IB Categories Archives: Topic 11: Animal physiology

11.4 – Reproduction

11.4 – Reproduction

11.4.1 – Annotate a light micrograph of testis tissue to show the location and function of interstitial cell (Leydig cells), germinal epithelium cells, developing spermatozoa and Sertoli cells

11.4.2 – Outline the processes involved in spermatogenesis within the testis, including mitosis, cell growth, the two divisions or meiosis and cell differentiation

In the testes of the male embryo, the primordial germ cells will differentiate to form the spermatogonia. These are still diploid cells.

Endless mitosis of the germinal epithelium cells produces many more spermatogonia which are found in the outer wall of the seminiferous tubule. This is a process that continues into adulthood, allowing for the continuous production of sperm.

These diploid cells grow larger to become primary spermatocytes (2n)

They will then enter meiosis I, dividing to become secondary spermatocytes (2n)

Following this, they enter meiosis II, during which they will divide to become two spermatids (n), making a total of four spermatids produced from each germinal epithelium cell.

The spermatids are then nurtured by the Sertoli cells to develop and differentiate into spermatozoa (n)

The sperm then detach from the Sertoli cells. The fluid of the seminiferous tubule carries them out of the testis and into the epididymis.

The final product of spermatogenesis is four sperm cells from each germinal epithelium cell.

11.4.3 – State the role of LH, testosterone and FSH in spermatogenesis

Luteinising Hormone – Secreted from the pituitary gland, and stimulates the secretion of testosterone by the testes.

Testosterone – Secreted from the interstitial cells in the testes. It stimulates the development of secondary spermatocytes into mature sperm.

Follicle Stimulating Hormone – This is secreted from the pituitary gland. It stimulates the primary spermatocytes to undergo meiosis I, forming secondary spermatocytes.

11.4.4 – Annotate a diagram of the ovary to show the location and function of germinal epithelium, primary follicles, mature follicle and secondary oocyte

The oogonia are formed by mitosis from the germinal epithelium and will then grow to become primary oocytes. Each menstrual cycle, a few primary follicles are formed from an oocyte and a layer of follicle cells. Meiosis I then occurs to produce the secondary oocyte and the first polar body.

The secondary oocyte will continue to mature in the follicle, or Graafian follicle. The oocyte will continue to mature, entering meiosis II, but stopping at prophase II. It will not develop further until a sperm enters the ovum.

The follicle burst open to release the secondary oocyte, with follicle cells, into the fallopian tube. This is ovulation. The remaining follicle becomes the corpus luteum, which temporarily produces progesterone. If the ovum is not fertilised, then this will degenerate.

11.4.5 – Outline the processes involved in oogenesis within the ovary, including mitosis, cell growth, the two divisions of meiosis, the unequal division of cytoplasm and the degeneration of polar body

In oogenesis, ova are produced in the ovaries.

When the female is still a foetus, diploid cells divide by mitosis in the germinal epithelium, creating more diploid cells (2n).

These cells then grow to become primary oocytes (2n)

The primary oocytes will then start meiosis I, but will stop during prophase I. The primary follicle consists of the primary oocyte and a layer of follicle cells around it.

In general, a female is born with about 400 000 primary follicles.

During each menstrual cycle, a primary follicle will develop. In this time, the primary oocyte completes meiosis I to form two haploid nuclei. However, due to unequal cytokinesis, the cytoplasm in divided unequally and the result is a large secondary oocyte and a small polar cell (n)

The secondary oocyte will then enter meiosis II, stopping in prophase II. Simultaneously, the surrounding follicle cells will proliferate and form follicle fluid.

At ovulation, the follicle will burst and release the secondary oocyte into the fallopian tubes.

The remaining follicle then becomes the corpus luteum, which secretes progesterone.

If the ovum is fertilised, then it will complete meiosis II with the sperm nucleus inside it and a second polar body is formed due to unequal cytokinesis. The first and second polar bodies will degenerate.

