Biology AS Unit 1 Edexcel

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  • Created by: Raeesa
  • Created on: 26-03-13 13:41


Water is a solvent, substances can dissolve in it. Most biological reactions take place in a solution, so water is essential. Water transports substances, substnces are more easily transported if they can dissolve.

H20 has a negatively charged oxygen atom and positively charged hydrogens, this makes water dipole. Cohesion is the attraction between molecules of the same type. Water is very cohesive because they're dipolar, this helps water to flow, making it great for transporting substances.

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Carbohydrates are made from monosaccharides.

Monosaccarides are joined together by glycosidic bonds in a condensation reaction. A hydrogen atom on one monosaccharide bonds to a hydroxyl group on the other, releasing a molecule of water. The reverse of this is a hydrolysis reaction. When two monosaccharides join together they form a dissacharide.

Maltose is made of two glucose molecules, with a glycosidic bond.                            Lactose is made up of glucose and galactose, with 1-4 glycosidic bonds.                  Sucrose is made up of glucose and fructose, with a 1-2 glycosidic bond. 

A polysaccharide is formed when more than two monosaccharides join together.

Amylose is made up of glucose molecules joined together by 1-4 glycosidic bonds.            

Amylopectin is made up of glucose with 1-4 and 1-6 glycosidic bonds, with side branches.                                                                                                      

Glycogen is made up of glucose with 1-4 and 1-6 glycosidic bonds and even more side brances.

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Starch is the main energy storage material in plants. 1)Cells get energy from glucose. PLants store excess glucose as starch. When a plant needs more glucose for energy it breaks down starch to release the glucose. 2)Starch is a mixture of two polysaccharides of alpha-glucose. -Amylose is a long, unbranched chain of glucose joined together with 1-4 glycosidic bonds. This gives it a coiled structure. This makes it compact, so its really good for storage because you can fit more in to a small space. -Amylopectin is a long, branched chain of glucose that contains 1-4 and 1-6 glycosidic bonds. Its side branch allow the enzymes that break down the molecule to get at the glycosidic bonds easily. This means that the clucose can be released quickly. 3) Starch insoluble in water, so water doesnt enter cells by osmosis. (Making them swell) Good for storage.

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Is the main energy store in animals. 1)Its structure is very similar to amylopectin, except it has loads more side branches. Energy can be released quickly, important for animals. 2)It is very compact so it is good for storage. 3)Glycogen is also insoluble in water so it doesnt cause cells to swell by osmosis. 4)Its a large molecule so it can store lots of energy.

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A triglyceride is made up of one glycerol molecule and 3 fatty acids. Fatty acides have long tails made of hydrocarbons, The tails are hydrophobic, this makes lipids insoluble in water. Like carbohydrates, triglycerides are formed by a condensation reaction and are broken up by hydrolysis reactions. The fatty acid tails and the glycerol molecule are joined together by ester bonds. A hydrogen atom on the glycerol molecule bonds to a hydroxyl group on the fatty acid, releasing a molecule of water. The reverse happens in hydrolysis. A molecule of water is added to each ester bond to break it apart.

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Saturated and Unsaturated Lipids

Saturate lipids are mainly found in animal fats and unsaturated lipids are mostly found in plants. Unsaturated lipids melt at lower tempuratures than saturated ones . Saturated lipids don't have double bonds between the carbon atoms in their hydrocarbon tails - every carbon is attached to at least two hydrogen atoms. Unsaturated lipids do have double bonds between the carbon atoms in their hydrocarbon tails. These double bonds cause the chain to kink. If they have more than two of them, the lipid is called polyunsaturated.

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Amino acids

The monomers of proteins are amino acids. A dipeptide is formed when two amino acids join together. A polypeptide is when more that two amino acids join together, Proteins are made up of many polypeptides. Dipeptides and polypeptides are formed by condensation reactions. All amino acids have a carboxyl group (-COOH) and an amino group (-NH2) attached to a carbon atom. The difference between amino acids is the R group. Amino acids are linked together by peptide bonds.

