Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Cell membrane transport

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After DNA replication, each DNA molecule is segre-

gated, i.e., separated from the other, and attached to a different

region of the internal face of the membrane. The formation of

a septum, or dividing internal wall, separates the cell into

halves, each containing a nucleotide. The process of splitting

the cell in two identical daughter cells is known as cytokinesis.

See alsoBacterial growth and division; Biochemistry; Cell

cycle (eukaryotic), genetic regulation of; Cell cycle and cell

division; Chromosomes, eukaryotic; Chromosomes, prokary-

otic; DNA (Deoxyribonucleic acid); Enzymes; Genetic regu-

lation of eukaryotic cells; Genetic regulation of prokaryotic

cells; Genotype and phenotype; Molecular biology and molec-

ular genetics

CCell membrane transportELL MEMBRANE TRANSPORT

The cell is bound by an outer membrane that, in accord with

the fluid mosaic model, is comprised of a phospholipid lipid

bilayer with proteins—molecules that also act as receptor

sites—interspersed within the phospholipid bilayer. Varieties

of channels exist within the membrane. There are a number of

internal cellular membranes that partially partition the inter-

cellular matrix, and that ultimately become continuous with

the nuclear membrane.

There are three principal mechanisms of outer cellular

membrane transport (i.e., means by which molecules can pass

through the boundary cellular membrane). The transport

mechanisms are passive, or gradient diffusion, facilitated dif-

fusion, and active transport.

Diffusion is a process in which the random motions of

molecules or other particles result in a net movement from a

region of high concentration to a region of lower concentra-

tion. A familiar example of diffusion is the dissemination of

floral perfumes from a bouquet to all parts of the motionless

air of a room. The rate of flow of the diffusing substance is

proportional to the concentration gradient for a given direction

of diffusion. Thus, if the concentration of the diffusing sub-

stance is very high at the source, and is diffusing in a direction

where little or none is found, the diffusion rate will be maxi-

mized. Several substances may diffuse more or less independ-

ently and simultaneously within a space or volume of liquid.

Because lightweight molecules have higher average speeds

than heavy molecules at the same temperature, they also tend

to diffuse more rapidly. Molecules of the same weight move

more rapidly at higher temperatures, increasing the rate of dif-

fusion as the temperature rises.

Driven by concentration gradients, diffusion in the cell

usually takes place through channels or pores lined by pro-

teins. Size and electrical charge may inhibit or prohibit the

passage of certain molecules or electrolytes (e.g., sodium,

potassium, etc.).

Osmosis describes diffusion of water across cell mem-

branes. Although water is a polar molecule (i.e., has overall par-

tially positive and negative charges separated by its molecular

structure), transmembrane proteins form hydrophilic (water lov-

ing) channels to through which water molecules may move.

Facilitated diffusion is the diffusion of a substance not

moving against a concentration gradient (i.e., from a region of

low concentration to high concentration) but which require the

assistance of other molecules. These are not considered to be

energetic reactions (i.e., energy in the form of use of adenosine

triphosphate molecules (ATP) is not required. The facilitation

or assistance—usually in physically turning or orienting a

molecule so that it may more easily pass through a mem-

brane—may be by other molecules undergoing their own ran-

dom motion.

Transmembrane proteins establish pores through which

ions and some small hydrophilic molecules are able to pass by

diffusion. The channels open and close according to the phys-

iological needs and state of the cell. Because they open and

close transmembrane proteins are termed “gated” proteins.

Control of the opening and closing mechanism may be via

mechanical, electrical, or other types of membrane changes

that may occur as various molecules bind to cell receptor sites.

Active transport is movement of molecules across a cell

membrane or membrane of a cell organelle, from a region of

low concentration to a region of high concentration. Since

these molecules are being moved against a concentration gra-

dient, cellular energy is required for active transport. Active

transport allows a cell to maintain conditions different from

the surrounding environment.

There are two main types of active transport; movement

directly across the cell membrane with assistance from trans-

port proteins, and endocytosis, the engulfing of materials into

a cell using the processes of pinocytosis, phagocytosis, or

receptor-mediated endocytosis.

Transport proteins found within the phospholipid

bilayer of the cell membrane can move substances directly

across the cell membrane, molecule by molecule. The sodium-

potassium pump, which is found in many cells and helps nerve

cells to pass their signals in the form of electrical impulses, is

a well-studied example of active transport using transport pro-

teins. The transport proteins that are an essential part of the

sodium-potassium pump maintain a higher concentration of

potassium ions inside the cells compared to outside, and a

higher concentration of sodium ions outside of cells compared

to inside. In order to carry the ions across the cell membrane

and against the concentration gradient, the transport proteins

have very specific shapes that only fit or bond well with

sodium and potassium ions. Because the transport of these

ions is against the concentration gradient, it requires a signifi-

cant amount of energy.

Endocytosis is an infolding and then pinching in of the

cell membrane so that materials are engulfed into a vacuole or

vesicle within the cell. Pinocytosis is the process in which

cells engulf liquids. The liquids may or may not contain dis-

solved materials. Phagocytosis is the process in which the

materials that are taken into the cell are solid particles. With

receptor-mediated endocytosis the substances that are to be

transported into the cell first bind to specific sites or receptor

proteins on the outside of the cell. The substances can then be

engulfed into the cell. As the materials are being carried into

the cell, the cell membrane pinches in forming a vacuole or

other vesicle. The materials can then be used inside the cell.

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