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Cell Transport Mechanisms
This MECQ travel
protocols can be
compared to the cell as
a restricted area
wherein movements of
materials in and out are
being controlled.

Illustration of the cell membrane implenting the
cell transport mechanism (travel protocols)

What is Cell Transport?
Cell transport refers to the movement of substances across
the cell membrane. Probably the most important feature of
a cells phospholipid membranes is that they are selectively
permeable. This feature allows a cell to control the
transport of materials, as dictated by the cells function.
The permeability of a membrane is dependent on the
organization and characteristics of the membrane lipids
and proteins. In this way, cell membranes help maintain a
state of homeostasis with cells (and tissues, organs and
organ systems) so that an organism can stay alive and
healthy.

2 Types of Cell Transport
1.) Passive Transport- involves the movement of material
along a concentration gradient (high concentration to
low concentration) because materials are moving down a
concentration gradient. It does require the expenditure
of energy
There are three main types of Passive Transport:
a.) Simple Diffusion- movement of small or lipophilic
molecules through the membrane
b.) Osmosis- movement of water molecules through
the membrane (dependent on solute
concentrations)
c.) Facilitated Diffusion- movement of large or charged
molecules via channel proteins that facilitate the
transport (e.g ions and sucrose)

2.) Active Transport- involves the movement of material
against a concentration gradient (low concentration to high
concentration) because materials are moving against the
gradient, it requires the expenditure of energy (e.g hydrolysis)
a.) Carrier Proteins or Protein Pumps
An important membrane adaption for active transport is the
presence of specific carrier proteins or pumps to facilitate
active transport.
b.) Bulk Transport
Like the active transport processes that move ions and small
molecules via carrier proteins, bulk transport is an energy
requiring (and, in fact, energy-intensive) process, hence it is a
type of active transport.
The movement of macromolecules such as
protein or polysaccharides into or out of the cell.

There are 2 types of Bulk Transport, exocytosis and
endocytosis and both require the expenditure of energy

Endocytosis- is a cellular mechanism where a cell
internalizes substances from the external environment.
This substances undergo certain processes of breaking
down to smaller elements either for use by the cell or
for elimination purposes. There are three types based
on mechanisms of particle. They include: Phagocytosis,
Pinocytosis and Receptor mediated endocytosis.
Exocytosis- is a process that is used to transport
materials from inside the cell to the external part of the
cell using energy. The mechanism uses special vesicles
filled with the particles of interest to transport
generally, in this mechanism of exocytosis, a special
vesicle bound to the cell membrane, containing the
cellular particles will expel the cell content to the
external part of the cell.

Diffusion is the movement of molecules from an area of
high concentration of the molecules to an area with a
lower concentration. The difference of concentrations of
the molecules in the two areas is called the concentration
gradient. Diffusion will continue until this gradient has
been climinated. Since diffusion moves materials from an
area of higher concentration to the lower, it is described
as moving solutes “down to concentration gradient ”. The
result of diffusion is an equal concentration or equilibrium
of molecules on both sides of the membrane. This process
does not require any energy input henc, it is a passive
transport, in fact a concentration gradient itself is a form
of a stored potential energy and this energy is used up as
the concentration equalize.
Diffusion

Factors that affect Diffusion
While diffusion will go forward in the presence of
concentration gradient of a substance, several factors
will affect the rate of diffusion
Extent of the concentration gradient: The greater the
difference in concentration, the more rapid the
diffusion. The closer of the distribution of material gets
to equilibrium, the slower the rate of diffusions
becomes.
Mass of the molecules diffusing: Heavier molecules
move more slowly; therefore, they diffuse more
slowly. The reverse is true for lighter molecules.

Temperature: Higher temperature increase the energy
and therefore the movement of the molecules,
increasing the rate of diffusion. Lower temperature
decrease the energy of the molecules, thus decreasing
the rate of diffusion.
Solvent Density: As the density of a solvent
increases, the rate of diffusion decreases.
Solubility: Nonpolar or lipid soluble pass through plasma
membranes more easily than polar materials, allowing a
faster rate of diffusion.
Surface area and thickness of the plasma membrane:
Increase surface area, increases the rate of diffusion,
whereas a thicker membrane reduces it.

Distance Travelled: The greater the distance that a
substance must travel, the slower the rate of
diffusion. This places an upper limitation on cell size.

Osmosis
Osmosis is the movement of water molecules from
a solution with a high concentration of water
molecules to a solution with a lower concentration
of water molecules, through a cell’s partially
permeable membrane.
A partially permeable membrane (sometimes
called a selectively permeable membrane) only
allows certain molecules or ions to cross it.

In the diagram above, the higher concentration of
water molecules to the left of the partially
permeable membrane makes it likely that a large
number of water molecules will collide with the
membrane and pass through it.

The lower concentration of water molecules
on the right-hand side of the partially
permeable membrane in the diagram makes it
likely that fewer water molecules will collide
with the membrane and pass through it.

Transport Protein
A transport protein is completely spans the
membrane and allows certain molecules or ions
to defuse across the membrane.
Transport proteins is also known as
transmembrane proteins, are membrane proteins
that aid in the facilitated diffusion or active
transport of ions across the hydrophobic lipid
bilayer.

3 types of Transport Protein
1.) Channel Protein
Is a type of transport protein
act like a pore in the
membrane that lets water
molecules or small ions
through quickly.

2.) A gated channel protein
Is a type of transport protein that opens a
“gate” allowing a molecule to pass
through the membrane.
Gated channels have a binding site that is
specific for a given molecule or ion. A
stimulus causes the "gate" to open or
shut. The stimulus may be chemical or
electrical signals, temperature, or
mechanical force, depending on the type
of gated channel.

3.) Carrier Protein
A carrier protein is a transport protein
that is specific for an ion, molecule, or
group of substances.
Carrier proteins "carry" the ion or molecule
across the membrane by changing shape
after the binding of the ion or molecule.
Carrier proteins are involved in passive
and active transport.

A model of a channel protein and carrier proteins

Active Transport
Is the energy requiring process of pumping molecules
and ions across membranes “uphill” against a
concentration gradient. To move these molecules
against their concentration gradient, a career protein
is needed.
In active transport, as career proteins are used to
move materials against their concentration gradient,
these proteins are known as pumps.

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