Cell Membrane and Transport

Structure andFunction

Plasma Membrane *Plasma Membrane is made of a bilayer of

Phospholipids.A phospholipid has a hydrophilic head and

hydrophobic tails, it is amphipathic.This creates a special dynamic that allows the

membrane to maintain an internal environment different from the external and therefore maintain internal homeostasis

Fluid Mosaic Model *4 Main Components of the plasma

membrane contribute to the model’s name.

1. Phospholipids form the bilayer. They are viscous and constantly rearranging

themselves laterally and flip-flopping this provides the Fluid part of the model.

The unsaturated and saturated tails prevent the phospholipids from tightly packing, or disassociating and falling apart.

Fluid Mosaic Model *2. Cholesterol is embedded into the bilayer. It acts as glue reducing the fluidity of the

membrane at moderate temps but at lower temps prevent the membrane from freezing.

The cholesterol is a temperature buffer, many winter plants increase the production of cholesterol in the winter to prevent their tissues from freezing.

Fluid Mosaic Model *3. Proteins are embedded in the bilayerProteins give the model its pattern

(mosaic).Proteins have many functions within the

membrane: Transport, enzymes, signals, cell to cell recognition and joining

Proteins can be integral/transmembrane or peripheral on the surface only.

Fluid Mosaic Model *4. Glycolipids and Glycoproteins These are short polymers of sugars attached to

lipids or proteins within the membraneFunction in cell to cell recognition and

signaling, organization of tissues, and function of immune system

These biological markers are in a unique pattern on every cell type, and essential to cell recognition ex. A sperm recognizes the ovum, and cells of the immune system recognize bacteria

Synthesis of membranesMembranes have distinct inside and outside

surfacesEach protein has a directional orientation

due to its functionVesicles are the source of membrane

renewal. As they fuse the inside of the vesicle becomes continuous with the outside of the cell. Remember Vesicles are generated by the ER.

So the proteins that end up on the outside of the cell start on the inside surface of the ER.

Other specialized structuresMicrovilli – extensions of the plasma membrane that

increase the absorption area of a kidney or intestinal cell.

Membrane junctions- weave cells together into tissuesTight junctions- integral proteins of adjacent cells fuse

together reducing extracellular spaceDesmosomes- create an internal network in which filaments

tie cells together to prevent tearing of cellular sheetsGap junctions- hollow channel in which the plasma

membranes of adjacent cells fuse so cytosol can be exchanged important in electrically excitable tissues like the heart and smooth muscle where ion passage helps synchronize activity

Selectively PermeableThe plasma membrane is selectively

permeableHydrophilic surface and hydrophobic core

prevents polar molecules from moving through the membrane. Even water which is very small can only navigate through slowly.

Proteins often provide a path in which molecules can pass into/out of the cell

Passive Transport *Diffusion is the movement of a substance across a

membrane from an area of high concentration to an area of low concentration with out spending energy, Osmosis is the diffusion of water.

Facilitated Diffusion uses Transport Proteins that provide a path for polar molecules and specific ions, each is specific to the substance it helps.

Some are channel proteins that allow substance to move easily through a tunnel down the center, others are carrier proteins that bind with the substance but do not utilize energy in the transport

Ex. Aquaporins, are channel proteins specific to water transport, also ion channels, and gated channels

Ex. Glucose transporters, are carrier proteins

Diffusion *Matter is in constant motion, thermal motion.

This property drives diffusion. Molecules move down their concentration

gradient until they reach dynamic equilibrium. “High to Low”

Concentration of solutes often drive the diffusion of water, osmosis

Remember the terms solute, solvent, solution?

Osmosis *Tonicity, the ability of a solution to cause a cell to

gain or lose water. It depends on the concentration of solutes that can not diffuse across the membrane

Isotonic- solute concentrations are sameHypertonic- solute concen. is higherHypotonic- solute concen. is lowerOsmoregulation, the control of water balanceA red blood cell placed in: pg 133 isotonic, stays the same, a state of dynamic

equilibrim is maintained. hypertonic will shrivel, as water rushed out of the

cell. hypotonic will swell/burst open as water rushes

into the cell.

Osmosis in plant and animal cells

Homeostatic balance (not Hbio)*If a patient has excess fluid in the extracellular spaces a

hypertonic solution may be administered to draw the fluid into the blood to then be eliminated by the kidney

If a patient is dehydrated, a hypotonic solution may be given to rehydrate the tissues (sport drinks usually work)

Filtration is a process which forces water and solutes through a capillary cell membrane using a pressure gradient

Blood holds a hydrostatic pressure forcing fluids with vital solutes out to bathe the cells

This occurs in your kidneys, pressure pushes waste solutes out of the blood into kidney where they are collected and eliminated as urine.

Osmosis and Plant cells *Unlike animal cells Plant cells have a cell wall. The

cell wall’s main function is support. This is assisted by the central vacuole and internal turgor pressure.

A cell that is turgid, rigid, has a high internal turgor pressure, which cancels out the water potential of a hypotonic solution and prevents the cell from bursting.

A cell that is flaccid, has lost pressure and may become limp and the plant may wilt.

A cell may die if placed in a hypertonic solution because the complete loss of turgor pressure causes plasmolysis, the plasma membrane pulls away from the cell wall and the cytoplasm shrivels.

Plasmolysis in Elodea plant

Active Transport *Work against a gradient and utilize cellular energy

(ATP)Carrier Proteins are phosphoralized by ATP and

change their shape, binding to the molecules they transport across the membrane.

Example: Sodium-Potassium Pump brings 2 K+ inside and 3 Na+ out (electrogenic pump)

This creates a membrane potential (+ outer surface charge and – internal surface charge) this drives other ion transport, cations in and anions out, this is a chemical force

Together this is called an electrochemical gradient. http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.html#

Active Transport cont’dProton Pump, actively transport H+ out of

the cell to generate electrochemical gradient essential in cell respiration and photosynthesis.

Cotransport, uses a proton pump to drive a carrier protein (utilizes H+ gradient instead of ATP)

see handout in packet

Bulk Transport *

(see handout in packet) http://www.johnkyrk.com/CellIndex.html

EndocytosisPhagocytosis cellular eating,ex

amoeba,WBCPinocytosis cellular drinking, nonspecific

intakeReceptor Mediated endocytosis, between

the neuronsExocytosis, secretion to export products ex. Neurons release neurotransmitters in bulk

Bulk Transport

Toxins interupt cellular pathways (not Hbio)*Toxins from Anthrax, Diphtheria, Tetanus,

and bacteria that cause Cholera or Botulism interrupt cell signal pathways by infiltrating the plasma membrane.

Cholera enters an intestinal cell via receptor -endocytosis and rides a “lipid raft” to the ER where it unfolds its deadly proteins causing disease

Botulism toxin causes paralysis by blocking the release via exocytosis, of acetylcholine, the neurotransmitter that links neurons to muscle cells.