General Principles of Intellectual Property: Concepts of Intellectual Proper...
The plasma membrane and material transport
1. 04. The plasma membrane & material transport.
Source: Campbell et al. (2011)
Ian Anderson
Saint Ignatius College Geelong
2. Knowledge and skills.
Describe the molecular structure of plasma
membranes.
Explain how the hydrophobic and hydrophilic
properties of phospholipids help to maintain the
structure of plasma membranes.
List the functions of membrane proteins.
Describe the different ways that molecules or
substances cross membranes, including
• Diffusion • Active transport
• Osmosis • Endocytosis
• Facilitated diffusion • Exocytosis
3. Plasma membrane.
The boundary that separates a cell from its
surroundings.
Maintains the internal environment of a cell by
controlling the movement of substances into and out
of cell (i.e. partially permeable).
Enabling the cytosol to have a different composition
from the surrounding environment.
~7-9 nm thick.
4. Fluid-mosaic model.
Composed of a double layer (bilayer) of lipids
(phospholipids), called a phospholipid bilayer.
The phospholipid molecules are in constant motion,
sliding past one another and the other molecules
embedded in the plasma membrane.
Proteins can
Traverse the plasma membrane and stick out from both
surfaces; be partially submerged in the inner or outer
surface; or be found on the surface.
Main roles are in helping transport molecules across the
plasma membrane; acting as attachment points for
other cells; and functioning as identity tags for cells.
5. Fluid-mosaic model.
Carbohydrates are attached to some proteins that
protrude to the outside of the cell (together called
glycoproteins).
Play an important role in cell-to-cell interactions
Cholesterol molecules can be found in the middle
between the two phospholipid bilayers.
Helps to stabilize the plasma membrane and keep it
flexible.
7. Phospholipids.
Lipids that contain a phospate group at one end of
each molecule.
Hydrophilic (‘water loving’) at their phosphate end.
Hydrophobic (‘water fearing’) along their fatty acid tail
region.
When in an aqueous solution, the phospholipid
molecules line up with their hydrophobic tails
pointing away from the solution phospholipid
bilayer.
9. Movement in and out of cells.
All cells must be able to exchange (take in and expel)
substances with their environment in order to survive,
grow and reproduce.
Movement of substances across the plasma membrane
depends on
Surface area available for exchange
Chemical properties of the substance being exchanges,
plus
Concentration gradients, temperature, etc.
10. Surface area-volume ratio.
Surface area-volume ratio of a cell influences the rate
of entry and exit of substances into and out of it.
As a structure increases in size, its SA:V decreases.
The SA:V ratio differs according to the shape of the
structure.
Ratio is highest in flattened shapes and lowest in
spheres.
12. Movement across the plasma membrane.
Plasma membrane is a partially permeable boundary.
i.e. only some dissolved materials are able to cross it.
Various processes used to allow materials to cross
plasma membrane
Diffusion
Facilitated diffusion
Osmosis
Active transport
Endocytosis and exocytosis
13. Diffusion.
The net movement of molecules along a concentration
gradient, from a region of high concentration to a
region of low concentration.
Does not require energy (i.e. passive).
Occurs due to the random movement of molecules.
Always occurs down a concentration gradient.
The larger the concentration gradient, the more rapid
the rate of diffusion.
14. Diffusion.
Diffusion across the plasma membrane will occur as
long as the molecule can pass through.
Lipid-soluble substances (such as vitamins A & D and
alcohol) diffuse through the lipid bilayer.
Small molecules such as water, O2 and CO2 can diffuse
between the phospholipid molecules.
Large molecules, polar molecules and small ions
generally are unable to diffuse across the plasma
membrane without assistance.
15. Facilitated diffusion.
The movements of a substance across the plasma
membrane from a region of higher concentration of a
substance to a region of lower concentration, assisted
(or facilitated) by specific membrane proteins.
Membrane proteins are either carrier proteins or ion
channels.
Carrier proteins attaches to the molecule, with the resultant
change in shape allowing the molecule to be shifted across the
plasma membrane and then released.
Ion channels act like gates by opening and closing the
channel.
Does not require energy.
16. Facilitated diffusion.
Facilitated diffusion is more rapid than simple
diffusion.
The protein channels through the plasma membrane
are specific for particular molecules.
Mainly involves substances that cannot diffuse across
the plasma membrane by dissolving in the lipid bilayer
of the membrane.
Polar molecules will diffuse through via carrier proteins
and small ions via the ion channels.
e.g. the movement of glucose across the membrane of red
blood cells.
18. Osmosis.
The net movement of free water molecules through a
partially permeable membrane, from a dilute to a more
concentrated solution.
A special case of diffusion (i.e. movement of water
molecules not the movement of solute molecules).
Osmotic gradient; osmotic pressure.
e.g. Absorption of water from food in the gut;
reabsorption of water in kidneys.
19. Osmosis.
Osmotic influence on cells.
Cells are affected by the
amount of dissolved materials
in the water that surrounds
them.
Hypertonic v hypotonic v
isotonic solutions.
Hypertonic soln = more dissolved
material & less water.
Hypotonic soln = less dissolved
material and more water.
Isotonic soln = both solutions the Source: Enger et al. (2011)
same concentration of water.
21. Active transport.
The net movement of dissolved substances into or out of a
cell against the concentration gradient.
Requires energy.
Enables cells to maintain stable internal conditions in spite
of extreme variation in the external surroundings.
Occurs through protein channels which are very selective
for specific ions and molecules.
e.g. Sodium ions are pumped out of cells and potassium ions
are pumped in, both against the concentration gradient.
Plant root cells take up nitrate ions from very dilute
solutions in the soil.
22. Active transport.
e.g. active transport of salt occurs in many freshwater
organisms.
Freshwater fish lose salt by diffusion across their skin-
cell plasma membranes into their surrounding
freshwater environment. Energy (in the form of ATP) is
used to actively transport salt molecules against the
concentration gradient.
Some molecules can be transported across plasma
membranes either by facilitated diffusion or active
transport e.g. glucose (most cells v liver cells).
24. Endocytosis & exocytosis.
Movement of large molecules and particles in bulk across
the plasma membrane.
Requires energy.
Exocytosis = movement of a substance out of a cell.
Transport vesicles migrate to the plasma membrane, fuse with
it, and release their contents.
e.g. Digestive enzymes in a transport vesicle.
Endocytosis = movement of a substance into a cell.
Molecules enter cells within vesicles that pinch inward from
the plasma membrane.
Phagocytosis v pinocytosis
e.g. white blood cells engulf microorganisms by phagocytosis.
25. Endocytosis & exocytosis.
Endocytosis and exocytosis in a white blood cell.
Source: Enger et al. (2011)
