BSCI330: Plasma Membrane and Plant Cell Wall REVIEW

Which of the following would you expect to diffuse freely through the plasma membrane?

Anything small and nonpolar (ex. steroid hormones) because the inside of the plasma membrane is hydrophobic.

Describe the nature and structure of phospholipids

Amphipathic (polar head and nonpolar tails)

One tail is saturated (no hydrogen bonds) and the other is unsaturated (one cis hydrogen bond creates a kink)—increases membrane fluidity

Phosphoglycerides

Two fatty acids + glycerol/glycerine + phosphate + polar head group

THREE TYPES:

• Phosphatidylethanolomine: Head is ETHANOL + AMINE; net neutral charge

• Phosphatidylserine: Head is AMINO ACID SERINE (amine +, carboxyl -, phosphate - = net - charge)

• Phosphatidylcholine: Head is tertiary amine; net neutral charge

Sphingolipid

Linker molecule sphinoglycine + had its own fatty acid chain (just add 1 more fatty acid with an AMIDE LINKAGE)

Sterols

Create a RIGID ring structure thru less flexible short bonds; increase stiffness to the membrane by adding more sterol.

HIGH TEMP: prevent from becoming too fluid

LOW TEMP: prevent from freezing solid

What is ergosterol?

Found in fungi!

Most antibiotics that target bacteria target differences between prokaryotic and eukaryotic cells.

Antifungal drugs target ergosterol

• nystatin and amphotericin B bind to ergosterol and form pores which allow ions to leak (disrupt function thru osmotic stress → cell dies)

• miconazole and lamisil target enzymes that synthesize ergosterol so that cells can’t synthesize new membrane —> stops proliferation

Phospholipids orientation in plasma membrane

Self-assemble into bilayer membrane

NO HYDROPHOBIC FORCE that causes them to cluster

Water molecules are most stable when interacting with hydrophillic heads, which means hydrocarbon tails associate with each other.

Explain the energetically unfavorable/favorable shape of bilayer

At the edges, water can interact with hydrophobic molecules, so it curls the plane into a sphere to avoid this.

What is the fluid mosaic membrane?

A non-uniform structure:

fluid - constituents can diffuse within lateral constraints

mosaic - not a continuous surface; made of discrete pieces (different lipid composition)

hydrocarbon tails can be different heights

Why is flip-flop rare?

The hydrophilic head would have to push through the center (can’t establish bonds that stabilize it); disorder of water overwhelms the membrane.

Lipid Rafts

Portion of lipid membranes where hydrocarbon tails are longer than the rest, resulting in protruding lipid heads

Membrane Asymmetry

The ganglioside is exclusive to the outer leaflet, whereas the phosphatidylserine is exclusive to the inner leaflet!

Voltage can be created on the inner leaflet to do work. ATP is used to maintain asymmetry.

Cell death can be signaled by asymmetry of the cell (phosphatidylserine on the outer leaflet).

Liposome

Technique used to study lipid membranes; spherical vesicle with a hallow center

Can form the the liposomes with different conditions (inside and outside) and test different membranes

Once liposome is formed, cannot change interior environment!

Black Membrane

Technique used to study lipid membranes; consists of a dish with two chambers and a wall between them—the wall has a hole/window

Planar lipid bilayer that is formed across this window

Does not necessarily mimic cellular conditions, but have access to change conditions on either side of the membrane and apply a voltage!

Membrane Proteins (function + types)

Function: molecular transport, signal transduction, structural support, anchorage to cytoskeleton, interactions with other cells

Types:
Peripheral - attached to surface

Integral - embedded into membrane

Transmembrane - crosses the membrane completely via transmembrane region

Plant Cell Wall

Can be separated from the plant cell

Made from carbohydrates (mainly cellulose)

Function of Cellulose

Provide tensil strength; resistance to stretching (one comparable to steel)

Pectin

Second set of complex carbohydrates in cell wall; short chain
Like a gel that resists the compressive force and distributes it

Lignin

Waterproofing agent

Tonicity in animal cells?

Hypotonic: Concentration of water is high outside → cell SWELLS

Hypertonic: Concentration water is low outside → cell SHRIVELS

Isotonic: stable environment

Turgor Pressure

Ability for plant cells to build up pressure from excess water (helps plants to grow)