The primary function of the lungs is to allow gas exchange to occur. Oxygen gas can diffuse into the blood from the air in the lungs. Oxygen of course is needed for the process of aerobic respiration that is happening in every cell all the time. Aerobic respiration produces carbon dioxide as a waste product. Carbon dioxide diffuses out the blood in the lungs into the air in the lungs. Hence the name gas exchange – one gas (oxygen) diffuses in, another (carbon dioxide) diffuses out.
This diagram above shows the bronchial tree – the branching network of tubes that carry air into the lungs. The trachea at the top branches into the right and left bronchi, then each in turn branch into smaller bronchi and finally into the smallest tubes called bronchioles. Bronchioles carry air into a cluster of tiny airsacs called alveoli (not ravioli as AZB told his F division today…)
Diffusion is the passive movement of molecules of a liquid or gas from a high concentration to a low concentration. So the first question is what ensures that there is an appropriate concentration gradient for each gas to diffuse?
In order to understand this, you have to remember that the blood going to the lungs is deoxygenated. The right ventricle pumps deoxygenated blood to the lungs in the pulmonary arteries. The tiny alveoli are then covered with capillaries and these join together to form the pulmonary veins. The pulmonary veins carry the oxygenated blood back to the left atrium of the heart. So the blood coming to the lungs will have a low oxygen concentration but a high carbon dioxide concentration.
How are the structure of alveoli adapted for efficient gas exchange?
- The alveoli in total provide a large surface area for the diffusion of oxygen and carbon dioxide. The total surface area of the alveoli in humans is approximately 90 m2 – the equivalent of two tennis courts…..
- The walls of the alveoli are very thin. The alveolus is lined with a single layer of cells, and of course the capillaries are also only one cell thick. So the distance for the diffusion of oxygen and carbon dioxide is very small (hence the rate of diffusion is very fast)
- The alveoli have a rich blood supply. Alveoli are lined by many capillaries.
- The surface of the alveolus is moist. Gas exchange surfaces are always moist as oxygen and carbon dioxide will diffuse more rapidly if they are dissolved in water.
- Alveoli also contain a cell that secretes surfactant. This molecule reduces the surface tension in the film of water that lines the alveolus, allowing air to move in and out more smoothly.
This posts addresses one of the commonest misconceptions you encounter as a biology teacher and it concerns a mistaken belief about the function of the roots of a plant.
The roots anchor the plant in the ground and so prevent it toppling over due to wind. But their main function is to do with the absorption of materials from the soil into the cells of the plant. The question is what exactly is taken up in the roots?
Well most people remember that water is absorbed in the roots by osmosis. The best candidates will remember the microscopic root hair cells in the root that massively increase the surface area for the uptake of water. This absorbed water is transported into the xylem tissue in the centre of the root and then moved up the plant to the leaves by transpiration pull.
Roots also absorb mineral ions from the soil by active transport. Active transport is the process where energy from respiration in the cell is used to pump material across the cell membrane against the concentration gradient. Mineral ions absorbed included nitrate ions (needed to make amino acids and proteins), magnesium ions (needed to make chlorophyll) and phosphate ions (needed to make DNA)
So where is the common misconception? This all seems sensible and fairly straightforward. Roots absorb water by osmosis and mineral ions by active transport.
Whenever root function is tested in exams, many candidates get in a pickle as they confuse mineral ions (nitrate, phosphate, magnesium, potassium) with food molecules. Plants do NOT absorb food molecules through their roots. There are very few food molecules such as glucose, amino acids, and lipids in soil. If there were, more animals would eat soil as a source of nutrition…… Plants do not need to absorb food molecules of course: the big idea you learn is that plants can make their own food molecules in the leaves in the process of photosynthesis.
So in your exam, if you ever find yourself writing anything that suggests that plants take in food through their roots, stop, take a deep breath, cross it all out and count yourself lucky you have prevented yourself from one horror answer at least!
This post is going to describe some of the ways molecules can cross the cell membrane. (For Eton students revising for Trials, diffusion and active transport are found in the F block syllabus, osmosis comes in E Block)
Diffusion is the simplest to understand. Diffusion does not even need a cell membrane to occur. In the example below the dye molecules will move randomly in the solution. As the dye starts in one place, these random movements will mean that slowly spread out until an equilibrium is reached. This movement of the dye from the region of high concentration to the low concentration is called diffusion.
When considering diffusion into a cell, if the cell membrane is permeable to a particular molecule then the random movements of the molecule will mean that there will be a net (overall) movement from the higher concentration to the lower concentration down the concentration gradient.
Key Points about diffusion:
- Always happens down a concentration gradient (from a high concentration to a lower one)
- Never requires any energy from the cell – it is a passive process
Active Transport is a process that will move molecules into a cell against the concentration gradient – i.e. from a low concentration to a high concentration. This “pumping” of the molecules against the gradient requires energy from the cell and of course this energy comes from respiration.
You can see from the diagram above that active transport is working against the concentration gradient, is using energy from inside the cell (actually a molecule made in mitochondria in respiration called ATP) and that a specific transport protein is involved in the cell membrane. This protein will have a binding-site that is specific for a particular molecule and the solute molecule to be transported will collide with the transport protein due to random movement. Energy from the cell can cause the transport protein to change shape such that the solute is released on the other side of the membrane.
