Cells need to take in useful substances and remove other substances, such as waste, in order to function effectively. Exchange of materials occurs between the cell and its surrounding environment across the cell membrane. Depending on the circumstances, this is achieved via three transport processes - diffusion, osmosis and active transport. To make exchange as efficient as possible, larger organisms have evolved specialised exchange surfaces.
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Diffusion is the process by which particles of a substance spread out from each other, moving from a region where they are in high concentration to a region of low concentration. In the same way as a ball will roll from a high point to a low point down a gradient, particles of a substance will move down a concentration gradient until they are evenly spread.
In order to do this, particles of a substance must be free to move. This is the case for particles of a gas or particles of a dissolved substance.
Diffusion allows substances to pass into or out of cells across the cell membrane - but they must be dissolved and there must be a concentration gradient present between the solutions on either side of the cell membrane.
Diffusion is a very important exchange process which is widely used in living organisms.
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Active transport is a transport process which is used to move dissolved molecules from low concentration to high concentration, against a concentration gradient. This process requires energy from respiration in order to take place.
Active transport is carried out by a series of protein carriers within the cell membrane. These have a binding site, allowing a specific dissolved substance to bind to the side of the membrane where it is at a lower concentration.
Energy from respiration then changes the shape of the protein so that it releases the substance onto the other side of the membrane.
Active transport has the advantage of allowing cells to absorb dissolved substances from very dilute solutions, which is otherwise an impossible process.
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Active transport 2
Root hair cells in plant roots use active transport to absorb mineral ions (such as nitrates) from the soil - even though there are lower concentrations of minerals in the soil than there are within the root hair cell.
Small intestine villi cells use active transport alongside diffusion to maximise the absorption of glucose and other substances, eg minerals.
Differences between diffusion and active transport;
The table below shows the major differences between active transport and diffusion.
Transports dissolved substances from high to low concentration
Transports dissolved substances from low to high concentration
Requires no additional energy input
Requires energy from respiration
Does not necessarily require protein carriers in the cell membrane
Requires protein carriers in the cell membrane
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Osmosis only applies to the movement of water into or out of a cell. The definition of osmosis is the movement of water molecules from a dilute solution (with a high proportion of water molecules) to a more concentrated solution (with a low proportion of water molecules) across a partially permeable membrane. A partially permeable membrane allows small, soluble molecules like water to pass through it freely - but prevents larger molecules from doing so. In a cell, the cell membrane acts as a partially permeable membrane.
On the left-hand side of the partially permeable membrane is pure water. On the right-hand side is a solution which is more concentrated, containing minerals, for example. Water molecules move across the membrane at random in both directions, but the higher concentration of water molecules on the left-hand side means that more water moves by osmosis from left to right than moves in the opposite direction. The overall (net) effect is that water levels on the left drop, whilst they rise on the right due to the overall movement of water from left to right. Eventually, the concentrations of water on either side of the partially permeable membrane become equal. This means that the molecules of water moving in either direction are equal, so effectively there is no overall (net) movement of water.
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the effect of osmosis on cells
If a cell has a more dilute solution inside it than outside it, then the overall movement of water is out of the cell. In animal cells this would cause the cell to shrivel up, whilst in plant cells this would cause the membrane and cytoplasm to shrink away from the cell wall, causing the plant cell to become flaccid (limp).
If a cell has a more concentrated solution inside it than outside it, then the overall movement of water is into the cell. In plant cells this causes the cell to begin to swell, and the cytoplasm and membrane push against the cell wall. The strong cell wall then resists further expansion, supporting the cell which becomes turgid (fully inflated).
However, animal cells do not have a cell wall so any large movement of water into the cell causes it to burst. For this reason, it is important that the concentration of water outside cells is constant.
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All living organisms rely on exchanges with the environment to survive. However, diffusion only works efficiently if the distance over which the substances have to diffuse is small and the organism has a large surface area compared to its volume. This is the case for small organisms.
For larger, more complex organisms – which have a small surface area:volume ratio and a bigger distance from the surface to the cells inside the body - diffusion alone is insufficient to meet the needs of all cells.
