Biology unit 2

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  • Created by: Megan
  • Created on: 28-03-13 17:30

Animal cell

Nucleus: contains genetic material that controls the activities of the cell. 

Cytoplasm: gel - like substance where most of the chemical reactions happen. It contains enzymes that control the these chemical reactions. 

Cell membrane: holds the cell together and controls what goes in and out. 

Mitochondria: these are where most of the reactions for respiration take place. Respiration releases energy that the cell needs to work.

Ribosomes: these are where proteins are made in the cell. 

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Yeast cells and Bacteria cells

Yeast: A yeast cell us a single celled microorganism. A yeast cell has a nucleus, cytoplasm, and a cell membrane surrounded by a cell wall. 

Bacteria: Bacteria cells are also single - celled microorganisms. A bacteria cell has cytoplasm and a cell membrane surrounded by a cell wall. The genetic material floats in the cytoplasm because bacteria cells don't have a nucleus.

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Plant cell

Plant cells usually have all the bits that animal cells have, plus:

Rigid cell wall: made of cellulose. It supports the cell and strengthens it.

Permanent vacuole: contains cell sap, a weak solution of sugar and salts. 

Chloroplasts: these are where photosynthesis occurs, which makes food for the plant . They contain a green substance called chlorophyll. 

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1. Diffusion is the spreading out of particles from an area of high concentration to an area of low concentration. 

2. Diffusion happens in both solutions and gases - that is because the particles in these subastances are free to move about randomly.

3. The simplest type is when different gases diffuse through eachother. (This is what is happening when the smell of perfume diffuses through the air in the room). 

4. The bigger the difference in concentration, the faster the diffusion rate. 

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cell membranes are kind of clever...

1. They're clever because they hold the cell together but they let stuff in and out as well.

2. Dissolves substances can move in and out of cells by diffusion. 

3. Only very small molecules can diffuse through cell membranes though - things like oxygen, glucose, amino acids and water.

4. Big molecules like starch and proteins can't fit through the cell membrane.

5. Just liek with diffusion in air, particles flow through the cell membrane from where there's a high concentration to where there's a low concentration. 

6. They're only moving sbout randomly, so they go both ways - but if there a lot more  particles on one side of the membrane, there's a net (overall) movement from that side. 

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Palisade leaf cells

1. Packed with chloroplasts for photosynthesis. More of them are are crammed at the top of the cell - so they're nearer the light. 

2. Tall shape means a lot of surface are exposed down the side for absorbind CO2 from the air in the leaf. 

3. Thin shape means tht you can pack loads of them in at the top of the leaf. 

Palisade leaf cells are grouped together at the top of the leaf where most of the photosynthesis  happens. 

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Guard cells

1. Are adapted to open and close pores.

2. Special kidney shape which opens and closes the stomata (pores) in a leaf. 

3. When the plant had lots of water the guard cells fill with it and go plump and turgid. This makes the stomata open, so gases can be exchanged for photosynthesis.

4. When the plant is short of water, the guard cells lose water and become flaccid, making the stomata close. This helps stop to much water vapour escaping.

5. Thin outer walls and thickened inner walls make the opening and closing work. 

6. They're also sensitive to light and close at night to save water without losing out on photosynthesis. 

7. Guard cells are therefore adapted to their function of allowing gas exchange and controlling water loss within a leaf.

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Red blood cells

1. Are adapted to carry oxygen. 

2. Concave shape gives a big surface area for absorbing oxygen. It also helps them pass smoothly through capillaries to reach body cells. 

3. They're packed with haemoglobin - the pigment that absorbs the oxygen. 

4. They have no nucleus, to leave even more room for haemoglobin. 

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Sperm and egg cells

1. Are specialised for reproduction.

2. The main function of an egg cell are to carry the female DNA and to nourish the developing embryo in the early stages. The egg cell contains huge food reserves to feed the embryo. 

3. When a sperm fuses with the egg, the egg's membrane instantly changes its structure to stop any more sperm getting in. This makes sure the offspring end up with the right amount of DNA.

4. The function of a sperm is basically to get the male DNA to the female DNA. It has a long tail and a streamlined head to help it swim to the egg. There are a lot of mitochondria in the cell to provide the energy needed. 

5. Sperm also carry enzymes in their heads to digest through the egg cell membrane. 

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Large multicellular organisms

1. Are made up of organ systems.

2. Specialised cells carry out a particular function.

3. The process by which cells become become specialised for a particular job is called differentiation

4. Differentiation occurs during the development of a multicellular organism. 

5. These specialised cells form tissues, which form organs, which form organ systems. 

6. Large multicellular organisms have different systems inside them for exchaging and transporting materials.

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Similar cells are organised into tissues

A tissue is a group of similar cells that work together to carry out a particular function. It can include more than one type of cell. In mammals, examples of tissues include:

1. Muscular tissue, which contracts to move whatever it is attached to.

2. Glandular tissue, which makes and secretes chemicals like enzymes and hormones. 

3. Epithelial tissue, which covers some parts of the body. 

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Tissues are organised into organs

An organ is a group of different tissues that work together to perform a certain function. For example, the stomach is an organ made of these tissues:

1. Muscular tissue, which moves the stomach wall to churn up the food.

2. Glandular tissue, which makes digestive juices to digest food.

3. Epithelial tissue, which covers the outside and inside of the stomach. 

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Organs are organised into organ systems

An organ system is a group of organs working together to perform a particular function. For example, the digestive system (found in humans and animals) breaks down food and is made up of these organs:

1. Glands which produce digestive juices.

2. The stomach and small intestine, which digest food.

3. The liver, which produces bile

4. The small intestine, which absorbs soluble food molecules. 

5. The large intestine, which absorbs water from undigested food, leaving faeces

The digestive system exchanges materials with the environment by taking in nutrients and releasing substances such as bile. 

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Plant cells

Plants are made of organs like stems, roots and leaves. These organs are made of tissues

For example, leaves are made of: 

1. Mesophyll tissue - this is where most of the photosynthesis in a plant occurs. 

2. Xylem and phloem - they transport things like water, mineral ions and sucrose around the plant. 

3. Epidermal tissue - this covers the whole plant. 

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Carbon dioxide + water ----> glucose + oxygen 

1. Photosynthesis is the process that produces food in plants and algae. The food it produces is glucose

2. Photosynthesis happens inside the chloroplasts. 

3. Chloroplasts contain a green substance called chlorophyll, which absorbs sunlight and uses its energy to convert carbon dioxide and water into glucose. Oxygen is also produced as a by - product. 

4. Photosynthesis happesns in the leaves of all green plants - this is largely what the leaves are for. 

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Limiting factor

1. Light, CO2 and temperature can become limiting factors. This just means that it's stopping photosynthesis from happening any faster

2. Which factor is limiting at a particular time depends on the environmental conditions

  • at night, it's obvious that light is the limiting factor
  • in winter, it's often the temperature 
  • If it's warm enough and bright enough, the amount of CO2 is usually limiting. 
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How plants use glucose

1. For respiration -  Plants manufacture glucose  in their leaves. They then use some of the glucose for respiration. This releases energy which enables them to convert the rest of the gluose into various other useful substances, which they can use to build new cells and grow. To produce some of these substances they also ned to gather a few minerals.

2. Making cell walls - Glucose is converted into cellulose for making strong cell walls. 

3. Making proteins - Glucose is combined with nitrate ions to make amino acids, which are then made into proteins.

4. Stored in seeds - Gluscose is turned into lipids for storing in seeds. Sunflower seeds, for example, contain a lot of oil - we get cooking oil and margarine from them. 

5. Stored as starch - Glucose is turned ino starch and stored in roots, stems and leaves, ready for use when photosynthesis isn't happening, like in the winter. Starch is insoluble which makes it much better for storing than glucose. 

