AS Biology 1

AQA AS Biology Unit 1

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Causes of Disease - Pathogens

A pathogen is an organism that causes disease e.g some bacteria, fungi and all viruses. Pathogens can penetrate an organisms interface with the environment. Organisms have 3 main interfaces -

  • Gas-Exchange system (respiratory)
  • Digestive system (food and water)
  • Skin

The body has natural defences which help to prevent pathogens entering the body -

  • A mucous layer that covers gas-exchange surfaces which forms a sticky barrier which is hard to penetrate.
  • Production of enzymes that can break down pathogens
  • Production of stomach acid which kills micro-organisms.

Pathogens cause disease by damaging host cells and producing toxins (harmful molecules) which enter into the body.

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Cause of Disease - Lifestyle

Lifestyle can affect the risk of developing some diseases.

Cancer - specific lifestyle factors that are known to contribute to cancer include -

  • Smoking : cause mouth, throat and lung cancer
  • Diet : low fat, high fibre diet rich in fruit and veg helps reduce cancer risk
  • Obesity
  • Exercise : people who regularly exercise have a lower cancer risk
  • Sunlight : more sunlight exposure, higher the cancer risk

Coronary Heart Disease (CHD) - lifestyle factors can increase the risk of CHD -

  • Smoking
  • High Blood Pressure
  • High blood cholesterol levels
  • Obesity : with a body mass index of 25+
  • Diet
  • Physical activity


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The Digestive System

Digestion breaks down large molecules into small molecules.

  • Oesophagus - carrys food from mouth to the stomach, has a thick muscular wall.
  • Stomach - muscular sac, inner layer which produces enzymes. It stores and digests food, especially proteins.
  • Small Intestine - long muscular tube, further digests food by enzymes produced in its gland, it is adapted for its purpose of putting digestion products back into the bloodstream by small villi and microvilli on its surface.
  • Large Intestine - absorbs water and forms faeces from undigested food.
  • Rectum - final part of the intestines, where faeces is stored before being removed.
  • Salivary Glands - produce saliva and amylase to break down starch into maltose.
  • Pancreas - produces carbohydrases, lipases and proteases, these are also made in the small intestine.

Physical digestion - food broken down by teeth and churned by stomach muscles to give food a larger surface area. Chemical digestion - enzymes break down large insoluble molecules by hydrolysis. There are 3 types of digestive enzymes - Carbohydrases break down carbs to monosaccharides,Lipases break down lipids to glycerol and fatty acids and proteases which break down proteins to amino acids.

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Carbohydrates

Carbs are polymers that contain C, H and O. Carbs are made from monosaccharides, glucose is a hexose sugar - a monosaccharide with 6 carbons. The monomers they are made from are monosaccharides e.g glucose, fructose, galactose.

Monosaccharides join together by condensation reactions to form di and polysaccharides. Di - 2 monosaccharides, poly - more than 2 monosaccharides. During the reaction H20 is released and a glycosidic bond is formed between the 2 monosaccharides.

Di and Polysaccharides are broken down during digestion.

  • Maltose is hydrolysed by maltase to produce glucose and glucose
  • Sucrose is hydrolysed by sucrase to proudce glucose and fructose
  • Lactose is hydrolysed by lactase to produce glucose and galactose

Use the Benedict's test for sugar. For reducing sugars (all monosaccharides) you add benedicts solution and in heat it will turn red and form a red precipitate if sugar is present. For non-reducing sugars you break them into monosaccharides by heat, then add HCl and NaCH then use the test for reducing sugars.

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Starch

Starch is made from two polysaccharides - amylose and amylopectin.

When it is digested it is broken down into maltose by amylase. Maltose is then broken down, by maltase, into glucose.

To test for the presence of starch we use the Iodine test. Take a solution and add iodine to it. If starch is present the solution will turn from orangey- brown to blue-black.

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Proteins

Proteins are made from long chains of amino acids (the monomers of proteins). Dipeptide - 2 amino acids, polypeptide - more than 2 amino acids. Proteins are made up of one or more polypeptides.

Different amino acids have different variable groups, normally R. Glycine variable group replaced by a H.

Polypeptides are formed by condensation reactions. The bonds formed between amino acids are called peptide bonds. During digestion they are broken by hydrolysis.

