Transcript Membrane Structure and Function

Cell Membrane
Chapter Outline
• 1) Plasma Membrane Structure and
Function
• 2) Permeability of the Plasma Membrane
• 3) Diffusion and Osmosis
• 4) Transport by Carrier Proteins
• 5) Exocytosis and Endocytosis
1) Plasma Membrane Structure
and Function
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Fluid-Mosaic Model of Membrane
1) Phospholipid bilayer  fluid
2) Proteins on/in membrane  mosaic
3) Cholesterol in animal cells and related
steroid in plant cells  strengthen
membrane and regulate fluidity
• 4) Glycoproteins and glycolipids  cellular
interaction/ communication
Types of Membrane Proteins
• Integral (or Transmembranal) proteins
have hydrophilic ends and a hydrophobic
midsection
• Peripheral proteins which are hydrophilic
Functions of Membrane Proteins
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1) Channel Protein
2) Carrier Protein
3) Cell Recognition Protein
4) Receptor Protein
5) Enzymatic Protein
Channel Protein
• Allow a specific molecule or ion to cross
the plasma membrane freely through
water filled pores
• No change in protein shape
• Most plasma membranes contain specific
channel proteins for common ions such as
Na+, K+, Cl-
A Simpson’s Moment
• Tetrodotoxin is a toxin
that works by
inhibiting a specific
Na+ channel protein.
• This toxin is made by
the pufferfish.
Carrier/ Transporter Protein
• Selectively combine and interact with a
specific molecule so that it can cross the
plasma membrane
• Change in shape of protein
• Amino acids, needed for synthesis of new
proteins, enter the cell via carrier proteins
• Na-K pump in nerve cells
Cell Recognition Protein
• Proteins that have attached carbohydrate
chains
• Carbohydrate chains can vary in number
of sugars, types of sugars, sequence of
sugars and branching of carbohydrate
chains
• Diversity of glycoprotein cell-to-cell
recognition, adhesion between cells,
reception of specific molecules
Cell Recognition Protein
• Major histocompatability (MHC) proteins
distinguish your body cells from other
person’s body cells
• Problem for organ transplant
Receptor Protein
• Has a specific shape that only a specific
molecule will bind to
• The binding causes a change in the shape
of the protein  cellular response
• Anti-diuretic hormone (ADH) attaches to
receptors in kidney  change in
permeability of membrane to water
Enzymatic Protein
• Catalyze a specific reaction
• Epithelial cells lining small intestine
manufacture lactase
2) Permeability of Plasma
Membrane
• Membrane is selectively permeable 
some molecules can cross membrane,
while other molecules can not
• Molecules that can cross the membrane
either cross by passive transport or active
transport
Passive Transport
• Does not use chemical energy (ATP)
• Molecules move along a concentration
gradient : areas of high concentration 
areas of low concentration
• Diffusion : No channel or carrier protein
• e.g. lipid soluble molecules, gases, water
• Facilitated diffusion: Channel or carrier
protein
• e.g. glucose, amino acids
Active Transport
• Chemical energy (ATP) is used by cell to
move molecules
• Carrier protein moves molecules across
membrane against concentration gradient:
areas of low concentration  areas of
high concentration
• e.g. sucrose, some amino acids, ions
3) Diffusion and Osmosis
• Diffusion: molecules diffuse down their
concentration gradient
• Diffusion is a physical phenomenon
• Osmosis: the diffusion of water across a
selectively permeable membrane due to
concentration differences
• Osmosis is a biological phenomenon
Lower Water
Concentration
Higher Water
Concentration
Osmosis Jones
Osmosis in Cells
• 1) Isotonic Solution
• 2) Hypotonic Solution
• 3) Hypertonic Solution
Isotonic Solution
• If the concentration of
solute (salt) is equal
on both sides, the
water will move back
and forth but it won't
have any result on the
overall amount of
water on either side.
• "ISO" means the
same
Hypotonic Solution
• The word "HYPO" means
less, in this case there
are less solute (salt)
molecules outside the cell
• Water will move into the
cell to dilute the solute
(salt) concentration until
an isotonic balance is
reached.
• The cell will gain water,
grow larger and
potentially burst in a
process called lysis
Hypertonic Solution
• The word "HYPER"
means more, in this case
there are more solute
(salt) molecules outside
the cell
• Water will move out of
the cell to dilute the
solute (salt) concentration
until an isotonic balance
is reached.
• The cell will lose water
and shrink in a process
called crenation
Animal
Cell
Plant
Cell
Osmosis and Diffusion
• Water always moves by diffusion across a
selectively permeable membrane from a
hypotonic solution (higher water potential)
to a hypertonic solution (lower water
potential)
4) Transport by Carrier Proteins
• Carrier proteins in the membrane combine
with specific molecules, which are then
transported across the membrane
• Carrier proteins are required for both
facilitated diffusion and active transport
Facilitated Diffusion
• Facilitated diffusion is passive transport
• The cell does not use energy to move
molecules across the membrane
• Molecules move through a carrier protein
from a region of higher concentration to a
region of lower concentration
Active Transport
• The cell uses energy (ATP) to move
molecules across the membrane against a
concentration gradient
• Molecules move through a carrier protein
from a region of lower concentration to a
region of higher concentration
• The Na-K pump is an example of active
transport
Na-K Pump
• 1) ATP binds to an active site on the protein
forming the Na-K pump, thus providing
energy for it.
• 2) 3 sodium ions (red balls) from the
cytoplasm bind to lock and key sites on the
Na-K pump.
• 3) The energized protein of the Na-K pump
changes shape, releasing the 3 sodium ions
to the extracellular environment
Na+ ion
Na-K Pump
• 4) Two potassium ions (from the
extracellular environment) bind to lock
and key sites on the protein of the Na-K
pump.
• 5) The protein of the Na-K pump
changes shape as the phosphate group
leaves the protein's active site.
• 6) Potassium ions are released into the
cytoplasm.
5) Exocytosis and Endocytosis
• Macromolecules such as proteins and
complex carbohydrates are too large to be
moved into or out of the cell through a
carrier protein
• Macromolecules are moved into or out of
the cell by vesicle formation
• Vesicle formation requires energy (ATP)
Exocytosis
• To export materials out of the cell:
• 1) a vesicle forms around the molecules,
• 2) the vesicle moves towards the cell
membrane,
• 3) the vesicle fuses with the cell
membrane, and
• 4) the molecules are released into the
external environment
Exocytosis
Endocytosis
• Molecules are taken into the cell by vesicle
formation:
• 1) the molecules are engulfed by the cell
membrane
• 2) a vesicle forms around the molecules,
and
• 3) the vesicle moves into the cytoplasm
Endocytosis
Forms of Endocytosis
• 1) Phagocytosis: endocytosis of a food
particle or another cell
• 2) Pinocytosis: endocytosis of a liquid or
very small particle
• 3) Receptor-mediated Endocytosis:
receptor proteins bind to a specific
molecules such as vitamin or hormone,
which are then brought in by endocytosis