1. Script for - MECHANISMS OF TRANSPORT ACROSS THE
CELL MEMBRANE
Slide #2- Transport and the Cell Membrane
Transport?
Definition: Any process wherein movement of matter and/or energy from one part of a
system to another occurs.
Importance of Transport?
To carry on life-sustaining and specialized activities, each cell must exchange materials
across the cell membrane with the homeostatically maintained internal fluid environment
that surrounds it, i.e. the ECF.
Transport Across the Cell Membrane Depends on?
Transport across the cell membrane depends on the permeability of the membrane.
A membrane can be either permeable, semi-permeable or impermeable.
If a substance can cross the membrane, the membrane is said to be permeable to that
substance, if a substance is unable to pass, the membrane is impermeable to it.
The plasma (cell) membrane is selectively permeable in that it permits some particles to
pass through while excluding others.
Properties of the Substance to Pass Through the Cell Membrane
Without any Assistance?
1. Relative solubility of the particle in lipid, i.e. lipid-solubility- so that it can pass
through the lipid bilayer, which forms the cell membrane.
2. The size of the particle, for water-soluble substances that should be of less than
o.8nm in diameter to actually pass through a protein channel that is part of the cell
membrane.
Uncharged/non-polar/highly lipid soluble molecules readily dissolve in the lipid bilayer
and permeate through the membrane. Examples are oxygen, carbon dioxide and fatty
acids.
Charged/polar/poorly lipid soluble substances cannot pass through the lipid bilayer and
require a passage to pass. Protein channels serve as the substitute pathway.

2. Slide #3- Structure of the Cell Membrane
Cell Membrane?
It is the external limiting, outer boundary of the cell. The semi-permeable cell membrane
helps control what substances enter or exit the cell. It is a delicate structure, which
encloses the cell, separating the contents of the cell from the surrounding environment.
Some cell organelle cells are membrane-bound and also have this cell membrane, e.g.
mitochondria, nucleus, endoplasmic reticulum, etc.
The cell membrane surrounds the cytoplasm of a cell and physically separates the
intracellular components from the extracellular environment, thereby serving a function
similar to that of skin. The cell membrane is known as the fluid mosaic model because
proteins are embedded in a sea of lipids giving a mosaic effect in fluid contributed by
lipids.
The functions of the cell membrane include, but are not limited to:
• Controlling what goes in and out of the cell.
• Excretion of metabolic products.
• Anchoring of the cytoskeleton to provide shape and size to the cell.
• Attaching to the extracellular matrix to help group cells together in the formation of tissues.
• Transportation of particles by way of ion pumps, ion channels, and carrier
proteins.
• Containing receptors that allow chemical messages to pass between cells and systems.
• Participation in enzyme activity important in such things as metabolism and immunity.
• Exchange of gases and nutrients.
• Repels negatively charged molecules inside the cell, keeping them in.
Lipid Bilayer?
Cholesterol prevents fatty acid chains from crystallizing, increasing membrane fluidity.
Membrane carbohydrates serve as self-identity markers that enable cells to identify and
interact with each other. Membrane proteins have a variety of functions such as to serve
as channels for water-soluble substances, to transfer specific substances across the
membrane, to act as receptor sites, to serve as docking-marker receptors (t-SNAREs)
which is involved in secretion, function as membrane bound enzymes, recognize self
from non-self cells, involved in cell-to-cell interaction and as cell adhesion molecules
that grasp extracellular matrix.

3. Slide #4- Membrane Proteins
Classifying Proteins?
1. Peripheral- on the periphery of the membrane either outside or inside usually on
the cytosolic surface, part of the cytoskeletal proteins to maintain cell shape.
2. Integral (a.k.a. transmembrane)- spans the membrane involved in transport of
substances in and out of the cell membrane
a. Carriers
b. Pumps
c. Channels
• Leaky
• Gated (Voltage, Ligand, Mechanically)
3. Lipid-anchored- Located covalently bound to single or multiple lipid molecules,
which hydrophobically insert into the cell membrane and anchor the protein. The protein
itself is not in contact with the membrane.
Conformational changes of gated-channels take place when certain stimuli are applied to
them.Carriers change their shape as they transport substances.Pumps use energy to get
substances across the membrane against the concentration gradient. Pumps have an ATP
binding site as well.
Classify Transport? (Same as in slide)
Passive transport involves net movement of a substance along its concentration gradient
not requiring metabolic energy. Active Transport involves net movement of a substance
along its concentration gradient requiring ATP. Bulk transport is to transport large polar
molecules or multimolecular particles.
Slide #5- Passive Transport
Diffusion is the net movement of material from an area of high concentration of that
material to an area with lower concentration. The difference of concentration between the
two areas is often termed as the concentration gradient, and diffusion will continue until
this gradient has been eliminated. Since diffusion moves materials from an area of higher
concentration to the lower, it is described as moving solutes "down the concentration
gradient" (compared with active transport, which often moves material from area of low
concentration to area of higher concentration, and therefore referred to as moving the
material "against the concentration gradient").

