AQA GCSE Biology Unit 2

  • Created by: Charlotte
  • Created on: 17-09-13 17:55

Animal Building Blocks

Animal Cells:                                                                                                                   Each cell is surrounded by a cell membrane. It holds the cell together and controls what goes in and out of the cell.

Cells contain organelles. These include the nucleus, mitochondria and ribosomes. The nucleus controls the cell's activities and is surrounded by cytoplasm. It contains DNA (genetic material that gives the cell instructions about what enzymes they need) Hundreds of chemical reactions take place in the cytoplasm, these reactions are controlled by enzymes.

Electron microscopes magnify 500000x and show mitochondria. Mitochondria use glucose in respiration to release energy for the cell. Ribosomes are the smallest organelles, and they build up proteins from simple compounds called amino acids. Proteins are molecules that are used to make other parts of the cell and other chemicals such as enzymes.

Specialised cells are cells that have particular features that relate to what they do. Muscle cells have fibrils and can contract and expand. Sperm cells have a tail to help them find an egg, and they have lots of mitochondria to give them energy for movement. Nerve cells have long fibres that can carry electrical impulses. Cytoplasm at each end aids communication with other cells.

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Plant and Alga Building Blocks

Plant Cells: Like animal cells, plant cells usually have a nucleus, cell membrane, mitochondria, cytoplasm, and ribosomes. Plant cells also have a cell wall made of a carbohydrate called cellulose. This cellulose is made of tiny fibres, and together these fibres become very strong and support the cell.  Some plant cells also have organelles called chloroplasts in their cytoplasm. Chloroplasts contain a green pigment called chlorophyll, which is used in photosynthesis to absorb the suns light energy.In photosynthesis, light energy is converted to chemical energy in the form of glucose as food for the plant.

In the centre of many plant cells there is a large, permanent space filled with liquid. This is a vacuole. The liquid is cell sap and contains sugars, salts and water. When it is full it presses out on the edges on the cell, making it firm. If the vacuole is not full then the cell is not so firm. This is the reason plants wilt when they do not have enough water.

Specialised plant cells: Palisade mesophyl cells are found in the leaf of the plant and have lots of chloroplasts, which trap lots of light energy, making them the main photosynthetic cells. Root hair cells have extensions into the soil so they can absorb water and mineral ions. They are long and narrow, and so can fit between soil particles.

Remember to distinguish clearly between cell wall and cell membrane.

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Bacteria and Yeast Cells

Bacteria are unicellular organisms- each cell can live on its own and carries out all seven characteristics of life (MRS GREN). Bacteria are known as microbes or microorganisms because a microscope is needed to see them. A bacterium consists of cytoplasm surrounded by membrane and cell wall. The cell wall is not made from cellulose. There is no nucleus and no other organelles apart from plasmids (small circles of DNA that have genes to make proteins) and the cytoplasm contains circular chromosomes that form loops of DNA which contains the cells genes.

Yeast: a single celled, microscopic fungus. Yeast is found on plant leaves, flowers and in soil. It is ten times the size of a bacterium. You can buy fresh, compressed yeast or sachet yeast for making bread. Yeast is used in breweries to produce the alcohol in beer. It has a nucleus, cytoplasm, mitochondria, a vacuole, a cell membrane and a cell wall.

PROKARYOTIC CELLS- No nucleus and no other organelles surrounded by a membrane. E.g. Bacteria

EUKARYOTIC CELLS- Membrane bound nucleus and other organelles. E.g. animal, plant and yeat cells.

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Getting In and Out of Cells

Diffusion of gases: Flowers release smelly particles that you can smell. You smell them because the particles spread through the air, sometimes many metres. The movement of these particles through the air is called diffusion. This is the AQA specifacation definition of diffusion:

Diffusion is the spreading of the particles of a gas, or of any substance in solution, resulting in a net movement from a region where they are of a higher concentration to a region with lower concentration. The greater the difference in concentration, the faster the rate of diffusion.

A difference in concentration of a substance in two different areas is called concentration gradient. A greater concentration in one place than the other means a faster rate of diffusion than if the concentrations in the two places are nearly the same. Even when there is a big concentration gradient, diffusion happens in both directions, there is an overall net movement of substances.

Living organisms use diffusion in respiration- they need oxygen which travels through cell membranes through diffusion. Plant and algal cells also require oxygen for respiration, but because their rate of respiration is slower, the rate of diffusion is slower too.

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Word equation:

water and carbon dioxide -----suns light energy and chlorophyll---- oxygen and glucose

Symbol equation:

H20 + CO2   -----suns light energy and chlorophyll------- O + C6H12O6

Balanced Symbol Equation:

6H20 + 6CO2  ------suns light energy +chlorophyll------- 6O + C6H12O6

A green pigment called chlorophyll, located within chloroplasts in the leaf, absorbs the suns light energy. This energy is used by converting carbon dioxide (found in the air), and water (absorbed by the roots) into glucose. Oxygen is produced as a by product and released into the atmosphere.