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organisms live in different places

1. A habitat is the place where an organism lives

2. The distribution of an organism is where an organism is found

3. Where a organism is found is affected by environmental factors such as: temperature, availability of water, availability of oxygen and cabon dioxide, availability of nutrients  and amount of light. 

4. An organism might be more common in one area than another due to differences in environmental factors between the two areas.

5. There are a couple of ways to study the distribution of an organism. You can: 

  • Measure how common an organism is in two sample areas and compare them.
  • Study how the distribution changes across an area, by placing quadrats along a transect. 
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Collecting environmental data

1. Reliability  

  • Quadrats and transects are pretty good tools for finding out how an organism is distributed.
  • But, you have to work hard to make sure your results are reliable - which means making sure they are repeatable and reproducible.
  • To make your results more reliable: take a large sample size and use random samples

2. Validility 

  • For your results to be valid they muust be reliable and answer the original quetion.
  • To answer the question, you need to control all the variables. 
  • The question you want to answer is whether a difference in distribution between two sample areas is due to a difference in one environmental factor. 
  • If youve controlled all other variables that could be affecting the distribution, you'll know whether difference in distribution is caused by the environmental factor or not. 
  • If you don't control the other variables you won't know whether any correlation you've found is because of chance, because of the environmental factor or because of a different variable. 
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Enzymes are catalysts produced by living things

1. Living things have thousands of different chemical reactions going on inside them all the time. These reactions need to be carefully controlled to get the amount of substance.

2. You can usually make a reaction happen more quickly by raising the temperature. This would speed up the useful reactions but also the unwanted ones too. There's also a limit to how far you can raise the temperature inside a living creature before its cells start getting damaged. 

3. Living things produce enzymes that act as biological catalysts. Enzymes reduce the need for high temperature and we only have enzymes to speed up the useful chemical reactions in the body. 


4. Enzymes are all proteins and all proteins are made up of chains of amino acids. 

5. As well as catalysts, proteins act as structural components of tissues, hormones and antibodies. 

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Enzymes have special shapes

1. So they can catalyse reactions.

2. Chemically reactions usually involve things either being split apart or joined together.

3. Every enzyme has a unique shape that fits onto the substance involved in a reaction.

4. Ezymes are really picky - they usually only catalyse one reaction.

5. This is because, for the enzyme to work, the substance has to fit its special shape. If the substance doesn't match the enzymes shape, then the reaction won't be catalysed. 

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Enzymes need the right conditions

1. Changing the temperature changes the rate of an enzyme - catalysed reaction. 

2. Like with any reaction, a higher temperature increases the rate at first. But if it gets too hot, some of the bonds holding the enzyme together break. This destroys the enzyme's special shape and so it won't work any more. It's said to be denatured.

3. Enzymes in the human body normally work best around 37C.

4. The pH also affects enzymes. If it's too high or too low, the pH interferes with the bonds holding the enzyme together. This changes the shape and dentures the enzyme.

5. All enzymes have an optimum pH that they work best at. It is often neutral pH 7, but not always. 

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Digestive enzymes

1. Starch, proteins and fats are BIG molecules. They're too big to pass through the walls of the digestive system. 

2. Sugars, amino acids, glycerol and fatty acids are much smaller molecules. They can pass easily through the walls of the digestive system. 

3. The digestive enzymes break down the big molecules into smaller ones. 

  • AMYLASE CONVERTS STARCH INTO SUGARS (made in: salivary glands, pancreas and small intestine) 
  • PROTEASE CONVERTS PROTEINS INTO AMINO ACIDS (made in: stomach, pancreas and small intestine) 
  • LIPASE CONVERTS LIPIDS INTO CLYCEROL AND FATTY ACIDS (made in: pancreas and small intestine) 
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1. Neutralises the stomach acid and emulsifies fats.

2. Bile is produced in the liver. It's stored in the gall bladder before it's released into the small intestine.

3. The hydrochloric acid in the stomach makes the pH too acidic for enzymes in the small intestine to work properly. Bile is alkaline - it neutralises the acid and makes conditions alkaline. The enzymes in the small intestine work best in these alkaline conditions. 

