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Doc Brown's Biology AQA GCSE BIOLOGY 3 Revision Notes Study Notes

AQA GCSE B3 Biology Unit B3.1 Movement of molecules in and out of cells

AQA GCSE BIOLOGY UNIT 3 Biology 3 for GCSE Triple Award Science or GCSE Biology

REVISION NOTES GUIDE SUMMARY: What do you need to know for the examinations? What do you need to able to do in the exams? In AQA GCSE Science A examinations HT means for higher tier students only. Sorry, but I don't have much time to answer questions, but if you see any apparent errors or wish to comment, please email me. All my notes, learning objectives, comments for exam revision are based on the official AQA GCSE Science A Key Stage 4 syllabus specification.

AQA GCSE Science BIOLOGY B3 Unit B3.1 Movement of molecules in and out of cells

  • Appreciate that we need to understand how biological and environmental systems operate when they are working well in order to be able to intervene when things go wrong.

  • Appreciate that modern developments in biomedical and technological research allow us to do so.

  • Know and understand that the cells, tissues and organs in plants and animals are adapted to take up and get rid of dissolved substances.

  • Know that different conditions can affect the rate of transfer.

  • Sometimes energy is needed for transfer to take place - active transport.

  • You should be able to use your skills, knowledge and understanding to:

    • evaluate the development and use of artificial aids to breathing, including the use of artificial ventilators,

    • evaluate the claims of manufacturers about sports drinks,

    • analyse and evaluate the conditions that affect water loss in plants.


AQA GCSE Science BIOLOGY Unit B3.1.1 Dissolved substances

  • a) Know and understand that dissolved substances move by diffusion and by active transport.

  • b) Know and understand that water often moves across boundaries by osmosis - a specific case of particle diffusion.

    • Know that osmosis is the diffusion or bulk movement of water from a dilute to a more concentrated solution through a partially permeable membrane (semi-permeable membrane) that allows the passage of water molecules.

    • A partially permeable membrane has extremely small pores or holes that only allow the tiniest of molecules like water through.

      • Other molecules eg even relatively small molecules like sugars or amino acids and any larger molecules like proteins or glycogen, will not pass through a semi-permeable membrane.

      • So, in living organisms, only water gets through and depending on the concentration of dissolved substance, osmosis can happen in either direction across a partially permeable membrane.

      • Although the water molecules (and any other particles) are moving around at random, there will be a net transfer of water in one direction at a time through a partially permeable membrane ...

      • ... the water will diffuse from a less concentrated solution to a more concentrated solution i.e. from the higher concentration of water molecules to a lower concentration of water molecules across the membrane

      • This osmosis diffusion can occur in either direction depending on the relative concentration of the solutes in the cell fluids or tissue fluids and concentrated solutions e.g. of sugars, will tend become diluted by water passing through the partially permeable membrane.

      • You can do simple experiments to demonstrate this by placing blocks or cylinders of potato into pure water and a series of sugar solutions (e.g. glucose) increasing in concentration (increasingly higher molarity mol/dm3).

      • The potato 'chips' will shrink and lose mass the more concentrated the sugar solution because the water will undergo osmosis and diffuse through the partially permeable membranes of the potato cells to try and dilute the sugar solution.

    • Use of the terms turgor and plasmolysis is not required.

  • c) Know and understand that differences in the concentrations of the solutions inside and outside a cell cause water to move into or out of the cell by osmosis.

    • The soft cell wall, or outer membrane of an animal cell, acts as a partially permeable membrane.

    • The water surrounding cells, the tissue fluid, contains the dissolved molecules the cell needs to survive eg sugars, amino acids, oxygen, as well as waste carbon dioxide etc.

    • If the cells are short of water ('partially dehydrated'), the concentration of dissolved substances increases, so water diffuses through the cell membrane into the cells to dilute the cell fluids until equilibrium is established. Conversely, if the cell solution is too dilute, then water will diffuse out from osmotic action across the semi-permeable membrane of the cell wall.

  • d) Know that most soft drinks contain water, sugar and ions.

  • e) Know and understand that sports drinks contain sugars to replace the sugar used in energy release during the activity.

    • They also contain water and ions to replace the water and ions lost during sweating.

  • f) Know and understand that if water and ions are not replaced, the ion / water balance of the body is disturbed and the cells do not work as efficiently.

  • g) Know and understand that substances are sometimes absorbed against a concentration gradient.

    • This means transfer occurs in the opposite direction to the natural direction of diffusion and osmosis.

    • Know that this requires the use of energy from respiration and this process is called active transport.

    • Know that active transport enables cells to absorb ions from very dilute solutions.

    • Active transport is required to absorb nutrients like amino acids, sugars like glucose etc. from the gut when the concentration in the gut is lower than their concentrations in the blood supply, and a healthy body requires these nutrients all the time.

