Surface area : volume ratio - affects how quickly substances are exchanged.

Smaller organisms have higher SA:VOL ratios

Singles celled organisms - substances can diffuse quickly directly into (or out of) the cell across the cell membrance. Diffusion is fast because of the small distances the substances have to travel.

Multicellular organisms - Diffusion across the outer membrane is too slow becuse some cells are deep inside the body - large distance to cross. Larger animals also have a low SA:VOL ratio - makes it difficult to exchange enough substances to supply a large volume of animal through a relatively small outer surface.

Multicellular organisms need specialised exchange organs.

They also need an efficent system to carry substances to and from their individual cells - MASS TRANSPORT

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Body size - the rate of heat loss from an organism depends on its surface area. If an organism has a small relative SA its harder to loose heat. With a larger relative surface area heat is lost much easier. Smaller organisms need a high metabolic rate to generate enough heat to stay warm.

Body shape - Animals (of any size) with a compact shape have a small SA relative to their VOL -minmising heat loss from their surface. A less compact shape have a larger SA relative to their VOL - increases heat loss from their surface.

Adaptations for heat exchange - The animals body shape is adapted to suit its environment. e.g. the artic fox has small ears & a round head to reduce its SA:VOL ratio and heat loss. the african bat- eared fox has large ears & a more pointed nose to increase its SA:VOL ratio and heat loss.

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  • Animals with a high SA:VOL tend to loose more water - this is a problem for animals living in hot environments. Some small dessert animals have kidney structure adaptations so that they produce less urine to compensate.
  • To support their high metabolic rates, smal mammals living in cold regions need to eat large amounts of high energy foods e.g seeds & nuts.
  • Smaller mammals may have thick layers of fur or hibernate when the weather gets really cold.
  • Larger organisms living in hot regions e.g elephants and hippos find it hard to keep cool because their heat loss is relatively slow. Elephants have developed large, flat ears which increase their SA - heat loss speeds up. Hippos spend much of the day in the water - a behavioural adaptation to help them to loose heat.
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Gas exchange occurs over a gas exchange surface - a boundary between the outside environment and the internal environment of an organism. O2 and CO2 need to diffuse across gas exchange surfaces as quickly as possible.

Gas exchange surfaces often have

  • Large surface areas
  • They're thin (often just one layer of epithelial cells) - this provides a short diffusion pathway across the gas exchange surface.
  • The organism also maintains a steep concentration gradient of gases across the exchange surface

All of these help increase the rate of diffusion.

In single celled organisms they absorb and release gases by diffusion through their cell surface membranes. As they have a relatively large SA, a thin surface and short diffusion pathway (O2 can take part in biochemical reactions as soon as it diffuses into the cell) THERES NO NEED FOR A SPECIALISED GAS EXCHANGE SYSTEM.

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The gas exchange surface is the gills.


  • Each gill is made of lots of thin plates called gill filaments - give a large SA for exchange of gases (increase the rate of diffusion).
  • The filaments are covered in lots of lamellae which increase the SA even more
  • The lamellae have lots of blood capillaries and a thin surface layer of cells to speed up diffusion, between the water and the blood.


  • In the gills, blood flows through the lamellae in one direction and water flows over them in the opposite direction.THIS IS A COUNTER CURRENT SYSTEM
  • The counter current  system means that the water with a relatively high O2 conc always flows next to blood with a lower conc of O2.
  • A steep conc gradient is mainstained between the water and the blood - as much O2 as possible diffuses from the water into the blood.
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The main gas exchange surface is the surface of the meophyll cells in the leaf.

They're well adapted for their function - they have a large SA.

Gases move in and out through special pores in the epidermis (mostly the lower epidermis) called stomata.

The stomata can open to allow exchange of gases, and close if the plant is loosing too much water.

Guard cells control the opening and closing of the stomata.

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Terrestrial insects have microscopic air-filled pipes called trachea which they use for gas exchange. Air moves into the trachea through pores on the surface called spiracles.

O2 travels down the conc gradient towards the cells.

Trachea branch off into smaller tracheoles  - this increase the SA, which have thin, permeable walls and go to individual cells -  this means that O2 diffuses directly into respiring cells. This minmises the diffusion distance.

The insects circulatory system doesnt transport O2

CO2 from the cells moves down its own conc gradient towards the spiracles to be released into the atmosphere

Insects use rhythmic abdomincal movements to move air in and out of the spiracles.

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Exchanging gases tends to make you loose water. Plants and insects have evolved adaptations to minimise water loss without reducing gas exchange too much.

If insects are loosing too much water, they close their spircales using muscles. They also have a waterproof, waxy cuticle all over their body and tiny hairs around their spiracles - these reduce evaporation.

