Chapter 5 Transport Mechanism and Cell Permeability

The plasma membrane is about what passes through it due to its molecular composition. It permits nutrients into the cell but keeps unwanted substances out. Similarly, vital cell proteins and other chemicals are maintained inside the cell, while excreta or wastes are expelled to the outside. Selective, or differential, permeability is the name for this feature. There are two primary modes of transport through the plasma membrane. Concentration or pressure variations drive movement in passive processes. The cell provides energy (ATP) to power the transport process inactive processes.

Passive Processes

Diffusion and filtration are two key passive membrane transport mechanisms. Every cell in the body uses diffusion to convey information. Filtration, on the other hand, usually happens only via capillary walls. Molecules have kinetic energy and are always moving. Molecules clash and ricochet off one another as they move about at tremendous speeds, changing direction with each collision. The kinetic energy of the molecules drives diffusion, and the speed of diffusion is determined by molecular size and temperature. Molecules that are smaller move quicker, and molecules that are larger move faster as the temperature rises.


When a concentration gradient (difference in concentration) exists, the random molecular movement has the net effect of distributing the molecules uniformly throughout the environment. The transfer of molecules from a location of higher concentration to a region of lower concentration is known as diffusion. Diffusion can be seen in a variety of nonliving systems. For example, if a bottle of ether was uncorked in the front of the laboratory, you’d be nodding shortly afterward as the ether molecules spread throughout the space. Another example is the capacity to detect a friend’s cologne as soon as he or she enters the room.

The plasma membrane, which acts as a physical barrier, influences particle diffusion into and out of cells. In general, chemicals that may dissolve in the lipid component of the plasma membrane, such as CO 2 and O 2, diffuse passively through the membrane. Simple diffusion is the unsupported diffusion of solutes (dissolved compounds) through a selectively permeable membrane.

A protein carrier molecule assists in the transport of certain chemicals across the plasma membrane, such as glucose. The carrier binds to the glucose and transports it across the membrane. Small ions pass through water-filled protein channels to cross the membrane. The chemicals travel through a passive transport method known as assisted diffusion in both circumstances. The substances travel from a higher concentration area to a lower concentration area, or down their concentration gradients, as in simple diffusion.


Osmosis is the passage of water across a selectively permeable membrane. Water travels down its concentration gradient during osmosis. Water concentration is inversely proportional to solute concentration. Both water and solutes will travel down their concentration gradients via the membrane if the solutes can diffuse over it. Water alone will move by osmosis and produce changes in the volume of the compartments on either side of the membrane if the particles in the solution are nonpenetrating solutes (unable to pass the membrane).


Filtration is a passive process in which hydrostatic (fluid) pressure forces water and solutes through a membrane.
Because the blood pressure in the capillaries is greater than the fluid pressure in the tubules, fluids and solutes filter out of the capillaries in the kidneys and into the renal tubules. Filtration is a non-selective process. The amount of filtrate (fluids and solutes) generated is nearly entirely determined by the pressure gradient (pressure difference between the two sides of the membrane) and the membrane pores’ size.

Active Processes

The mechanism is active whenever a cell employs the bond energy of ATP to transfer substances across its borders.
Actively moving substances are often unable to pass through diffusion. They might not be lipid-soluble; they might be too big to pass through membrane channels, or they might have to travel against a concentration gradient rather than with it. Active transport and vesicular transport are the two forms of active processes.

Active Transport 

Active transport, like carrier-mediated facilitated diffusion, necessitates carrier proteins that combine precision with the drug being carried. Active transport can be primary, with ATP hydrolysis driving it directly, or secondary, with energy stored in ionic gradients driving it indirectly. Most of the time, compounds move against concentration, electrochemical, or both gradients. Amino acids and sugars are two examples of chemicals that are transported into cells by such transporters. Both solutes are lipid insoluble and too big to enter via membrane channels, but they are required for cell survival. Active transport ejects sodium ions (Na +) from cells. In an A&P laboratory, active transport is difficult to study, hence it will not be discussed further here.

Vesicular Transport

Fluids containing big particles and macromolecules are carried across cellular membranes inside membranous sacs known as vesicles in vesicular transport. Vesicular transport, like active transport, transports things into and out of cells (endocytosis) ( exocytosis). By transferring chemicals into, across, and out of cells, as well as moving substances from one region or membrane organelle to another, vesicular transport can combine endocytosis and exocytosis. Vesicular transport necessitates energy, which is normally in the form of ATP, and all types of vesicular transport use protein-coated vesicles to some degree.

Phagocytosis, pinocytosis, and receptor-mediated endocytosis are the three forms of endocytosis. The cell engulfs some relatively large or solid item, such as a clump of bacteria, cell debris, or inanimate particles, in phagocytosis (“cell eating”). When a particle interacts with receptors on a cell’s surface, pseudopods, or cytoplasmic extensions, develop and circulate around it. A phagosome is an endocytotic vesicle that results from this process. The phagosome then unites with a lysosome and its contents are digested in most situations. Exocytosis is the process through which indigestible materials are evacuated from the cell. Only macrophages and a few other white blood cells in the human body can phagocytose. These cells aid in the body’s defense against disease-causing microbes and cancer cells.

The cell “gulps” a drop of extracellular fluid containing dissolved molecules in pinocytosis (“cell drinking”), also known as fluid-phase endocytosis. The process is nonspecific since no receptors are engaged. Unlike phagocytosis, pinocytosis is a common cellular function that allows them to sample the extracellular fluid. It is especially crucial in nutrient-absorbing cells, such as those lining the intestines.

Receptor-mediated endocytosis is the most common mechanism for selective endocytosis of most macromolecules. Plasma membrane proteins that bind just specific compounds are the receptors for this mechanism. Cells can use this highly selective method to concentrate material that is only present in trace levels in the extracellular fluid. The ingested vesicle may merge with a lysosome, which digests or releases its contents, or it may be carried across the cell by exocytosis, which releases its contents. Because it provides a rapid way to transport chemicals from blood to extracellular fluid, the latter situation is common among endothelial cells lining blood arteries. Enzymes, insulin and other hormones, cholesterol (associated with a transport protein), and iron are among the substances taken up via receptor-mediated endocytosis.

Exocytosis is a vesicular transport mechanism in which chemicals are ejected from the cell and released into the extracellular fluid. The substance to be evacuated from the cell is first encased in a secretory vesicle, which is a protein-coated vesicle. The vesicle usually migrates to the plasma membrane, fuses with it, and then ruptures, emptying its contents outside the cell. Hormone secretion, neurotransmitter release, mucus secretion, and trash ejection all involve exocytosis.