3 Exchange Substances 6.1-7.9 Flashcards
1
Q
Two types of exchange
A
Passively and actively
2
Q
Passive exchange
A
No metabolic energy is required, by diffusion and osmosis
3
Q
Active exchange
A
Metabolic energy is required, by active transport
4
Q
Surface area to volume ratio
A
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.
5
Q
Organism’s have evolved one or of the following features:
A
- 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)
6
Q
Features of specialised exchange surfaces
A
- 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
7
Q
Gas exchange in insects
A
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
8
Q
Respiratory gases move in and out of the tracheal system (insects) in 3 ways:
A
- 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
9
Q
Gas exchange in single celled organisms
A
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
10
Q
Structure of the gills
A
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
11
Q
Why is the counter current flow important for ensuring that max gas exchange is achieved (fish)
A
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
12
Q
Adaptions in leaves for rapid diffusion
A
- 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
13
Q
Gas exchange in leaves
A
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
14
Q
Stomata
A
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
15
Q
Leaf structure
A
Waxy cuticle Upper epidermis Mesophyll cells Air spaces Lower epidermis Guard cells and stomata
16
Q
Limiting water loss in insects adaptations
A
– 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
17
Q
Thick cuticle adaptation
A
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.
18
Q
Rolling up of leaves adaptation
A
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
19
Q
Limiting water loss in plants adaptations
A
Thick cuticle Rolling up of leaves Hairy leaves Stomata in pits or grooves A reduced SA to V ratio of the leaves
20
Q
Hairy leaves adaptation
A
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
21
Q
Stomata in pits or grooves adaptation
A
Traps still, moist air next to the leaf and reduces the water potential gradient
22
Q
A reduced SA:V ratio of the leaves adaptation
A
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
23
Q
The volume of oxygen that has to be absorbed and the volume of carbon dioxide that must be removed are large in mammals because:
A
– 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
24
Q
Mammalian lungs
A
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.
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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
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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
27
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
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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
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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
30
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 ).
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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
32
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
33
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
34
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.
35
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
36
Major parts of the digestive system
```
Oesophagus
Stomach
Ileum
Large intestine
Rectum
Salivary glands
Pancreas
```
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Oesophagus
Carries food from the mouth to the stomach
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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
39
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
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Large intestine
Absorbs water. Most of the water that is absorbed is water from the secretions of many digestive glands
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Rectum
The final section of the intestine. The faeces are stored here before periodically being removed via the anus in a process called egestion
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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
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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
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Two stages digestion takes place in
Physical breakdown and chemical digestion
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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
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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.
47
Which three enzymes are particularly important in chemical digestion?
Carbohydrases, lipases and proteases
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Carbohydrases
Hydrolyse carbohydrates, ultimately to monosaccharides
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Lipases
Hydrolyse lipids (fats and oils) into glycerol and fatty acids
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Proteases
Hydrolyse proteins, ultimately to amino acids
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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
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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
53
In addition to maltose what two other common disaccharides in the diet are hydrolysed?
Sucrose and lactose
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Sucrose
Is found in many natural foods, especially fruit
| The disaccharide is hydrolysed by a membrane-bound disaccharidase– sucrase
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Lactose
Is found in milk
| The disaccharide is hydrolysed by a membrane-bound disaccharidase – lactase
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Sucrase
A disaccharidase
Hydrolyses the single glycosidic bond in the sucrose molecule. This hydrolysis produces two monosaccharides glucose and fructose
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Lactase
A disaccharidase
Hydrolyses the single Glycosodic bond in the lactose molecule. This hydrolysis produces the two monosaccharides glucose and galactose
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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
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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
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endopeptidases
Hydrolyse peptide bonds between amino acids in the central region of a protein molecule forming a series of peptide molecules
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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
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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
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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
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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
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Absorption of amino acids and monosaccharides
The digestion of proteins produces amino acids, while that of carbohydrates produces monosaccharides such as glucose, fructose and galactose. The methods of absorbing these products are the same, namely diffusion and co-transport.
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Absorption of triglycerides
Once formed during digestion, monoglycerides and fatty acids remain in association with the bile salts that initially emulsified the lipid droplets. The structures formed are called micelles. They are tiny, being around 4 to 7 nm in diameter. 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. Here the micelles breakdown, releasing the monoglycerides and fatty acids. As these are nonpolar molecules, they easily diffuse across the cell surface membrane into the epithelial cells
Once inside the epithelial cells, monoglycerides and fatty acids are transported to the endoplasmic reticulum where they are re-combined to form triglycerides. Starting in the endoplasmic reticulum and continuing in the Golgi apparatus, the triglycerides associate with cholesterol and lipoproteins to form structures called chylomicrons. Chylomicrons are special particles adapted for the transport of lipids.
