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Chapter 3 Cell Membrane

Terms in this set (30)

Plasma Membrane
defines extent of cell

separates intracellular fluid from extracellular fluid

Structure: Lipid Bilayer with protein molecules "plugged into" or dispersed in it
Lipid Bilayer
Composed of:
-phospholipids (mostly)
-cholesterol
-glycolipids
Phospholipids
Polar "head" that is positively charged
HYDROPHILIC

Non-polar "tail" that is made of two fatty acid chains
HYDROPHOBIC

-the majority of membrane phospholipids are unsaturated, which gives kinks to their tails
Cholesterol
Polar region (its hydroxyl group)

Non-polar Region (its fused ring system)

Function: It wedges its platelike hydrocarbon rings between the phospholipid tails
STABILIZE THE MEMBRANE
INCREASES THE MOBILITY OF THE PHOSPHOLIPIDS AND FLUIDITY OF THE MEMBRANE
Glycolipids
lipids with attached sugar groups

Where: out cell mmebrane surface

Their sugar groups make the glycolipid molecule polar, whereas the fatty acid tails are nonpolar
Membrane Proteins
responsible for the specialized membrane functions

Types:
-Integral
-Peripheral
Integral Proteins
Firmly inserted in the lipid bilayer
Most are TRANSMEMBRANE PROTEINS that span the width of the membrane and protrude on both sides

All have hydrophilic and hydrophobic regions
(allows them to both interact with the fatty acid tails and the water inside and outside the cell)

Functions:
-Enzymes
-Most involved in Transport

TRANSPORT:
1) cluster together to form channels or pores through which water-soluble molecules can move (small ones). Bypasses the lipids
2) carriers, bind to substance and ferry it across the membrane
3)receptor for hormones and other chemical messengers and relay messages to the cell interior
Peripheral Proteins
Attach rather loosely to integral proteins
EASILY REMOVED
Tethered to cytoskeleton

Include a network of filaments that helps support the membrane from its cytoplasmic side

Functions:
-Enzymes
-Motor Proteins involved in mechanical functions (changing the cell shape during cell division and muscle contraction)
-Link Cells together

Many of the proteins branching out in the extracellular fuid are glycoproteins--GLYCOCALYX (carbohysdrate rich fuzzy sticky area at the surface of the cell

GLYCOCALYX provides highly specific biological markers by which approaching cells recognize one another
example: sperm and egg
Cells are bonded together by:
1) Glycoproteins in the glycocalyx act as an adhesive
2) Wavy contours of the membranes of adjacent cells fit togther in a toungue-and-groove fashion
3)special membrane junctions are formed (MOST IMPORTANT)
Tight Junctions
Definition: series of integral protein molecules in the plasma membranes of adjacent cells fuse together

IMPERMEABLE JUNCTION THAT ENCIRCLES THE CELL

-prevent molecules from passing through the extracellular space between adjacent cells

Example: lining of digestive tract
Junctions:
-Tight
-Desmosomes
-Gap
Desmosomes
-"anchoring junctions"

-mechanical couplings scattered along the sides of the cells next to each other that keep them from separating

-anchoring junctions bind cells next to each other and form a tensions reducing network of fibers. guy wires
Gap Junctions
Definition: Communicating junctions allow ions and small molecules to pass from ne cell to th net for cell to cell communcation

-The cells are connected by hollow cylinders called connexons
-The many types of connexon proteins vary the selectivity gap junction channels.

Example: present in electrically excitable tissues (heart and smooth muscle) BECAUSE ION PASSAGE FROM CELL TO CELL HELP SYNCHRONIZE THEIR ELECTRICAL ACTIVITY AND CONTRACTION
Passive Processes
substances cross the membrane without any energy input from the cell

-Diffusion
-Filtration
-Simple Diffusion
-Facilitated Diffusion
-Osmosis
Diffusion
-molecules move along concentration gradient

-the greater the difference in concentration of the diffusing molecules and the ions between the two area, the more collisions occur and the faster the net diffusion of the particles
-SPEED OF DIFFUSION: influences by size of molecules and tempterature
(Solution reaches EQUILIBRIUM with molecules moving equally in all directions (no NET movement)

Examples: movement of ions across cell membranes, movement of neurotransmitters between two nerve cells

A molecule will diffuse through the membrane if it is:
-lipid soluble
-small enough to pass through membrane channels
-assited by a carrier molecule
Simple Diffusion
-unassisted diffusion of lipid-coluble or very small particles

