Proteins

Effedor

Figure PG.10 G protein activation and de-activation

Second Messenger Enzymes

The production of second messengers; cyclic AMP, cyclic GMP, inositol triphosphate and diacylglycerol takes place at the cell membrane. The enzymes responsible for their production are adenylyl cyclase, guanylyl cyclase and tyrosine kinase. The activities of these enzymes are controlled by various pathways, which involve both activation and inhibition. Second messenger effects are multiple and widespread intracellularly (see page 251).

Membrane Transport of Substances

Cell membranes control the movement of a wide range of particles and substances between the intra- and extracellular spaces. These include gases, ions, water, proteins and intracellular granules or debris. Different components of the membrane are associated with different mechanisms of transport. The phospholipid bilayer areas of the membrane allow diffusion of water, small molecules and lipid soluble substances. Transmembrane proteins provide active mechanisms for transport and allow ion diffusion via channels. Examples of substances transported across membranes are shown in Figure PG. 11.

Membrane Transport Mechanisms

Various mechanisms exist for the transport of substances across the cell membrane. Mechanisms such as diffusion and osmosis are passive and do not require the expenditure of energy. Active transport is mediated by integral membrane proteins and uses energy often in the form of ATP. Membrane transport mechanisms include:

• Ion channel diffusion

• Facilitated diffusion

• Primary active transport

• Secondary active transport

• Exo- and endocytosis Diffusion

This describes the resultant movement of solute molecules due to their random thermal motion. It is a passive process and net movement of the solute occurs when a concentration gradient is present (from a high to a low concentration). Certain molecules can diffuse across the phospholipid bilayer areas of a cell membrane.

EXAMPLES OF SUBSTANCES TRANSPORTED ACROSS MEMBRANES

Substance

Size nm

Site

Mechanism

Water

0.13

Lipid bilayer

Osmosis

Oxygen

0.12

Lipid bilayer

Diffusion

Nitrogen

0.12

Lipid bilayer

Diffusion

Carbon dioxide

0.12

Lipid bilayer

Diffusion

Sodium ions

0.19

Lipid bilayer

Ion diffusion

ATPase pump

Active transport

Potassium ions

0.23

Ion channel

lon diffusion

ATPase pump

Active transport

Calcium ions

0.17

Ion channel

Ion diffusion

ATPase pump

Active transport

Urea

0.23

Lipid bilayer

Diffusion

Steroid

<1.0

Lipid bilayer

Diffusion

Fatty acid

<1.0

Lipid bilayer

Diffusion

Glucose

0.38

Transport proteins

Faciliatated diffusion

Proteins

>7.5

Vesicles

Exocytosis

Endocytosis

Figure PG.11

Figure PG.11

In general, the rate of diffusion through a membrane, D, is dependent on the concentration gradient (C1 - C2), the area of membrane exposed, A, and the permeability constant kp:

The permeability constant, kp depends on the local temperature and the characteristics of the membrane; molecular properties also affect it. The phospholipid bilayers are relatively impermeable to ions and large polar (hydrophilic) molecules, but permeable to small polar molecules and lipophilic substances. Thus, the rate of diffusion across the cell membrane:

• Increases with concentration gradient

• Increases with surface area

• Increases with temperature

• Increases with lipid solubility

• Decreases with molecular weight

• Decreases with electrical charge of particle

A cell membrane acts as a diffusion barrier to solute molecules and can reduce their rates of diffusion by a factor of between 103 and 106, compared with free diffusion rates in water.

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