Transport across the Cell Membrane
- Ankur Jyoti Saikia
- B. tech part[IV]
- Dept. of Pharmaceutical Engineering and Technology.
- BL 101 [Biology].
- 09th July 2018.
- Prof. Vinod Tiwari
It refers to the collection of mechanisms that regulate the passage of solutes such
as ions and small molecules through biological membranes, which are lipid bilayers that
contain proteins embedded in them. The regulation of passage through the membrane is
due to selective membrane permeability - a characteristic of biological membranes which
allows them to separate substances of distinct chemical nature. In other words, they can be
permeable to certain substances but not to others.
The movements of most solutes through the membrane are mediated by membrane
transport proteins which are specialized to varying degrees in the transport of specific
molecules. As the diversity and physiology of the distinct cells is highly related to their
capacities to attract different external elements, it is postulated that there is a group of
specific transport proteins for each cell type and for every specific physiological stage. This
differential expression is regulated through the differential transcription of the genes coding
for these proteins and its translation, for instance, through genetic-molecular mechanisms,
but also at the cell biology level: the production of these proteins can be activated
by cellular signaling pathways, at the biochemical level, or even by being situated
in cytoplasmic vesicles.[
Two types of transport process occur across the membrane.
1. Non-mediated transport.
2. Mediated transport.
Non-mediated transport occurs through the simple diffusion process and the driving force for the
transport of a substance through a medium depends on its chemical potential gradient. Whereas
mediated transport requires specific carrier proteins. Thus, the substance diffuses in the direction
that eliminates its concentration gradient; at a rate proportional to the magnitude of this gradient and
also depends on its solubility in the membrane’s non-polar core.
Mediated transport is classified into two categories depending on the thermodynamics of the system:
1. Passive-mediated transport or facilitated diffusion: In this type of process a specific molecule
flows from high concentration to low concentration.
2. Active transport: In this type of process a specific molecule is transported from low concentration to
high concentration, that is, against its concentration gradient.
Substances that are too large or polar diffuse across the lipid bilayer on their own through membrane
proteins called carriers, permeases, channels and transporters. Unlike active transport, this process
does not involve chemical energy. So the passive mediated transport is totally dependent upon the
permeability nature of cell membrane, which in turn, is function of organization and characteristics of
membrane lipids and proteins.
Types of passive transport:
• Facilitated transport.
Passive mediated transport:
Transport Medium: Diffusion
Diffusion is a process of passive transport in which molecules move from an area of higher
concentration to one of lower concentration.
• Substances diffuse according to their concentration gradient; within a system, different
substances in the medium will each diffuse at different rates according to their individual
• After a substance has diffused completely through a space, removing its concentration
gradient, molecules will still move around in the space, but there will be no net movement
of the number of molecules from one area to another, a state known as dynamic equilibrium.
• Several factors affect the rate of diffusion of a solute including the mass of the solute, the
temperature of the environment, the solvent density, and the distance traveled.
• Diffusion: The passive movement of a solute across a permeable membrane
• Concentration gradient: A concentration gradient is present when a membrane separates two
different concentrations of molecules.
Transport Medium: Osmosis
Osmosis is the movement of water across a membrane from an area of low solute concentration
to an area of high solute concentration.
• Osmosis occurs according to the concentration gradient of water across the membrane, which
is inversely proportional to the concentration of solutes.
• Osmosis occurs until the concentration gradient of water goes to zero or until the hydrostatic
pressure of the water balances the osmotic pressure.
• Osmosis occurs when there is a concentration gradient of a solute within a solution, but the
membrane does not allow diffusion of the solute.
• Solute: Any substance that is dissolved in a liquid solvent to create a solution.
• Osmosis: The net movement of solvent molecules from a region of high solvent potential to a
region of lower solvent potential through a partially permeable membrane.
• Semipermeable membrane: A type of biological membrane that will allow certain molecules or
ions to pass through it by diffusion and occasionally by specialized facilitated diffusion.
Tonicity, which is directly related to the osmolarity of a solution, affects osmosis by determining the
direction of water flow.
• Osmolarity describes the total solute concentration of a solution; solutions with a low solute
concentration have a low osmolarity, while those with a high osmolarity have a high solute
• Water moves from the side of the membrane with lower osmolarity (and more water) to the side
with higher osmolarity (and less water).
• In a hypotonic solution, the extracellular fluid has a lower osmolarity than the fluid inside the
cell; water enters the cell.
• In a hypertonic solution, the extracellular fluid has a higher osmolarity than the fluid inside the
cell; water leaves the cell.
• In an isotonic solution, the extracellular fluid has the same osmolarity as the cell; there will be no
net movement of water into or out of the cell.
• Osmolarity: The osmotic concentration of a solution, normally expressed as osmoles of solute
per litre of solution.
