Cell Membrane
and Transport
HOW THE CELL ABSORBS AND EXCRETES VARIOUS MOLECULES
Cell Membrane
Cell membrane separates the components of a cell from its environment (what
surrounds the cell) and controls the passage of materials into and out of a cell.
Composed of a phospholipid bilayer (75-95%), and proteins (5-25%).
Hydrophilic headsAttracted to water
Hydrophobic tailsRepelled from water
Membrane Proteins
Membrane proteins are used for
Communication
I.D. Tag
Anchors
Gates
Pumps
Cell Membrane Structure
Fluid Mosaic Model- molecules and proteins are “free floating” along the
phospholipid bilayer.
Fluid Mosaic Model
3 Types of Proteins Found in Cell Membrane
Channel Proteins- allow molecules or ions to enter the cell. They can be open
or gated.
Receptor Proteins- relay information from outside the cell to inside the cell.
Marker Proteins- act as the name tag of the cell.
Factors Affecting Transport
Molecules size and charge
The cell membrane is permeable to small neutral molecules, H2O, gases, and lipids.
The cell membrane is impermeable to charged molecules. Proteins are needed to
move these across the membrane.
Concentration gradient
Molecules like to move from areas of high concentration to areas of low
concentration
Electrical gradient
The inner membrane of the cell is slightly negative (-) to the outer surface of the cell.
Therefor (+) charged molecules will be attracted into the cell.
Concentration Gradient- the difference in concentration
across a space.
Passive Transport
High
Low
• “Down” concentration gradient.
No energy needed.
Active Transport
Low
High
• “Against” concentration gradient.
Requires energy.
Moving Down the Concentration Gradient
Moving molecules down their concentration gradient is known as
Passive Transport. The cell uses no energy to move the
molecules.
Processes that used passive transport are…
Diffusion
Osmosis
Facilitated Diffusion
Diffusion
Diffusion is the movement of molecules from an area of higher
concentration to an area of lower concentration.
When the concentration of molecules is
the same throughout the space it occupies
they are in a state of Equilibrium.
No energy needed
Osmosis
Osmosis is the process by which water molecules diffuse across a cell
membrane from an area of higher concentration to an area of lower
concentration.
The direction of movement of water molecules depends on the relative
concentration of solutes on the two sides of the semi-permeable membrane.
There are three osmolality's
◦ Hypotonic
◦ Hypertonic
◦ Isotonic
Hypotonic Solutions
When the concentration of solute molecules outside the cell is lower than the
concentration inside the cell, the solution outside the cell is hypotonic to the
cytosol.
Water molecules will have a net movement into the cell
causing the cell to swell.
The force that water molecules exert against
cell wall is called osmotic pressure.
If a cell takes in to much water it can burst.
This is termed cytolysis.
the
Hypotonic Solution
Plant cell
Animal Cell
Hypertonic Solutions
When the concentration of solute molecules outside the cell is higher than the
concentration in the cytosol, the solution outside is hypertonic.
Water will have a net movement out of the cell
causing the cell to shrivel.
Plasmolysis is when osmotic pressure is lost
due to water leaving the cell. This is what
causes plants to wilt.
Hypertonic Solutions
Plant Cell
Animal Cell
Isotonic
When the concentration of solutes outside and inside the cell are equal, the
outside solution is said to be isotonic to the cytosol.
There will be no net movement.
Isotonic Solutions
Plant Cell
Animal Cell
What will happen to the cell?
10% Solute
90% Water
5% Solute
95% Water
What will happen to the cell?
10% Solute
90% Water
10% Solute
90% Water
What will happen to the cell?
10% Solute
90% Water
30% Solute
70% Water
Facilitated Diffusion
This type of transport is for molecules that may not be soluble or are to large
to pass through the membrane pores.
Requires protein channels or carrier proteins which move the molecules down
their concentration gradient.
Protein channels are specific to one type of molecule
Some are always open, others are gated and require a trigger to open and
close.
No energy needed