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Protein Structures

Primary structure - Held together by the peptide bonds between amino acids. Secondary structure - Held together by hydrogen bonds that form between nearby amino acids. These bonds create alpha helix chains or beta pleated sheets. Tertiary structure - This is affected by a few different kinds of bonds. -Ionic interactions. These are weak attractions between negative and positive charges on different parts of the molecult. -Disulphide bonds. Whenever two molecules of the amino acid cysteine come close the sulfur atom is one bonds to the sulfer in the other, forming a disulphide bond. -Hydrophobic and hydrophilic interactions. This affects how the protein folds up. -Hydrogen bonds. Quaternary structure - this tends to be determined by the tertiart structure of the individual polypeptide chains being bonded together. It can be influenced by all bonds mentioned above.

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3D protein structures

Globular proteins are round, compact made up of multiple polypeptide chains. The chains are coiled up so that hydrphilic parts of chains are on the outside of the molecule are hydrophobic parts of the chains face inwards. This makes the proteins soluble so they're easily transported in fluids. Eg. haemoglobin is a globular protein made of 4 polypeptide chains. It carries oxygen around the body in the blood. It is soluble so it can be easily transported in the blood. It has iron containing haem groups that bind to oxygen. Fibrous proteins are made up of long, insoluble polypeptide chains that are tightly coiled round each other to form a rope shape. The chains are held together by lots of bonds (disulphide and hydrogen) which made them strong. Because they're strong, they are found is supportive tissue. Eg. collagen is a strong, fibrous protein that forms supportive tissue in animals.

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They catalyse metabolic reactions in your body. Can be intracellular or extracellular. They are globular proteins.Enzymes have an active site which has a specific shape. The active site is the part of the enzyme where the substrate molecules bind to. Enzymes are highly specific due to their 3D structure. A certain amount of energy needs to be supplied to a chemical reaction before a reaction will start. This is called the activation energy. Enzymes lower this, making reactions happen at a lower tempurature. This speeds up the rate of reaction. When a substrate fits into the enzymes active site it forms an enzyme-substrate complex. This is what lowers the activation energy. Induced fit theory: As the substrate binds the active site changes shape slightly to complete the fit, this locks the substrate even more tightly to the enzyme.

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Enzymes only catalyse one reaction. This is because only one substrate will fir into the active site, The active sites shape is determined by the enzymes 3D structure. Each different enzyme has a different 3D structure and so a different shaped active site. If the substrate shape doesnt match the active site, the reaciton wont be catalysed. The more enzyme molecules there are in a solution the more likely a substrate molecule is to collide with one and form an enzyme-substrate complex. So increasing the concentration of the enzyme increases the rate of reaction. But if the amount of substrate is limited there comes a point when theres more than enough enzyme molecules to deal with all the available substrate so adding more enzyme has no further effect.

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Mass Transport Systems

Multicellular organisms need mass transport systems. In single-celled organisms, glucose and oxygen can diffuse directly into the cell across the cell membrane. The diffusion rate is quick because of the short distance the substance have to travel. 

In multicellular organisms, diffusion across the outer membrane would be too slow because of the large distance the substance would have to travel to reach all of the cells. So they have mass transport systems, which are used to carry raw materials from specilised exchange organs to the body cells and to remove metabolic waste. In mammals the mass transport system is the circulatory system where blood is used to transport substances around the body. 

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The Heart

The left ventricle of the heart has thicker, more muscular walls than the right ventricle because it needs to contract powerfully to pump blood all the way around the body. The right side only needs to get blood to the lungs which are nearby.

The ventricles have thicker walls than the atria because they have to push blood out of the heart whereas the atria just need to push blood to the ventricles.

The atrioventricular valves link the atria to the ventricles and stop blood flowing backwards into the atria when the ventricles contract. 

