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© James Torrance, James Fullarton, Clare Marsh, James Simms, Caroline Stevenson

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Preface

This book has been written to act as a valuable resource for students studying Higher Grade Human Biology. It provides a core text which adheres closely to the SQA syllabus for Higher Human Biology introduced in 2018. Each section of the book matches a section of the revised syllabus; each chapter corresponds to a content area. In addition to the core text, the book contains a variety of special features:

Learning Activities

Each chapter contains an appropriate selection of learning activities in the form of Case Studies, Related Topics, Research Topics, Related Information, Related Activities and Investigations, as laid down in the SQA Course Support Notes. These non-essential activities are highlighted throughout in yellow for easy identification. They do not form part of the basic mandatory course content needed when preparing for the final exam but are intended to aid understanding and to support research tasks during course work.

Testing Your Knowledge

Key questions incorporated into the text of every chapter and designed to continuously assess Knowledge and Understanding. These are especially useful as homework and as instruments of diagnostic assessment to check that full understanding of course content has been achieved.

What You Should Know

Summaries of key facts and concepts as ‘Cloze’ Tests accompanied by appropriate word banks. These feature at regular intervals throughout the book and provide an excellent source of material for consolidation and revision prior to the SQA examination.

Applying Your Knowledge and Skills

A variety of questions at the end of each chapter designed to give students practice in exam questions and foster the development of Skills of Scientific Inquiry and Investigation (for example, selection of relevant information, presentation of information, processing of information, planning experimental procedure, evaluating, drawing valid conclusions and making predictions and generalisations). These questions are especially useful as extensions to class work and as homework.

Updates and syllabus changes: important note to teachers and students from the publisher

This book covers all course arrangements for Higher Human Biology but does not attempt to give advice on any ‘added value assessments’ or ‘open assignments’ that form part of a final grade in Higher Human Biology (2018 onwards).

Please remember that syllabus arrangements change from time to time. We make every effort to update our textbooks as soon as possible when this happens, but – especially if you are using an old copy of this book – it is always advisable to check whether there have been any alterations to the arrangements since this book was printed. You can check the latest arrangements at the SQA website (www.sqa.org.uk), and you can also check for any specific updates to this book at www.hoddereducation.co.uk/scottish-curriculum.

We make every effort to ensure accuracy of content, but if you discover any mistakes please let us know as soon as possible – see our contact details on the back cover.

1 Division and differentiation in human cells

Somatic cells

A somatic cell is any type of cell in the human body other than those cells that are involved in reproduction.

Division of somatic stem cells

Somatic stem cells divide during growth and tissue repair. In humans, each somatic stem cell contains 46 chromosomes and is diploid (i.e. contains 23 pairs of homologous chromosomes). Prior to nuclear division (mitosis), the genetic material undergoes DNA replication (see Chapter 2) and becomes doubled in quantity for a brief time. Nuclear division quickly follows and the genetic material is divided equally between two daughter nuclei. Therefore, each somatic stem cell formed receives an identical copy of the full set of 46 chromosomes and the diploid chromosome number is maintained. This process ensures that each somatic stem cell receives a complete set of the genetic information.

Germline cells

Germline cells are the cells involved in reproduction. They include haploid gametes (sperm and ova) and the diploid cells that divide to form these gametes.

Division of germline stem cells

Diploid germline stem cells first divide by mitosis. The nucleus of each of these cells contains 23 pairs of homologous chromosomes and this genetic material becomes doubled by DNA replication. Following nuclear division, each identical diploid germline stem cell formed receives 23 pairs of homologous chromosomes and the chromosome number is maintained.

Each of these diploid germline stem cells then undergoes a second form of nuclear division called meiosis. This involves a further doubling of the genetic material followed by two rounds of nuclear division. The first round separates the homologous chromosomes; the second separates the chromatids. The result is the formation of four haploid gametes, each containing 23 single chromosomes.

The human life cycle alternates between haploid and diploid cells as summarised in Figure 1.1.

Cellular differentiation

A multicellular organism such as a human being consists of a large number of cells. Rather than each cell carrying out every function for the maintenance of life, a division of labour occurs and most of the cells become differentiated. Cellular differentiation is the process by which an unspecialised cell becomes altered and adapted to perform a specialised function as part of a permanent tissue.

