Scientific Article

Edexcel A2 biology revision cards for the synoptic elements of the June 2012 Unit 5 scientific article 'A source of thinspiration'

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  • Created on: 21-06-12 11:47


  • Cancers occur when the rate of cell multiplication is faster than the rate of cell death. This causes the growth of a tumor, often in tissues with a high rate of mitosis, such as the lung, bowel, gut or bone marrow. Cancers are thought to be caused by damage to DNA, particually tumor repressor genes and oncogenes.
  • Oncogenes code for proteins that stimulate the transition from one stage in the cell cycle to the next.
  • Tumor repressor genes produce suppressor proteins that stop the cell cycle.
  • There is a very complex network of signals and inhibitors that interact to control the cell cycle. There needs to be damage to more than one part of the cell control system for cancer to occur, making cancer of any particular cell unlikely but, due to the high volume of dividing cells, some mutations occur which can lead to cancer.
  • It is suggested that there is an inherited predisposition for cancer.
  • Carcinogens are environmental factors that have been linked to cancer. These can include;
    • Smoking
    • UV light exposure
    • Diet (linked to free radicles)
    • Some viral infections
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Brain Structure

  • The Cerebrum allows sight, thought, learning and the ability to feel emotion.
    • The cerebrum is the largest part of the brain.
    • It's divided into two halves called the cerebral hemispheres.
    • The cerebrum has a thin outer layer called the cerebral cortex. The cortex has a large surface area so it's highly folded to fit inside the skull.
    • The cerebrum is involved in vision, learning, thinking and emotions.
    • Different parts are involved in different functions, e.g. the back of the cortex is involved in vision and the front is involved in thought.
  • The Hypothalamus controls body temperature
    • the hypothalamus is found just beneath the middle part of the brain.
    • The hypothalamus automatically maintains body temperature at the normal level (thermoregulation)
    • It produces hormones that control the pituitary gland - a gland just below the hypothalamus.
  • The Medulla controls breathing rate and heart rate
    • The medulla is at the base of the brain, at the top of the spinal cord.
    • It automatically controls breathing rate and heart rate.
  • The Cerebellum coordinates movement
    • The cerebellum is underneath the cerebrum and it's folded cortex
    • It's important for coordinating movement and balance.
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Brain Function

Thalamus - is responsible for routing all incoming sensory information to the correct part of the brain.

Hippocampus - involved in laying down long term memory

Basal Ganglia - collection of neurones that lie deep within each hemisphere and are responsible for selecting and initiating stored programs for movement.

Midbrain - relays information to the cerebral hemispheres, including auditory information to the temporal lobe and visual information to the occipital lobe.

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Brain Imaging

  • To investigate the structure and function of the brain, and to diagnose medical conditions, you need to look inside the brain
  • This can be done with sugary, buts its risky and invasive
  • The brain can be visualised using non-invasive methods such a scanners.
  • There are three main types of brain scans;
    • Computed Tomography (CT) scans
    • Magnetic Resonance Imaging (MRI) scans
    • Functional Magnetic Resonance Imaging (fMRI) scans
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Computed Tomography (CT) Scans

CT scanners use X-rays to produce cross-section images of the brain. Dense structures in the brain absorb more radiation than less dense structures so show up a lighter colour.

Uses of CT

  • Investigating brain structure - CT scans show major structures in the brain.
  • Investigating brain function - A CT scan doesn't show brain function - it only shows brain structure. But if a CT scan shows a diseased or damaged brain structure and the patient has lost some function, the function of that part of the brain can be worked out.
  • Medical diagnosis - CT scans can be used to diagnose medical problems because they show damaged or diseased areas of the brain. 
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Magnetic Resonance Imaging (MRI) Scans

MRI scanners use a really strong magnetic field and radio waves to produce cross-section images of the brain.

