The diffusion of water through the plasma membrane is of such importance to the cell that it is given a special name: osmosis. This page will examine how ions and small molecules are transported across cell membranes. The transport of macromolecules through membranes is described in Endocytosis.
6. Lipids
Phospholipid bilayer
Forms boundary to isolate cell contents from
environment
Restricts passage of hydrophilic substances across
the membrane
Cholesterol
Increases bilayer strength, flexibility
Reduces membrane fluidity
Reduces permeability to water-soluble substances
7. Proteins
Transport proteins
Regulate movement of water soluble substances
Channel proteins have pores that allow passage of ions
and small water-soluble molecules
Carrier proteins bind to molecules and change shape for
delivery across membrane
8. Proteins
Receptor proteins
Docking site for molecules
outside the cell
Trigger internal cellular
response
Recognition proteins
Identification tags
Oligosaccharides aid in
cell-cell recognition
Cell-surface attachment
sites
9. The membrane is
permeable to:
H2O
Gases (O2, CO2, N2)
Lipids
Small, neutral molecules
(such as urea)
The membrane is
impermeable to:
- Small, charged
molecules
- “large molecules” such
as amino acids, glucose
and larger
these compounds
must go through
channels present in the
membrane in order to
enter or exit the cell
Factors affecting transport
- cell membrane
- Chemical gradient
- Electrical gradient
- Rate of transport
10. Compound moves from
an area of high
concentration to low
concentration (or
concentration gradient)
All compounds
permeable to the
phospholipid bilayer
will move this way
11. Figure 4.3
Positive ions are
attracted to
negative ions and
vice versa
Ions are repelled by
ions of the same
charge (+ against +
and – against -)
12. Both chemical and
electrical forces
(electrochemical force)
drive the movement of
compounds across the
cell membrane
13. Movement of substances across the cell
membrane
Passive transport
Substances move from [high][low]
No energy input required
Simple Diffusion, Facilitated Diffusion,
Osmosis
Active transport
Substances move from [low][high]
Requires energy input
Protein carriers, Endocytosis, Exocytosis
14. Compounds move toward
the area of lower
concentration
Compounds permeable to
the cell membrane will
move through diffusion.
(Compounds unable to pass
through the membrane will
only pass if membrane
channels open)
17. In facilitated diffusion small polar molecules and
ions diffuse through passive transport proteins.
No energy needed
Most passive transport proteins are solute specific
Example: glucose enter/leaves cells through
facilitated diffusion
Passive transport
protein
Higher concentration
Lower concentration
18. Carrier protein
has binding site
for molecule
Molecule enters
binding site
Carrier protein changes
shape, transporting
molecule across membrane
Carrier protein resumes
original shape
(Inside Cell)(Inside Cell)(Inside Cell)(Inside Cell)
(Outside(Outside
Cell)Cell)
(Outside(Outside
Cell)Cell)
DiffusionDiffusion
ChannelChannel
ProteinProtein
DiffusionDiffusion
GradientGradient
Molecule inMolecule in
TransitTransit
19. Diffusion of water across a differentially
permeable membrane
Water moves from [high] [low]
20. Compounds move from area of low
concentration toward area of higher
concentration
ATP (energy) is needed pump
26. Active process for movement of large
molecules and organisms
Substance is taken in by vesicle formed from
cell membrane
Phagocytosis: solid substance in vesicle
Pinocytosis: liquid droplets in vesicle
Receptor-Mediated Endocytosis: incoming
substance binds to receptor
27. Substance is expelled after being
enclosed in a vesicle within the cell
Used to move large molecules out of the
cell
29. Glucose (CARBOHYDRATE) absorption - Na+ dependent glucose transporter.
Glucose enters the enterocyte “arm-in- arm” with sodium (Na+) through a special
protein transporter (Na+ - glucose co-transporter). Na+ concentration in the cell is
low. Na+ therefore moves from the high concentration intestinal lumen into the cell,
taking glucose with it. After glucose passes through the cell, it moves out alone
through another transporter protein (by facilitated diffusion) at the basal end. The
Na+ is pumped out actively. Both enter the capillary blood circulation.
Fructose (carbohydrate) absorption Facilitated diffusion Fructose is a simple sugar
found in fruits. It enters with the help of a transporter protein embedded into the apical
membrane of the enterocyte. After passing through the cell, fructose leaves by a
similar mechanism and is taken up by the blood.
ABSORPTION OF BIOMOLECULES
30. Fat (triglyceride) absorption Transporter proteins This is a complex process. Trigly-
cerides are compounds in which 3 fatty acids of variable length are bound to a glycerol (an
alcohol). Digestive enzymes split off two fatty acids so glycerol is left with one
(monoglyceride).
