide with active sites more frequently. 18 TOPIC 3  4. At the optimum temperature, enzyme activity is the highest. 5. As the temperature increases beyond the optimum temperature, enzyme activity drops sharply. This is because enzymes are made of proteins, which are denatured at high temperatures. The enzyme loses its 3-dimensional structure and active site conformation due to the breaking of the weak bonds that hold the structure together. 6. At extremely high temperatures, the enzyme is completely denatured and the rate of reaction drops to zero. 3.14 Effects of pH on the rate of enzyme-catalysed reactions 1. The graph showing the effects of pH on the rate of enzyme-catalysed reactions is shown in the graph below: at optimum pH rate of reaction 4 5 6 7 8 9 pH Effect of pH on the rate of reaction of amylase 2. Enzyme activity is the highest at the optimum pH of the enzyme. 3. As the pH increases or decreases from the optimum, enzyme activity sharply decreases. This is because the hydrogen bonds and ionic bonds that hold the 3-dimensional structure are disrupted. The shape of the active site is changed as the enzyme is denatured. 4. At extreme pH levels, the enzyme is completely denatured and the rate of reaction drops to zero. 5. The optimum pH for each enzyme differs. For example, pepsin works best under the acidic conditions in the stomach, while intestinal enzymes work best under alkaline conditions. Biological Molecules 19  TOPIC Nutrition in Humans 4 Objectives Candidates should be able to: (a) describe the functions of main regions of the alimentary canal and the associated organs: mouth, salivary glands, oesophagus, stomach, duodenum, pancreas, gall bladder, liver, ileum, colon, rectum, anus, in relation to ingestion, digestion, absorption, assimilation and egestion of food, as appropriate (b) describe peristalsis in terms of rhythmic wave-like contractions of the muscles to mix and propel the contents of the alimentary canal (c) describe the functions of enzymes (e.g. amylase, maltase, protease, lipase) in digestion, listing the substrates and end-products (d) describe the structure of a villus and its role, including the role of capillaries and lacteals in absorption (e) state the function of the hepatic portal vein as the transport of blood rich in absorbed nutrients from the small intestine to the liver (f) state the role of the liver in carbohydrate metabolism fat digestion breakdown of red blood cells metabolism of amino acids and the formation of urea breakdown of alcohol (g) describe the effects of excessive consumption of alcohol: reduced self-control, depressant, effect on reaction times, damage to liver and social implications 20 TOPIC 4 4.1 Overview of the digestive system salivary gland tongue salivary duct salivary gland buccal cavity (mouth cavity) epiglottis pharynx oesophagus diaphragm liver stomach gall bladder spleen bile duct pyloric sphincter duodenum pancreas pancreatic duct colon jejunum caecum appendix ileum rectum anus The human digestive system 1. Human digestion takes place in the mouth, stomach and small intestine. 2. The alimentary canal consists of the mouth, the oesophagus, the stomach, the small and large intestines and the anus. 3. Other organs associated with digestion include the liver, pancreas, gall bladder and salivary glands. Nutrition in Humans 21  4.2 The mouth 1. Food enters the body through the mouth, or buccal cavity. Physical and chemical digestion takes place in the mouth. In the mouth: (a) Teeth start to break the food into smaller pieces. This makes food easier to swallow and also increases the surface area to volume ratio of the food for the digestive enzymes to work on more efficiently. (b) Salivary glands secrete saliva which moistens the food and makes it easier to swallow. Saliva also contains salivary amylase, an enzyme which breaks down starch into maltose. The optimum pH of salivary amylase is 7. (c) The tongue rolls the food into a bolus, which is then swallowed. 4.3 The oesophagus 1. The food passes through the pharynx and enters the oesophagus. The oesophagus is a muscular tube that leads to the stomach. 2. It is made up of two layers of smooth muscle. The external layer is the longitudinal muscle and the inner layer is the circular muscle. These muscles found along much of the entire length of the alimentary canal. 3. These muscles contract and relax alternately to cause wave-like contractions known as peristalsis. 4. Food moves along the oesophagus due to peristalsis. 5. Digestion of starch by salivary amylase continues in the oesophagus. 4.4 The stomach 1. The food reaches the stomach, which is a muscular bag with elastic walls. 2. The stomach walls form deep pits that contain gastric glands. These glands secrete mucus which protects the stomach walls. They also secrete gastric acid and pepsinogen. 3. Peristalsis in the stomach churns the food to break the food up and mix it thoroughly with gastric juice. 4. Gastric acid is hydrochloric acid with pH 2. Gastric acid (a) stops the activity of salivary amylase by denaturing it, (b) changes the inactive form of pepsin, pepsinogen, into the active form, pepsin, and (c) kills germs and bacteria. 