Topic 6: organisms exchange substances with their environment

Exchange takes place at the surface of an organism but the cells that make up its volume use these substances. Small organisms have a larger surface area compared to their volume which allows efficient exchange across their body surface. 

As organisms get bigger, their volume increases faster than their surface area, which means simple diffusion is no longer sufficient enough to meet the needs of relatively active organisms. As a result, they have evolved to have: 

• A flattened shape so that no cell is too far away from the exchange surface, e.g. leaves 
• Specialised exchange surfaces with large surface area to increase the surface area to volume ratio, e.g. lungs in humans/gills in fish

The size and metabolic rate of an organism will determine how much materials are exchanged. Organisms with a high metabolic rate will need to exchange more materials so require a larger surface area to volume ratio. 

Single-celled organisms are small and therefore have a large surface area to volume ratio. Oxygen diffuses in and carbon dioxide diffuses out across the cell-surface membrane, which is their exchange surface. If the organism has a cell wall, this is not an additional barrier to exchange. 

For gas exchange, insects have an internal network of tubes called tracheae, supported by strengthened rings to prevent them collapsing and further divide into tracheoles. These extend throughout all the body tissues of the insect so oxygen in atmospheric air is brought directly to the tissues as there is a short diffusion pathway between a tracheole and any body cell. Air moves into the tracheae through tiny pores in their body surface called spiracles. Gases move through the tracheal system: 

• along a diffusion gradient. During cellular respiration, oxygen is used so its concentration is lower towards the ends of the tracheoles, which creates a concentration gradient, so oxygen from air diffuses along the trachae and trachioles to the cells. CO2  produced by cells creates a gradient in the opposite direction. 
• via mass transport. The contraction of muscles in insects can squeeze the trachea, which enable mass movements of gases in and out thus speeding up the exchange of respiratory gases. 
• in tracheoles filled with water. During major activity, cells surrounding the tracheoles respire anaerobically, which produces lactate. This lowers the water potential of cells as it is soluble therefore water moves into cells from the tracheoles via osmosis. As a result, the volume in the tracheoles decreases so more air moves into them. This means the final diffusion is in gas phase which is faster than liquid but this also leads to more evaporation of water. 

As the tracheal system relies on diffusion to exchange gases, it requires a short diffusion pathway, which limits the size that insects can attain. 

Gills are made up of gill filaments stacked into a pile, with gill lamellae at right angles to them, increasing the surface area. Water enters through the mouth, is forced over the gills (located behind the head) and leaves through an opening on each side of the body. 

The countercurrent exchange principle: The flow of water over the gill lamellae flows in the opposite direction to blood within them so blood loaded with oxygen meets water that has maximum oxygen concentration, and blood with little oxygen meets water that has had most, but not all, its oxygen removed. This maintains a diffusion gradient across the entire width of the gill lamellae so oxygen diffuses from the water into the blood at all times. 

If blood and water flowed in the same direction with blood with low oxygen concentration meeting water with high concentration (parallel flow), eventually an equilibrium would be reached that only 50% of oxygen from water diffuses into the blood because the diffusion gradient is maintained across half of the distance across the gill lamellae. 

Gas exchange with the external air changes depending on the rates of respiration and photosynthesis in the plant leaf because the gases produced by one process can be used by another. When photosynthesis is taking place, most of the CO2 comes from the external air, although some comes respiration; some of the oxygen from photosynthesis is used in respiration, but most of it diffuses out. When photosynthesis is not taking place, oxygen diffuses in and CO2 diffuses out of the leaf for respiration. There is no specific transport system for exchange, which is by diffusion. It takes place in the gas phase and no cell is far away from the exchange surface. 