Proteins have 4 structural levels -

  • Primary - sequence of amino acids in the polypeptide chain.
  • Secondary - Hydrogen bonds form between the amino acids, chain coils into a helix (a glucose) or a pleated sheet (b glucose)
  • Tertiary - Coiled chain is coiled further as more bonds are formed between different parts of the polypeptide chain.
  • Quarternary - several different polypeptide chains held together by bonds, quarternary structure is the way the chains are assembled together.
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Protein Functions

A proteins shape determines it function. Eg. Haemoglobin is compact and soluble for easy transport and for carrying O2.

Proteins have a variety of functions -

  • Enzymes - they are spherical in shape due to the tight folding polypeptides and they are soluble.
  • Antibodies - immune response. Made of a mixture of light and heavy polypeptide chains.
  • Transport proteins - cell membrane. The proteins fold up and form a channel, they then can transport molecules and ions.
  • Structural proteins - strong, long chains. Eg. Keratin, which is found in nails and hair.

To test for proteins you use the Biuret test. Take an alkaline and add NaOH, then add CuSo2. If a protein is present a purple layer forms, if there is no protein the solution stays blue.

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Enzyme action 1

Enzymes are biological catalysts. They have an active site with a specific shape which substrate molecules bind to.

Enzymes lower the Activation Energy of a reaction, meaning reactions can happen at a lower temperature therefore they speed up the rate of reaction.

Enzyme-substrate complexs are what lower the A.E. - it brings the substrates closer together, reducing repulsion so allowing easier bonding. When enzymes catalysing breakdowns, the active site puts strain on the substrate bonds making it easier to break.

Lock and Key model - early model, where substrate fits the enzymes active site like a key to a lock.

Induced fit model - newer model, as substrate binds the acitve site changes shape slightly to complete the fit. This is more particular as the substrate has to fit the enzymes the right way and it has to make the active site change shape in the right way.

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Enzyme action 2

Enzyme properties relate to their tertiary structure.

Enzymes are very specific, only one substrate will fit their active site. Active site shape is determined by tertiary structure. If the tertiary structure is altered, active site will change shape meaning the substrate wont fit the enzyme - it will be denatured.

Tertiary structure can be altered by pH, temperature and substrate concentration.

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Factors affecting Enzyme Activity

Temperature - rate of reaction increases as temp. increases. More heat means more energy which means the molecules move faster which means more collisions. If the temp. is too high enzymes bonds break changing the shape of the active site, denaturing it.

pH - enzymes work best at the optimum pH. Above and below the optimum pH, the bonds holding enzymes shape are broken, changing active site shape, denaturing it.

Substrate Concentration - this affects the rate of reaction up to a point. High substrate conc., faster the rate of reaction, more substrates to collide with enzymes. When all the active sites are full adding more substrate makes no difference, saturation point.

Enzyme activity can be inhibited.

  • Competitive - competitive inhibitor molecules are a similar shape to substates, they bind to the active site blocking it so no reaction can take place.
  • Non-competitive - these molecules bind to the enzyme away from the active site, this causes the active site to change shape so the substrate will no longer fit, denaturing the enzyme so no reactions can take place.
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Animal Cell Structure

All cells contain organelles. Different organelles have different functions.

  • Plasma Membrane - made of lipids and proteins, controls movement in and out of cells. Also has receptor molecules.
  • Nucleus - contains nuclear envelope, chromatin and nucleolus. Pores allow movement between nucleus and cytoplasm, nucleolus makes ribosomes.
  • Lysosome - round, surrounded by membrane. Contains digestive enzymes used to digest invading cells and break down components.
  • Ribosome - small, floats in cytoplasm or attached to rough endoplasmic reticulum, site where proteins are made.
  • Endoplasmic Reticulum - rough, smooth. System of membranes enclosing a fluid filled space. Smooth - synthesises and processes lipids. Rough - folds and processes proteins made by ribosomes.
  • Golgi apparatus - fluid filled flattened sacs, processes and packages new lipids and proteins, also makes lysosomes.
  • Microvilli - folds in the plasma membrane, increase surface area.
  • Mitochondrion - double membrane, inner one folded as Cristae, inside is the matrix - contains respiration enzymes.
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Analysis of Cell Components

Magnification is size, resolution is detail.

magnification = length of image/ length of specimen


  • Light microscope - light, low resolution, max resolution 0.2 microm, max magnification x1500
  • Electron microscope - electrons, high resolution, max resolution 0.0001 microm, max magnification x1500000

Electron microscopes are either scanning or transmission -

  • SEMS - Scan beam of electrons through specimen, electrons are knocked off and gathered in a cathode ray tube which forms the image. 3-D, used on thick specimens but has a low resolution.
  • TEMS - Transmit beam of electrons through specimen. Denser parts absorb more electrons - look darker. High resolution, only used on thin specimens.
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Cell Fractionation

Cell fractionation separates organelles.