Differentiation in human cells

A human being begins life as a fertilised egg (zygote), as shown in Figure 1.2. The zygote divides repeatedly by mitosis and cell division to form an embryo. Like the cells in an adult, each embryonic cell possesses all the genes for constructing the whole organism. However, unlike those in adult cells, all the genes in cells at this early stage are switched on or have the potential to become switched on.

As embryological development proceeds, the unspecialised cells of the early embryo undergo differentiation and become specialised in structure and biochemical properties, making them perfectly adapted for carrying out a particular function. For example, a motor neuron (see Figure 1.2) is a type of nerve cell that possesses an axon (a long, insulated cytoplasmic extension). This structure is ideally suited to the transmission of nerve impulses. Similarly the cells of the epithelial lining of the windpipe are perfectly suited to their job of sweeping dirty mucus up and away from the lungs.

Selective gene expression

Once a cell becomes differentiated, it only expresses the genes that code (see Chapter 3) for the proteins characteristic of that particular type of cell. For example, in a nerve cell, genes that code for the formation of neurotransmitter substances are switched on and continue to operate but those for mucus are switched off. The reverse is true of the genes in a goblet cell in the lining of the windpipe. Only a fraction of the genes in a specialised cell are expressed (e.g. 3–5% in a typical human cell).

Stem cells

Stem cells are unspecialised cells that can:

•  reproduce (‘self-renew’) themselves by repeated mitosis and cell division while remaining undifferentiated and unspecialised

•  differentiate into specialised cells when required to do so by the multicellular organism that possesses them.

Embryonic stem cells

An early human embryo consists of a ball of embryonic stem cells (see Figures 1.3 and 1.4). All of the genes in an embryonic cell have the potential to be switched on, so the cell is capable of differentiating into almost all of the cell types (more than 200) found in the human body. Because of this ability, embryonic stem cells are described as being pluripotent (see also page 9).

Tissue stem cells

Tissue stem cells are found in locations such as skin and red bone marrow (see Figure 1.3). They have a much narrower differentiation potential than embryonic stem cells because many of their genes are already switched off. However, they are able to replenish continuously the supply of certain differentiated cells needed by the organism such as skin, intestinal lining and blood.

Tissue stem cells can only give rise to a limited range of cell types closely related to the tissue in which they are normally located. However, they are involved in the growth, repair and renewal of all the cell types found in that particular tissue and are therefore described as being multipotent. For example, blood stem cells in red bone marrow give rise to red blood cells, white blood cells (various forms of phagocytes and lymphocytes) and platelets. (Also see the Related Topic – Origin of blood cells below.)

Related Topic

Origin of blood cells

Tissue stem cells in red bone marrow (see Figure 1.5) give rise to red blood cells, platelets and specialised white blood cells including phagocytes, natural killer cells and B and T lymphocytes – the cells of the immune system.

Testing Your Knowledge 1

1  a)  Briefly describe how chromosome number is maintained in a somatic cell at cell division. (2)

    b)  What is the difference between the terms haploid and diploid? (2)

    c)  What is a germline cell? (1)

    d)  Briefly describe how a diploid germline cell produces haploid gametes. (2)

2  a)  Define the term cellular differentiation. (2)

    b)  In what way is a ciliated epithelial cell a good example of a specialised cell? (1)

    c)  A goblet cell in the lining of the windpipe produces mucus but not insulin. Explain briefly how this specialisation is brought about with reference to genes. (2)

3  a)  Give TWO characteristics of stem cells. (2)

    b)   i)   Name TWO types of stem cell found in humans.

          ii)  For each type, identify ONE location where these cells could be found. (4)

    c)  Which type of stem cell is capable of differentiating into all the types of cell that make up the organism to which it belongs? (1)

Research value of stem cells

Much of the research to date has been carried out using stem cells from embryos of mice and humans. Human stem cells will renew themselves when grown in optimal culture conditions, provided that certain growth factors are present. In the absence of these growth factors, the stem cells differentiate rapidly.

By investigating why stem cells continue to multiply in the presence of a certain chemical yet undergo differentiation in its absence, scientists are attempting to obtain a fuller understanding of cell processes such as growth and differentiation. It is hoped that this will lead in turn to a better understanding of gene regulation (including the molecular biology of cancer).