Uses of MRI

  • Investigating brain structure - you can see the structure of the brain in a lot more detail than an MRI scanner than with a CT scanner, and you can clearly see the difference between normal and abnormal (diseased or damaged) brain tissue.
  • Investigating brain function - This is done in the same way as CT scans.
  • Medical diagnosis - MRI scans can also diagnose medical problems because they show damaged or diseased sections of the brain
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Functional Magnetic Resonance Imaging (fMRI)

fMRI scanners are like MRI scanners, but they show changes in brain activity as they actually happen:

  • More oxygenated blood flows to active areas of the brain (to supply the neurones with oxygen and glucose.
  • Molecules in oxygenated blood react differently to a magnetic field than those in deoxygenated blood.
  • So the more active areas of the brain can be identified on an fMRI scan.

Uses of fMRI

  • Investigating brain structure - An fMRI scan gives a detailed picture of the brains structure, similar to to an MRI scan.
  • Investigating brain function - rMRI scans are used to research the function of the brain as well as its structure. If a function is carried out whilst in the scanner, the part of the brain that's involved in that action will be more active.
  • Medical diagnosis - fMRI scans are really useful to diagnose medical problems because they show up damaged or diseased areas of the brain and they allow the study of conditions caused by abnormal activity in the brain.
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Basal Metabolic Rate (BMR)

A constant supply of energy is needed to maintain essential body processes, such as the pumping of the heart, breathing and maintaining a constant body temperature. These processes go on all the time, even 'at rest'. The energy needed for these essential processes is called the basal metabolic rate (BMR) and varies between individuals. BMR is higher in:

  • Males
  • Heavier people
  • Younger people
  • More active people.
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Obesity occurs when the amount of energy consumed, outweighs the amount of energy used. There are two main ways of indicating obesity; Body mass index (BMI) and waist to hip ratio.

  • BMI - to calculate BMI, body mass (in Kg) is divided by height (in meters) squared. BMI does't have an exact correlation with fat levels in the body and may not be accurate for athletes, people over 60, or those with long term health conditions. A BMI over 30 is classified as obese, above 40 is severely obese.
  • Waist-to-hip ratio - suggested to be a more accurate indicator of obesity. It is calculated by dividing the wist circumference by hip circumference. A man is considered obese with a ratio over 0.90 and a women over 0.85.

 Consequences of obesity

  • Increased risk of CHD and stroke
  • Increased risk of type II diabetes
  • Raised blood pressure
  • Elevated blood lipid levels 
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Energy Balance

  • Organisms need a supply of energy, so that they can grow, move and reproduce - in animals this comes in the form of food.
  • Energy budget is a term used to describe the amount of energy taken in by an organism (in food) and the amount of energy used up by an organism.
  • Ideally a person should take in the same amount of energy as they use up - their energy budget should be balanced. If there's an imbalance in the energy then it will affect the person's weight.

Weight gain:

  • If energy intake is higher than energy output, the excess energy will be turned into fat reserves in the body, so the person will gain weight.
  • If the energy difference is a lot and its sustained over a long period of time, the person could become obese.

Weight loss:

  • If energy intake is lower than energy output, the body will have to get more energy from somewhere - it'll turn to some of its fat reserves into energy, so the person will loose weight.
  • If this energy difference is large and is sustained over a long period of time, the person is likely to become underweight.
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  • Triglycerides are a kind of lipid and is made up of one molecule of glycerol with three fatty acids attached to it.
  • Fatty acid molecules have lone hydrocarbon tails that are hydrophobic
  • These tails make lipids insoluble in water
  • All fatty acids consist of the same basic structure, but the hydrocarbon tail varies.
  • Triglycerides  are formed by the condensation reaction and broken down by hydrolysis reactions.
  • Three fatty acids and a single glycerol molecule are joined together by ester bonds.
  • A hydrogen atom on the glycerol molecule bonds a hydroxl group on the fatty acid, releasing a molecule of water.
  • The reverse happens in hydrolysis.
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  • There are two types of lipids - saturated and unsaturated
  • Saturated lipids are found in many animal fats and unsaturated lipids are mainly found in plants.
  • Unsaturated lipids melt at lower temperatures than saturated ones.
  • The difference between these two types of lipids is their hydrocarbon tails.
  • Saturated - saturated lipids don't have any double bonds between the carbon atoms in their hydrocarbon tails.
  • Unsaturated - unsaturated lipids have do have double bonds between the carbon atoms in the hydrocarbon tails. These double bonds cause the chain to kink.
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High and Low Density Lipoproteins

Cholesterol is a lipid made in the body and some is needed for the body to function normally.