• Hypotonic: Having a lower osmotic pressure than another; a cell in this environment causes
water to enter the cell, causing it to swell.
• Hypertonic: having a greater osmotic pressure than another.
• Isotonic: having the same osmotic pressure.
Transport Medium: Facilitated transport
Facilitated diffusion is a process by which molecules are transported across the plasma membrane
with the help of membrane proteins.
• A concentration gradient exists that would allow ions and polar molecules to diffuse into the cell,
but these materials are repelled by the hydrophobic parts of the cell membrane.
• Facilitated diffusion uses integral membrane proteins to move polar or charged substances across
the hydrophobic regions of the membrane.
• Channel proteins can aid in the facilitated diffusion of substances by forming a hydrophilic
passage through the plasma membrane through which polar and charged substances can pass.
• Channel proteins can be open at all times, constantly allowing a particular substance into or out of
the cell, depending on the concentration gradient; or they can be gated and can only be opened by
a particular biological signal.
• Carrier proteins aid in facilitated diffusion by binding a particular substance, then altering their
shape to bring that substance into or out of the cell.
• Facilitated diffusion: The spontaneous passage of molecules or ions across a biological
membrane passing through specific transmembrane integral proteins.
• Membrane protein: Proteins that are attached to, or associated with the membrane of a cell or
Transport Medium: Filtration
Filtration is the process of the movement of water and solute molecules across the cell membrane due
to hydrostatic pressure generated by the system. Depending on the size of the membrane pores, only
solutes of a certain size may pass through it. The membrane pores of the Bowman's capsule in the
kidneys are very small, and only albumins (smallest of the proteins) can filter through. On the other
hand, the membrane pores of liver cells are extremely large, to allow a variety of solutes to pass
through and be metabolized.
Active mediated transport
Active transport is the movement of a substance against its concentration gradient (i.e. from low to
high concentration). It is an endergonic process that, in most cases, is coupled to the hydrolysis of ATP.
Types of active transport:
1. Primary active transport: Primary active transport, also called direct active transport, directly uses
energy to transport molecules across a membrane. Example: Sodium-potassium pump, which helps to
maintain the cell potential.
2. Secondary active transport: Secondary active transport or co-transport, also uses energy to transport
molecules across a membrane; however, in contrast to primary active transport, there is no direct
coupling of ATP; instead, the electrochemical potential difference created by pumping ions out of the
cell is instrumental. The two main forms of active transport are antiport and symport.
Types of Primary active transport: [carrier proteins for active transport]
• Antiport: In antiport two species of ion or solutes are pumped in opposite directions across a
membrane. One of these species is allowed to flow from high to low concentration which yields the
entropic energy to drive the transport of the other solute from a low concentration region to a high
one. Example: the sodium-calcium exchanger or antiporter, which allows three sodium ions into the
cell to transport one calcium out.
• Symport: Symport uses the downhill movement of one solute species from high to low
concentration to move another molecule uphill from low concentration to high concentration
(against its electrochemical gradient). Example: glucose symporter SGLT1, which co-transports one
glucose (or galactose) molecule into the cell for every two sodium ions it imports into the cell.
Types of Secondary active transport:
• Endocytosis: Endocytosis is the process by which cells absorb larger molecules and particles from the
surrounding by engulfing them. It is used by most of the cells because large and polar molecules cannot
cross the plasma membrane. The material to be internalized is surrounded by plasma membrane, which
then buds off inside the cell to form vesicles containing ingested material.
• Phagocytosis or “cell eating,” is a mechanism whereby the cell can ingest solid particles. Phagocytosis
is the process by which certain living cells called phagocytes engulf larger solid particles such as
bacteria, debris or intact cells. Certain unicellular organisms, such as the protists, use this particular
process as means of feeding. It provides them part or all of their nourishment. This mode of nutrition is
known as phagotrophic nutrition. In amoeba, phagocytosis takes place by engulfing the nutrient with the
help of pseudopods, that are present all over the cell, whereas, in ciliates, a specialized groove or
chamber, known as the cytostome, is present, where the process takes place. When the solid particle
binds to the receptor on the surface of the phagocytic cell such as amoeba, then the pseudopodia extends
and later surrounds the particle. Then their membrane fuses to form a large intracellular vesicle called
phagosome. These phagosomes fuse with the lysosome, forming phagolysosomes in which ingested
material is digested by the action of lysosomal enzymes. During its maturation, some of the internalized
membrane is recycled to plasma membrane by receptor mediated endocytosis.
• Pinocytosis, or “cell drinking,” allows the cell to consume solutions. An infant’s intestinal lining ingests
breast milk by pinocytosis, allowing the mother’s protective antibodies to enter the baby’s bloodstream.
• Exocytosis: The process by which the cells direct the contents of secretory vesicles out of the cell
membrane is known as exocytosis. These vesicles contain soluble proteins to be secreted to the
extracellular environment, as well as membrane proteins and lipids that are sent to become components
of the cell membrane. It is the final step in the secretory pathway that typically begins in the
endoplasmic reticulum (ER), passes through the Golgi apparatus, and ends at the outside of the cell.
Some of the examples include secretion of proteins like enzymes, peptide hormones and antibodies
from cells and release of neurotransmitter from presynaptic neurons.
• Types of exocytosis:
1. Constitutive exocytosis: Secretory materials are continuously released without requirement of any
specific kind of signal.
2. Regulated exocytosis: Regulated exocytosis requires an external signal, a specific sorting signal on
the vesicles for release of components. It contains a class of secretory vesicles that fuse with plasma
membrane following cell activation in presence of signal. Examples of regulated exocytosis are
secretion of neurotransmitter, hormones and many other molecules.
Transport Medium: Channels
The integral proteins involved in facilitated transport are collectively referred to as transport proteins;
they function as either channels for the material or carriers. In both cases, they are transmembrane
proteins. Channels are specific for the substance that is being transported. Channel proteins have
hydrophilic domains exposed to the intracellular and extracellular fluids; they additionally have a
hydrophilic channel through their core that provides a hydrated opening through the membrane layers.
Passage through the channel allows polar compounds to avoid the nonpolar central layer of the plasma
membrane that would otherwise slow or prevent their entry into the cell.
Channel proteins are either open at all times or they are “gated,” which controls the opening of the
channel. The attachment of a particular ion to the channel protein may control the opening or other
mechanisms or substances may be involved. In some tissues, sodium and chloride ions pass freely
through open channels, whereas in other tissues, a gate must be opened to allow passage.
Cells involved in the transmission of electrical impulses, such as nerve and muscle cells, have gated
channels for sodium, potassium, and calcium in their membranes. Opening and closing of these
channels changes the relative concentrations on opposing sides of the membrane of these ions, resulting
in the facilitation of electrical transmission along membranes (in the case of nerve cells) or in muscle
contraction (in the case of muscle cells).
Example - Aquaporins are channel proteins that allow water to pass through the membrane at a very
Transport Medium: Carrier Proteins
This protein binds a substance and, in doing so, triggers a change of its own shape, moving the bound
molecule from the outside of the cell to its interior; depending on the gradient, the material may move in
the opposite direction. Carrier proteins are typically specific for a single substance. This adds to the
overall selectivity of the plasma membrane. The exact mechanism for the change of shape is poorly
understood. Proteins can change shape when their hydrogen bonds are affected, but this may not fully
explain this mechanism. Each carrier protein is specific to one substance, and there are a finite number of
these proteins in any membrane. This can cause problems in transporting enough of the material for the
cell to function properly.
To move substances against the membrane’s electrochemical gradient, the cell utilizes active transport,
which requires energy from ATP.
• The electrical and concentration gradients of a membrane tend to drive sodium into and potassium
out of the cell, and active transport works against these gradients.
• To move substances against a concentration or electrochemical gradient, the cell must utilize energy
in the form of ATP during active transport.
• Primary active transport, which is directly dependent on ATP, moves ions across a membrane and
creates a difference in charge across that membrane.
• Secondary active transport, created by primary active transport, is the transport of a solute in the
direction of its electrochemical gradient and does not directly require ATP.
• Carrier proteins such as uniporters, symporters, and antiporters perform primary active transport and
facilitate the movement of solutes across the cell’s membrane.
• Adenosine triphosphate: a multifunctional nucleoside triphosphate used in cells as a coenzyme,
often called the “molecular unit of energy currency” in intracellular energy transfer
• Active transport: movement of a substance across a cell membrane against its concentration
gradient (from low to high concentration) facilitated by ATP conversion
• Electrochemical gradient: The difference in charge and chemical concentration across a
Moving Against a Gradient
To move substances against a concentration or electrochemical gradient, the cell must use energy. This
energy is harvested from adenosine triphosphate (ATP) generated through the cell’s metabolism. Active
transport mechanisms, collectively called pumps, work against electrochemical gradients. Small
substances constantly pass through plasma membranes. Active transport maintains concentrations of
ions and other substances needed by living cells in the face of these passive movements. Much of a
cell’s supply of metabolic energy may be spent maintaining these processes.
Because active transport mechanisms depend on a cell’s metabolism for energy, they are sensitive to
many metabolic poisons that interfere with the supply of ATP.
Two mechanisms exist for the transport of small-molecular weight material and small molecules.
Primary active transport moves ions across a membrane and creates a difference in charge across that
membrane, which is directly dependent on ATP. Secondary active transport describes the movement of
material that is due to the electrochemical gradient established by primary active transport that does not
directly require ATP.
Example - most of a red blood cell’s metabolic energy is used to maintain the imbalance between
exterior and interior sodium and potassium levels required by the cell.