Exchange

- Exchange takes place at the surface of an organism and for it to be effective the exchange surfaces of the organism must be large in comparison with its volume.

- Small organisms have a high surface area to volume ratio to allow efficient gas exchange.

- As an organism becomes larger, their volume increases faster than their surface area (low surface area to volume ratio) and because of this, simple diffusion of substances across the outer surface can only meet the needs of inactive organisms.

- Therefore, for substances to reach all of the organism, they have specialised features.

- Surface area= area of one size x how many sides there are.

- Volume= length x width x height

- Surface area to volume ratio= surface area ÷ volume, in ratio form (for example if the surface area= 6cm2 and volume= 1cm3 then the surface area would be 6/1= 6.0:1)

1. Large surface area to volume ratio= increases the rate of exchange.

2. Very thin= so that the diffusion distance is short and therefore materials are able to cross the exchange surface rapidly.

3. Selectively permeable= to allow selected materials to cross.

4. Transport system= ensures the movement of the internal medium.

HOW TO CALCULATE DIFFUSION

- diffusion= surface area x difference in concentration ÷ length of diffusion path.

- When cells are respiring, oxygen is used up so its concentration towards end of tracheoles falls, creating a diffusion gradient that causes oxygen to diffuse from the atmosphere along the trachae and tracheoles and into the cells.

- Carbon dioxide is produced by cells in respiration, creating diffusion gradient in opposite direction, causing carbon dioxide to diffuse along tracheoles from the cells to the atmosphere.

- Contraction of muscles can squeeze trachea, enabling mass movement of air in and out, speeding up the exchange of respiratory gases.

- During periods of major activity, muscles around tracheoles respire, producing lactae that lowers water potential of muscles cells so water in ends of tracheoles decreases in volume and draws air further in them, meaning final diffusion pathway is of gas, not liquid so diffusion is more rapid.

- Gases enter and leave trachea by spiracles on body surface; these may be opened/closed by valve. When open, water vapour can evaporate from the insect, but are kept closed most of the time to prevent water loss.

- Gills are located behind the head of a fish.

- Gill filaments= make up gills and are stacked up in a pile, resembling pages of a book.

- Gill lamellae= located to right angle of gill fillaments and increase surface area of the gills.

- Water is taken in through the mouth and forced over the gills and out through an opening on each side of the body.

- The flow of water over the gill lamellae and the flow of the blood will flow in opposite directions- this is known as a 'countercurrent flow.'

- Blood that is already well loaded with oxygen meets water which has its maximum concentration of oxygen.

- Therefore, diffusion of oxygen from the water to the blood takes place.

- Moreover, blood with little oxygen in it meets water which has had most, but not all, of its oxygen removed.

- Diffusion of oxygen from the water to the blood takes place once again.

- As a result, a diffusion gradient for oxygen uptake is maintained across the width of the gill lamellae and this way about 80% of the oxygen available in the water is absorbed into the blood of the fish.

- If the flow of the water and the blood would have been in the same direction then the diffusion gradient would only be maintained across part and only 50% of the available oxygen would be absorbed by the blood.

- No living cell is far from the external air, and therefore a source of oxygen and carbon dioxide.

- Diffusion takes place in the air, which is more rapid than in water.

- The air spaces inside a leaf have a very large surface area compared with the volume of living tissue (large surface area to volume ratio).

- There is no specific transport system for gases, which move in and through the plant by diffusion.

- Stomata in the leaves= means that no cell is far from a stoma and therefore the diffusion pathway is short.

- Numerous interconnecting air spaces in the mesophyll= gases can readily come into contact with mesophyll cells.

- Large surface area of mesophyll cells= allows for rapid diffusion.

MINUTE PORES THAT OCCUR MANILY ON THE UNDERSIDE OF LEAVES

- Each stomata is surrounded by guard cells, which can open and close the stomatal pore.

