Transport across cell membrane

Substances move through the cell membrane by two major processes:

1. Passive : requires no energy
2. Active: consume energy in the form of ATP

Passive Transport Processes

Here substances move across the cell membrane without any energy expenditure by the cell. It includes

• Dffusion
• Osmosis

Diffusion

Diffusion is a passive process (i.e. no external source of energy is required) by which molecules move from areas of high concentration to areas of low concentration (down their ‘chemical gradient’).

It is of two types:

1. Simple diffusion
2. Facilitated diffusion

Simple diffusion

It occurs because the heat content of the solution keeps the solvent and the solute particles of the solution in constant motion.

Net movement stops when the concentration of the molecules is equal everywhere within the solution.

Factors affecting diffusion

• Distance: Greater distance, longer time for diffusion.
• Size of the gradient: Larger the concentration gradient (difference), faster the diffusion occurs.
• Temperature: Higher the temperature, faster is the diffusion rate.
• Molecular size: Smaller size, more diffusion
• Lipid solubility: Lipid soluble molecules diffuse rapidly, Water soluble molecules slow.
• Surface area: Larger the membrane surface area available for diffusion, the faster is the diffusion rate.

Facilitated Diffusion

It is a carrier-mediated process that enables molecules that are too large to flow through membrane channels by simple diffusion.

Eg:  Transport of glucose into RBCs, muscles and adipose tissue in the presence of insulin.

Follow saturation kinetics

Osmosis

Osmosis is the passive flow of the solvent e.g. water across a selectively permeable membrane, into a region where there is higher concentration of a solute to which the membrane is impermeable

Osmotic pressure

Pressure required to prevent osmosis

Note: The colloidal osmotic pressure due to the plasma colloids is called oncotic pressure.

Active Transport Processes

Substances are transported against their chemical and electrical gradient.

This form of transport requires energy (in form of ATP).

It includes:

• Primary active transport processes,
• Secondary active transport processes,
• Carrier type processes; and
• Vesicular transport processes

Primary active transport processes

They directly use the energy obtained from the hydrolysis (breakdown) of adenosine triphosphate (ATP).

Eg:

• Sodium-potassium (Na-K) pump
• Calcium (Ca) pump and
• Potassium hydrogen (K-H) pump

Sodium-potassium pump or Na-K ATPase

Mechanism of operation

• Step 1: Binding of 3Na ions and ATP to a crrier protein inside the cell causes change in shape of protein resulting in 3Na ions to move out of the cell.
• Step 2: When 2K ions bind to the carrier protein on the outside of the cell, this causes second change in shape of protein resulting in 2K ions to move into the cell.

Functions of Na-K ATPase pump

• Na-K pump is responsible for maintaining the high K and low Na concentrations in the cell.
• It helps in regulation of normal cell volume and pressure.
• It is one of the major energy using process in the body and accounts for a large part of the basal metabolism (BMR).

Secondary active transport Processes

In some tissues, the active transport of Na is coupled (linked) to the transport of other substances.

Eg: glucose and amino acids are reabsorbed from the proximal renal tubules or absorbed from the intestinal lumen only if Na binds to the protein and is transported down its electrochemical difference at the same time.

Carrier and Transport Proteins

Types

1. Uniporters are carriers that transport a single particle in one direction, such as the facilitated diffusion of glucose.
2. Symporters (co-transporters) are carriers that transport two particles together in the same direction, such as the secondary active transport of glucose.
3. Antiporters (counter transporters) are carriers that transport molecules in opposite directions i.e. they exchange one substance for another. For example, Na-K pump which moves 3Na out of the cell in exchange for 2K that moves into the cell.

Vesicular Transport Processes

Many substances are transported across the cell membrane by endocytosis and exocytosis.

Endocytosis: 3 types

• Phagocytosis (Cell eating)
• Pinocytosis (cell-drinking)
• Receptor-mediated endocytosis

Phagocytosis (Cell eating)

It is the process by which extracelllar substances (bacteria, dead tissue, foreign particles etc.) are (eaten) by the cells with formation of vesicle.

Pinocytosis (Cell eating)

In this process the substance ingested is in solution form.

Receptor-mediated endocytosis

Here the material to be transported first builds to a receptor, and then the receptor-substance complex is ingested by endocytosis.

Exocytosis:

Here substances secreted by the cell are trapped within vesicles or granules which fuse with the cell membrane and release their contents to the ECF. This leaves the contents of the vesicles or granules outside the cell and the cell membrane intact. It requires Ca2+ and energy

Hormones, digestive enzymes and synaptic transmitters are examples of substances transported out of the cell by this process.