Chylomicrons move out of the epithelial cells by exocytosis. They enter lymphatic capillaries called lacteals that are found at the centre of each Villus. From here, the chylomicrons pass, via lymphatic cells, into the blood system. The triglycerides in the chylomicrons are hydrolysed by an enzyme in the endothelial cells of blood capillaries from where they diffuse into cells
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Haemoglobin molecules
The haemoglobin is a group of molecules found in a wide variety of organisms. Haemoglobin is a protein molecule with the Quaternary structure that has evolved to make it efficient at loading oxygen under one set of conditions but unloading it under a different set of conditions.
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Structure of a haemoglobin molecule
Primary structure – sequence of amino acids in the four polypeptide chains
Secondary structure – in which each of these polypeptide chains is coil into a helix
Tertiary structure – in which each polypeptide chain is folded into a precise shape – an important factor in its ability to carry oxygen
Quaternary structure- in which all four polypeptides are linked together to form an almost spherical molecule. Each polypeptide is associated with a haem group – which contains a ferrous (Fe2+) ion. Each ion can combine with a single oxygen (O2) molecule making a total of four O2 molecules that can be carried by single haemoglobin molecule in humans
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Loading and unloading oxygen
The process by which haemoglobin binds with oxygen is called loading, or associating. In humans this takes place in the lungs
Process by which haemoglobin releases its oxygen is called unloading, or dissociating. In humans this takes place in the tissues
Haemoglobins with a high affinity for oxygen take up oxygen more easily, but release it less easily. Haemoglobins with a low affinity for oxygen take up oxygen less easily, but release it more easily
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Role of haemoglobin
To transport oxygen. To be efficient at transporting oxygen haemoglobin must:
- Readily associate with oxygen at the surface where gas exchange takes place
- Readily dissociate from oxygen at those tissues requiring it
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Oxygen dissociation curves
When haemoglobin is exposed to different partial pressures of oxygen, it doesn’t bind the oxygen evenly. The graph of the relationship between the saturation of haemoglobin with oxygen and the partial pressure of oxygen is known as the oxygen dissociation curve.
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Explanation for the shape of the oxygen dissociation curve
The shape of the haemoglobin molecule makes it difficult for the first oxygen molecule to bind to one of the sites on its four polypeptide subunits because they are closely united. Therefore at low oxygen concentrations, little oxygen binds to haemoglobin. The gradient of the curve is shallow initially.
However, the binding of the first oxygen molecule changes the Quaternary structure of the haemoglobin molecule, causing it to change shape. This change makes it easier for the other subunits to bind to an oxygen molecule. In other words, the binding of the first oxygen molecule induces the other subunits to bind to an oxygen molecule.
It therefore takes a small increase in the partial pressure of oxygen to bind the second oxygen molecule than it did to bind the first one. This is known as positive cooperativity because binding of the first molecule makes binding of the second easier and so on. The gradient of the curve steepens.
The situation changes, however, after the binding of the third molecule. While in theory it’s easier for haemoglobin to bind the fourth oxygen molecule, in practice it is harder. This is simply due to probability. With the majority of the binding sites occupied, it’s less likely that a single oxygen molecule will find an empty site to bind to. The gradient of the curve reduces and the graph flattens off
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Oxygen dissociation curves and affinity
The further to the left the curve, the greater is the affinity of haemoglobin for oxygen (so it loads oxygen readily but unloads it less easily)
The further to the right the curve, the lower is the affinity of haemoglobin for oxygen (so it loads oxygen less readily but unloads it more easily)
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Effects of carbon dioxide on oxygen dissociation curves
Haemoglobin has a reduced affinity for oxygen in the presence of carbon dioxide. The greater the concentration of carbon dioxide, the more readily the haemoglobin releases its oxygen.
This explains why the behaviour of haemoglobin changes in different regions of the body
Dissolved carbon dioxide is acidic and the low pH causes haemoglobin to change shape.
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Loading, transport and unloading of oxygen
At the gas exchange surface carbon dioxide is constantly being removed
The pH is slightly raised due to the low concentration of carbon dioxide
The higher pH changes the shape of haemoglobin into one that enables it to load oxygen readily
The shape also increases the affinity of haemoglobin for oxygen, so it is not released while being transported in the blood to the tissues
In the tissues, carbon dioxide is produced by respiring cells
Carbon dioxide is acidic in solution, so the pH of the blood within the tissues is lowered
The lower pH changes the shape of haemoglobin into one with a lower affinity for oxygen
Haemoglobin releases its oxygen into the respiring tissues
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Behaviour of haemoglobin in different regions of the body
– At the gas exchange surface (e.g. the lungs), the concentration of carbon dioxide is low because it diffuses across the exchange surface and is excreted from the organism. The affinity of haemoglobin for oxygen is increased, which, coupled with the high concentration of oxygen in the lungs, means oxygen is readily loaded by haemoglobin. The reduced carbon dioxide concentration has shifted the oxygen dissociation curve to the left.