- Therefore, this means they can control the rate of gas exchange and this is so important because terrestial organisms lose water by evaporation.

- Plants have evolved to balance the needs of gas exchange and control of water loss; they do this by closing stomata at times when they want to reduce water loss.

- The problem for all terrestial organisms is that water evaporates very easily from the surface of their bodies and they therefore become dehydrated.

- However, efficient gas exchange requires a thin and permeable surface with a large area and these features conflict with the need to conserve water.

- Insects have the following adaptations that reduce water loss:

1) SMALL SURFACE AREA TO VOLUME RATIO= minimises the area over which water is lost.

2) WATERPROOF COVERINGS= all over their body surfaces, in insects this 'covering' is an outer skeleton made out of chitin that is conserved with a waterproof cuticle.

3) SPIRACLES= openings of the trachea that can be closed to reduce water loss.

- Plants have a waterproof coverings but can not have a small SA:V ratio because they photosynthesise, which requires a larfe leaf area for the capture of light and gas exchange.

- Xerophytes= plants adapted to living in areas with very short water supply.

- Modifications plants have to reduce water loss:

1) A THICK CUTICLE= the thicker the cuticle, the less water that can escape.

2) ROLLING UP OF LEAVES= most leaves have their stomata confined to the lower epidermis, the rolling of leaves in this way protects the lower epidermis from the outside. This region becomes saturated with water vapour and so has a very high water potential.

3) HAIRY LEAVES= a thick layer of hair on leaves traps moist air next to the leaf surface, the water potential gradient between the inside and outside of leaf is reduced and less water is lost.

4) REDUCED SURFACE AREA TO VOLUME RATIO= by having small leaves and circular in cross-section, the rate of water loss is dramatically reduced.

- Lungs are site of gas exchange and are located inside the body because:

1. Air is not dense enough to support and protect the delicate structures.

2. The body as a whole would otherwise lose a great deal of water and dry out.

- The lungs are supported by a ribcage, and the ribs are able to move due to the muscles that surround them.

- The lungs are ventilated by a tidal stream of air, ensuring that the air within them is constantly replenished.

1) LUNGS= a pair of lobed structures made up of many highly branched tubules (called bronchioles) which end in tiny air sacs (called alveoli.)

2) TRACHEA= flexible airway that is supported by rings of cartilage; this prevents the trachea from collapsing as the air pressure inside falls when breathing in. Tracheal walls are made up of muscle and lined with ciliated epithelium and goblet cells.

3) BRONCHI= two divisions of the trachea, each leading to 1 lung. Similar in structure to the trachea but also produce mucus to trap dirt particles and have cilia that move the mucus towards the throat. The larger bronchi are supported by cartilage.

4) BRONCHIOLES= series of branching subdivisions of the brochi and their walls are made up of muscle lined with epithelial cells, allowing them to contrict so they can control the flow of air in and out of the alveoli.

5) ALVEOLI= air sacs at the end of bronchioles, they are lined with epithelium and the elastic fibres allow them to stretch as they fill with air when breathing in, they then spring back during breathing out to expel carbon dioxide rich air.

- Breathing= a process that has the role of maintaining the diffusion of gases across the alveolar epithelium as air is constantly moved in and out of the lungs.

- Inspiration= when the air pressure of the atmosphere is greater than the air pressure inside the lungs, air is therefore forced into the lungs.

- Expiration= when the air pressure in the lungs is greater than the air pressure in the atmosphere, air is forced out of the lungs.

- The pressure changes within the lugs is brought about by the movement of the following muscles

1. Diaphragm= sheet of muscle that separates the thorax from the abdomen.

2. Internal intercostal muscles= contraction of them leads to expiration.

3. External intercostal muscles= contraction of them leads to inspiration.

1. External intercostal muscles contract and the internal intercostal muscles relax.

2. Ribs are pulled upwards and outwards, this increases the volume of the thorax.

3. Diaphragm muscles contract, causing it to flatten and this also helps to contribute to the increase of the volume of the thorax.