Models

A model organism is one that is suitable for laboratory research because its biological characteristics are similar to those of a group of related (but often unavailable) organisms. For example, mice are used in research as model organisms for humans since they possess many genes equivalent to the genes in humans that are responsible for certain inherited diseases. Research work on the models helps to provide an understanding of the malfunctioning of these genes and may lead the way to the development of new treatments.

Similarly stem cells, which are genetically identical to differentiated somatic cells, can be used in research as model cells to investigate:

•  the means by which certain diseases and disorders develop

•  the responses of cells to new pharmaceutical drugs.

Therapeutic use of stem cells

Cornea repair

In recent years, scientists have shown that corneal damage by chemical burning can be successfully treated using stem cell tissue. This can be grown from the patient’s own stem cells located at the edge of the cornea. In many cases, the person’s eyesight can be restored following grafting of the stem cell tissue from the healthy eye to the surface of the damaged eye.

Skin graft

In a traditional skin graft, a relatively large section of skin is removed from a region of the person’s body and grafted to the site of injury. This means that the person has two bodily areas that need careful treatment and time to heal.

A skin graft using stem cells only requires a small sample of skin to be taken to obtain stem cells. Therefore the site needs much less healing time and suffers minimum scarring. The sample is normally taken from an area close to and similar in structure to the site of injury. Enzymes are used to isolate and loosen the stem cells which are then cultured. Once a suspension of new stem cells has developed, they are sprayed over the damaged area to bring about regeneration of missing skin.

Bone marrow transplant

Tissue stem cells present in bone marrow and blood are routinely used in bone marrow transplantation to treat cancers of the blood.

Case Study Future potential therapeutic uses of stem cells

Treatment of burns

Recently, human embryonic stem cells grown on synthetic scaffolds have been used to treat burn victims. The stem cell tissue provides a source of temporary skin while the patient is waiting for grafts of their own skin to develop. This potential use of stem cells is expected to be developed further in the future.

Treatment of degenerative conditions

Embryonic stem cells are able to differentiate into any type of cell in the body. Therefore, they are believed to have the potential to provide treatments in the future for a wide range of disorders and degenerative conditions such as diabetes, Parkinson’s disease and Alzheimer’s disease (see Figure 1.6) that traditional medicine has been unable to cure. Already scientists have managed to generate nerve cells from embryonic stem cells in culture. It is hoped that this work will eventually be translated into effective therapies to treat neurological disorders such as multiple sclerosis. However, the use of embryonic stem cells raises questions of ethics.

Ethical issues

Ethics refers to the moral values and rules that ought to govern human conduct. The use of stem cells raises several ethical issues. For example, the extraction of human embryonic stem cells to create a stem cell line (a continuous culture) for research purposes results in the destruction of the human embryo. Many people believe strongly that this practice is unethical (see Case Study – Embryonic stem cell debate).

Ethical issues are also raised by the use of induced pluripotent stem cells (see Case Study – Sources of stem cells) and by the use of nuclear transfer technique (see Case Study – Nuclear transfer technique).

Case Study Embryonic stem cell debate

At present the creation of a human embryonic stem cell line using cells from a human embryo (of no more than 14 days) results in the destruction of the embryo. The ethical debate about the use of embryonic stem cells most commonly rests on the controversial question: ‘Is a human embryo of less than 2 weeks a human person?’

People on one side of the debate believe that the embryo is definitely a human person and argue that fatally extracting stem cells from it constitutes murder. People on the other side of the debate feel certain that the embryo is not yet a person and believe that removing stem cells from it is morally acceptable.

The people who are against stem cell research using human embryos often support their case with the following claims:

•  A human life begins when a sperm cell fuses with an egg cell and it is inviolable (i.e. it is sacred and must not be harmed).

•  A unique version of human DNA is created at conception.

•  A fertilised egg is a human being with a soul.

•  Stem cell research violates the sanctity of life.

The people who are in favour of stem cell research using human embryos often support their case with the following arguments:

•  An embryo is not a person although it has the potential to develop into a person.

•  At 14 days or less an embryo is not sentient (i.e. it does not have a brain, a nervous system, consciousness or powers of sensation).

•  The death of a very young embryo is not of serious moral concern when it has the potential to benefit humanity (particularly people whose daily lives are compromised by debilitating medical conditions).

•  Abortion is legal in many countries including the UK. Destroying a 14-day-old embryo is far less objectionable to most people than terminating a fetus at 20 weeks.

•  Stem cell research uses embryos that were generated for IV fertilisation but were not used and would be destroyed as a matter of course.

