The plasma membrane functions

The functions of the plasma membrane include:

Isolation

Regulation of exchange with the environment

Sensitivity to the environment

Structural support
Plasma Membrane

Physical barrier extracellular fluids

Helps in maintaining homeostasis

separates
intracellular
fluids
from
Plays a dynamic role in cellular activity – selectively
permeable
Fluid Mosaic Model

Double
bilayer
phospholipids

of
Phospholipids
have
hydrophobic tails and
hydrophilic heads
Hydrophilic head
Hydrophobic
tails
+ CH3
CH2 N CH
C3
H
CH2
Phosphate
3
O
group
O P O
–
OC
C
C
H
H H
2
O2
O
O
O
C
C
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH
CH
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH3
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH3
The plasma membrane includes proteins

Integral proteins


Within the membrane
Peripheral proteins

Bound to inner or outer surface of the
membrane
The plasma membrane includes proteins

Anchoring proteins (stabilizers)


Recognition proteins (identifiers)


Bind and respond to ligands (ions, hormones)
Carrier proteins


Catalyze reactions
Receptor proteins


Label cells as normal or abnormal
Enzymes


Attach to inside or outside structures
Transport specific solutes through membrane
Channels

Regulate water flow and solutes through membrane
Structures on the plasma membrane surfaces
Microvilli, Cilia,
Stereocilia
Specialized
junctions
Features of Apical Surface of Epithelium - Microvilli

Projections that increase surface area

Folding of the plasma membrane
http://cellbio.utmb.edu/microanatomy/epithelia/epith_lec.htm
Features of Apical Surface of Epithelium - Cilia

These structures are designed for motility.

Epithelia that need to move substances across their
surface (like mucous in the air passages) have cilia.

Each cilium or flagellum has a basal body located at its
base.

Basal bodies anchor the cilia or flagella and are
thought to be responsible for their formation.

They look like centrioles and are believed to be
derived from them

Flagella: (ex)


spermatoza
Extra long cilia
Moves cell
http://www.lbl.gov/Science-Articles/Archive/sabl/2006/Jul/02.html
Cell junctions – 3 groups

Tight junction


Gap junction


designed to restrict the movement of material
between the cells they link
create cytoplasmatic communication bridges
between cells
Anchoring junction

attach cells to one another or to extracellular matrix
Membrane Junctions
Tight junction
Gap junction
http://www.phschool.com/science/biology_place/biocoach/biomembrane2/junctions.html
Anchoring
junction
Tight Junctions

An intercellular junction between
cells in which the outer layers of
the cell membranes fuse,

reducing the ability of larger
molecules and water to pass
between the cells.

Tight junctions prevent the
free movement of molecules
between cells in the intestine
and allow the intestinal cell to
control absorption
Gap junctions

Example – intercalated discs in the heart, electrical
synapses
Cell transport mechanisms How things enter and leave the cell
2 groups of movement


Passive transport – no energy is needed

Diffusion

Carrier-mediated
Active transport – requires ATTP

Pumps

Vesicular transport
Characteristics of selectively
permeable membranes
EXTRACELLULAR
FLUID
Plasma
membrane
Passive mechanisms
do not require ATP.
Diffusion is
movement driven
by concentration
differences.
CYTOPLASM
Materials may cross
the plasma membrane
through active or
passive mechanisms.
Active mechanisms
require ATP.
Carrier-mediated
transport involves
carrier proteins, and
the movement may
be passive or active.
Vesicular transport
involves the
formation of
intracellular
vesicles; this is an
active process.
Passive transport

All molecules in the body are in constant motion
regardless of the presence of a membrane (kinetic
energy)

Motion stops only at absolute zero

By international agreement, it is defined as 0K on the
Kelvin scale, −273.15°C on the Celsius scale and
−459.67°F on the Fahrenheit scale

When a membrane is present the movement in a certain
direction can be limited or changed

A molecule will move in a certain direction until collide
with another molecule. When this happens, the direction
of the movement will change
Diffusion

Diffusion is the tendency for molecules to spread out
evenly into the available space

The driving force is kinetic energy

Slow in air and water but important over small distances

Although each molecule moves randomly, diffusion of a
population of molecules may exhibit a net movement in
one direction


Depends on a concentration gradient. (What is a
concentration? A concentration gradient?)
At dynamic equilibrium, as many molecules cross one
way as cross in the other direction
Factors Affecting Diffusion

Distance (inversely related)

Molecule size (inversely related)

Temperature (directly related)

Gradient size (directly related)

Electrical forces

Attraction of opposite charges (+,–)

Repulsion of like charges (+,+ or –,–)
Diffusion

The movement of molecules will happen in ALL
directions

What is usually important is the net rate of diffusion
in a certain direction

The net movement will be from high to low
concentration until equilibrium is reached

At equilibrium, the net movement is equal in all
directions
When a membrane is present

Membrane can be:



Freely permeable (this does not apply to plasma
membrane) – allows passage of all substances
Selectively permeable – permits passage of some
materials and prevents passage of others
Impermeable – cells can be impermeable to
specific substances, but no living cell has a
completely impermeable membrane
Permeability characteristics of membranes
Freely permeable membranes
Ions
Protein
Selectively permeable membranes
Carbohydrates
Ions
Protein
—
Lipids
Freely permeable membranes
allow any substance to pass without
difficulty.
Carbohydrates
Ions
Protein
—
Water
Impermeable membranes
—
Water
Water
Lipids
Selectively permeable membranes,
such as plasma membranes, permit the
passage of some materials and prevent
the passage of others.
Carbohydrates
Lipids
Nothing can pass through impermeable
membranes. Cells may be impermeable
to specific substances, but no living cell
has an impermeable membrane.
Selectively permeable membranes

