AP Bio Topics 3 & 4 Plasma Membrane and Membrane Permeability

Why are the heads of phospholipids hydrophilic?

Because they’re polar and water is polar

Plasma Membrane

Separates the internal cell environment from the external cell environment. Composed primarily of phospholipids. Forms a bilayer

What does it mean for the Phospholipid heads to be amphipathic?

Have both hydrophilic and hydrophobic regions

Polar hydrophilic heads

(phosphate regions) oriented toward aqueous environments

Nonpolar hydrophobic tails

(fatty acid regions) are facing inwards away from the aqueous environment (Hydrophobic interior causes the membrane to be selectively permeable)

Fluid Mosaic Model

A model to describe the structure of cell membranes

Fluid: (Fluid Mosaic Model)

membrane is held together by weak hydrophobic interactions and can therefore move and shift. Temperature affects fluidity.

Unsaturated hydrocarbon tails

Helps maintain fluidity at low temps, kinked tails prevent tight packing of phospholipids.

Cholesterol

Helps maintain fluidity at high and low temps. High temp: reduces movement. Low temp: reduces tight packing of phospholipids.

Mosaic:

Comprised of many macromolecules

There are two major categories of proteins in the membrane:

Integral proteins and peripheral proteins

Integral proteins:

Proteins that are embedded into the lipid bilayer. Can be hydrophilic, hydrophobic, or both. Determined by R groups. Hydrophilic (charged or polar side chains). Hydrophilic regions make up the interior of the channel or poor hydrophilic regions are exposed to cytosol. hydrophobic equals nonpolar side chains, hydrophobic regions, make up the protein service and interact with fatty acids on the interior of the membrane.

Peripheral proteins:

Proteins that are not embedded into the lipid bilayer, loosely bonded to the surface

Selective permeability:

The ability of membranes to regulate the substances that enter exit. Due to hydrophobic interior (non-polar hydrophobic tails). easy passage across the membrane: small nonpolar, hydrophobic molecules (hydrocarbons CO2 02N2) Difficult passage or protein, assisted passage: hydrophilic, polar molecules, large molecules, ions because of the non-polar hydrocarbon tails

Cell walls

Some organisms also have cell walls that cover their plasma membrane (bacteria, Archaea, fungi and plants). Function of cell walls: provides structural boundary (defines and supports the self shape), permeability barrier for some substances, protection from osmotic lysis (cell bursting with excess water goes into cell)

Plants

The cell wall is composed of cellulose. Thicker than plasma membranes. Contains plasmodesmata, which are whole like structures in the cell wall with cytosol that connect adjacent cells.