4. It emulsifies fats. In other words it breaks the fat into tiny droplets. This gives a much bigger surface area of fat for the enzyme lipase to work on - which makes its digestion faster. 

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Respiration involves many reactions, all of which are catalysed by enzymes. These are really important reactions, as respiration releases the energy that the cell needs to do just about everything. 

1. Respiration is not breathing in and breathing out. 

2. Respiration is the process of releasing energy from the breakdown of glucose - and it goes on in every cell of your body. 

3. It happens in plants too. All living things respire It's how they release energy from their food. 


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Aerobic respiration

1. Aerobic respiration is respiration using oxygen. It's the most efficient way to release energy from glucose. 

2. Aerobic respiration goes goes on all the time in plants and animals

3. Most of the reactions in aerobic respiration happen inside mitochondria.


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Respiration releases energy

You need to know four examples of what the energy released by anaerobic respiration is used for: 

1. To build up larger molecules from smaller ones. 

2. In animals, to allow the muscles to contract.

3. In mammals and birds the energy is used to keep their body temperature steady. 

4. In plants, to build sugars, nitrates and other nutrients into amino acids, which are then built up into proteins.

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Exercise increases the heart rate

1. Muscles are made of muscle cells. These use oxygen to release energy from glucose, which is used to contract the muscles.

2. An increase in muscles activity requires more glucose and oxygen to be supplied to the muscle cells. Extra carbon dioxide needs to be removed from the muscle cells. For this to happen the blood has to flow at a faster rate. 

3. This is why physical activity: 

  • Increases your breathing rate and makes you breathe more deeply to meet the demand for extra oxygen
  • Increases the speed at which the heart pumps.
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1. Some glucose from food is stored as glycogen.

2. Glycogen's mainly stored in the liver, but each muscle also has its own store.

3. During vigorous exercise muscles use glucose rapidly, so some of the stored glycogen is converted back to glucose to provide more energy. 

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Anaerobic respiration

1. When you do vigorous exercise and your body can't supply enough oxygen to your muscles, they start doing anaerobic respiration instead of aerobic. 

2. Anaerobic just means without oxygen. It's the incomplete breakdown of glucose, which produces lactic acid


3. This is not the best way to convert glucose into energy because lactic acid builds up in the muscles, which gets painful. It also causes muscle fatigue - the muscles get tired and they stop contracting efficiently.

4. Another downside is that anaerobic respiration does not release nearly as much energy as aerobic respiration - but it's useful in emergencies. 

5. The advantage it that at least you can keep on using your muscle for a while longer. 

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Oxygen debt

1. After resorting to anaerobic respiration, when you stop exercising you'll have an oxygen debt. 

2. In other words you have to repay the oxygen that you didn't get to your muscles in time, because your lungs, heart and blood couldn't keep up with the demand earlier on. 

3. This means you have to keep breathing hard for a while after you stop, to get more oxygen into your blood. Blood flows through your muscles to remove the lactic acid by oxidising it to harmless CO2 and water. 

4. While high levels of CO2 and lactic acid are detected in the blood, the pulse and breathing rate stay high to try and rectify the situation.

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Enzymes are used in biological detergents

1. Enzymes are the biological ingredients in biological detergents and washing powders.

2. They're mainly protein-digesting enzymes (proteases) and fat - digesting enzymes (lipase).

3. Because the enzymes break down animal and plant matter, they're ideal for removing stains like food or blood.

4. Biological detergents are also more effective at working at low temperatures than other types of detergents.

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Enzymes are used to change foods

1. The proteins in some baby foods are pre-digested using protein-digesting enzymes (proteases), so they're easier for the baby to digest. 

2. Carbohydrate-digesting enzymes (carbohydrates) can be used to turn starch sugar into sugar syrup.

3. Glucose syrup can be turned into fructose syrup using an isomerase enzyme. Fructose is sweeter, so you can use less of it - good for slimming foods and drinks. 