Cholesterol needs to be attached to a protein to be moved around, so the body forms lipoproteins. These come in two forms:

  • HDLs - mainly protein that transport cholesterol from body tissues to the liver where it is recycled or excreted. Their function is to reduce the total blood cholesterol when the level is too high.
  • LDLs - mainly lipid that transport cholesterol from the liver to the blood, where it circulates until needed by cells.
  • Their function is to increase total blood cholesterol when levels are too low.

High total blood cholesterol level and high LDL level have both been linked to an increased risk of CVD. 

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Blood Vessels

  • Arteries carry blood from the heart to the rest of the body. They're thick walled, muscular and have elastic tissue in the walls to cope with the high pressure caused by the heartbeat. The endothelium is folded, allowing the artery to expand - this also helps it to deal with high pressure. Arteries have narrow lumen.
  • Veins take blood back to the heart. They've a wider lumen than equivalent arteries, with very little elastic or muscle tissue. Veins contain valves to stop the blood flowing backwards. Blood flow through the veins is helped by contraction of body muscles surrounding them.
  • Capillaries are the smallest of the blood vessels. They are where metabolic exchange occurs. there are networks of capillaries in tissue (called capillary beds) that increase the surface area for exchange. Capillary walls are only one cell thick, which speeds up diffusion of substances into and out of cells.
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Atheroma Formation

  • The wall of an artery is made up of several layers.
  • The endothelium is usually smooth and unbroken
  • If damage occurs to the endothelium (e.g. high blood pressure) there will be an inflammatory response.
  •  The white blood cells and lipids from the blood, clump together under the endothelium to form fatty deposits.
  • Over time, more white blood cells, lipids and connective tissue build up and harden to form a fibrous plaque called an atheroma.
  • This plaque partially blocks the lumen of the artery and restricts blood flow, which causes blood pressure to increase.
  • The hardening of arteries, caused by atheromas, is called atherosclerosis.
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  • Atheromas develop within the walls of the arteries.
  • An atheroma can rupture the endothelium of an artery, damaging the artery wall and leaving a rough surface.
  • This triggers thrombosis (blood clotting) - a blood clot forms at the site of the rupture.
  • This blood clot can cause a complete blockage of an artery, or it can become dislodged and block a blood vessel elsewhere in the body.
  • The blood flow to tissues supplied by the blocked blood vessel will be severely restricted, so less oxygen will reach those tissues, resulting in damage.
  • Heart attack, stroke and deep vein thrombosis are three forms of cardiovascular disease that can be caused by blood clots.  
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Formation of Blood Clots

Thrombosis is used by the body to prevent excessive blood loss when a blood vessel is damaged. A series of reactions occurs that lead to the formation of thrombosis:

  • A protein called thromboplastin is released from the damaged blood vessel.
  • Thromboplastin triggers the conversion of prothrombin (a soluble protein) into thrombin (an enzyme).
  • Thrombin then catalyses the conversion of fibrinogen (a soluble protein) to fibrin (solid insoluble fibers).
  • The fibrin fibers tangle together and form a mesh in which platelets (small fragments of cells in the blood) and red blood cells get trapped  - this forms a blood clot.

Blood clots can cause heart attacks, stroke and deep vein thrombosis.

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Coronary Heart Disease

CHD is when the coronary arteries have loads of atheromas in them, which restricts blood flow to the heart. The atheromas also increase the risk of blood clots forming, leading to an increased risk of heart attack. 

