Ch 5 Membrane
Structure and Function
Control the movement of materials
into and out of the cell.
Membrane Structure

Phospholipids: arranged in
a bilayer due to polarity of
molecule.
 Glycolipids: Similar to
phospholipids mono or
polysaccharides instead of
phosphates. Cell to cell
communication
 Cholesterol: lipid found in
animal plasma
membranes, reduces
permeability and provides
rigidity.
Membrane Structure Contd
 Membrane
Proteins: Integral proteins
largely determine the membrane’s
function(s).


Channel proteins, Carrier proteins, Receptor
proteins, Enzymatic proteins, Glycoproteins.
Learn these!
Membrane structure
 Fluid
Mosaic Model: The composition of
phospholipids will contribute to the degree
of permeability, and the proteins are free to
move laterally w/in the plasma membrane.
QOD
1. How would you expect the saturation
levels of membrane fatty acids to differ in
plants adapted to cold environments and
plants adapted to hot environments?
Answers
1.
Plants adapted to cold environments would be
expected to have more unsaturated fatty acids
in their membranes, since those remain fluid at
lower temperatures. Plants adapted to hot
environments would be expected to have more
saturated fatty acids, which would allow the
fatty acids to “stack” more closely, making the
membranes less fluid and therefore helping
them to stay intact at higher temperatures.
Permeability of the
Plasma Membrane
Selective Permeability
Some substances can freely move across
the membrane whereas others cannot.
 Hydrophobic (nonpolar) molecules, such
as hydrocarbons, carbon dioxide, and
oxygen, can cross with ease.
 Small Polar molecules (H2O) can pass
between the phospholipids
Selective Permeability contd
 Macromolecules
cannot pass through
membrane

Transported by vesicle formation
 Ions

and charged molecules cannot
Carrier and channel proteins transport these.
Transport across the plasma membrane
includes:

Passive transport
 Facilitated transport
 Active transport
 Membrane assisted
Passive Transport
 Diffusion:
The spontaneous tendency of a
substance to move down its concentration
gradient from a more concentrated to a
less concentrated area.

The diffusion of a substance across a
biological membrane = passive transport
Note that each substance diffuses down its own concentration
gradient, unaffected by the concentration differences of other
substances
Passive Transport contd


1.
2.
Osmosis: The diffusion of water across
the plasma membrane.
Solutions: a liquid that is a homogeneous
mixture of two or more substances. Has
2 parts
Solvent: The dissolving agent of a
solution (liquid portion) . Water is the
most versatile solvent known.
Solute: A substance that is dissolved in a
solution.
Passive Transport contd
Tonicity: The ability of a solution to cause a cell
within it to gain or lose water. 3 types
 Hypertonic: A solution with a higher
concentration of solute than inside the cell.


Hypotonic: A solution with a lower concentration
of solute than inside the cell.


Water diffuses out.
Water diffuses into.
Isotonic: A solution with equal concentrations of
solute on both sides of the membrane.

No net movement of water into or out of.

Turgid: A walled cell becomes turgid if it has a
greater solute concentration than its surroundings,
resulting in entry of water. (turgor pressure)
(Tonicity?)
 Plasmolysis: A phenomenon in walled cells in
which the cytoplasm shrivels and the plasma
membrane pulls away from the cell wall when the
cell loses water to a hypertonic environment.
Crenate
Transport by Proteins
 Molecules
that cannot diffuse across the
plasma membrane, can be transported by
integral membrane proteins.
 Channel & Carrier Proteins: Specific to the
molecule they transport

Required for facilitated and active transport.
Facilitated Transport
An integral protein (channel or carrier)
assists the movement of a molecule down
its concentration gradient.
Facilitated Diffusion
 Increases
the rate
at which the
solute crosses the
plasma membrane

Faster than
diffusion
Active Transport
The movement of molecules against their
concentration gradient
Active Transport
 Movement
from low concentration to high
concentration.
Requirements:
 Transport protein (specific) aka pumps
 Energy (ATP)
The sodium–potassium pump: a
specific case of active transport.
-pumps ions against steep
concentration gradients
-Sodium ion concentration (represented
as [Na+]) is high outside the cell and
low inside, while potassium ion
concentration ([K+]) is low outside
the cell and high inside.
-The pump oscillates between two
conformational states in a pumping
cycle that moves three sodium ions
out of the cell for every two
potassium ions pumped into the cell.
- ATP powers the changes in
conformation by phosphorylating the
transport protein (that is, by
transferring a phosphate group to
the protein).
Fig 5.11
Proton pumps, the main electrogenic pumps of plants,
animals, fungi, and bacteria, are membrane proteins that
store energy by generating voltage (charge separation)
across membranes of mitochondria and chloroplasts.
Membrane Assisted
Transport
The formation of vesicles by the plasma
membrane to enable the transport of
macromolecules (too big for transport proteins)
- Bulk transport
Membrane Assisted Transport
“out of
cell” Vesicles
(formed by Golgi)
fuse with plasma
membrane to
secrete specific
molecules to
external
environment.
 Exocytosis:
Membrane Assisted Transport
“into cell” invagination and
pinching off of plasma membrane to form
a vesicle to take in certain substances into
cell.
 Types:
 Endocytosis:


Phagocytosis: “cell eating” solids such as food
particles or other cells.
Pinocytosis: “cell drinking” liquids.