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Using enzymes in industry

Enzymes are really useful in industry. They speed up reactions without the need for high temperatures and pressures. You need to know the advantages and disadvantages


  • They're specific, so they only catalyse the reaction you want them to.
  • Using lower temperatures and pressures means a lower cost as it saves energy
  • Enzymes work for a long time, so after the initial cost of buying them, you can continually use them. 
  • They are biodegradable and therefore cause less environmental pollution.


  • Some people can develop allergies to the enzymes. 
  • Enzymes can be denatured by even a small increase in temperature. They're also susceptible to poisons and changes in pH. This means the conditions in which they work must be tightly controlled
  • Enzymes can be expensive to produce. 
  • Contamination of the enzyme with other substances can affect the reaction. 
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1. Are really long molecules of DNA.

2. DNA stands for deoxyribose nucleic acid. 

3. It contains all the instructions to put an organism together and make it work.

4. It's found in the nucleus of animal and plant cells, in really long molecules called chromosomes

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A gene codes for a specific protein

1. A gene is a section of DNA. It contains the instructions to make a specific protein. 

2. Cells make proteins by stringing amino acids together in a particular order. 

3. Only 20 amino acids are used, but they make up thousands of different proteins.

4. Genes simply tell cells in what order to put the amino acids together. 

5. DNA also determines what proteins the cell produces

6. That in turn determines what type of cell it is. 

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Everyone has unique DNA

Almost everyone's DNA is unique. The only exceptions are identical twins, where the two people have indeticial DNA, and clones.

DNA fingerprinting is a way of cutting up a person's DNA into small sections and then seperating them. Every person's genetic fingerprint has a unique pattern. This means you can tell people apart by comparing samples of their DNA. 

DNA fingerprinting is used in...

1. Forensic science - DNA taken from a crime scene is compared with a DNA sample taken from a suspect. 

2. Paternity testing - To see if a man is the father of a particular child. 

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1. In a cell that's not dividing, the DNA is spread out in long strings. 

2. If the cell gets a signal to divide, it needs to duplicate its DNA - so there's one copy for each new cell. The DNA is copied and forms X-shaped chromosomes. Each arm of the chromosome is an exact dupllicate of the other. 

3. The chromosomes then line up at the centre of the cell and cell fibres pull them apart. The two arms of each chromosome go to opposite ends of the cell. 

4. Membranes form around each of the sets of chromosomes. These become the nuclei of the two new cells.

5. Lastly, the cytoplasm divides. 

6. You now have two new cells containing exactly the same DNA - they're identical. 

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Asexual reproduction

1. Also uses mitosis. 

2. Some organisms also reproduce by mitosis, e.g. strawberry plants form runners in this way, which become new plants. 

3. This is an example of asexual reproduction. 

4. The offspring have exactly the same genes as the parent - so there's no variation

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1. Have half the usual number of chromosomes.

2. During sexual reproduction, two cells called gametes combine to form a new individual. 

3. Gametes only have one copy of each chromosome. This is so that you can combine one sex cell from the mother and one sex cell from the father and still end up with the right number of chromosomes in body cells. For example, human body cells have 46 chromosomes. The gametes have 23 chromosomes each, so that when an egg and sperm combine, you get 46 chromosomes again. 

4. The new individual will have a mixture of two sets of chromsomes, so it will inherit features from both parents. This is how sexual reproduction produces variation

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1. As with mitosis, before the cell starts to divide, it duplicates its DNA - one arm of each chromosome is an exact copy of the other arm. 

2. In the first division in meiosis (there are two divisions) the chromosome pairs line up in the centre of the cell.

3. The pairs are then pulled apart, so each new cell only has one copy of each chromosome. Some of the father's chromosomes and some of the mother's chromosomes go into each new cell. 

4. In the second division the chromsomes line up again in the centre of the cell. It's a lot like mitosis. The arms of the chromosomes are pulled apart

5. You get four gametes each with only a single set of chromosomes in it. 

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Embryonic stem cells

1. You know that differentiation is the process by which a cell changes to become specialised for its job. In most animal cells, the ability to differentiate is lost at an early stage, but lots of plant cells don't ever lose their ability. 

