Question

Biological Membranes ?

Answer 1

Plasma membrane is the boundary that separates the living cell form its surroundings

Cellular membranes

-transport, enzymatic activity, signal transduction, cell – cell recognition, inter-cellular joining, attachment to the Cytoskeleton and extracellular matrix.

Membranes have a distinct inside and outside faces
the asymmetrical distribution of proteins, lipids and associated carbohydrates in the plasma membrane is built by the ER and Golgi apparatus.

Membrane structure results in selective permeability

A cell must exchange materials with its surroundings, a process controlled by the plasma membrane
Plasma membranes are selectively permeable regulating the cells molecular traffic.
Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly.
Polar molecules, such as sugars, do not cross the membrane easily.

Passive transport: Diffusion across a membrane with no energy investment

Diffusion is the tendency for molecules to spread out evenly into the available space.
Although each molecule moves randomly, diffusion of a population of molecules may exhibit a net movement in one direction.
Ay dynamic equilibrium, as many molecules cross one way as cross in the other direction.

Effect of Osmosis on water balance

Osmosis is the diffusion of water across a selectively permeable membrane
Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration.


Water Balance of cells without walls

Tonicity is the ability of a solution to cause a cell to gain or lose water.
Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane.
Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water
Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water.

Transport Proteins

Transport proteins allow passage of hydrophilic substances across the membrane.
Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel.
Channel proteins called aquaporins facilitate the passage of water.

Facilitated Diffusion: Passive transport aided by proteins

In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane.
Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane
Channel proteins include
- Aquaporins, for facilitated diffusion of water
- Ion channels that open or close in response to a stimulus (gated channels).

Active transport uses energy to move solutes against their gradients

Facilitated diffusion is still passive because the solute moves down its concentration gradient
Some transport proteins, however, can move solutes against their concentration gradients
Active Transport moves substances against their concentration gradient
Active transport requires energy, usually in the form of ATP
Active transport is performed by specific proteins embedded in the membrane

Ion pumps maintain membrane potential

Membrane potential is the voltage difference across a membrane
Voltage is created by differences in the distribution of positive and negative ions
Two combined forces, collectively called electrochemical gradient, drive the diffusion of ions across the membrane:
- A chemical force (the ion's concentration gradient)
- An electrical force (the effect of the membrane potential on the ion's movement).
An electrogenic pump is a transport protein that generates voltage across a membrane
The sodium – potassium pump is the major electrogenic pump of animals cells
The main electrogenic pump of plants, fungi, and bacteria is a proton pump

Bulk transport by Exocytisis and Endocytosis

Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins
Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles
Bulk transport requires energy
In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents
In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane.