Biology - Scion · PDF filevi Catch Up Biology 5.7 Introns and exons 29 ... 26.7 Muscles and...

P hil iP Br a dl e y & J a ne C a lv er t

se cond ed i t ion

Biology f o r t h e m e d i C a l s C i e n C e s

Catch Up Biology covers the basic principles and concepts in biology that you will need if you are studying medicine or a related subject, or one of the biomedical sciences. The book focuses on human biology and covers: • the basic molecules of life, such as proteins, carbohydrates, nucleic

acids • cells, tissues and processes, including energy metabolism, cell division,

epithelial and connective tissues • the key mammalian systems, for example, homeostasis, the endocrine,

respiratory and digestive systems

Throughout the book the authors highlight clinical examples so that you can see the relevance of basic biology to your course. The book also contains questions (and answers) so that you can test your understanding of the subject as you work through the book. This new edition features two new chapters on microorganisms and on genetic disease.

Catch Up Biology is the ideal book to refresh your understanding of the basic concepts of biology.

r e v ie w s of t he f ir s t ed i t ion:“… extremely useful at university, a very in depth book, just what’s required”

“I have enjoyed reading this book and have found it highly informative. I have studied Biology in a formal context, but this was some years ago. I bought this book in order to better inform my grasp of biology before applying for a

Medical Degree”

MI T CH F R Y A ND EL IZ A BE T H PA GE

SE COND ED I T ION

CHEMISTRY F O R T H E L I F E A N D M E D I C A L S C I E N C E S

M. H A R R I S , G . TAY L OR & J . TAY L OR

SE COND ED I T ION

MATHS & STATS F O R T H E L I F E A N D M E D I C A L S C I E N C E S

Biology

BiologyBradley & Calvert

second edition

w w w. s c i o n p u b l i s h i n g . c o m

9 781904 842880

ISBN 978-1-904842-88-0

Contents

Preface xi

Acknowledgements xii

The molecules of life

1 Water and life 1 1.1 The properties of water 1 1.2 Water in the human body 2 1.3 Test yourself 3

2 Proteins 5 2.1 Introduction 5 2.2 Primary structure 7 2.3 Secondary structures 7 2.4 Tertiary structure 9 2.5 Quaternary structure 10 2.6 Domains 10 2.7 Functions of proteins 11 2.8 Conformational change 11 2.9 Test yourself 12

3 Carbohydrates 13 3.1 Introduction 13 3.2 Monosaccharides 13 3.3 Glycosidic bond 15 3.4 Polysaccharides 16 3.5 Glycoconjugates 17 3.6 Functions of carbohydrates 18 3.7 Test yourself 18

4 Lipids 19 4.1 Introduction 19 4.2 Fatty acids 19 4.3 Triglycerides and phospholipids 20 4.4 Cholesterol 21 4.5 Functions of lipids 21 4.6 Test yourself 22

5 Nucleic acids and genes 23 5.1 Introduction 23 5.2 DNA and RNA 24 5.3 DNA synthesis 26 5.4 RNA synthesis 26 5.5 The genetic code 26 5.6 Protein synthesis 27

CatchUp_Biology.indb 5 22/04/2013 10:40

vi Catch Up Biology

5.7 Introns and exons 29 5.8 Regulation of gene expression 29 5.9 Test yourself 31

Cells and tissues

6 The cell 33 6.1 Introduction 33 6.2 Eukaryotic cells 33 6.3 Cell specialisation 37 6.4 Test yourself 37

7 Microorganisms 39 7.1 Introduction 39 7.2 Bacteria 40 7.3 Viruses 42 7.4 Fungi 45 7.5 Other infectious agents 45 7.6 Treatment of infectious disease 46 7.7 Test yourself 47

8 Energy metabolism 49 8.1 Introduction 49 8.2 The citric acid cycle 50 8.3 Release of energy from fats and proteins 51 8.4 Oxidative phosphorylation 51 8.5 Anaerobic respiration 53 8.6 Test yourself 53

9 Membrane transport 55 9.1 Introduction 55 9.2 Osmosis 55 9.3 Facilitated diffusion 58 9.4 Active transport 58 9.5 Exocytosis and endocytosis 59 9.6 Test yourself 60

10 Cell division and mitosis 61 10.1 Introduction 61 10.2 Cell cycle 61 10.3 Control of cell division 63 10.4 Cell division and differentiation 64 10.5 Test yourself 65