11.4.6 – Draw and label a diagram of a mature sperm and egg 

11.4.7 – Outline the role of the epididymis, seminal vesicle and the prostate gland in the production of semen

Epididymis – The sperm reach the epididymis after the leave the testes. At this stage, they are unable to swim. The sperm are stored here to continue to mature and become able to swim.

Seminal VesiclesProduce and store fluids that are released for ejaculation, mixed with the sperm to increase the total volume of the ejaculate. This fluid contains nutrients such as fructose, providing energy for the sperm, as well as mucus to protect the sperm when they reach the vagina.

Prostate GlandProduce and store fluids that are released for ejaculation, mixed with the sperm to increase the total volume of the ejaculate. This fluid contains mineral ions and is alkaline to protect the sperm from the acidic environment of the vagina.


11.4.8 – Compare the processes of spermatogenesis and oogenesis, including the number of gametes and the timing of the formation and release of gametes

11.4.9 – Describe the process of fertilisation, including the acrosome reaction, penetration of the egg membrane by a sperm and the cortical reaction z

Fertilisation takes place in the fallopian tubes. A secondary oocyte is released from the ovaries at about day 14 of the menstrual cycle and enters the oviducts. The sperm enter the female body during sexual intercourse through the vagina, and then travel up into the fallopian tubes. Only a few sperm will reach this stage, as many will die due to the high acidity of the female system, along with other factors.

When the sperm and the ovum meet, the sperm must pass between the follicle cells that surround it. They will then arrive at the jelly coat or the zona pellucida. In the acrosome of the sperm, there are digestive enzymes that break down the coat to form a path.

The process of changing the head of the sperm is called capacitation.

The head of the sperm will then fuse with the plasma membrane of the ovum, allowing the nucleus to enter.

Once a sperm has entered the ovum, the cortical granules are then released across the membrane through exocytosis to prevent any more sperm from entering. If another sperm does enter, then the zygote would not survive.

At this point, the secondary oocyte will recommence meiosis II, forming a second polar body. Both the first and second polar bodies are then released and degenerate.

Fertilisation is complete once the nuclei of the ovum and the sperm fuse together. The cytoplasm will divide to form diploid cells of the embryo.



11.4.10 – Outline the role of HCG in early pregnancy

Human Chorionic Gonadotrophin (HCG) is secreted once the embryo is implanted in the wall of the uterus. This prevents the corpus luteum from degenerating, but causes it to grow to continue producing oestrogen and progesterone. This maintains the pregnancy. Eventually, the placenta will take the place of the corpus luteum in secreting oestrogen and progesterone.

11.4.11 – Outline early embryo development up to the implantation of the blastocyst

The fertilisation of the ovum happens in the fallopian tube. When the zygote (fertilised ovum) is formed, it travels down the fallopian tube, dividing by mitosis as it does so.

When the zygote first begins to divide, it does so through cleavage of the cell. At this stage, the mass and size of the embryo do not change. The result is that, by the time it reaches the uterus, the embryo has undergone several mitotic divisions to become a hollow ball of small cells called blastomeres, which organise themselves to form the blastocyst, which is also filled with fluid.

The embryo will usually implant in the endometrium at about days 7-14, at which point the blastocyst contains approximately 100 cells. This is called implantation. A few blastomeres will group together to form the inner cell mass, which later become the foetus. The endometrium provides nutrients to embryo.

11.4.12 – Explain how the structure and functions of the placenta, including its hormonal role in secretion of oestrogen and progesterone, maintain pregnancy

The placenta is made up of maternal and fetal membrane tissues, allowing the blood of the mother and child to come close enough together to allow for exchange. The umbilical cord is formed from an artery and a vein, and connects the foetus to the placenta. The placenta also protects the baby from bacteria; however some viruses are still able to cross it. Exchange takes place through both active transport and diffusion.

In addition to the exchange of materials, the placenta also has important functions in the production of hormones. In early pregnancy, it produces HCG in order to maintain the corpus luteum. The placenta will later replace the corpus luteum to produce progesterone and oestrogen. This is important for ensuring that the pregnancy is maintained.


