Transport across plasma membrane

Transport across plasma membrane is carried out by two different processes diffusion and active transport, they are further divided into various methods.

(A) Diffusion

• Simple diffusion
• Facilitated diffusion
• Osmosis

(B) Active transport

Movement of large molecules across the membrane

• Endocytosis
• Exocytosis

Simple Diffusion or Passive Transport:

• It is a passive process where the solute molecules in a solution are carried in the direction of their concentration gradient i.e. from higher concentration to lower concentration without the utilization of energy.
• Substances move across the cell membrane by three basic mechanisms.
• The molecules remain in the aqueous phase and diffuse through aqueous channels or pores in the membrane.
• The molecule leaves the aqueous phase on one side of the membrane, dissolves in the lipid bilayer and crosses it and again enters the aqueous phase on the opposite side of the membrane.
• The molecules combine with carrier molecules and help them across the cell membrane.

Facilitated Diffusion:

• It is also called carrier-mediated diffusion.
• The carrier protein facilitates the diffusion of the substances to the other side of the membrane.
• Energy is not required for such transfer.
• Many lipid insoluble substances like certain vitamins, glucose, urea cross the membrane by this process.
• The transfer is in the direction of concentration gradient, from higher concentration to lower concentration.
• This transfer is achieved through the structural changes in the protein when it binds with the material to be transferred.

Osmosis:

• It is defined as the movement of solvent molecules across a semi-permeable membrane
  from an area of higher concentration to an area of lower concentration.
• Osmosis occurs only when a membrane is permeable to water but it is not permeable to certain solutes.
• Consider a U-shaped tube containing a selectively permeable membrane that separates the left and right arms of the tube.
• A volume of water is poured into the left arm, and the same volume of a solution containing a solute that cannot pass through the membrane is poured into the right arm.
• Because the water concentration is higher on the left arm and lowers on the right arm, the net movement of water molecules (osmosis) occurs from left to right.
• Water moves down its concentration gradient.
• At the same time, the membrane prevents diffusion of the solute from the right arm into the left arm.
• As a result, the volume of water in the left arm decreases, and the volume of solution in the right arm increases.
• Hydrostatic pressure: Pressure excreted by a liquid is known as hydrostatic pressure which forces the water molecules to move back into the left arm.
• Osmotic pressure: Pressure excreted by the solute on a semipermeable membrane through which it cannot penetrate.
• The osmotic pressure of a solution is proportional to the concentration of the solute particles that cannot cross the membrane.
• The higher the solute concentration, the higher is the solution’s osmotic pressure.

Active Transport:

• When the material is transported out against the concentration gradient i.e. from lower concentration to higher concentration with the utilization of energy then the process is called active transport.
• Energy is obtained from the hydrolysis of ATP.
• Active transport is of two types:

(a) Primary Active Transport:

• Energy is derived from the hydrolysis of ATP which changes the shape of carrier protein.
• The carrier protein pumps a substance across a plasma membrane against its concentration gradient.

(b) Secondary Active Transport:

• In this process, the electrochemical potential difference created by pumping ions out of the cell is used to transport molecules across the membrane.
• The electrochemical gradient is used to drive other substances across the plasma membrane against their concentration gradient.
• A symport is an active transport protein that transports two different molecules across the cell membrane at the same time.
• The material transferred along with some ions is called symport or co-transport.
• An anti-port is an active transport protein that transports two molecules in opposite directions against their concentration gradients.
• A uni-port is an active transport protein that transports a single molecule across the cell membrane.

Endocytosis:

• It is a transport mechanism that involves engulfing extracellular materials within a segment of the cell membrane to form a vesicle called corpuscular or vesicular transport.
• For example, Macromolecular nutrients like fats and starches, oil-soluble vitamins A, D, E, K, and drugs such as insulin.
• Endocytosis includes two types of processes:

(a) Phagocytosis: It is a form of endocytosis in which the cell engulfs large solid particles, such as worn-out cells, whole bacteria, or viruses.

(b) Pinocytosis: It is a form of endocytosis in which tiny droplets of extracellular fluid are taken up.

Exocytosis:

• The undigested substance called the residual body is excreted through the cell membrane by a process called exocytosis.
• The undigested substances produced within the cytoplasm may be enclosed in a membrane to form a vesicle called an exocytic vesicle.
• These cytoplasm exocytic vesicles fuse with the internal surface of the plasma membrane.
• The vesicle then ruptures releasing its content into the extracellular space and its membranes are left behind and refused.