The body and its constituents Enzvmes Enzyme Many of the bodys chemical reactions can be reproduced in a test-tube. Surprisingly, the rate at which the reactions Substrates then occur usually plummets to the extent that, for all practical purposes, chemical activity ceases. The cells of the body have developed a solution to this apparent prob. lem-they are equipped with a huge array of enzymes Enzymes are proteins which act as catalysts for biochemi- cal reactions-that is, they speed the reaction up but are Active site Product not themselves changed by it, and therefore can be used over and over again. Enzymes are very selective and will Figure 2.13 Action of an enzyme: A Enzyme and substrates usually catalyse only one specific reaction. The mole- cule(s) entering the reaction is called the substrate and it binds to a very specific site on the enzyme, called the From a physical point of view, substances will alway travel from an area of high concentration to one of low active site. Whilst the substrate(s) is bound to the active site the reaction proceeds, and once it is complete the oncentration, assuming that there is no barrier in the way, Between two such areas, there exists a concentration product(s)of the reaction breaks away from the enzyme gradient and movement of substances occurs dowr and the active site is ready for use again(Fig. 2. 13) o ener es can catalyse both synthesis and breakde reactions, and their names(almost always! )end in -ase. required for such movement; this process is therefore described as passive Net movement ot substance high concentration low concentration MOVEMENT OF SUBSTANCES There are many examples in the body of substances WITHIN THE BODY phill, 1. e against the concentration gradient; in this case, chemical energy is required, usually in the form of ATP. These processes are described as active Movement of substances across cell membranes by active Learning outcomes transport is described on page 34 assive movement of substances in the body proceeds After studying this section, you should be able to usually in one of two m or osmoSIs m compare and contrast the processes of osmosis and diffusion Diffusion m using these concepts, describe how molecules move Diffusion refers to the movement of a chemical substance within and between body compartments from an area of high concentration to an area of low con- centration, and occurs mainly in gases, liquids and solu tions. This process enables the transfer of oxygen from Within the body, it is essential that substances(e. g. mole- the alveoli of the lungs(high concentration) through the cules, electrolytes)move around. Nutrients absorbed in alveolar and capillary walls into the blood (low concen- the small intestine must move, or they will never reach tration). Sugar molecules heaped at the bottom of a cup the tissues they are destined to nourish. Waste substances of coffee which has not been stirred will, in time, become must travel from the tissues to their exit points from the evenly distributed throughout the liquid by diffusion body.To enter the body from inhaled air, oxygen gas (Fig. 2.14). The process of diffusion is speedes s ne of the capillary to get into the blood Communication diffusing substance is increased molecules, such as hormones, have to travel from the site Diffusion can also occur across a semi of production to their destination. Water itself, the princi- brane, such as the plasma membrane; in this case, onl pal constituent of the body, has to move in order to be those molecules able to cross the membrane can diffuse able to be distributed throughout the body fluids and through. For example, the capillary wall is effectively a keep solutes at appropriate physiological concentrations, semipermeable membrane; whereas water can travel thus maintaining homeosta freely in either direction across it, large proteins in the
The body and its constituents 26 Enzymes Many of the body's chemical reactions can be reproduced in a test-tube. Surprisingly, the rate at which the reactions then occur usually plummets to the extent that, for all practical purposes, chemical activity ceases. The cells of the body have developed a solution to this apparent problem—they are equipped with a huge array of enzymes. Enzymes are proteins which act as catalysts for biochemical reactions — that is, they speed the reaction up but are not themselves changed by it, and therefore can be used over and over again. Enzymes are very selective and will usually catalyse only one specific reaction. The molecule(s) entering the reaction is called the substrate and it binds to a very specific site on the enzyme, called the active site. Whilst the substrate(s) is bound to the active site the reaction