4. Slide #6- Simple Diffusion
It means that kinetic movement of molecules or ions occurs through a membrane opening
or through intermolecular spaces without any interaction with carrier proteins in the
membrane. The amount of substances available, the velocity of kinetic motion, and the
number and sizes of openings in the membrane through which the molecules or ions can
move determine the rate of diffusion.
The protein channels involved in simple diffusion are distinguished by 2 important
characteristics: they are often selectively permeable to certain substances and many of the
channels can be opened or closed by gates.
Selectively permeability depends the channel itself-diameter, shape and nature of its
electrical charges and chemical bonds along its inside surface. Example: The Na Channel
has a strongly negative charge on the inside, which can pull small-dehydrated Na ions
actually pulling them away from their hydrating water molecules. The K Channel on the
other hand are smaller and not negatively charged but have different chemical bonds. The
hydrated for of K ions are real small therefore it can pass through the K channels easily
as a hydrated structure. These are leaky channels.
Conformational changes of gated-channels take place when certain stimuli are applied to
them. Voltage-gated channels open and close on electrical stimulus (e.g. action potential)
whereas the ligand-gated channels respond to chemical stimuli (e.g. neurotransmitters,
hormones) and mechanically gated channels respond to mechanical stimuli (e.g. pressure,
touch, pain).
An example of simple diffusion is how oxygen, a non-polar lipid soluble compound,
diffuses across the respiratory membrane. There is a high concentration of oxygen in the
lungs and a low one in tissues since it has given up most of its oxygen to the respiring
tissues, therefore oxygen diffuses across the membrane from the lungs into the blood.

5. Slide #7- Facilitated Diffusion
It is movement of molecules across the cell membrane via special transport proteins that
are embedded within the cellular membrane. Many large molecules, such as glucose, are
insoluble in lipids and too large to fit through the membrane pores. Therefore, it will bind
with its specific carrier proteins, and the complex will then be bonded to a receptor site
and moved through the cellular membrane. The carrier facilitates diffusion of the
substance to the other side. It is like a revolving door- a person enters from one side and
the door revolves to the other side of the door, where the person leaves.
Carrier-mediated transport systems are of two kinds- facilitated diffusion and active
transport. The three characteristics that determine the kind and amount of substance to be
transported through the cell membrane are:
1. Specificity- each carrier protein is specialized to transport a specific substance or
closely related compounds. Example, the cysteine carriers found in the kidney
membranes that remove the essential amino acid from urine and transport it back
to the blood
2. Saturation- There is a limit to the amount of a substance that can be transported
across the membrane via a carrier in a given time; that is, a limited number of
carrier binding sites are available within a particular plasma membrane for a
specific substance. The limit is known as the transport maximum. Until it is
reached, the numbers of carrier binding sites occupied by a substance and,
accordingly, the substance’s rate of transport across the membrane are directly
related to its concentration. The more of a substance available to be transported,
the more will be transported. When transport maximum has been reached, the
carrier is saturated (all binding sites are occupied) and the rate of transport across
the membrane is maximal. Further increases in the substance’s concentration are
not accompanied by corresponding increases in the rate of transport. Example,
glucose carrier mechanism is capable of actively reabsorbing up to 375 mg of
glucose per minute before it reaches its maximum transport capacity, the rest
being excreted in urine.
3. Competition- several closely related compounds may compete for a ride across
the membrane on the same carrier. If a given binding site can be occupied by
more than one type of molecule, the rate of transport of each substance is less
when both molecules are present when either is present by itself. In other words,
when a carrier can transport 2 closely related substances, the presence of both
diminishes the rate of transport of either. Example, the case of alanine and glycine
amino acids.
An example of facilitated diffusion is by which glucose enters cells. The cells
metabolize glucose almost rapidly as it enters the cells from the blood. There is,
therefore, a continuous gradient for net diffusion of glucose into the cells. However,
glucose is polar and uses a glucose carrier.

