3 Exchange Substances 6.1-7.9 Flashcards
Two types of exchange
Passively and actively
Passive exchange
No metabolic energy is required, by diffusion and osmosis
Active exchange
Metabolic energy is required, by active transport
Surface area to volume ratio
Exchange takes place at the surface of an organism, but the materials absorbed are used by the cells that mostly make up its volume. For exchange to be effective, the exchange surface of the organism must be large compared with its volume.
Organism’s have evolved one or of the following features:
- a flattened shape so that no cell is ever far from the surface (e.g. a flatworm or a leaf)
- specialised exchange surfaces with large areas to increase the surface area to volume ratio (e.g. lungs in mammals, gills in fish)
Features of specialised exchange surfaces
- large SA relative to the V of the organism’s which increases the rate of exchange
- very thin so that the diffusion distance is short and therefore materials cross the exchange surface rapidly
- selectively permeable to allow selected materials to cross
- movement of the environment medium, for example, air, to maintain a diffusion gradient
- a transport system to ensure the movement of the internal medium, for example, blood, in order to maintain a diffusion gradient
Gas exchange in insects
Insects overcome water loss
Insects l have evolved an internal network of tubes (tracheae)- supported by rings to prevent collapsing
Tracheae divided into smaller tubes (tracheoles); extend throughout all body tissues
Respiratory gases move in and out of the tracheal system (insects) in 3 ways:
- along a diffusion gradient: cells respiring; O2 is used up so conc towards end of tracheoles falls. CO2 produced by cells. Creates diffusion gradient in opposite direction. As diffusion in air is much more rapid than in water, respiratory gases are exchanged quickly.
- mass transport: contraction of muscles in insects squeeze trachea, enabling mass movements of air in and out; further speeds up exchange of respiratory gases
- ends of tracheoles are filled with water: muscle cells around tracheoles respire carry out some anaerobic respiration, produces lactate, is soluble; lowers water potential of muscle cells. Water moves into cells from tracheoles by osmosis
Gas exchange in single celled organisms
Single celled organisms are small; have large SA:V
Oxygen is absorbed by cell surface membrane
CO2 from respiration diffuses out across their body surface
Structure of the gills
Gills made up of gill filaments; these are stacked up in a pile
At a right angle to them are gill lamellae; increase SA of gills
Water taken in through 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 flow of blood within them are in opposite directions - known as counter current flow
Why is the counter current flow important for ensuring that max gas exchange is achieved (fish)
If the water and blood flowed in the same direction, far less gas exchange would take place
The arrangement means that blood that is already well loaded with oxygen meets water, which has its max conc of oxygen. Therefore diffusion of oxygen from water to the blood takes place
Blood with little oxygen in it meets meets water which has most, but not all of its oxygen removed
Adaptions in leaves for rapid diffusion
- many small pores (stomata) and so no cell is far from a stoma and therefore the diffusion pathway is short
- numerous interconnecting air spaces that occur throughout the mesophyll so that gases can readily come in contact with mesophyll cells
- large SA of mesophyll cels for rapid diffusion
Gas exchange in leaves
No living cell is far from the external air, and therefore a source of oxygen and carbon dioxide
Diffusion takes place in the gas phase (air), which makes it more rapid than if it were in water
Stomata
Minute pores that occur mainly, but not exclusively, on the leaves, especially the underside. Each stoma is surrounded by a pair of special cells (guard cells)
These cells open and close the stomatal pore. In this way they can control the rate of gaseous exchange. This is important because terrestrial organisms lose water by evaporation. Plants have evolved to balance the conflicting needs of gas exchange in control of water loss. They do this by closing stomata at times when water loss would be excessive
Leaf structure
Waxy cuticle Upper epidermis Mesophyll cells Air spaces Lower epidermis Guard cells and stomata
Limiting water loss in insects adaptations
– Small surface area to volume ratio to minimise the area over which water is lost
– waterproof coverings over the body surfaces. In the case of insects this covering is a rigid outer skeleton of chitin that is covered with a waterproof cuticle
– spiracles are the openings of the tracheae at the body-surface and these can be closed to reduce water loss. This conflicts with the need for oxygen and so occurs largely when the insect is at rest
Thick cuticle adaptation
although the waxy cuticle forms a waterproof layer, up to 10% of water loss can still occur by this route. The thicker the cuticle, the less water can escape by this means.