  • Diet can increase risk of CVD if its high in saturated fats and salt.
  • High blood pressure increased risk of CVD - excessive alcohol consumption, stress and diet can all increase blood pressure.
  • Smoking increases risk of CVD due to carbon monoxide, nicotine and the decrease on antioxidants in the blood.
  • Inactivity increases blood pressure and so increases risk of CVD.
  • Genetics can lead to a predisposition to CVD
  • Increased Age increases risk of CVD
  • Gender - men are three times more likely to suffer from CVD than pre-menopausal women.
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Protein Structure

Proteins have four structural levels:

Primary Structure - this is the sequence of amino acids in the polypeptide chain.

Secondary Structure - the polypeptide chain doesn't remain flat and straight. Hydrogen bonds form between the amino acids in the chain. This makes it automatically coil into an alpha helix of fold into a beta pleated sheet - this is a secondary structure.

Tersiary Structure - the coiled or folded chain of amino acids is often coiled and floded further. More bonds form between different parts of the polypeptide chain. For proteins made form a single polypeptide chain, the tertiary structure forms their final 3D structure.

Quaternary Structure - some proteins are made of several different polypeptide chains held together by bonds. The quaternary structure is the way these polypeptide chains are assembled together. For proteins made form more than one polypeptide chain, the quaternary structure is the protein's final 3D structure.

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Structure of DNA

  • DNA is a polynucleptide made up of many nucleotides that contain deoxyribose(sugar) a phosphate and a base.
  • There are 4 possible bases; AdenineThymineGuanine and Cytosine.
  • DNA exists as a double helix with hydrogen bonding between the bases encased in a sugar-phosphate backbone.
  • Each base can only join with one particular partner (complementry base paring) A ->T G -> C (to remember At The Golf Course)
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mRNA copy made in the nucleus

  • RNA polymerase attaches to the DNA at the beginning of the gene
  • The H-bonds between the DNA strands breakuntwisting the DNA.
  • 1 Strand becomes an antisense strand to make a mRNA copy
  • RNA polymerase lines up free nucleotides alongside the antisense strand.Complementary base paring = mRNA strand being a reversed copy of DNA (except T --> U in RNA)
  • Pared RNA nucleotides join together forming a mRNA strand.
  • RNA polymerase moves along the DNA, separating the strands and assembling the mRNA strand.
  • RNA polymerase move on, H-bonds in DNA reform and DNA strands re-twist into double helix
  • When RNA polymerase reaches a stop codon, it stops making mRNA and detaches from DNA.
  • mRNA moves out of the nucleus through a nuclear pore and attaches to a ribosome in the cytoplasm ready for translation.
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 - mRNA modified before translation.

 - Genes contain both introns (non-coding) and exons (coding)

 - During transcription both the introns and exons are copied into mRNA

 - The introns are removed by a process called splicing

 - Splicing is the removal of introns and joining up exons

 - Splicing takes place in the nucleus

 - Exons can be joined up in different orders to form different mRNA strands

 - This allows more than one amino acid sequence (and so more than 1 protein) to    come from one gene

 - After splicing the mRNA leaves the nucleus for translation.

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Occurs on ribosomes in the cytoplasm

Amino acids are joined together forming a protein

1. mRNA attaches to a ribosome and transfer RNA (tRNA) carry amino acids to the ribosome.

2. tRNA, with complimentary anticodon, attaches itself to the first codon bycomplimentary base paring

3. A second tRNA molecule attaches itself to the next codon on the mRNA in the same way.

4. The amino acids join together via a peptide bond the tRNA molecule moves away leaving its amino acid behind.

5. This process continues, forming a polypeptide chain, until a stop codon on the mRNA strand.

6. The polypeptide chain (protein) moves away from the ribosome

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Muscle Fibres

  • Skeletal muscles are made up of large bundles of long cells called muscle fibres.
  • The cell membrane of muscle fibre is called the sarcolemma.
  • Parts of sarcolemma fold inwards across the muscle fibres and stick into the sarcoplasm (a muscle cells cytoplasm). These folds are called transverse (T) fibules and they help to spread electrical impulses through the sarcoplasm.
  • A network of internal membranes called the sarcoplasmic reticulum runs through the sarcoplasm. the sarcoplasm reticulum stores and releases calcium ions that are needed for muscle contraction.
  • Muscle fibres have lots of mitochondria to provide ATP thats needed for muscle contraction.
  • Muscle fibres are multinucleate
  • Muscle fibres have long cylindrical orgonelles called microfibrils. They are made up of protein and are highly specalised for contraction.
  • Skeletal muscles are made up of two types of muscle fibres - slow and fasttwitch. Different muscles have different proportions of slow and fast twitch fibres.
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Slow Twitch Muscle Fibres