11 Reproduction 67 11.1 Introduction 67 11.2 Sexual reproduction 68 11.3 Fertilisation 70 11.4 Reproductive and therapeutic cloning 71 11.5 Test yourself 72

12 Inheritance 73 12.1 Introduction 73


Contents vii

12.2 Definition of terms 73 12.3 Mutations 74 12.4 Mendelian inheritance 74 12.5 Monohybrid inheritance 75 12.6 Dihybrid inheritance 76 12.7 Linkage 77 12.8 Autosomal and sex-linked genes 78 12.9 Genetic fingerprinting 78 12.10 Evolution by natural selection 78 12.11 Test yourself 80

13 Genetic disease 81 13.1 Introduction 81 13.2 Cystic fibrosis: an autosomal recessive disease 81 13.3 Huntington disease: an autosomal dominant disease 82 13.4 Haemophilia A: an X-linked recessive disease 83 13.5 Asthma: a disease caused by multiple genes 84 13.6 Genetic testing 85 13.7 Gene therapy and genetic diseases 85 13.8 Test yourself 86

14 Epithelial tissues 87 14.1 Introduction 87 14.2 Classification 87 14.3 Adhesion 88 14.4 Test yourself 89

15 Connective tissues 91 15.1 Introduction 91 15.2 Glycosaminoglycans 91 15.3 Fibres 92 15.4 Cells 93 15.5 Test yourself 93

16 Excitable tissues 95 16.1 Introduction 95 16.2 Membrane potential 95 16.3 Muscle 97 16.4 Nerve 101 16.5 Test yourself 105

Systems

17 Homeostasis 107 17.1 Introduction 107 17.2 Regulation of plasma glucose 108 17.3 Thermoregulation 109 17.4 Test yourself 111

18 The endocrine system 113 18.1 Introduction 113 18.2 Types of hormone 114


viii Catch Up Biology

18.3 Cell signal receptors 114 18.4 Second messenger systems 115 18.5 Endocrine glands 116 18.6 Hypothalamo-pituitary axis 117 18.7 Posterior pituitary 119 18.8 Other endocrine glands 120 18.9 Test yourself 121

19 The nervous system 123 19.1 Introduction 123 19.2 Structure of nervous system 123 19.3 The brain 124 19.4 The spinal cord 126 19.5 Peripheral nervous system 127 19.6 Test yourself 129

20 The cardiovascular system 131 20.1 Introduction 131 20.2 Blood 131 20.3 Plasma 132 20.4 Red blood cells 133 20.5 Platelets 134 20.6 The heart 134 20.7 The circulatory system 137 20.8 Test yourself 138

21 The respiratory system 139 21.1 Introduction 139 21.2 Structure of the respiratory system 139 21.3 Respiration 141 21.4 Gaseous exchange 142 21.5 Control of respiration 143 21.6 Test yourself 144

22 The digestive system 145 22.1 Introduction 145 22.2 Oral cavity 145 22.3 Structure of the GI tube 146 22.4 The stomach 147 22.5 Small intestine and accessory glands 148 22.6 Large intestine 150 22.7 Test yourself 151

23 The reproductive system 153 23.1 Introduction 153 23.2 Male reproductive system 153 23.3 Spermatogenesis 154 23.4 Female reproductive system 154 23.5 Oogenesis and the menstrual cycle 155 23.6 Copulation and fertilisation 157 23.7 Implantation and pregnancy 157 23.8 Birth 158


Contents ix

23.9 Test yourself 158

24 The urinary system 159 24.1 The kidney 159 24.2 The bladder 162 24.3 Test yourself 162

25 The immune system 163 25.1 Immune responses to infection 163 25.2 Inflammation 165 25.3 Lymphocytes and the specific immune response 165 25.4 Diseases of the immune system 166 25.5 Test yourself 167

26 The musculoskeletal system 169 26.1 Introduction 169 26.2 Bone 169 26.3 Regulation of calcium levels 170 26.4 Cartilage 171 26.5 The skeleton 171 26.6 Synovial joints 171 26.7 Muscles and locomotion 172 26.8 Test yourself 174

Answers to ‘test yourself’ questions 175

Glossary 179

Index 203


3.1 Introduction

Carbohydrates, as the name suggests, are molecules made up from carbon and water (hydrogen and oxygen). Carbohydrates serve a number of important functions. They are a major source of energy for life. They are also important structural molecules in many organisms. Additionally, carbohydrates can be attached to other biological molecules, such as proteins, and in doing so modify the properties of the molecule. Probably the most abundant organic molecule on earth is the carbohydrate cellulose, a major constituent of plant cell walls. Sugars are small carbohydrates and these can be joined together to form oligosaccharides and polysaccharides. Different sugars can be joined together in many different ways to give rise to a very diverse array of structures.