Plants stomata are usually kept open during the day to allow gaseous exchnage. Water enters the guard cells, making them turgid this opens the stomatal pore. If the plants starts to get dehydrated, the guard cells lose water and become flacid, which closes the pore.

Some plants are adapted for life in warm, dry or windy habitats where water loss is a problem - XEROPHYTES.

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Examples of Xerophyttic adaptations include;

  • Stomata sunk in pits to trap water vapour, reducing the conc gradient of water between the leaf and the air - reduces evaporation of water from the leaf.
  • A layer of hairs on the epidermis to trap water vapour around the stomata.
  • Curled leaves with the stomata inside, protecting them from wind ( windy conditions increase the rate of diffusion and evaporation)
  • A reduced number of stomata, there are fewer places for the water to escape.
  • Thicker, waxy, waterproof cuticles on leaves and stems to reduce evaporation.
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As you breathe in, air enters the trachea (windpipe). The trachea splits into two bronchi - one bronchus leading to each lung. Each bronchus then branches off into smaller tubes called bronchioles, these end in small sacs called alveoli. This is where gases are exchanged. The ribcage, intercostal muscles and diaphragm all work together to move air in and out.

INTERCOSTAL MUSCLES - Found between the ribs. 2 layers of intercostal muscles - interior & exterior.

Ventilation - consists of inspiration (breathing in) and expiration (breathing out)


  • The external intercostal and diaphragm muscles contract
  • This causes the ribcage to move upwards and outwards and the diaphragm to flatten, inc the vol of the thoractic cavity (the space where the lungs are)
  • As this increases, the lung pressure decreases (to below atmospheric pressure)
  • Air always flows from an area of high pressure to and area of low pressure ( dwon a pressure gradient) - air flows down the trachea into the lungs
  • Its an acitve process, it requires energy.
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  • The external intercostal and diaphragm muscles relax.
  • The ribcage moves downwards and inwards and the diaphragm becomes curved again.
  • The vol. of the thoracic cavity decreases, causing the air pressure to increase (to above atmospheric pressure).
  • Air is forced down the pressure gradient and out of the lungs.
  • Normal expiration is a passive process - no energy required.
  • Expiration can be forced though (blowing out candles on a bday cake)
  • During forced expiration, the external intercostal muscles relax and internal intercostal muscles contract, pulling the ribcage further down and in.
  • During this time, the movement of the two sets of intercostal muscles is said to be antagonsitic (opposing)
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Each alveolous is made from a single layer of thin,flat cells called alveolar epithelium.

  • Theres a huge number of alveoli in the lungs, theres a big surface area for exchanging O2 and CO2.
  • The alveoli are surronded by a network of capillaries.
  • O2 diffuses out of the alveoli , across the alveolar epithelium and the capillary endothelium ( a type of epithelium that forms the capillary wall) and into haemoglobin in the blood.
  • CO2 diffuses into the alveoli from the blood and is breathed out.

O2 from the air moves down the trachea, bronchi and broncioles into the alveoli - this occurs down a pressure gradient. Once in the alveoli, the O2 diffuses across the alveolar epithelium, then the capillary epithelium ending up in the capillary itself - this happens down a diffusion gradient

The alveoli have a thin exchange surface - this means there are short diffusion pathway. They also have a large surface area due to the large number of alveoli. Theres also a steep conc gradient of O2 and CO2 between the alveoli and the capillaries - increases the rate of diffusion This is constantly maintained by the flow of blood and ventilation.

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Lung diseases affect both ventilatiom (breathing) and gas exchnage in the lungs.

  • TIDAL VOLUME - the vol. of air in each breath - usually between 0.4dm3 and 0.5dm3
  • VENTILATION RATE - the no. of breaths per min. For a healthy person at rest its about 15 breaths.
  • FORCED EXPIRATORY VOLUME (FEV1) - the max vol. of air that can be breathed out in 1 second
  • FORCED VITAL CAPACITY (FVC) - the max vol. of air its possible to breathe forcefully of the lungs after a really deep breath in.