11.4.13 – State that the foetus is supported and protected by the amniotic sac and amniotic fluid

During gestation, the embryo is protected by the amniotic sac and fluid. The embryo is a very small, delicate structure; hence such protection from injury is essential. It is able to float in the amniotic fluid to support it and protect it from shock.

11.4.14 – State that materials are exchanged between the maternal and foetal blood in the placenta

The placenta is attached to the foetus, allowing for the exchange of materials between the mother and child. This occurs through diffusion and active transport. These materials include:

  • Respiratory gases – Including oxygen diffuses across the membrane to the foetus, with carbon dioxide diffusing back.
  • Water – Enters the placenta by osmosis
  • Glucose – Enters by facilitated diffusion
  • Ions and Amino Acids – Transported across the membrane using active transport
  • Excretory Products – This includes urea, which leave the foetus
  • Antibodies – These enter the foetus’ bloodstream from the mother to protect it from the same diseases, called passive immunity.

The placenta acts as a barrier to bacteria, but not all viruses

11.4.15 – Outline the process of birth and its hormonal control, including the changes in progesterone and oxytocin levels and positive feedback

Just before birth, the concentration of progesterone decreases dramatically. Since progesterone inhibits the contraction of the muscles in the uterus, this effect is stopped and the contractions can begin.

Oxytocin is released from the pituitary gland to relax the fibres of the bones of the pelvic girdle. The cervix begins to dilate. Oxytocin will also cause contractions.

A positive feedback loop is established: as the cervix stretches further, more oxytocin is released to continue the contractions. This stops once birth is complete and the cervix stops stretching.

Contractions move down towards the cervix to push the baby out, becoming more and more powerful and frequent as time goes on.

The placenta and the remaining umbilicus are discharged afterwards in a period called the afterbirth.

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11.3 – The Kidney

11.3 – The Kidney

11.3.1 – Define excretion

The removal of the waste products of metabolic pathways from the body

All living things excrete their waste, along with any excess metabolites.

11.3.2 – Draw and label a diagram of the kidney

The two kidneys act as filters for the blood, removing harmful toxins. Blood flow through the kidneys is extremely high because of the large number of capillaries. The medulla has a high osmolarity because it contains lots of salt. The nephrons are surrounded by veins, but they do not touch.

11.3.3 – Annotate a diagram of a glomerulus and associated nephron to show the function of each part

The glomerulus is a group of branching capillaries.

11.3.4 – Explain the process of ultrafiltration, including blood pressure, fenestrated blood capillaries and basement membrane

Ultrafiltration takes place in the Malpighian body. Here, there are many porous capillaries which are not selective and therefore allow most substances through.

Blood Pressure

Unfiltered blood is transported to the nephron in the renal artery. The blood pressure increases in the glomerulus because the branches become narrower. The high blood pressure forces water, urea, salts and other solutes form the blood in the glomerulus to the lumen of the Bowman’s capsule.

The porous capillaries and podocytes, or special cells, filter blood by being permeable to water and small solutes but not blood and other plasma proteins. The process in non-selective, and the resulting filtrate contains salts, glucose, vitamins and nitrogenous waste. On the end of the nephron, there is an invagination that accommodates the glomerulus.

On the end of the nephron, there is an invagination that accommodates the glomerulus.

Fenestrated Blood Capillaries

The fenestrated blood capillaries form a path of low resistance out of the glomerulus as they have gaps between the cells that form the vessels. These gaps are only visible through an electron microscope.

Basement Membranes

Not all of the contents of the blood are forced out during the process, which is prevented by basement membranes. These act as filtration barriers, preventing cells and large proteins or polypeptides from passing through. These are instead retained in the circulating blood.

11.3.5 – Define osmoregulation

The control of the water balance of the blood, tissue or cytoplasm of a living organism.

The collecting duct is important for the process of osmoregulation

11.3.6 – Explain the reabsorption of glucose, water and salts in the proximal convoluted tubule, including the roles of microvilli, osmosis and active transport

The proximal convoluted tubule has microvilli, which give it greater surface area to absorb glucose, amino acids and mineral ions from the filtrate back into the capillary network. It is the longest section of the nephron. Urea is also transported by diffusion.