The semi lunar valves link the ventricles to the pulmonary artery and aorta, and stop blood blowing back into the heart after the ventricles contract.

The cords attach the atroventricular valves to the ventricles to stop them being forced up into the atria when the ventricles contract.

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Blood Vessels

Arteries carry blood away from the heart to the rest of the body. They're thick walled, muscular and have elastic tissue in the walls to cope with thee high pressure caused by the heartbeat. They have a folded endothelium, sallowing the artery to expand - this also helps cope with high pressure.

Veins take blood back to the heart. Veins contain valves to stop the blood flowing backwards. Blood flow through the veins is helped by contractions of the body muscles surrounding them. 

Capillaries are the smallest of the blood vessels. They are where metabolic exchange occurs - substances are exchanged between cells and the capillaries. There are networks of capillaries in tissue called capillary beds, which increase the surface area for for exchange. Capillary walls are only one cell thick, which speed up diffusion of substances into and out of cells.

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Ventricular Diastole/Atrial Systole:

The ventricles are relaxed. The atria contract. There is higher pressure inside the chamber.

Ventricular Systole/Atrial diastole:

The atria are relaxed. The ventricles contract. The pressure is higher in the ventricles, forcing the AV valves to shut. The pressure in the ventricles is also higher than in the aorta and pulmonary artert, which opens the SL valves.

Ventricular Diastole/Atrial Diastole:

The ventricles and the atria both relax. The SL valves close. Blood returns to the heart and the atria fill again. As the ventricles relax, the pressure falls so AV valvues open.

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If damage occurs to the endothelium by high blood pressue, there will be an inflammatory response, white blood cells will move into the area and form fatty streaks. Over time more white blood cells and lipids and connective tissue will build up and harden to form fibrous plaque called an atheroma. This partially blocks the lumen of the artery, increasing blood pressure. 

An atheroma can rupture the endothelium of an artery damaging the wall and leaving a rough surface. This triggers thrombosis - a blood clot forms at the site of the rupture. The blood clot can cause a complete blockage of the arteries. The blood flow to tissues supplied by the cloked blood vessel will be severly restricted so less oxygen will reach those tissues, resulting in damage. 

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Blood Clots

1) A protein called thromboplastin is released from the damaged blood vessel.

2)Thromboplastin triggers the conversion of prothrombin into thrombin.

3)Thrombin then catalyses the conversation of fibrinogen to fibrin.

4)The fibrin fibres tangle together and form a mesh in which platelets and red blood cells get trapped - this forms a blood clot. 

Heart Attacks: The heart muscles is supplied with blood by the coronary arteries. This blood contains oxygen needed by the heart muscle cells to carry out respiration. If a coronary artery artery becomes completely blocked by a blood clot an area of the heart muscle will be totally cut off from its blood supply so it wont recieve any oxygens. This causes a heart attack. 

HDL: Maily proteins, transport cholestrol from body tissue to the liver where its recycld or excreted. Their function is to reduce total blood cholestrol when the level is too high. 

LDL: mainly lipid, transport form liver to the blood, function to increase total blood cholestrol.

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Treatment of CVD

Antihypertensives reduce high blood pressure: These drugs include diuretics (which cause more urine to be produced, so reduce the volume of blood, beta-blockers (which reduce the stregth of the heartbeat) and vasodilators (which widen the blood vessels. The different types of antihypertensives work in different ways, so they can be given in combination to reduce blood pressure. Palpitations, abnormal heart rhythms, fainting, headaches and drowsiness are all side effects from the blood pressure being too low. Other side effects include allergic reactions and depression. 

Plant statins reduce cholestrol in the blood: Plants contain chemicals called stanols and sterols. These reduce blood chlostrol in humans by reducing the amount of chlostrol absorbed from the gut. They can reduce the risk of CVD. They can reduce the absorbtion of some vitamins from the gut.

