Unicellular organisms have a small volume to surface area ratio, and are small, meaning they have a short diffusion distance. This makes them ideal for diffusion, active transport and osmosis without a transport system.
2.50 understand the need for a transport system in multicellular organisms
Larger, multicellular organisms, need a transport system to move necessary substances (e.g. mineral ions, water, oxygen, sugars) around the organism. If they didn't have transport systems, because the volume to surface area ratio is large, it would take a long time for all of the nutrients to reach all the cells. Without transport systems, multicellular organisms would not be able to live, they wouldn't have enough nutrients to work.
Flowering plants:
2.51 describe the role of phloem in transporting sucrose and amino acids between the leaves and other parts of the plant
Phloem tubes are columns of living cells. Their cell walls at either end form small holes to allow substances to pass through (sieve plates). The tubes allow dissolved sugars and amino acids to be transported through the plant. This movement is known as translocation.
2.52 describe the role of xylem in transporting water and mineral salts from the roots to other parts of the plant
Xylem tubes are made up of hollow, dead cells. There are no cell walls at either end, allowing water and dissolvedd mineral ions to flow through the plant freely. The tube has thick, reinforced cell walls, that have a secondary function of support.
2.53 explain how water is absorbed by root hair cells
Water in drawn into the root hair cells by osmosis. The cells have long 'hairs', hence the name. This increases the surface area, making them specialised for absorbing water and mineral ions.
2.54 understand that transpiration is the evaporation of water from the surface of a plant
Transpiration is the evaporation and diffusion from leaves, the vapour escaping from the stomata. Water molecules are cohesive (they stick together), so a tension pull is produced, causing more water to travel up to the leaf, creating a transpiration stream.
2.55 explain how the rate of transpiration is affected by changes in humidity, wind speed, temperature and light intensity
More humid = Less transpiration. This is because there is a less steep concentration gradient so the water vapour doesn't diffuse away.
More windy = More transpiration. The water vapour is blown away from the leaf more quickly so diffusion is sped up.
More heat = More transpiration. Higher temperatures mean more evaporation, and the particles move more quickly so diffusion is faster.
More light = More transpiration. More light means more photosynthesis, so the stomata open wide. When it begins to get darker, the stomata begin to close. Open stomata allow water to escape.
2.56 describe experiments to investigate the role of environmental factors in determining the rate of transpiration from a leafy shoot
A potometer can be used to measure the rate of transpiration.
- Set up apparatus so it is watertight, allowing a bubble to form in the capillary tube. The shoot should be cut diagonally to maximise surface area. Take note of where the bubble starts.
- Start a stopwatch and record the distance moved by the bubble per unit of time (e.g. cm/h)
- Record on a graph, and see how it changes with different variables.
All of the variables should cause it to change proportionally.
Humans:
2.57 describe the composition of the blood: red blood cells, white blood cells, platelets and plasma
The blood is made up of four main components: Plasma (54.3%), red blood cells (45%), white blood cells and platelets (0.7%).
2.58 understand the role of plasma in the transport of carbon dioxide, digested food, urea, hormones and heat energy
Plasma is about 93% water, and 7% proteins. It is a pale yellow liquid, and is the medium in which most substances are transported around the body. It transports white blood cells and platelets, as well as red blood cells which transport oxygen around the body, it transports amino acids and glucose absorbed from the gut to bodily cells, urea from liver to kidneys where it is removed, then excreted, hormones from different glands around the body to send chemical messages around the body, and heat energy.
2.59 explain how adaptations of red blood cells, including shape, structure and the presence of haemoglobin, make them suitable for the transport of oxygen
Red blood cells have a bi-concave shape, and are small. This means they have a high surface area to volume ratio, ideal for absorbing and releasing oxygen. Haemoglobin is the chemical contained in red blood cells that bonds with oxygen in the lungs to form oxyhaemoglobin, allowing them to carry oxygen around the body. They also have no nucleus to decrease volume.
