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  • 3 Movement in and out of cells

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    • #3
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      • #4
        Active transport





        Sometimes substances are required to be move against the Concentration Gradient, or faster than they would by Passive Transport. In these cases, Active Processes are used, which require energy.

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        • Energy is needed because particles are being moved against a concentration gradient, in the opposite direction from which they would naturally move (by diffusion)
        There are many occasions when cells need to take in substances which are only present in small quantities around them.
        E.g. root hair cells in plants take in nitrate ions from the soil. Their concentration are often higher inside the root hair cell than in the soil, so the diffusion gradient is from the root hair à the soil. Despite this, the root hair cells still can take nitrate ions in, by active transport.






        The importance of active transport: energy-consuming process by which substances are transported against a concentration gradient, e.g. ion uptake by root hairs and glucose uptake by epithelial cells of villi.






        Two big differences between diffusion and active transport:
        • direction of movement (down or up a gradient)
        • use of energy for movement
        The active transport is carried out by ‘carrier proteins’ in the membrane, which bind to the solute molecule, change shape and carry the molecule across the membrane.



        Extended Only How Protein Molecules Move Particles

        • Active transport works by using carrier proteins embedded in the cell membrane to pick up specific molecules and take them through the cell membrane against their concentration gradient:
          1. Substance combines with carrier protein molecule in the cell membrane
          2. Carrier transports substances across membrane using energy from respiration to give them the kinetic energy needed to change shape and move the substance through the cell membrane
          3. Substance released into cell
        QUESTIONS






        Figure above shows root hair cells.
        1. Explain how the presence of root hair cells on roots enables the efficient absorption of water and minerals. [2 marks]
        2. Root hair cells can absorb mineral ions by diffusion and active transport.
        a)Define the term active transport [2 marks]
        b)Explain why respiration rates may increase in root hair cells during the uptake of mineral ions [1 mark]

        Answers

        1. - Large number of root hair cells give a large surface area to the root.
        - Mitochondria are present to provide energy for active transport.
        2. a) active transport is absorption of a substance into a cell or across a membrane
        - against (up) a concentration gradient.
        - using energy


        b) active transport requires energy
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        • #5
          Osmosis


          Osmosis is the diffusion of water molecules from a region of their higher concentration to a region of their lower concentration, through a partially permeable membrane.





          Osmosis is a special form of diffusion and always involves the movement of H2O across a membrane. Osmosis is:
          • the movement of H2O
          • across a selectively permeable membrane
          • down a water potential gradient.
          In the picture below

          - The concentration of sugar molecules is higher on the concentrated solution (L) and lower on the diluted one (R).

          - The concentration of water molecules is higher on the (R) and lower on the (L) (a lot of place is taken up by sugar molecules).








          It is confusing to talk about the 'concentration of water', so we can say that a diluted solution (R) has a high water potential and a concentrated solution (L) has a low water potential.

          There is a water potential gradient between the 2 sides. The water molecules diffuse down this gradient, from a high water potential (R) to a low water potential (L).










          Cell membranes
          • partially permeable (let some substances pass through, but not others).
          • separate 2 solutions: cytoplasm and solution around the cell.
          • If the solutions are of different concentrations, osmosis will occur.




          Effect of Osmosis on plant and animal cells


          1. When placed in H2O:

          Concentration of H2O outside the cell is higher than inside it. Cells will take in H2O by osmosis:
          • plant cells become turgid (swollen) but do not burst (have tough cell wall which is fully permeable).
          • animal cells will burst (no cell wall).





          2. When placed in concentrated sugar or salt solutions:

          Concentration of H2O inside the cell is higher than outside it. H2O get out of the cells by osmosis:
          • plant cells become flaccid (soft and limp), cytoplasm is no longer pressed against the cell wall. The plant loses it firmness and begin to wilt.
          • animal cells shrink, become crenated.












          Common misconceptions

          Sugar and salt do not move by osmosis. Cell membranes prevent them entering or leaving the cell.





          Try this

          A potato was set up as shown in the figure below (left-hand side). The investigation was left for several hours. The results are shown on the right-hand side of the figure.



          1. Describe what happened to
          a. the water in the disk
          b. the salt solution in the hollow in the potato. [2 marks]
          2.
          a. Name the process that is responsible for the changes that have occurred. [1 mark]
          b. Explain why these changes have occurred. [3 mark]
          c. Where does this process occur in a plant? [1 mark]
          d. What is the importance to the plant of this process? [1 mark]
          Answers

          1. a. The volume of water in the dish decreased.
          b. The volume of salt solution in the potato increased.
          2. a. Osmosis
          b. 3 points from:

          - there was a higher concentration of water in the dish than in the potato
          - so water moved into the potato.
          - from a high concentration of water to a lower concentration of water
          - by osmosis.

          c. Root hairs, or in the roots.
          d. Osmosis enables the plant to absorb water to maintain cell turgidity (or to replace water lost by transpiration).


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          • #6
            Diffusion

            Diffusion is the net movement of molecules from a region of its higher concentration to a region of its lower concentration. Molecules move down a concentration gradient, as a result of their random movement.









            For living cells, the principle of the movement down a concentration gradient is the same, but there is one problem:

            The cell is surrounded by a cell membrane, which can restrict the free movement of the molecules à This is a selective permeable membrane: the composition of the membrane (lipid and protein) allows some molecules to cross with ease, but others with difficulty or not at all. The simplest sort of selection is based on the size of the molecules.

            Importance of gaseous and solute diffusion

            Diffusion helps living organisms to:
            • obtain many of their requirements
            • get rid of many of their waste products
            • gas exchange for respiration
            Examples
            • CO2 uses by plants for photosynthesis is diffuses from the air into the leaves, through the stomata (pores at the surface of leaves). There is a lower concentration of CO2 inside the leaf, as the cells are using it up. O2 (waste product of photosynthesis diffuses out in the same way).
            • Flowering plants use diffusion to attract pollinators like bees.
            • Some of the products of digestion are absorbed from the ileum of mammals by diffusion.

            Site of diffusion Substance Description
            Alveoli of lungs O2 Alveoli à Blood capillaries
            CO2 Blood capillaries à Alveoli
            Stomata of leaf O2 Air spaces of leaf à Atmosphere

            Gas exchange between alveolar spaces and capillaries.



            Factors favoring diffusion
            • Distance (the shorter the better), e.g. thin walls of alveoli and capillaries.
            • Concentration gradient (the bigger the better). This can be maintained by removing the substance as it passes across the diffusion surface. (Think about oxygenated blood being carried away from the surface of alveoli).
            • Size of the molecules (the smaller the better).
            • Surface area for diffusion (the larger the better).
            • Temperature (molecules have more kinetic energy at higher temperature).

            Importance of water as a solvent
            • Most cells contain about 75% of water.
            • Many substances move around a cell dissolved in water.
            • Many important reactions take place in water.





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