Selective based on:
1.
2.
Characteristics of material to pass

Size

Electrical charge

Molecular shape

Lipid solubility
Characteristics of membrane

What lipids and proteins present

How components are arranged
Diffusion through cell membrane


Diffusion is divided into 2 types:
1. Simple diffusion – the movement of particles through the
membrane with no assistance

Nonpolar / lipid-soluble substances that diffuse directly
through the lipid bilayer

Gases readily diffuse through lipid bilayer. (Ex.
movement of oxygen inside cells and CO2 outside)

Diffusion of water and other lipid-insoluble molecules
happens via protein channels

The channels are highly selective as a result of the
diameter, shape, charge and chemical bonds
Diffusion of lipid-soluble materials
Diffusion of lipid-insoluble materials
Diffusion through cell membrane


2. facilitated diffusion - Assisted by carrier protein

Materials are bound to specific proteins and move
through water-filled protein channels (big polar
molecules; ex. – glucose)

The facilitated diffusion rate depends on the rate in
which the carrier protein molecule can undergo
changes that allow passage
Carrier Proteins

Are integral transmembrane proteins

Show specificity for certain polar molecules

Their number will influence the amount that can be
transferred through the membrane
Osmosis

Osmosis is a simple diffusion of water.

It occurs through a selectively permeable membrane

Occurs when the concentration of a water is different on
opposite sides of a membrane

Membrane must be freely permeable to water, selectively
permeable to solutes
Osmosis – osmolality, osmolarity and osmotic pressure

Osmolality (molecular weight) - One osmole is 1 gram
molecular weight

Osmolarity (concentration) - One osmole in one liter

Osmotic pressure – defined by the concentration of
solute particles in a solution

Is defined by the number of particles, not their size
or nature

Each particle in a solution, regardless of its
mass, exerts the same pressure against the
membrane
Effects of Solutions of Varying Tonicity

Tonicity – description of how the solution affects a
cell



Isotonic – solutions with the same solute
concentration as that of the cytosol
Hypertonic – solutions having greater solute
concentration than that of the cytosol
Hypotonic – solutions having lesser solute
concentration than that of the cytosol
Passive Membrane Transport: Filtration

The passage of water and solutes through a membrane
by hydrostatic pressure

Pressure gradient pushes solute-containing fluid from a
higher-pressure area to a lower-pressure area

Depending on the size of the membrane pores

only solutes of a certain size may pass through it.
Transport that uses ATP

A movement that can be against concentration gradient

Uses ATP to move solutes across a membrane

Two types:

Active transport - use of carrier proteins

Vesicular transport
Types of Active Transport

2 types according to the source of energy used for the
transport

Primary active transport

The energy for the transport derived directly from a
high energy molecule – ATP

The hydrolysis of ATP causes phosphorylation of a
transport protein that in turn changes its shape.

That change “promotes” the passage of materials
(ex. Sodium-potassium pump)
Fig. 7-16-7
EXTRACELLULAR
FLUID
Na+
[Na+] high
[K+] low
Na+
Na+
Na+
Na+
Na+
Na+
Na+
CYTOPLASM
1
Na+
[Na+] low
[K+] high
P
ADP
2
ATP
P
3
P
P
6
5
4
Types of Active Transport

Secondary active transports – one ATP-powered pump
can drive secondary transport of other solutes.

The energy is derived from the energy stored in
creating the concentration gradient

This concentration difference was created by the
primary active transport that used ATP

Secondary transport, like the primary, depends on
carrier proteins, but without the need of energy
Fig. 7-19
–
+
H+
ATP
–
H+
+
H+
Proton pump
H+
–
H+
+
–
H+
+
H+ Diffusion
of H+
Sucrose-H+
cotransporter
H+
Sucrose
–
–
+
+
Sucrose
Active transport


Symport system – two substances are moved
across a membrane in the same direction
Antiport system – two substances are moved
across a membrane in opposite directions (Na/K)
Vesicular Transport


Transport of large particles and macromolecules across plasma
membrane using vesicles and ATP
Endocytosis – enables large particles and macromolecules to enter the
cell. Few types:

Receptor-mediated endocytosis – selective process that depends
on the binding of extracellular material to a specific receptor

This binding initiates the endocytosis

Phagocytosis – “cell eating”; endocytosis of solid objects

pseudopods engulf solids and bring them into the cell’s interior

Happens in specialized cells

Pinocytosis – “cell drinking”; endocytosis of liquids.

This is not a selective process and does not involve receptor
Vesicular Transport



Exocytosis
–
moves
substance from the cell
interior to the extracellular
space
Transcytosis – moving
substances into, across, and
then out of a cell
Vesicular trafficking –
moving substances from
one area in the cell to
another
Passive Membrane Transport – Review
Process
Energy Source
Simple diffusion Kinetic energy
Facilitated
diffusion
Kinetic energy
Osmosis
Kinetic energy
Filtration
Hydrostatic
pressure
Example
Movement of O2 through
membrane
Movement of glucose into
cells
Movement of H2O in & out
of cells
Formation of kidney filtrate
Active Membrane Transport – Review
Process
Energy Source
Example
Active transport of solutes ATP
Movement of ions across
membranes
Exocytosis
ATP
Neurotransmitter secretion
Endocytosis
ATP
White blood cell
phagocytosis