6. Slide #8- Factors influencing the Rate of Net Diffusion of a
Substance across a Membrane (Fick’s Law of Diffusion)
(Same as in slide)
Movement of ions is also affected by their electrical charge. Like charges repel each
other whereas opposite charges attract. Cations move towards negatively charged areas
and Anions move towards positively charged areas.
Slide #9- Osmosis
Most cell membranes are permeable to water, and since the diffusion of water plays such
an important role in the biological functioning of any living being, a special term has
been coined for it -- osmosis.
The osmotic pressure exerted by particles in a solution, whether they are molecules or
ions, is determined by the number of particles per unit volume of fluid, not by the mass of
the particles. The reason for this is that each particle in a solution, regardless of its mass,
exerts, on average the same amount of pressure against the membrane. Large particles
having greater mass move at slower velocities and vice versa.
The tonicity of a solution refers to the effect on cell volume of the concentration of non-
penetrating solutes in the solution surrounding the cell.
1. If the medium is hypotonic — a dilute solution, with a higher water concentration
than the cell — the cell will gain water through osmosis.
2. If the medium is isotonic — a solution with exactly the same water concentration
as the cell — there will be no net movement of water across the cell membrane.
3. If the medium is hypertonic- a concentrated solution, with a lower water
concentration than the cell- the cell will lose water by osmosis.

7. Slide #10- Active Transport and Primary Active Transport
Active transport requires a carrier to expend energy to transfer its passenger uphill
against a concentration gradient. Some ions most be kept at higher concentrations in the
ICF than in the ECF or vice versa. This is possible only by active transport. For example,
sodium ions should be of higher concentration in the ECF and potassium ions should be
higher in concentration in the ICF.
The hydrogen-ion pump present in gastric glands of the stomach and the late distal
tubules and cortical collecting ducts of the kidneys exhibits primary active transport.
Hydrogen ions are pumped into the stomach lumen to increase the acidic environment
best for the activity of enzymes of the stomach.
The Sodium- Potassium Pump transports sodium from the inside and to the outside
simultaneously with potassium going in the opposite direction. By phosphorylating the
carrier on the intracellular side, the affinity for sodium to the carrier increases.
Phosphorlylation is accomplished by the breakdown of ATP into ADP and inorganic
phosphate, the inorganic phosphate binding to a specific site on the carrier molecule.
Phosphorlyation also induces change in the shape of the carrier, leading to the deposition
of sodium to the other side. The following dephosphorylation, affinity of the carrier to
potassium increases on the extracellular side, changing the shape of the carrier, restoring
it back to its original shape, depositing potassium on the other side, i.e. into the ECF.
This pump establishes a concentration gradient which is necessary in generation of
electrical impulses, helps regulate cell volume and also indirectly serves as the energy
source in secondary active transport.
Slide #11- Secondary Active Transport
Example of Cotransport in glucose absorption in intestines- a cotransport carrier at the
luminal border simultaneously transfers glucose against a concentration gradient and
sodium down the gradient from the lumen to the cell. No energy is directly used by the
cotransport carrier to move glucose uphill. Instead, operation of the carrier is driven by
the sodium concentration gradient (low sodium in ICF compared to lumen) established by
the energy-using sodium-potassium pump. The sodium-potassium pump actively
transports sodium out of the cell at the basolateral border, keeping the ICF sodium
concentration lower than the luminal concentration.
Example of Counter transport- sodium ions again attempts to diffuse to the interior of the
cell because of their large concentration gradient. However, this time, the substance to be
transported is on the inside of the cell and must be transported to the outside. Once both
sodium and the other substance to be transported have bonded to the carrier, a
conformational change takes place and the substance is transported to the outside while
sodium enters the inside. Examples are of the sodium-calcium (in all cells mostly) and
the sodium-hydrogen (in proximal tubules of kidneys) counter-transport.

8. Slide #12- Bulk Transport
Steps of endocytosis-
1. Cell membrane surrounds subject to be ingested by putting out pseudopodia.
2. Pseudopodia fuse over the surface.
3. Pinching of the membrane-enclosed vesicle so that engulfed material is trapped
within the cell.
There is membrane gain since vesicle membrane joins and adds to the cell membrane.
Steps of exocytosis-
1. A membrane-enclosed vesicle formed within the cell fuses with the cell
membrane.
2. The vesicle opens up outside the cell membrane.
3. Release of the substance.
Filtration is movement of water and solute molecules across the cell membrane due to
hydrostatic pressure generated by the cardiovascular system. Depending on the size of the
membrane pores, only solutes of a certain size may pass through it. For example, the
membrane pores of the Bowman's capsule in the kidneys are very small, and only
albumins, the smallest of the proteins, have any chance of being filtered 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.
Osmotic colloid pressure- in blood plasma, the dissolved compounds have an osmotic
pressure. A small portion of the total osmotic pressure is due to the presence of large
protein molecules and that is osmotic colloid pressure.
Hydrostatic Pressure- is the pressure exerted by fluid on the walls that contain it.