5. Pepsin is a protease. The optimum pH for pepsin is about 2. 22 TOPIC 4  6. Food leaves the stomach in small quantities at regular intervals, and enters the small intestine through the pyloric sphincter as a semi-liquid mass known as chyme. The pyloric sphincter is a ring of muscle at the base of the stomach that allows chyme to pass into the small intestine in small amounts at a time. Allowing the food to pass into the small intestine in small quantities ensures that the food can be completely digested by the enzymes in the intestines. If the person had a heavy meal, the contents of the stomach may be emptied over a period of up to three hours. 4.5 The small intestine 1. The small intestine is divided into three parts: the duodenum, jejunum and ileum. 2. Food is moved through the small intestine by peristalsis. 3. In the duodenum, chyme from the stomach mixes with digestive juices from the pancreas, liver, gall bladder and intestinal glands. 4. The pancreas produces pancreatic juice, which is an alkaline solution containing trypsinogen, pancreatic amylase and pancreatic lipase. Pancreatic juice enters the duodenum through the pancreatic duct. 5. Intestinal juice contains intestinal lipase, enterokinase, erepsin, maltase, lactase, sucrase and several other enzymes. 6. All enzymes in the small intestine have an optimum pH under alkaline conditions. 7. Bile, an alkaline greenish-yellow fluid, is produced by the liver and stored in the gall bladder. It passes into the small intestine through the bile duct. Bile breaks up large fat droplets into smaller fat droplets in a process called emulsification. This increases the surface area to volume ratio of the fats for lipases on work on and speeds up fat digestion. 8. Action of enzymes involved in carbohydrate digestion in the small intestine: pancreatic amylase starch maltose maltase maltose 2 molecules of glucose sucrase sucrose glucose + fructose lactase lactose glucose + galactose 9. Action of enzymes involved in fat digestion in the small intestine: lipase fats 3 fatty acids + glycerol Nutrition in Humans 23  10. Action of enzymes involved in protein digestion in the small intestine: enterokinase trypsinogen trypsin trypsin proteins polypeptides peptidases / erepsin polypeptides amino acids Note: Enterokinase converts the inactive form of trypsin, trypsinogen, into trypsin. 11. Food is completely digested in the small intestine. The jejunum and ileum function mainly to absorb nutrients and water. 12. Nutrients have to be absorbed into the body from the lumen of the small intestine. The small intestine is adapted for this role by having: (a) An inner wall with large circular folds (b) Finger-like projections on the inner wall called villi (c) Each epithelial cell on the villi has smaller projections called microvilli 13. These adaptations increase the surface area of the small intestine, resulting in a larger surface for absorption. 14. The villi have thin walls (one-cell thick) so that food molecules diffuse over a shorter distance. 15. Within each villus is a network of capillaries and a small vessel called a lacteal. 16. Nutrients are absorbed across the wall of the small intestine and into the capillaries or lacteal. The lacteal transports fats away from the small intestine while the capillaries transport sugars and amino acids. lacteal capillary network villus epithelium A villus 17. The transport of food away from the small intestine sets up a concentration gradient for diffusion. 18. Glucose and amino acids are absorbed by diffusion or active transport depending on the concentration gradient. 19. Fatty acids and glycerol are absorbed by the epithelial cells of the villi and recombined within those cells to form fats, which are transported into a lacteal. 24 TOPIC 4  20. Water is absorbed by passive diffusion throughout the length of the small intestine and mineral salts are absorbed in the ileum. 21. The food eventually leaves the small intestine and enters the large intestine. 4.6 The large intestine 1. The large intestine or colon is shaped like an inverted U and has the function of absorbing the remaining water and mineral salts that have not been absorbed by the small intestine. Note that most of the water that was present in the small intestine (from liquid in ingested food as well as the water content in intestinal mucus and digestive juices) had been absorbed by the small intestine. 2. The undigested waste matter moves along the large intestine by peristalsis, getting progressively drier. 3. The undigested waste matter comprises mainly cellulose, which is indigestible to humans. 4. The waste matter ends up at the rectum where it is stored before it can be eliminated from the body through the anus. The elimination of waste material is called egestion. 4.7 Transport of products of digestion 1. As absorption takes place in the small intestine, the blood in the capillaries of the villi becomes very rich in simple sugars and amino acids. 2. The blood capillaries of the villi converge into a large blood vessel called the hepatic portal vein, which leads to the liver. 