proceeds, and once it is complete the product(s) of the reaction breaks away from the enzyme and the active site is ready for use again (Fig. 2.13). Enzymes can catalyse both synthesis and breakdown reactions, and their names (almost always!) end in ~ase. MOVEMENT OF SUBSTANCES WITHIN THE BODY Learning outcomes After studying this section, you should be able to; • compare and contrast the processes of osmosis and diffusion • using these concepts, describe how molecules move within and between body compartments. Within the body, it is essential that substances (e.g. molecules, electrolytes) move around. Nutrients absorbed in the small intestine must move, or they will never reach the tissues they are destined to nourish. Waste substances must travel from the tissues to their exit points from the body. To enter the body from inhaled air, oxygen gas must move across first the alveolar wall and then the wall of the capillary to get into the blood. Communication molecules, such as hormones, have to travel from the site of production to their destination. Water itself, the principal constituent of the body, has to move in order to be able to be distributed throughout the body fluids and keep solutes at appropriate physiological concentrations, thus maintaining homeostasis. Figure 2.13 Action of an enzyme: A. Enzyme and substrates. B. Enzyme-substrate complex. C. Enzyme and product. From a physical point of view, substances will always travel from an area of high concentration to one of low concentration, assuming that there is no barrier in the way. Between two such areas, there exists a concentration gradient and movement of substances occurs down the concentration gradient, or downhill. No energy is required for such movement; this process is therefore described as passive. Net movement of substance high concentration - low concentration There are many examples in the body of substances moving uphill, i.e. against the concentration gradient; in this case, chemical energy is required, usually in the form of ATP. These processes are described as active. Movement of substances across cell membranes by active transport is described on page 34. Passive movement of substances in the body proceeds usually in one of two main ways — diffusion or osmosis. Diffusion Diffusion refers to the movement of a chemical substance from an area of high concentration to an area of low concentration, and occurs mainly in gases, liquids and solutions. This process enables the transfer of oxygen from the alveoli of the lungs (high concentration) through the alveolar and capillary walls into the blood (low concentration). Sugar molecules heaped at the bottom of a cup of coffee which has not been stirred will, in time, become evenly distributed throughout the liquid by diffusion (Fig. 2.14). The process of diffusion is speeded up if the temperature rises and/or the concentration of the diffusing substance is increased. Diffusion can also occur across a semipermeable membrane, such as the plasma membrane; in this case, only those molecules able to cross the membrane can diffuse through. For example, the capillary wall is effectively a semipermeable membrane; whereas water can travel freely in either direction across it, large proteins in the
Introduction to the chemistry of life solute molecule A Before diffusion Figure 2. 14 The process of diffusion: a spoonful of sugar in a cup Figure 2.15 The process of osmosis. Net water movement when a ed blood cell is suspended in solutions of varying concentrations tonicity): A Isotonic solution, B, Hypotonic solution. C. Hypertonic plasma and red blood cells are too large to cross and therefore remain in the blood BODY FLUIDS Osmosis Osmosis is the movement of water down its concentra Learning outcomes tion gradient across a semipermeable membrane when equilibrium cannot be achieved by diffusion of solute After studying this section, you should be able to molecules. This is usually because the solute molecules a define the terms intra- and extracellular fluid are too large to pass through the pores in the membrane The force with which this occurs is called the osmotic pres- a using examples, explain why homeostatic control sure. Water crosses the membrane down its concentration of the composition of these fluids is vital to body function gradient from the side with the lower solute concentra- tion to the side with the greater solute concentration. This dilutes the more concentrated solution, and concentrates The total body water in adults of average build is about the more dilute solution. Osmosis proceeds until equilil of body weight. This proportion is higher in young rium is reached, at which point the solutions on each side people and in adults