Leaves have the following adaptations for gas exchange: 

• Stomata- no cell is far away from a stoma so the diffusion pathway is short
• Many interconnecting air-spaces throughout the mesophyll so gases readily come in contact with mesophyll cells 
• Large surface area of mesophyll cells for rapid diffusion

Efficient gas exchange requires a thin, permeable surface with a large surface area, which conflicts with the need to conserve water as it easily evaporates from their body surface. The insect has to balance needs of exchanging respiratory gases with limiting water loss. Insects have evolved the following adaptations to reduce water loss: 

• Small surface area to volume ratio to minimise the area over which water is lost 
• Rigid outerskeleton made of chitin which is covered with a waterproof cuticle
• Spiracles which can be closed to reduce water loss. This mostly occurs when the insect is at rest because it conflicts with the need for oxygen. 

Water loss in plants occurs by transpiration. Plants cannot have a small surface area to volume ratio to reduce water loss because they need to be able to capture as much sunlight as possible and exchange gases for photosynthesis. Xerophytic plants are adapted to living in areas with a short supply of water in the following ways:

• A thick cuticle, which forms a waterproof barrier. The thicker the cuticle, the less the water can escape
• Rolling up of leaves, which protects the lower epidermis containing many stomata and traps a region of still air. This region becomes saturated with water vapour so has high water potential. There is no water potential gradient between the outside and inside of the leaf so no net movement of water. This is also the same for having stomata in pits or grooves. 
• Hairy leaves, also trap still moist air and reduce water loss by evaporation. 
• Reduced surface area to volume ratio by having smaller leaves balance against the needs for photosynthesis 

Humans have evolved lungs to ensure efficient gas exchange between the air and their blood. They are located within the body because the density of the air is not dense enough to support and protect these delicate structures, and because the body would lose a lot of water and dry out. They are supported by the ribcage. 

• Trachea= flexible airway supported by rings of cartilage to prevent it from collapsing when pressure is low in the lungs during inspiration. Tracheal walls are made of muscles cells lined up with ciliated epithileum and goblet cells. 
• Bronchi= 2 divisions of the trachea, which each lead to one lung. They are similar in structure to the trachea although the amount of cartilage decreases as they get narrower.
• Bronchioles= branching subdivisions of the bronchi. Their walls are made of muscle which allows them to constrict to control the  flow of air in and out of the alveoli. 
• Alveoli= minute air-sacs at the end of bronchioles. They are lined with epithelium and have collagen and elastic fibres between each other. Elastic fibres allow them to stretch and recoil. The alveolar membrane is the gas-exchange surface. 

Breathing/ventilation maintains the diffusion of gases across the alveolar epithelium. The pressure changes within the lungs are brought about by the movement of 3 sets of muscles: the diaphragm, the internal and external intercostal muscles. 

Inspiration is breathing in. It is an active process. The external intercostal muscles contract, while the internal intercostal muscles relax. The volume of the thorax is increased in the following ways: the ribs are pulled upwards and outwards; the diaphragm muscles contract causing it to flatten. The increased volume results in reduced pressure inside the lungs, causing air to be forced into the lungs as atmospheric pressure is now greater than pulmonary pressure. 

Expiration is breathing out, which is a passive process. The internal intercostal muscles contract while the external muscles relax. The volume of the thorax is decreased when the ribs move downwards and inwards, and the diaphragm muscles relax so it is pushed up again. This increases the pulmonary pressure so air is forced out because the atmospheric pressure is lower. 

Each alveolus has a network of capillaries which are made up of a single layer of thin cells.

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

• red blood cells are slowed as they pass through pulmonary capillaries allowing more time for diffusion 
• distance between alveolar air and RBCs reduces as RBCs are flattened against capillary walls 
• walls of alveoli and capillaries are very thin therefore diffusion distance is short 
• alveoli and pulmonary capillaries have a very large total surface area 
• breathing movements constantly ventilate the lungs and the action of the heart constantly circulates blood around the alveoli. These ensure a steep concentration gradient of gases to be exchanged is maintained
• blood flow through the pulmonary capillaries maintains a concentration gradient