Can you think of another area of the iGCSE syllabus which features collisions between a specific binding-site on a protein and a certain other molecule? Linking ideas is a key characteristic of the A* Biologist!
Osmosis is the hardest of these processes to understand properly, especially as an iGCSE student when you are often told an over-simplified account that does not make sense…. Let’s try to simplify it in a way that does make sense.
Firstly it is only water molecules that can move by osmosis into and out of cells – never anything else. Indeed osmosis is the only way water can cross a membrane – it never moves by diffusion or active transport.
Osmosis is a passive process – it never needs any energy from the cell’s respiration and the only energy involved is the kinetic energy of the water molecules.
Osmosis can only occur through a partially permeable membrane. All cell membranes are partially permeable and this means they let small molecule like water through but prevent the diffusion of the larger solute molecules.
The water molecules on both sides of the membrane in the diagram above will be moving around randomly. They will occasionally hit one of the pores in the membrane and so pass across the membrane. This movement will be happening from left to right and from right to left.
The presence of the sucrose (solute) in the solution on the right means that some of the water molecules on that side of the membrane are less able to move. This is because they are temporarily attracted to the solute molecules by weak hydrogen bonds. So their kinetic energy is reduced and this makes them less likely to randomly collide with the pores in the membrane. The presence of the solute on the right means that water molecules on the left on average are more likely to collide with the membrane than the water molecules on the right and this leads to an overall movement from left to right. This net movement of water molecules from the dilute solution to the more concentrated solution through the partially permeable membrane is called osmosis.
This diagram has the two solutions reversed so in which direction will osmosis happen here? Thats right from right to left. You can see the hydrogen bonds attracting water molecules to the solute – these are the ones that lower their kinetic energy overall.
You might even have been taught about osmosis with reference to the water potential of a solution. The water potential of a solution is just a measure of how much kinetic energy the water molecules in a solution possess. So a dilute solution will have a high water potential, a concentrated solution (with lots of dissolved solute) a lower water potential.
Osmosis is the
- net movement of water
- through a partially permeable membrane
- from a solution with a high water potential (a dilute solution) to a solution with a lower water potential (a concentrated solution)
- Oxygen diffuses from the air in the alveolus into the blood
- Carbon Dioxide diffuses from the air spaces in the leaf into the palisade mesophyll cells of the leaf
- Glucose diffuses from the blood into an actively-respiring muscle
- Nitrates are pumped from the soil into root hair cells by active transport
- In the kidney, glucose and other useful molecules are pumped from the nephron back into the blood by active transport.
- In nerve cells, sodium and potassium ions are pumped across the cell membrane to set up the gradients needed for a nerve impulse
- Water enters root hair cells from the soil by osmosis
- In the kidney, water is reabsorbed from the nephron by osmosis.
- In the large intestine, water is reabsorbed from the colon back into the blood by osmosis
There are many many more examples of each process, but this should be enough to be going on with…….
There are two syllabus points in bold (only tested in paper 2) that refer to embryonic and foetal development. The first asks you to understand the role of the placenta in supplying the developing foetus with nutrients and oxygen and the second concerns the role of amniotic fluid in protecting the developing embryo.
The placenta is in many ways a remarkable organ. It contains a mixture of maternal cells from the uterine lining and embryonic cells, but these cells from two genetically different individuals are capable of sticking together to form the placenta. The placenta is only present in the uterus once an embryo has successfully implanted a week or so after fertilisation has happened in the Fallopian Tubes. The placenta is linked to the foetus via the umbilical cord, a structure that contains an umbilical artery and vein carrying foetal blood to and from the placenta.
There is a key idea here that is very important. There is no mixing of maternal and foetal blood in the placenta. This would be disastrous for both mother and baby for a whole variety of reasons. The maternal blood is at a much higher pressure than the foetal blood and if the foetus were connected to the maternal circulatory system directly, its blood vessels would burst. The foetus and mother can have different blood groups of course and you may now that some blood groups are incompatible and can trigger clotting. So it is essential that there is never any mixing of blood. But what happens in the placenta is that mother’s blood empties into spaces in the placenta and babies’ blood is carried by the umbilical artery into capillaries that are found in finger-like projections called villi. This means there is a large surface area and a thin barrier between the two bloods and so exchange of materials by diffusion is possible.
The main function of the placenta then is to allow the exchange of materials between the foetal and maternal circulations. The developing foetus inside its mother’s uterus has no direct access to oxygen nor food molecules of course yet both are needed to allow healthy development. The foetus also needs a mechanism to get rid of the waste molecule, carbon dioxide that is being produced in all its cells all the time. Until the kidneys mature fully the foetus also has to get rid of urea, another excretory molecule that could build up to toxic concentrations unless removed from the growing foetus.
A few interesting points:
You will see that antibodies are small enough to cross the placenta. This gives the baby a passive immunity that can protect it for a short time from any pathogens it encounters.
Drugs such as alcohol and nicotine can cross the placenta. This is why it is so vital that pregnant mothers do not smoke and drink to ensure that the foetus’ development is not affected by these drugs.