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Photosynthesis: Limiting Factors

  • Light Intensity
  • Carbon Dioxide Concentration
  • Temperature

If one or more of these is low the rate of photosynthesis is slowed down or limited. If you look at each factor seperately, you find that increasing the amount of one factor will increase the level of photosynthesis, but only for a short time. After a while the increase will level off, because photosynthesis is being limited by another factor. If you increase temperature too much, the enzymes which control photosynthesis will break down, or denature. This stops photosynthesis.

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Glucose Produced in Photosyntheis

The glucose produced during photosyntheis is used in many ways by the plant. Some is used during respiration to produce energy. It can also be converted into insoluble starch for storage until it is needed. You can test to see if a plant is photosyntheising well by seeing how much starch is present in the leaf.

Glucose is also used to produce fat or oil, to produce cellulose, which strengthens the cell wall, and to produce proteins

When producing proteins, plants also use nitrate ions absorbed in the soil.

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Distribution of Organisms

Physical factors that may affect the distribution of organisms:

  • temperature
  • availability of nutrients
  • amount of light
  • availability of water
  • availability of oxygen and carbon dioxide

You can get quantative data (numerical data) about the distribution of organisms by using a quadrat. A quadrat is a square shape that is placed either systematically or randomly and the number of organisms within it is counted. There are two ways of using a quadrat:

Random sampling: this involves randomly throwing the quadrat multiple times and recording your findings.

Sampling along a transect: This involves placing the quadrat along even, measured intervals and recording your findings.

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Proteins are very important molecules as they take part in most chemical reactions in cells and are part of the structure of most organelles.

All proteins contain the same four elements: carbon, hydrogen, oxygen, and nitrogen. Proteins are large molecules, and they are made from a chain of smaller molecules called amino acids. There are 20 different types of amino acids, and they join together in a long chain (a polypeptide) in a specific order to create a specific protein. Because there are 20 different amino acids, there are thousands of different combinations of chain, so many many different types of proteins can be formed. The polypeptide chains then bend and fold to form a very specific 3D shape, that enables other molecules to fit into the protein.  Proteins act as:

  • Structural components of tissues such as muscle- these proteins have been formed into minute fibres made from very long chains of amino acids that have been folded into a simple shape
  • Hormones- e.g. insulin that controls your blood sugar level
  • Antibodies- protein molecules with a very precise 3D shape. They are produced by white blood cells to fight off pathogens
  • Catalysts that speed up the rate of chemical reactions. Biological catalysts are called enzymes. Enzymes are proteins
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During a chemical reaction, chemical bonds are broken or formed, and a new substance is produced. Enzymes speed up this reaction by making it easier for the reacting substances to come together and be rearranged.

The starting substance of the reaction is called a substrate and the finishing substance is called the product. Enzymes lock onto substrates. Each enzyme has a very specific shape, so will only fit onto one type of substrate, like a key that fits into a lock. When the substrate has reacted, the product no longer fits into the enzyme and so removes itself. The enzyme is unchanged by the reaction so it is free to react other substrates.

Most enzymes work best around body temperature: 37c. If the temperature goes above this, the enzymes will denature. This means that the specific 3D shape of the enzyme changes, so the substrate cannot fit into the enzyme. They no longer speed up the reaction.

pH also has an effect on enzymes. Different enzymes work best at different pH levels, for instance stomach enzymes work best in low pH because the stomach is an acidic environment. Intestinal enzymes work best in a high pH because the intestine is an alkaline environment. Stomach enzymes would not work well in the intestine.

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Digestive Enzymes

Molecules of proteins, starches and fats are much too big to be directly absorbed into your body- they need to be broken down. The process of digestion means breaking these larger molecules into smaller ones. This process is catalysed, sped up, by enzymes. Digestive enzymes are produced by special glands and tissues in the gut. Enzymes are made inside cells, but they move outside of cells into the gut where they can come into contact with food molecules. Each different food needs a different type of enzyme to break it down.

  • Amylase: this enzyme breaks down starch into sugars. It is produced in the salivary glands, pancreas and small intestine. Starch molecules are made up of a chain of glucose molecules, and amylase breaks them down into many seperate glucose molecules.
  • Protease: protease enzymes catalyse the breakdown of protein into amino acids. Protease is produced by the stomach, pancreas and small intestine. Protein molecules are made up of long chains of amino acids, and protease breaks these down into many seperate amino acids.
  • Lipase: this breaks down lipids (fats and oils) into fatty acids and glycerol. Lipase is produced by the pancreas and small intestine.

pH in the gut is important as different enzymes work best in different pH levels. The stomach produces hydrochloric acid because the protease enzymes there work best in an acidic solution. The liver produces bile which is stored in the gall bladder. When released into the small intestine, bile neutralises the acid produced in the stomach so that intestinal enzymes can work in an alkaline condition.