– In rapidly respiring tissues (e.g. the muscles), the concentration of carbon dioxide is high. Affinity of haemoglobin for oxygen is reduced, which, coupled with the low concentration of oxygen in the muscles, means that oxygen is readily unloaded from the haemoglobin into the muscle cells. The increased carbon dioxide concentration has shifted the oxygen dissociation curve to the right.
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How is it that the more active a tissue, the more oxygen is unloaded?
The higher the rate of respiration, the more carbon dioxide the tissue produces, the lower the PH, the greater the haemoglobin shape change, the more readily oxygen is unloaded, the more oxygen is available for respiration.
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What happens when a haemoglobin reaches a tissue with a low respiratory rate?
Only unloads one of the four oxygen molecules
| The blood returning to the lungs will therefore contain haemoglobin that is still 75% saturated with oxygen
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What happens when haemoglobin reaches a tissue that is very active?
It unloads three of its four oxygen molecules
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What is the saturation of haemoglobin at atmospheric pressure?
Around 97%
| In practice not all haemoglobin molecules are loaded with their maximum four oxygen molecules
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Lugworm oxygen dissociation curve
The lugworm is not very active and spends almost all its life in a U-shaped borrow. Most the time it is covered by seawater, which it circulates through its borrow. Oxygen defuses into the lugworms blood from the water and it uses haemoglobin to transport oxygen to its tissues.
When the tide goes out, lugworm can no longer circulate a fresh supply of oxygenated water through its borrow. As a result, the water in the burrow contains progressively less oxygen as the lugworm uses it up. The lugworm has to extract as much oxygen as possible from the water in the burrow if it is to survive until the tide covers it again.
The dissociation curve is shifted far the left of that of a human. This means that the haemoglobin of the lugworm is fully loaded with oxygen even when there is little available in its environment.
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Llama oxygen dissociation curve
It lives at high altitudes. At these altitudes the atmospheric pressure is lower and so the partial pressure of oxygen is also lower. It is therefore difficult to load haemoglobin with oxygen.
Llamas also have a type of haemoglobin has a higher affinity for oxygen than human haemoglobin. In other words it is shifted to the left of that of human haemoglobin
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Why do large organisms have a transport system?
With increasing size, the surface area to volume ratio decreases to a point where the needs of the organism cannot be met by the body surface alone. Specialist exchange surfaces are therefore required to absorb nutrients and respiratory gases, and remove excretory products. These exchange surfaces are located in specific regions of the organism. The transport system is required to take materials from cells to exchange surfaces and vice versa. Materials have to be transported between exchange surfaces and the environment. They also need to be transported between different parts of the organism. As organisms have evolved into larger and more complex structures, the tissues and organs of which they are made have become more specialised and dependent upon one another. This makes the transport system all the more essential
84
Whether or not there is a specialised transport medium, and whether or not it is circulated by a pump, depends on what two factors?
– The surface area to volume ratio
– how active the organism is
The lower the SA:V, and the more active the organism, the greater is the need for a specialist transport system with a pump
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Features of transport systems
– A suitable medium in which to carry materials, for example blood. This is normally a liquid based on water because water readily dissolves substances and can be moved around easily, but can be a gas such as air breathed in and out of the lungs.
– A form of mass transport in which the transport medium is moved around in bulk over larger distances- more rapid than diffusion
-A closed system of tubular vessels that contains the transport medium and forms a branching network to distribute it to all parts of the organism.
-A mechanism for moving the transport medium within vessels. This requires a pressure difference between one part of the system and another.
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Circulatory Systems in mammals
Mammals have a closed, double circulatory system in which blood is confined to vessels and passes twice through the heart for each complete circuit of the body.
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Why does the blood pass through the heart twice?
because, when blood is passed through the lungs, its pressure is reduced. If it were to pass immediately to the rest of the body its low pressure would make circulation very slow.
Blood is therefore returned to the heart to boost its pressure before being circulated to the rest of the tissues.
As a result, substances are delivered to the rest of the body quickly, which is necessary as mammals have a high body temp and hence a high rate of metabolism.
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The vessels that make up the circulatory system of mammals are divided into what three types?
Arteries
Veins
Capillaries
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Structure of the human heart
Two separate pumps laying side by side. Pump on the left deals with oxygenated blood from the lungs, while the right deals with deoxygenated blood from the body.