  • Muscle fibres contract slowly
  • Muscle fibres for posture contain a large proportion on slow muscle fibres.
  • Good for endurance activities
  • Can work for long periods of time without exhausting.
  • Energyis releases through aerobic respiration.
  • Contain lots of mitochondria and blood vessils to suply the muscles with oxygen.
  • Reddish in colour because of richness of myoglobin (red, oxygen storing protein).
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Fast Twitch Muscle Fibres

  • Muscle fibers contract very quickly.
  • Muscles used for fast movement. 
  • Good for short bursts of speed and power.
  • Tire easily.
  • Energy is released quickly through anaerobic respiration using glycogen.
  • Few mitochondria/blood vessels.
  • White due to lack of myoglobin.
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Sliding Filament Theory and Myosin Filaments

Sliding filament theory

  • Myosin and actin filaments slide over one another to make scarcomeres contract - the myofiloments themselves don't contract.
  • The simultaneous contraction of lots of sarcomeres means the myofibrils and muscle fibers contract.
  • Sarcomeres return to their original length when the muscle relaxes.

Myosin Filaments

  • Myosin filaments have globular heads that are hinges so they can go back and forth.
  • Each myosin head has a binding site fir actin and a binding site for ATP.
  • Actin filaments have binding sites for myosin heads called actin-myosin binding sites.
  • Two other proteins called tropomyosin and tropin are found between actin filaments. These proteins are attached to each other and they help myofilaments move past each other.
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Muscle Contraction

  • In a resting (unstimulated) muscle the actin-myosin binding site is blocked by tropomyosin, which is held in place be troponin. This means that the myofilaments can't slide slide past each other because the myosin heads can't bind to the actin-myosin binding sites on the actin filaments.
  • When an action potential from a motor neurone stimulates a muscle cell, it depolarises the sarcolmma. Depolarisation spreads down the T-tubles to the sacroplasmic reticulum. This causes the sarcoplasmic reticulum to release stored calcium ions (Ca2+) into the sacrcoplasm, calcium ions bind to troponin, causing it to change shape. This pulls the attached tropomypsin out of the actin-myosin binding site on the actin filaments, exposing the binding site, which allows the mysoin head to bind. The bond formed when a myosin head binds to an actin filament is called the actin-myosin cross bridge.
  • Calcium ions also activate the enzyme ATPase, which brakes down ATP (into ADP + Pi) to provide the energy needed for muscle contraction. The energy released from ATP moves the myosin head, which pulls the actin filament away.
  • ATP also provides energy to break the actin-mysoin cross bridge, so the myosin head detaches from the actin filament after it has moved. The myosin then reattaches to a different binding site further along the actin filament. A new actin-myosin cross bridge is formed and the cycle is repeated during which many cross bridges form and break very rapidly, pulling the actin filaments along. This shortens the sacromere, causing the muscle to contract. This cycle will continue as long as Ca2+ ions are present and bond to troponin.
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Muscle Contraction Continued

  • When the muscle stops being stimulated, calcium ions leave their binding sites on the troponin molecules and are moved by active transport back into the sarcoplasmic reticulum (requires ATP).
  • The tropin molecules return their original shape, pulling the attached tropomyosin molecules with them. This means the tropomyosin once again block the actin binding sites.
  • Muscles aren't contracted because no myosin heads are attached to actin filamnets.
  • The actin filaments slide back to their relaxed position, which lengthens the sacromere.
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Muscles and Movement