2. Some cells are undifferentiated. They can develop into different types of cell depending on what instructions they're given. These cells are called stem cells

3. Stem cells are found in early human embryos. They're exciting to doctors and medical researchers because they have the potential to turn into any kind of cell at all. This makes sense if you think about it - all the different types of cell found in a human being have to come from those few cells in the early embryo. 

4. Adults also have stem cells, but they're only found in certain places, line bone marrow. These aren't as versatile as embryonic stem cells - they can't turn into any cell type at all, only certain ones. 

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Stem cells

1. Medicine already uses adult stem cells to cure disease. For example, people with some blood diseases can be treated by bone marrow transplants. Bone marrow contains stem cells that can turn into new blood cells to replace faulty ones. 

2. Scientists can also extract stem cells from very early human embryos and grow them. 

3. These embryonic stem cells could be used to replace faulty cells in sick people - you could make beating heart muscle cells for people with heart disease, insulin-producing cells for people with diabetes, nerve cells for people paralysed by spinal injuries, and so on. 

4. To get cultures of one specific type of cell, researchers try to control the differentiation of the stem cells by changing the environment they're growing in. 

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Against stem cell research

1. Some people are against stem cell research because they feel the human embryos shouldn't be used for experiments since each one is a potential human life.

2. Others think that curing patients who already exist and who are suffering is more important than the rights of embryo

3. One fairly convincing argument in favour of this point of view is that the embryos used in the research are usually unwanted ones from fertility clinics which, if they weren't used for research, would probably just be destroyed. But of course, campaigners for the rights of embryos usually want this banned too. 

4. These campaigners feel that scientists should concentrate more on finding and developing other sources of stem cells, so people could be helped without having to use embryos.

5. In some countries stem cell research is banned, but it's allowed in the UK as long as it follows strict guidelines. 

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Your chromosomes control your gender

There are 22 matched pairs of chromosomes in every human body cell. The 23rd pair are labelled ** and XY. They're the two chromosomes that decide whether you turn out male or female



When making sperm, the X and Y chromsomes 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

A similar thing happens when making eggs. But the original cell has two X-chromosomes, so all the eggs have one X-chromosome. 

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The work of Mendel

Mendel reached these three important conclusions about heredity in plants

1. Characteristics in plants are determined by hereditary units

2. Hereditary units are passed on from both parents, one unit from each parent.

3. Hereditary units can be dominant or recessive - if an individual has both dominant and the recessive unit for a characteristic, the dominant characteristic will be expressed. 

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Fossils form in rocks in one of three ways


  • Things like teeth, shells, bones which don't decay easily, can last a long time when buried.
  • They're eventually replaced by minerals as they decay, forming a rock-like substance shaped like the original hard part. 
  • The surrounding sediments also turn to rock, but the fossil stays distinct inside the rock and eventually someone digs up.


  • Sometimes, fossils are formed when an organism is buried in a soft material like clay. The clay later hardens around it and the organism decays, leaving a cast of itself. 
  • Things like footprints can be pressed into materials, leaving an impression when it hardens.


  • In amber and tar pits there's no oxygen or moisture so decay microbes can't survive.
  • In glaciers it's too cold for the decay microbes to work. 
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The fossils record contains many species that don't exist any more - these species are said to be extinct. Dinosaurs and mammoths are extinct animals, with only fossils to tell us they existed at all. 

Species become extinct for these reasons:

  • The environment changes too quickly
  • A new predator kills them all
  • A new disease kills them all 
  • They can't compete with another species for food
  • A catastrophic event happens that kills them all 
  • A new species develops 
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1. A new species is a group of similar organisms that can reproduce to give fertile offspring.

2.  Speciation is the development of a new species.

3. Speciation occurs when populations of the same species become so different that they can no longer breed together to produce fertile offspring

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