below average weight. It is lower in he elderly and in obesity in all age groups. About 22% of said to be isotonic Osmosis can be illustrated using the body weight is extracellular water and about 38% is semipermeable membrane of the red blood cell as an intracellular water(Fig. 2.16) example The concentration of water and solutes in the plasma maintained within a very narrow range because if the Extracellular fluid plasma water concentration rises, i.e. the plasma The extracellular fluid(ECF)consists of blood, plasma, becomes more dilute than the intracellular fluid within lymph, cerebrospinal fluid and fluid in the interstitial centration gradient across the membranes and into the fluid) bathes all the cells of the body except the outer a the red blood cells, then water will move down its con- spaces of the body. Interstitial or intercellular fluid(tisst red blood cells. This may cause the red blood cells to ers of skin. It is the medium through which substances swell and burst. In this situation, the plasma is said to be pass from blood to the body cells, and from the cells to hypotonic. Conversely, if the plasma water concentration blood. Every body cell in contact with the ECF is directly falls so that the plasma becomes more conc centrated dependent upon the composition of that fluid for its well than the intracellular fluid within the red blood cells being Even slight changes can cause permanent damage, (the plasma becomes hypertonic), water passively moves and any change is therefore resisted by the body, through y osmosis from the blood cells into the plasma and one or more of its many control mechanisms; this is shrinkage of the bloc od cells occurs (Fig215) homeostasis. For example, a fall in plasma calcium levels
Introduction to the chemistry of life Figure 2.14 The process of diffusion: a spoonful of sugar in a cup of coffee. Figure 2.15 The process of osmosis. Net water movement when a red blood cell is suspended in solutions of varying concentrations (tonicity): A. Isotonic solution. B. Hypotonic solution. C. Hypertonic solution. plasma and red blood cells are too large to cross and therefore remain in the blood. Osmosis Osmosis is the movement of water down its concentration gradient across a semipermeable membrane when equilibrium cannot be achieved by diffusion of solute molecules. This is usually because the solute molecules are too large to pass through the pores in the membrane. The force with which this occurs is called the osmotic pressure. Water crosses the membrane down its concentration gradient from the side with the lower solute concentration to the side with the greater solute concentration. This dilutes the more concentrated solution, and concentrates the more dilute solution. Osmosis proceeds until equilibrium is reached, at which point the solutions on each side of the membrane are of the same concentration and are said to be isotonic. Osmosis can be illustrated using the semipermeable membrane of the red blood cell as an example. The concentration of water and solutes in the plasma is maintained within a very narrow range because if the plasma water concentration rises, i.e. the plasma becomes more dilute than the intracellular fluid within the red blood cells, then water will move down its concentration gradient across the membranes and into the red blood cells. This may cause the red blood cells to swell and burst. In this situation, the plasma is said to be hypotonic. Conversely, if the plasma water concentration falls so that the plasma becomes more concentrated than the intracellular fluid within the red blood cells (the plasma becomes hi/pertonic), water passively moves by osmosis from the blood cells into the plasma and shrinkage of the blood cells occurs (Fig. 2.15). BODY FLUIDS Learning outcomes After studying this section, you should be able to; • define the terms intra- and extracellular fluid • using examples, explain why homeostatic control of the composition of these fluids is vital to body function. The total body water in adults of average build is about 60% of body weight. This proportion is higher in young people and in adults below average weight. It is lower in the elderly and in obesity in all age groups. About 22% of body weight is extracellular water and about 38% is intracellular water (Fig. 2.16). Extracellular fluid The extracellular fluid (ECF) consists of blood, plasma, lymph, cerebrospinal fluid and fluid in the interstitial spaces of the body. Interstitial or intercellular fluid (tissue fluid) bathes all the cells of the body except the outer layers of skin. It is the medium through which substances pass from blood to the body cells, and from the cells to blood. Every body cell in contact with the ECF is directly dependent upon the composition of that fluid for its wellbeing. Even slight changes can cause permanent damage, and any change is therefore resisted by the body, through one or more of its many control mechanisms; this is homeostasis. For example, a fall in plasma calcium levels 27