All of these can be figured out from the graph produced by a spirometer


  • When someone becomes infected with tb bacteria, immune system cells build a wall around the bacteria in the lungs - this forms tubercles
  • Infected tissues die and the gaseous exchange surface is damaged - tidal vol is decreased. This means less air can be inhaled, to compensate patients have to breathe faster (ventilation rate is increased)
  • Common symptoms include a persistent cough, coughing up blood and mucus, chest pains.
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  • Fibrosis is the formation of scar tissue in the lungs - can be the result of an infection or exposure to substances e.g asbestos or dust.
  • Scar tissure is thicker and less elsatic than normal lung tissure - the lungs are less able to expand & cant hold as much air
  • Tidal vol. is reduced & FVC is also reduced
  • Theres a reduction in the rate of gaseous exchange - diffusion is slower across a thciker scarred membrane
  • Symptoms of fibrosis include shortness of brethe, a dry cough, chest pain
  • Fibrosis sufferers have a faster ventilation rate than normal - to get enough air into their lungs to oxygenate their blood.
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  • A respiratory condition where the airways become inflamed & irritated - usually because of an allergic reaction to pollen & dust
  • During an attack the smooth muscle lining the bronchioles contracts and a large amount of mucus is produced.
  • This causes constriction of the airways, making it difficult for the sufferer to breathe properly.Air flow in and out of the lungs is severly reduced - less O2 enters the alveoli and moves in the blood.
  • Reduced air flow means that FEV1 is serverly reduced (less air can be breathed out in 1 second)
  • Symptoms include wheezing, a tight chest and shortness of breath
  • During an attack the symptoms come on very suddenly - they can be relieved by drugs (inhalers) which cause the muscle in the bronchioles to relax, opening up the airways
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  • A lung disease caused by smoking or long - term exposure to air pollution - foreign particles in the smoke becomes trapped in the alveoli.
  • This causes inflammation, which attracts phagocytes to the area. The phagocytes produce an enzyme that breaks down elastic (a protien found in the walls of the alveoli)
  • Elastin is elastic - helps the alveoli return to their normal shape after inhaling and exhaling air
  • Loss of it means that the alveoli cant recoil to expel air as well (it remains trapped)
  • It also leads to destruction of the alveoli walls, whcih reduces the SA of the alveoli - the rate of gaseous exchange decreases.
  • Symptoms include shortness of breath and wheezing
  • Sufferers have an increased ventillation rate as they try to increase the amount of air reaching their lungs.
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Food is broken down into smaller molecules during digestion

  • The large biological molecules (strach, proteins) are too big to cross cell membrances - they cant be absorbed from the gut into the blood.
  • During digestion, these large mol are broken down into smaller mol e.g glucose, amino acids - these can move across cell membranes. This means they can be easily absorbed from the gut into the blood to be transported around the body for use by the body cells
  • Large biological molecules are polymers which can be broken down into smaller molecules (monomers) using hydrolysis reactions - these break bonds by adding water
  • During H carbohydrates are broken down into disaccharides and then monosaccharides. Fats are broken down into fatty acids then monoglycerides. Proteins are broken down into amino acids

A variety of digestive enzymes are produced by specialised cells in the digestive system of mammals - these enzymes are released into the gut to mix with food. Enzymes only work with specific substrates, diff enzymes are needed to catalyse the breakdown of diff food molecules.

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  • Amylase is a digestive enzyme that catalyses the conversion of STARCH (polysaccharide) into the smaller sugar MALTOSE (a disaccharide). This involves the hydrolysis of the glycosidic bonds in starch
  • Its produced by the salivary glands & also the pancrea (releases amylase into the small intestine)
  • Membrane - bound disaccharides are enzymes that are attached to the cell membranes of epithelial cells lining the ileum (final part of the small intestine)
  • They help to break down disaccharides (maltose, sucrose & lactose) into monosaccharides (glucose, fructose & galactose) - involves the hydrolysis of glycosidic bonds.
  • Maltose (Disaccharide) & maltase (disaccharidase) = glucose & glucose (monosaccharides)
  • Sucrose (Disaccharide) & sucrase (disaccharidase) = glucose & fructose (monosaccharides)
  • Lactose ( Disaccharide) & lactase (disaccharidase) = glucose & galactose (monosaccharides)
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  • Lipase enzymes catalyse the breakdown of lipids into monoglycerides & fatty acids - involves the hydrolysis of the ester bonds in lipids.
  • Lipases are made in the pancreas - they work in the small intestine.
  • Bile salts are produced by the liver & emulsify lipids - cause them to form small droplets
  • Bile salts are really important in the process of lipid digestion - several small lipid droplets have a bigger SA than a single large droplet. The formation of small droplets greatly increases the SA of lipid thats available for lipases to work on.
  • Once its been broken down, the monoglycerides & fatty acids stick with the bile salts to form micelles (tiny structures)
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Endopeptidases and exopeptidases are enzymes that catalyse the conversion of proteins into amino acids by hydrolysing the peptide bonds between amino acids.


  • Act to hydrolyse peptide bonds within a protein
  • Trypsin & chymotrypsin are 2 examples of endopeptidases - they're synthesised in the pancreas and secreted into the small intestine
  • Pepsin is another endopeptidase. Its released into the stomach by cells in the stomach lining.
  • Pepsin only works in acidic conditions - these are provided by HCL in the stomach.