Rolling up of leaves adaptation
most leaves have their stomata mainly on the lower epidermis. The rolling of leaves in a way that protects the lower epidermis from the outside helps to trap a region of still air within the rolled leaf. This region becomes saturated with water vapour and so has very high water potential. There’s no water potential gradient between the inside and outside of the leaf and therefore no water loss
Limiting water loss in plants adaptations
Thick cuticle Rolling up of leaves Hairy leaves Stomata in pits or grooves A reduced SA to V ratio of the leaves
Hairy leaves adaptation
A thick layer of hairs on leaves, especially on the lower epidermis, traps still, moist air next to the leaf surface. The water potential gradient between the inside and the outside of the leaves is reduced and therefore less water is lost by evaporation
Stomata in pits or grooves adaptation
Traps still, moist air next to the leaf and reduces the water potential gradient
A reduced SA:V ratio of the leaves adaptation
By having leaves that are small and roughly circular in cross section, like pine needles, rather than leaves that are broad and flat, the rate of water loss can be considerably reduced. This reduction in SA is balanced against the need for a sufficient area for photosynthesis to meet the requirements of the plant
The volume of oxygen that has to be absorbed and the volume of carbon dioxide that must be removed are large in mammals because:
– They are relatively large organisms with a large volume of living cells
– they maintain a high body temperature which is related to them having a high metabolic and respiratory rates
Mammalian lungs
The site of gas exchange in mammals. They are located inside the body because air is not dense enough to support and protect these delicate structures and the body as a whole would otherwise lose a great deal of water and dry out
The lungs are supported and protected by a bony box called the rib cage. The ribs can be moved by the muscles between them. The lungs are ventilated by a tidal stream of air, thereby ensuring that the air within them is constantly replenished.
Lungs function and structure
The lungs are a pair of lobed structures made up of a series of highly branched tubules, called bronchioles, which end in tiny air sacs called alveoli
Trachea structure and function
The trachea is a flexible airway that is supported by rings of cartilage. The cartilage prevents the trachea collapsing as the air pressure inside falls when breathing in. The tracheal walls are made up of muscle, lined with ciliated epithelium and goblet cells
Bronchi structure and function
The bronchi are two divisions of the trachea, each leading to one lung. They are similar in structure to the trachea and, like the trachea, they also produce mucus to trap the particles and have cilia that move the dirt-laden mucus towards the throat. The larger bronchi are supported by cartilage, although the amount of cartilage is reduced as the bronchi get smaller
Bronchioles structure and function
The bronchioles are a series of branching subdivisions of the bronchi. The walls are made of muscle lined with epithelial cells. This muscle allows them to contact so that they can control the flow of air in and out of the alveoli
Alveoli structure and function
The alveoli are minute air sacs, with a diameter of between 100 and 300 micro metres, at the end of the Bronchioles. Between the alveoli there are some collagen and elastic fibres. The alveoli are lined with Epithelium. Elastic fibres allow the alveoli to stretch as they fill with air when breathing in. They then springback during breathing out in order to expel the carbon dioxide-rich air. The alveolar membrane is the gas exchange surface
Ventilation
Breathing
To maintain diffusion of gases across the alveolar epithelium, air is constantly moved in and out of the lungs
When the air pressure of the atmosphere is greater than the air pressure inside the lungs, air is forced into the lungs. This is called inspiration (inhalation).
When the air pressure in the lungs is greater than that of the atmosphere, air is forced out of the lungs. This is called expiration (exhalation ).