The body and its constituents cause the heart to stop beating. Calcium levels in the ECF Extracellular fluid (ECF are only one of the many parameters under constant, careful adjustment by the homeostatic mechanisms of (Plasma 2.51, interstitial fluid 9.5) the body Total body water Intracellular fluid The composition of intracellular fluid (ICF)is largely controlled by the cell itself, because there are selective intracellular fluid (ICF) 28 litres uptake and discharge mechanisms present in the cell membrane. The composition of ICF can therefore be very different from ECF. Thus, sodium levels are nearly ten times higher in the ECF than in the ICE. This concentra tion difference occurs because although sodium diffuses Figure 2.16 Distribution of body water in a 70 kg person into the cell down its concentration gradient there is a pump in the membrane which selectively pumps it back out again. This concentration gradient is essential for the causes tetany(abnormal spasmodic muscle contractions) function of excitable cells (mainly nerve and muscle) and convulsions(fits), because of increased excitability Conversely, many substances are found inside the cell of muscle and nervous tissue. Rising blood calcium in significantly higher amounts than outside, e.g. ATP, depresses muscle and nerve function, and can even protein and potassium
The body and its constituents Figure 2.16 Distribution of body water in a 70 kg person. causes tetany (abnormal spasmodic muscle contractions) and convulsions (fits), because of increased excitability of muscle and nervous tissue. Rising blood calcium depresses muscle and nerve function, and can even cause the heart to stop beating. Calcium levels in the ECF are only one of the many parameters under constant, careful adjustment by the homeostatic mechanisms of the body. Intracellular fluid The composition of intracellular fluid (ICF) is largely controlled by the cell itself, because there are selective uptake and discharge mechanisms present in the cell membrane. The composition of ICF can therefore be very different from ECF. Thus, sodium levels are nearly ten times higher in the ECF than in the ICF. This concentration difference occurs because although sodium diffuses into the cell down its concentration gradient there is a pump in the membrane which selectively pumps it back out again. This concentration gradient is essential for the function of excitable cells (mainly nerve and muscle). Conversely, many substances are found inside the cell in significantly higher amounts than outside, e.g. ATP, protein and potassium. 28
The cells, tissues and organisation of the bod The cell: structure and functions Disorders of cells and tissues 53 Neoplasms or tumours 53 Pla Causes of neoplasms 53 ell division 32 Effects of tumours 55 Mutation 33 Causes of death in malignant disease 55 Transport of substances across cell Tissues 35 Epithelial tissue 3 Connective tissue 36 Muscle tissue 40 Tissue regeneration 42 Membranes 43 Glands 43 Organisation of the body 44 The skeleton 44 Axial skeleton 44 Cavities of the body 49 Abdominal cavity 50
The cells, tissues and organisation of the body The cell: structure and functions Disorders of cells and tissues 53 30 Plasma membrane 30 Organelles 31 Cell division 32 Mutation 33 Transport of substances across cell membranes 33 Tissues 35 Epithelial tissue 35 Connective tissue 36 Muscle tissue 40 Nervous tissue 42 Tissue regeneration 42 Membranes 43 Glands 43 Organisation of the body 44 Anatomical terms 44 The skeleton 44 Axial skeleton 44 Appendicular skeleton 48 Cavities of the body 49 Cranial cavity 49 Thoracic cavity 49 Abdominal cavity 50 Pelvic cavity 51 Neoplasms or tumours 53 Causes of neoplasms 53 Growth of tumours 54 Effects of tumours 55 Causes of death in malignant disease 55
The body and its constituents Cells are the smallest functional units of the body. They are grouped together to form tissues, each of which has a endopl Ribosomes specialised function, e.g. blood, muscle, bone. Different CMN tissues are grouped together to form organs, e.g. heart, stomach, brain. Organs are grouped together to form sys- tems, each of which performs a particular function that maintains homeostasis and contributes to the health of the individual (p. 5). For example, the digestive system is responsible for taking in, digesting and absorbing food nd involves a number of organs, including the stomach nd intestines THE CELL: STRUCTURE AND Secretory Plasma FUNCTIONS Figure 3. 1 The simple cell. Learning outcomes After studying this section you should be able to a describe the structure of the plasma membrane u explain the functions of the following organelles: nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, lysosomes. microtubules and microfilaments m outline the two types of cell division a define the term'mutation Phospholipid Membrane protein a compare and contrast active, passive and bulk transport of substances across cell membranes ead-h The human body develops from a single cell called the zygote, which results from the fusion of the ovum( female Figure 3.2 The plasma membrane egg cell)and the spermatozoon(male germ cell). Cell multiplication follows and, as the fetus grows, cells with Plasma membrane different structural and functional specialisations zygote. Individual cells are too small to be seen with the of Phospholipids(fatty (g. 3.2)consists of two layers develop, all with the same genetic make-up as the The plasma membrane substances(p. 24) with some naked eye. However, they can be seen when thin slices of protein molecules embedded in them. Those that extend tissue are stained in the laboratory and magnified by a all the way through the membrane may provide channels that allow the passage of, for example, electrolytes and A cell consists of a plasma membrane inside which there non-lipid-soluble substances are a number of organelles floating in a watery fluid The phospholipid molecules have a head which is called cytosol(Fig. 3. 1). Organelles are small structures electrically charged and hydrophilic(meaning water lov with highly specialised functions, many of which are ing)and a tail which has no charge and is hydrophobic contained within a membrane. They include: the (meaning water hating). The phospholipid bilayer nucleus, mitochondria, ribosomes, endoplasmic reticulum, arranged like a sandwich with the hydrophilic heads Golgi apparatus, Lysosomes, microfilaments and microtubules, aligned on the outer surfaces of the membrane and the
The body and its constituents Cells are the smallest functional units of the body. They are grouped together to form tissues, each of which has a specialised function, e.g. blood, muscle, bone. Different tissues are grouped together to form organs, e.g. heart, stomach, brain. Organs are grouped together to form systems, each of which performs a particular function that maintains homeostasis and contributes to the health of the individual (p. 5). For example, the digestive system is responsible for taking in, digesting and absorbing food and involves a number of organs, including the stomach and intestines. THE CELL: STRUCTURE AND FUNCTIONS 30 Learning outcomes After studying this section you should be able to: • describe the structure of the plasma membrane • explain the functions of the following organelles: nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, lysosomes, microtubules and microfilaments • outline the two types of cell division • define the term 'mutation' • compare and contrast active, passive and bulk transport of substances across cell membranes. The human body develops from a single cell called the zygote, which results from the fusion of the ovum (female egg cell) and the spermatozoon (male germ cell). Cell multiplication follows and, as the fetus grows, cells with different structural and functional specialisations develop, all with the same genetic make-up as the zygote. Individual cells are too small to be seen with the naked eye. However, they can be seen when thin slices of tissue are stained in the laboratory and magnified by a microscope. A cell consists of a plasma membrane inside which there are a number of organelles floating in a watery fluid called cytosol (Fig. 3.1). Organelles are small structures with highly specialised functions, many of which are contained within a membrane. They include: the nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, lysosomes, microfilaments and microtubules. Figure 3.1 The simple cell. Figure 3.2 The plasma membrane. Plasma membrane The plasma membrane (Fig. 3.2) consists of two layers of phospholipids (fatty substances (p. 24)) with some protein molecules embedded in them. Those that extend all the way through the membrane may provide channels that allow the passage of, for example, electrolytes and non-lipid-soluble substances. The phospholipid molecules have a head which is electrically charged and hydrophilic (meaning 'water loving') and a tail which has no charge and is hydrophobic (meaning 'water hating'). The phospholipid bilayer is arranged like a sandwich with the hydrophilic heads aligned on the outer surfaces of the membrane and the