4. The increased volume of the thorax results in a reduction of pressure in the lungs.

5. Atmospheric pressure is now greater than pulmonary pressure, this therefore forces air into the lungs.

1. The internal intercostal muscles contract and the external intercostal muscles relax.

2. Ribs move downwards and inwards, this decreases the volume of the thorax.

3. Diaphragm muscles relax and so it is pushed up again by the contents of the abdomen that were compressed during inspiration and the volume of the thorax is once again further decreased.

4. The decreased volume of the thorax leads to an increased pressure in the lungs.

5. The pulmonary pressure is now greater than the atmosphoric pressure and so this means air is forced out of the lungs.

- There are around 300 million alveoli in each human lung and their total SA is around 70m2.

- Around each alveolus is a network of pulmonary arteries so narrow that red blood cells are flattened against the thin capillary walls in order to squeeze through.

- Diffusion of gases between the alveoli and blood will be very rapid because:

1. Red blood cells are slowed as they pass through the pulmonary capillaries; this allows more time for diffusion.

2. The distance between the alveolar air and red blood cells is reduced as the red blood cells are flattened against the capillary walls.

3. Walls of alveoli and capillaries are very thin so the diffusion distance is therefore short.

4. Blood flow through the pulmonary capillaries maintains a concentration gradient.

1) OESOPHAGUS= carried food from the mouth to the stomach.

2) STOMACH= a muscular sac with an inner layer that produces enzymes and has the role of storing and digesting foods.

3) ILEUM= a long muscular tube that further digests food by enzymes that are produced by its walls and glands. The inner walls are folded into villi, giving them a larger surface area and the overall role of the ileum is to absorb the products of digestion into the bloodstream.

4) LARGE INTESTINE= absorbs water; most of which is from secretions of digestive glands.

5) RECTUM= stores faeces before they are removed via the **** in a process called egestion.

6) SALIVARY GLANDS= situated near the mouth and pass their secretions via a duct in the mouth; these secretions contain amylase which hydrolyses starch into maltose.

7) PANCREAS= large gland situated below the stomach that produces pancreatic juice which contains proteases to hydrolyse proteins, lipase to hydrolyse lips and amylase to hydrolyse starch.

- If the food is large it is broken down into smaller pieces by the teeth.

- This makes it possible to ingest the food and provides a large surface area for chemical digestion- the second stage of digestion.

- Food is churned by the muscles in the stomach wall, physically breaking up the food.

- Large and insoluble molecules ar hydrolysed to smaller, soluble ones and is carried out by enzymes; all digestive enzymes function by hydrolysis.

- Enzymes are specific so more than one enzyme is needed to hydrolyse a large molecule.

- Usually one enzyme hydrolyses a large molecule into sections and then hydrolysed into smaller molecules by one or more additional enzymes.

- There are 3 important types of digestive enzymes:

1. CARBOHYDRASES= hydrolyse carohydrates into monosaccharides.

2. LIPASES= hydrolyse lipids into glycerol and fatty acids.

3. PROTEASES= hydrolyse proteins into amino acids.

1. Saliva enters the mouth from salivary glands and is mixed with food during chewing.

2. Salivary amylase starts hydrolysing any starch in the food to maltose as well as containing mineral salts that keep the pH at optiumum of around 7.

3. Food is swallowed and enters stomach where the acidic conditions denature the amylase, preventing any further hydrolysis of the starch.

4. The food is then passed into the small intestine where is mixes with pancreatic juice- pancreatic amylase continues the hydrolysis of any remaining starch to maltose.

5. Muscles in the intestine wall push food along the ileum, and epithelial lining produces maltase which hydrolyses the maltose from starch breakdown into a-glucose.

