B2.1: Membrane Structure

Cell Membrane

Bilayer of phospholipids that form a continuous barrier, separates cell from environment and controls passage of particles

Phospholipid Structure

Amphipathic, both hydrophilic and lipophilic

Polar head composed of glycerol and phosphate

Two nonpolar tails composed of fatty acids

Phospholipid bilayer

In aqueous environment, phospholipids spontaneously arrange into _________ (spherical)

Hydrophobic tails attracted to each other, hydrophilic heads H-bond with cytosolic (inside cell) and extracellular (outside cell) fluids

Fluid-Mosaic Model

Phospholipid bilayer with embedded proteins

fluid - phospholipids are free and move laterally

mosaic - different types of proteins (like tiles in a mosaic)

Fatty Acids in Cell Membrane

Phospholipid bilayer is sustained by hydrophobic interaction, drifts laterally but not transversely (not inside to outside of membrane)

Fatty Acid Structure

Saturated - straight, backs tightly, increases density of membrane, decreases fluidity and permeability

Unsaturated - Kinks, bent, packs loosely, decreases density, increases fluidity and permeability

*Ratio of saturated and unsaturated fatty acids are regulated, must be fluid but intact, must be permeable but not perforated*

Steroid

4 fused carbon rings, 3 cyclohexane rings, 1 cyclopentane ring

Cholesterol

Maintains membrane fluidity and stability in animal cells

Steroid rings align w/ membrane's fatty acid tails, hydroxyl groups align w/ membrane's phosphate heads

Commonly located between saturated fatty acids to create gaps between phospholipids, not needed btwn unsaturated bc already gap

*not solid or liquid, but liquid-ordered state*

Cholesterol Properties

Controls membrane fluidity by separating phosphate tails

membrane too fluid means too much permeability which means too much diffusion

Steroids buffer against temperature changes

in high temp - maintains impermeability

in low temp - stops crystallization

*also secures peripheral proteins*

Glycoproteins and Glycolipids

Involved in Cell to cell recognition

Glycoproteins

conjugated quaternary proteins embedded in membrane with carbs projecting out, ex ABO glycoproteins

Glycolipids

Lipid embedded in hydrophobic core w/ carbs projecting out, ex. antigen

Glycocalyx

The external surface of a plasma membrane that is important for cell-to-cell communication

formed w/ glycoproteins and glycolipids, its a carbohydrate-rich layer outside of membrane, anchored by proteins/lipids

-aqueous solution in gaps btwn carbs

Adjacent glycocalyces can fuse, helps to attach cells together to form tissues and attaches to extracellular matrix as an anchor

Tissue Formation

Cell-adhesion molecules (CAMs)

Proteins with some parts embedded and other parts protruding out

Junctions form when CAMs of adjacent cells bind their extracellular parts together

When CAMs bond...

same types of CAMs leads to tissue, different types of CAMs leads to junction

Some prevent extracellular movement, other promote it

ex. CAMs of benign tumors prevent metastasis (relocation, therefore tumor won't move)

Phospholipid Bilayer Permeability

Held together by hydrophobic interaction, membrane core has low permeability to polar solutes

Semi-Permeability of Phospholipid Bilayer

Only certain solutes will freely cross, dependent on size and polarity

small or nonpolar will cross

large or polar will not cross

NOT selective, won't choose specific things to let cross

-specific movement done by proteins channels and pumps

-ex. Cl- channels only allow Cl- ions to cross

Passive Transport

Particles are in continuous, random motion ******

they move down their own concentration gradient from higher to lower concentration, no ATP involved

They move towards equilibrium (the net movement, at least) and once reached, there is no NET movement (but molecules keep moving)

Passive Transport Types

simple diffusion, osmosis, facilitated diffusion (w/ channels)

Simple Diffusion

Movement of Particles directly through membrane, occurs only when phospholipid bilayer is permeable to particle

Factors:

-Temperature, rate of particle movement

-size, smaller particles face less resistance

-gradient, steeper concentration gradients = faster diffusion rate