Each has two chambers: atrium and ventricle
The ventricles pump blood away from the heart and into the arteries. The atria receives blood from the veins
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Atrium
Thin-walled and elastic and stretched as it collects blood
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Ventricle
Has a much thicker muscular wall as it has to contract strongly to pump blood some distance, either to the lungs or the rest of the body
The right pumps blood only to the lungs and has a thinner muscular wall than the left. Left muscular wall to contract to create enough pressure to pump blood to the rest of the body
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Valve that prevents backflow of blood from left ventricle to left atrium
Left atrioventricular (bicuspid) valve
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Valve that prevents backflow of blood from right ventricle to right atrium
Right atrioventricular (tricuspid) valve
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Vessels connecting the heart to the lungs
Pulmonary vessels
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The vessels connected to the four chambers of the heart
Pulmonary artery
Vena cava
Pulmonary vein
Aorta
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Aorta
Connected to the left ventricle and carried oxygenated blood to all parts of the body except the lungs
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Vena cava
Connected to the right atrium and brings deoxygenated blood back from the tissues of the body (except the lungs)
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Pulmonary artery
Connected to the right ventricle and carries deoxygenated blood to the lungs, where its oxygen is replenished and its carbon dioxide is removed. Unusually for an artery, it carries deoxygenated blood
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Pulmonary vein
Connected to the left atrium and brings oxygenated blood back from the lungs. Unusually for a vein, it carries oxygenated blood
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Supplying the heart muscle with oxygen
The heart muscle is supplied by its own blood vessels, called coronary arteries, which branch off the aorta shortly after it leaves the heart. Blockage of these arteries, for example by a blood clot, leads to myocardial infarction, or heart attack, because an area of the heart muscle is deprived of blood and therefore oxygen also. The muscle cells in this region are unable to respire and so die
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Myocardial infarction
Blockage of coronary arteries, for example by a blood clot, leads to myocardial infarction, or heart attack, because an area of the heart muscle is deprived of blood and therefore oxygen also. The muscle cells in this region are unable to respire and so die
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Risk factors associated with cardiovascular disease
Smoking
High blood pressure
Blood cholesterol
Diet
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Smoking associated with cardiovascular disease
- Carbon monoxide: combines easily but irreversibly with haemoglobin in rbc to form carboxyhaemoglobin. It thereby reduces the oxygen carrying capacity of the blood. To supply the equivalent quantity of oxygen to the tissues, the heart works harder. Leads to raised blood pressure that increases the risk of coronary heart disease and strokes. Also the reduction in the oxygen carrying capacity of blood means that it may be insufficient to supply the heart muscle during exercise. This leads to chest pains or myocardial infarction (heart attack)
- Nicotine stimulates the production of the hormone adrenaline, which increases heart rate and raises blood pressure. As a consequence there is a greater risk of smokers suffering coronary heart disease or a stroke. Nicotine makes platelets in blood more ‘sticky’ and this leads to a higher risk of thrombosis and hence of strokes and myocardial infarction
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What two main constituents of tobacco smoke increase the likelihood of heart disease?
Carbon monoxide
| Nicotine
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High blood pressure associated with cardiovascular disease
As there is already a higher pressure in the arteries, the heart must work harder to pump blood into them and is therefore more prone to failure
Higher blood pressure within the arteries means that they are more likely to develop an aneurism (weakening of the wall) and burst, causing haemorrhage
To resist the high pressure within them, the walls of the arteries tend to become thickened and may harden, restricting the flow of blood
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Blood cholesterol associated with cardiovascular disease
Cholesterol is an essential component of membranes. As such, it is an essential biological molecule which must be transported in the blood. It is carried in the plasma as tiny spheres of lipoproteins. There are two main types:
- High-density lipoproteins, which remove cholesterol from tissues and transport it to the liver for excretion. They help protect arteries against heart disease
- low-density lipoproteins, which transport cholesterol from the liver to the tissues, including the artery walls, which they infiltrate, leading to the development of atheroma, which may lead to heart disease
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Diet associated with cardiovascular disease
High levels of salt raise blood pressure
| High levels of saturated fat increase low-density lipoprotein Levels and hence blood cholesterol conc
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Cardiac cycle
The heart undergoes a sequence of events that is repeated in humans around 70 times each minute when at rest
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What are the two main phases to the beating of the heart?