6. Other disaccharides involved in digestion include; 1. Sucrase, which hydrolyses the glycosidic bond to produce glucose and fructose and 2. Lactase, which hydrolyses the glycosidic bond to produce glucose and galactose.

LIPIDS ARE HYDROLYSED BY LIPASES

- Lipases are produced in the pancreas.

- They work by hydrolysing the ester bond found in glycerides to form fatty acids and monoglycerides.

- Emulsification= lipids are split up into tiny droplets called micelles by bile salts that are produced by the liver.

- Emulsification increases the surface area of the lipids so that lipases actions is made to be much more quick.

PEPTIDASES (PROTEASES) HYDROLYSE PROTEINS.

- There are a number of different peptidases involved in the hydrolysis of proteins that take place during protein digestion;

1. ENDOPEPTIDASES= hydrolyse the peptide bonds between amino acids in the central region of a protein molecule and this forms a series of peptide molecules.

2. EXOPEPTIDASES= hydrolyse the peptide bonds on the terminal amino acids of the peptide molecules formed by endopeptidases, they then therefore release dipeptides and single amino acids.

3. DIPEPTIDASES= hydrolyse the bond between the 2 amino acids of a dipeptide, these enzymes are membrane-bound, meaning that they are part of the cell-surface membrane of the epithelial cells lining the ileum.

- Ileum is adapted to carry out its function of absorbing the products of digestion:

1. The wall is folded and possess villi, which are finger-like projections that are around 1mm long.

2. Villi have thin walls, lined with epithelial cells of which is a rich network of blood capillaries.

3. The villi considerably increase the surface area of the ileum and therefore, increase the rate of absorption.

4. Villi are located at the interface between the lumen of the intestines and the blood and other tissues inside the body.

VILLI ARE PART OF A SPECIALISED EXCHANGE SURFACE ADAPTED FOR THE ABSORPTION OF THE PRODUCTS OF DIGESTION.

1. INCREASE SURFACE AREA= increases the rate of diffusion.

2. VERY THIN WALLED= reducing the distance over which diffusion takes place.

3. CONTAIN MUSCLE= meaning that they are able to move and this helps to maintain a diffusion gradient as their movement mixes the contents of the ileum ensuring that as the products of digestion are absorbed, new material rich in the products of digestion replaces it.

4. WELL SUPPLIED WITH BLOOD VESSELS= allows blood to carry away absorbed molecules, maintaining a diffusion gradient.

5. EPITHELIAL CELLS LINING VILLI POSSESS MICROVILLI= further increase the surface area for absorption.

- Once formed monoglycerides and farrt acids remain in association with the bile salts that emulsified the lipid droplets, the structures formed are called micelles.

- Through the movement of material within the lumen of the ileum, the micelles come into contact with the epithelial cells lining the villi of the ileum.

- Therefore, the micelles will break down and release monoglycerides and fatty acids which diffuse easily across the cell-surface membrane into the epithelial cells.

- Once inside the epitheial cells, monoglycerides and fatty acids are transported to the endoplasmic reticulum where they are recombined to form triglycerides.

- Triglycerides associate with cholesterol and lipoproteins to form structures called chylomicrons which are special particles adapted for the transport of lipids.

- Chylomicrons move out of epithelial cells by exocytosis and enter the lymphatic capillaries before the triglycerides in the chylomicrons are hydrolysed by an enzyme in the endothelial cells of blood capillaries.

BILE SALTS ARE INVOLVED IN THE ABSORPTION OF FATTY ACIDS

1. One end of the bile salt molecule is lipophillic (soluble in fat) and the other end is hydrophillic (soluble in water).

2. Bile salt molecules arrange themselves with their lipophillic ends in fat droplets, leaving their lipophobic ends sticking out.

3. Therefore, fat droplets are preventing from sticking to each other and form large droplets, leaving only tiny ones- micelles.

4. In this form, fatty acids reach the epithelial cells of the ileum where they break down and release fatty acids for absorption.