  • Act to hydrolyse peptide bonds at the ends of protein molecules - they remove single AA from proteins
  • Dipeptidases work specifically on dipeptides - they act to seperate the two AA that make up a dipeptide by hydrolysing the peptide bond between them.
  • These are often located in the cell - surface membrane of epithelial cells in the small intestine.
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The products of digestion are absorbed across cell membranes


  • Glucose is absorbed by active transport with Na+ inos via a co-transporter protein.
  • Galactose is absorbed in the same way
  • Fructose is absorbed via facillitated diffusion through a diff transporter protein

Monoglycerides & fatty acids

  • Micelles help to move monoglycerides & fatty acids towards the epithelium
  • Because micelles constantly break up & reform they can 'release' monoglycerides and fatty acids allowing them to be absrobed - whole micelles are not taken up across the epithelium.
  • Monoglycerides and fatty acids are lipid soluble, so can diffuse directly across the epithelial cell membrane.
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  • Reb blood cells contain haemoglobin (Hb).
  • Hb is a large protein with a quarternary structure.
  • Each chain has a haem group - these contain a Fe2+, this gives haemoglobin its red colour.
  • Hb has a high affintity for oxygen - each molecule can carry four O2 molecules

In th lungs, O2 joins to haemolgobin to form oxyhaemoglobin. This is a reverisble reaction - when oxygen dissociates from the oxyhaemoglobin near the body cells, it converts back into haemolgobin.Haemoglobin saturation depends on the partial pressure of oxygen. The partial pressure of oxygen (pO2) is a measure of O2 conc. The greater the conc of dissolved oxygen is cells, the higher the partial pressure. This is the same principal with CO2. Haemoglobin's affinity for oxygen varies depending on the partial pressure of oxygen.

  • Oxygen loads onto haemoglobin (forms oxyhaemoglobin) when theres a high pO2. Oxyhaemoglobin unloads its O2 when theres a lower pO2.
  • Oxygen enters blood capillaries at the alveoli in the lungs. Alveoli have a high pO2, O2 froms OHG.
  • During respiration, oxygen is used up - lowering the pO2. Red blood cells deliver OHG to respiring tissues, where O2 is unloaded.
  • Hb then retunrs to the lungs to pick up more oxygen. 
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Dissociation curves show how affinty for oxygen varies - they show how saturated the haemoglobin is at any given partial pressure. 

Where pO2 is high e.g. in the lungs - Hb has a high affinity for oxygen - it will readily combine - so it has a high saturation of oxygen

Where pO2 is low e.g. in respiring tissues - Hb has a low affinity for oxygen - it releases oxygen rather than combines with it - has a low saturation of oxygen

The graph is 'S' shaped - when Hb combines with the first O2 mol, its shape alters in a way that makes it easier for other mol to join.  - the curve has a steep section as its really easy for oxygen mol to join, and shallow bits where its harder. 

When the curve is steep, a small change in pO2 causes a big change in the amount of oxygen carried by the Hb. 

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Carbon Dioxide also affects oxygen unloading - Hb gives up its oxygen more readily at higher partial preesures of CO2.

  • When cells respire, CO2 is produced = pCO2 is raised.
  • This increases the rate of oxygen unloading - so the dissociation curve 'shifts' right 
  • The saturation of blood with oxygen is lower for a given pO2 - more O2 is being released. 


Different organisms have diff types of Hb with diff oxygen transporting capacities - this is an adaptation that helps the organism to survive in a particular environment. 

  • Organisms that live in environments with a low conc of oxygen have Hb wwith a higher affinity for oxygen - it holds onto it more. The dissociation curve is to the left of ours.
  • Organisms that are very active and have a high oxygen demmand have Hb with a lower affinity for oxygen - the curve is to the right of the human one.
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The circulattory system is a mass transport system Multicellular organisms, have a low surface area: vol ratio, they need specialised transport systems.The circulatory system consists of the heart and blood vessels. The heart pumps blood through blood vessels (arteries, aterioles, veins and capillaires) to reach different parts of the body.Blood transport respiratory gases, products of digestion, metabolic wastes and hormones round the body. 

Different blood vessels are adapted for different functions

Arteries - carry blood from the heart to the rest of the body. Their walls are thick and muscular & have elastic tissues to stretch and recoil as the heart beats, the inner lining (endothelium) is folded, allowing the artery to strecth - both of these qualities help to maintiain the high pressure. Only the pulmonary artery carries deoxygenated blood to the lungs - all the other arteries carry oxygenated blood.

Arteries divide into smaller vessels called arterioles - these form a network throughout the body. Blood is directed to the different parts of the body by muslces inside the arterioles. They contract to restrict blood flow or relax to allow full blood flow

Veins - take blood back to the heart under low pressure. They have a wider lumen , with very little elastic or muscle tissue. They contain valves to stop blood flowing backwards. Blood flow through the veins is helped by contraction of the body muslces. Only the pulmonary vein carries oxygenated blood to the heart from the lungs, all the other veins carry deoxygenated blood. 

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Substances are exchanged between blood and body tissues at capillaries 

Arterioles branch into capillaries. Substances e.g. glucose and oxygen are exchanged between cells and capillaries 

Capillaries are adapted for efficent diffusion

  • Theyre always found near cells in exchange tissues - theres a short diffusion pathway
  • Their walls are only one cell thick - also shortens the diffusion pathway.
  • Theres a large number of capillaries, networks of capillaries are called capillary beds - this increases the surface area for exchange. 

Tissue fluid is the fluid that surrounds cells in tissues, its made from small molecules that leave the blood plasma e.g. water, oxygen and nutrients. Tissues fluid is formed in blood. Unlike blood, tissue fluid doesnt contain red blood cells or big proteins because theyre too large to be pushed out through the capillary walls. Cells take in oxygen and nutrients from the tissues fluid and release metabolic waste into it.

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In a capillary bed , substances move out of the capillaries into the tissue fluid by pressure filtration;

  • At the start of the bed, nearest the arteries the hydrostatic (liquid) pressure inside the capillaries is greater than the hydrostatic pressure in the tissue fluid. 
  • The difference in hydrostatic pressure means an overall outward pressure forces fluid out of the capillaries and into the spaces around the cell, forming tissue fluid.
  • As the fluid leaves, the hydrostatic pressure reduces in the capillaries - the hydrostatic pressure is lower at the venule end if the capillary bed (end nearer the veins)
  • Due to fluid loss & an increasing conc of plasma proteins, the water potential at the venule end of the capillary bed is lower than the water potential in the tissue fluid. 
  • Water then re-enters the capillaries from the tissue fluid at the venule end by osmosis.

Any excess tissue fluid is drained into the lymphatic system which transports this excess fluid from the tissues and dumps it back into the circulatory system.

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The heart consists of two muscular pumps. The right side pumps dexoygenated blood to the lungs and the left side pumps oxygenated blood to the whole body.

Each bit of the heart is adapted to do its job effectively.

  • The left ventricle of the heart has thicker, more muscular walls than the right ventricle- it needs to contract more powerfully to pump blood all the way round the body, the right side only needs to get blood to the lungs. 
  • The ventricles have thicker walls than the atria - they have to push blood out of the heart whereas the atria just need to push blood a short distance into the ventricles
  • The atrioventircular valves link the atria to the ventricles and stop blood flowing back into the heart after the ventricles contract. The valves only open one way - whether theyre open or closed depends on the relative pressure of the heart chambers, if the pressure is higher behind a valve its forced open. But if the pressure is higher in front of the valve its forced shut. This keeps blood unidirectional.
  • The cords attach the atriocentricular valves to the ventricles to stop them being forced up into the artia when the ventricles contract.
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The cardiac cycle pumps blood around the body

Ventricles relax atria contract

The ventricls are relaxed, the atria contract decreasing the volume & increasing the pressue inside the chambers - this pushes the blood into the ventricles. Theres a slight increase in ventricular pressure and chamber volume as the ventricles recieve the ejected blood.

Ventricles contract, atria relax

The atria relax. The ventricles contract (decreasing their volume & increasing their pressure). The pressure becomes higher in the ventricles than in the atria, which forces the AV valves to shut preventing back-flow. The pressure in the ventircles is also higher than in the aorta and pulmonary artery, which forces open the SL valves and blood is forced out into these arteries.

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Most cardiovascular disease starts with Atheroma formation

The wall of an artery is made up of several layers. The endothelium (inner lining) is usually smooth and unbroken, if damage occurs to the endothelium e.g by high blood pressure white blood cells (mostly macropahges) and lipids from the blood clump together under the lining to form fatty streaks. Over time, this builds up along with connecitve tissue, this hardens to form a fibrous plaque - atheroma. Atheroma's partially block the lumen of the artery and restrict blood flow - causing blood pressure to increase. Coronary heart disease occurs when the cornonary arteries have lots of atheromas in them - restricting blood flow to the heart muscle - this can lead to myocardial infarction.

Atheromas increase the risk of aneurysm (a balloon like swelling of the artery) and thrombosis (formation of a blood clot) 

  • Atheroma plaques damage and weaken arteries, they also narrow arteries and increae bp. When blood travels through a weakened artery at high pressure it may push the inner layers of the artery through the outer elastic layer to form an aneurysm. This may burst, causing a haemorrhage.
  • An atheroma plaque can rupture the endothelium (inner lining) of an artery. This damages the artery wall and leaves a rough surface. Platelets and fibrin accumulate at the site of damage and form a blood clot. This can cause a complete blockage of the artery, or become dislodged and block a blood vessel elsewhere in the body. Debris from the rupture can cause another blood clot to form further down the artery.
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Interrupted blood flow to the heart can cause a myocardial infarction.