Contraction (systole)
| Relaxation (diastole)
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Relaxation of the heart (diastole)
Blood returns to the atria of the heart through the pulmonary vein (from the lungs) and the vena cava (from the body). As the atria fill, the pressure in them rises. When this pressure exceeds that in the ventricles, the atrioventricular valves open allowing the blood to pass into the ventricles. The passage of blood is aided by gravity. The muscular walls of both the atria and ventricles are relaxed at this stage. The relaxation of the ventricle walls causes them to recoil and reduces the pressure within the ventricle. This causes the pressure to be lower than that in the aorta and pulmonary artery, and so the semilunar valves in the aorta and pulmonary artery close, accompanied by the characteristic dub sound of the heartbeat
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Two stages of systole
Contraction of the atria (atrial systole)
| Contraction of the ventricles (ventricular systole)
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Contraction of the atria (atrial systole)
The contraction of the atria walls, along with the recoil of the relaxed ventricle walls, forces the remaining blood into the ventricles from the atria. Throughout this stage the muscles of the ventricle walls remains relaxed
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Contraction of the ventricles (ventricular systole)
After a short delay to allow the ventricles to fill with blood, their walls contract simultaneously. This increases the blood pressure within them, forcing shut the atrioventricular valves and preventing backflow of blood into the atria. The lub sound of these valves closing is a characteristic of the heartbeat. With the atrioventricular valves closed, the pressure in the ventricles rises further. Once it exceeds that of the aorta and pulmonary artery, blood is forced from the ventricles into these vessels. The ventricles have thick muscular walls which means they contract forcefully. This creates the high-pressure necessary to pump blood around the body. The thick wall of the left ventricle has to pump blood to the extremities of the body while the relatively thin the wall of the right ventricle has to pump blood to the lungs
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What valves are in control of bloodflow?
Atrioventricular valves
Semilunar valves
Pocket valves
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Atrioventricular valves
Between the left atrium and ventricle and the right atrium and ventricle. These prevent back flow of blood when contraction of the ventricles means that ventricular pressure exceeds atrial pressure. Closure of these valves ensures that, when the ventricles contract, blood within them moves to the aorta and pulmonary artery rather than back to the atria
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Semilunar valves
In the aorta and pulmonary artery. These prevent back flow of blood into the ventricles when the pressure in these vessels exceeds that in the ventricles. This arises when the elastic walls of the vessels recoil increasing the pressure within them and when the ventricles relax reducing the pressure within the ventricles
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Pocket valves
In veins that occur throughout the venous s system. These ensure that when the veins are squeezed, e.g. when skeletal muscles contract, blood flows back towards the heart rather than away from it
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Design of valves
They are made up of a number of flaps of tough, but flexible, fibrous tissue, which are cusp-shaped, in other words like deep bowls. When pressure is greater on the convex side of these cusps, rather than on the concave side, they move apart to let bloodflow pass between the cusps. When pressure is greater on the concave side than on the convex side, blood collects within the bowl of the cusps. This pushes them together to form a tight fit that prevents the passage of blood
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Cardiac output
Volume of blood pumped by one ventricle of the heart in one minute. Is usually measured in decimetres cubed per minute and depends upon two factors:
– the heart rate (the rate at which the heart beats)
– the stroke volume (volume of blood pumped out at each beat)
Cardiac output = heart rate X stroke volume
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Changes in ventricular pressure
It’s low at first, but gradually increases as the ventricles fill with blood as the atria contract. The atrioventricular valves close and pressure rises dramatically as the thick muscular walls of the ventricles contract. As pressure rises above that of the aorta, blood is forced into the aorta past the semilunar valves. Pressure falls as the ventricles empty and the walls relax
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Changes in atrial pressure
It’s always relatively low because the thin walls of the atrium cannot create much force. It is highest when they are contracting, but drops when the atrioventricular valve closes and its walls relax. The atria then fill with blood, which leads to a gradual buildup of pressure until a slight drop when the atrioventricular valve opens and some blood moves into the ventricle
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Changes in aortic pressure
Rises when ventricles contract as blood is forced into the aorta. It then gradually falls, but never below around 12 kPa, because of the elasticity of its walls, which creates a recoil action – essential if blood is to be constantly delivered to the tissues. The recoil produces a temporary rise in pressure at the start of the relaxation phase.
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Changes in ventricular volume
Rises as the atria contract and the ventricles fill with blood, and then drops suddenly as blood is forced out into the aorta when the semilunar valve open. Volume increases again as the ventricles fill with blood
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Electrocardiogram
During the cardiac cycle, the heart undergoes a series of electrical current changes. These are related to the waves of electrical activity created by the sinoatral node and the hearts response to these. If displayed on a cathode ray oscilloscope, these changes can produce a trace known as an electrocardiograph. Doctors can use this trace to provide a picture of the heart’s electrical activity and hence his health.
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Arteries
Carry blood away from the heart and into arterioles
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Arterioles
Smaller arteries that control bloodflow from arteries to capillaries
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Capillaries
Are tiny vessels that link arterioles to veins
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Veins
Carry blood from capillaries back to the heart
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What is the basic layered structure of arteries, arterioles and veins?
– Tough fibrous outer layer that resists pressure changes from both within and outside
– muscle layer that can contract and so control the flow of blood
– elastic layer that helps to maintain blood pressure by stretching and springing back (recoiling)
– Thin inner lining (endothelium) that is smooth to reduce friction and thin to allow diffusion
– lumen that is not actually a layer but the central cavity of the blood vessel through which the blood flows
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Difference between arterioles and arteries?
Arterioles are smaller in diameter and have a relatively larger muscle layer and lumen.
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Artery structure related to function
– The muscle layer is thick compared to the veins. This means smaller arteries can be constricted and dilated in order to control the volume of blood passing through them
– The elastic layer is relatively thick compare to the veins because it’s important that blood pressure in the arteries is kept high if blood is to reach the extremities of the body. Elastic wall is stretched at each beat of heart. It then springs back when the heart relaxes. The stretch and recoil action helps to maintain high-pressure and smooth pressure surges created by the beating of the heart
– The overall thickness of the wall is great. This also resists the vessel bursting under pressure
–there are no valves because blood is under constant high-pressure due to the heart pumping blood into the arteries. Therefore tends not flow backwards
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Arteriole structure related to function
– The muscle layer is relatively thicker than in arteries. The contraction of this muscle layer allows constriction of the lumen of the arteriole. This restricts the flow of blood and so controls its movement into the capillaries that supply the tissues with blood.
– The elastic layer is relatively thinner than in arteries because blood pressure is lower
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Vein structure related to function
– The muscle layer is relatively thin compared to arteries because veins carry blood away from tissues and therefore the constriction and dilation cannot control the flow of blood to the tissues
– the elastic layer is relatively thin compared to arteries because the low pressure of blood within the veins will not cause them to burst and pressure is too low to create a recoil action.
– The overall thickness of the wall is small because there is no need for a thick wall as the pressure within the veins is low. It also allows them to be flattened easily, aiding the flow of blood within them
– There are valves at intervals throughout to ensure blood does not flow backwards, which it might otherwise do because the pressure is so low. When body muscles contract, veins are compressed, pressurising the blood within them. Valves insure that this pressure directs the blood in One Direction only: towards the heart
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Capillary structure related to function
– The walls consist mostly of the lining layer, making them extremely thin, so the distance over which diffusion takes place is short. This allows for rapid diffusion of materials between the blood and the cells.
– They are numerous and highly branched, providing a large surface area for exchange
– They have a narrow diameter and so permeate tissues, which means that no cell is far from a capillary and there is a short diffusion pathway.
– the lumen is so narrow that red blood cells are squeezed flat against the side of the capillaries. This brings them even closer to the cells to which they supply oxygen. This again reduces the diffusion distance.
– There are spaces between the lining (endothelial) cells that allow white blood cells to escape in order to deal with infections within tissues.
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Tissue fluid
Watery liquid that contains glucose, amino acids, fatty acids, ions in solution and oxygen.
They supply all of the substances to the tissues.
In return, it receives carbon dioxide and other waste materials from the tissues.
Tissue fluid is therefore the means by which materials are exchanged between blood and cells and, as such, it bathes all of the cells of the body.
It’s formed from blood plasma, and the composition of blood plasma is controlled by various homoeostatic systems.
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Tissue fluid formation
Blood pumped by the heart passes along arteries, then the narrower arterioles and, finally, the even narrower capillaries. Pumping by the heart creates a pressure, called hydrostatic pressure at the arterial end of the capillaries. This hydrostatic pressure causes tissue fluid to move out of the blood plasma. The outward pressure is however opposed by two other forces:
-Hydrostatic pressure of the tissue fluid outside the capillaries which resists outward movement of liquid
– The lower water potential of the blood, due to the plasma proteins, that causes water to move back into the blood within capillaries.
However, the combined effect of all these forces is to create an overall pressure that pushes tissue fluid out of the capillaries at the arterial ends.
This pressure is only enough to force small molecules out of the capillaries, leaving all the cells and proteins in the blood because these are too large to cross the membranes. This type of filtration under pressure is called ultrafiltration
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Return of tissue fluid to the circulatory system
Once tissue fluid has exchanged metabolic materials with the cells it bathes, it is returned to the circulatory system. Most tissue fluid returns to the blood plasma directly by the capillaries.
Not all the tissue fluid can return to the capillaries; the remainder is carried back via the lymphatic system.
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Return of tissue fluid to the circulatory system via the capillaries
The loss of the tissue fluid from the capillaries reduces the hydrostatic pressure inside them
As a result, by the time the blood has reached the venous end of the capillary network its hydrostatic pressure is usually lower than that of the tissue fluid outside.
