Cell membrane Structure and Function
Or The Fluid mosaic model and cellular transportation
The Cell Membrane
Described by the fluid mosaic model
Fluid- because phospholipid bilayer gives membrane an oily fluidic quality.
Mosaic because of all the different little bits that are also parts of the membrane
Structure
Phopholipid bilayer
Two layer of phospholipids
Mosaic bits
Proteins
Channel proteins
-always open
-allow materials to pass through membrane freely
ie) chlorine channels
Carrier Protein- selectively transports materials
-specific to certain materials
Na/K pumps for nerve cells
Cell Recognition protein
-these proteins have chain of carbohydrates attached
-usedfor cell indentification
-involved in immune response and rec of “foreign” cells in body- transplant rejection
receptor proteins
-sensitive to certain materials , cause cell to change behavior when activated by signal
ie) liver cells store glucose when signalled by hormone insulin
Enzymatic proteins
-these proteins aloe reactions to occor. Ie- enxymatic proteins on the cells lining the
small intestine are resp for the break down of materials
Cholesterol
- stabilitizes and strengthen the phospholipid bilayer
Overall Function of Membrane
-maintains consistent environment within cell
How? By controlling the movement of material in and out of the cell
We say that the Plasma Membrane is differentially permeable. Some things move
through it and others cannot.
Some molecules freely move through the membrane
Ex) water, O2, CO2, lipid soluble materials
Some require a membrane protein for transportation
Ex) charged ions, amino acids, some sugars
Another factor in movement of materials is the concentration gradient
General Rule: Liquids and gases will move from areas of high to low concentration, or
down a concentration gradient
Called Diffusion
Eventually distribution will be even, but movement will continue anyway.
Examples of diffusion- tea in water, perfume in air, pollen in air. Sugar in coffee,
Movement of gases:
In water or liquid we use the terms solution, solvent and solute.
Example: salt water solution- salt is the solute and the solute moves from hi to low conc
Factors that effect the rate of diffusion: temp, pressure,
Osmosis is diffusion of water is diffusion of water through a membrane. Water moves
from areas of more to less water.
Figure 4.6 water moves across the membrane at the bottom of the tube.
What happens in the diagram?
Why?
How does this relate to animal cells?
Saline solution for hydration
If you have low blood pressure in the hspital they my give youa saline solution for
rehydration. How would the concentration of water in the solution compare to that of
your blood?
It should be the same
This is called isotonic.
Cells in isotonic solutions don’t have a net loss or gain of water.
Saline=.9 5 solution o salt
What would happen is they gave you pure water? How would the water move if it had
less solute in it?
Water would move into the cells and cause them to swell and perhaps burst. A solution
that is less concentrated thn another is said to be hypotonic Cell bursting is call lysis.
What is the concentration of saline was more concentrated.
Water out of cell, cell shrinking=crenation. The cell would be in a hypertonic solution.
Transportation by a carrier protein
Special proteins move the materials
Facilitated Diffusion
Mov’t from hi to low- aided by a protein
Ex) glucose. Amino acids, during the absorption process of digestion
Active Transport
Movement is from lo to hi in order to create a build up of a material
Requires a transport protein and ATP energy
Ex) conc iodine inside the thyroid gland
Happens in the kidney to make sure that all glucose is taken out of the urine
In the Na’K pump of the neuron- this sets up the electro-chemical gradients needed for
nerve impulses
Vesicle Formation
Exocytosis (exiting the cell) and Endocytois( entering the cell)
Requires energy
Exocytosis- materials leave cell via a vesicle
Vesicle is made of same arterial as cell membrane; vesicle fuses to membrane and bursts
and releases its contents
Ie) cells of the pancreas produce and release insulin
Endocytosis

Two types
o Pinocytosis- cell drinking
o Phagocytosis- cell eating
Both involve the formation of a vesicle around material that then brought into the cell
Pino:
Very materials and the solute around them.
Sometimes its receptor mediated pinocytosis
Membrane have coated pits with receptors that bind with specific materials
Phago
Formation of a ves around large matter
Ie) white blood cells eat bacteria
Cell Transport Questions and Assignments
Can you explain with details why cells are said to be selectively permeable?
Why are cells so small?
Provide examples of each type of cellular transportation in the body and explain what
factors affect the rate of transportation
Diffusion
Movement of respiratory gasesCO2 from tissue cell to blood
O2 from lungs to blood
Rate- faster if the temp, conc, pressure, is higher
Also faster for smaller materials
Electrochemical gradients- neg ions will move towards pos environments, and vice versa
Osmosis
What is it called when an animal cell shrinks? (crenation)
What situation would cause this? Something is ________tonic to something else)
Cell bursting is called_____________. This happens when as cell is placed in an
environment, which is _________________.
Facilitated Diffusion
Uses ___________ and ____________ proteins
What are some features of these types of proteins?
Examples of their activity?
Examples of facilitated diffusion? Where? Why?
Movement of glucose into all cells- glucose can’t directly diffuse
Rate will depend on the number of transport proteins
Active Transport
How is this different from facilitated diffusion? Similar?
From low to high conc, requires ATP
Examples
Accumulation of iodine inside thyroid
Glucose from intestine
Glucose from nephron
Sodium from urine to blood in kidneys
What type of proteins are often needed for active transport?
Name some examples; say where they are found, and what they do.
Na/K pump in neurons
Vesicle Formation
Find specific examples when exocytosis, pinocytosis and phagocytosis are used by the
body.