3.2 Monosaccharides

The simplest sugars are monosaccharides, which have the general formula (CH2O)n. According to the value of n, sugars are described as trioses, pentoses, hexoses etc. An important sugar in energy metabolism is glucose. This is a hexose with the formula C6H12O6. Monosaccharides such as glucose can occur as mirror image D and L isomers (enantiomers), similar to amino acids (see Fig. 3.1). D or L refers specifically to the arrangement of groups at the asymmetric carbon furthest from the aldehyde group (carbon 5 in glucose).

BASIC CONCEPTS:

Carbohydrates range in structure from simple sugar molecules, such as glucose, to polymers of sugar molecules, such as those that constitute the rigid wall around all plant cells. Carbohydrates are a major source of energy for the body and so knowledge of their basic structure is very important in understanding energy metabolism. Carbohydrates can be attached to other molecules, including proteins and lipids, and are present on the surface of cells where they can act as recognition molecules.

03 Carbohydrates


14 Catch Up Biology

D-glucose L-glucose

H

H

H

H

H

H

HO

HH

OH

HO

OH

OH

HO

HO

OH

1CHO

6CH2OH

2

3

4

5

1CHO

6CH2OH

2

3

4

5

C

C

C

C

C

C

C

C

Figure 3.1. D and L glucose

Other sugars, including mannose and galactose, share the same chemical formula as glucose but are not mirror image forms (Fig. 3.2). Glucose frequently assumes a ring form, as shown in Fig. 3.2, by the reaction of the aldehyde group on carbon 1 with the hydroxyl group on carbon 5. The glucose ring can also be found in either alpha or beta form, according to the arrangement of the OH group attached to the first carbon atom. In alpha

CH2OH CH2OH

CHO

H

H

H

H

H

C

C

C

C

H

OH

OH

OH

OH C

C

C

C

CHO

OH

OH

H

H

OH

OH

Glucose Mannose

CH2OHCH2OH

HH

H

H

H

H

H

HH

OH

OH

OH

OH

OH OH

H

OH

OH

β form α form

O O

(b)

(a)

Figure 3.2. (a) Glucose and mannose in open chain form; (b) Glucose in ring form


Chapter 3 Carbohydrates 15

glucose the OH group is below the plane of the ring, whereas in beta glucose it is above the plane of the ring. This minor structural difference has important consequences for the properties of the molecules and their polymers. Starch is a polymer of alpha glucose molecules and cellulose is a polymer of beta glucose. Note that humans have digestive enzymes that can degrade starch but are unable to digest cellulose.

Sugars may also contain additional groups. Glucosamine is a derivative of glucose in which an amino (NH2) group replaces one of the OH groups. The amino group frequently has an additional acetyl group (COCH3) attached to it, forming N-acetyl glucosamine. Another important glucose derivative is glucuronic acid, which contains a carboxylic acid group. Sugar derivatives such as N-acetyl-glucosamine and glucuronic acid are present in glycosaminoglycans, which are an important component of the extracellular matrix (see Chapter 15).

3.3 Glycosidicbond

When sugars join together to form oligosaccharides or polysaccharides this creates a glycosidic bond. A glycosidic bond is formed by the interaction of an OH group on one sugar with an OH group on another sugar. This involves the elimination of water and is therefore a condensation reaction. Enzymes that can break down glycosidic bonds are termed glycosidases. In comparison to the peptide bond, which is always formed between the NH2 and COOH groups on the alpha carbon of an amino acid, there is much greater diversity in the way in which glycosidic bonds can form. Different carbon atoms can be involved, and an individual sugar can participate in multiple glycosidic bonds to give rise to branched structures. The bond is described according to the particular carbon atoms involved and whether the hydroxyl groups are in the alpha or beta position. In starch the glucose units are joined by a-1,4 linkages (Fig. 3.3), which means that the reaction involves the OH group on carbon 1 of one sugar reacting with the OH group on carbon 4 of the next. Glycosidases are highly specific for a particular type of glycosidic linkage.