Therefore tissue fluid is forced back into the capillaries by the higher hydrostatic pressure outside
In addition, the plasma has lost water and still contains proteins. It therefore has a lower water potential than the tissue fluid
As a result, water leaves the tissue by osmosis down a water potential gradient
The tissue fluid has lost much of its oxygen and nutrients by diffusion into the cells that it bathed, but it has gained carbon dioxide and waste materials in return
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Return of tissue fluid to the circulatory system via the lymphatic system
This is a system of vessels that begin in the tissues. Initially they resemble capillaries (except they have dead ends), but they gradually merge into larger vessels that form a network throughout the body. These larger vessels drain their contents back into the bloodstream by two ducts that join veins close the heart
The contents of the lymphatic system (lymph) are not moved by the pumping of the heart. Instead they moved by:
– hydrostatic pressure of the tissue fluid that has left the capillaries
– Contraction of body muscles that squeeze the lymph vessels – valves in the lymph vessels ensure that the fluid inside them moves away from the tissues in the direction of the heart
– contraction of body muscles that squeeze the length vessels – valves
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Xylem vessels
Hollow, thick-walled tubes that transport water
| Walls contain lignin
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Transpiration
Process where the main force that pulls water through the xylem vessels in the stem of a plant is the evaporation of water from leaves
Energy for this is supplied by the sun. It’s therefore passive
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Movement of water out through stomata
The humidity of the atmosphere is usually less than that of the air spaces next to the stomata. As a result there us a water potential gradient from the air spaces through the stomata to the air. Provided the stomata are open, water vapour molecules diffuse out of the air spaces into the surrounding air. Water lost by diffusion from the air spaces is replaced by water evaporating from the cell walls of the surrounding mesophyll cells. By changing the size of the stomatal pores, plants can control their rate of transpiration.
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Movement of water across the cells of a leaf
Water is lost from mesophyll cells by evaporation from their cell walls to the air spaces of the leaf. This is replaced by water reaching the mesophyll cells from the xylem either via cell walls or via the cytoplasm. In the case of the cytoplasmic route the water movement occurs because:
-mesophyll cells lose water to the air spaces by evaporation due to heat supplied by the sun
-these cells now have a lower water potential and so water enters by osmosis from neighbouring cells
– loss of water from these neighbouring cells lowers the water potential
– they, in turn, take in water from their neighbours by osmosis
In this way, a water potential gradient is established that pulls water from the xylem, across the leaf mesophyll, and finally out into the atmosphere.
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Movement of water up the stem in the xylem
The main factor that is responsible for the movement of water up the xylem, from the roots to the leaves, is cohesion-tension.
- water evaporates from mesophyll cells due to heat from the sun leading to transpiration
- water molecules form hydrogen bonds between one another and hence tend to stick together. This is known as cohesion
- water forms a continuous, unbroken column across the mesophyll cells and down the xylem
- as water evaporates from the mesophyll cells in the leaf into the air spaces beneath the stomata, more molecules of water are drawn up behind it as a result of cohesion
- a column of water is therefore pulled up the xylem as a result of transpiration. This is called the transpiration pull
- transpiration pull puts the xylem under tension, that is, there is a negative pressure within the xylem, hence the name cohesion-tension theory
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What pieces of evidence support the cohesion-tension theory?
- change in the diameter of tree trunks according to the rate of transpiration. During the day, when transpiration is at its greatest, there is more tension (more neg pressure) in the xylem. This pulls the walls of the xylem vessels inwards and causes the trunk to shrink in diameter.
- if a xylem vessel is broken and air enters it, the tree can no longer draw up water. This is because the continuous column of water is broken and so the water molecules can no longer stick together
- when a xylem vessel is broken, water doesn’t leak out, as would be the case if it were under pressure. Instead air is drawn in, which is consistent with it being under tension
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Measurement of water uptake using a potometer
About 99% of the water taken up by a plant is lost during transpiration, which means that the rate of uptake is almost the same as the rate at which transpiration is occurring. We can then measure water uptake by the same shoot under different conditions e.g. various humidities, wind speeds or temps.
- a leafy shoot is cut under water. Care is takes to not get water on the leaves
- the potometer is filled completely with water, making sure there are no air bubbles
- using a rubber tube, the leafy shoot is fitted to the potometer under water
- the potometer is removed from under the water and all joints are sealed with water proof jelly
- an air bubble is introduced into the capillary tube
- the distance moved by the air bubble in a given time is measured a number of times and the mean is calculated
- using the mean value, the volume of water loss is calculated
- th volume of water loss against the time in minutes can be plotted on a graph
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Specialised plant cells
Root hair cell
| Xylem vessels
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Root hair cell
The exchange surfaces in plants that are responsible for the absorption of water by osmosis and mineral ions by active transport
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Translocation
The process by which organic molecules and some mineral ions are transported from one part of the plant to another
In flowering plants, the tissue that transports biological molecules is called phloem
Can happen in any direction
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Phloem
Transports organic molecules and mineral ions via translocations
Made up of sieve Tube elements, long thin structures arranged end to end. The end walls are perforated to form sieve plates.
