Edexcel Categories Archives: B3

Sex determination

Sex determination

  • Women have two X chromosomes and men have one X and one Y. The 23rd pair of chromosomes is called the sex chromosomes.
  • When making sperm, the X and Y chromosomes are drawn apart in the first division in meiosis. There’s a 50% chance each sperm cell gets an X-chromosome and a 50% chance it gets a Y-chromosome. All egg cells have one X chromosome.
  • Punnett squares can be used to calculate the probability of the genders of people.


Sex-linked inheritance

  • Most of our chromosomes come in matching pairs. The two chromosomes in a pair have the same genes in the same places but may have different alleles.
  • However, the X chromosome is much larger than the Y chromosome. As well as this, there are more genes on the X chromosome than on the Y chromosome. This means that males will only have one copy of most of the genes on the X chromosome.
  • One gene found only on the X chromosome codes for a substance that clots blood. The normal allele, H, allows the blood to clot. The recessive allele, h, prevents normal clotting and causes a disease called haemophilia.
  • Haemophilia is an example of a sex-linked genetic disorder.

  • The probability of the inherited alleles can be found the same way as with normal genes, except we must show which chromosomes the genes are on.
  • Each time this couple have a child, the probability that it will be a boy with haemophilia is 1 in 4. This can be written 1:3 or as 25%.
  • The gene for red-green colour blindness is also on the X chromosome. The normal allele of this gene gives normal colour vision, but there is a recessive allele that means a person cannot distinguish red from green.


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  • Egg cells and sperm cells are gametes and are heavily specialised for their functions.
  • The main functions of the egg is carry the female DNA and nourish the developing embryo:
    • 1) The egg contains nutrients in the cytoplasm to feed the embryo.
    • 2) As soon as the egg is fertilised with a sperm, the egg’s membrane quickly changes its structure to prevent any more sperm getting in. This is to ensure the offspring have the correct amount of DNA.
    • 3) The egg contains a haploid nucleus, like sperm, so when they join together the zygote will have the diploid number of chromosomes.
  • The function of the sperm is to transport the male’s DNA to the female’s egg so that their DNA can combine.
    • 1) Sperm are small and have tails so they can swim to the egg.
    • 2) Sperm have a lot of mitochondria in their middle section to provide the energy needed to swim this distance.
    • 3) Sperm also have an acrosome at the front of the head where they store the enzymes they need to digest their way through the membrane of the egg cell.
    • 4) They contain a haploid nucleus – they contain one copy of each chromosome.
  • The two nuclei from the sperm and egg fuse to form a zygote. The zygote is diploid and divides repeatedly to form an embryo which embeds in the uterus lining to grow and develop.

Fertility treatment

  • Some women have levels of FSH that are too low to cause their eggs to mature. This means that no eggs are released and the woman can’t get pregnant.
  • Hormones: Women can be given extra hormones such as FSH and LH to stimulate egg release in their ovaries. However, it doesn’t always work and too many eggs could be stimulated, resulting in multiple pregnancies. The babies also tend to be born earlier than usual, increasing the risk of problems at birth or later.
  • IVF: This stands for in vitro fertilisation. Some of the woman’s egg cells are taken from her ovaries and fertilised in a dish with her partner’s sperm cells. One or two embryos are then put into her uterus to develop. IVF babies are born early more often than naturally conceived babies which may causes problems at birth or later.
  • Surrogate mothers: If the woman cannot grow an embryo in her own uterus, another woman can grow it for her using her own eggs or the surrogate mother’s eggs with the man’s sperm. The surrogate mother gives birth to the baby. Handing the baby over the couple may cause problems if the surrogate mother has developed a strong bond with the baby and does not want to give it up.
  • Egg donation: If the woman’s ovaries aren’t producing eggs, eggs can be used from a woman who agrees to donate hers. This woman will be given hormones to make her ovaries release the eggs. The IVF is carried out using sperm from the first woman’s partner. A few women who donate eggs react really badly to the hormones used to collect them.
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The menstrual cycle has four stages

The menstrual cycle has four stages

  • Stage 1: Day 1 is when the bleeding starts. The uterus lining breaks down and is released.
  • Stage 2: The lining of the uterus builds up again, from day 4 to day 14, into a thick spongy layer of blood vessels ready to receive the fertilised egg.
  • Stage 3: An egg is released from the ovary (ovulation) at about day 14.

