Chapter 5: Membrane Structure, Synthesis and Transport

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Vocab from Ch5

Phospholipids

composed of hydrophobic fatty acid tails and hydrophilic phosphate heads

Proteins and carbohydrates

carbohydrates vary in number; most membranes are 50% protein by mass

leaflet

half of a phospholipid bilayer; strikingly asymmetrical

Integral membrane protein

protein that cannot be released without dissolving the membrane

Transmembrane protein

most common integral membrane protein; has at least one region physically inserted into the hydrophobic region of the bilayer

Transmembrane segments

Stretches of nonpolar amino acids that span or traverse the membrane from one leaflet to the other

Lipid-anchored protein

Integral membrane protein that has amino acid side chain covalently attached to a lipid molecule

Peripheral membrane proteins

AKA extrinsic proteins; do not interact with hydrophobic interior rather noncovalently bound to protruding integral proteins or polar phosphate heads

Lipid raft

group of lipids that float together in a unit within a larger sea of lipids; high amount of cholesterol

Factors that affect membrane fluidity

1) Shorter tails= less likely to interact --> more fluidity
2) Double bonds present in acyl tails --> unsaturated --> more fluidity
3) presence of cholesterol --> less fluid at high temps, more fluid at low temps

Cholesterol

Short and rigid planar molecule produced by animal cells that stabilizes the membrane (plants have similar phytosterols)

Larry Frye and Michael Edidin Experiment

In 1970, Larry Frye and Michael Edidin fused human and mouse cells, cooled some, and incubated then cooled others then exposed them to labeled mouse antibodies revealing lateral movement in incubated cells

glycosylation

process of covalently attaching a carbohydrate to a lipid or protein resulting in a glycolipid or glycoprotein

role of carbohydrates

1) sometimes serve as recognition signals for other cellular proteins
2) play a role in blood type

Cell coat AKA glycocalyx

carbohydrate-rich zone on the surface of certain animal cells which sheilds them from mechanical and physical damage

FFEM Freeze Fracture electron microscopy

used to analyze the interiors of phospholipid bilayers; invented by Russel Steere in 1957; the sample is frozen in liquid Nitrogen and split with a knife

Ways new lipids can be transfered to diff membranes

1) diffuse laterally to the nuclear envelope
2) transported via vesicle to the golgi, lysosomes, vacuoles, or plasma membrane
3) lipid exchange proteins

Lipid exchange proteins

extract a lipid from one membrane, diffuse through the cell, and insert into another membrane

Lipid Synthesis

Occurs at the cytosolic leaflet of the ER

Transmembrane segment composition

If a polypeptide has a stretch of 20 hydrophobic amino acids that forms an alpha helix, it will be a transmembrane segment

N-Linked glycosylation

Involves attachment of a carbohydrate to the amino acid the Nitrogen of asparagine in a polypeptide chain; also occurs in archaea

O-Linked glycosylation

Involves addition of a string of sugars to the Oxygen atom of serine or threonine side chains in polypeptides; occurs only in the Golgi apparatus; important for production of proteoglycans

Proteoglycans

help organize the extracellular matrix; make up mucus

Highly permeable to phospholipid bilayer

1) Gases (CO2, N2, O2)
2) very small, uncharged molecules (ETHANOL)

Moderately permeable to phospholipid bilayer

1) Water
2) Urea (H2NCONH2)

Low permeability to phospholipid bilayer

1) Polar organic molecules
2) Sugars

Not permeable to phospholipid bilayer

1) Ions (Na+, K+, Mg+, Ca2+, Cl-)
2) Polar charged molecules and macromolecules (amino acids, ATP, proteins, polysaccharides, nucleic acids)

electrochemical gradient

Dual gradient that has both electrical and chemical components; occurs with solutes that have a net positive or negative charge

Crenation

shriveling as a result of being placed n a hypertonic solution

plasmolysis

in PLANT cells, when in hypertonic solution, the inner membrane shrinks away from the cell wall

Turger

when cell membran= is pressed up against the cell wall

Channels

Open passageway for facilitated diffusion; most are gated

Ligand-gated channels

controlled by noncovalent binding of small molecules (ligands)

Transporters (AKA carriers)

bind to solutes in a hydrophilic pocket and undergo a conformational change; slower than channels; principal method for organic molecules, hormones and neurotransmitters; key role in export

Uniporters

transporters that bind a single ion or molecule

Symporters (AKA cotransporters)

transporters that bind two or more ions or molecules and transport them in the same direction

Antiporters

Transporters that bind two or more ions or molecules and transport them in the opposite direction

Primary active transport

use of a pump

pump

a type of transporter that directly uses energy to transport a solute against a gradient

secondary active transport

use of pre-existing gradient to drive active transport of another solute

Na+/K+ ATPase

Proposed by Jens Skou; Antiporter that transports 3 Na+ out of the cell and 2 K+ into the cell using energy from ATP

electrogenic pump

pump that generates an electrical gradient

exocytosis

vesicle is loaded with cargo, released from the golgi, and fuses with the plasma membrane. Protein coat wraps around as vesicle is formed and is shed before the vesicle fuses with the plasma membrane

endocytosis

1) cargo binds to receptor and the receptors aggregate, the receptors cause coat proteins to bind the surrounding membrane. The plasma membrane invaginates as coat proteins cause a vesicle to form
2) The vesicle is released into the cell
3) The protein coat is shed
4) The vesicle fuses with an internal organelle such as a lysosome
5) cargo is released into the cytosol

pintocytosis

formation of membrane vesicles to internalize the extracellular fluid

phagocytosis

giant endocytosis (like engulfing a bacterium)

myelin

a fatty substance that helps insulate neurons and speeds the transmission of nerve impulses; has 3% carbohydrate

lamella

a thin membrane that is one of the calcified layers that form bones; has no carbohydrates

hydrophobic exclusion

The tendency of nonpolar molecules to aggregate together when placed in water. Exclusion refers to the action of water in forcing these molecules together; relevant to imbedded proteins

Which transmembrane protein processes drugs?

G-protein coupled receptors and channels ABC transporters and solute carriers are also targets for drug therapy

Cotranslational imports

sorting signals (first thing to think of)

proteoglycans

a glycoprotein consisting of a small core protein with many carbohydrate chains attached, found in the extracellular matrix of animal cells.

Hyperosmotic

solution with the higher SOLUTE concentration

Hypo-osmotic

solution with the lower SOLUTE concentration

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