The heart muscle is supplied with blood by the coronary arteries - this contains the oxygen needed by heart muscle cells to carry out respiration. If the coronary artery becomes completely blocked (by a blood clot) an area of the heart muscle will be totally cut off from its blood supply, recieving no oxygen. This causes a myocardial infarction - known as a heart attack.

A heart attack can cause damage and death of the heart muscle. Symptoms include pain in the chest and upper body, shortness of breath and sweating. If large areas of the heart are affected complete heart failure can occur, which is often fatal.

High blood cholesterol and poor diet increase the risk of cardiovascular disease

  • if blood cholesterol level is high then the risk is increased. This is because cholesterol is one of main constituents of fatty deposits that form atheromas - these can lead to increased blood pressure and blood clots. This could block the flow of blood to coronary arteries which could cause a myocaridal infarction. Diets high in sat fat and salt increase the risk of cardiovascular disease.
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Cigarette smoking (nicotine and CO2) also increases the risk of cardiovascular disease

  • nicotine increases the risk of high blood pressure
  • CO2 reduces the oxygen carrying capacity of haemoglobin - so reduces the amount of oxygen available to tissues which may lead to heart attack.
  • Smoking also decreases the amount of antioxidants in the blood which are important for protecting cells from damage. Fewer antioxidants means cell damage in the coronary artery walls is more likely which can lead to atheroma formation.

High blood pressure also increases the risk of cardiovascular disease

  • HBP increases the risk of damage to the artery walls, damaged walls have an increased risk of atheroma formation causing further increase in blood pressure.
  • Atheromas can also cause blood clots to form - a blood clot could block flow of blood to the heart muscle possibly resulting in myocardial infarction.
  • Anything that increases bp also increases the risk of cardiovascular disease e.g. being overweight, not exercising and excessive alcohol consumption.
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Xylem tissue transports water and mineral ions in solutions, these move up the plants from the roots to the leaves.

Phloem tissue transports organic substances like sugars (also in solution) both up and down the plant.

Both of these are mass transport systems - they move substances over large distances

Xylem vessels transport the water and ions. They are very long, tube-like structures fomred from dead cells joined end to end - they have no end walls which makes an interrupted tube that allows water to pass up through the middle easily.

Cohesion and tension help water move up plants from roots to leaves, against the force of gravity. Water evaporates from the leaves at the 'top' of the xylem - this creates tension which pulls more water into the leaf. Because water molecules are cohesive, when some are pulled into the leaf other follow. This means the whole column of water in the xylem from the leaves down to the roots, moves upwards & water enters the stem through the roots. This is known as the COHESION-TENSION theory of water transport. 

Transpiration is the evaporation of water from a plants surface, especially the leaves. Water evaporates from the moist cell walls and accumulates in the spaces between cells in the leaf. When the stomata open, it moves out of the leaf down the concentration gradient (more water inside the leaf).

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Four main factors affect transpiration rate 

  • Light - the lighter it is the faster the transpiration rate. The stomata open when its light to let in CO2 for photosynthesis.
  • Temp - the higher the temp the faster the transpiration rate. Warmer water molecules have more energy so they evaporate from the cells inside the leaf faster. This increases the conc gradient between the inside and outside of the leaf making water diffuse out of the leaf faster
  • Humidity - the lower the humidity, the faster the transpiraiton rate. If the air around the plant is dry, the conc gradient between the leaf and the air is increased, increasing transpiration.
  • Wind - the windier it is, the faster the transpiration rate. Lots of air movement blows away water molecules from around the stomata - increasing the conc gradient, increasing the rate of transpiration. 
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A potometer can be used in estimate transpiration rate.

  • cut a shoot underwater - prevents air from entering. Cutting it at a slant increases thr SA
  • assemble the potometer in water and insert the shoot underwater, so no air can enter.
  • remove the apparatus from the water but keep the end of the capillary tube submerged in a beaker of water
  • check the app is watertight and airtight 
  • dry the leaves, allow time for the shoot to acclimatise and then shut the tap
  • remove the end of the tube from the beaker of water until one air bubble has formed, then put the end of the tube back into the water
  • record the starting position of the air bubble. 
  • start a stopwatch and record the distance moved by the bubble per unti time e.g. per hour. The rate of air bubble movement is an estimate of the transpiration rate
  • only change one variable at a time, e.g. temp, all others must be kepy constant. 