Stage 4: The lining is then maintained for 14 days, until day 28. If no fertilised egg has landed on the uterus wall by day 28 then the spongy lining starts to break down again and the whole cycle starts over.


Hormones and the menstrual cycle

  • FSH (follicle-stimulating hormone): This causes a follicle (an egg and its surrounding cells) to mature in one of the ovaries. It stimulates oestrogen production.
  • Oestrogen: This causes the uterus lining to thicken and grow. A high level stimulates an LH surge.
  • LH (luteinising hormone): LH stimulates ovulation at day 14 – the follicle ruptures and the egg is released. The remains of the follicle is also stimulated to develop into a structure called a corpus luteum which secrets progesterone.
  • Progesterone: This maintains the lining of the uterus. It inhibits the production of FSH and LH. When progesterone levels fall, and there is a low oestrogen level, the uterus lining breaks down. A low progesterone level allows FSH to increase and the whole cycle starts again.
  • If the egg is fertilised and is implanted in the uterus (pregnancy) the level of progesterone will stay high to continue to maintain the uterus lining.
  • The uterus lining has a thick spongy layer of blood vessels – this blood supply allows the placenta to develop. The placenta delivers the baby with oxygen, glucose and nutrients it needs to grow and removes its waste products (urea and carbon dioxide).

Negative feedback controls levels of hormones

  • The different hormones in the blood during the menstrual cycle are controlled by negative feedback. FSH is controlled by negative feedback for example:
    • FSH stimulates the ovary to release oestrogen.
    • Oestrogen inhibits further release of FSH from the pituitary gland.
    • After FSH has caused a follicle to mature, negative feedback keeps FSH levels low. This ensures no more follicles mature.


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The kidneys

The kidneys

  • The Kidneys help maintain the internal environment by:
    • Filtering the blood
    • Reabsorbing all the sugar
    • Reabsorbing the dissolved ions needed by the body
    • Reabsorbing as much water as the body needs
    • Releasing urea, excess salts and excess water as urine.
  • Blood is brought to the kidneys from the renal arteries. Urea and other substances are filtered out of the blood to form urine.
  • The cleaned blood flows out of the renal veins and the urine is excreted.

Treating kidney failure


  • Renal dialysis is an artificial kidney machine which filters the blood.
  • The word ‘dialysis’ means ‘splitting into two’ and refers to the way a patient’s blood is purified by separating off the unwanted waste products such as urea by the dialysis membrane.
  • The kidney dialysis machine works like a real kidney.
  • The filter in the machine is visking tubing. The tiny holes allow small molecules such as water, ions, and urea pass through but not big molecules such as cells and proteins.
  • One side of the membrane the patient’s blood flows and the other side a special dialysis fluid which has to be changed every few hours.
  • To achieve dialysis it’s usually easier to use a vein that is closest to the skin and also has a large lumen. However, the pressure in a vein is low so has to be attached to an artery for the necessary pressure.
  • The kidney dialysis is a complex and expensive piece of apparatus. A kidney transplant is preferred.

Kidney transplants

  • Humans can live happily with one kidney but when both kidneys fail they would die in a week because of the poisonous waste building up in the blood.
  • The problem with kidney transplants is that they kidney is rejected by the person’s immune system and they begin to attack as it has foreign antigens. We say the organ has been rejected.
  • There is a way to avoid this: using a kidney from a close relative as they would have similar antigens.
  • The patient will need life-long medication to prevent the kidney being rejected. This suppresses the immune system so the patient may catch colds more easily.

How kidneys work

  • Each kidney contains thousands of tiny, microscopic tubes called nephrons.


    • Blood flows in a network of capillaries called a Glomerulus which runs inside the Bowman’s capsule of the nephron.
    • The high pressure of the blood squeezes small molecules through the tiny holes such as water, urea, ions and glucose. Big molecules cannot fit through the holes and stay in the blood. This process is called filtration.


    • Some substances are reabsorbed if they are useful, like glucose. All the glucose is selectively reabsorbed – it is moved out of the nephron back into the blood against the concentration gradient.
    • Sufficient water is reabsorbed, according to the level of ADH. The process of maintaining the right water content in the body is Osmoregulation.

    Release of wastes

    • Urea and excess water are not reabsorbed. They continue out of the nephron, into the ureter and down to the bladder as urine. Urine is released through the urethra.

    Controlling water content

    • The hormone responsible for controlling the water content of the blood is called Antidiuretic hormone – ADH.
    • This is found in the pituitary gland and is secreted by it.
    • The control of water content is a good example of negative feedback. This is where a change in a factor leads to the opposite change happening to keep things fairly constant, even if they keep changing.