-nonpolar and lipid soluble substances diffuse directly through the lipid bilayer

-oxygen always diffuses from blood into cells

carbon dioxide always diffuses from cells into blood
Facilitated Diffusion
-Lipid insuluble solutes pass through the lipid bilayer
1) bind to protein carriers in the membrane and is ferried across
2) moves though water filled protein channels

Carriers: transmembrane integral proteins that show specificity for molecules of a certain polar substance or class of substances that are too large to pass through membrane channels (sugars and amino acids)

Channels: transmembrane proteins that serve to transport substances (ions or water) through aqueous channels from one side of the membrane to the other. Channels are selective due to pore size and the chargesof the amino acids lining the channel
Some channels are always open while some channels are gated

Unlike simple diffusion, the rate of facilitated diffusion can be mediated because the permeability of the membrane can be changed by regulating the activity or number of ion carriers or channels
Osmosis
-the diffusion of water across a semi-permeable membrane

-Can move directly through the plasma membrane
-can move through water-specific channels created by transmembrane proteins called aquaporins

-Water moves along concentration gradient (moves to where there are more solutes)

OSMOLARITY: the total concentration of all solute particles in a solution

HYDROSTATIC PRESSURE: the back pressure exrted by water against the membrane)

The greater the amount of nondiffusable, or nonpenetrating sulutes in a cell, the higher the osmotic pressure and the reater the hysrostati =prassure
Tonicity
the ability of a solution to change the shape or tone of cells by altering their internal water volume

ISOTONIC: solutions with the same concentrations of nonpenetrating solutes as those found in cells
-celle retain normal shape
-no net loss or gain of water
=the body's extracellular fluids and most intravenous solutions are isotonic

HYPERTONIC: solutions with a higher concentration of nonpenetrating solutes than seen in the cell
-cells immersed in hypertonic solution shrink and lose some of their water
-Example: a strong saline solution

HYPTONIC: solutions are more dilute- contain a lower concentration of nonpenetrating solutes
-cells plump up quickly as water rushes into them
-water continues to enter cells until they burst or lyse
-Example: distilled water (contains no solutes EXTREME)
Active Transport
WHENEVER A CELL USES THE BOND ENERGY OF ATP TO MOVE SOLUTES ACROSS THE MEMBRANE

-Requires carrier proteins that combine SPECIFICALLY and REVERSIBLY with the transported substances
-Requires ATP

-Ion Pump
-Endocytosis
-those in which the cell ingests small patches of the plasma membrane and moves subtances from the cell exterior to the cell interior
1.) Phagocytosis
2.) Pinocytosis
3.) Receptor mediated endocytosis
-Exocytosis
Endocytosis
Vesicular transport processes in which the cell ingests small patches of the plasma membrane and MOVES SUBSTANCES FROM THE EXTERIOR OF THE CELL TO THE INTERIOR

Types:
-Phagocytosis
-Pinocytosis
-Receptor-mediated endocytosis
Phagocytosis
-the cell engulfs a relatively large or solid material (clump of bacteria, cell debris, inanimate particles)

-particle binds to receptors on cell surface
-cytoplasmic extensions called pseudopods engulf particle (forms PHAGOSOME)
-Phagosome fuses with lysosome and its contents are digested
-indigestible contents are ejected from the cell by exocytosis

Phagocytes:
INGEST AND DISPOSEOF BACTERIA, FOREIGN SUBSTANCES, AND DEAD TISSUE CELLS

Movement: move by amoeboid motion (flowing of cytoplasm into temporare pseudopods to creep around)

-macrophages
-certain white blood cells
Pinocytosis
(fluid-base endocytosis) (cell drinking)

-a bit of infolding plasma membrane (clathrin coated pit) surround small volume of extracellular fluid containing dissolved molecules
-droplet enters cell and fuses with endosome (vesicle)

UNLIKE PHAGOCYTOSIS, PINOCYTOSIS IS A ROUTINE ACTIVITY OF MOST CELLS
It gives them a nonselective way of sampling the extracellular fluid
-Particularly important in cells that absorb nutrients, such as cell that line the intestines
Receptor Mediated Endocytosis
-the main mechanism for the specific ENDOCYTOSIS and TRANSCYTOSIS of most macromolecules
-very selective
-allows cells to concentrate material that is present in very small amound of extracellular fluid so it's convenient

-extracellular substances bind to specific receptor proteins in regions of clathrin-coated pits
-allows cell to ingest and concentrate specific substances (ligands) in protein coated vesicle
-ligands released inside cell
OR
-ligands combined with lysosome to digest contents.

