Edexcel AS Biology Unit 1

revision cards for AS biology SNAB edexcel

  • Created by: millie
  • Created on: 15-05-11 19:40

Transport and circulation


Water is an excellent solvent and this is why it is the medium transport in all living things. This solvent ability comes from the fact that water molecules have one end that is slightly positive (hydrogen) and one end that is slightly negative (oxygen).

  • Ionic substances such as sodium chloride dissolve easily in water because the positively and negatively charged ions are separated due to the dipole nature of water.
  • Molecules like glucose and amino acids also have charged groups and therefore dissolve easily.
  • Water is a liquid at room temperature
  • It takes a lot of energy to heat water up and turn it into a gas.
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Transport and circulation

The structure and function of blood vessels


  • thick wall - withstands high blood pressure
  • smooth muscle - alters diameter of lumen
  • elastic fibres - to allow stretch and recoil
  • smooth layer of endothelial cells - low friction to ease blood flow


  • Very thin wall - allows rapid exchange between blood and tissues


  • thin wall - blood under low pressure
  • wide lumen 
  • valves - prevent back flow
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Cardiac cycle and heart rate

Sequence of events on the right side of the heart

  • Oxygenated blood drains into the left atrium from lungs along the pulmonary vein
  • Raising of blood pressure in the atrium forces open the left atrioventricular valve
  • Left atrial systole forces more blood through the valve
  • As soon as left atrial systole is over, the left ventricular muscles start to contract. This forces the left AV valve closed and opens the valve in the mouth of the aorta
  • Blood then leaves through the aorta to the body
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Cardiac cycle and heart rate

Sequence of events on the left side of the heart

  • Deoxygenated blood enters from the body along the vena cava
  • The right atrial muscle contracts and the right AV valve opens due to the pressure difference
  • The blood enters the right ventricle and it contracts, forcing the semilunar valve in the pulmonary artery open and the blood leaves through this valve along the artery into the lungs.
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Blood clotting

  • Clot formation is stimulated when there is damage to a blood vessel
  • Damage exposes collagen fibres to which platelets attach
  • The platelets release a clotting factor called thromboplastin
  • in the presence of vitamin k and calcium, thromboplastin converts prothrombin into thrombin
  • Thrombin then converts the soluble fibrinogen into insoluble fibrin
  • Fibrin forms a network of fibres, which trap cells to form a clot
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1. The endothelial lining becomes damaged for reasons such as high blood pressure or toxins from smoking

2. There is an inflammatory response and white blood cells move into the artery wall, cholesterol builds up which leads to a formation of atheroma

3. Calcium salts and fibres build up which leads to the formation of plaque and the artery narrows

4. The narrowing of the artery leads to high blood pressure, which means it is more likely for more plaques to form

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Treatment of CVD

Diuretics - Increase the volume of urine, lowers blood pressure and volume. Can lead to dizziness, nausea, muscle cramps.

Beta blockers - block response of heart to hormones and make contractions less frequent and powerful. Possible link with diabetes

ACE inhibitors - Block the production of angiotensin, which normally causes aterial constriction and rise in blood pressure. Can lead to dizziness, impaired kidney function, cough, heart arrhythma

Statins - Lower cholesterol level in the blood by blocking the liver enzyme. Can lead to muscle cramps, nausea, constipation and diarrhoea, liver failure

Anticoagulants - reduce risk of clot formation. Can lead to uncontrollable bleeding

Platelet inhibitory e.g. aspirin- Make platelets less sticky

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Structure and function of carbohydrates

Sucrose - glucose + fructose. main transport sugar in plants. soluble

Lactose - glucose + galactose. 'milk sugar'. soluble

Maltose - glucose + glucose. soluble

The bonds that form between monosaccharides are glycosidic bonds. In the reaction that forms a glycosidic bond there is a loss of one molecule of water, this is called a condensation reaction.

Hydrolysis breaks the glycosidic bond, adding back the water molecule.

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Structure and function of lipids

Lipids are insoluble in water but soluble in organic substances such as ethanol. Triglycerides are lipids made of one glycerol molecule and three fatty acids, joined by an ester bond.

All glycerol molecules are the same, the fatty acids vary in:

  • the length of the hydrocarbon chain
  • the absence or presence, and number of double bonds
  • the mix of fatty acids
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Structure and function of lipids

Saturated (fats)

  • strong intermolecular bonds
  • solid at room temperature
  • straight chain molecule

Monounsaturated (oils) (one double bond)

  • intermolecular bonds weaker because of kinked shape
  • liquid at room temperature
  • molecule with one kink in chain

Polyunsaturated (oils) (more than one double bond)

  • intermolecular bonds weaker because of kinked shape
  • liquid at room temperature
  • molecule with x kinks in chain (x= no. of double bonds)
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Risk factors of CVD


  • Tendency to high blood pressure and poor cholesterol metabolism
  • Arteries that are more easily damaged
  • Mutations in genes that affect relative HDL:LDL levels in blood

Gender - Oestrogen can give women some protection

Ageing - Elasticity and width of arteries decrease with age

Diet - Saturated fats, cholesterol and lipoprotein levels

High blood pressure - should not be sustained 140/90

Smoking - chemicals physically damage artery linings and cause them to constrict

Inactivity - regular exercise reduces risk

Obesity - Increases rick of diabetes type II and CVD

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Population studies on risk factors

Cohort studies

  • large number of people followed
  • long period of time
  • monitored to see if they develop the condition
  • various risk factors the participants have been exposed to are looked at

Case-control studies

  • A group with the condition (cases) is compared with a group who do not have it (control)
  • Past history is investigated to try to identify factors leading to one group having the disease and the others not
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Structure of amino acids and proteins

An amino acid consists of a central carbon atom attached to an amino group, a carboxylic acid, a hydrogen atom and a variable side group.

