This document discusses the structure and function of the plasma membrane. It notes that the plasma membrane is selectively permeable and controls what enters and exits the cell through passive and active transport mechanisms. The plasma membrane is made up of a phospholipid bilayer with integral and peripheral proteins. Passive transport mechanisms like simple diffusion, facilitated diffusion, and osmosis move substances down concentration gradients without energy expenditure. Active transport mechanisms move substances against concentration gradients using energy from ATP.
2. substances that are
required by cells
• Glucose
• H2O
• O2
• Mineral
substances to be
eliminated from cells
• ammonia
• urea
• CO2
Cell membranes are like gates.
3. The main function of a plasma membrane is to
control what enters and what exits a cell.
• keeps certain substances inside and other
substances on the outside.
• This function is critical. If needed molecules
(such as those used in protein synthesis) were
free to leave, then death of the cell would quickly
occur.
the necessity for movement of
substances across the plasma
membrane:
4. PLASMA MEMBRANE
Membrane lipids are organized in a bilayer (two
sheets of lipids making up a single membrane)
approximately 60 to 100 Å thick
5. Plasma Membrane Structure
Plasma membrane phospholipid bilayer
combination of phospholipids & proteins
http://www.bio.davidson.edu/people/macampbell/111/memb-swf/membranes.swf
FLUID MOSAIC
MODEL
• not fixed in any rigid pattern
• float around in the membrane
Proteins
6. These molecules form a phospholipid bilayer
• hydrophilic (polar) heads facing their aqueous
surroundings (e.g., the cytosol)
• hydrophobic tails facing each other
Hydrophobic end (repels water)
Hydrophilic polar heads (attracted to water)
the charges on the
phosphate and amino
groups (in red) make that
portion of the molecule
hydrophilic.
Amphiphilic lipids~major constituents of cell membranes
7. Integral and peripheral membrane proteins
PERIPHERAL PROTEINS - lie on the surface of the membrane.
INTEGRAL PROTEINS - extend into and sometimes completely through the
membrane.
• marker proteins ~ like nametags that identify the cell to other cells
• transport proteins ~ responsible for shipping and receiving. They move materials in and
out of the cell
• receptor proteins ~ serve as binding or attachment sites, especially for hormones or
other molecular messengers
8. thin film on the inside of an egg
Semipermeable membrane – lipid bilayer
selectively permeable membrane
partially permeable membrane
differentially permeable membrane
• Some materials freely pass - the membrane is
permeable to such molecules and whether they are
inside or outside of the cell depends on other factors
• Some materials are excluded
• Some materials enter or leave the cell only by the using
cell energy
9. 1. Small hydrophobic molecules, such as CO2, O2 and small
lipids, dissolve in the membrane and pass through readily.
2. Tiny polar molecules, such as H2O and alcohol, can also
slip between the phospholipid molecules.
3. Ions and most nutrient molecules do not move freely
through membranes, but are often carried by thetransport
protein channels, either with or without the use of energy.
4. Most large molecules are excluded and must be
manufactured within the cell, or moved by significant
alterations of the membrane itself.
10. • Fluid
Any substance that can move or change shape in response to
external forces without breaking apart. Gases and liquids are
fluids.
• Concentration
The number of molecules of a substance in a given volume
• Gradient
A physical difference between two regions so that molecules will
tend to move from one of the regions toward the other.
Concentration, pressure and electrical charge gradients are
common in cells.
11. Passive Transport
involves moving things
through membranes
without the expenditure of
cell energy down gradients.
e.g: diffusion.
TRANSPORTATION
Active Transport
the pumping of
molecules or ions
through a membrane
against their
concentration
gradient.
12. to where
there is
less of it
Diffusion
the net movement of a substance from
where there is
more of it along
a concentration
gradient
until molecules are
equally distributed
(and the gradient
no longer exists).
Passive Transport
13. • passive transport (requires no energy)
• without the involvement of specific carrier proteins
• The Rate of Diffusion can be affected by:-
- Temperature (Higher temperature, faster molecule movement)
- Molecule size (Smaller molecules often move more easily)
- Concentration (Initial rate faster with higher concentration)
- Electrical and pressure gradients of the two regions (Greater
the gradient differential, the more rapid the diffusion (again, initially))
• Materials that may move freely through membranes by
simple diffusion include:
- H20 (water)
- CO2 (carbon dioxide)
- O2 (oxygen)
- Uncharged & some small lipid-soluble molecules (alcohol)
http://www.bbc.co.uk/education/asguru/biology/01cellbiology/05pathways/06passivesimple/index.shtml
Passive Transport
Simple Diffusion
14. Simple Diffusion
* Small polar molecules
cannot readily dissolve
in the hydrophobic
bilayer but can pass
through the membrane
via "holes" formed by
intrinsic proteins forming
channels.
* The proteins are thus
called channel/ transport
proteins.
Passive Transport
15. Facilitated Diffusion
• passive transport (No energy is involved)
• with the help of membrane transport proteins, which temporarily bind
to the substance to be moved through the membrane
• carrier proteins and channel proteins are involved (has a specific
receptor site for that substance)
• Materials that move through membranes by facilitated diffusion include:
- small polar molecules (e.g. glucose)
- Many small ions
- Amino acids
Passive Transport
16. the substance
being deposited
on the other
side of the
membrane.
The protein
then returns
to its original
shape ready
to operate
again
the substance
binds to the
carrier protein
undergoes a
conformational
(shape) change
Passive Transport
17. Osmosis
• a special case of
diffusion.
• diffusion of water
molecules from an area of
high water concentration
to an area of low water
concentration,
• i.e. from an area of low
solute concentration to an
area of high solute
concentration
Passive Transport
Water molecule
selectively permeable membrane
solute
19. • A hypotonic solution has a LOWER concentration
of solutes
• A hypertonic solution has a HIGHER
concentration of solutes
• An isotonic solution is a solution that has a
concentration of solutes equal to its environment
So how does this affect cells?
Passive Transport
20. Human Red Blood Cells Typical Plant Cell
HypertonicHypotonic Isotonic HypertonicHypotonic Isotonic
The net movement of water is
always FROM a hypotonic solution
(higher concentration of water) TO
a hypertonic solution (lower
concentration of water )
There is NO NET
MOVEMENT in isotonic
solutions
Passive Transport
21. Active Transport
• against the concentration gradient
• typically requires two carrier protein active sites:-
- to recognize the substance to be carried, and
- to release ATP (adenosine triphosphate) to provide
the energy for the protein carriers or "pumps".
Active Transport
http://www.bbc.co.uk/education/asguru/biology/01cellbiology/05pathways/08active/index.shtml
24. Comparison of passive and active transport
TELL ME THE
ANSWER PLEASE
Similarities? Differences?
25. Discuss the processes of passive transport
and active transport in living organisms:
a) gaseous exchange in the
alveoli and blood capillaries
simple diffusion
b) absorption of digested food in the
villus
facilitated
diffusion
26. c) absorption of water by root hairs of a plant
Discuss the processes of passive transport
and active transport in living organisms:
27. d) ion intake by root hairs of a plant
active transport
Discuss the processes of passive transport
and active transport in living organisms:
