The Cell Membrane

1
The Cell Membrane
2
Where? What?

• Plants AND animals
• Surrounds the cell
• Barrier between outside environment and cell
• Involved in signaling, transport, structural
  integrity, adhesion
• Has protein receptors

3
What are the main players?

• Phospholipids (bilayer)
• Cholesterol
• Proteins
• Glycolipids
• Carbohydrates

4
(No Transcript)
5
The Fluid Mosaic Model

• Phospholipid bilayer
• Hydrophilic head exposed to the outside
• Hydrophobic tail hides inside
• Membrane proteins are randomly dispersed in
  phospholipid bilayer

6
Fluid Mosaic Model
7
Fluidity of the Membrane

• Membrane is NOT stiff/rigid
• The lipids and proteins can drift throughout the
  membrane
• Cholesterol makes the membrane stronger by
  limiting the movement of phospholipids

8
Membrane as a Mosaic

• Lipid bilayer has membrane proteins embedded in
  it
• Integral proteins
• Go through the membrane (both sides)
• Peripheral proteins
• attached to the surface of the membrane
• Include the receptor proteins

9
Selective Permeability of the Cell Membrane

• The cell membrane can choose what enters and
  exits a cell
• Gatekeeper of the Cell

10
End todays notes

• Now you will construct a cell membrane model
  using the materials supplied to you by your
  teacher.
• You must build the model on top of a sheet of
  white paper, label it with the appropriate labels
  and take a picture of it.
• Make sure Ms. O sees your model before you
  destroy it!! (and that your name is visible in
  the picture)
• On a separate sheet of paper, indicate the part
  involved in each of the functions listed

11
(No Transcript)
12
membrane modeling necessary parts

• On a separate sheet of paper after you have
  finished your membrane model, indicate the part
  involved in
• Transport
• Enzymatic activity
• Signal transduction
• Cell-cell recognition
• Intercellular joining
• Attachment to the cytoskeleton and extracellular
  matrix
• (fig 7.10 in text book is a good reference for
  this part)

• Label the
• Phospholipid
• Hydrophobic tail
• Hydrophilid head
• Cholesterol
• Integral protein
• Peripheral protein
• Receptor protein
• Glycoprotein
• Glycolipid
• Carbohydrate chain
• Interior and exterior of cell

13
Permeability of the membrane and transport across
it!!
14
Transport 2 types

• Passive
• Diffusion
• Active
• Sodium/Potassium pump

15
Nonpolar molecules

• Hydrophobic
• Hydrocarbons, carbon dioxide, oxygen
• Dissolve in the lipid bilayer and cross easily

16
Polar molecules

• Crossing is impeded by hydrophobic interior of
  the membrane
• Cross very slowly
• ions and polar molecules such as glucose

17
Transport proteins

• Carrier proteins change shape to shuttle
  substances across the membrane
• Channel Proteins have Hydrophilic channels
• Aquaporins allow passage of water
• Greatly increase rate of water passage
• Where do you think in your body do you have cells
  with high s of aquaporins? (cells that would
  need to extract water)
• Both allow for greater and faster passage of
  substances across the membrane

18
plasmolysis

• The wilted plant plasma membrane shrinks away
  from cell wall
• Can lead to plant DEATH ?

19
plasmolysis
20
What are some applications and consequences of
membrane permeability?

• Why do supermarkets spray their produce with
  water to make them look crisp?

21
Membrane potential

• There is a voltage across a membrane
• Cytoplasmic side is negative compared to the
  extracellular side

22
Electrochemical gradient

• Electrical force and concentration gradients
  drive diffusion
• ? This is called the ELECTROCHEMICAL GRADIENT

23
Passive Transport

• does NOT require the input of energy by the cell

24
3 types of passive transport

• Diffusion
• Osmosis
• Facilitated Diffusion

25
Diffusion

• Movement of molecules from high to low
  concentration until equilibrium is reached.

