Topic 5' The Plasma Membrane

1
Topic 5. The Plasma Membrane

• Structure Function

September 26, 2005 Biology 1001
2
5.1 Obtaining Raw Materials for Metabolism

• All organisms, whether autotrophs or
  heterotrophs, must obtain certain molecules from
  the extracellular environment for the chemical
  reactions that occur in the cell
• Eg. Heterotrophs need organic molecules like
  glucose, water and minerals such as Na or Ca2
• Autotrophs need CO2, H2O minerals
• Such materials must pass through the cell wall
  and the plasma membrane
• The plasma membrane acts as a selectively
  permeable barrier screening certain materials
  from entering the cell while allowing others to
  enter at different rates

Note Section 5.2 was covered in Topic 3
3
Section 5.3 The Structure and Function of the
Plasma Membrane

• Plasma membranes of cells consist mostly of
  lipids and proteins, with some carbohydrates as
  well
• The most abundant lipids are the phospholipids
• The phospholipids and most membrane proteins are
  amphipathic molecules, that is they have both
  hydrophilic (polar) and hydrophobic (non-polar)
  regions
• The membrane is a fluid structure with a mosaic
  of various proteins embedded in or attached to a
  bilayer of phospholipids this is the fluid
  mosaic model

4
The fluid mosaic model of membranes
Figure 7.3
5
More features of the plasma membrane

• Carbohydrates are located on the external
  (extracellular) side of the membrane where they
  are covalently bonded to lipids (as glycolipids)
  or proteins (as glycoproteins) and function in
  cell-cell recognition
• The two faces of the membrane also differ in
  lipid composition and protein constituents
  membranes are sided

6
Features of the fluid mosaic model

• The fluidity
• Both the lipids and proteins drift laterally, the
  lipids at a faster rate
• Fluidity is important to maintain membrane
  permeability and for proteins to function
  properly
• Cholesterol acts a temperature buffer to
  maintain fluidity
• Also, the proportion of unsaturated fatty acids
  can change
• The mosaicness
• Membranes are a collage of different kinds of
  proteins
• These proteins determine the specific functions
  of the membrane
• They can be transmembrane proteins or peripheral
  proteins

7
PASSIVE TRANSPORTDIFFUSION OSMOSIS

• What determines the direction of transport?
• Diffusion is the tendency for the molecules of a
  substance to spread out evenly into the available
  space as a result of thermal motion (heat energy)
• The substance therefore moves spontaneously down
  its concentration gradient from an area of
  greater concentration to an area of lesser
  concentration
• Only the concentration of the particular
  substance is important
• Such passive transport requires no energy
  expenditure by the cell, and accounts for much of
  the traffic across cell membranes

8
Diffusion
9
Osmosis The Diffusion of Water

• Osmosis is a special case of diffusion the
  diffusion of water molecules down their
  concentration gradient

Note When attempting to determine the direction
of movement of water molecules, one must consider
the concentration of water relative to the total
concentration of solutes
10
Water Balance of Cells

• What happens to a cell when it is placed in a
  solution?
• Terminology
• Tonicity is the ability of a solution to cause a
  cell to lose or gain water
• A solution is isotonic to a cell if it has the
  same concentration of solutes
• A solution is hypertonic to the cell if it
  contains more total solutes
• A solution is hypotonic to the cell if it
  contains less total solutes
• A cell without a wall does best in a isotonic
  environment it loses water and shrivels in a
  hypertonic one, it gains water and lyses in a
  hypotonic one
• A cell with a wall maintains turgidity in a
  hypotonic environment plasmolyzes in a
  hypertonic one, and is flaccid in an isotonic
  environment

11
OsmoregulationThe Control of Water Balance
Figure 7.13
12
Membranes Are Selectively Permeable

• Not all substances pass through the cell membrane
  and those that do do so at different rates
• Two features contribute to this selective
  permeability the polarity of the molecule and
  the presence or absence of transport proteins for
  particular molecules
• Small non-polar molecules such as O2 pass through
  relatively easily
• Larger or polar molecules like H20 and glucose
  pass much more slowly
• In the latter case transport proteins may assist
  the passage
• Transport proteins work by creating a hydrophilic
  channel through the membrane, or by carrying the
  molecule across the membrane

13
Passive Transport Facilitated Diffusion

• Diffusion is related to the direction of movement
  of molecules down their concentration gradient
• But we still have to consider the selective
  permeability of the membrane not all substances
  pass at the same rate
• Substances will diffuse at different rates due to
  differing size and polarity, and the presence or
  absence of transport proteins
• When transport proteins are required but the
  direction of movement is still determined by
  diffusion down a concentration gradient, it is
  called facilitated diffusion
• Both diffusion (including osmosis!) and
  facilitated diffusion are passive transport
  processes

14
Active Transport

• Certain transport proteins carry substances
  against their concentration gradient
• This is called active transport because the cell
  must expend energy
• Enables the cell to maintain internal
  concentrations of molecules different from the
  environmental concentrations
• The classic example is the sodium-potassium pump
  of animal cells which exchanges sodium for
  potassium across the membrane to keep sodium
  higher outside and potassium higher inside

15
Active Transport
Passive Vs. Active Transport
Figure 7.16 The Na-K Pump
16
Bulk Transport Across Plasma Membranes

• For larger molecules and particles transport
  involves vesicles
• During exocytosis, the cell secretes particles by
  fusing vesicles to the plasma membrane
• During endocytosis, the cell uptakes particles by
  surrounding them with plasma membrane that
  pinches off to form an intracellular vesicle