As larger organisms evolved, specialised organ systems - with surfaces across which substances could be exchanged efficiently - also evolved. These specialised organ systems were needed in order to transport substances around the organisms.
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Organ systems 2
Adaptations of exchange surfaces
Common features of exchange surfaces include:
- having a large surface area for greater exchange – achieved by having a folded surface
- having a thin exchange surface for a short diffusion distance
Animals also further maximise the efficiency of exchange by:
- having a good blood supply due to an extensive capillary network in exchange organs – this distributes the exchanged materials to all cells of the body and can help to maintain a concentration gradient
- maintaining ventilation of the surface (at gas exchange surfaces) through breathing - this always ensures that a concentration gradient is maintained
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Exchange in the digestive system
Villi are folds within the wall of the small intestine across which digested food molecules are exchanged between the gut and the bloodstream. This exchange takes place by diffusion and active transport.
Villi are adapted for the maximum absorption of digested food molecules because:
- the folded villi greatly increase the surface area of the intestine
- the villi are made of a single layer of thin cells
- beneath the villi is an extensive blood capillary network to distribute the absorbed food molecules
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Sports drinks; During extended periods of exercise, an athlete’s body changes:
- the athlete uses up much of the glucose in their body during respiration
- the athlete generates heat as they respire more
- the athlete sweats more to try to cool themselves down (this sweating results in the loss of water and mineral ions, eg sodium)
Maintaining the correct balance of mineral ions is essential for cells to function efficiently and effectively. If the water and ion content of the body changes, it can cause too much water to move into or out of its cells - possibly leading to them becoming damaged. During prolonged exercise, not only are ions and water lost, but the loss of water occurs at a faster rate than the loss of ions - which can disturb this balance and lead to cells dehydrating.It is therefore important that athletes replace the lost water and mineral ions and replenish the glucose which has been used during respiration.Water is only able to rehydrate the body. It does not replace the lost ions and glucose. Most soft drinks contain water, sugar and mineral ions, but not at the concentrations which are most effective at maintaining an athlete’s performance.However, sports drinks contain water, sugar and mineral ions at levels which are most effective at maintaining performance - rehydrating the athlete as well as replacing the glucose and maintaining the correct ion/water balance for cells to function effectively. This helps the athlete to continue exercising for longer.
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Evaluating sports drinks
Sports drinks manufacturers often make claims about the performance benefits of using their branded sports drinks, but it is important that these claims are evaluated based on valid data from controlled trials of a large sample of athletes.
Different manufacturers put slightly different amounts of sugar and mineral ions in their sports drinks, and therefore each brand will potentially have differing effects on an athlete’s performance.
Gaseous exchange in the lungs
To supply the cells of our body with a continuous supply of oxygen for respiration and to remove the carbon dioxide generated by respiration, we have evolved a specialised exchange surface for gas exchange within the breathing system. The efficiency of this system is further improved by ventilation of this exchange surface and by having an efficient blood supply - both of which maintain a suitable concentration gradient.
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The lungs are part of the breathing system which is adapted for two functions:
ventilation – the movement of air into and out of the lungs
gas exchange – the 'swapping’ of gases between the alveolar air and the blood
The lungs are located within the upper part of your body called the thorax. They are surrounded by the ribcage (which protects them) and in between the ribs are intercostal muscles which play a role in ventilating the lungs. Beneath the lungs is a muscular sheet called the diaphragm. This separates the lungs from the abdomen of the body and also plays a role in ventilating the lungs.
Within the lungs is a network of tubes through which air is able to pass. Air is firstly warmed, moistened and filtered as it travels through the mouth and nasal passages. It then passes through the trachea and down one of the two bronchi and into one of the lungs. After travelling into the many bronchioles, it finally passes into some of the millions of tiny sacs called alveoli, which have the specialised surfaces for gas exchange.
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When you inhale:
The intercostal muscles contract, expanding the ribcage outwards and upwards.
The diaphragm contracts, pulling downwards to increase the volume of the chest.
Pressure inside the chest is lowered and air is sucked into the lungs.
When you exhale:
The intercostal muscles relax, the ribcage drops inwards and downwards.
The diaphragm relaxes, moving back upwards, decreasing the volume of the chest.