3. The blood from the small intestine travels to the liver via the hepatic portal vein. The composition of blood in this vein varies greatly throughout the day depending on whether absorption of nutrients is occurring in the small intestine. 4.8 Role of the liver in carbohydrate metabolism 1. The liver is involved in carbohydrate metabolism and regulation of blood glucose concentration. 2. When the glucose level in blood is high, the islets of Langerhans in the pancreas secrete insulin, which is a hormone that stimulates the liver cells to convert glucose into glycogen. The liver cells convert excess glucose in the blood from the hepatic portal vein into glycogen, which is stored in the liver. 3. When the glucose level in blood is low, the islets of Langerhans secrete glucagon, which is a hormone that stimulates the liver cells to convert stored glycogen in the liver back into glucose. The glucose is released into the blood leaving the liver, which supplies glucose to the body cells. Nutrition in Humans 25  4.9 Role of the liver in fat metabolism 1. The liver produces bile, an alkaline liquid which helps fat digestion by emulsifying fats. 2. It oxidises fats to produce energy. 3. It converts excess carbohydrates and proteins to fatty acids and glycerol which are exported and stored as fatty tissue. 4.10 Role of the liver in breakdown of red blood cells 1. Aging red blood cells are removed by the spleen. 2. Haemoglobin from the red blood cells is brought to the liver, where it is broken down. The iron from the haemoglobin is stored in the liver while the other metabolic by-products of the breakdown form bile pigments. 4.11 Role of the liver in protein metabolism 1. The liver is involved in the synthesis of plasma proteins e.g. albumin, and blood clotting factors e.g. fibrinogen. 2. The liver is responsible for the deamination of excess amino acids, which refers to the removal of the amino group (–NH2) from an amino acid. 3. The amino group is converted into ammonia, which is toxic to cells, before it is further converted to urea by enzymes in the liver, and subsequently removed in urine. 4. The remnants of the amino acid are converted to glucose. 4.12 Role of the liver in detoxification 1. The liver breaks down toxic substances for excretion in urine or bile. 2. It also breaks down alcohol to acetaldehyde through the action of an enzyme called alcohol dehydrogenase. 3. Acetaldehyde is then converted to harmless acetic acid by acetaldehyde dehydrogenase. 4. Alcohol irritates oesophageal, stomach and intestinal linings. Excessive alcohol consumption can lead to inflammation and ulcers. 5. Excessive alcohol consumption can also lead to inflammation, scarring and destruction of liver cells. 6. The liver cells are replaced with fibrous scar tissue in a disease called cirrhosis of the liver, leading to loss of liver function. 26 TOPIC 4  TOPIC Nutrition in Plants 5 Objectives Candidates should be able to: (a) identify and label the cellular and tissue structure of a dicotyledonous leaf, as seen in transverse section using the light microscope and describe the significance of these features in terms of their functions, such as the distribution of chloroplasts in photosynthesis stomata and mesophyll cells in gaseous exchange vascular bundles in transport (b) state the equation, in words and symbols, for photosynthesis (c) describe the intake of carbon dioxide and water by plants (d) state that chlorophyll traps light energy and converts it into chemical energy for the formation of carbohydrates and their subsequent uses (e) investigate and discuss the effects of varying light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis (e.g. in submerged aquatic plant) (f) discuss light intensity, carbon dioxide concentration and temperature as limiting factors on the rate of photosynthesis 5.1 External leaf structure 1. The leaf blade or lamina is thin, with a large surface area to volume ratio, increasing sunlight absorbed for photosynthesis and diffusion of carbon dioxide and oxygen. 2. The leaf stalk or petiole holds the leaf away from the stem so that the leaf can get more sunlight. 3. The mid-rib and vein network carry food substances away from the leaves, and water and mineral salts to the leaves. Nutrition in Plants 27  5.2 Internal leaf structure 1. The diagram below shows the cross section of a dicotyledonous leaf as seen under a microscope: cuticle upper vascular epidermis bundle chloroplast palisade mesophyll xylem phloem spongy mesophyll lower epidermis guard stoma cell Cross section of a dicotyledonous leaf 2. The upper epidermis is a single layer of irregular, closely packed cells covered by a layer of waxy cuticle. The cuticle protects the epidermis and prevents excessive water loss through evaporation. It is transparent to allow sunlight to pass through. Epidermal cells contain no chloroplasts. 3. Palisade mesophyll cells are columnar in shape and closely packed. They contain a lot of chloroplasts to increase absorption of sunlight for photosynthesis. 