The cell and it's membrane, Lecture notes of Physiology

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THE CELL/ PLASMA MEMBRANE

AND

ITS TRANSPORT MECHANISMS

LECTURE OUTLINE

• Review the structure of lipid bilayer
• Factors that affect movement of materials across

the membrane

• Osmosis
• Passive transport – simple diffusion & facilitated

diffusion

• Classes of carrier proteins
• Active transport
• The Na+/ K+ ATPase/ pump
• Endocytosis
• Receptor-mediated endocytosis
• Exocytosis

STRUCTURE OF THE LIPID

BILAYER

structure of lipid bilayer

Although the internal fluid environment around the cells, ECF, is kept

stable by homeostasis, the fluid inside the cell, ICF, is distinctly different

from the ECF.

Each cell type maintains an intracellular composition which is unique for

that cell type.

This unique difference between the composition of the ICF and ECF is

due to the plasma membrane.

The ability of the plasma membrane to regulate materials passing

through it is a function of its chemical structure and physiological

activity.

The plasma membrane consists of a thin layer of lipids and proteins with

a limited number of carbohydrates.

The most abundant membrane lipids are the phospholipids.

Cholesterol, also a lipid, can be found in the plasma membrane between

the phospholipid molecules.

Cholesterol prevents the phospholipids from crystalizing and provides

rigidity which helps stabilize the plasma membrane.

The more cholesterol in the plasma membrane, the less the membrane

fluidity.

SHOWS THE PHOSPHATE HEAD AND FATTY ACID CARBON TAIL

FUNCTIONAL MEMBRANE PROTEINS

Factors that affect movement of materials across the membrane

3. The surface area of the membrane - The greater the surface area of the

membrane the faster the rate of diffusion of a substance.

4. The molecular weight of a substance - diffusion of a substance is

dependent on the collision of the molecules. When light weight molecules

collide they are knocked further than heavy molecules. Thus, the heavier

the molecules the slower the rate of diffusion.

5. The distance through which diffusion takes place - the greater the

distance to diffuse the slower the rate of diffusion.

Diffusion can be represented by a basic equation, often referred to as

Fick's Law.

Factors that affect movement of materials across the membrane

Fick's Law of Diffusion

J= -D * ∆C/∆x

• It was described that the flux was equal to

the negative diffusivity times the change in

concentration divided by the change in

distance.

Factors that affect movement of materials across the membrane

3. C – Concentration – Concentration is the amount of mass in a given

volume.

The symbol ∆C refers to the change in concentration from when the

object had not diffused at all, to the final concentration when the object

was done diffusing.

Units: amount of substance / volume. Example: mol / cm

3

, mol/L

∆x – Distance – This refers to the distance that the object is diffusing.

The symbol ∆x refers to the distance between where the object started

and where it ended up after it diffused.

Units: the units of length. Example: m, cm, ft

Therefore, returning to the equation, J = -D * ∆C/∆x

The diffusion constant D,is dependent on the temperature, viscosity of

the fluid and the size of the particles in question.

TRANSPORT MECHANISMS ACROSS THE CELL MEMBRANE

1. OSMOSIS

Large quantities of water molecules constantly move across cell

membranes by simple diffusion, but, in general, net movement of water

into or out of cells is negligible.

There are, however, many cases in which net flow of water occurs across

cell membranes and sheets of cells.

An example of great importance to is the secretion of and absorption of

water in the small intestine. In such situations, water still moves across

membranes by simple diffusion - osmosis.

Osmosis is the net movement of water across a selectively permeable

membrane driven by a difference in solute concentrations on the two

sides of the membrane.

In osmosis is water flows from the solution with the lower solute

concentration into the solution with higher solute concentration.

This means that water flows in response to differences in molarity

across a membrane irrespective of the size of the solute particles.

Equilibrium is reached once sufficient water has moved to equalize the

solute concentration on both sides of the membrane, and at that point,

net flow of water ceases.

OSMOSIS

2. PASSIVE TRANSPORT- Simple diffusion

A molecule or ion that crosses the membrane by moving down a

concentration or electrochemical gradient and without expenditure of

metabolic energy is said to be transported passively.

1. SIMPLE DIFFUSION – driven by the kinetic energy of the molecules

being transported

All molecules and ions are in constant motion and it is the energy of

motion - kinetic energy - that drives passive transport.

Transport of uncharged species across a membrane is dictated by the

prevailing concentration gradient.

For ions and charged molecules, the electrical potential across the

membrane also becomes critically important. Together, gradients in

concentration and electric potential across the cell membrane constitute

the electrochemical gradient that governs passive transport mechanisms.

Proteins are suspended in the inner layer, although the more hydrophilic

areas of these proteins "stick out" into the cells interior as well as the

outside of the cell.

These integral proteins, also known as gateway proteins, function as

binding sites for substances to be brought into the cell, through channels

that will allow materials into the cell via a passive transport mechanism.