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 poresContains 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 Ribosomessmall 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 attachedSmooth / 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 / FlagellaCilia: fine hair like projections which assist in movement along the cell membraneFlagella: one or two longer tail like projections

Cell MembraneCell 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 MembraneTransport 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 MembraneNet 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 MembraneOsmosis• 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 MembraneCarrier-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 MembraneVesicular Transport (Bulk Transport)An active process where materials move into/out of cell enclosed in vesiclesVesicles: 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 AreaAs objects get smaller their surface area to volume ratio increases.