DISECTING PLANTS - using cross sections of the stem, sections are cut as thinly as possible, sections are then stained (TBO) which turns xylem vessels blue green. This allows you to see the position of the cells and their structure.

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Phloem tissue is adapted for transporting solutes. Phloem tissue transports solutes round plants. Its also arranged in tubes.  Sieve tube elements are living cells that form the tube for transporting solutes - they dont have a nucleus or organelles. Theres a companion cell for each sieve tube element, they carry out living functions for sieve cells e.g. providing the energy needed for the active transport of solutes.

Translocation is the movement of solutes (e.g. sucrose) to where theyre needed in a plant, sometimes called assimilates. Its an energy requiring process that occurs in the phloem. Translocation moves solutes from 'sources' to 'sinks'. The source of a solute is where its made. The sink is the area where its used up. e.g. the source for sucrose is usually the leaves, and the sinks are the other parts of the plant, especially the food storage organs and the meristems (areas of growth) in the roots, stems and leaves. Enzymes maintain a conc gradient from the source to the sink by changing the solutes at the sink (by breaking them down or making them into something else) - ensures there always a lower conc at the sink than the source e.g. in potatoes, sucrose is converted to starch in the sink areas - always a lower conc of sucrose at the sink than inside the phloem. This makes sure a constant supply of new sucrose reaches the sink from the phloem.

Translocation of solutes can be demonstrated experimentally.By suppyling a part of a plant with an organic substance that has radioactive label e.g. C14. The radioactive carbon will be incroporated with organic substances and moved around the plant by translocation. This movement can be tracked by autoradiography, the plant is then killed and placed on photographic film - radioactive = film is black. The results demonstrate the translocation of substances from source to sink over time. 

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The mass flow hypothesis explains phloem transport

  • active transport is used to actively load the solutes from companion cells into the sieve tubes of the phloem at the source
  • this lowers the WP inside the sieve tubes, so water enters by osmosis from the xylem and companion cells
  • this creates a high pressure inside the sieve tubes at the source end of the phloem
  • at the sink end, solutes are removed from the phloem to be used up
  • this increases the WP inside the sieve tubes, so water also leaves the tubes by osmosis
  • this lowers the pressure inside the sieve tubes
  • the result is a pressure gradient from the source end to the sink end
  • the gradient pushes solutes along the sieve tubes towards the sink 
  • when they reach the sink the solutes will be used or stored. 

Evaluating the mass flow  - If a ring of bark (which includes the phloem but not xylem) is removed from a woody stem, a bulge forms above the ring - this fluid has a higher conc of sugars than the fluid from below the ring - there is evidence that theres a downflow flow of sugars. However, the sieve plates would create a barrier to mass flow, a lot of pressure would be needed for the solutes to get through at a reasonable rate 

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The translocation of solutes can be demonstrated experimentally

  • plants are supplied with organic substances that have radioactive labels e.g. CO2 with C14
  • this will then by incorporated into organic substances produced by the leaf (e.g. sugars procued by photosynthesis) which will be moved around by translocation.
  • autoradiography tracks these movements. To reveal where the radioactive tracer has spread to in a plant, the plant is killed (e.g. by freezing it in liquid nitrogen) and then placed on photographic film - the radioactive substance is present wherever the film turns black
  • the results demonstrate the translocation of substances from source to sink over time - e.g autoradiographs of plants killed at different times show an overall movement of solutes (e.g. products of photosynthesis) from the leaves towards the roots. 
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The translocation of solutes can be demonstrated experimentally

  • plants are supplied with organic substances that have radioactive labels e.g. CO2 with C14
  • this will then by incorporated into organic substances produced by the leaf (e.g. sugars procued by photosynthesis) which will be moved around by translocation.
  • autoradiography tracks these movements. To reveal where the radioactive tracer has spread to in a plant, the plant is killed (e.g. by freezing it in liquid nitrogen) and then placed on photographic film - the radioactive substance is present wherever the film turns black
  • the results demonstrate the translocation of substances from source to sink over time - e.g autoradiographs of plants killed at different times show an overall movement of solutes (e.g. products of photosynthesis) from the leaves towards the roots. 
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DNA is stored differently in different organisms

Nuclear Eukaryotic DNA is linear and wound around histone proteins and coiled up to make a compact chromosome.

DNA molecules are shorter and circular in prokaryotes. Its still in the form of chromosomes but isnt wound around histones - it condenses to fit in the cell by supercoiling, 

A gene is a sequence of DNA bases that codes for either a polypeptide or functional RNA

The sequence of amino acids in a ploypeptide forms the primary structure of a protein.- the order of bases in a gene determined the order of amino acids in a polypeptide.

Each amino acid is coded for by a sequence of three bases in a gene - a triplet.