    • If the concentrations of nutrients in the gut is higher than that in the blood stream, then the nutrients will naturally diffuse into the blood stream because of the direction of the concentration gradient (more concentrated ==> less concentrated).

    • If the concentration gradient flow is in the direction of blood stream (higher) to gut (lower), then respiration powered active transport must be used to work against the natural diffusion flow.

    • Remember that absorption by diffusion down the concentration gradient through membranes doesn't require energy from respiration

  • h) Know and understand that many organ systems are specialised for exchanging materials.

    • Know that the effectiveness of an exchange surface is increased by:

      • having a large surface area,

      • being thin, to provide a short diffusion path,

      • (in animals) having an efficient blood supply,

      • (in animals, for gaseous exchange) being ventilated.

  • i) Know that gas and solute exchange surfaces in humans and other organisms are adapted to maximise effectiveness.

    • It is essential that the transfer processes of moving sugars, amino acids, oxygen etc. into cells and the removal of waste products, can happen as efficiently as possible.

    • Therefore exchange surfaces have evolved to maximise the rate of transfer of wanted and unwanted chemicals.

    • To increase the probability of exchange the exchange surface needs to be ...

      • a large surface area to increase diffusion rate eg alveoli in lungs, villi in intestine

      • a thin layer so diffusion times are short - cell membranes are usually quite thin,

      • lots of thin blood vessels to bring in essential molecules for life and carry waste molecules away eg the thin bronchiole tubes in the lungs,

    • -

  • j) Know that the size and complexity of an organism increases the difficulty of exchanging materials.

    • One reason for this increased difficulty in exchanging materials is that the distance from the exchange surface is getting further away from where the nutrients and oxygen are needed and the waste to be removed.

  • k) Know and understand that in humans:

    • the surface area of the lungs is increased by the alveoli - tiny sacs of the end of the tiny bronchiole tubes in the lungs,

    • the surface area of the small intestine is increased by villi.

  • l) Know and understand that the villi in the small intestine provide a large surface area with an extensive network of thin blood capillaries to absorb the products of digestion by diffusion and active transport.

    • The tissue lining in the small intestine is covered with millions of protuberances called villi, which poke up from the intestine surface into the partially or wholly digested food /mush'.

    • This considerably speeds up the absorption process because the villi massively increase the effective digested food absorbing surface area of the small intestine.

    • Each villus (of the millions of villi) has single layer of surface cells and each villus contains a multitude of fine blood capillaries into which the small digested food molecules can rapidly diffuse into, a good blood supply is needed to efficiently carry the digested food away to where they are needed.


AQA GCSE Science BIOLOGY Unit B3.1.2 Gaseous exchange

  • a) Know and understand that the lungs are in the upper part of the body (thorax), protected by the ribcage and separated from the lower part of the body (abdomen) by the diaphragm.

    • You should be able to recognise these structures on a diagram.

    • The ribcage physically protects the lungs from being easily crushed and damaged.

    • To increase the efficiency of gas exchange in the lungs the bronchus divides in two (the bronchi), so each lung gets a good supply of air. Each bronchus divides and divides into many bronchioles with a tiny sac at the end of each one - the alveoli - which considerably increases the area for oxygen and carbon dioxide gas exchange.

  • b) Know and understand that the breathing system takes air into and out of the body so that oxygen from the air can diffuse into the bloodstream for respiration, and waste carbon dioxide from respiration, can diffuse out of the bloodstream into the air.

    • This gas exchange happens in the lungs which has millions of tiny air sacs called alveoli at the ends of the finest bronchiole tubes. Surrounding the alveoli are small arteries (fine capillaries) bringing 'dark red' deoxygenated blood to the lungs. The gas exchange occurs on the membrane surfaces of the alveoli and the fine blood vessels. Here, oxygen, from breathing in, is transferred from the air in the alveoli into the fine veins which carry the 'bright red' oxygenated blood away to where it is needed in the rest of the body and simultaneously carbon dioxide goes in the opposite direction from the deoxygenated blood into the alveoli and breathed out.

      • The alveoli are well designed by evolution to perform this gas exchange efficiently. To increase the probability to transfer gas molecules, they have a huge surface area because of their tiny sac like structure, the cell membrane lining is moist to dissolve gases, the sac walls are thin to reduce diffusion to time and an excellent blood supply of numerous tiny blood vessels - vein and artery capillaries.

  • c) Know and understand that to make air move into the lungs the ribcage moves out and up and the diaphragm becomes flatter.

    • Know these changes are reversed to make air move out of the lungs.

    • Know the movement of air into and out of the lungs is known as ventilation.

      • You should be able to describe the mechanism by which ventilation takes place, including the relaxation and contraction of muscles leading to changes in pressure in the thorax.

      • As you breathe in, the intercostal muscles contract expanding the rib cage, and the diaphragm also contracts making it flatter, both of which increase the volume of the thorax.

        • This has the effect of decreasing the pressure in the lungs and allowing air to be easily drawn in, the air will flow in naturally, due to the pressure difference between the air in the lungs (lower pressure) and the 'outside' air (higher pressure).

