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Key Words

  • Chromosome - Threadlike structure in the nucleus of a cell made from molecules of DNA. Chromosomes carry genes. 
  • Allele- Different versions of the same gene
  • Homozygous- An individual with two differrent alleles for a particular gene
  • Genotype- A description of the genes an organism has. 
  • Gene- A section of DNA giving the instructions for a cell about how to make one kind of protein.
  • DNA-The chemical that makes up chromosomes
  • Environment-Everything that surrounds you
  • Embryonic stem cells- Unspecialised cell in the very early embryo that can dived to form any type of cell, or even a new individual
  • Adult stem cells-Unspecialised cell that can develop into any specialised cell. 
  • Clones - A new cell/ idividual made by asexual reproduction (one parent). A clone has the same genes as its parent.
  • Phenotype- A description of the physical characteristics that an organism has. 
  • Heterozygous-An individual with two different alleles for a particular gene the same 
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What are genes?

Every living organism is made from cells. Most cells have a nucleus. Inside the nucleus are chromosomes. Chromosomes are made from very long molecules of DNA 

Chromosomes contain thousands of genes. Genes control which proteins a cell makes, including:

  • Strucutural proteins to build the body, for example collagen
  • Functional proteins to take part in chemical reactions, for example enzymes, 

Your characteristics depend on your genes and the environment

  • Some depend on one gene only e.g. dimples
  • Some depend on several genes working together e.g. eye colour
  • Some depend on environment only e.g. scars
  • Some depend on genes and the environment e.g. weight
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Why do family members look alike?

Human cells contain pairs of chromosomes. One chromosome in each pair came from the mother's egg and the other came from the father's sperm. 

In the two chromosomes of a pair, the same genes are in the same place. The two genes in a pair can be differnet. Different versions of the same genes are called alleles. For each gene a person has either two identical alleles or two different alleles. 

If both alleles for a gene are the same, you are homozygous for the gene. If both alleles are different you are heterozygous. 

You have similarities to your parents because you developed from a fertelised egg that got alleles from your mum and dad. 

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What makes us different?

Brothers and sisters are not identical. This is because they inherit different combinations of alleles from their parents. 

Alleles can be dominant or recessive. For example the allele that gives you straight thumbs( T) is dominant. The curved thumb allele (t) is allele is recessive. If you inherit either one or two dominant alleles (TT or Tt) you will have straight thumbs. If you inherit two receessive alleles (tt) you will have curved thumbs.

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What causes inherited disorders?

Huntingtons disease develops after 35 it is fatal. 


Tremor and clumsiness 

Memory loss and concentration problems 

Mood changes 

One faulty dominant allele causes Huntington's disease. 

Children with cystic fibrosis make thick, sticky mucus. They have:

  • Difficulty breathing
  • Difficulty digesting food. 
  • Frequent chest infections

A faulty recessive allele causes cystic fibrosis 

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What are clones?

Bacteria, plants and some animals can reproduce asexually (with one parent) to form clones. Clones are individuals with identical genes. Only environmental factors can cause differences between clones. 

Plants that produce bulbs or runners are clones of each other. Animals do not usually form clones but there are exceptions:

  • Identical twins are clones. They form when the cells of an embryo seperate to make two embryos. 
  • Scientists make clones by removing an egg cell nucleus. They take a nucleus from an adultbody cell of the organism they want to clone and transfer it to the empty egg cell. They grow the embryo for a few days and implant it into a uterurus.
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B2: How do our bodies resist infection?

Immune system 



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How do our bodies resist infection?

Inside your body, conditions are ideal for microorganisms like bacteria and viruses. So they reprouduce quickly. Some bacteria and viruses cause infectuius deseases. They five you diseases symptoms by damaging cells or making poisons (toxins). 

White blood cells try to destroy harmful microorganisms. They are part of your immune system. One type if wgute blood cell engulfs and digests harmful microorganisms. Another type of white blood cell makes antibodies. 

Some one the white blood cells that make each antibody stay in your blood. These are memory cells. If the same microoganism invades your body in future, memory cells recongnise its antigens. The memory cells quickly make the correct antibodies. The invaders are destroyed before you feel ill.You are immune to the disease. 

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How do vaccines work?

Vacciens prevent you getting diseases. A vaccine contains a safe form of a disease- causing microorganism. When a vaccine is injected nto your body, white blood cells make antibodies against the microorganism. Your body stores some of these memory cells. If an active form of the microorganism enters your blood in future, memory cells quickly make antibodies. You do not get ill. 

Having vaccinations is not risk free. Different people have different side- effects from vaccinanations because they have different genes. 

To prevent epidemics of infectious diseases,a high percentage of the population must be vaccinated.If they are not, large numbers of the disease-causing microorganism will remain in infected people, and people who cannont be vaccinated are likely to catch the disease. 

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How do antibiotics work?

Antimicrobials are chemicals that may kill bacteria, fungi and viruses. 

Some type of antimicrobial inhibit the reprodction of microorganisms. 

Antibiotics are effective against bacteria, but not viruses. 

Over timne, some bacteria and fungi become resitant to antimicrobials. To reduce antibiotic resistance, people must:

  • Finish all the tablets, even if they feel better
  • Only use antibiotics when necessary

Antimicrobial resistance develops when random chages in the genes of bacteria or fungi make new variteties htat the antimicrobial cannot kill or inhibit.


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How are new drugs trialled?

In most himan trials on ill people. one group of patientstake the new drug. Another group patients are controls. The controls take either the existing treatment for the illness or a placebo/ 

A placebo looks like the new treatment, but has no drugs in it. Placebos are rarely used in human trials because people who take them miss out on the benefits of both existing and new treatments. 