Possible solutions to the problem

In the future the ethical issues raised by the use of embryonic stem cells may become less heated if advances in induced pluripotent stem cell technology (see page 9) and increased use of stem cells from amniotic fluid (see page 9) reduce the need for their use.

Case Study Sources of stem cells

Donated embryos

At present patients undergoing infertility treatment may agree to donate any extra embryos that are not required for their treatment to medical science. These very early embryos provide an immediate source of embryonic stem cells for research. In addition long-term cultures originally set up using cells isolated from donated embryos and continued for many years provide a further source of embryonic stem cells. However, the number of human embryonic cells available for research remains limited and this restricts the ability of scientists to carry out research work in this important area.

Amniotic fluid

Scientists continue to search for new sources of stem cells. One of these is amniotic fluid. The stem cells that it contains can be harvested from the fluid removed from pregnant women for amniocentesis tests (see Figure 1.7). The stem cells obtained are capable of differentiating into many types of specialised cell such as bone, muscle, nerve and liver. One advantage of using stem cells from amniotic fluid is that it does not involve the destruction of a human embryo.

Induced pluripotent stem cells

A totipotent stem cell is one that is able to differentiate into any cell type and is capable of giving rise to the complete organism. A pluripotent stem cell is a descendent of a totipotent stem cell and is capable of differentiating into many different types of cell.

Induced pluripotent stem cells are, strictly speaking, not true stem cells. They are differentiated cells (e.g. from human skin) that have been genetically reprogrammed using transcription factors (see Chapter 3) to switch some of their turned-off genes back on again. As a result they act as stem cells and can be used for research. However, the viruses used as vectors to deliver the transcription factors have been shown to cause cancers in mouse models. Therefore a significant amount of research is essential in this area before induced pluripotent cells can be considered for use as part of a routine medical procedure.

Case Study Nuclear transfer technique

This technique involves removing the nucleus from an egg (see Figure 1.8) and then replacing it with a nucleus from a donor cell. Some cells constructed in this way divide normally, producing undifferentiated stem cells.

Using this technique, a nucleus from a human cell (e.g. skin) can be introduced into an enucleated animal cell (e.g. an egg cell from a cow), as shown in Figure 1.9. The cell formed is called a cytoplasmic hybrid cell. Once it begins to divide, stem cells can be extracted after 5 days and used for research. However, they are not 100% human and must not be used for therapeutic procedures.

Some people feel that it is unethical to mix materials from human cells with those of another species even if the hybrid cells formed are used strictly for research purposes only. Other people support the production of cytoplasmic hybrid cells because it helps to relieve the shortage of human embryonic stem cells available for research. In addition, they point out that the practice allows the nucleus from a diseased human cell (e.g. from a patient with a degenerative disease or cancer) to be introduced into the enucleated animal egg. This may allow scientists to study the gene expression in these cells, observe how the disease develops and eventually develop new treatments that disrupt the disease process.

Cancer cells

A cancer is an uncontrolled growth of cells. Cell division in normal healthy cells is controlled by factors such as cell cycle regulators and external chemical signals. Cancerous cells do not respond to these regulatory signals.

Tumours

Cancer cells divide uncontrollably to produce a mass of abnormal cells called a tumour. A tumour is described as benign if it remains as a discrete group of abnormal cells in one place within an otherwise normal tissue. Most benign tumours (e.g. warts – see Figure 1.10) do not cause problems and can be successfully removed.

A tumour is said to be malignant if some of its cells lose the surface molecules that keep them attached to the original cell group, enter the circulatory system and spread through the body. This enables them to invade other tissues and do harm by ‘seeding’ new tumours in other parts of the body (see Figure 1.11).

Genetic errors

Most cancers originate from a cell that has undergone a succession of mutations to the genes involved in the control of cell division. As these genetic errors accumulate, a point is reached where control of cell division is lost. The cell can now divide excessively, unhindered by any regulatory signal or control mechanism.

Normally it takes a very long time for a cell to accumulate, and be affected by, several different mutations. It is for this reason that the risk of cancer increases with age. However, the risk is also increased by exposure to agents that cause genetic damage such as smoking, pollution and excessive exposure of skin to ultraviolet radiation (see Figure 1.12). Cancer is particularly common in skin, lung and bowel tissue because the mutation rate is higher than normal in these cells that have a high frequency of division.