Associated with the sieve tube elements are cells called companion cells.
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Transport of organic substances in the phloem
Happens via translocation.
Having produced sugars during photosynthesis, the plant transports them from the sites of production, known as sources, to the place where they will be used directly or stored for future use – known as sinks.
As sinks can be anywhere in a plant, the translocation of molecules in phloem can be in either direction
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What does phloem transport?
Organic molecules- sucrose and amino acids
| Inorganic ions- potassium, chloride, phosphate and magnesium ions
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Mechanism of translocation
Mass flow theory
Divided into three phases:
1. Transfer of sucrose into sieve elements from photosynthesising tissue
2. Mass flow of sucrose through sieve tube elements
3. Transfer of sucrose from the sieve tube elements into storage or other sink cells
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Translocation- 1. Transfer of sucrose into sieve elements from photosynthesising tissue
- sucrose is manufactured form the products of photosynthesis in cells with chloroplasts
- the sucrose diffuses down a conc gradient by facilitated diffusion from the photosynthesising cells into companion cells
- hydrogen ions are actively transported from companion cells into the spaces within cells walls using ATP
- these hydrogen ions then diffuse down a conc gradient through carrier proteins into the sieve tube elements
- sucrose molecules are transported along with the hydrogen ions in a process known as co-transport proteins
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Translocation-2. Mass flow of sucrose through sieve tube elements
- the sucrose produced by photosynthesising cells (source) is actively transported into the sieve tubes
- this causes the sieve tubes to have a lower (more negative) water potential
- as the xylem has a much higher (less neg) water potential, water moves from the xylem into the sieve tubes by osmosis, creating a high hydrostatic pressure within them
- at the respiring cells (sink), sucrose is either used up during respiration or converted to starch for storage
- these cells therefore have a low sucrose content and so sucrose is actively transported into them from the sieve tubes lowering their water potential
- due to this lowered water pot, water also moves into these respiring cells, from the sieve tubes, by osmosis
- the hydrostatic pressure of the sieve tubes in this region is therefore lowered
- as a result of water entering the sieve tube elements at the source and leaving at the sink, there is a high hydrostatic pressure at the source and a low one at the sink
- there is therefore a mass flow of sucrose solution down this hydrostatic gradient in the eve tubes
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Translocation-3. Transfer of sucrose from the sieve tube elements into storage or other sink cells
The sucrose is actively transported by companion cells, out of the sieve tubes and into the sink cells
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Ringing experiments
A section of the outer layers (protective layer and phloem) is removed around the complete circumference of a woody stem while it is still attached to the rest of the plant. After a period of time, the region of the stem immediately above the missing ring of tissue is seen to swell. Samples of the liquid that has accumulated in this swollen region are found to be rich in sugars and other dissolved organic substances. Some non-photosynthetic tissues in the region below the ring (towards the root) are found to wither and die, while those above the ring continue to grow
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Ringing experiments observations suggest that removing the phloem around the stem has led to:
- the sugars of the phloem accumulating above the ring, leading to swelling in this region
- the interruption of flow of sugars to the region below the ring and the death of tissues in this region
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Conclusion from ringing experiments
The phloem is responsible for translocations sugars in plants. As the ring of tissue removed had not extended into the xylem, its continuity had not been broken
If it were the tissue responsible for translocating sugars you would not have expected sugars to accumulate above the ring nor tissues below it to die
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Tracer experiments
Radioactive isotopes are useful for tracing the movement of substances in plants. E.g. the isotope 14C can be used to make radioactively labelled carbon dioxide (14CO2). If a plant is then grown in an atmosphere containing 14CO2, the 14C isotope will be incorporated into the sugars produced during photosynthesis.
These radioactive sugars can be traced as they move within the plant using autoradiography
When placing a cross section of plant stem on X-ray film, film becomes blackened where it has been exposed to the radiation produced by the 14C in the sugars. The black regions are found to correspond to where phloem tissue is in the stem
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Evidence that translocation of organic molecules occurs in phloem
- when phloem is cut, a solution of organic molecules flow out
- plants provided with radioactive CO2 can be shown to have radioactively labelled carbon in phloem after a short time
- aphids are a type of insect that feed on plants. They have needle-like mouth parts which penetrate the phloem. They can therefore be used to extract the contents of the sieve tubes
- the removal of a ring of phloem from around the whole circumference of a stem leads to the accumulation of sugars above the ring and their disappearance below it