4. Spongy mesophyll cells are irregular in shape with numerous intercellular air spaces around them to allow for fast diffusion of carbon dioxide, which enters the leaf through the stomata, to all photosynthetic cells. They contain fewer chloroplasts than palisade mesophyll cells. They are covered with a thin film of moisture so that carbon dioxide can dissolve in it. 5. Within the mesophyll layers are the vascular bundles containing xylem and phloem tissue. This brings the transport tissue into close contact with the photosynthetic tissue, allowing water and mineral salts to be distributed to the photosynthetic cells efficiently and food products to be brought to other parts of the plant. 6. The lower epidermis is also a single layer of closely-packed cells covered by a layer of waxy cuticle. 7. Guard cells are bean-shaped, chloroplast-containing cells located in the lower epidermis. They control the opening and closing of the stoma (plural: stomata), the gap between the guard cells. The stomata allow carbon dioxide to diffuse in, oxygen to diffuse out and water vapour to escape. 28 TOPIC 5 5.3 Mode of operation of guard cells epidermal cells guard cell with chloroplasts stoma stoma close open Open stoma Closed stoma 1. Plants open their stomata during the day for carbon dioxide intake and close their stomata during the night to minimise water loss through transpiration. 2. Guard cells control the opening and closing of stomata through regulation of water potential within themselves. 3. Photosynthesis in guard cell chloroplasts provides the energy for the uptake of potassium ions into the cell. 4. This lowers the water potential within the guard cells, causing water to enter by osmosis. 5. Each guard cell has a thicker cellulose cell wall on the side surrounding the stomata, as compared to the side adjacent to neighbouring epidermal cells. When water enters the cell, the side away from the stoma, being thinner, expands more than the side framing the stoma. This causes the cells to become curved such that the stoma opens. 6. When there is excessive water loss, even during the day, the guard cells lose turgor and become flaccid, causing the stoma to close. 5.4 Intake of carbon dioxide 1. When carbon dioxide within the leaf is used up by photosynthesis, the concentration of carbon dioxide in the leaf becomes lower than that in atmospheric air. 2. Carbon dioxide diffuses into the intercellular air spaces of the spongy mesophyll layer through the stomatal openings. 3. The mesophyll cells exposed to the intercellular air spaces are covered by a thin film of water. Carbon dioxide dissolves in it and diffuses into the cells. Nutrition in Plants 29  5.5 Intake of water 1. The vascular bundles in the stem pass through the petioles and enter the leaves. 2. Within the leaves, they branch throughout the mesophyll layers, forming leaf veins. 3. Water from the roots travels through the xylem vessels in the vascular bundles and enters the leaves. 4. Once out of the xylem vessels, water travels from cell to cell through osmosis. 5.6 Photosynthesis 1. Photosynthesis is the process by which plants convert carbon dioxide and water into sugars using sunlight as energy in the presence of chlorophyll. 2. Equation for photosynthesis: light energy 6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O chlorophyll light energy carbon dioxide + water glucose + oxygen + water chlorophyll 3. Photosynthesis is split into 2 stages: light-dependent stage and light-independent stage. 4. Light-dependent stage: (a) Light energy is absorbed by chlorophyll and used to split water into hydrogen and oxygen atoms in a process called photolysis. (b) The oxygen atoms combine to form oxygen gas which is a product of photosynthesis. (c) Other high-energy molecules are generated for use in the light-independent stage to convert carbon dioxide into glucose. 5. Light-independent stage: (a) The chemical energy stored during the light reactions as high-energy molecules is used in a series of reactions to convert carbon dioxide into carbohydrate. (b) Hydrogen from the light reactions is used as a reducing agent in the process. (c) The carbohydrate formed in this stage is converted to glucose and other carbohydrates by enzymes. (d) No light energy is required in this stage. 30 TOPIC 5 5.7 Limiting factors on rate of photosynthesis 1. Light intensity, carbon dioxide concentration and temperature are limiting factors on the rate of photosynthesis. 2. At a constant temperature and carbon dioxide concentration, the rate of photosynthesis increases with increasing light intensity until it reaches a plateau. 3. When the plateau is reached, light is no longer the limiting factor in the reaction. The concentration of carbon dioxide becomes the limiting factor. 4. Increasing the concentration of carbon dioxide raises the plateau reached. 