CELLS TAKEN IN BY RECEPTOR MEDIATED ENDOCYTOSIS:
-enzymes
-insulin (and some other hormones)
-low density lipoproteins (such as cholesterol attached to a transport protein)
-Iron

UNFORTUNATELY VIRUSES AND TOXINS USE THIS ROUTE TO ATTACK OUT CELLS
Vesicular Transport
Fluids containing large particles and macromolecules are tranported across cellular membranes inside membranous sacs called vesicles
EXOCYTOSIS and ENDOCYTOSIS use these

The 'coated pit' is most often coated in Clathrin
-CLATHRIN IS A PROTEIN COATING FOUND ONT HE SYTOPLASMIC FACE OF THE PIT. The Clathrin coat (clathrin and some accessory proteins) acts both in cargo selection and in deforming the membrane to produce the vesicle

OTHER TYPES OF PROTEIN COATINGS:
-Caveolae
Involed in receptor mediated cytosis called POTOSIS
These are smaller than clathrin coated vesicles, and their cage-like protein coat is thinner and composed of a different protein called CAVEOLIN
These vesicles appear to provide sites for cell signaling and cross talk between signaling pathways
Closely associated with lipid rafts that are platforms for G-proteins, receptors for hormones (insulin), and enzymes involved in cell regulation

-Coatomer (COP1 or COP2) Proteins
Used in most types of intracellular VESICULAR TRAFFICKING, in which wesicles transport substances between organelles
Exocytosis
-stimulated by a cell surface signal
Binding of a hormone to a membrane receptor
change in membrane voltage

Functions:
-accounts for hormone secretion
-neurotransmitter release
-mucous secretion
-ejection of wastes

How it Works:
-the substance to be removed from the cell is enclosed in a protein-coated membranous sac called a vesicle
-vesicle migrates to plasma membrane and fuses with it, then ruptures
-sac contents spill out of cell

-v-SNARES (vesicle) --recognize certain plasma membrane proteins, and bind to t-SNARES (target)
-this binding causes the membranes to 'corkscrew' together and fuse, rearranging the lipid monolayers without mixing them
-membrane material added by exocytosis is removed by endocytosis--the reverse process
Membrane Potential
Definition: voltage across a membrane
-voltage: electrical potential energy resulting from the separation of oppositely charged particles

In Cells: Oppositely charged particles are ions and the membrane is the barrier that keeps them apart

In their resting state, all cells exhibit RESTING MEMBRANE POTENTIAL.
--For this reasons all cells are said to be polarized

Polarized: Inside is negative compared to its outside (charge separation)
What Causes Resting Membrane Potential
How it Comes About:
-diffusion causes ionic imbalances that polarize the membrane

Resting Membrane Potential is determined by the concentration gradient of K+ and the differential permeability of the plasma membrane to K+ and other ions

K+ diffuses out of cell along its concentration gradient, but the net charge inside the cell gets increasingly negative and pulls K+ back
AT THIS POINT K+'s CONCENTRATION GRADIENT IS EXACTLY BALANCED BY THE ELECTRICAL GRADIENT (membrane potential)
ONE K+ ENTERS THE CELL AS ONE LEAVES

Na+ also contributes to Resting Membrane Potential
Na+ is attraction to the interior of cells by its concentration gradient
HOWEVER, the membrane is much more permeable to K+, which means K+ still helps determine Resting Membrane Potential
How Resting Potential is Maintained
-Through ion pumps

-If more Na+ enters, more Na+ is pumped out

-The Na+-K+ pump couples Na+ and K+ transport and each turn of the pump ejects
3Na+ out of the cell and 2K+ back in

Na+-K+ pump maintains both the membrane potential (charge separation) and the osmotic balance
If Na was not continuously taken out of cells, the osmotic gradient would cause cells to burst because too much water would go into them
Electrochemical Gradients
Ions diffuse according to electrochemical gradients

recognizes the effect of both electrical and concentration (chemical) forces

Example: Although diffusion of K+ across the membrane is aided by the membrane's greater permeability to it and by the ion's concentratino gradient, its diffusion is resisted somewhat by the positive charge on the cell's exterior

Aided: Easy diffusion because of permeability
Resisted: positive charge on outside of cell (electrochemical gradient)

Na+
Aided: drawn into cell by steep electrochemical gradient
Resisted: cell's relative impermeability to it
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