The plasma membrane maintains the internal environment of cells through selective permeability. It is composed of a phospholipid bilayer with embedded and attached proteins. Molecules move across the membrane via diffusion, facilitated diffusion, osmosis, active transport, and endocytosis/exocytosis. These processes allow for the exchange of materials while maintaining different internal and external compositions.

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.

6. Fluid-mosaic model.
Source: Enger et al. (2011)

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.

8. Phospholipids.
Source: http://bioweb.wku.edu/courses/biol115/wyatt/biochem/lipid/Lipid_2.asp

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.

11. Surface area-volume ratio.

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.

17. Facilitated diffusion.
Source: Campbell et al. (2011)

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.

20. Osmosis.

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).

23. Active transport.
Source: Walpole et al. (2011)

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)