Transport across plasma membrane

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  • 1. Transport across the plasma membrane ALBIO9700/2006JK
  • 2. Diffusion and facilitated diffusion• Diffusion – the net movement of molecules (or ions) from a region of their higher concentration to a region of their lower concentration (molecules move down a concentration gradient)• Molecules tend to reach an equilibrium where evenly spread• The rate at which substance diffuses across a membrane depends on: – The ‘steepness’ of the concentration gradient. The greater the difference in concentration, the greater the difference in the number of molecules passing in the 2 directions and hence the faster the net rate of diffusion – Temperature. Diffusion takes place faster in high temperatures due to higher kinetic energy – The surface area. The greater the surface area, the more molecules or ions can cross it at any one moment (faster diffusion) – The nature of the molecules or ions. Substances with large molecules tend to diffuse more slowly. Non-polar molecules diffuse more easily through cell membranes as they are soluble in non-polar phospholipid tails ALBIO9700/2006JK
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  • 4. • Facilitated diffusion – diffusion of a substance through protein channels in a cell membrane• The proteins provide hydrophilic areas that allow the molecules or ions to pass through a membrane that would otherwise be less permeable to them• Rate depends on how many appropriate channels there are in the membrane and on whether they are open or not ALBIO9700/2006JK
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  • 6. Active transport• The energy-consuming transport of molecules or ions across a membrane against a concentration gradient made possible by transferring energy from respiration• Like facilitated diffusion, active transport is achieved by special transport proteins, each of which is specific for a particular type of molecule or ion• However, it requires energy (molecule ATP) because movement occurs up a concentration gradient• Energy used to make the transport protein (carrier protein) change its 3D shape, transferring the molecules or ions across the membrane in the process ALBIO9700/2006JK
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  • 8. Bulk transport• Transport of large quantities of materials into cells (endocytosis) or out of cells (exocytosis)• Exocytosis – process by which materials are removed from cells• Endocytosis – engulfing of the material by the plasma membrane to form a small sac or ‘endocytotic vacuole’ – Phagocytosis: bulk uptake of solid material – Pinocytosis: bulk uptake of liquid ALBIO9700/2006JK
  • 9. Osmosis• Solution = solute + solvent• Partially permeable membrane – membrane which allows only certain molecules through• If solution B has higher concentration of solute molecules than solution A – solution B is more concentrated than solution A/solution A is more dilute than solution B• Osmosis involves net movement of water molecules only ALBIO9700/2006JK
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  • 11. • Water potential and solute potential – Water potential: the tendency of water molecules to move from one place to another (psi, ψ) – Water always moves from a region of higher water potential to a region of lower water potential (down concentration gradient) – Osmosis: the movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane – Pure water has highest water potential (solutes lower the water potential) – Solute potential: the amount that the solute molecules lower the water potential of a solution. Always –ve (ψs). ALBIO9700/2006JK
  • 12. • Osmosis in animals – If the water potential of the solution surrounding the cell is too high, the cell swells and bursts – If it is too low, the cell shrinks – In animal cells ψ = ψs (water potential is equal to solute potential)• Pressure potential – The greater the pressure applied, the greater the tendency for water molecules to be forced back – Increasing the pressure increases the water potential – Pressure potential: contribution made by pressure to water potential (ψp) – The pressure potential makes the water potential less negative and is therefore positive ALBIO9700/2006JK
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  • 14. • Osmosis in plant cells – Different from animal cells because of rigid and strong cell wall – Cell wall prevents the cell from bursting (lysis) – When fully inflated with water: turgid – Water potential is a combination of solute potential and pressure potential (ψ = ψs + ψp) – When protoplast shrinks and pulls away from the cell wall: plasmolysis – Incipient plasmolysis: the point at which pressure potential has just reached zero and plasmolysis is about to occur ALBIO9700/2006JK
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  • 16. Exchange surfaces• Gaseous exchange in mammalian lungs – Gaseous exchange surface: where oxygen from the external environment can diffuse into the body, and carbon dioxide can diffuse out – In humans: alveoli in the lungs ALBIO9700/2006JK
  • 17. • Uptake of mineral ions in a plant root – Specialised exchange surface: root hairs (very thin extensions of the cells that make up the outer layer or epidermis of a root) – Root hairs make contact with thin layer of water coating each soil particle and absorbs it by osmosis – Lower concentration of solutes in the water in the soil than there is inside the root hair cell – Water potential higher outside the root hair and water moves passively down the water potential gradient into the cells – Mineral ions also absorbed by facilitated diffusion if concentration is higher outside root hair – By active transport (carrier proteins and energy) when concentration is relatively low in soil ALBIO9700/2006JK
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