16 Catch Up Biology

CH2OH

CHOH

H

C4 41

4 41 1

1

HO

H

C

OHC

H

O

HC

OH

CH2OH

CHOH

H

HO

C C

C

H

O

HC

OH

CH2OH

CHOH

H

C C

H

OH

C

H

O

HC

OH

CH2OH

CHOH

H

C

HO

OH

C

HC

H

dehydration

H2O

O

HC

OH

H

O

+

Figure 3.3. Formation of an a-1,4 glycosidic bond

3.4 Polysaccharides

Polysaccharides consist of many monosaccharide subunits covalently linked together. They may involve one or more different sugar units and can be either linear or branched in structure. Starch is a storage polysaccharide of plants that plays an important role in the human diet. Starch has two components – amylose, which is a linear polymer of glucose joined by a-1,4 linkages, and amylopectin, which is similar but also contains branches due to additional a-1,6 linkages occurring at points along the chain (Fig. 3.4). The enzyme that digests starch is called amylase and is present in saliva and in the small intestine.

CH2OH

O

O

O

O O O

OO

CH2OH6'CH2

1'

1' 4'

1,6 linkage

1,4 linkage

Figure 3.4. a-1,4 and a-1,6 linkages in glycogen



Mammals store carbohydrates in the form of glycogen and this can be found in the liver and in muscle. Glycogen is similar in structure to amylopectin but is more branched.

Cellulose is an unbranched polymer of glucose linked by b-1,4 glycosidic bonds and is an important structural component of the cell walls of plants. Humans do not have an enzyme that can hydrolyse this type of linkage and cannot use cellulose as an energy source. However, cellulose does play an important role in digestive function as a source of roughage, or insoluble matter, helping to maintain the consistency of the faeces and to stimulate mucus secretion as waste matter passes through the large intestine. Ruminants, such as cattle and sheep, are able to make use of cellulose because they harbour bacteria that secrete the enzyme cellulase in a part of their digestive tract known as the rumen.

3.5 Glycoconjugates

Other molecules may also have sugars attached to them – a process known as glycosylation. Many proteins are glycosylated, particularly those that are found in cell membranes or that are secreted from the cell. Glycoproteins contain one or more oligosaccharides attached to the protein backbone, either at the hydroxyl group of serine or threonine residues (when they are described as O-linked) or via the amide group of asparagine (N-linked). The relative amount of carbohydrate in glycoproteins varies enormously, from merely 1 or 2% to over 70%. Mucin is a very large and heavily glycosylated molecule that forms a major constituent of mucus and contributes to its gel-like properties. Mucin contains a very high proportion of carbohydrate in the form of O-linked oligosaccharides. Lipids may also be glycosylated.

CLINICAL EXAMPLE: ABO BLOOD GROUPS

The human ABO blood group antigens are oligosaccharides present on glycoproteins and glycolipids on red blood cells. Blood group specificity is defined by the oligosaccharide component. Enzymes called glycosyl transferases are required for the synthesis of oligosaccharides. These are highly specific and whether an individual produces the blood group oligosaccharide O, A or B will depend on which glycosyl transferases they have inherited from their parents. If an individual with type O blood were to receive a transfusion of type A blood then their immune system would react against the A oligosaccharide, destroying the transfused cells and leading to serious consequences.


18 Catch Up Biology

3.6 Functionsofcarbohydrates

Carbohydrates serve a range of biological functions, several of which have been referred to above. They are an important part of the diet, providing a ready source of energy. In the form of polysaccharides such as starch and glycogen they play a role in storage. They are also important structural components of cells (e.g. cellulose) and influence the mechanical properties of tissues or secretions (e.g. glycosaminoglycans, mucins).

Sugars are also important recognition molecules. Within the cell, the carbohydrates on glycoproteins play a role in ensuring that these molecules are transferred to the appropriate cellular compartments. Carbohydrates present on cell surface glycoproteins and glycolipids also interact with proteins called lectins. This type of interaction plays an important role in the trafficking of white blood cells to sites of infection (see Chapter 25). Oligosaccharides also play an important role in the recognition of an egg by a sperm (see Chapter 11).