Pressure inside the chest increases and air is forced out.
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When a person stops breathing on their own, mechanical ventilation can be used until the patient is able to recover and again breathe independently. This is done by machines called ventilators - which fall into two main types:
Negative pressure ventilators - the patient is placed in an airtight machine from the neck down, and a vacuum is created around the thorax. This creates a negative pressure, which leads to the expansion of the thorax and a decrease in pressure. As a result, air is drawn into the lungs. As the vacuum is released, the elasticity of the lungs, diaphragm and chest wall cause exhalation.
Positive pressure ventilators - air is forced into the lungs through a tube which is inserted into the trachea. As the ventilator pumps air in, the lungs inflate. When the ventilator stops, the elasticity of the lungs, diaphragm and chest wall cause exhalation.
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The table below lists some of the pros and cons of using artificial ventilators.
Artificial ventilatorUsesAdvantagesDisadvantages Negative pressure Developed and used from the 1920s to treat polio sufferers
Effective at treating many polio patients over the years
Patient is confined to the machine
The vacuum on full-body machines can affect the abdomen, leading to the pooling of blood in lower parts of the body
Positive pressure Used extensively since the 1950s
Useful during operations, where surgeons need access to the body
- Effective at ventilating the lungs
Long-term ventilation requires the tube to be surgically inserted into the trachea through the neck
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Within the alveoli, an exchange of gases takes place between the gases inside the alveoli and the blood.
Blood arriving in the alveoli has a higher carbon dioxide concentration which is produced during respiration by the body’s cells. However, the air in the alveoli has a much lower concentration of carbon dioxide, meaning there is a concentration gradient which allows carbon dioxide to diffuse out of the blood and into the alveolar air.
Similarly, blood arriving in the alveoli has a lower oxygen concentration (as it has been used for respiration by the body’s cells), while the air in the alveoli has a higher oxygen concentration. Therefore, oxygen moves into the blood by diffusion and combines with the haemoglobin in red blood cells to form oxyhaemoglobin.
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To maximise the efficiency of gas exchange, the alveoli have several adaptations:
They are folded, providing a much greater surface area for gas exchange to occur.
The walls of the alveoli are only one cell thick. This makes the exchange surface very thin - shortening the diffusion distance across which gases have to move.
Each alveolus is surrounded by blood capillaries which ensure a good blood supply. This is important as the blood is constantly taking oxygen away and bringing in more carbon dioxide - which helps to maintain the maximum concentration gradient between the blood and the air in the alveoli.
Each alveolus is ventilated, removing waste carbon dioxide and replenishing oxygen levels in the alveolar air. This also helps to maintain the maximum concentration gradient between the blood and the air in the alveoli.
Exchange system in plants
Like all living organisms, plants must exchange materials with their environment. These exchanges include absorbing water and minerals from the soil and absorbing carbon dioxide from the air for photosynthesis. Therefore plants have specialised exchange surfaces which maximise the efficiency of these exchanges.
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Exchanges in the roots
The role of the roots is to absorb water from the soil by osmosis and dissolve mineral ions from the soil by active transport.The mineral ions are transported around the plant where they serve a variety of functions, whilst the water is transported to be used as a reactant in photosynthesis, as well as to cool the leaves by evaporation and support the leaves and shoots by keeping cells rigid.To maximise the efficiency of absorption, roots have specialised cells called root hair cells which are found just behind the tip of the root.
Root hair cells have several adaptations:
the tube-like protrusion provides a greater surface area across which water and mineral ions can be exchanged
the tube-like protrusion can penetrate between soil particles, reducing the distance across which water and mineral ions must move
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Diffusion in the leaves
One of the main functions of leaves is as a major site of photosynthesis – to produce glucose from water and carbon dioxide with the input of energy from sunlight.
To perform this function effectively, leaves are adapted to maximising the absorption of carbon dioxide and sunlight.
Larger surface area to absorb light and carbon dioxide
Short diffusion distance for carbon dioxide to diffuse into leaf cells, and oxygen to diffuse out of leaf cells
Can open to allow diffusion of carbon dioxide into the leaf from the atmosphere, and the diffusion of oxygen and water vapour out of the leaf