  • Movement involves skeletal muscles, tendons, ligaments and joints.
  • Skeletal muscles are the type of muscle you move.
  • Skeletal muscles are attached to bones by tendons.
  • Ligaments attach bones to other bones and hold them together.
  • Skeletal muscles contract and relax to move bones at a joint.
  • Muscles that work together to move a bone are called antagonistic pairs.
  • The muscle that extends a joint is called an extensor and the muscle that flexes a joint when it contracts is called felxor.
  • Muscles can only contract.
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Aerobic Respiration

  • Aerobic respiration is the process where a large amount of energy is released by splitting glucose into CO2 (which is released as a waste product) and H2 (which combines with atmospheric CO2 to produce H2O)
  • The energy is used to phosphorylate ADP to ATP. ATP is then used to provide energy for all the biological processes around the cell.
  • There are four stages in aerobic respiration - glycolysis, the link reaction, the krebs cycle and oxidative phosphorylation.
  • The first three stages are a series of reactions. The products from these reactions are used in the final stage to produce loads of ATP.
  • Each reaction in respiration is controlled by a different enzyme.
  • The first stage happens in the cytoplasm of the cell and the other three take place in the mitrochondria.
  • Coenzymes are used in respiration. For example;
    • NAD and FAD trasnfer hydrogen from one molecule to another - this means they can reduce or oxidise a molecule.
    • Coenzyme A transfers acetate between molecules.
  • All cells use glucose to respire, but organisms can also break down other complex organic molecules, which can then be respired.
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Anaerobic Respition

Lactate fermentation

  • Anaerobic respiration doesn't use oxygen.
  • It doesn't involve the link reaction, krebs cycle or oxidative phosphorylation.
  • There are two two types of anaerobic respiration.
  • Lactate fermentation occurs in animals and produces lactate
    • Glucose is converted to pyruvate via glycolysis
    • Reduced NAD (from glycolysis) transfers H to pyruvate to form lactate and NAD.
    • NAD can then be reused in glycolysis.
  • The lactate production regenerates NAD. This means glycolysis can continue even when their isn't much O2 around, so a small amount of ATP can still be produced to keep some biological processes going.

Lactic acid needs to be broken down

  • After a period of anaerobic respiration lactic acid builds up. Animals can break it down in two ways;
    • Cells can convert lactic acid back to pyruvate (when cells re-enter aerobic respiration at the krebs cycle).
    • Liver cells can convert the lactic acid back to glucose (Which can then be respired or stored)
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Control of Heart Rate

Cardiac muscle controls the regular beating of the heart.

Cardiac muscle is 'myogenic' - it can contract and relax without receiving signals from neurons. This pattern of contractions controls the regular heart beat.

  • The process starts in the sinoatrial node (SAN) which is in the wall of the right atrium.
  • The SAN is like a pacemaker - it sets the rhythm of the heartbeat by sending out waves of electrical activity to the atrial walls.
  • This causes the right and left atria to contract at the same time.
  • A bind of non-conducting collagen tissue prevents the waves of electrical activity from being passed directly from the atria to the ventricles.
  • Instead, these waves of electrical activity are transferred by the SAN to the atrioventricular  node (AVN).
  • The AVN is responsible for passing the waves of electrical activity into the bundle of His. But, there's a slight delay before the AVN reacts, ensuring the ventricles contract after the atria have emptied.
  • The bundle of His is a group of muscle fibers responsible for conducting the electrical activity to the finer muscle fibers in the right and left ventricle walls, called pukyne fibres.
  • Pukyne fibers carry the waves of electrical activity into the right and left ventricles, causing them to contract simultaneously, from the bottom up . 
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Control of Heart Rate

Heart rate is controlled by the cardiovascular control center in the medulla of the brain.

Decreased blood pH causes an increase in heart rate

  • A decrease in blood pH (caused by an increase in CO2) is detected be chemoreceptors.
  • The chemoreceptors send nerve impulse to the medulla.
  • The medulla sends nerve impulses to the SAN to increase the heart rate.

Increased blood pressure causes a decrease in heart rate.