Primary structure - amino acids are able to join to other amino acids to form chains (polypeptides). The bond is a peptide bond.

Secondary structure - Hydrogen bonds form which can hold together a helical structure, and a sheet made up of polypeptides laid out parallel to eachother.

Tertiary structure - further folding forms a 3D shape.

Quaternary structure - Two or more polypeptide chains held together by hydrogen bonds.

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Enzymes are globular proteins - each enzyme has a specific 3D shape including an active site. Only molecules with a specific shape (the substrate) fit.

The lock and key hypothesis:

1. Random movement causes the enzyme and substrate to collide, and thr substrate enters the active site.

2. Enzyme-substrate complex forms. Charged groups attract, distorting the substrate and aiding bond breakage or formation.

3. Products are released from the active site leaving the enzyme unchanged and ready to accept another substrate

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Cell membranes

Membrane structure

  • phospholipids are like triglyceride lipids, they are polar with a hydrophilic head and hydrophobic tail.
  • when suspended in water, phospholipids naturally form bilayers with the tails inside and the heads outside
  • proteins form a mosaic amongst the lipids
  • carbohydrates are found only on the outside of the membrane
  • proteins in the membrane act as channels for movement of substances in and out
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Transport across cell membranes

Types of movement


  • directly through phospholipid bilayer
  • passive
  • net movement down concentration gradient

Facilitated diffusion

  • same as diffusion but requiring a channel protein


  • diffusion of free water molecules
  • through a partially permeable membrane
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Transport across cell membranes

Active transport

  • all kinds of molecules possible
  • through carrier proteins
  • needs energy from the breakdown of ATP
  • up a concentration gradient

Exo and endocytosis

  • large particles of all kinds
  • bulk transport using vesicles made from cell surface membrane 
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Structure and role of DNA and RNA

Structure of nucleotides

  • a phospate group
  • a base
  • a sugar: ribose in RNA, deoxyribose in DNA

the components are joined in a condensation reaction

RNA is a single strand of nucleotides. in DNA, two separate strands are held together by hydrogen bonds.

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The gene and the genetic code

  • the order of the bases on one strand of DNA is the genetic code
  • this code is formed from triplets of bases
  • each triplet codes for an amino acid
  • the sequence of triplets codes for the sequence of amino acids that will form a polypeptide, which will fold up to form a protein
  • a gene is a sequence of bases on one of the strands of a DNA double helix molecule which codes for a polypeptide chain (chain of amino acids)
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DNA replication and protein synthesis

How does DNA replication occur?

During replication the two strands of DNA unwind and split apart. Free nucleotides line up along each strand. The enzyme polymerase bonds the nucleotides together as a phosphodiester bond forms between each deoxyribose and adjacent phosphate group. These are linked by hydrogen bonds.

1. Hydrogen bonds between the bases break, allowing the DNA up 'unzip'

2. DNA nucleotides pair up with their complementary bases. DNA polymerase links adjacent molecules.

3. Two identical daughter strands are created.

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DNA replication and protein synthesis

Protein synthesis

The genetic code in DNA is in the nucleus, but the proteins formed using the code are made in the cytoplasm. So the DNA is copied, making a molecule of messenger RNA in a process called transcription. the mRNA passes into the cytoplasm through nuclear pores and is used to make a polypeptide in a process called translation.

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Transcription and translation


The DNA unwinds and hydrogen bonds between base pairs split to separate the two strands. Only one strand is used in the formation of mRNA (template strand). Ribonucleotides are paired with their complement on the template strand. They are then joined up by RNA polymerase to form a strand of mRNA


the mRNA carries the genetic message in the same base sequence language as the DNA. tRNA translates the base sequence on the mRNA into the protein amino acid sequence. Each tRNA molecule carries an amino acid to the mRNA, where the amino acid joins others carried by other tRNAs to build a polypeptide.

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Genes, mutations and CF


Mutations are a change in the base sequence on the DNA, and this can give rise to a change in amino acid sequence in the protein.

Mutations that occur during DNA replication are the most dangerous as they are passed onto new cells. In body cells they may lead to cancer and in gametes they can be passed to offspring and cause genetic disorders like CF

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Genes, mutations and CF

cystic fibrosis is a genetic disorder cause by a mutation of a single gene. the sticky mucus formed in CF can cause problems in:

gas exchange

  • mucus accumulates in the lungs and bacteria trapped in mucus increase risk of infection
  • mucus can reduce surface area for gas exchange


  • mucus blocks the pancreatic duct, so digestive enzymes cant reach the small intestine 


  • mucus can block the cervix in women and the sperm duct in men which stops sperm from reaching the egg
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Gene therapy

gene therapy is the insertion of a normal allele of a gene into cells to replace a faulty allele that causes an inherited disorder

somatic therapy

1. identify the gene involved

2. make copies of the normal allele and insert into a vector - the two most common vectors are viruses and liposomes

3. use the vector to insert the allele into the target cells

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Genetic screening

pre natal DNA testing

amniocentesis a syringe is used and amniotic fluid is withdrawn which contains fetal cells. the amiotic fluid is analysed 

chronic villus sampling syringe is used to take a sample of embryonic tissue from the placenta and it is analysed

pre implantation genetic diagnosis

  • embryos created through IVF are tested and only the healthy ones are implanted into the mother
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