    ADH: not enough water

    • Step 1: The brain senses there is not enough water in the blood.
    • Step 2: The pituitary gland secrets more ADH.
    • Step 3: The ADH causes the kidneys to reabsorb more water.
    • Step 4: A small volume of concentrated urine is produced.

    ADH: too much water

    • Step 1: The brain senses there is too much water in the blood.
    • Step 2: The pituitary gland secretes less ADH.
    • Step 3: This makes the kidneys reabsorb less water.
    • Step 4: A large volume of dilute urine is produced.
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Monoclonal antibodies

Monoclonal antibodies

  • Monoclonal antibodies: many identical antibodies.
  • These were invented for making large quantities of antibodies for identifying particular substances.
  • A lymphocyte can divide over and again to make identical clones of itself. However, once it starts making antibodies it becomes a B-lymphocyte and it can’t divide anymore.
  • To get around this problem a B-lymphocyte can be fused with a tumour cell (which divides very quickly) to produce a hybridoma.
    • Step 1: A mouse/rat is immunised by injection of antigen to stimulate the production of antibodies targeted against X.
    • Step 2: The antibody forming cells, B-lymphocytes, are isolated from the mouse’s blood.
    • Step 3: A tissue culture of tumour cells is grown.
    • Step 4: The B-lymphocytes and tumour cell are fused together which forms a hybridoma.
    • Step 5: Grown in a culture, to produce multiple hybridomas from the original one.
    • Step 6: The antibodies produce from the hybridomas are isolated and used later.


Using monoclonal antibodies

  • These are often used in medicine. An example is pregnancy testing. Antibodies are placed on a strip that bind with the hormone “human chorionic gonadotrophin” (hCG). This is found in the urine of women in the early stages of pregnancy.
  • The strip is dipped into some urine and if there is any hCG in it, it binds with the monoclonal antibodies on the strip and causes a colour change.
  • Monoclonal antibodies can be made radioactive to find cancer in the body.
  • Monoclonal antibodies can be developed in the lab to stick to the special substances found on cancerous cells and platelets. The antibodies are labelled with a radioactive element and then put in the patient. A picture of the patient’s body is then taken with a special camera that detects radioactivity.
  • Anywhere there are cancer cells or blood clots would show up as a bright spot as antibodies bind with the tumour markers on these cells.
  • This information can be interpreted by doctors to find where the cancer is, what size it is and how fast it is spreading.
  • Drugs can be attached to monoclonal antibodies to deliver the drug only to cells that need to be destroyed. This means less of the drug is needed, as none of it is wasted in parts of the body that are healthy. Healthy cells are much less of a risk of being harmed.
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White blood cells

White blood cells

  • There are two types of white blood cells. Phagocytes and Lymphocytes. They both get rid of pathogens.
  • Phagocytes get rid of pathogens by phagocytosis:
    • Step 1: encounters foreign body or pathogen
    • Step 2: engulfs the foreign body or pathogen
    • Step 3: breaks down with enzymes
    • This is a non-specific immune response and will engulf ANY foreign body.
  • Lymphocytes, specifically B-Lymphocytes produce antibodies to fight pathogens.
    • Step 1: B-Lymphocyte encounters the pathogen.
    • Step 2: The cell produces antibodies matching the antigen on the pathogen.
    • Step 3: The antibodies fit onto the antigens and cause them to “clump”
    • Step 4: The pathogen is absorbed and digested by the white blood cells.

Primary response

  • When a foreign body first enters the system, the antigen is not recognised immediately. It takes the body a while for the lymphocytes to multiply and secret enough antibodies to destroy the pathogens.
  • Memory lymphocytes remain in the blood for a long time and remember a specific antibody.
  • The person becomes immune to that particular pathogen; their immune system can now respond quickly to an infection.

