Biology Notes

AQA revision guide used

Higher Tier

Includes: Life Processes, Enzymes and Homeostasis and Genetics

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  • Created by: Georgia
  • Created on: 14-05-12 13:04

Plant and Animal Cells

Plant and Animal Cells

Nucleus - contains genetic material which controls the acitvites of the cell.

Cytoplasm - contains enzymes that control the chemical reactions which takes place here.

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

Mitochondria - reactions for respiration take place. Repiration releases energy that cell needs to work.

Ribosomes - Proteins are made.

Plant cells have all of the above plus these extra bits below!

Cell wall - supports cell and strengthens it. Made of cellulose.

Permanent Vacuole - contains cell sap, weak solution of sugar and salts.

Chloroplasts - where photosynthesis occurs. Makes food for plant and contains chlorophyll.

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Cells, Tissues, Organs and Systems

  • cells have structures that are specialised to carry out the function.
  • Similar cells are grouped together to create a tissue.
  • Different tissues create an organ.
  • Organs have a particular job.
  • Groups of organs create an organ system.
  • Groups of organs or organ systems create an organism.


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Specialised Cells

Palisade leaf cells

  • Adapted for photosynthesis, have lots of chloroplasts.
  • Tall shape = lots of surface area for absorbing CO2. 
  • Thin shape means lots can be packed into the top of a leaf. 
  • Grouped together = palisade layer = leaf tissue where most photosynthesis happens.

Guard Cells

  • Adapted to open and close pores. 
  • Kidney shape which opens and closes the stomata.


  • Thin outer walls and thickened inner walls make the opening and closing work.
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Specialised cells 2

  • Sensitive to light and close at night to save water without losing out on photosynthesis.


  • Allowing gas exchange and controlling water loss.

Red blood cells

  • Concave shape gives big surface area for absorbing oxygen. 
  • Packed with haemoglobin - pigment which absorbs oxygen.
  • No nucleus to leave even more room for haemoglobin.
  • Blood = tissue
  • Adapted to carry oxygen.

Sperm and egg cells

  • Specialised for reproduction.
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Specialised cells 3

  • Egg = to carry female DNA, nourish the developing embryo, contains huge food reserves to feed the embryo.
  • Sperm = to carry male DNA, to get the male DNA to the female DNA, has a lot of mitochondria to provide energy.
  • Sperm carries enzymes in their head to digest through egg cell membrane.
  •  When sperm fuses egg, egg's membrane changes structure to stop further sperm entering it.
  • Making sure embryo has correct amount of DNA.
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Diffusion is the passive movement of particles from a high concentration area to a low concentration area. In liquids and gases.

  • The bigger the diffrence in concentration the faster the diffusion rate.
  • Cell membranes hold cell together but also let stuff in and out. This is done by diffusion and osmosis.
  • Only small molecules can diffuse through cell membranes. E.g glucose, amino acids, water and oxygen.
  • Rate of diffusion depends on 3 things: 

Distance = substances diffuse more quickly when they haven't as far to move.

Concentration gradient = diffuse faster if there's a big difference in concentration.

Surface area = more surface available for molecules to move across, the faster they can get from one side to the other.

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Osmosis is the movement of water molecules across a partially permeable membrane from a region of high water concentration to a region of low water concentration.

  • A partially permeable membrane is just one with very small holes in it. So only tiny molecules like water can pass through.
  • Molecules move randomly and pass through both ways.
  • Due to more water molecules on one side than the other theres always a steady net flow of water into the region with fewer molecules.
  • Strong sugar solutions get more dilute. Trying to even up the concentration on either side of the membrane.
  • Water moves in and out of cells by osmosis.
  • High sugar solution outside the cell means water will move out of the cell to dilute it.
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The equation:Carbon dioxide + water (+ sunlight absorbed by chlorophyll) = Glucose + Oxygen

  • Produces food in plants, the food it produced is glucose.
  • Happens in the leaves of all green plants. Happens within chloroplasts.
  • Chloroplasts contain a substance called chlorophyll which absorbs sunlight and uses the energy to convert co2 + H2O into glucose. Oxygen is also produced.
  • Four things are needed for photosynthesis:

Rate of photsynthesis:

  • Depends on 3 things: Temperature, light and carbon dioxide levels.
  • Can be controlled by an artificial environment e.g greenhouse.
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How plants use the glucose


  • Use glucose for respiration, this releases energy which enables them to convert rest of glucose into other useful substances. Which enables them to build new cells and grow.
  • To produce some of the substances they also need a few minerals from the soil.

Making fruits

  • Glucose + fructose both turned into sucrose for storing in fruits. This makes it taste nice so the animals eat then spread the seeds all over the place in their poo.

Making cell walls

  • Glucose is converted into cellulose for making cell walls.

Making proteins

  • Glucose is combined with nitrates (from the soil) to make amino acids which are made into proteins.
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How plants use glucose 2

Stored in seeds

  • Glucose is turned into lipids (fats and oils) for storing in seeds.

Stored as starch

  • Glucose is turned into starch and stored in roots, stems and leaves for when photosynthesis isn't happening.
  • Starch is insoluble which makes it better for storing because it doesn't have an effect by osmosis like glucose would.
  • Potato and carrot plants store starch underground throughout winter so a new plant can grow the following spring.
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Minerals for healthy growth


  • Needed for making amino acids which are then used to make proteins.
  • Lack of nitrates in the soil means the plant will start to show stunted growth.


  • Needed to make chlorophyll which is needed for photosynthesis.
  • If there is no magnesium in the soil the leaves will turn yellow. As magnesium is needed to make chlorophyll which gives leaves their green colour.


  • Monoculture is where just one type of crop is grown in the same field year after year. Soil becomes defficient in certain minerals which the crop uses most of.
  • Causes poor growth and gives a reduced yield. Meaning poor crop unless fertiliser to replace these minerals is used.
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Enzymes and digestion

Digestive enzymes break down big molecules into smaller ones. 

  • Starch, proteins and fats are BIG molecules. Too big to pass through the walls of the digestive system.


  • Sugars, amino acids, glycerol and fatty acids are much smaller molecules.


  • Convert starch into simple sugars.
  • Found in salivary glands, pancreas and small intestine.


  • Converts proteins into amino acids.
  • Made in the stomach, pancreas and small intestine.
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Enzymes and digestion 2


  • Converts fats into glycerol and fatty acids.
  • Produced in pancreas and small intestine.

Bile neutralises the stomach acid and emulsifies fats.

  • Bile is produced in the liver and then stored in the gall bladder before it is released in the small intestine.

Liver - > gall bladder - > small intestine

  • Emulsifies fats, breaks the fat into tiny droplets. Giving a much bigger surface area of fat for the enzymes lipase to work on - makes digestion faster.
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The digestive system

Breakdown of food is catalysed by enzymes

  • Enzymes used in the digestive system are produced by specialised cells in glands and in the gut lining.
  • Different enzymes catalyse the breakdown of different food molecules.

Salivary Glands - produce amylase enzyme in the saliva.

Liver - Where bile is produced. Neutralises stomach acid and emulsifies fats.

Gall bladder - Bile is stored, before released into small intestine.

Large intestine - excess water is absorbed from the food.

Stomach - pummels food with muscular walls, produces protease enzyme (pepsin) and produces hydrochloric acid to kill bacteria and create right pH.

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The digestive system 2

Pancreas - Produces protease, amylase and lipase enzymes. Releases these into small intestine.

Small intestine - Produces protease, amylase and lipase enzymes to complete digestion. Where food is absorbed out of the digestive system into the body.

Rectum - Where faeces are stored before they exit the body through excrete.

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Biological catalysts - Enzymes

Enzymes are catalysts produced by living things

  • Living things have thousands of different chemical reactions going on inside them all the time. Enzymes are biological catalysts.
  • Reactions can be carefully controlled to get right amounts of substances. Make reaction quicker by rising the temperature. If too high it causes damage to the cells and it becomes denatured.
  • A catalyst is a substance which increases the speed of a reaction without being changed or used up in the reaction.
  • Enzymes are all proteins. Proteins are made up of chains of amino acids which are folded into unique shapes so they can catalyse reactions.
  • Chemical reactions usually involve things either being split apart of joined together. Every enzyme has a unique shape that fits onto the substance involved in a reaction.
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Enzymes and Respiration

Enzymes help build amino acids and proteins.

  • Respiration involves many reactions, all which are catalysed by enzymes.
  • Respiration is the process of releasing energy and the breakdown of glucose - goes on in every cell in your body.
  • Happens in plants as well. All living things respire.

Aerobic respiration

  • Needs lots of oxygen, respiration using oxygen. Most efficient way to release energy from glucose.
  • Happens within mitochondria.

Glucose + Oxygen -> carbon dioxide + water + energy

  • Anaerobic respiration happens without oxygen and releases much less energy.
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Respiration releases energy for

  • To build up larger molecules from smaller ones. (Like proteins and amino acids.)
  • In animals, to allow the muscles to contract - enabling them to move.
  • In mammals and birds the energy is used to keep their body temperature steady.
  • In plants, to build sugars, nitrates and other nutrients into amino acids which are then built up to create proteins.
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Uses of Enzymes

Biological Detergents

  • Protein - digesting enzymes (proteases) and fat-digesting enzymes (lipases).
  • They remove stains.

Change foods

  • Proteins in baby food in some baby are pre digested using proteases, so they are easy to digest.
  • Carbohydrate-digesting (carbohydrases) turn starch syrup into sugar syrup.
  • Glucose syrup can be turn into fructose syrup using isomease. Fructose is a sweetener, meaning it's slimming products.

