Cell Structure & TRANSPORT

1. Cell Structure & TRANSPORT

2. Cell Structure

3. 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

4. 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 • - Nucleoluscontains RNA (protein synthesis) • - Filled with nucleoplasm • Ribosomes • small spherical structures which assist in protein production • (amino acids  polypeptides / proteins) Cell Structure

5. 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

6. 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 Structure

7. 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

8. 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

9. 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 concwhich 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

10. 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

11. 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.gglucose • Active Transport: Movement of substances across the membrane, requiring energy, against the conc gradient. • E.gamino acids. • Fig 3.7 Cell Membrane

12. 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. Cell Membrane