The Plasma Membrane

1
The Plasma Membrane

• Transport Across the Membrane

2
Intro. To the Plasma Membrane

• separates the living cell from its surroundings.
• 8 nm thick, controls traffic into and out of the
  cell.
• selectively permeable, allowing some substances
  to cross more easily than others
• Major macromolecules in membranes are lipids,
  proteins, and some carbohydrates
• Made of a bilayer of phospholipids. With polar
  heads, hydrophilic, and non-polar tails,
  hydrophobic.

3
Intro Cont.

• The membrane is represented as a fluid mosaic
  model, a fluid environment with a mosaic of
  proteins and carbs. embedded or attached that
  serve several functions.

4
Membrane Movement and Cholesterol

• Most of the lipids and some proteins can drift
  laterally in the plane of the membrane, but
  rarely flip-flop from one layer to the other.
• Cholesterol is wedged between phospholipids
  molecules in the plasma membrane of animals
  cells. It restrains the movement of the
  phospholipids in warm temps. and maintains
  fluidity by preventing tight packing at cold
  temps.

5
Cells can Change their Membrane Composition

• Cells can modify the lipid make-up of membranes
  to compensate for changes in fluidity caused by
  changing temperatures.
• Ex, winter wheat, increases the percentage of
  unsaturated phospholipids in the autumn.
• This lets them prevent their membranes from
  solidifying during winter.

6
A Mosaic of Proteins

• Membranes are very complex and dynamic containing
  many different parts.
• Proteins decide most of the membranes functions.
• Contain lipids and carbohydrates also
• The collection of molecules in the membrane vary
  from membrane to membrane
• All of the structures in the membrane serve
  various functions like cell recognition proteins.
• Read Pg. 84-85

7
2 Types of Proteins

• Peripheral proteins are not embedded in the lipid
  bilayer, they are loosely bounded to the surface.
  
• Integral proteins penetrate, often completely
  spanning the membrane (a transmembrane protein).
• Where they contact the core, they have
  hydrophobic regions with nonpolar amino acids
• Where they are in contact with the cytoplasm,
  they have hydrophilic regions of amino acids

8
Protein Functions
9
Carbohydrates in the Membrane

• Used for cell to cell recognition, the ability of
  a cell to distinguish one type of neighboring
  cell from another.
• important in cell sorting and organization as
  tissues and organs in development.
• Basis of immune response. Ex. WBC and T-cell
  response
• Membrane carbohydrates are usually branched
  oligosaccharides with fewer than 15 sugar units
• vary from species to species, individual to
  individual, and even from cell type to cell type
  within the same individual. Ex. ABO group

10
Crossing the Membrane

• steady traffic of small molecules and ions moves
  across the plasma membrane in both directions
• Ex, sugars, amino acids, and other nutrients
  enter a muscle cell and waste products leave
• membranes are selectively permeable so all this
  traffic is under some control. Esp. the large
  molecules.
• Passage is controlled in part due to the
  hydrophobic core of the membrane. So other
  hydrophobic molecules cross easily while polar
  molecules and ions have difficulty.
• Proteins assist and control the transport of
  ions and polar molecules.

11
Transport Proteins

• ions and polar molecules can cross the lipid
  bilayer by passing through transport proteins
  that span the membrane.
• Some transport proteins have a hydrophilic
  channel
• Others bind molecules and carry passengers across
  the membrane physically
• Each transport protein is specific
• Ex. Gluclose transport in liver. Not fructose.

12
Passive Transport

• Requires no energy from the cell
• Substances pass by diffusion or osmosis.
• Diffusion is the movement of substance from a
  high concentration to a low concentration.
• This occurs due to kinetic theory. The movement
  of single particle is random but on a whole they
  move form high to low.
• Osmosis is the movement of water from high to
  low.

13
Diffusion of 1 Solute
14
Passive Transport Cont..

• substances will diffuse from where it is more
  concentrated to where it is less concentrated,
  down its concentration gradient. NO ATP NEEDED
• Each substance diffuses down its own
  concentration gradient, independent of the
  concentration gradients of other substances.
• KEY concentration gradient represents potential
  energy and drives diffusion. When there is a
  concentration gradient substances move when there
  is not the do not move.

15
Osmosis, Water Movement

• Movement of water from high to low concentration.
• 3 types of solutions
• solution with the higher concentration of solutes
  is hypertonic.
• solution with the lower concentration of solutes
  is hypotonic.
• Solutions with equal solute concentrations are
  isotonic
• direction of osmosis is determined only by a
  difference in total solute concentration

16
Osmosis Examples
17
Osmosis Example Cell Survival Depends on
Osmoregulation
18
Facilitated Diffusion

• Still diffusion so requires no energy or ATP
• Many polar molecules and ions that are normally
  stopped by the lipid bilayer of the membrane
  diffuse passively with the help of transport
  proteins.
• passive movement of molecules down its
  concentration gradient via a transport protein is
  called facilitated diffusion.

19
Transport Proteins

• much in common with enzymes.
• They can have specific binding sites for the
  solute.
• Transport proteins can become saturated when they
  are tmoving passengers as fast as they can.
• Transport proteins can be inhibited by molecules
  that resemble the normal substrate.

20
Channel Proteins

• provide corridors allowing a specific molecule or
  ion to cross the membrane.
• Allow for fast transport
• water channel proteins, aquaprorins, make
  possible massive amounts of diffusion

21
Gated Channels

• open or close depending on the presence or
  absence of a physical or chemical stimulus.
• Ex. neurotransmitters bind to specific gated
  channels on the receiving neuron, these channels
  open.
• This allows sodium ions into a nerve cell.
• When the neurotransmitters are not present, the
  channels are closed.

22
Active Transport

• Requires ATP, ENERGY.
• move solutes against their concentration
  gradient, from the side where they are less
  concentrated to where they are more concentrated.
• Vital for a cell to keep up its internal
  concentrations of small molecules that would
  normally diffuse away.
• done by specific proteins embedded in the
  membranes.

23
Sodium-Potassium Pumps.

• maintains the gradient of sodium (Na) and
  potassium ions (K) across the membrane.
• Typically, animal cells have higher
  concentrations of K and lower concentrations of
  Na inside the cell
• sodium-potassium pump uses the energy of one ATP
  to pump three Na ions out and two K ions in
  against the gradient.

24
Sodium-Potassium Pump Action
25
Examples
26
Exocytosis and Endocytosis

• Ways of getting large molecules in and out of the
  cell.
• Phagocytosis is cell eating and involves solids.
• Pinocytosis is cell drinking and involves
  liquids.

27
(No Transcript)
28
(No Transcript)
29
Receptor Mediated Endo. Very Specific