The pressure changes within the lungs are brought about by the movement of three sets of muscles:
-The diaphragm, which is a sheet of muscle that separates the thorax and abdomen
-Intercostal muscles, which lie between the ribs. There are two sets of intercoastal muscles:
+the internal intercostal muscles, whose contraction leads to expiration
+the external intercostal muscles, whose contraction leads to inspiration
Inspiration
Breathing in is an active process (it uses energy) and occurs as follows:
– the external intercostal muscles contract, while the internal intercostal muscles relax
– ribs are pulled upwards and outwards, increasing the volume of the thorax
– The diaphragm muscles contract, causing it to flatten, which also increases the volume of the thorax
– the increased volume of the thorax results in a reduction of pressure in the lungs
– atmospheric pressure is now greater than pulmonary pressure, and so it is forced into the lungs
Expiration
Breathing out is a largely passive process (it doesn’t require much energy) and occurs as follows:
– the internal intercostal muscles contract, while the external intercostal muscles relax
– the ribs move downwards and inwards, decreasing the volume of the thorax
– The diaphragm muscles relax and so it is pushed up again by the contents of the abdomen that were compressed during inspiration. The volume of the thorax is therefore further decreased
– the decreased volume of the thorax increases the pressure in the lungs
– the pulmonary pressure is now greater than that of the atmosphere, and so air is forced out of the lungs
Role of the alveoli in gas exchange
There are about 300 million alveoli in each human lung. The total surface area is around 70 m² – about half the area of a tennis court. Each alveolus is lined with epithelial cells only 0.05 micro metres to 0.3 micro metres thick thick. Around each alveolus is a network of pulmonary capillaries, so narrow that red blood cells are flattened against the thin capillary walls in order to squeeze through. These capillaries have walls that are only a single layer of cells thick.
Diffusion of gases 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 the alveolar air and red blood cells is reduced as the red blood cells are flattened against the capillary walls
– The walls of both alveoli and capillaries are very thin and therefore the distance over which diffusion takes place is very 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. Together, this is sure that a steep concentration gradient of the gases to be exchanged is maintained
– blood flow through the pulmonary capillaries maintains a concentration gradient
Major parts of the digestive system
Oesophagus Stomach Ileum Large intestine Rectum Salivary glands Pancreas
Oesophagus
Carries food from the mouth to the stomach
Stomach
Muscular sac with an inner layer that produces enzymes. Its role is to store and digest food, especially proteins. It has glands that produce enzymes which digest protein
Ileum
A long muscular tube. Food is further digested in the ileum by enzymes that are produced by its walls and by glands that pour their secretions into it. The inner walls of the ileum are folded into villi, which gives them a large surface area. The surface area of the stability is further increased by millions of tiny projections, called microvilli, on the epithelial cells of each villus. This adapts the ileum for its purpose of absorbing the products of digestion into the bloodstream
Large intestine
Absorbs water. Most of the water that is absorbed is water from the secretions of many digestive glands
Rectum
The final section of the intestine. The faeces are stored here before periodically being removed via the anus in a process called egestion
Salivary glands
Situated near the mouth. They pass their secretions via a duct into the mouth. The secretions contain the enzyme amylase, which hydrolyses starch into maltose
Pancreas
A large gland situated below the stomach. It produces a secretion called pancreatic juice. This secretion contains proteases to hydrolyse proteins, lipase to hydrolysed lipids and amylase to hydrolyse starch
Two stages digestion takes place in
Physical breakdown and chemical digestion
Physical breakdown
If the food is large, it’s broken down into smaller pieces by means of structures such as the teeth. This not only makes it possible to ingest the food but also provides a large surface area for chemical digestion. Food is churned by the muscles in the stomach wall and this also physically breaks it up
Chemical digestion
Chemical digestion hydrolyses large, insoluble molecules into smaller, soluble ones. It is carried out by enzymes. All digestive enzymes function by hydrolysis. Hydrolysis is the splitting up of molecules by adding water to the chemical bonds that hold them together. Enzymes are specific and so it follows that more than one enzyme is needed to hydrolyse a large molecule. Usually one enzyme hydrolyses a large molecule into sections and the sections are then hydrolysed into smaller molecules by one or more additional enzymes.
Which three enzymes are particularly important in chemical digestion?
Carbohydrases, lipases and proteases
Carbohydrases
Hydrolyse carbohydrates, ultimately to monosaccharides
Lipases
Hydrolyse lipids (fats and oils) into glycerol and fatty acids
Proteases
Hydrolyse proteins, ultimately to amino acids
Carbohydrate digestion in chemical digestion
It usually takes more than one enzyme to completely hydrolyse a large molecule. Typically one enzyme hydrolyses the molecule into smaller sections and then other enzymes hydrolyse the sections further into the monomers. These enzymes are usually produced in different parts of the digestive system. It is obviously important that enzymes are added to the food in the correct sequence. This is true of starch digestion
First the enzyme amylase is produced in the mouth and the pancreas. Amylase hydrolyses the alternate glycosidic bonds of the starch molecule to produce the disaccharide maltose. The maltose is in turn hydrolysed into the monosaccharide a-glucose by the second enzyme, a disaccharide called maltase. Maltase is produced by the lining of the ileum
Digestion of maltose
Saliva enters the mouth from the salivary glands and is thoroughly mixed with the food during chewing. Saliva contains salivary amylase. This starts hydrolysing any starch in the food to maltose. It also contains mineral salts that help to maintain the pH around neutral. This is the optimum pH for salivary amylase to work.
