Membrane Structure and Function

What is a plasma membrane and how is it composed?

It is membrane at the boundary of every cell, composed of a phospholipid bilayer and proteins.

What is meant by fluid-mosaic model of a membrane?

In 1972, S.J.Singer and G. Nicolson proposed that membrane proteins are dispersed, individually inserted into the phospholipid bilayer with their hydrophilic regions protruding. This molecular arrangement would maximixe contact of hydrophilic regions of proteins and phospholipis with water in the cytosol and extracellular fluid, while providing their hydrophobic parts with a nonaqueous environment. In this fluid-mosaic model, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

What are protein functions in the membrane?

- Transport

- Enzymatic activity

- Receptor sites for signals

- Cell adhesion

- Cell-cell recognition

- Attachment to the cytoskeleton

Facts:

- A membrane is held together primarily by hydrophobic interactions (much weaker than covalent bonds).

- More than 50 kinds of proteins have been found so far in the plasma membrane of red blood cells.

- The Davson-Danielli sandwich model of the membrane has been replaced by the fluid mosaic model, in which amphipathic proteins are embedded in the phospholipid bilayer.

- Short chains of sugars are linked to proteins and lipids on the exteror side of the plasma membrane, where they interact with surface molecules of other cells

- One solute’s “downhill” diffusion drives the other’s “uphill“ transport

Key terms:

Integral proteins = transmembrane protein eith hydrophobic regions that extend into and often completely span the hydrophobic interior of the membrane and with hydrophilic regions in contact with the aqueous solution on either side of the membrane

Peripheral proteins = proteins loosely bounded to the surface of a membrane or to part of an integral protein and not embedded in the lipid bilayer

Glycolipids = molecules formed of membrane carbohydrates covalently bonded to lipids

Glycoproteins = membrane carbohydrates + proteins

Aquaporins = channel proteins (facilitate the passage of water molecules through the membrane)

Diffusion = the movement of molecules of any substance so they spread out evenly into the available space (passive transport)

Osmosis = the diffusion of water

Facilitated diffusion = the spontaneous passage of molecules or ions across a membrane with the assistance of specific transmembrane transport proteins

Passive transport = no energy required / invested

Active transport = energy required

Summary:

The Davison Danielli sandwich model of the cell membrane (1935) has been replaced by the fluid mosaic model, in which amphipathic proteins are embedded in the phospholipid bylayer. Proposed by Singer and Nicolson in 1972.

The plasma membrane controls the processes of the exchange of molecules and ions of the cell with its surrounding.

Besides enzymatic activity, receptor sites for signals, cell adhesion, cell-cell recognition and attachment to the cytoskeleton, the protein function in the membrane is transport, but the question is how do materials get across the cell membrane?! ... There are two ways of transport and these are passive and active. Passive (diffusion, osmosis, facilitated diffusion) does not require cellular energy. The way of movement of the atoms and ions is from higher to lower concentration until the equilibrium is reached. Active transport (Carrier-Mediated, endocytosis, exocytosis), unlike passive, requires cellular energy (ATP).

Extra:

osmosis (video)