3.7 TestyourselfThe answers are given on p. 175.

Question 3.1What is the chemical formula for glucose?

Question 3.2What is the name of the bond that joins sugars together in a polysaccharide?

Question 3.3What is the enzyme that degrades starch called?

Question 3.4What are the enzymes that synthesise oligosaccharides?

Question 3.5What polymer is used to store carbohydrates in

animals?


P HIL IP BR A DL E Y & J A NE C A LV ER T

SE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T IONSE COND ED I T ION

BIOLOGY F O R T H E M E D I C A L S C I E N C E S

Catch Up Biology covers the basic principles and concepts in biology that you will need if you are studying medicine or a related subject, or one of the biomedical sciences. The book focuses on human biology and covers: • the basic molecules of life, such as proteins, carbohydrates, nucleic

acids • cells, tissues and processes, including energy metabolism, cell division,

epithelial and connective tissues • the key mammalian systems, for example, homeostasis, the endocrine,

respiratory and digestive systems

Throughout the book the authors highlight clinical examples so that you can see the relevance of basic biology to your course. The book also contains questions (and answers) so that you can test your understanding of the subject as you work through the book. This new edition features two new chapters on microorganisms and on genetic disease.

Catch Up Biology is the ideal book to refresh your understanding of the basic concepts of biology.

R E V IE W S OF T HE F IR S T ED I T ION:“… extremely useful at university, a very in depth book, just what’s required”

“I have enjoyed reading this book and have found it highly informative. I have studied Biology in a formal context, but this was some years ago. I bought this book in order to better inform my grasp of biology before applying for a

Medical Degree”

MI T CH F R Y A ND EL IZ A BE T H PA GE

CHEMISTRY F O R T H E L I F E A N D M E D I C A L S C I E N C E S

M. H A R R I S , G . TAY L OR & J . TAY L OR

MATHS & STATS F O R T H E L I F E A N D M E D I C A L S C I E N C E S

BIOLOGY

BIOLOGYBRADLEY & CALVERT

w w w. s c i o n p u b l i s h i n g . c o m

9 781904 842880

ISBN 978-1-904842-88-0

Contents

Preface xi

Acknowledgements xii


1 Water and life 1 1.1 The properties of water 1 1.2 Water in the human body 2 1.3 Test yourself 3

2 Proteins 5 2.1 Introduction 5 2.2 Primary structure 7 2.3 Secondary structures 7 2.4 Tertiary structure 9 2.5 Quaternary structure 10 2.6 Domains 10 2.7 Functions of proteins 11 2.8 Conformational change 11 2.9 Test yourself 12

3 Carbohydrates 13 3.1 Introduction 13 3.2 Monosaccharides 13 3.3 Glycosidic bond 15 3.4 Polysaccharides 16 3.5 Glycoconjugates 17 3.6 Functions of carbohydrates 18 3.7 Test yourself 18

4 Lipids 19 4.1 Introduction 19 4.2 Fatty acids 19 4.3 Triglycerides and phospholipids 20 4.4 Cholesterol 21 4.5 Functions of lipids 21 4.6 Test yourself 22

5 Nucleic acids and genes 23 5.1 Introduction 23 5.2 DNA and RNA 24 5.3 DNA synthesis 26 5.4 RNA synthesis 26 5.5 The genetic code 26 5.6 Protein synthesis 27



vi Catch Up Biology

5.7 Introns and exons 29 5.8 Regulation of gene expression 29 5.9 Test yourself 31

Cells and tissues

6 The cell 33 6.1 Introduction 33 6.2 Eukaryotic cells 33 6.3 Cell specialisation 37 6.4 Test yourself 37

7 Microorganisms 39 7.1 Introduction 39 7.2 Bacteria 40 7.3 Viruses 42 7.4 Fungi 45 7.5 Other infectious agents 45 7.6 Treatment of infectious disease 46 7.7 Test yourself 47

8 Energy metabolism 49 8.1 Introduction 49 8.2 The citric acid cycle 50 8.3 Release of energy from fats and proteins 51 8.4 Oxidative phosphorylation 51 8.5 Anaerobic respiration 53 8.6 Test yourself 53

9 Membrane transport 55 9.1 Introduction 55 9.2 Osmosis 55 9.3 Facilitated diffusion 58 9.4 Active transport 58 9.5 Exocytosis and endocytosis 59 9.6 Test yourself 60

10 Cell division and mitosis 61 10.1 Introduction 61 10.2 Cell cycle 61 10.3 Control of cell division 63 10.4 Cell division and differentiation 64 10.5 Test yourself 65

11 Reproduction 67 11.1 Introduction 67 11.2 Sexual reproduction 68 11.3 Fertilisation 70 11.4 Reproductive and therapeutic cloning 71 11.5 Test yourself 72

12 Inheritance 73 12.1 Introduction 73

Cells and tissues


Contents vii

12.2 Definition of terms 73 12.3 Mutations 74 12.4 Mendelian inheritance 74 12.5 Monohybrid inheritance 75 12.6 Dihybrid inheritance 76 12.7 Linkage 77 12.8 Autosomal and sex-linked genes 78 12.9 Genetic fingerprinting 78 12.10 Evolution by natural selection 78 12.11 Test yourself 80

13 Genetic disease 81 13.1 Introduction 81 13.2 Cystic fibrosis: an autosomal recessive disease 81 13.3 Huntington disease: an autosomal dominant disease 82 13.4 Haemophilia A: an X-linked recessive disease 83 13.5 Asthma: a disease caused by multiple genes 84 13.6 Genetic testing 85 13.7 Gene therapy and genetic diseases 85 13.8 Test yourself 86

14 Epithelial tissues 87 14.1 Introduction 87 14.2 Classification 87 14.3 Adhesion 88 14.4 Test yourself 89

15 Connective tissues 91 15.1 Introduction 91 15.2 Glycosaminoglycans 91 15.3 Fibres 92 15.4 Cells 93 15.5 Test yourself 93

16 Excitable tissues 95 16.1 Introduction 95 16.2 Membrane potential 95 16.3 Muscle 97 16.4 Nerve 101 16.5 Test yourself 105

Systems

17 Homeostasis 107 17.1 Introduction 107 17.2 Regulation of plasma glucose 108 17.3 Thermoregulation 109 17.4 Test yourself 111

18 The endocrine system 113 18.1 Introduction 113 18.2 Types of hormone 114

Systems


viii Catch Up Biology

18.3 Cell signal receptors 114 18.4 Second messenger systems 115 18.5 Endocrine glands 116 18.6 Hypothalamo-pituitary axis 117 18.7 Posterior pituitary 119 18.8 Other endocrine glands 120 18.9 Test yourself 121

19 The nervous system 123 19.1 Introduction 123 19.2 Structure of nervous system 123 19.3 The brain 124 19.4 The spinal cord 126 19.5 Peripheral nervous system 127 19.6 Test yourself 129

20 The cardiovascular system 131 20.1 Introduction 131 20.2 Blood 131 20.3 Plasma 132 20.4 Red blood cells 133 20.5 Platelets 134 20.6 The heart 134 20.7 The circulatory system 137 20.8 Test yourself 138

21 The respiratory system 139 21.1 Introduction 139 21.2 Structure of the respiratory system 139 21.3 Respiration 141 21.4 Gaseous exchange 142 21.5 Control of respiration 143 21.6 Test yourself 144

22 The digestive system 145 22.1 Introduction 145 22.2 Oral cavity 145 22.3 Structure of the GI tube 146 22.4 The stomach 147 22.5 Small intestine and accessory glands 148 22.6 Large intestine 150 22.7 Test yourself 151

23 The reproductive system 153 23.1 Introduction 153 23.2 Male reproductive system 153 23.3 Spermatogenesis 154 23.4 Female reproductive system 154 23.5 Oogenesis and the menstrual cycle 155 23.6 Copulation and fertilisation 157 23.7 Implantation and pregnancy 157 23.8 Birth 158


Contents ix

23.9 Test yourself 158

24 The urinary system 159 24.1 The kidney 159 24.2 The bladder 162 24.3 Test yourself 162

25 The immune system 163 25.1 Immune responses to infection 163 25.2 Inflammation 165 25.3 Lymphocytes and the specific immune response 165 25.4 Diseases of the immune system 166 25.5 Test yourself 167

26 The musculoskeletal system 169 26.1 Introduction 169 26.2 Bone 169 26.3 Regulation of calcium levels 170 26.4 Cartilage 171 26.5 The skeleton 171 26.6 Synovial joints 171 26.7 Muscles and locomotion 172 26.8 Test yourself 174