Using enzymes in industry takes a lot of control

  • They speed up reactions without the need for high temperatures and pressures.

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Advantages and disadvantages of using enzymes in i


  • Specific, only catalyse the reaction you want it too.
  • Lower temperatures and pressure means lower costs and saves energy.
  • Enzymes work for a long time, so after initial cost you can continually use them.
  • Biodegradable and therefore less environmental pollution.


  • People can develop allergies to enzymes.
  • Enzymes can be denatures by even a small increase in temperature.
  • Contamination of the enzyme with other substances can affect the reaction.
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Homeostasis is the maintenance of a constant internal environment.

Six things that need to be controlled:

  • The body temperature.
  • Water content.
  • Ion content.
  • The blood sugar levels.
  • Carbon dioxed (wastage)
  • Urea, made from excess amino acid. (wastage)

Body temperature must be carefully controlled.

  • Enzymes work best at a certain temp. In the human body at 37 degrees.
  • Thermoregulatory centre in the brain acts as personal thermostat.
  • Contains receptors that are sensitve to temp. of blood flowing through brain.
  • Recieves impulses from skin about skin temp.
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When your too hot or too cold


  • Hairs lie flat.
  • Sweat is produced by sweat glands and evaporates, removing heat.
  • Blood vessels supplying skin dilate so more blood flows close to the surface of the skin. Making it easier for heat to be transferred from blood to the environment.


  • Hairs stand up to trap insulating layer of air.
  • No sweat is produced.
  • Blood vessels constrict to close off skins blood supply. 
  • Muscles contract, which needs respiration which releases energy as heat.
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Kidneys and Homeostasis

Act as filters to 'clean the blood.' 3 main roles:

  • Remove urea from the blood. -

Proteins cannot be stored by the body so any excess amino acids are converted into fats and carbs to be stored.

Process occurs in the liver. Urea is poisonous and produced as a waste product from reactions.

  • Adjust ions in the blood. -

Ions such as sodium are taken into body as food then absorbed into blood.

Ions are lost in sweat and excess ions removed by kidneys.

  • Adjust water content of the blood.

Taken in via food and drink, lost in: urine, sweat and air breathed out. NEEDS a CONSTANT BALANCE.

Hot day: lots of sweat, less urine (dark and concentrated) Cold day: less sweat,more urine (pale and dilute.)

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Controlling blood sugar

Insulin controls blood sugar levels.

  • Eating foods rich in carbs adds a lot of glucose into the blood from the gut.
  • Normal reactions of cells removes glucose from blood.
  • Vigorous excercise removes a lot of glucose from the blood.
  • Levels of glucose in blood must be kept steady. Changes are monitored and controlled by the pancreas using insulin.
  • Blood glucose level: HIGH - insulin added / LOW - insulin not added.

Diabetes (type 1) pancreas stops making enough insulin meaning it can rise to a level that can kill them however it can be helped. OR diabetics can have a pancreas transplant.

  • Avoiding foods rich in carbs or exercising after carbs.
  • Injecting insulin - how much depends on diet and how active person is.
  • Check blood sugar using glucose monitoring device.
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DNA and Cell division

Chromosones are really long molecules of DNA.

  • DNA contains all the instructions to put an organism together and make it work. Found in the nucleus of animal/plant cells, in chromosones.
  • Gene codes for a specific protein. Gene is a section of DNA. They tell cells what order to put amino acids together, determing the type of cell.
  • Cells make proteins by putting amino acids together in a specific area. Only 20 amino acids are used, making up 1000's of proteins.


  • Makes new cells for growth and repair. Body cells normally have 2 copies of each chromosone, 23 pairs in total.
  • When a body cell divides it needs to make new cells identical to original, same no. of chromosones.
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Cell division

  • Mitosis is when a cell reproduces itself by splitting to form two identical offspring.
  • When cell needs to divide it duplicates its DNA. Cell fibres then pull the 'arms' of the chromosomes apart. Each arm is a duplicate.
  • The chromosomes line apart and the cell fibres pull them apart. Two arms of each chromosome go to opposite ends of cell.
  • Membranes form around each of the sets of chromosomes - become nuclei of two new cells. Cytoplasm then divides.
  • Two new cells containing exactly same DNA are produced.

Asexual reproduction uses mitosis. Offspring have exactly same genes. No variation.


  • Gametes have half number of chromosomes. Happens within sexual reproduction.
  • Meiosis produces cells which have half the normal number of chromosomes.
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stem cells continued

  • Embryos researched are usually unwanted ones from fertility clinics, so otherwise may be destroyed.
  • Campaigners feel scientists should concentrate more on finding and developing other sources of stem cells.
  • Countries stem cell research is banned but allowed in UK as long as sticking to strong guidlines.
  • All MEN have: XY chromosomes. WOMEN have: ** chromosomes.
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