The mineral ions are transported by active transport, facilitated diffusion and exchange of ions.

About 80% of the water is reabsorbed by osmosis.

Active transport is used for the transport of glucose and amino acids, so there are a large number of mitochondria to provide the ATP required.

11.3.7 – Explain the roles of the loop of Henle, medulla, collecting duct and ADH (vasopressin) in maintaining the water balance of the blood

Loop of Henlé

In the descending limbs of the Loop of Henle are permeable to water, but not to sodium ions. The sodium ions are instead diffused into the loop. The water is drawn out of the filtrate by osmosis. This decreases the concentration of the fluids in the medulla, whilst the filtrate becomes more concentrated.

Water loss from the body is minimised by expelling more concentrated urine. The glomerular filtrate flows through the medulla, the loop of Henle, and the out through the cortex. The loop of Henle increases the solute concentration of the medulla, as it creates and area of high solute concentration in the cells and tissues of the medulla. The volume of filtrate is eventually reduced.

On the other hand, the ascending limbs are not permeable to water, but to sodium ions, which are pumped from the filtrate into the medulla via active transport. As a result, the concentration of sodium ions in the medulla is increased. The concentration of the filtrate is reduced.


If this hormone is released, the pores of the distal convoluted tubule and collecting duct open, making them permeable to water.

The blood moves into the distal convoluted tubule where ions are exchanged, and then on to the collecting duct. The permeability of these is increased by the hormone vasopressin (ADH), which opens pores in the cell membranes of these tubules.

11.3.8 – Explain the differences in the concentration of proteins, glucose and urea between blood plasma, glomerular filtrate and urine

All of these products move in the following sequence:

During this process, urea is eliminated from the body through the urine. However, not all of the urea that leaves the collecting duct will be taken up by the loop of Henle, so there is only 50% total reabsorption. Glucose, on the other hand, is reabsorbed from the filtrate back into the blood. There is 100% reabsorption of it. Whilst proteins are found in the blood plasma, they do not move into the glomerular filtrate. If proteins are found in urine, then this means that blood pressure is too high and there is damage nephritis of the Bowman’s capsule.

11.3.9 – Explain the presence of glucose in the urine of untreated diabetic patients

People with diabetes experience erratic blood glucose levels, and will often rise above normal levels, especially after meals. As a result, the kidney is not able to reabsorb all the glucose from the blood, so some still remains in the urine. This is because the pumps of the proximal convoluted tubule are not able to reabsorb such high concentrations of glucose (higher than 90mg per 100mL).

As a result, the concentration of glucose in the person’s urine can indicate whether they are diabetic. If they are not treated, glucose will continue to be present.

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11.2 – Muscles and Movement

11.2 – Muscles and Movement

11.2.1 – State the roles of bones, ligaments, muscles, tendons and nerves in human movement

Bones – Act as anchors for the muscles, and levers to control the movement of muscles, support and protect body parts

Ligaments – Connect bone to bone to prevent them from becoming dislocated. They are made up of strong fibres.

Muscles – Contract to allow for movement. Skeletal muscles attach to the bones, and are found in pairs called antagonistic pairs.

Tendons – Attach muscles to bones, made up of connective tissue

Nerves – Bundles of nerve cells that send messages through the body to specific places. They stimulate the contraction of muscles and altogether, the CNS coordinates movement

Combined, these different parts allow for the body to move at the joints. The movable joints in the body are called synovial joints because there is synovial fluid between the bones to keep them lubricated

11.2.2 – Label a diagram, of the human elbow joint, including cartilage, synovial fluid, joint capsule, named bones and antagonistic muscles (biceps and triceps)

11.2.3 – Outline the functions of the structure in the human elbow named in 11.2.2

Biceps – the flexor muscle attached to the radius, bends the elbow as the triceps relaxes

Triceps – extensor muscle attached to ulna, straightens the elbow

Humerus – attachment for muscles to form a system of levers

Radius – transmits force from the biceps through the forearm

Ulna – bone that transmits force from triceps through forearms

Capsule – seals the joint, contains synovial fluid, but does not restrict movement