2.60 describe how the immune system responds to disease using white blood cells, illustrated by phagocytes ingesting pathogens and lymphocytes releasing antibodies specific to the pathogen
Phagocytes are able to detect foreign substances and objects in the body, and engulf them. They are non-specific, so they attack anything. This is why people with organ transplants must take immunosuppressants; the phagocytes would otherwise attack the foreign cells and reject the organ.
Lymphocytes detect the antigens found on the surface of pathogens, and release the corresponding antibody: a specific protein that will attack the pathogens, and mark them for destruction by other white blood cells. Once the infection has been fought off, some antibodies remain in the blood so they are easily able to detect and reproduce the same antibody when in contact with the disease.
2.61 understand that vaccination results in the manufacture of memory cells, which enable future antibody production to the pathogen to occur sooner, faster and in greater quantity
In a vaccine, dead or inactive pathogens, or a harmless part of a pathogen or even just the genetic material is injected into the bloodstream. This sample contains antigens, which creates an immune response that allows the lymphocytes to produce antibodies to attack the antigens. Once this harmless 'infection' is fought off, these lymphocytes contain a 'memory' of how to produce the particular antibody, and are able to fight off real infection later on, with quick and large-scale production of antibodies.
Some people have weaker immune systems, so vaccines can cause problems. If they have an immune condition, are taking immunosuppressants, or are too young or too old, they cannot have an immunisation and thus are unprotected. This is why it is so important for the vast majority to be vaccinated, so vulnerable people are protected too.
2.62 understand that platelets are involved in blood clotting, which prevents blood loss and the entry of micro-organisms
Platelets are small fragments of cells that clump together, held in a web-like structure of proteins, called fibrin, over damaged areas. This is known as blood clotting, and prevents blood from escaping from the damaged vessels, as well as stopping potentially harmful microorganisms from getting in.
2.63 describe the structure of the heart and how it functions
The heart is made up of four chambers; the left atrium, the left ventricle, the right atrium and the right ventricle. These chambers are each attached to their own blood vessel.
The right atrium is connected to the vena cava. It takes in deoxygenated blood from the body and passes it through the tricuspid valve to the right ventricle.
The right ventricle pumps deoxygenated blood at a low pressure to the lungs (through the semi-lunar valve to the pulmonary artery). This must be low pressure to fit through the tiny capillaries in the lungs without bursting them.
The left atrium receives oxygenated blood from the pulmonary vein, after it has passed through the lungs. The blood is then passed through the biscuspid valve into the left ventricle.
The left ventricle has a thick muscular wall that allows blood to be pumped around the body at high pressure. It squeezes strongly so the oxygenated blood passes through the semi lunar valve into the aorta.
2.64 explain how the heart rate changes during exercise and under the influence of adrenaline
More exercise means more respiration, meaning there is more carbon dioxide being released into the bloodstream, and a higher demand for oxygen. This causes the heart to pump faster so more oxygen can reach the muscles.
Adrenal glands are hormonal glands that release a chemical called adrenaline. This is released when the organism is threatened. It sends a signal to the brain to make the heart pump faster in order to get more oxygen to the bodily tissues for action (fight or flight)
2.65 describe the structure of arteries, veins and capillaries and understand their roles
Arteries are the blood vessels that carry blood away from the heart. This is usually oxygenated, but not in the case of the pulmonary artery. Arteries are under high pressure, as it has come directly from the heart. The walls are strong and elastic (with thick layers of muscle), and they are thick compared to the size of the lumen, and no valves. This allows them to withstand and maintain high blood pressure.
Veins carry blood back to the heart. They usually contain deoxygenated blood, but again the pulmonary vein is an exception. They have thin walls and low pressure, but contain valves to prevent the blood travelling backwards. They have large lumen to help the blood flow despite the low pressure.
Capillaries are really tiny blood vessels. Their walls are just one cell thick, making them ideal for diffusion. They carry blood close to every cell to provide them with the necessary nutrients. They have very small lumen.
2.66 understand the general structure of the circulation system to include the blood vessels to and from the heart, the lungs, the liver and the kidneys.
In humans, the blood circulates in two systems. This is called double circulation.
The two systems work together in oxygenating the blood and pumping it around the body.
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