The food is swallowed and enters the stomach, where the conditions are acidic. This acid denatures the amylase and prevents further hydrolysis of the starch. After time the food is passed into the small intestine, where it mixes with the secretion from the pancreas called pancreatic juice.
The pancreatic juice contains pancreatic amylase. This continues the hydrolysis of any remaining starch to maltose. Alkaline salts are produced by both the pancreas and the intestinal wall to maintain the pH around neutral so that the amylase can function.
Muscles in the intestinal wall push the food along the ileum. It’s epithelial lining produces the disaccharide maltase. Maltase is not released into the lumen of the ileum but is part of to the cell surface membranes of the epithelial cells that line the ileum. It is therefore referred to as a membrane-bound disaccharidase. The maltase hydrolyses the maltose from starch breakdown into a-glucose
In addition to maltose what two other common disaccharides in the diet are hydrolysed?
Sucrose and lactose
Sucrose
Is found in many natural foods, especially fruit
The disaccharide is hydrolysed by a membrane-bound disaccharidase– sucrase
Lactose
Is found in milk
The disaccharide is hydrolysed by a membrane-bound disaccharidase – lactase
Sucrase
A disaccharidase
Hydrolyses the single glycosidic bond in the sucrose molecule. This hydrolysis produces two monosaccharides glucose and fructose
Lactase
A disaccharidase
Hydrolyses the single Glycosodic bond in the lactose molecule. This hydrolysis produces the two monosaccharides glucose and galactose
Lipid digestion in chemical digestion
Lipids are hydrolysed by enzymes called lipases. Lipases are enzymes produced in the pancreas that hydrolyse the Ester bond found in triglycerides to form fatty acids and monoglycerides. A monoglyceride is a glycerol molecule with a single fatty acid molecule attached.
Lipids are firstly split up into tiny droplets called micelles by bile salts, which are produced by the liver. This process is called emulsification and increases the surface area of the lipids so that the action of lipases is speeded up
Protein digestion
Proteins are large, complex molecules that are hydrolysed by a group of enzymes called peptidases (proteases). There are a number of different peptidases:
Endopeptidases, exopeptidases and dipeptidases
endopeptidases
Hydrolyse peptide bonds between amino acids in the central region of a protein molecule forming a series of peptide molecules
Exopeptidases
Hydrolyse the peptide bonds on the terminal amino acids of the peptide molecules formed by endopeptidases. In this way they progressively release dipeptides and single amino acids
Dipeptidases
Hydrolyse the bonds between the two amino acids of a dipeptide. Dipeptidases are membrane-bound, being part of the cell surface membrane of the epithelial cells lining the ileum
Structure of the ileum
The ileum is adapted to the function of absorbing the products of the digestion. The wall of the ileum is folded and possesses fingerlike projections, about 1 mm long, called Villi. They have thin walls, lined with epithelial cells on the other side of which is a rich network of blood capillaries. The villi considerably increase the surface area of the ileum and therefore accelerate the rate of absorption.
Villa are situated at the interface between the lumen (cavity) of the intestines (in effect outside the body) and the blood and other tissue inside the body. They are part of a specialised exchange surface adaptation of the absorption of the products of digestion
How does the villi’s properties increase the efficiency of absorption?
They increase the surface area for diffusion
They are very thin walled, thus reducing the distance over which diffusion takes place
They contain muscle and so are able to move. This helps to maintain diffusion gradients because their movement mixes the contents of the ileum. This insures that, as the products of digestion are absorbed from the food adjacent to the Villi, new material rich in the products of digestion replaces it
They are well supplied with blood vessels so that blood can carry away absorbed molecules and hence maintain a diffusion gradient
The epithelial cells lining the Villi possess microvilli. These are fingerlike projections of the cell surface membrane that further increase the surface area for absorption