Anticoagulants reducte the formation of blood clots: eg. warfin and heparin. Can treat people who already have blood clots, however cant get rid of existing ones. If person is injured, the reduction of blood cholestrol could cause excessive bleeding. 

Platelet Inhibitory: eg. aspirin. can treat those with CVD already. Side effects: rashes, diarrhoea, nausea, liver function problems, excessive bleeding. 

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Cell Membrane Structure

Phospholipid molecules form a continuous double layer. This bilayer is fluid because the phopholipds are constantly moving. Protein molecules are scattered through the bilayer, like tiles in a mosaic. Because the phospholipid bilayer is fluid, the proteins can move around within it. Some proteins have a poysaccharide (carbohydrate) chain attached - these are called glycoproteins. Some lipids have a polysaccharide chain attached - these are glycolipids. Cholestrol is also present in the membrane. Its between the phospholipids forming bonds with them. This makes the membrane more rigid.

Increasing tempurature increases membrane permeability. Increasing alcohol concentration increases membrane permeability.

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Transport across the cell Membrane

Diffusion is the net movement of particles from an area of high ocncentration to an area of lower concentration, down a concentration gradien. Gas exchange surfaces are adapted from efficient diffusion. Most gas exchange surfaces have a large surface area and are thin.

In mammals oxygen diffuses out of the alveoli, across the alvolar epithelium and capillary epithelium and into the blood. Carbon dioxide diffuses into the alveoli from the blood and is breathed out. Many alveoli provide a large surface area for diffusion to occur across. The alveolar epithelium and capillary endothelium are each only one cell thick giving a short diffusion pathway. All the alveoli have a good blood supply from capillaries. They constantly take away oxygen and bring more carbon dioxide maintaining the concentration gradient. Breathing in and out refreshes the air in the alveoli, keeping the concentration gradients high. 

Osmosis is the diffusion of water molecules across a partially permeable membrane, from an area of high concentration to an area of low concentration.

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Transport across the Cell Membrane

Facillitated Diffusion: Carrier proteins move large molecules into or out of the cell, down their concentration gradient. Different carrier proteins facilitate the diffusion of different molecules. First a large molecule attaches to a carrier protein in the membrane, then the protein changes shape. This releases the molecult on the opposite side of the membrane. Channel proteins form pores in the membrane for charged particles to diffuse through down their concentration gradient.

Active transport: Sililar to FD, except ATP is used to move the solute against the concentration gradient.

Cells can take in substances by endocytosis. A cell can surround a substance with a section of its cell membrane. The membrane then pinches off to form a vesicle inside the cell containing the ingested substance.

Cells can secrete substances by exocytosis. Vesicles fuse eith the cell membrane and release their contents outside the cell.

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Structure of DNA and Replication

DNA and RNA are polynucleotides. Each mononucleotide is made up of a pentose sugar (with 5 carbon atoms) a phosphate group and a nitrogen base. The sugar in DNA is deoxyrybose and in RNA its ribose sugar. Each mononucleotide has the same sugar and phosphate, the base can vary. In DNA there are 4 basesm adenine, thymine, cytosine and guanine. In RNA uracil replaces thymine. The mononucleotides are joiined together through condensation reactions between the phosphate group of mononuclotide and the sugar of another, water is a by-product.

Two complementary DNA strands join together by hydrogen bonding between the bases. Adenine always pairs with thymine and guanine always pairs with cytosine. The two DNA strands wind up to form the DNA double helix. 

DNA can copy itself. The helix unzips to form two single strands, each original strand acts as a template for a new strand. Free floating mononucleotides join to each original template strrand by complamentary base pairing. The mononucleotides on the new strands are joined together by the enzyme DNA polymerase. Hydrogen bonds form between the bases on the original and new strand.  

