Cell Biology
Lecture 4
Plasma Membrane Transport
Permeable
Permeable
Slightly permeable
Impermeable
Impermeable
Impermeable
1. Simple Diffusion
•In
simple diffusion, small noncharged
molecules or lipid soluble molecules pass
between the phospholipids to enter or leave
the cell, moving from areas of high
concentration to areas of low concentration
(they move down their concentration
gradient).
Small hydrophobic molecules cross
membrane by simple diffusion
Gases (O2, CO2), lipids and small
uncharged molecules (urea, ethanol) are
transported by simple simple diffusion.
 They do not need transport proteins.
 No metabolic energy is consumed
because the movement is from a high to a
low concentration gradient.

2. Osmosis

Osmosis is a type of simple diffusion in
which water molecules diffuse through a
selectively permeable membrane from areas
of high water concentration to areas of
lower water concentration. (Note that the
more particles dissolved in a solution, the
less water there is in it, so osmosis is
sometimes described as the diffusion of
water from areas of low solute
concentration to areas of high solute
concentration).
Hypertonic and hypotonic solutions

A hypertonic solution has more solutes
and less water than a hypotonic solution.
For example the solution inside the cell is
hypertonic to the solution outside the
cell. During osmosis, water moves from
the hypotonic solution (more water, less
solutes) to the hypertonic solution (less
water, more solutes).
Which of the following is
hypertonic solution?
Membrane proteins mediate transport of
most molecules and all ions across
biomembranes




Few molecules (gases, water, ethanol, urea)
can pass through pure phospholipid bilayer.
Transport of most molecules require the
help of transport proteins.
All transport proteins are transmembrane
proteins.
Allow the movement of hydrophilic
substance without their coming into contact
with the hydrophobic interior of the
membrane
3. Facilitative Diffusion

In facilitated diffusion, substances move into or
out of cells down their concentration gradient
through protein channels in the cell membrane.
Simple diffusion and facilitated diffusion are
similar in that both involve movement down the
concentration gradient. The difference is how the
substance gets through the cell membrane. In
simple diffusion, the substance passes between
the phospholipids; in facilitated diffusion there are
a specialized membrane channels. Charged or
polar molecules that cannot fit between the
phospholipids generally enter and leave cells
through facilitated diffusion.

homeostasis maintains constant conditions in a living
organism. Membrane control is essential for this.

Passive transport
This is movement across the membrane without any energy
input. There are 3 forms: diffusion, facilitated diffusion and
osmosis
Diffusion:
- particles are in constant motion (Brownian or heat-kinetic
motion)
-the overall effect of this random motion over time is
diffusion (or the movement of substances from an area of
high concentration to an area of lower concentration, thus
resulting in equal concentration in both areas)
-for this to occur oacross the membrane, the membrane
must be permeable to the substances (apolar particles and
small molecules like H2O, O2 and CO2)
Active Transport
 transport of a substance against the
concentration gradient
 requires energy
 must use transport proteins and ATP
 a substance binds the transport protein and
energy is released from ATP to allow the
protein to change shape or move and
release the substance on the other side of
the membrane.
 Example: Na+/K+ pump - this is especially
important in nerve and muscle cell

Types of transport proteins
Uniport transport of Glucose and
Water
‘’The protein-mediated transport of small
hydrophillic molecules across a
membrane’’
 Most animals require glucose as a source
of energy.
 They employ a glucose uniporter to take
up glucose from the blood or extra
cellular fluid down its conc. Gradient.

Features of Uniport transport
• The rate of diffusion is far higher than
simple diffusion through a pure
phospholipids.
•Transport occurs via limited number of
uniporter molecules and not throughout
the phospholipid bilayer.
•Transport is specific.
GLUT1



Glucose transporter found in the plasma membrane of
erythrocytes. 12 GLUTS are encoded by human
genomes.
Glucose conc. is high in the extracellular medium than
in cell.
Like other uniporters, GLUT1 alternates between 2
confirmational states.
Osmotic pressure causes water to
move across membranes
Osmotic pressure is directly proportional
to the difference in the conc. of the total
number of solute molecules on each side
of the memberane.
 Aquaporins are a family of membrane
proteins that allow water and a few other
small uncharged molecules (glycerol) to
cross biomembrane, 11 aquaporins are
known in humans

ATP-powered Pumps and
Intracellular Ionic Environment
They use the energy released by
hydrolysis of ATP.
 Have one or more binding sites for ATP
located on subunits of the protein facing
the cytosol.
 These proteins do not hydrolyse ATP into
ADP and Pi unless ions or other
molecules are simultaneously
transported.

Classes of pumps