DNA (deoxyribonucleic acid) is a complex molecule present in all living cells. It is the molecule of inheritance and it stores genetic information in its sequence of bases. This sequence determines the organism’s genotype and the structure of its proteins.

Testing Your Knowledge 2

1  a)   i)   Name ONE medical condition that is routinely treated using tissue stem cells.

          ii)  From where in the human body are these cells obtained? (2)

    b)  Give an example of a medical condition that might be treated in the future using stem cells. (1)

    c)  Why can the stem cells used to treat the medical condition you gave as your answer to a) not be used to treat patients who have the condition you gave as your answer to b)? (2)

2  One definition of the word ethical is ‘in accordance with principles that are morally correct’. Briefly explain why stem cell research using human embryos raises ethical issues. (2)

3  Identify TWO characteristics of cancer cells. (2)

Applying Your Knowledge and Skills

1  Figure 1.13 represents the processes of cell division and cellular differentiation in an animal.

Match W, X, Y and Z with the following terms: specialised cell, zygote, tissue stem cell and embryonic stem cell. (3)

2  Figure 1.14 shows a possible use of stem cells in the future.

    a)  Match blank boxes P, Q, R, S, T and U with the following possible answers. (5)

          i)   stem cells induced to differentiate

          ii)  nucleus removed and retained

          iii) stem cells removed and cultured in laboratory

          iv) egg lacking nucleus retained

          v)  matching tissue transplanted to patient without fear of rejection

          vi)  nucleus inserted into egg

    b)  Which of the following is not represented in Figure 1.14? (1)

          A  cytoplasmic hybrid cell

          B  amniotic stem cell line

          C  undifferentiated stem cells

          D  nuclear transfer technique

    c)   i)   Name a source of the donor egg cells used at present in this line of research.

          ii)  Why does this prevent the series of events shown in the diagram being put into practice? (2)

3  The procedure that was adopted to produce ‘Dolly the sheep’ is shown in Figure 1.15.

    a)  What name is given to the technique employed to create the original cell that gave rise to Dolly? (1)

    b)  Why is Dolly said to be the result of a cloning procedure? (1)

    c)   i)   Did Dolly develop a black face or a white face?

          ii)  Explain your answer. (2)

    d)   i)   What was the chance of Dolly being a ram?

A  0

B  1 in 1

C  1 in 2

          ii)  Explain your choice of answer. (2)

4  The four people shown in Figure 1.16 all support the use of human embryos in stem cell research.

    a)  Who is making a statement based on fact rather than expressing an opinion? (1)

    b)   i)   Do you consider this person’s statement to be a convincing or an unconvincing argument in support of the use of embryonic stem cells for research?

          ii)  Justify your answer. (2)

5  Table 1.1 refers to a country’s lung cancer death rates.

    a)  By how many deaths per 100 000 was the lung cancer death rate for region B greater than that for region A in i) 2002? ii) 2007? (2)

    b)   i)   How many people per million died of lung cancer in region A during 2004?

          ii)  What percentage of people in the whole country died of lung cancer in 2009? (2)

    c)   i)   Which region’s data reflect more closely the trend shown by the country as a whole? Explain how you arrived at your answer.

          ii)  In what way does the overall trend for the other region differ from that of the country as a whole? (3)

6  The data in Table 1.2 show the results of a survey carried out on people aged 45–64 years in a northern European country who died of melanoma skin cancer.

    a)  Draw a line graph of the data for the northern European country. (3)

    b)  Identify the dependent and the independent variables from your graph. (1)

    c)  What overall trend does the graph show? (1)

    d)   i)   Which reading in the table is most likely to be a source of error?

          ii)  Justify your answer with reference to the data in the table. (3)

    e)  How many northern European people of this age group per million died of melanoma cancer in 1998? (1)

    f)   i)   Calculate the percentage increase in deaths per 100 000 for the northern European country between 1990 and 1992.

          ii)  Calculate the percentage decrease in deaths per 100 000 for all Europe between 1996 and 1997. (2)

    g)   i)   Which of the following would be the most likely value of deaths per 100 000 for the northern European country for the year 2000?

A  7.8

B  5.2

C  1.9

          ii)  Explain your choice of answer. (2)

    h)  Suggest TWO reasons why death due to melanoma cancer is now less common among the people of this northern European country than it was in the 1990s. (2)