Genes that dont code for a polypeptide code for functional RNA instead - functional RNA is RNA molecules other than mRNA e.g. tRNA or rRNA 

A cells genome is the complete set of genes in the cell.

A cells proteome is the full range of proteins that the cell is able to produce. 

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In eukaryotice DNA genes that do code for polypeptides contain sections that dont code for amino acids - these are called introns and are removed during protein snythesis (these arent present in prokaryotice DNA)

Exons are sections of a gene that do code for polypeptides.

Eukaryotic DNA also contains regions of multiple repeats outside of genes - these are DNA sequences that repeat over and over e.g. CCTTCCTTCCTT - these dont code for amino acids either - theyre called non - coding repeats.

A gene can exsist in more than one form - these are called alleles - the order of bases in each allele is slightly different so they code for slightly different versions of the same polypeptide.

Alleles coding for the same characteristic will be found at the same fixed position (locus) on each chromosome in a homolygous pair.

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Theres more than one type of RNA 

Messenger RNA (mRNA)

  • made during transcription
  • carries the genetic code from the DNA to the ribosomes where its used to make a protein during translation
  • single ploynucleotide strand 
  • groups of three adjacent bases are called codons/triplets/base triplets

Transfer RNA (tRNA)

  • involved in translation
  • carries the amino acids used to make proteins to the ribosomes
  • single polynucleotide strand that folded into a clover shape - h bonds hold it in this specific shape
  • every tRNA molecule has a specific sequence of three bases at one end called an anticodon
  • they also have an amino acid bonding site at the other end. 
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  • RNA polymerase (enzyme) attaches to the DNA double helix at the beginning of a gene
  • h bonds between the two DNA strands break, seperating the 2 strands and the DNA uncoils exposing some of the bases
  • one of the strands is then used as a template to make an mRNA copy
  • the RNA polymeras lines up free RNA nucleotides alongside the exposed bases on the template strand - the free bases are attracted to the exposed bases.
  • specific, complimentary base pairing means that the mRNA strand ends up being a complimentary copy of the DNA template strand (except T is replaced with U)
  • once they have paired up theyre joined together by RNA polymerase, forming an mRNA molecule
  • Rna polymerase moves along the DNA, seperating the strands and assembling the mRNA strand. The DNA then recoils back into a double helix
  • when RNA polymerase reaches the stop signal it stops making mRNA and detaches from the DNA.
  • In eukaryotes, mRNA moves out of the nucleus through a nuclear pore and attaches to a ribosome in the cytoplasm.
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In eukaryotes, introns and exons are both copied into the mRNA during transcription - this mRNA is known as pre-mRNA. Splicing then occurs and introns are removed and exons are joined together - forming mRNA. Splicing happens in the nucleus.In prokaryotes, mRNA is produced directly from the DNA - without splicing taking place because theres no introns. 


  • occurs at the ribosomes in the cytoplasm
  • mRNA attaches itself to a ribosome and transfer RNA molecules carry amino acids to it. ATP provides energy for the bond between amino acid and tRNA to form
  • a tRNA molecule (carrying an amino acid) with an anticodon thats complimentary to the first codon on the mRNA attaches itself to the mRNA by specific base pairing.
  • A second tRNA molecule attaches itself to the next codon on the mRNA in the same way
  • the two amino acids attached to the tRNA molecules are joined by a peptide bond, the first tRNA molecule moves away, leaving its amino acid behind.
  • a third tRNA molecule binds to the next codon on the mRNA, its amino acids bind to the first two and the second tRNA molecule moves away
  • this process continues producing a ploypeptide chain until theres a stop signal on the mRNA molecule
  • the polypeptide chain moves away from the ribosome and translation is complete.
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The genetic code is non overlappying - each base triplet is read in sequence, seperate from the triplet before and after it, Base triplets dont share their bases. 

Its also degenerate - there are more possible combinations of triplets than there are amino acids. Some amino acids are codef for by more than one base triplet.

Its also universal - the same specific base triplets code for the same amino acids in all living things

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Amino acids 

  • AA are absorbed in a similar way to glucose & galactose.
  • Na+ ions are actively transported out of the epithelial cells into the ileum itself.
  • They then diffuse back into the cells through sodium - dependent transporter proteins in the epithelial cells membranes, carrying the AA with them..
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Ventricles relax, atria relax

The ventricles and atria both relax. The higher pressure in the PA and aorta closes the SL valves to prevent back-flow into the ventricles. Blood returns to the heart and atria fill again due to the higher pressure in the vena cava and PV - this starts to increase the pressure of the atria. As the ventricles continue to relax, their pressure falls below the pressure of the atriaand so the AV valves open - allowing blood to flow passively. The atria contract and the whole process starts again.

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