Secondary response

  • If the same pathogen enters again, the immune system will produce a quicker and stronger immune response.
  • The secondary response often gets rid of pathogens before you show any symptoms.
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  • Edward Jenner founded the idea of vaccination. He realised that people who got cowpox didn’t get smallpox. In 1796 Jenner took pus from a cowpox blister and rubbed it into the skin of an 8-year-old boy. He got a mild fever but that was all. He did the same with the pus from a smallpox blister. He didn’t get smallpox. The cowpox vaccine had made him immune to smallpox.
  • Smallpox is caused by a virus. All viruses and cells have chemicals on their outer surfaces called antigens. Our bodies can recognise foreign antigens and therefore try to destroy these foreign antigens and cells.
  • A vaccine contains a harmless version of a pathogen or parts of it. Your body recognises it as a foreign body and your immune system kicks into action. B-lymphocytes respond to the pathogen by producing antibodies. By trial and error, the antibody that fits the antigen on the pathogen is found. Once this happens, the B-Lymphocytes produce it in large numbers.
  • Some of these lymphocytes become memory lymphocytes. The way the body responds to infection is called the immune response. Making someone immune to a disease is called immunisation.

Are vaccines safe?

  • All young children in the UK are offered vaccinations to dangerous childhood diseases such as measles and whooping cough. However, sometimes children can get a reaction to the vaccine.
  • About 20% of children may get a mild fever or a rash from the measles vaccine and 1 in a million get a dangerous reaction.
  • Due to media scares about the risks of immunisation, many parents are persuaded not to have their children immunised.


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Microorganism growth

Microorganism growth

  • Much is done to keep numbers of bacteria in milk as low as possible; the milking equipment is sterilised, the cow’s udders are washed before milking and most milk is cooled rapidly to slow the growth of bacteria. It is then pasteurised.
  • If bacteria have everything they need (food, oxygen and warmth) they can grow into a colony of millions in a few days.
  • We say the population of bacteria grows exponentially. In the right conditions bacteria can divide by binary fission every 20 minutes.
  • The number of bacteria in a sample can be measured using resazurin dye which turns blue in high levels of oxygen. As the concentration of oxygen decreases it changes to lilac, mauve then pink and eventually colourless. The more bacteria there are, the more quickly oxygen levels fall.
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Plant defences

Plant defences

  • Plants can be attacked by insects or pathogens. If this happens to crops, crop yield can be severely reduced as the plants are eaten, which will increase the price of the crop in the long run.
  • As a result, plants can produce chemicals to protect the plant when they are attacked by pathogens. This can be done in different ways:
    • Attract parasites to kill the insects.
    • Put the insect off (doesn’t taste nice)
    • Attract predators to attack the insects eating the plants.
  • Although the plant may have to use a lot of energy to make the chemicals it is worth it.


  • Milkweed plant produces a toxic chemical that stops insects from eating due to the taste.
  • Corn plant, when attacked by caterpillars produces a chemical that attracts a parasitic wasp. The wasp lays eggs in the skin of the caterpillar. When the larvae hatch they eat the caterpillar from the inside out, preventing further attack.
  • Wheat seedlings produce a chemical when attacked by insects which attract aphids that eat the insects.
  • Young lupin leaves produce poisonous chemicals called alkaloids. These make the leaves poisonous to insect pests or larger herbivores that might want to eat them.
  • Potatoes are often attacked by a fungus-like organism called potato blight that destroys their leaves, thereby killing the plant. Some varieties of potato produce chemicals that kill it.
  • Plants are a key source of food for people. If pathogens destroy a crop our food supply is at risk. An example of this is the famine caused by potato blight in Ireland in 1845 and 1846 that killed over 1 million people.

Drugs from plants

  • Many chemicals in plants have been found to help cure diseases in people.

  • New possibilities are being researched all the time. For example, research suggests that the alkaloids found in potato plants could be developed as valuable treatments for cancer. This is one of the reasons the rainforest should be preserved as there may be many new species of plant with chemicals with curing properties we haven’t utilised yet.

Louis Pasteur

  • Louis Pasteur found microorganisms were responsible for some diseases and food going off. As a result, he proposed the idea of keeping microorganisms away from food in order to prevent diseases and preserve foods.
  • Methods of doing this are called aseptic techniques. This includes pasteurisation of milk.


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  • This is controlled by plant genes. The way in which living organisms respond to changes in day length.
  • An example of this is deciduous trees as they lose their leaves in winter as the days become shorter.
  • The plants use the change in day length to help them grow or flower at the right time. Responses to changing day length are called photoperiodism.
  • Plants can tell when it is spring from winter as the days get longer. As a result, their seeds sense this and germinate even if the plant has already died.
  • Some plants grow throughout winter but grow faster in lengthening days as a response. In autumn, the days get shorter and the plants stop growing to prepare for winter.
  • Plant species are often synchronised when releasing their pollen as it is an important stage for creating the next generation. For example, millions of crop plants harmonise their life cycles to produce grains at the same time.
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