Answers to ‘test yourself’ questions 175

Glossary 179

Index 203


3.1 Introduction

Carbohydrates, as the name suggests, are molecules made up from carbon and water (hydrogen and oxygen). Carbohydrates serve a number of important functions. They are a major source of energy for life. They are also important structural molecules in many organisms. Additionally, carbohydrates can be attached to other biological molecules, such as proteins, and in doing so modify the properties of the molecule. Probably the most abundant organic molecule on earth is the carbohydrate cellulose, a major constituent of plant cell walls. Sugars are small carbohydrates and these can be joined together to form oligosaccharides and polysaccharides. Different sugars can be joined together in many different ways to give rise to a very diverse array of structures.

3.2 Monosaccharides

The simplest sugars are monosaccharides, which have the general formula (CH2O)n. According to the value of n, sugars are described as trioses, pentoses, hexoses etc. An important sugar in energy metabolism is glucose. This is a hexose with the formula C6H12O6. Monosaccharides such as glucose can occur as mirror image D and L isomers (enantiomers), similar to amino acids (see Fig. 3.1). D or L refers specifically to the arrangement of groups at the asymmetric carbon furthest from the aldehyde group (carbon 5 in glucose).

BASIC CONCEPTS:

Carbohydrates range in structure from simple sugar molecules, such as glucose, to polymers of sugar molecules, such as those that constitute the rigid wall around all plant cells. Carbohydrates are a major source of energy for the body and so knowledge of their basic structure is very important in understanding energy metabolism. Carbohydrates can be attached to other molecules, including proteins and lipids, and are present on the surface of cells where they can act as recognition molecules.

03 Carbohydrates03


14 Catch Up Biology

D-glucose L-glucose

H

H

H

H

H

H

HO

HH

OH

HO

OH

OH

HO

HO

OH

1CHO

6CH2OH

2

3

4

5

1CHO

6CH2OH

2

3

4

5

C

C

C

C

C

C

C

C

Figure 3.1. D and L glucose

Other sugars, including mannose and galactose, share the same chemical formula as glucose but are not mirror image forms (Fig. 3.2). Glucose frequently assumes a ring form, as shown in Fig. 3.2, by the reaction of the aldehyde group on carbon 1 with the hydroxyl group on carbon 5. The glucose ring can also be found in either alpha or beta form, according to the arrangement of the OH group attached to the first carbon atom. In alpha

CH2OH CH2OH

CHO

H

H

H

H

H

C

C

C

C

H

OH

OH

OH

OH C

C

C

C

CHO

OH

OH

H

H

OH

OH

Glucose Mannose

CH2OHCH2OH

HH

H

H

H

H

H

HH

OH

OH

OH

OH

OH OH

H

OH

OH

β form α form

O O

(b)

(a)

Figure 3.2. (a) Glucose and mannose in open chain form; (b) Glucose in ring form



glucose the OH group is below the plane of the ring, whereas in beta glucose it is above the plane of the ring. This minor structural difference has important consequences for the properties of the molecules and their polymers. Starch is a polymer of alpha glucose molecules and cellulose is a polymer of beta glucose. Note that humans have digestive enzymes that can degrade starch but are unable to digest cellulose.

Sugars may also contain additional groups. Glucosamine is a derivative of glucose in which an amino (NH2) group replaces one of the OH groups. The amino group frequently has an additional acetyl group (COCH3) attached to it, forming N-acetyl glucosamine. Another important glucose derivative is glucuronic acid, which contains a carboxylic acid group. Sugar derivatives such as N-acetyl-glucosamine and glucuronic acid are present in glycosaminoglycans, which are an important component of the extracellular matrix (see Chapter 15).

3.3 Glycosidic bond

When sugars join together to form oligosaccharides or polysaccharides this creates a glycosidic bond. A glycosidic bond is formed by the interaction of an OH group on one sugar with an OH group on another sugar. This involves the elimination of water and is therefore a condensation reaction. Enzymes that can break down glycosidic bonds are termed glycosidases. In comparison to the peptide bond, which is always formed between the NH2 and COOH groups on the alpha carbon of an amino acid, there is much greater diversity in the way in which glycosidic bonds can form. Different carbon atoms can be involved, and an individual sugar can participate in multiple glycosidic bonds to give rise to branched structures. The bond is described according to the particular carbon atoms involved and whether the hydroxyl groups are in the alpha or beta position. In starch the glucose units are joined by a-1,4 linkages (Fig. 3.3), which means that the reaction involves the OH group on carbon 1 of one sugar reacting with the OH group on carbon 4 of the next. Glycosidases are highly specific for a particular type of glycosidic linkage.