  • Homogenisation - breaking up the cells. Grind or vibrate cells which breaks the membrane and releases the organelles.
  • Filtration - filtered to get rid of large cell/tissue debris.
  • Ultracentrifugation - separating the organelles. The fragments are poured into a tube. The tube is spun in a centrifuge, the heaviest organelles fall to the bottom and form a thick sediment - pellet. The other organelles float on top - supernatent. The supernatent is then poured into another tube and the process is repeated removing the pellet each time until all organelles are separated.

Organelles heaviest to lightest -

Nuclei, Mitochondria, Lysosomes, Endoplasmic Reticulum, Ribosome

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Plasma Membranes

Substances are exchanged across plasma membranes. It controls what enters of leaves the cell. Plasma Membranes are mostly made of lipids (phospholipids), proteins and carbohydrates (usually attached to proteins or lipids). The fluid mosaic model shows the plasma membrane with its phospholipid bilayer.

Triglycerides are a kind of lipid, made of 1 glycerol and a hydrocarbon tail of 3 fatty acids, the tail is hydrophobic making it insoluble. They are formed by condensation, the bond formed between glycerol and the fatty acids is called an ester bond.

Fatty acids can be saturated (no double C bond) or unsaturated (has double C bonds, causing the chain to kink)

Phospholipids are like triglycerides except their 3rd fatty acid is replaced with a phosphate group, which is hydrophilic, placed on the left side of the glycerol.

Use the emulsion test for lipids (fats). Shake your test substance with ethanol and then pour into water, any lipid will show as a milky emulsion.

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Exchange across Plasma Membrane

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. It is a passive process. Concentration gradient is the path of diffusion - particles diffuse down a concentration gradient.

Rate of diffusion depends on -

  • Concentration gradient (higher gradient, faster the rate)
  • Thickness of exchange surface (thinner surface, faster the rate)
  • Surface area (larger surface area, faster the rate) Eg. Microvilli, found on plasma membrane, increase the surface area meaning more particles can be exchanged in the same amount of time.

Osmosis is the diffusion of water molecules across a partially permeable membrane from an area of higher water potential (high conc. of H20) to an area of lower water potential (low conc. of H20). Plasma membrane is partially permeable so water can diffuse across but larger solute molecules can't. Pure water has the highest water potential.

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Facilitated Diffusion across the Plasma Membrane

Facilitated diffusion uses carrier proteins and proteins channels to transport large, solute or charged particles into the plasma membrane, as they can not diffuse through it. It is also a passive process.

  • Carrier proteins - move large molecules in and out of the cell down their concentration gradient. The molecules attach themselves to the protein, this then makes the protein change shape and they release the molecules on the opposite side.
  • Protein channels - these are pores in the plasma membrane, they diffuse charged particles down their concentration gradient. Different protein channels diffuse different charged particles.
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Exchange across Plasma Membranes 2

Active transport moves substances against its concentration gradient. Carrier proteins are involved in active transport, same as in facilitated diffusion but now it uses cell energy ATP. Co-transporters are a type of carrier protein. They bind two molecules at the same time and use one molecules concentration gradient to move the other against its concentration gradient.

Carbohydrate digestion products are absorbed in different ways.

  • Some glucose diffuses across the intestinal epithelium into the blood
  • Some glucose enters the intestinal epithelium by active transport with sodium ions. Na ions are actively transported into the blood, so Na ions then diffuse from the small intestine to the cell down their concentration gradient by co-transporter protein. The co-transporter carries glucose into the cell with the Na. Now their are lots of glucose in the cell so glucose diffuses into the blood down its concentration gradient through a protein channel by facilitated diffusion.
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Fluid Mosaic Model

The fluid mosaic model explains membrane properties -

  • Membrane is a good barrier against most water soluble molecules due to its hydrophobic tails.
  • Membrane controls what enter and leave the cell
  • Membrane allows cell communication by receptor proteins - vital for proper body function.
  • Membrane allows cell recognition so the body doesnt attack its own cells. Glycoproteins and glycolipids tell the white blood cells its your own)
  • Membrane is fluid - constantly moving - cholestrol molecules fit in between phopholipids and makes the membrane more rigid and prevents it from breaking up.
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