Topic 4

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  • Topic 4
    • Endocytosis and Exocytosis
      • En- Phaagocytosis: solid substances, sometimes whole organisms are taken into a cell through in folding of the surface membrane
      • En- Pinocytosis: similar to phagocytosis but infoldngs in the membrane much smaller so transports liquids
      • Exo- reverse of endocytosis, vesicules and vacuoles move towards the surface membrane fuse with it and release the contents to outside the cell
    • Xylem and Phloem
      • Mass flow hypothesis: dissolved sucrose moves in phloem by means of pressure gradients between source and sinks
        • mass flow of the phloem towards sink when hydrostatic pressure is lower due to the sucrose
      • Uptake and loss of water
    • Gas Exchange
      • Mammals Alveoli  have a large surface area
      • Insects Tracheae from spiracles, into tracheoles then organs
      • Plants gained and lossed through the concentration of potassium ions
      • Fish     counter current  flow
    • Surface Area to Volume Ratio
      • Small Organisms
        • High SA-V ratio diffusion is adequate as diffusion over a short distance
      • Large Organisms
        • Low SA-V ratio diffusion is not adequate so have evolved organs for gas exchange
      • Rate of diffusion
    • Heart and Blood
      • Heart and blood vessels
      • Single and double circulation
      • CARDIAC CYCLE - diastole (relaxed); pressure increases in atrium so AV opens; systole of atrium; then ventricle systole; AV close SL open
      • Control of the heartbeat; SAN impulses across walls of atrium, AVN through Bundle of His, Purkyne fibers.
      • Pressure changes and ECG
        • Platelets form and plug and release clotting factors including thromboplastin
          • Prothrombin > active from thrombin, catalysed through thromboplastin
            • Thrombin acts on soluble fibrogen into insoluble fibrin forming a mesh to cover the wound
              • Platelets and blood called get trapped in mesh forming in a blood clot
      • Haemoglobin: Bohr Effect, when levels of CO2 are high as the tissues Hb bind with O2 less well so Hb gives O2 up more easily
    • Cell Surface Membrane
      • Glycoproteins and glycolipids often act as receptors and antigens, they are lipids or proteins with a carbohydrate chain attached
    • Passive Movement
      • Diffusion: small non charged molecules can diffuse through the lipid bilayer of the cell surface membrane. high to low conc
      • Facilitated diffusion: large charged or polar molecules move thrugh a channel in the bilayer known as carrier proteins.
      • Osmosis: the diffusion of water
        • the turgor pressure is that of the inside of the cells, so plants cells should be turgid and ridged
        • Water potential = turgor pressure+ osmotic potential
          • Turgor pressure= hydrostatic pressure caused by the cell contents pushing against the cell wall
            • Osmotic potential= the potential of water to move out of a solution across a partially permeable membrane
              • water potential= measure for water to move out of a solution by osmosis
    • Active Transport
      • 1- proteins in the membrane act as carriers (specific to the molecule)
        • 2- the release of energy from the hydrolysis of ATP causes a change of shape in the carrier protein moving the molecule from one side to the other
          • 3- the protein then returns to its original shape (passively) to allow more molecules to enter.
    • Tissue Fluid and Formation
      • Derived from blood and contain everything that plasma does but no proteins and platelets, l
      • Lymph is derived from tissue fluid and is similar but has a lower concentration of nutrients


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