The Cell : Membrane Transport

Plasma Membrane

e boundary that separates the living cell from its surroundings

Selective Permeability

allowing some substances to cross it more easily than others

Phospholipids

amphipathic molecules, containing hydrophobic and hydrophilic regions

fluid mosaic model

a membrane is a fluid structure with a “mosaic” of various proteins embedded in it

Proteins

determine most of the membrane’s specific functions

Peripheral proteins

proteins that are bound to the surface of the membrane

Integral Proteins

proteins that penetrate the hydrophobic core

Six

How many functions of membrane proteins are there ?

Transport, Enzymatic Activity, Signal Transduction, Cell - Cell Recognition, Intercellular joining & Attachment to cytoskeleton & ECM

Give the 6 functions of membrane proteins

Exocytosis

These release secretory proteins from the cell

Hydrophobic (non - polar) molecules

such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly

Hydrophilic molecules

including ions and polar molecules do not cross the membrane easily

Passive Transport

diffusion of a substance across a membrane with no energy investment.

Diffusion

the tendency for molecules to spread out evenly into the available space

dynamic equilibrium,

as many molecules cross the membrane in one direction as in the other.

Concentration Gradient

the measurement of how the concentration of something changes from one place to another ; region along which the density of a chemical substance increases or decreases

Osmosis

the diffusion of water across a selectively permeable membrane ; Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides

Tonicity

ability of surrounding solution to cause a cell to gain or lose water

Isotonic Solution

A solute concentration is the same as that inside the cell; no net water movement across the plasma membrane ; no changes will happen to the cell

Hypertonic Solution

Solute concentration is greater than that inside the cell; cell loses water ; cell becomes shriveled

Hypotonic solution

Solute concentration is less than that inside the cell; cell gains water ; cells swells

Osmoregulation

The control of solute concentrations and water balance, is a necessary adaptation for life in such environments

Facilitated Diffusion

the passive movement of molecules along the concentration gradient

Transport Proteins

speeds the passive movement of molecules across the plasma membrane

Carrier Proteins

undergo change ; subtle in shape that translocates the solute binding site across the membrane

Channel Proteins

provide corridors that allow a specific molecule or ion to cross the membrane

Ions channels and Aquaporins

Two types of Channel proteins

Ion Channels

facilitate the diffusion of ions

Aquaporins

facilitate the diffusion of water

Active Transport

moves substances against their concentration gradients ; allows cells to maintain concentration gradients that differ from their surroundings.

ATP

is required by Active transport

Membrane Potential

the voltage difference across a membrane

Voltage

created by difference in the distribution of positive and negative ions across a membrane

Electrochemical gradient

drive the diffusion of ions across a membrane

Electrogenic pump

is a transport protein that generates voltage across a membrane

Electrical Force

(the effect of the membrane potential on the Ion’s movement)

Chemical Force

(the Ion’s concentration gradient)

Sodium-potassium pump

the major electrogenic pump of animal cells.

Proton Pump

main electrogenic pump of plants, fungi, and bacteria

Small Molecules & water

enter or leave the cell through the lipid bilayer or via transport proteins

Large Molecules

such as polysaccharides and proteins, cross the membrane in bulk via vesicles

Energy

Bulk transport requires (-----)

Exocytosis

transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell

Exocytosis

secretory cells use (------) to export their products

Endocytosis

the cell takes in macromolecules by forming vesicles from the plasma membrane