Monoglycerides and all fatty acids use trans-port proteins to enter into the enterocyte.
Fatty acids that have a carbon chain of less than 12 carbons (short chain) leave the cells
and enter the blood circulation. The fate of long chain fatty acids is more complex.
Within the enterocyte, they are attached again to glycerol and form triglycerides (re-
esterification). Then they are combined with proteins and form lipoprotein particles. These
are transported in vesicles to the basal membrane and ejected from the enterocyte by
exocytosis (binds to a cell surface receptor and leaves).
Lipoproteins do not enter the blood but are taken up by the lymphatic system that
drains later into the blood circulation.
31. Protein (amino acids) absorption Na+ dependent amino acid transporters Amino
acids are also dragged along by sodium, similarly to glucose. However, there are
at least 7 transporter proteins due to the varied chemical structure of different
amino acids. Outbound traffic of amino acids goes through 5 known transport
systems. Amino acids are also taken up by the blood circulation.
Notas do Editor
Eukaryotic Cell
This refers to the types of processes used by cells to get materials in and out across the cell membrane.
The membrane is said to be “semi-permeable.”
This means that it will allow spontaneous passage of some materials but others must use special processes to get across.
A concentration gradient is a difference in the number of molecules or ions of a given substance in two adjoining regions.
Molecules constantly collide and tend to move according to existing concentration gradients.
The net movement of like molecules down a concentration gradient (high to low) is simple diffusion.
Gradients in temperature, electric charge, and pressure, can influence movements.
Passive transport: Follows a concentration gradient
Passive in the sense that the process does not require energy.
It just happens naturally.
Diffusion is passive transport.
Due to the random movement of molecules in liquids & gases (Brownian motion).
A concentrated material will tend to become more uniformly dispersed in the space made available to it.
Water can diffuse across the cell membrane unassisted.
Facilitated transport: Also follows a concentration gradient, but requires gateway to get through.
In this case, the particular materials would not spontaneously cross the pure phospholipid membrane, even if encouraged by a concentration gradient.
A facilitator molecule is produced in the membrane that serves as a gateway for the material.
The material can now cross through the gateway spontaneously whenever a concentration gradient exists.
Active transport: Flow up-hill against a concentration gradient; requires energy expenditure to keep going.
The required energy is almost always provided directly by ATP.
The ATP donates a phosphate to a specific gateway molecule which then “pumps” the desired molecule across the membrane, even if it goes against a concentration gradient.
The ATP energy is used to drive the pump.
Shows successive stages in the dispersal of red dye molecules after being dropped into water.
Note that the molecules to be transported (little balls) are in higher concentration outside the cell than inside the cell.
The molecules are of a type that cannot get to the inside of the cell by going directly through the membrane.
The protein channel provides a passage way for the molecules to follow the concentration gradient.
Osmosis is the passive movement of water across a differentially permeable membrane in response to solute concentration gradients, pressure gradients, or both.
Special case of diffusion.
Involves the diffusion of water across a membrane that has solutions of differing concentration on opposite sides of the membrane.
The water moves more abundantly toward the side with the highest concentration of dissolved materials.
The concentration of water is obviously lower on the side with the highest concentration of dissolved materials.
So the water is simply diffusing from a higher concentration of water to a lower concentration of water.
Example: Pure water on one side; sugar solution on other (ƒ5-5).
Sugar molecules can’t cross, but crowd membrane pores on syrupy side.
Water crosses faster toward sugar.
Net movement of water ====> sugar solution
There is a net increase in volume on the side that has the higher concentration of sugar.
The side of the membrane with the lower concentration of sugar experiences a loss of volume.
This persistent movement can result in a build up of pressure known as osmotic pressure.
Note that the concentration of the molecules is higher outside the cell than inside, but that the molecules are moving out, against the gradient. This always requires energy, thus the term “active” transport.
Here are some special adaptations of endocytosis.
Pinocytosis is when a vesicle forms around and then surrounds and engulfs the liquid from outside the cell.
Phagocytosis is when the vesicle forms around a solid particle like in slide 29.
Receptor-mediated Endocytosis uses special receptor molecules that;
Bind to specific molecules outside the cell
Aggregate together as more and more of the receptors bind target molecules
Cause the membrane to warp inwards where they aggregate
Eventually pinch off to form a “coated” vesicle.
This shows the steps in exocytosis.
A vesicle on the inside of the cell contains a cell product destined for export.
The membrane of the vesicle merges with the plasma membrane, and the contents of the vesicle are now outside the cell.