Cells & Membranes

Prokaryotic vs. Eukaryotic Cells Why would it benefit a cell have to membrane-bound organelles? –Different “compartments” for specific reactions –Allows incompatible reactions to occur at the same time

Limits to Cell Size Surface Area to Volume Ratio –As a cell increases in size, the volume increases faster than the surface area (SA:V ratio also becomes smaller) Why is this significant? –If ratio is too small  flow of nutrients (in) & flow of wastes (out) aren’t fast enough to keep up with metabolic activities of the cell –Larger cells also have more trouble moving materials through cytoplasm

Protein Traffic in a Cell Secretory Protein = protein that is going to leave the cell Pathway: –Ribosome  Rough ER  vesicle  Golgi Apparatus  vesicle  Plasma (Cell) Membrane

Movement within a cell Involves specialized proteins “walking” across microtubules or other proteins Requires ATP as an energy source Examples include: –Vesicle movement within a cell –Cilia and flagella movement –Muscle contraction

Membrane Properties fluid = phospholipids move a lot laterally but not much flip-flopping Membranes need to stay fluid to function properly. Fluidity of the membrane can be adjusted by –Temperature –Amount of cholesterol

Fluidity of the membrane Effect of temperature –the higher the temperature, the more fluid the membrane will be –At a certain temperature, each membrane will become more solidified (less fluid) This depends on the types of fatty acids in the membrane (saturated or unsaturated)

Fluidity of the membrane Effect of cholesterol –At medium temperatures - reduces fluidity - prevents lateral movement of phospholipids –At low temperatures -prevents solidification -- lowers the temperature at which a membrane solidifies -Prevents phospholipids from being packed closely

Membrane Properties Mosaic = proteins, carbohydrates embedded in membrane Integral/Transmembrane proteins often have both hydrophilic and hydrophobic regions. Membrane proteins have many different functions. Carbohydrates are often involved in cell-cell recognition. hydrophobic hydrophilic

Why must a plasma membrane be selectively permeable? To control what enters the cell Maintain a certain environment within the cell (Dynamic Homeostasis– things constantly moving in and out to maintain certain concentrations, conditions)

Passive Transport Require NO energy Small, nonpolar molecules, O 2, CO 2 Moves with the concentration gradient –From high to low concentrations Osmosis (diffusion of water) –occurs across the membrane and through aquaporin (channel proteins)

Passive Transport Facilitated Diffusion –Channel Proteins –Carrier Proteins What would you expect the amino acid composition to be on the channel portion of the protein? –Polar and hydrophilic (so that polar/hydrophilic molecules can get across)

Active Transport Requires energy Ions, larger molecules Moves against the gradient –From low to high concentrations Ex. Sodium-Potassium pump –Requires ATP to work

Transport using vesicles Endocytosis –Cell “eating”– taking molecules in Exocytosis –Expelling molecules from a cell