The digestive is the exchange surface through which food substances are absorbed. It consists of:

• The oesophagus carries food  from the mouth to the stomach
• The stomach- muscular sac with an inner layer that produces enzymes. Its role is to store and digest food, especially proteins. It has glands which produce enzymes to digest protein. 
• The ileum- long muscular tube in which food is further digested by enzymes that are produced by its walls and glands that secrete into it. Its inner walls are folded into villi, which gives them a large surface area. This is further increased by microvilli on the epithelial cells of each villus. Overall, this makes it suited to its purpose of absorbing products of digestion into the bloodstream. 
• The large intestine- absorbs water, most of which is from secretions of many digestive glands
• The rectum- the final section of the intestines where faeces are stored before periodically being removed via the an*s in the process of egestion. 

• Saliva enters the mouth from salivary glands and mixes with food during chewing
• Salivary amylase starts hydrolysing any starch in food to maltose. Saliva also contains mineral salts that help maintain pH around neutral, which is the optimum pH for salivary amylase
• Food is swallowed and enters the stomach, where conditions are acidic. The acid denatures the amylase so prevents further hydrolysis of the starch. 
• Food is passed into the ileum where it mixes with pancreatic juice. This contains pancreatic amylase which continues further hydrolysis of any remaining starch. Alkaline salts are produced by the pancreas and intestinal wall to maintain pH
• Muscles in the intestine wall push food along the ileum. Its epithelial lining produces maltase, which is a membrane-bound disaccharidase. It hydrolyses the maltose from starch breakdown into a-glucose.   

The glycosidic bond in sucrose is hydrolysed by sucrase to produce glucose and fructose, whilst the one in lactose is hydrolysed by lactase to produce glucose and galactose. Monosaccharides are absorbed into the ileum by co-transport and diffusion. 

Lipids (fats and oils) are first split up into micelles (tiny droplets) by bile salts produced in the liver in the process of emulsification. This increases the surface area of the lipids so that the action of lipases is faster. Ester bonds in triglycerides are hydrolysed by lipases to form fatty acids and monoglycerides (glycerol molecule with a single fatty acid).

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. The micelles break down, releasing the monoglycerides and fatty acids, which easily diffuse across the cell-surface membrane into the epithelial cells because they are non-polar molecules. Once inside the epithelial cells, they are transported to the endoplasmic reticulum (ER) where they are recombined to form triglycerides. Triglycerides move from the ER to the Golgi apparatus, associating with cholesterol and lipoproteins to form chylomicrons. Chylomicrons are special particles adapted for the transport of lipids. They move out of the epithelial cells by exocytosis and enter lymphatic capillaries called lacteals found at the centre of each villus. From here, the chylomicrons pass into the blood system. The triglycerides in the chylomycrons are hydrolysed by an enzyme in the endothelial cells of blood capillaries from where they diffuse into cells. 

Proteins are hydrolysed by peptidases/proteases. Endopeptidases hydrolyse peptide bonds between amino acids in the central region of the protein molecule. Exopeptidases hydrolyse peptide bonds on the terminal amino acids of the peptide molecules formed by endopeptidases. After this, dipeptidases hydrolyses the peptide bond between dipeptides. Dipeptidases are membrane-bound--a part of the cell-surface membrane of the epithelial cells lining the ileum. In this way, dipeptides and amino acids are progressively released. 

Amino acids are absorbed in the ileum by co-transport and diffusion. 

Its function is to absorb the products of digestion. It is folded into villi, which have microvilli on the surface of each epithelial cell, increasing the surface area and rate of absorption. 

• They increase surface area for diffusion
• Very thin walled, so reduce diffusion distance. 
• Contain muscle so are able to move, which helps maintain a diffusion gradient because it mixes the contents of the ileum. This ensures that as products are absorbed, new material rich in products of digestion replaces it. 
• Well supplied with blood vessels so that blood can carry away absorbed molecules and hence maintain a diffusion gradient.