Synovial Fluid – lubricates the join, reduces friction, nourishes the cartilage

Synovial Membrane – Secretes synovial fluid

Tendon – attaches muscle to bone

Cartilage – the flexible covers ends of bones to reduce friction between bones

Ligaments – Hold all the bones in their correct positions

11.2.4 – Compare the movements of the hip joint and the knee joint 


11.2.5 – Describe the structure of striated muscle fibres, including the myofibrilswith light with light and dark bands, mitochondria, the sarcoplasmic reticulum, nuclei and the sarcolemma

Each muscle is made up of bundles of muscle fibres. Each fibre is then made up of even smaller structures, called myofibrils. The myofibrils are sarcomeres attached end to end, which contain light and dark bands, causing the myofibril to appears striped. In between the myofibrils, there are mitochondria to provide the energy for muscle contraction. The sarcoplasmic reticulum surrounds each myofibril.

11.2.6 – Draw and label a diagram to show the structure of a sarcomere, including Z lines, actin filaments, myosin filaments with heads, and the resultant light and dark bands

11.2.7 – Explain how skeletal muscle contracts, including the release of calcium ions from the sarcoplasmic reticulum, the formation of cross-bridges, the sliding of actin and myosin filaments, and the use of ATP to break cross-bridges and re-set myosin heads

A nerve impulse is sent and reaches the neuromuscular junction, causing the neurotransmitter acetylcholine to be released into the synapse. This triggers the depolarisation of the sarcolemma, which is the plasma membrane on the muscle fibre. As a result, calcium ions are released from the sarcoplasmic reticulum as bind to troponin.

Troponin displaces tropomyosin, which exposes binding sites for myosin to form crossbridges with thin actin filaments. The actin is pulled to the midline. ATP is then hydrolysed at the myosin head, causing it to detach from the actin binding site, then reattach further along the filament. The entire cycle repeats.

11.2.8 – Analyse electron micrographs to find the state of contraction of muscle fibres

Look for the narrower light ands, which will show that the muscle has contracted

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11.1 – Defence Against Infectious Diseases

11.1 – Defence Against Infectious Diseases

11.1.1 – Describe the process of blood clotting

Clotting is an important process as it prevents the body from losing too much blood from wounds, as well as preventing pathogens from entering the body and maintaining blood pressure. The process begins by releasing platelets the tissue. The platelets are cell fragments that circulate in the bloodstream with the red blood cells and white blood cells (the erythrocytes and leukocytes).

The clotting factors, which are released from the platelets or damaged tissue, activate prothrombin to become the enzyme thrombin. This is then responsible for catalysing the conversion of soluble fibrinogen into the insoluble protein fibrin. Fibrinogen is constantly found in the bloodstream. Red blood cells become trapped in the fibrin, preventing the flow of blood.

Once platelets reach it, they will produce sticky extensions and connect to each other. As they contract, the liquid is forced out and a clot is formed.

11.1.2 – Outline the principle of challenge and response

Challenge and Response

The principle of challenge and response means that immunity to a specific pathogen is only developed once it is encountered in the body. In other words, the body needs to be challenged by a pathogen. The response begins when the immune system recognises the antigens on the pathogen and begins taking steps to fight it. The white blood cells recognise the antigen, engulf it, and then reform the antigen on their cell membrane. The cell then goes to the lymph nodes, where the B-cells are found.

The white blood cells will locate the B-cell that corresponds to the antigen, and present the antigen to be B-cell. It is possible for more than one B-cell may be required, called polyclonal response. All these B-cells will start cloning to form different types of plasma cell, making different antibodies to fight all of the antigens on the pathogen.

Clonal Selection

This process involves identifying the B-cell that corresponds to the antigen. The B-cell will then clone itself to make plasma cells, which will in turn produce and excrete the appropriate antibodies.

T killer cells, which are cytotoxic cells, are also formed to detect and destroy cells that are infected.

