Membrane Structure And Function PDF

Cell Membrane Transport  Objectives On successful learning of this topic, you should be able to: Describe in words and diagrams the structure of the cell membrane Explain factors which affect permeability of the membrane to water and other substances. Describe the...

Cell Membrane Transport  Objectives On successful learning of this topic, you should be able to: Describe in words and diagrams the structure of the cell membrane Explain factors which affect permeability of the membrane to water and other substances. Describe the various mechanisms involved in transport of substances across the cell membrane. Define: osmosis, diffusion, facilitated diffusion, endocytosis, exocytosis, active transport, water potential Cell Membrane Structure  Cell membranes Only about 7nm (10-10M) wide Function in: Protection reception of stimuli communication transport. Offer some control over what is allowed into the cell. Referred to as being selectively permeable because it allows some molecules to pass easily but restricts others.  Transport Across Cell Membranes Transport across cell membranes must occur because the cell needs to: Obtain nutrients Excrete waste substances Secrete useful substances Generate the ionic gradients essential for nervous and muscular activity Maintain a suitable internal pH and ionic concentration for enzyme activity. Transport Across Cell Membranes Four basic mechanisms of cell membrane transport Passive (do not require ATP) Diffusion + Facilitated diffusion Osmosis Active (energy consuming) Active transport Bulk transport (endocytosis and exocytosis)  Passive Transport - Diffusion 1. Diffusion Passive movement of molecules or ions (solute) from a region where they are in high concentration to a region where they are in low concentration. ie. down/along a diffusion gradient. Lipid bilayer prevents polar molecules (like glucose and amino acids) from passing but allows lipid soluble molecules to pass. May be facilitated by membrane proteins Passive Transport - Diffusion  Passive Transport - Diffusion Diffusion rate affected by: steepness of the concentration gradient surface area of membrane distance to be diffused size and charge of diffusing molecule  Factors affecting the rate of diffusion The steepness of the diffusion gradient is the difference in concentration of the molecule between the its point of origin and the destination. Steeper the gradient = faster rate of diffusion because there are comparatively more molecules available for movement from origin and much less at the destination. Factors affecting the rate of diffusion  Factors affecting the rate of diffusion Greater surface area of the membrane through which diffusion takes place = greater rate of diffusion. Microvilli increase the surface area of animal cells for absorption purposes  Factors affecting the rate of diffusion Smaller distance over which diffusion occurs = greater rate of diffusion. Rate decreases rapidly with increasing distance Diffusion only effective over very short distances. (membrane approx. 7nm wide  Factors affecting the rate of diffusion The smaller molecules/ions = greater rate of diffusion. e. The charge of diffusing molecule The smaller the charge of the diffusing molecule = greater rate of diffusion.  Transmembrane Transport by Diffusion Molecules transported across membrane by diffusion include: Respiratory gases O2 & CO2 diffuse rapidly H2O molecules (although polar) are small enough to pass between hydrophobic phospholipid molecules (transient hydrocarbon pores) Ions and larger polar molecules such as amino acids, sugars, fatty acids and glycerol diffuse across slowly since they are repelled by the hydrophobic region of the membrane.  Diffusion - Facilitated Facilitated diffusion Used for some ions and polar molecules Occurs via special transmembrane proteins - channels & carriers; also, several proteins can combine to form a channel. Diffusion occurs through channels in either direction. Ion channels allow the passage of ions and are usually gated (exist in an open or closed state)  Diffusion - Facilitated Channel proteins and carrier proteins contain water-filled hydrophobic channels or pores whose shape is specific for a particular ion or molecule. Channel proteins have a fixed shape Carrier proteins undergo rapid changes in shape. Diffusion - Facilitated  Passive Transport - Osmosis 2. Osmosis Passive movement of free water molecules (solvent in the cell) from a region of their high concentration to a region of their low concentration through a selectively permeable membrane. Differences in water potential cause water to move across membranes.  Osmosis - Water potential Water potential (psi – Ψ) The tendency of water molecules to move from one place to another (leave or enter a system). Water always moves from a region of higher water potential to a region of lower water potential. The highest value for Ψ is zero (pure water). Pure water (Ψ =0) will move to a region that has solutes dissolved in the water (Ψ is negative).  