5. Increasing the temperature over a certain range has little effect at low light intensities but increases the rate of photosynthesis at high light intensities. high carbon dioxide concentration rate of low carbon dioxide photosynthesis concentration light intensity Effect of light intensity on the rate of photosynthesis 6. Both light and dark reactions involve enzymes which would be denatured at a high temperature. rate of under 0.03% CO2 photosynthesis and fixed light intensity temperature / °C Effect of temperature on the rate of photosynthesis Nutrition in Plants 31  TOPIC Transport in Flowering Plants 6 Objectives Candidates should be able to: (a) identify the positions and explain the functions of xylem vessels, phloem (sieve tube elements and companion cells) in sections of a herbaceous dicotyledonous leaf and stem, under the light microscope (b) relate the structure and functions of root hairs to their surface area, and to water and ion uptake (c) explain the movement of water between plant cells, and between them and the environment in terms of water potential. (Calculations on water potential are not required.) (d) outline the pathway by which water is transported from the roots to the leaves through the xylem vessels (e) define the term transpiration and explain that transpiration is a consequence of gaseous exchange in plants (f) describe and explain the effects of variation of air movement, temperature, humidity and light intensity on transpiration rate how wilting occurs (g) define the term translocation as the transport of food in the phloem tissue and illustrate the process through translocation studies 6.1 Transport vessels 1. Vascular tissues of the plant consist of xylem vessels and the phloem. 2. Xylem vessels are elongated hollow tubes that are made of xylem cells linked end to end. Xylem cells are dead at maturity. 3. Functions of xylem tissue: (a) Conduct water and mineral salts from the roots to the leaves (b) Mechanical support 4. Adaptations to these functions include: (a) Absence of protoplasm and cross-walls which could impede water flow through the lumen (central space) (b) Deposition of lignin on the cell walls which strengthens vessel walls, providing support 32 TOPIC 6  lumen lumen lumen lumen lignin Xylem vessels 5. The phloem tissue consists of sieve tube elements and companion cells. 6. Sieve tube elements are elongated thin-walled living cells. They have degenerate protoplasm, which means they lack organelles such as the nucleus, ribosomes and the large central vacuole. 7. Sieve tube elements are arranged end to end, with porous walls called sieve plates between them. 8. There is one companion cell closely associated with each sieve tube element. Companion cells contain nuclei, cytoplasm and numerous mitochondria, and are responsible for performing the metabolic functions of the sieve tube elements. 9. The function of the phloem is to conduct sugars and amino acids from the leaves to other parts of the plant. 10. Adaptations to this function include: (a) Porous sieve plates that allow uninterrupted flow of food substances through the sieve tubes (b) Numerous mitochondria in the companion cells that provide energy for them to help load sieve tube members with sugar Transport in Flowering Plants 33  sieve plate sieve tube cell companion cell sieve plate Phloem vessels 6.2 Position of vascular tissue in dicotyledonous stems 1. In dicotyledonous stems, the vascular bundles are arranged in a ring around a central pith. 2. Between the ring of vascular tissue and the epidermis is the cortex. The epidermis is covered by waterproof cuticle that minimises water loss in the stem. 3. Within the vascular bundles, the phloem tissue is found on the side facing the cortex and the xylem on the side facing the pith. Between the xylem and phloem is a layer called the cambium. Cambium cells can differentiate into new xylem and phloem tissues. 4. Food is stored in the cortex and pith. pith xylem vascular cambium bundle phloem epidermis cortex Transverse section of a stem 34 TOPIC 6 6.3 Position of vascular tissue in dicotyledonous roots 1. The outermost layer of the root is the piliferous layer. It is a single layer of cells bearing root hairs. 2. The layer below the epidermis is called the cortex. It consists of storage tissue. 3. The central region of the root contains xylem and phloem tissues. The xylem radiates from the centre, with phloem tissues alternating between them. endodermis epidermis root hair cortex phloem xylem Transverse section of a root 6.4 Root hair cells 1. Root hairs are tubular outgrowths of root epidermal cells. Each root hair is usually an outgrowth of a single epidermal cell, so they are one-cell thick. 2. Being long and narrow, they have a large surface area to volume ratio for rapid absorption of water and minerals. 3. The cell surface membrane controls the water potential of the cell sap. The cell sap has a lower water potential than the soil solution, causing osmosis to take place. soil particles cellulose cell wall root hair vacuole nucleus Root hair cell Transport in Flowering Plants 35  6.5 Absorption of water and minerals by root hair cells 1. Soil particles are usually coated with water and dissolved mineral salts. 2. The cell sap in the root hair cells contains sugars and ions that cause it to be at a lower water potential than soil solution. 3. Water moves across the partially permeable cell surface membrane from the soil solution into the cell sap by osmosis. 4. The cell sap now has a higher water potential than the cell sap in the adjoining cell. 5. Water moves across the cell surface membranes into the adjoining cell by osmosis. 