Memory Cells

The body’s memory cells allow it to have long-term protection against diseases, even after the B-cells and antibodies are gone. They are produced from B-cells after clonal selection, so the corresponding pathogen must attack the body before immunity can be formed.

11.1.3 – Define active and passive immunity

Active Immunity – Immunity due to the production of antibodies by the organism itself after the body’s defence mechanisms have been stimulated by antigens.

Passive Immunity – When the antibodies are acquired from another organism in which active immunity has been stimulated, including via the placenta, colostrums or by injecting antibodies.

It is also helpful to know these definitions:

Natural Immunity – Immunity due to infection
Artificial Immunity – Immunity due to inoculation with a vaccine

11.1.4 – Explain antibody production

Our immune systems are able to make a huge range of antibodies to fight against disease, however it is not possible for all of these to remain in the body at once. Instead, the B-cells are stored, which can then begin producing antibodies if they are necessary. The whole process of building up enough antibodies takes only a few days.

The first stage is when the macrophages, or white blood cells, encounter the antigen. They take it in, and then attach it to the MHC proteins in the plasma membrane.

The helper T-cells can recognise antigens using the receptors on their membranes, and bind to the macrophage that carries it. The macrophage will then pass on a signal that causes the helper T-cell to be activated.

The helper T-cells use a similar process to activate the appropriate B-cell. The helper T-cell will locate a B-cell that has antibodies that correspond to the antigen it is carrying. The receptors bind, and the B-cell is activated.

B-cells use the process of clonal selection to produce the required antibodies. If a B-cell encounters their corresponding antigen, they begin cloning themselves to make more B cells, which then produce the antibody to fight off the disease. These clones are called plasma cells. They have a greater volume of cytoplasm and a large amount of rough endoplasmic reticulum for the synthesis of antibodies.

However, it is possible for one antigen to be destroyed by multiple antibodies. As a result, more than one type of clone cell is formed in the process called polyclonal selection.

Once the plasma cells begin producing large amounts of antibodies, which are secreted by exocytosis, the body has become immune to the disease.

Memory cells are formed at the same time as helper T-cells and B-cells, but these last much longer, after the antibodies and activated B-cells have disappeared. Since they remain, they allow the body to have a more rapid response if the same disease attacks the body.

11.1.5 – Describe the production of monoclonal antibodies and their use in diagnosis and in treatment

Monoclonal antibodies are produced in large quantities and can then be used for many purposes. The process is as follows:

  • The antigens that bind to the desired antibody are injected into an animal
  • The animal’s B-cells that produce this antibody are then extracted from the animal
  • Tumour cells are obtained, which will divide endlessly
  • The B-cells and tumour cells are fused to become hybridoma cells. As a result, the cells will continue to divide and grow, producing the antibody as they do so.
  • The antibodies can then be extracted and purified for use

Monoclonal antibodies are used to treat diseases, such as anthrax. This disease is caused by bacteria that secrete poisons. The monoclonal antibodies can be injected to neutralise the toxins until the body has time to respond to the disease and begin producing its own antibodies.

Another application is in the detection of disease, including malaria. The monoclonal antibodies are able to bind to the antigens. By collecting a sample and placing it on a test plate coated in antibodies, the presence of the antigen can be detected. An enzyme is usually added that causes the plate to change colour in the presence of the antibodies bound to the antigens. In addition, it is possible to see the level of infection and distinguish the strain of the disease.

11.1.6 – Explain the principle of vaccination

When we are vaccinated, we are injected with a modified form of the disease-causing microorganism. This initiates a response from the body, even though the disease and its symptoms do not develop. The pathogen is either dead or weakened (or attenuated). Some vaccines come in the form of an inactivated toxin.

Since the microorganism still has the same antigen, it causes the B-cells to be activated and begin cloning. The memory cells then form so that if the pathogen is encountered again, the body can respond quickly and produce antibodies.

Vaccines are usually injected, although some can be ingested. Multiple vaccines sometimes need to be given to produce a sufficient response in the body.

11.1.7 – Discuss the benefits and dangers of vaccination


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