Lecture 3
Cell Biology
1. Cell membrane structure and function.
2. Movement across membrane.
Prepared by Mayssa Ghannoum
Overview

The plasma membrane is the edge of life.
The boundary that separates the living cell from it
surroundings.

It controls traffic into and out of the cell it surrounds.

It exhibits selective permeability;
(allows some substances to cross it more easily than others.)

The plasma membrane and its proteins not only acts as an outer
boundary but also enable the cell to carry out its functions.

To better understand how membranes work, we’ll examine their
architecture.
Membrane structure and function
Cellular membranes are fluid mosaics of lipids and proteins

The major ingredients in cellular membranes are: lipids and
proteins. Less are carbohydrates.

The most abundant lipids in membranes are phospholipids.

A phospholipid is an amphipathic molecule, meaning it has both a
hydrophilic region and a hydrophobic region.
Hydrophilic: have affinity for water (soluble in water)
Hydrophobic : don’t have affinity for water (soluble in lipids)
 The
plasma membrane consist of a double
layer (bilayer) of phospholipids with a
“mosaic” of various proteins attached to or
embedded in it.
 In
the interior of the membrane, the
phospholipids tails are hydrophobic.
 In
contact with the aqueous solution on either
side of the membrane, the phospholipid
heads are hydrophilic.
 This
structure of the plasma membrane is
called:
Fluid mosaic model
The Fluidity of Membranes

Membranes are not static sheets of molecules locked rigidly in
place.
 Most of the lipids and some of the proteins can shift about laterally.
 The lateral movement of phospholipids within the membrane is
rapid.
 Proteins are much larger than lipids and move more slowly.
Membrane proteins and their functions
 In
the membrane, different proteins are embedded in the fluid
matrix of the lipid bilayer.
 Phospholipids form the
main fabric of the membrane, but
proteins determine most of the membrane’s functions.

there are two major populations of membrane proteins:
Integral proteins and peripheral proteins.
 Integral proteins
penetrate the hydrophobic core of the lipid
bilayer.
 Peripheral proteins
are not embedded in the lipid bilayer at all;
they are attached to the surface of the membrane.
Membrane proteins functions
 There
are six major functions
performed by proteins of the plasma
membrane:
1. Transport
2. Enzymatic activity
3. Signal transduction
4. Cell-cell recognition
5. Intercellular joining
6. Attachment to the cytoskeleton
and extracellular matrix (ECM).
The Role of Membrane carbohydrates in cell-cell
Recognition
recognition is the cell’s ability to distinguish one
type of neighboring cell from another.
 Cell-cell
 Cells
recognize other cells by binding to surface
molecules, often to carbohydrates, on the plasma
membrane.
 Membrane
carbohydrates are usually short, some are
bonded to lipids forming molecules called glycolipids.
Most are bounded to proteins forming glycoproteins.
Four steps of Synthesis of Membranes
The synthesis of membrane proteins and lipids is in
the endoplasmic reticulum. Carbohydrates are added
to proteins making them glycoproteins.
2. Inside the Golgi apparatus, the glycoproteins
undergo further carbohydrate modification and
lipids acquire carbohydrates and become
glycolipids.
3. The transmembrane proteins, glycolipids and
secretory proteins are transported in vesicle to the
plasma membrane.
4. The vesicles fuse with the membrane, releasing
secretory proteins from the cell.
1.
Membrane structure results in selective permeability
 The
fluid mosaic model of the plasma membrane helps
regulate the cell’s molecular traffic.
 A steady
traffic of small molecules and ions moves across
the plasma membrane in both directions.
 sugars,
amino acids, and other nutrients enter the cell, and
metabolic waste products leave it.
 The
cell takes in oxygen for use in cellular respiration and
expels carbon dioxide.
 Also,
the cell regulates its concentrations of inorganic ions,
such as Na+, K+, Ca²+ and Cl¯.
The permeability of the lipid bilayer

Nonpolar molecules, such as hydrocarbons, carbon dioxide and
oxygen, are hydrophobic and can dissolve in the lipid bilayer of
the membrane and cross it easily.

The hydrophobic core of the membrane makes difficult the
direct passage of ions and polar molecules, which are
hydrophilic, through the membrane.

Water, an extremely small polar molecule does not cross rapidly
the membrane.

The lipid bilayer is then only one aspect of the gatekeeper
system responsible for the selective permeability of cell.