6. This process continues until the water enters the xylem vessels and moves up the plant. cortex phloem xylem piliferous layer root hair water entering the root hair The path of water through the root 6.6 Transportation of water from the roots to the leaves 1. Water travels from the roots to the leaves against gravity through 3 primary mechanisms: (a) Root pressure (b) Transpiration (c) Capillary action 2. Root cells pump mineral salts into the xylem vessels using active transport. This causes the water potential of the xylem vessels to be lower than the water potential in the cortex cells. Water moves into the xylem vessels by osmosis, creating a pressure that forces water to move upwards. This is called root pressure. 3. Root pressure is not the main mechanism for movement of water in most plants as it can only force water to travel a short distance. 4. Transpiration is the loss of water vapour from the stomata of the leaves through diffusion. 36 TOPIC 6  5. The stomata have to be open for carbon dioxide intake due to photosynthesis. This allows the loss of water vapour from the intercellular air spaces in the leaves as the air outside has a lower water vapour concentration than the air inside the leaf. Transpiration is the necessary cost of carbon dioxide intake. 6. However it is also responsible for the transpiration pull, which is the main force that causes water to travel upwards in plants. 7. Transpiration pull is the suction force caused by transpiration that pulls water up the xylem. 8. Capillary action is the tendency of water to travel up the narrow xylem tubes due to the interactions between water molecules and the xylem walls. This is usually observed in young plants with narrow veins and is not significant in larger plants. 6.7 Factors affecting the rate of transpiration 1. Water vapour in the intercellular air space diffuses out of the stomata. 2. Evaporation from the thin film of water that coats the mesophyll cells replaces the water lost through transpiration. 3. As water evaporates from the mesophyll cells, the water potential of the cell sap decreases. The mesophyll cells absorb water from neighbouring cells closer to the vascular bundles by osmosis. These cells, in turn, absorb water from the xylem vessels. 4. This creates a suction force that pulls the entire column of water up the xylem vessels. 5. Factors affecting transpiration are: (a) Humidity of the surroundings – Humidity affects the concentration gradient of water vapour between the intercellular air spaces in the leaf and the external environment. The higher the humidity, the higher the concentration of water vapour in the external air. The diffusion gradient for water vapour is less steep so the rate of transpiration is lowered. (b) Air movement – Wind removes the water vapour that accumulates outside the stomata due to transpiration. This maintains the steep diffusion gradient of water vapour. The rate of transpiration will remain high as long as water vapour is continually being removed by wind. (c) Temperature – Heat increases the rate of evaporation and also increases the movement of water molecules. The higher the temperature, the higher the rate of evaporation as well as the rate of movement of water vapour, and thus, the higher the rate of transpiration. (d) Light intensity – Light intensity causes stomatal opening. Since transpiration takes place mainly through the stomata, the rate of transpiration will increase with increased light intensity. 6. Wilting takes place when the rate of transpiration exceeds the rate of water intake by the roots. Plant cells lose water and become flaccid. Transport in Flowering Plants 37  section of leaf water evaporates from surfaces of mesophyll cells into the intercellular air space water vapour diffuses stoma out of the leaf through the stomata The path of water in a leaf 6.8 Translocation 1. Translocation is the transport of sugars from the leaves to other parts of the plant. This is done by the phloem tissues. The leaves, which supply sugar, are known as the source while other parts of the plant which require sugar are known as the sink. 2. Energy is required for this process as the mode of uptake of sugars into sieve tube elements in the leaves is active transport. 3. At the end of the sieve tube where sugars are being unloaded for use, sugars are also removed from the sieve tube by active transport. 6.9 Translocation studies 1. To show that translocation occurs in the phloem, radioactive carbon dioxide may be introduced to the plant. After a few hours, slices of tissues are removed from the stems to determine where radioactivity, which indicates the presence of radioactive sugars, first appears. 2. Translocation occurs from source to sink and the direction of the movement may be upwards or downwards. To study the direction of translocation in a plant, a ring of bark, containing the phloem, is cut away from the stem. A few days later, a bulge has formed on top of the cut. This is formed due to an accumulation of phloem sap, as it is unable to move downwards towards the roots. 3. When an aphid is introduced to a plant, it will insert its proboscis into the stem to feed. The rest of the aphid is removed from its proboscis and phloem sap will continue to exude from the free end of the proboscis, which shows that there is pressure in phloem sap. This pressure is formed due active loading of sugar at the source, which will cause water to enter the phloem to generate a region of high pressure, and active unloading of sugar at the sink, which will cause water to exit the phloem, generating a region of low pressure. 