16 Catch Up Biology

CH2OH

CHOH

H

C4 41

4 41 1

1

HO

H

C

OHC

H

O

HC

OH

CH2OH

CHOH

H

HO

C C

C

H

O

HC

OH

CH2OH

CHOH

H

C C

H

OH

C

H

O

HC

OH

CH2OH

CHOH

H

C

HO

OH

C

HC

H

dehydration

H2O

O

HC

OH

H

O

+

Figure 3.3. Formation of an a-1,4 glycosidic bond

3.4 Polysaccharides

Polysaccharides consist of many monosaccharide subunits covalently linked together. They may involve one or more different sugar units and can be either linear or branched in structure. Starch is a storage polysaccharide of plants that plays an important role in the human diet. Starch has two components – amylose, which is a linear polymer of glucose joined by a-1,4 linkages, and amylopectin, which is similar but also contains branches due to additional a-1,6 linkages occurring at points along the chain (Fig. 3.4). The enzyme that digests starch is called amylase and is present in saliva and in the small intestine.

CH2OH

O

O

O

O O O

OO

CH2OH6'CH2

1'

1' 4'

1,6 linkage

1,4 linkage

Figure 3.4. a-1,4 and a-1,6 linkages in glycogen



Mammals store carbohydrates in the form of glycogen and this can be found in the liver and in muscle. Glycogen is similar in structure to amylopectin but is more branched.

Cellulose is an unbranched polymer of glucose linked by b-1,4 glycosidic bonds and is an important structural component of the cell walls of plants. Humans do not have an enzyme that can hydrolyse this type of linkage and cannot use cellulose as an energy source. However, cellulose does play an important role in digestive function as a source of roughage, or insoluble matter, helping to maintain the consistency of the faeces and to stimulate mucus secretion as waste matter passes through the large intestine. Ruminants, such as cattle and sheep, are able to make use of cellulose because they harbour bacteria that secrete the enzyme cellulase in a part of their digestive tract known as the rumen.

3.5 Glycoconjugates

Other molecules may also have sugars attached to them – a process known as glycosylation. Many proteins are glycosylated, particularly those that are found in cell membranes or that are secreted from the cell. Glycoproteins contain one or more oligosaccharides attached to the protein backbone, either at the hydroxyl group of serine or threonine residues (when they are described as O-linked) or via the amide group of asparagine (N-linked). The relative amount of carbohydrate in glycoproteins varies enormously, from merely 1 or 2% to over 70%. Mucin is a very large and heavily glycosylated molecule that forms a major constituent of mucus and contributes to its gel-like properties. Mucin contains a very high proportion of carbohydrate in the form of O-linked oligosaccharides. Lipids may also be glycosylated.






18 Catch Up Biology

3.6 Functions of carbohydrates

Carbohydrates serve a range of biological functions, several of which have been referred to above. They are an important part of the diet, providing a ready source of energy. In the form of polysaccharides such as starch and glycogen they play a role in storage. They are also important structural components of cells (e.g. cellulose) and influence the mechanical properties of tissues or secretions (e.g. glycosaminoglycans, mucins).

Sugars are also important recognition molecules. Within the cell, the carbohydrates on glycoproteins play a role in ensuring that these molecules are transferred to the appropriate cellular compartments. Carbohydrates present on cell surface glycoproteins and glycolipids also interact with proteins called lectins. This type of interaction plays an important role in the trafficking of white blood cells to sites of infection (see Chapter 25). Oligosaccharides also play an important role in the recognition of an egg by a sperm (see Chapter 11).

3.7 Test yourselfThe answers are given on p. 175.

Question 3.1What is the chemical formula for glucose?

Question 3.2What is the name of the bond that joins sugars together in a polysaccharide?

Question 3.3What is the enzyme that degrades starch called?

Question 3.4What are the enzymes that synthesise oligosaccharides?

Question 3.5What polymer is used to store carbohydrates in

animals?


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