AP Bio Cell Transport

plasma membrane

composed of phospholipid bilayer with embedded proteins and cholesterol molecules

fluid-mosaic model (of the plasma membrane)

describes the fluidity of the membrane and its components. Phospholipids and embedded proteins move freely around one another, forming a “fluid-mosaic”

plasma membrane functions

separates the internal cytoplasm frmo the external environment of the cell

controls the movement of substances in and out of the cell

allows cells to communicate with one another and interact with their external environment

Plama membrane components

liquid component referred to as a phospholipid bilayer

hydrophilic polar heads of the phospholipid molecules line the internal (cytoplasmic) and external (extracellular) surface of the membrane

hydrophobic fatty-acid tails of the phospholipids are sandwiched in between

protein molecules

float around like icebergs in a sea of phsopholipids

peripheral proteins

found on the inner membrane surface

integral proteins

partially or wholly embedded (transmembrane) in the membrane, amphipathic - have both hydrophobic and hydrophilic regions, portion embedded within the phospholipid is hydrophobic, while the portion of the protein exposed to the interior or exterior of the cell is hydrophilic

cholesterol

embedded in the phospholipid bilyaer controls the fluidity of the membrane (makes the membrane more or less fluid depending on the temperature)

At warm temperatures, retrains movement of phospholipids

At cool temperatures, it maintains fluidity by preventing tight packing

glycoproteins

proteins with attached carbohydrate chains

glycolipids

phospholipids with attached carbohydrate chains (golgi adds on chains after the protiens and lipids are produced by the ER)

Channel proteins

allow passage of specific ions or molecules through membrane via a channel in the protein (tunnel)

carrier proteins

combine with the molecule or ion to be transported and assist its passage through membrane (opens and closes)

cell recognition proteins

eg. glycoproteins, help cells identify one another

receptor proteins

shaped in such a way that specific molecules bind to it, allow a cell to respond to signals from other cells

enzymatic proteins

catalyze specific reactions

junction proteins

attach adjacent cells so that a tissue can fulfill a function

Permeability of plasma membrane

small, hydrophobic (nonpolar) molecules (eg. CO2, O2, and glycerol) freely cross the membrane by passing through the phospholipid bilayer

Polar or charged substances (eg. sugars and ions) do not cross the membrane and rely on transport proteins

Cell size

smaller cells have a large SA/V ratio = more efficiency of transporting material in and out of the cell

passive transport

movement of substances across the cell membrane without energy

both sides get closer to equilibrium (same concentration)

high to low concentration 

diffusion, osmosis, and facilitated transport

Diffusion

net movement of molecules down a concentration gradient

molecules move both ways along the gradient, but net movement is from high to low concentration

NET movement stops when the molecules reach a dynamic equilibrium

molecules still move both ways, but at equal rates

solute concentration is uniform - no concentration gradient

Osmosis