38 TOPIC 6  TOPIC Transport in Humans 7 Objectives Candidates should be able to: (a) identify the main blood vessels to and from the heart, lungs, liver and kidney (b) state the role of blood in transport and defence red blood cells – haemoglobin and oxygen transport plasma – transport of blood cells, ions, soluble food substances, hormones, carbon dioxide, urea, vitamins, plasma proteins white blood cells – phagocytosis, antibody formation and tissue rejection platelets – fibrinogen to fibrin, causing clotting (c) list the different ABO blood groups and all possible combinations for the donor and recipient in blood transfusions (d) relate the structure of arteries, veins and capillaries to their functions (e) describe the transfer of materials between capillaries and tissue fluid (f) describe the structure and function of the heart in terms of muscular contraction and the working of valves (g) outline the cardiac cycle in terms of what happens during systole and diastole. (Histology of the heart muscle, names of nerves and transmitter substances are not required.) (h) describe coronary heart disease in terms of the occlusion of coronary arteries and list the possible causes, such as diet, stress and smoking, stating the possible preventative measures 7.1 Overview of the human circulatory system 1. The components of the circulatory system are the heart, blood vessels and blood. 2. Blood passes through the heart twice in a complete circuit. This is termed double circulation. 3. Double circulation consists of: (a) Systemic circulation – Carries oxygenated blood (oxygen-rich) from the heart to all body organs and returns oxygen-poor blood to the heart (b) Pulmonary circulation – Carries deoxygenated blood (oxygen-poor) from the heart to the lungs for gaseous exchange before returning blood to the heart for transport to the body organs via systemic circulation Transport in Humans 39  4. The three main types of blood vessels are: (a) Arteries – Vessels that carry blood away from the heart to body organs. Arteries branch into arterioles and then into capillaries. (b) Capillaries – Microscopic vessels that connect between the arteries and veins. They converge into venules which converge into veins. They form networks called capillary beds that are present in most body tissues. (c) Veins – Vessels that return blood to the heart 5. The main vessels of the human circulatory system are: (a) Pulmonary arteries that supply oxygen-poor blood from the heart to the lungs (b) Pulmonary veins that bring oxygen-rich blood from the lungs to the heart (c) Aorta that supplies oxygen-rich blood from the heart to the rest of the body. The aorta branches into: coronary arteries which supply cardiac tissue, an anterior branch leading to the head and arms and a posterior branch (dorsal aorta) leading to abdominal organs and legs. (d) Branches of the dorsal aorta include: (i) Hepatic artery from the heart to the liver (ii) Arteries to the alimentary canal (iii) Renal arteries from the heart to the kidneys (e) Vena cava consists of an anterior branch which returns blood from the head and arms to the heart and a posterior branch. (f) Posterior vena cava collects blood from the posterior parts of the body, such as from: (i) Hepatic veins from the liver to the heart (ii) Renal veins from the kidneys to the heart 40 TOPIC 7  (g) Hepatic portal vein transports blood from the alimentary canal to the liver. Blood from the liver is returned to the heart via the hepatic vein. head and arms pulmonary pulmonary artery vein lungs anterior vena cava posterior aorta vena cava heart hepatic vein hepatic artery liver hepatic digestive tract portal vein renal artery renal vein kidneys trunk and legs The human circulatory system Transport in Humans 41  7.2 Components of blood 1. Blood is a connective tissue consisting of 45% cells suspended in 55% plasma. 2. Plasma is a clear yellowish liquid consisting mostly of water. It contains soluble proteins such as albumin and fibrinogen, as well as dissolved substances such as nutrients, waste products and ions. 3. Cellular elements in blood include: (a) Red blood cells (erythrocytes) which function to transport oxygen. Adaptations to this function are: (i) Flattened, biconcave shape without nucleus or organelles at maturity, increasing the surface area to volume ratio for faster diffusion of oxygen (ii) Contains haemoglobin, an iron-containing protein which is able to bind reversibly with oxygen (iii) Flexibility to turn bell-shaped in order to pass through the narrow lumen of the capillaries (b) White blood cells (leukocytes) are responsible for fighting infections in the body. There are two main types of white blood cells: (i) Phagocytes have lobed (bi-lobed, tri-lobed, multi-lobed) nuclei and granular cytoplasm. They engulf and digest foreign particles such as bacteria. (ii) Lymphocytes have a large rounded nucleus and a small amount of cytoplasm. They produce