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Protein Synthesis

Transcription: The hydrogen bonds between the two DNA strands in a gene break. One of the strands is then used as a template to make an RNA copy, called mRNA. The template strand is called the antisense strand. The mRNA moves out of the nucleous through a nuclear pore and attaches to a ribosome. 

Translation: tRNA molecules carry carry amino acids to the ribosome and attach themselves on to the mRNA by complementary base pairing. The amino acids are joined together by peptide bonds and the tRNAs move away leaving the amino acids to make a polypeptide chain. The polypeptide chain moves away from the ribosome and translation is complete.

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Genetic Terms

Gene: A sequence of bases on a DNA molecule that codes for a protein, which results in a characteristic.

allele: A different version of a gene.

Genotype: the alleles a person has

Phenotype: the characteristics the alleles produce.

Dominant: an allele whos characteristic appears in the phenotype even when thers only one copy.

Recessive: An allele whos characterisitc only appears in the phenotype if two copies are present. 

Homozygote: An organism that carries two copies of the same allele

Heterozygote: An organism that carries two different alleles.

Carrier: Someone who has one dominant and one recessive allele. 

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Cystic Fibrosis

Cystic fibrosis is caused by a mutation in the gene that codes for the CFTR protein.

CTFR is a carrier protein.

It transports chloride ions out of cells and into the mucus. This causes water to move into the mucus byb osmosis.

Mutant CFTR protein is much less efficient at transporting chloride ions out of the cell, so less water moves out by osmosis.This makes the mucus of people with CF abnormally thick and sticky.

This mucus causes problems in the respitratory, digestive and reproductive systems.

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Cystic Fibrosis

Respiratory System: The cilia are unable to move the mucus towards the throat because its so thick and sticky. This means the mucus builds up in the airways. Some airways can become completely blocked buy the mucus, gas exchange cant take place in the area below the blockage. This means that the surfae area available for gas exchange is reduced causing breathing difficulties.

Digestive System: The tube that connects the pancreas to the small intestine can become blocked with mucus preventing digestive enzymes produced by the pancreas from reaching the small intestine. This reduced the sufferers ability to digest food, and so fewer nutrients can be absorbed. The mucus can cause cysts to form in the pancreas. These inhibit the production of enzymes, which also reduces the ability to digest food and absorb nutrients. The mucus lining the small intestine is abnormally thick. This inhibits the absorption of nutrients.

Reproductive System: The tubes connecting the testicles to the penis are absent in some sufferers and can be blocked by mucus. Sperm Cant reach penis. Thickened cervical mucus can prevent the sperm from reaching the egg. 

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Genetic Screening

Identification of Carriers: Carrier testing is offered to individuals with a family history of genetic disorders. Carrier testing allows people to make informed decisions about things like whether to have children and whether to carry out prenatal testing. Finding out you're a carrier may cause emotional distress. The tests aren't always accurate. Results can be used by employers or life insurance companies resulting in genetic discrimination.

Preimplantation Genetic Diagnosis: it involves screening embyos for genetic disorders before they're implanted into the woman. it reduced the chances of having a baby with genetic disrders. It can be used to find out other characteristics, concern of future designer babies. False results could provide incorrect information.

Prenatal Testing: Amniocentesis: 15-16 weeks of pregnancy. A sample of amniotic fluid is obtained using a very fine needle. Fetal cells containing DNA can be analysed.

Choronic villus sampling: 8-12 weeks, Cells taken from chorionic villi, using needle.

Increases chance of miscarriage. False results. Unethical to abort fetus. 

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Gene Therapy

Alleles are inserted into vectors such as viruses, plasmids or liposomes. 

Stomatic Therapy: Changing the alleles in body cells, particularly the cells that are most affected by the disorder. it doesnt effect sex cells though so any offspring could still inherit the disease.

Germ line therapy: changing the alleles in sex cells. This means that every cell of any offspring produced from these cells will be affected by the gene therapy and they wont suffer from the disease. Germ line therapy in humans is currently illegal though.

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