  • Pressure receptors in the aorta wall and the carotid sinuses (at the start of the carotid arteries carrying blood to the brain) detect changes in blood pressure.
  • If the pressure is too high, the pressure receptors send nerve impulses to the cardiovascular center, which sends nerve impulses to the SAN, to slow down the heart rate.
  • If the pressure is too low, pressure receptors send nerve impulses to the cardiovascular centre, which sends nerve impulses to the SAN to speed up heart rate.
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Homeostasis is the maintenance of a constant temperature

  • Your external environment and what you're doing (e.g. exercising) can you can affect you internal environment - the blood and tissue that surrounds your cells (e.g. exercise increases body temperature)
  • Homeostasis involves control systems that keep your internal environment roughly constant - your internal environmental is kept in a state of equilibrium.
  • Keeping your internal environment at a constant temperature is vital for your cells to function normally and to stop them being damaged. For example, if body temperature becomes too high (e.g. 40C), enzymes may become denatured. The enzyme molecules vibrate to much, which breaks the hydrogen bonds that hold them in their 3D shape. The shape of the enzymes active site is changed and it no longer works as a catalyst. This means metabolic reactions are less efficient.
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Gene Expression

Stem cells become specialised because different genes in their DNA become active (they express different genes).

  • Stem cells all contain the same genes, but not all of them are expressed because not all of them are active.
  • Under the right conditions, some genes are activated and other genes are inactivated.
  • mRNA is only transcribed from active genes.
  • The mRNA from active genes is then translated into proteins.
  • These proteins modify the cell - they determine the cell structure and control the cell processes (including the activation of more genes, which produce more proteins).
  • Changes to the cell produced by these proteins cause the cell to become specalised (differentiate). These changes are difficult to reverse, so once a cell has differentiated it stays specialised.
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Stem Cells


  • The ability to produce all cell types, including all the specalised cells in an organism and extraembryonic cells.


  • The ability of a stem cell to produce all the specialised cells in an organism (but not extraembryonic cells)


  • The ability of a cell to give rise to a limited number of several different types of cell.
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  • Glucose is a type of carbohydrate that comes in two forms - alpha glucose and beta glucose.
  • It is a monosaccharide with 6 carbon atoms in each molecule.
  • It's structure is related to its function as the main energy source in animals & plants. Structure makes it soluble = easily transported, it's chemical bonds contain lots of energy.


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Drug Testing

 Drug testing is very controlled. Before a drug is allowed for use it must pass all 3 stages of drug testing. A drug must be tested on human tissues in a lab and then animals before it is allowed to go to clinical trials.

  • Phase 1 - this involves testing a new drug on a small group of healthy individuals. It's done to find out things like safe dosage, if there's any side effects, how the body reacts to the drug.
  • Phase 2 - if a drug passes phase 1 it will then be tested on a larger group of people (this time patients) to see how well the drug actually works.
  • Phase 3 - During this phase the drug is compared to existing treatments. It involves testing the drug on hundreds, or even thousands, of patients. Patients are randomly split into two groups - one group receives the new treatment and the other receives the existing treatment. This allows scientists to tell if the new drug is any better than the old drug.

Double blind trials - phase 2 and 3 clinical trials are usually double blind - neither the patients or the doctors know who's been given the placebo or old drug. This reduces bias in the results because the attitudes of the doctors and patients can't effect the results.

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Hormones and Nerves

Nervous control

  • Electrical transmission by nerve impulses and chemical transmission at synapse
  • Fast acting
  • Usually associated with short-term changes
  • Action potentials carried by neurones with connections to specific cells
  • Response is often very local, such as a specific muscle cell or gland.

Hormone control

  • Chemical transmission through the blood
  • Slower acting
  • Can control long term changes
  • Blood carries the hormone to all cells, but only the target cells are able to respond.
  • Response may be widespread, such as in growth and development
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Sympathetic Nervous System

  • Part of the peripheral nervous system
  • Part of the Autonomic nervous system
  • Prepares the body for flight or flight responses
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