Proteins built into the membrane play key roles in regulating
transport.
Transport Proteins
 Hydrophilic
substances can avoid contact with the lipid
bilayer by passing through transport proteins that span the
membrane.
 Some
transport proteins, called channel proteins, function by
having a hydrophilic channel that certain molecules use as a
tunnel through the membrane.
 The

water channel proteins are known as aquaporins.
Other transport proteins, called carrier proteins, change
shape in a way that shuttles the passengers across the
membrane.
 A transport
protein is specific for the substance it moves,
allowing only a certain substances to cross the membrane.

The selective permeability of a membrane depends on
both the discriminating barrier of the lipid bilayer and
the specific transport proteins built into the membrane.
 At
a given time, what determines whether a particular
substance will enter the cell or leave?
To answer this question, We will explore the modes of
membrane traffic.
Movement Across Membranes
 The
different ways of transporting molecules across the
membrane are:

Passive transport (Diffusion/Osmosis)
 Facilitated Diffusion
 Active transport
 Exocytosis and Endocytosis
Passive Transport
 Passive
transport is diffusion of a substance across a
membrane with no energy investment.
 The
rule of diffusion is: In the absence of other forces, a
substance will diffuse from where it is more concentrated to
where it is less concentrated.
Each substance will diffuse down its concentration
gradient.
 Diffusion
is a spontaneous process, needing no input of
energy.

Note that each substance diffuses down its own concentration gradient
unaffected by the concentration differences of other substances.
The diffusion of solutes across a membrane
Osmosis
 The
diffusion of water across a
selective permeable membrane is
called: Osmosis
 Water
diffuses across the membrane
from an area of higher to lower free
water concentration;
 water
moves from the area of lower
solute concentration to that of higher
solute concentration until the solute
concentrations on both sides of the
membrane are equal.
Facilitated Diffusion: Passive transport Aided by
Proteins

Many Polar molecules and ions impeded by the lipid bilayer of the
membrane diffuse passively with the help of transport proteins that
span the membrane. This phenomenon is called facilitated
diffusion.

Channel protein: has a channel through
which water or other solute can pass

Carrier protein: alternate between two
shapes, moving a solute across
the membrane during the shape change.
Active Transport

Active transport uses energy to move solutes against their
gradients.

The transport proteins involved in active transport are all carriers
proteins, rather than channel proteins.

Active transport enables a cell to maintain internal concentrations
of small solutes that differ from concentrations in its environment.

An animal cell has a much higher concentration of potassium ions
and a much lower concentration of sodium ions.

The plasma membrane helps maintain these gradients by pumping
sodium out of the cell and potassium into the cell.

ATP supplies the energy for most active transport.

An example of one
transport system that
works in active
transport is the
sodium-potassium
pump.
which exchange
sodium (Na+) for
potassium (K+) across
the plasma membrane
on animal cells.
Bulk Transport across the
membrane occurs by exocytosis
and endocytosis.
Water and small molecules enter and leave
the cell by passive or active transport.
 Large molecules, such as proteins and
polysaccharides, generally cross the
membrane in bulk by mechanisms that
involve packaging in vesicles.
These mechanisms require energy.

Exocytosis
 Definition
of exocytosis: Transport vesicles migrates to the
plasma membrane, fuse with it, and release their contents.

Mechanism?
1. A transport vesicle that has budded from the Golgi apparatus
moves along microtubules of the cytoskeleton to the plasma
membrane.
2. When the vesicle membrane and plasma membrane come into
contact, the lipid molecules of the two bilayers rearrange
themselves so that the two membranes fuse.
3. The contents of the vesicle then spill outside of the cell.
4. The vesicle membrane becomes part of the plasma membrane.
Endocytosis
 In
endocytosis, molecules enter cells within vesicles that
pinch inward from the plasma membrane.

The three types of endocytosis are:
Phagocytosis
Pinocytosis
Receptor-mediated endocytosis
Phagocytosis
 A cell
engulfs a particle
by wrapping around it
and packaging it within
a membrane enclosed
sac that can be large
enough to be classified
as vacuole.
Pinocytosis
 The
cell “gulps” droplets
of extracellular fluid into
tiny vesicles.
It’s not the fluid itself
that is needed by the cell,
but the molecules
dissolved in the droplets.

Pinocytosis is
nonspecific in the
substances it transports.
Receptor-Mediated Endocytosis
This type of endocytosis enables the cell to acquire a bulk
quantities of specific substances, even though those substances
may not be very concentrated in the extracellular fluid.
 Embedded in the membrane are proteins with specific receptors
sites exposed to the extracellular fluid.
 The receptors proteins are clustered in regions of the membrane
called coated pits.
 The specific substances bind to these receptors.
 When binding occurs, the coated pit forms a vesicle containing
the specific molecules.
 After this ingested material is liberated from vesicle, the
receptors are recycled to the plasma membrane by the same
vesicle.

Receptor-Mediated Endocytosis