Osmosis - Solute potential (Ψs) Solute potential (Ψs) The presence of solute molecules in water lowers Ψ (makes it more negative). The presence of solute molecules prevent water from moving. Water surrounds the solutes so less free water is available for movement. Eg. H+ on Cl- , O- on Na+ The extent to which solute molecules lower the Ψ is called solute potential (Ψs) Water molecules are attracted to solutes, so Ψs pulls in water molecules. Ψs has a negative value.  Osmosis - Pressure Potential (Ψp) Pressure potential (Ψp) As water molecules enter a plant cell, the wall pushes back on the cytoplasm as it expands. This pressure is called the pressure potential and tends to force water out of a cell. Ψp is given a positive value. Water potential (Ψ) = solute potential (Ψs) + pressure potential (Ψp)  Osmosis - Cell status -hyperosmotic Hyperosmotic cell - has a higher solute concentration than that found in the solution outside the cell. The extracellular environment would therefore be hypotonic, having less solute (= more solvent [Water] molecules) that inside the cell The cell would gain water as water moves from the area of higher water potential (outside) to inside the cell.  Osmosis - Cell status Hypotonic solution If an animal cell is placed in a hypotonic solution Water would leave the solution and enter cell. The cell would increase in volume and eventually burst. If a plant cell is placed in a hypotonic solution The rigid cell wall will resist swelling, so the cell won’t burst  Osmosis Hypoosmotic cell - has a lower solute concentration than that found in the extracellular environment Extracellular environment would therefore be hypertonic (more solutes). Water would move out of the cell to the area of lower water potential.  Osmosis Cells in Hypertonic Solutions If an animal cell was placed in a hypertonic solution There would be more free water molecules in the cell than outside and so water would move out of the cell. The cell would therefore shrink (crenate). If a plant cell was placed in a hypertonic solution It would lose water; As the volume decreases, the cell shrinks, and the cytoplasm moves away from the cell wall (plasmolysis).  Osmosis Isosmotic cell /Isotonic solution (iso = equal) - the outer and internal environment of a cell are osmotically equal. The movement of water into and out of a cell is equal; therefore, there is no net gain or loss of water by osmosis.  Control of Osmosis Animal cells Develop osmoregulatory mechanisms to maintain the balance of water to prevent excessive shrinkage or bursting when the concentration of surrounding solutions change. Plant cells Presence of cell wall; hormone (abscisic acid); control of transpiration  Transmembrane Transport -Active 3. Active Transport Movement of molecules or ions across a membrane against a concentration gradient (from region of lower to higher conc.) Requires chemical energy since it involves movement against the natural tendency to diffuse in the opposite direction. Energy directly or indirectly supplied by hydrolysis of ATP. NaK pump is also an enzyme, ATPase, (Adenosine triphosphatase) which hydrolyzes ATP (removes phosphate) and utilizes the energy released to move the ions against their concentration gradient. [coupled reaction]  Active Transport Movement is usually in one direction only, unlike diffusion which is reversible. Performed by carrier transmembrane proteins. E.g., sodium-potassium pump; found in eukaryotic, mostly animal cells Na+/K+ pump is essential for osmotic balance of animal cells (osmoregulation). Bacteria, fungi and plants have cell walls and do not need the pump. Sodium Potassium Pump  Transmembrane Transport Endocytosis and Exocytosis Bulk transport (Endocytosis & Exocytosis) Active process involving the bulk transport of materials through membranes whether into cells (endocytosis) or out of cells (exocytosis).  Transmembrane Transport - Endocytosis Endocytosis Large molecular weight materials are enclosed within infoldings or extensions of the cell surface membrane to form a vesicle. 2 types - phagocytosis and pinocytosis leading to the formation of vesicles called phagosomes and pinosomes respectively.  Bulk transport - Endocytosis Types of endocytosis Phagocytosis (cell eating) – material taken up is in the solid form. Cells specializing in phagocytosis are called phagocytes and are said to be phagocytic. E.g., white blood cells that engulf bacteria. Pinocytosis (cell drinking) – material taken up is in liquid form. Vesicles (pinosomes) are extremely small. Endocytosis  Bulk transport - Exocytosis Reverse of endocytosis Waste materials (e.g., solid undigested remains from phagocytic vacuoles) and useful material are secreted by the cell. Vacuoles with secretory cellular products migrate from the interior cytoplasm to the cell surface where they fuse with the plasma membrane and discharge their products to the exterior. Exocytosis Bulk transport – Endocytosis and Exocytosis