Fluid Mosaic Model

A mosaic of protein molecules drifting laterally in a fluid bilayer of phospholipids

The cellular membrane is primarily fluid as the hydrophobic interactions are not very strong for holding the components of the two layers in one place.  The cell membrane members are constantly in motion.

However, some components are held in one place by their attachment to the cytoskeleton.

Others move along cytoskeletal fibers along a fixed route.

Phospholipids with unsaturated hydrocarbon tails have kinks in their tail that prevent close packing of the phospholipids.

Cholesterol acts as a temperature buffer.  It restrains some of the movement of the phospholipids when it is hot and prevents them from packing too tightly when it is cold.

Both ends of the protein will be hydrophylic and extend into the aqueous solutions inside and outside the cell.

The center region of the protein is non-polar and usually coiled into alpha helices

[image]

1. Transport through the membrane
2. Enzymatic activity
3. Signal Transduction
4. Cell Cell Recogntion
5. Intercellular joining
6. Stability - attachment to the cytoskeleton & ECM

[image][image]

important in sorting of cells into tissues and organs in a developing embryo and rejection of foreign cells by the immune system.

Cells recognize other cells by bonding with their glycolipids or glycoproteins

Iso means same

If a cell is immersed in an environment that is isotonic to the cell there will be no net movement of water across the plasma membrane

Hyper means more

If the outside of the cell contains more nonpenetrating solutes (i.e. less water) then it is hypertonic to the cell.

water will leave the cell 

Hypo means less

If the outside of the cell contains less nonpenetrating solutes (i.e. less water) than the interior of the cell then it is hypotonic to the cell.

water will enter the cell 

A cell is lysed when it absorbs so much water it explodes

 refers to the breaking down of a cell, often by viral, enzymic, or osmotic (water diffusion) mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a "lysate".

Passive transport is a movement of biochemicals and other atomic or molecular substances across membranes.

Unlike active transport, it does not require an input of chemical energy, being driven by the movement of molcules from a higher gradient to a lower gradient.  

The rate of passive transport depends on the permeability of the cell membrane, which, in turn, depends on the organization and characteristics of the membrane lipids and proteins.

The four main kinds of passive transport are diffusion, facilitated diffusion, filtration and osmosis.

The four main kinds of passive transport are

diffusion

facilitated diffusion

filtration

osmosis

Diffusion is the net movement of material from an area of high concentration to an area with lower concentration.

The difference of concentration between the two areas is often termed as the concentration gradient, and diffusion will continue until this gradient has been eliminated.

Since diffusion moves materials from an area of higher concentration to the lower, it is described as moving solutes "down the concentration gradient" 

Facilitated diffusion, also called carrier-mediated diffusion, is the movement of molecules across the cell membrane via special transport proteins that are embedded within the cellular membrane.

Many large molecules, such as glucose, are insoluble in lipids and too large to fit through the membrane pores. Therefore, it will bind with its specific carrier proteins, and the complex will then be bonded to a receptor site and moved through the cellular membrane.

Facilitated diffusion is a passive process: The solutes move down the concentration gradient and don't use energy to move.

Filtration is movement of water and solute molecules across the cell membrane due to hydrostatic pressure generated by the cardiovascular system (blood pressure from your heart pumping).

Depending on the size of the membrane pores, only solutes of a certain size may pass through it. For example, the membrane pores of the Bowman's capsule in the kidneys are very small, and only albumins, the smallest of the proteins, have any chance of being filtered through. On the other hand, the membrane pores of liver cells are extremely large, to allow a variety of solutes to pass through and be metabolized.

Active transport is the movement of all types of molecules across a cell membrane against its concentration gradient (from low to high concentration).

In all cells, this is usually concerned with accumulating high concentrations of molecules that the cell needs, such as ions, glucose and amino acids.

If the process uses chemical energy, such as from adenosine triphosphate (ATP), it is termed primary active transport. Secondary active transport involves the use of an electrochemical gradient. 

Sodium Ions (Na⁺) bind inside the sodium potassium protein pump

ATP phosphorylates (transfers a phosphate group) to the protein causing it to change its shape so that it opens to the extracellular side of the membrane.

The new shape no longer binds the sodium so it is released.  Potassium (K⁺) binds to the protein and causes the phosphate to be released.

Loss of the phosphate causes the protein to revert to its original shape and it unbinds and releases the potassium into the intracellular space

[image]

An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consist of two parts, the electrical potential and a difference in the chemical concentration across a membrane.

In a resting nerve cell the quantity of Sodium Ions (Na⁺) is much lower inside the cell than outside it.  This causes the inside to be more negative than the outside which creates an electrical potential. 

When the cell is activiated gated channels open and the Sodium ions fall down their electrochemical gradient (towards the more negative area inside the cell)

any biological system that generates an electrochemical potential across a membrane by pumping one ion across the membrane without the concomitant movement of another ion of the same charge in the opposite direction.

A sodium potassium pump is electrogenic because for every three sodium ions it pumps out of the cell it only pumps two potassium ions back in.  therefore with each turn it becomes more positive on the inside creating an electrical potential.

The coupling of the "downhill" diffusion of one substance to the "uphill" transport of another against its own concentration gradient.

An example, is the coupling of a proton pump (active transport protein pump that pumps Hydrogen ions out of the cytoplasm) that requires ATP to fuel to forcefully move the Hydrogen ions to a Sucrose-H+ cotransporter.

The proton pump moves the H+ ions out of the cell which creates a more negative space inside the cell.  The H+ ions will flow back to the inside of the cell because of the polarity attraction.  The Sucrose-H+ cotransporter attaches a Sucrose molecule to the Hydrogen ion and uses this electrical energy to transport Sucrose.

[image]

Water and small solutes enter and leave the cell by diffusing through the lipid bilayer or by transport proteins.

How do larger molecules like proteins and polysaccharides get across the cell membrane

A transport vesicle that has budded from teh golgi apparatus moves along microtubules of the cytoskeleton to the plasma membrane.  when the vesicle membrane and the plasma membrane come into contact, the lipid molecules of the two bilayers fuse.  

The vesicle becomes part of the plasma membane and the large molecules are secreted into the extracellular space.

Phagocytosis - cellular eating

Pinocytosis - cellular drinking

receptor mediated endocytosis

Cholesterol travels in the blood in particles called low density lipoproteins (LDLs) which are complexes of lipids and proteins.

The LDLs are ligands which bind to LDL receptors on plasma membranes.  The cell then takes in the cholesterol via Receptor-mediated endocytosis.

In phagocytosis a cell engulfs a particle by wrapping pseudopodia around it and packaging it wihtin a membrane-enclosed sac that can be large enough to be classified as a vacuole.  Teh particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes.

[image]