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Enzymes at Home and in Industry

Some microorganisms, e.g. bacteria, produce enzymes that pass out of the cells. This enables the enzymes to be used for many different functions. Microorganisms can be grown quickly and cheaply, and as they rapidly multiply, they can produce lots of enzymes very quickly.

Enzymes in industry: In baby foods, proteases 'pre-digest' the proteins, this makes the food softer and easier for babies to eat and digest. Carbohydrase enzymes are used to convert starch into sugar syrup. Starch is cheaper than pure sources of sugar, so this means cakes and sugary drinks can be produced for less money. Isomerase is an enzyme that converts glucose syrup to fructose syrup, which is much sweeter. This means leass can be used, saving money.

In industry, enzymes are used to bring about reactions at normal temperatures and pressures that would otherwise require expensive, energy-demanding equipment. However, most enzymes are denatured at high temperatures and many are costly to produce. 

Enzymes in the home: Biological detergents contain lipase and protease enzymes. Biological detergents can be used at much lower temperatures, more effectively, than other types of detergents. However, some people use non-biological detergents, rather than biological detergents, as they can be allergic to the enzymes used in them.

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Aerobic Respiration

Without respiration organisms would die- it is carried out in plant and animal cells constantly to produce the energy that organisms need to carry out functions that enable them to survive.

glucose + oxygen ------------------ carbon dioxide + water + energy given out

The reactions in respiration are controlled by enzymes. Aerobic respiration uses oxygen from the air and glucose from digested food in the gut. Most of aerobic respiration occurs inside the mitochondria. Cells that use a lot of oxygen, such as muscle cells, usually have more mitochondria than other types. 

Energy produced in respiration is used in many ways. It is used:

  • To build larger molecules from smaller ones
  • In animals, to enable muscles to contract
  • In mammals and birds, to maintain a steady body temperature in colder surroundings
  • In plants, to build up sugars, nitrates and other nutrients into amino acids which are then built up into proteins.

Keeping body temperature steady is important as otherwise enzymes denature.

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Changes During Exercise

Muscles need energy to be able to contract and expand. Respiration provides this energy. The rate of respiration increases the harder you exercise, as the harder you exercise the more energy your muscles need. 

Glucose moves into the mitochondria from the blood when you exercise. However, if there isnt enough glucose, your body uses stores of glucose in your muscle and liver cells. This glucose has been stored as glycogen, and is converted back to glucose when needed.

Muscles store glucose as glycogen, which can then be converted back to glucose for use during exercise.

During exercise:

  • Your heart rate increases, because your blood needs to circulate faster so it can supply all the extra oxygen and sugar your cells need and remove all the extra carbon dioxide being produced.
  • The rate and depth of your breathing increases, because oxygen and carbon dioxide in the blood need to be exchanged faster with air in the lungs. Increasing depth and rate of breathing increases the volume of air you breathe.
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Anaerobic Respiration

Exercising for a long time causes muscles to fatigue. This means they stop contracting efficiently. This causes an increasing weakness and pain or cramps. The causes of fatigue are not well understood, but one cause of fatigue or pain is the build up of lactic acid in the muscles. Blood flowing through the muscles removes lactic acid. Lactic acid builds up becuase of anaerobic respiration.

If you exercise for a long time, aerobic respiration often does not provide enough energy. Then the body carries out anaerobic respiration, which is:

glucose---------lactic acid + energy given out

Anaerobic respiration breaks down glucose, but it provides much much less energy per glucose molecule than aerobic respiration. Much less energy is released than during aerobic respiration because the breakdown of glucose is incomplete. 

The Oxygen Debt: You breathe deeply and quickly for a while after exercise to repay the oxygen debt. If anaerobic respiration has occured, there will be lactic acid in the muscle cells. This must be transported to the liver and oxidised with the extra oxygen from the heavy breathing, so that it can be stored and used for aerobic respiration in the future.

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Cell Division- Mitosis

Body cells have two sets of chromosomes (that contain genetic information in the form of DNA). Humans have 46 chromosomes in every single body cell, so thats 2 sets of 23, apart from gametes (sex cells) which have just 23 chromosomes (a single set).

Body cells divide by mitosis. Mitosis occurs for two reasons: growth, and producing replacement cells (e.g. if you fall and cut yourself, new skin cells are needed).

There are two main stages in mitosis:  1. Copies of the genetic material are made- so in humans, that means that the 46 chromosomes in two pairs become 92 chromosomes in four pairs.  2. One copy of each chromosome (so in humans, thats 46 chromosomes) move to the edge of the cell and the cell divides creating two identical cells with the same number of chromosomes as the original (in humans, 46).

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Cell Division- Meiosis

The only cells that divide by meiosis are gametes (sex cells). In humans these are the sperm and egg- so meiosis happens in the reproductive organs, the testes in the man and the ovaries in the woman. 

Meiosis begins with a cell that contains two sets of chromosomes (2 sets of 23 in humans). Each chromosome is copied, so there are 4 sets of chromosomes. The cell divides in two, then in two again, so that there are 4 cells all with 1 set of chromosomes (1 set of 23 in humans).

Fertilisation- During fertilisation in humans, an egg cell fuses with a sperm cell. As these cells have been formed through meiosis, they both have 1 set of 23 chromosomes. When they fuse, the chromosomes from each cell come together in a new nucleus. This means that the nucleus contains 2 sets of 23 chromosomes- 46 chromosomes. This cell divides by mitosis, forming a bundle of cells with genetic information from the sperm and egg.

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Mitosis and Meiosis Diagram


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Differentiated and Stem Cells

In animals, most cells are differentiated and specialised to do a certain job. When a cell is fertilised, it divides multiple times by mitosis, to form an embryo. These early cells can differentiate to form any kind of cell. These cells are called stem cells, and the ones in an embryo are called embryonic stem cells. As cell division continues throughout pregnancy, more and more cells become specialised, until you are born, at which point almost every single cell is specialised.

In adults, stem cells are found in some places, such as in bone marrow. These stem cells can usually only differentiate into a limited range of other cells. The stem cells in bone marrow, for example, usually only differentiate to form different kinds of blood cells. These stem cells can be used to treat some diseases and disorders- bone marrow stem cells are being used to treat luekaemia, a cancer of blood cells. Treatment with stem cells may be able to help conditions as paralysis, as new nerve cells can be formed. Using embryonic stem cells is an easy option medically as they are easier to work with, but putting cells from one body into another means that the patient would have to take heavy medication for the rest of their life.  Using stem cells that are created from the patients own cells avoids this problem. 

There are many ethical issues with stem cell treatment, as some people believe that destroying an embryo after using its cells is murdering a new life. However, most of the embryos used are ones that are unwanted after fertility treatments, so would be destroyed anyway.

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Genes and Alleles

The nucleus contains chromosomes, which are made up of very tightly packed coils of genes. Each gene is a length of DNA. DNA has a double helix structure (like a twisted ladder). A certain gene codes for a particular sequence of amino acids, and therefore a particular protein. Proteins control many characteristics e.g. eye colour, hair colour. Each person has unique DNA, apart from identical twins. 

Some characteristics are controlled by a single gene. Eacg gene may have a different form: these are called alleles. In humans, one set of alleles comes from the egg and one set from the sperm, so variation occurs, because the alleles are not the same. For instance, the mothers allele of the eye colour gene may code for blue eyes, the fathers allele of the eye colour gene may code for brown eyes. The characteristic that you inherit will be based on how the alleles of the gene in each chromosome pair react.

Because everyone has unique DNA, it can be used to identify individuals. This process is called DNA profiling/fingerprinting. Children have profiles that match half of each parent's profile. Children in the same family may have some or many similarities on their profiles.

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Recessive/Dominant Alleles

Alleles are dominant or recessive:

The characteristic controlled by a dominant allele develops if the dominant allele is present on one or both chromosomes in a pair. For instance, if a mother passed on a dominant allele of a characteristic, and the father had a recessive allele of a characteristic, the child would inherit the mothers characteristic because she passed on the dominant allele. If both alleles were dominant the child would also inherit the dominant characteristic.

The characteristic controlled by a recessive allele develops only if the recessive allele is is present on both chromosomes in a pair. For instance, if the mother and the father both passed on the recessive allele of a characteristic, then that characteristic would be passed on to the child. But if the mother had the recessive allele of a characteristic and the father had a dominant allele of the characteristic, the fathers characteristic would be passed to the child because not both alleles were recessive.

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A well written set of notes with highlighted key words that covers a wide range  of topic such as cells,photosynthesis, respiration and DNA. They would need to teamed up with some diagrams of key structures and graphs and would then make a very useful resource. Making a glossary or a set of flashcards of the key terms would be a helpful revision exercise.



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This is amazing thank you so much! These notes are really helping me with my current GCSE revision. Very useful!



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Very helpful. Nicely laid out and the colour really helps bring out the important information.

Thank you very much (I have an exam at the end of the week, this will come in handy for last minute revision)

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