26
Diffusion

• Each substance diffuses down its OWN
  concentration gradient and is unaffected by
  concentration gradients of other substances

27
Diffusion

• Does all movement stop once equilibrium is
  reached??
• NO!!
• Equal rates in all directions

28
What substances may diffuse across membrane?
Nonpolar (non-charged) molecules and small polar
molecules
29
Osmosis

• The passive transport of water across a
  selectively permeable membrane
• Hyper-, hypo-, iso- tonic
• RELATIVE TERMS!!
• Always referring to solute concentration

• Water moves from areas of lower concentration of
  solutes (hypotonic) to areas of higher solute
  concentration (hypertonic)

30
Osmosis in Plant and Animal Cells

• Animal Cells
• Cell crenate/shrinks
• Cytolysis
• Occurs when a cell is in a hypotonic solution
• Water goes from solution into cell

Plant Cells Turgid- vacuole swells Flaccid-vacuol
e shrinks (plasmolysis)
31

• If a paramecium swims from a hypotonic to an
  isotonic environment, will its contractile
  vacuole become more or less active?

32
Facilitated Diffusion

• Does NOT require an input of energy
• Solute is still moving down its concentration
  gradient
• diffusion of large molecules across the cell
  membrane using transport proteins
• Glucose ions

33
Facilitated Diffusion

• Transport proteins are specific for their solutes
• Some are gated channels
• Chemical or electrical stimulus causes them to
  open

34
Example

• Which direction will sucrose move?
• Which direction will glucose move?
• Which direction will fructose move?

35
Water Potential Abbreviated by the letter ?
(psi)

• Measures the tendency of H2O to diffuse from one
  compartment to another

36
Water moves from

• an area of higher water potential (or higher free
  energy) to an area of lower water potential
  (lower free energy)

37
(No Transcript)
38
End of passive transport notes

• Osmosis case study

39
Active Transport

• The pumping of solutes against their gradients

40
Active transport

• Requires an input of energy by the cell
• Used so cells can stockpile extra supplies
  (storage of any substance in a very high
  concentration i.e. Iodine stored in thyroid
  glycogen stored in liver
• Sodium/Potassium Pump

41
Electrogenic Pumps

• Voltage across membranes stored energy that can
  be used for cellular work
• Sodium-Potassium Pump
• 3 Na OUT of the cell for every 2 K pumped in
• Net transfer of one positive charge from
  cytoplasm to extracellular fluid

42
Na/K pump

• helps maintain resting potential, avail
  transport, and regulate cellular volume
• Moves ions into and out of the cell (Na/K)
• Extremely important in nerve cell signal
  conduction
• For most animal cells, it is responsible for 1/3
  of the cell's energy expenditure
• For nerve cells 2/3s of expenditure

43
Sodium-Potassium Pump
44
Na/K pump

• http//highered.mcgraw-hill.com/sites/0072495855/s
  tudent_view0/chapter2/animation__how_the_sodium_po
  tassium_pump_works.html
• (animation)

45
Na/K pump

• What did you learn?
• Whats involved?
• How many sodium ions in for how many potassium
  ions out?
• Whats the point?

46
Cotransport

• Substance that has been pumped (H) across a
  membrane can do work as it leaks back by
  diffusion
• Another substance (sucrose here) hitches a ride
  
• So, here, sucrose is transported with the help of
  the flow of the H ions
• Animation

47
Endocytosis Exocytosis

• The movement of large molecules (polysaccharides,
  proteins, etc.) across the membrane using
  vesicles
• Endocytosis cell takes in macromolecules
• Exocytosis cell secretes macromolecules

48
Endocytosis

• Cell takes in macromolecules by forming vesicles
  made from the plasma membrane

49
Types of Endocytosis

• Phagocytosis cell eating
• Large molecules
• Pinocytosis cell drinking
• Small molecules liquids
• Receptor-mediated endocytosis seeks out
  specific molecules

50
Exocytosis

• The cell secretes macromolecules by the fusion of
  vesicles with the plasma membrane
• Used to release hormones, chemical signals, etc.

51
Active transport guide

• Must include a description and picture for each
• Active transport
• Sodium potassium pump
• Endocytosis
• Exocytosis
• Cotransport