      • In breathing out, the intercostal muscles relax (ribcage contracts), the diaphragm relaxes and moves up, so the combined effect is to increase the air pressure in the lungs and air is expelled.

      • Artificial ventilators move air into and out of a persons lungs. This may be because some injury or medical condition or undergoing an operation, which prevents them from breathing normally.

        • This used to be done by a large 'capsule' called an 'iron lung' which encased the whole body of the patient except for the head.

          • The pressure in the capsule is mechanically lowered to allow the lungs to expand and take in air and then raised to make the lungs contract and expel air.

          • However the blood flow in the lower part of the body can be poor and giving rise to poor circulation side effects.

        • Modern ventilators work by pumping air in a go/stop cycle, using a mouth piece connection, directly into the lungs to expand them and push out the ribcage.

          • When the pump temporarily stops, the ribcage relaxes, contracting the lungs and expelling the air.

          • This is a much more convenient method with a wide range of applications, and, it doesn't interfere with the body's blood supply, but there can be problems if the alveoli (may burst) can't cope with the artificially increased air supply.


AQA GCSE Science BIOLOGY Unit B3.1.3 Exchange systems in plants

  • a) Know and understand that in plants:

    • carbon dioxide enters leaves by diffusion and then diffuses into cells where photosynthesis takes place,

    • most of the water and mineral ions are absorbed by roots.

  • b) Know and understand that the surface area of the roots is increased by root hairs and the surface area of leaves is increased by the flattened shape and internal air spaces.

    • The fine root hairs considerably increase the surface area of the roots for absorbing water and minerals.

      • Root hair cells are very elongated combining into fine hair-like structures, which greatly increases surface contact with the soil from which most of the plant's water and mineral intake are absorbed.

      • As long as the water concentration is higher in the soil, the root hairs will naturally absorb water by osmosis.

      • The concentration of minerals in the root hair cells is higher than in the moisture surrounding the roots, so a problem, because the plants cells would naturally lose essential mineral ions by osmotic diffusion, not good!

      • Therefore, active transport systems must be used by the plant to counteract the natural direction of diffusion from a high mineral concentration in the plant cells to a low mineral concentration in the soil moisture.

      • Energy from respiration is usually required to absorb minerals into the roots from the soil moisture by working against the concentration gradient.

    • Beneath the apparently flat surface of a leaf is quite a porous layer of air spaces between the outer layers of cells - particularly on the underside of leaves - quite often the lower surface of a leaves feel rougher and 'roughness' means a more disrupted surface of a larger surface area.

  • c) Know and understand that plants have stomata (tiny pores or holes) to obtain carbon dioxide from the atmosphere and to remove oxygen produced in photosynthesis.

    • Carbon dioxide can diffuse in through the stomata and oxygen can diffuse out.

    • Since carbon dioxide is being used up in photosynthesis, the concentration gradient enables more carbon dioxide to diffuse in through the stomata.

    • Water, and the oxygen produced in photosynthesis diffuse out.

  • d) Know and understand that plants mainly lose water vapour from their leaves.

    • Know that most of the loss of water vapour takes place through the stomata.

    • Know that evaporation is more rapid in hot, dry and windy conditions.

    • Know that if plants lose water faster than it is replaced by the roots, the stomata can close to prevent wilting.

    • The process of water movement from the roots through the xylem and out of the leaves is called transpiration.

  • e) Know that the size of stomata is controlled by guard cells, which surround them.

    • The size of the opening of the stomata must be controlled by the guard cells or a plant might lose too much water and wilt.

    • Water will diffuse out and evaporate away much faster in less humid-drier, hotter or windier weather conditions.

    • The guard cells will respond to the ambient conditions ie close up the stomata if the rate of water loss is to great for water to be replenished from the roots.


  • Practical work in unit 3.1 to help develop your skills and understanding may have included the following:

    • use sensors, eg spirometers, to measure air flow and lung volume

    • investigating potato slices in different concentrations of liquid in terms of mass gain and mass loss - this is to illustrate the process of osmosis.

    • designing an investigation to measure the mass change of potato when placed in a series of molarities of sucrose solution

    • investigating the relationship between concentrations of sugar solution and change in length of potato strips

    • placing shelled eggs in different concentrations of liquid to observe the effect

    • placing slices of fresh beetroot in different concentrations of liquid to observe the effect, and then taking thin slices to observe the cells

    • observing guard cells and stomata using nail varnish

    • observing water loss from plants by placing in a plastic bag with cobalt chloride paper.


keywords: gcse AQA active transport osmosis biological & environmental systems exchange surface large area thin efficient blood supply gaseous exchange ventilation lungs alveoli small intestine villi Unit B3.1 Movement of molecules in and out of cells Dissolved substances Gaseous exchange systems in plants separate triple award science Unit 3 BIOLOGY 3

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