There are three types of human trials: 

In double blind trials, neither patiets nor doctors know who is in which group. 

In blind trials, doctors know who is in which group, but patients do not. 

In an open label trial both the ptient and their doctor know whether the patient is given the new drug. These trials are used when the patient cannot affect the outcome of using the drug.

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The heart

Your heart pumps blood around your blood. It is a double pump. 

  • The right lower chamber pumps blood to your lungs. 
  • The left lower chamber pumps blood to the rest of your body. 

Your heart is made from muscle. It has its own blood supply. The blood brings oxygen and glucose to the heart. Heart cells use these as a supply of energy. 

Blood travels around your body through arteries, veins and capillaries. 

Arteries take blood from the heart to your body. Their walls withstand the high pressure created by the pumping heart.  They have a thick layer of fat and thick outer wall to withstand the high pressure. 

Veins have thin walls and a thin layer of muscle that can be squashed when you move. This pushes blood to the heart. 

Capillaries take blood to and from tissues. Their very thin walls allow oxygen and food to diffuse into cells and out of cells

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Keeping a constant internal environment is called homeostasis

Body control systems have: 

  • Receptors (skin) to detect changes in the environment. 
  • Processing centres (brain)to recience information and coordiante responses automatically
  • Effectors (muscles and glands)to pruduce responses 

How do we control water levels? 

Cells only work properly if the concentration of their contents are correct. So their water levels must be constant. Your kidneys get rid of waste products by excretion. They also control water levels in your body. They do this by responding to water levels in blood plasma. 

Water levels in your blood plasma may go down because of sweating after exercise, eating salty food, not drinking much water. 

When water levels in blood plasma are low, your idney makes less urine. The urine is concentrated so it is dark. When water levels is blood plasma are high, your kidneys make lots of dilute urine.

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Kidneys are part of a negative feedback system/ 

  • Receptors in the brain detect changes in concentration in blood plasma. 
  • If the concentration is too high, the pituitary gland in the brain releases ADH (hormone) into the blood stream. 
  • The ADH travels to the kidneys(effectors). The more ADH that arrives, the more water the kidneys reabsorb into the body. So the more concentrated the urine. 

Drugs affect the amount of urine you make: 

  • Alcohol leads to big volumes of dilute urine, so you may get dehydrated.
  • This is because alcohol stops the pituitary gland releasing ADH 

Taking ecstasy leads to small volumes of concentrated urine. 

This is because Ecstasy makes the pituitary gland release more ADH

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A species is a group of organisms that can breed together to produce fertile offspring. 

Living organisms have features that help them survive intheir environments. These are adapations. Adaptions increase a species' chance of survival by making it more likely that individuals will survive and reproduce. 

How do species interact? 

Within a place where an organism lives-its habitat- there is competion for resources. Animal species may compete for food or shelter. Plants compete for space and light. 

Species in a habitat rely on each other, and on their environment. for food and other needs. The species are interdependent. A food web shows what eats what in a habitat. 

Changes to the food web affect other species. The fox population may decrease. The population of mice, slugs, beetles and grogs then may increase. Badger numbers may increase because there are fewer foxes to compete with. 

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How do plants use energy

Plants absorb 1-3% of the light energy from the Sun that falls on their leaves. They use this energy for photosynthesis. This produces the chemical that makes us plant cells and store energy. Plants are producers. 

Animals, bacteria and fungi depend on plants for food. So nearly all life is dependant on energy from the Sun.

How is energy transferrred? 

Energy is transfered between organisms:

  • When animals (consumers) eat other organisms 
  • When decay organisms (Decomposers and detrivores) eat other dead organisms and waste materials

  Only a small percentage of the energy at each stage of a food change is passed on. The rest of the energy: 

  • Is used for life processes 
  • Escapes to the surroundings as heat 
  • Is excreted as waste and passed on to decomposers 
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The carbon cycle

Plants take carbon dioxide out of the atmospher by photosynthesis. This makes glucose. Animals and plants break down glucose in respiration. This returns carbon dioxide to the atmospher. Microorganisms break down the molecules of dead organisms by decomposition. The combustion of wood and fossil fuels adds carbon dioxide to the atmosphere. 

How is nitrogen recycled? 

Microorganisms are vital in the nitrogen cycle. Decomposer bacteria break down proteins in dead organisms. 

The following processes are part of the nitrogen cycle: 

  • Nitrogen fixing bacteria in some plant roots convert nitrogen into nitrogen compounds including nitrates 
  • Plants use nitrates to make proteins. Animals digest plant proteins and use them to make animal proteins. 
  • Denitrifying bacteria break down nitrates in the soild and release nitrogen to the air. This is denitrification.
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Monitoring the environment

Scientists use indicators to measure environmental change. Living indicators include: 

  • Phytoplankton- measures ocean temperature change 
  • Lichens to monitor air quality 
  • Mayfly nypmphs to monotor oxygen levels in rivers. 

Non-Living indicators include: 

  • Nitrate levels in streams, rivers and lakes 
  • Carbon dioxide levels and temperature in air and oceans 
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How has life evolved

Life began about 3500 million years ago. All species evolved from simple living things. Fossils and DNA analysis of living organisms provide evidence of evolution. 

There is a variation between individuals of a species. Genetic variation is caused by mutations. Mutated genes in sex cells can be passed on to offspring. This occasionly produces new characteristics. Over time,evolutions makes species change. New species may develop,yoo. The changes below may cause evolution 

Mutations and natural selection 

Environmental changes 


Natural selection is different from selective breeding in which humans choose characteristics for a plant or animal 

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