antibodies to protect the body from pathogens. (c) Platelets (thrombocytes) are small cell fragments which have no nuclei. They play a role in blood clotting. Erythrocytes Phagocyte Lymphocyte 42 TOPIC 7 7.3 Role of blood in transport 1. Blood plasma transports: (a) Simple sugars, amino acids, fatty acids and glycerol from the capillaries in the small intestine (b) Waste products of metabolism from tissues: (i) Carbon dioxide in the form of bicarbonate ions. Carbon dioxide enters the blood from body tissues by diffusion into red blood cells, which contain the enzyme carbonic anhydrase to convert it to hydrogen carbonate. The hydrogen carbonate then diffuses out of red blood cells to be carried in plasma. In the lungs, the reverse occurs. (ii) Nitrogenous waste products such as urea, uric acid and creatinine to the kidneys to be removed (c) Hormones from the glands to target tissues (d) Heat from muscles and liver throughout the body 2. Red blood cells transport: (a) Oxygen as oxyhaemoglobin (b) A small amount of carbon dioxide bound to haemoglobin 7.4 Transport of oxygen by red blood cells 1. As air enters the lungs, oxygen dissolves in the fluid covering the moist epithelium of the alveoli. 2. The oxygen diffuses into the capillaries of the lungs where they bind reversibly with haemoglobin in red blood cells to form oxyhaemoglobin. 3. When blood is transported to oxygen-poor respiring tissues, oxyhaemoglobin releases its oxygen which then diffuses into tissue cells. 7.5 Immune function of white blood cells 1. Phagocytosis refers to the ingestion of harmful foreign particles, bacteria and dead or dying cells by certain types of white blood cells called phagocytes. 2. When phagocytes detect a foreign particle, it engulfs it by stretching itself around the particle and enclosing it. It then digests the particle and kills it. 3. After phagocytosis, these cells die and form pus. 4. Antibodies are special proteins found in blood and other bodily fluids that help phagocytes identify and neutralise foreign particles. Antibodies also activate other immune responses. Transport in Humans 43  5. When pathogens enter the blood, they stimulate lymphocytes to produce antibodies. 6. Antibodies may be present in the blood long after infection has been cured, conferring immunity to that particular infection. 7.6 Tissue rejection 1. Tissue rejection occurs when the transplanted tissue is not accepted by the body of the transplant recipient. 2. During tissue rejection, the tissues of the transplanted organ are treated as foreign bodies by the recipient’s immune system and are attacked by phagocytes. This causes the transplanted tissue to fail. 3. Prevention of tissue rejection: (a) Required tissue can be transplanted from genetically-similar donors. (b) Tissue can be transplanted from one part of the body to another, e.g. skin grafting, as the tissue will be recognised as the recipient’s own tissue. (c) Immunosuppressive drugs can be taken to suppress the immune system of the recipient. Associated problems include: (i) Lowered resistance to infection (ii) Having to continue taking the drugs for their entire lifespan 7.7 Blood clotting 1. The blood clotting process begins at the site of injury when blood vessels are damaged. 2. Platelets are activated, and the damaged tissue and activated platelets release thrombokinase. 3. Thrombokinase converts plasma protein, prothrombin, into thrombin in the presence of calcium and vitamin K. 4. Thrombin converts fibrinogen, a soluble plasma protein, to fibrin, an insoluble protein that forms long threads. 5. Fibrin forms a mesh across the damaged surface and traps red blood cells, forming a clot. 6. The clot prevents further blood loss, and also restricts the entry of pathogens into the blood. 44 TOPIC 7 7.8 Blood groups 1. There are 4 blood groups: A, B, AB and O. This classification is based on certain proteins present on the surfaces of red blood cells. 2. These proteins can be recognised by antibodies present in the blood plasma as either foreign or ‘self’. 3. If they are recognised as foreign, an immune response will be mounted against the foreign blood, resulting in agglutination, where the red blood cells clump together and are marked for phagocytosis. 4. When no agglutination occurs, it shows the blood can be accepted by the recipient. 5. Transfusion results between the different blood groups are shown below: Recipient Donor A B AB O A Accepted Rejected Accepted Rejected B Rejected Accepted Accepted Rejected AB Rejected Rejected Accepted Rejected O Accepted Accepted Accepted Accepted 7.9 Blood vessels and their functions 1. Arteries are blood vessels which carry blood away from the heart. 2. They have thick, muscular and elastic walls that can withstand the surge of the high pressure blood pumped out of the heart. 3. The arterial wall is divided into three layers. The outer layer is a protective layer consisting of connective tissue and elastic fibre. The middle layer consists of smooth muscle and more elastic fibres and the innermost layer next to the lumen consists of the endothelium, a single layer of flattened cells. 4. All arteries carry oxygenated blood with the exception of the pulmonary