Cell Structure & TRANSPORT

Cell Structure

Cell Structure Cytoplasm Fluid material in which the cell contents are suspended 75% - 90% water   Cytoskeleton framework of protein fibres Microtubules: hollow rods which keep organelles in place + move them around cell Microfilaments: (smaller) move materials around cell or move whole cell

Cell Structure Organelles Nucleus - Surrounded by a nuclear membrane (double membrane) with nuclear pores Contains genetic material (DNA) - DNA determines which enzymes (proteins) will be made by cell therefore controlling the functioning of the cell - Nucleolus contains RNA (protein synthesis) - Filled with nucleoplasm   Ribosomes small spherical structures which assist in protein production (amino acids  polypeptides / proteins)

Cell Structure Endoplasmic reticulum - Network of channels formed by pairs of parallel membranes which connect with cell and nuclear membranes - Provide surface for chem.. reactions and channels allow molecule storage and transport Rough / Granular ER: Ribosomes attached Smooth / Agranular ER: No ribosomes    Golgi Apparatus - series of flattened membranous bags which function to modify proteins and package them for secretion - addition of sugar, sulphate, phosphate, fatty acids - Ends of golgi apparatus pinch off to form vesicles with proteins inside   

Cell Structure Lysosomes - small spheres bonded by a membrane from the golgi apparatus - contain digestive enzymes (breaks down proteins, lipids, nucleic acids and some carbs) Mitochondria - spherical or sausage shaped structures with a double membrane (the inner one is folded) - contains enzymes which are involved in chem. reactions which release energy    Centrioles - pair of cylindrical structures (which are at right angles) made up of a ring of very fine tubes - involved in cellular reproduction   Cilia / Flagella Cilia: fine hair like projections which assist in movement along the cell membrane Flagella: one or two longer tail like projections

Cell Membrane Cell membrane Arranged in bilayer: Semi-permeable membrane / differentially permeable / selectively permeable: allow some materials to pass but not all. Composed of lipids (phospholipids) with tails pointing inward & proteins throughout Fig 3.2 Transport may be Passive: doesn’t require energy or; Active: requiring energy

Cell Membrane Transport may occur in three main ways: 1. Net Diffusion — (passive) the random movement of ions and molecules until they are evenly distributed, and osmosis (passive), a special case of diffusion specifically involving water movement. 2. Carrier-mediated transport — (passive or active) Special proteins that bind to an ion/molecule to help it move through membrane. 3. Vesicular transport — (active) process where materials are moved in membrane-bound sacs.

Cell Membrane Net Diffusion The movement of particles from a high concentration to a low concentration until at equilibrium as a result of straight and random movements/collisions. Particles moving away from high conc are in fewer collisions than particles moving towards and therefore continue in a straight line. Diffusion gradient / Concentration gradient: difference in conc which brings about diffusion. (If conc gradient is larger, diffusion occurs faster.) Fig 3.4 Fat soluble substances (e.g alcohol, steroids) and gases (CO2, O2) can diffuse directly through phospholipid layer Water, ions and small molecules can diffuse through protein membrane channels Fig 3.5

Cell Membrane Osmosis Special case of diffusion Diffusion of the SOLVENT (often water) through a semi permeable membrane (differentially permeable membrane) Semi permeable membrane: membrane which allows some substances to pass through but not all. Often movement of solvent creates two different fluid levels on either side of the membrane creating osmotic pressure Fig 3.6 

Cell Membrane Carrier-Mediated Transport Proteins which bind to one particular ion/molecule and assist it in moving across a membrane. Facilitated Diffusion: Passive process allowing larger molecules to diffuse, through a process of binding and being released from special proteins. Speed is limited by the number of binding proteins. E.g glucose Active Transport: Movement of substances across the membrane, requiring energy, against the conc gradient. E.g amino acids. Fig 3.7

Cell Membrane Vesicular Transport (Bulk Transport) An active process where materials move into/out of cell enclosed in vesicles Vesicles: Bubble surrounded by a membrane.   Endocytosis: Movement of a substance from the outside of the cell to the inside by the folding of the cell membrane around the substance forming a vesicle (fluid filled sac) Phagocytosis: solid particles (specialised cells only) Pinocytosis: liquid particles     Fig 3.8    Exocytosis: Movement of a substance from the inside of the cell to the outside by the migration of the vesicle. (may be waste or a substance needed elsewhere) Fig 3.9 Surface Area As objects get smaller their surface area to volume ratio increases.