Transcript Plasma Membrane

The Plasma Membrane and
Homeostasis
FLUID MOSAIC MODEL
Homeostasis – Maintaining
a Balance
 Cells must keep the proper
concentration of nutrients and water
and eliminate wastes.
 The plasma membrane is selectively
permeable – it will allow some things
to pass through, while blocking other
things.
 Amphipathic: hydrophobic & hydrophilic
regions
 Singer-Nicolson: fluid mosaic model
COMPONENTS OF CELL MEMBRANE
 Phospholipids: membrane
fluidity
 Cholesterol: membrane
stabilization
 “Mosaic” Structure:
 Integral proteins:
transmembrane proteins
 Peripheral proteins: surface
of membrane
 Membrane carbohydrates :
cell to cell recognition;
oligosaccharides (cell
markers); glycolipids;
glycoproteins
Structure of the Plasma
Membrane
 Lipid bilayer – two sheets of lipids
(phospholipids).
 Found around the cell, the nucleus,
vacuoles, mitochondria, and
chloroplasts.
 Embedded with proteins and
strengthened with cholesterol
molecules.
What’s a Phospholipid?
 It’s a pair of fatty acid chains and a
phosphate group attached to a glycerol
backbone.
 Polar (water-soluble) heads face
out and the nonpolar fatty acids
hang inside.
Membrane Proteins
 Transport: what
can enter/leave
cell.
 Serve as enzymes
 Signal transduction
(ie. Hormones)
 Intercellular joining
 Cell-cell recognition
(T-cells)
 ECM attachment
Cellular Transport
 Diffusion – movement of particles from
an area of high concentration to an area
of low concentration.
 Caused by Brownian motion (movement of
particles because of the movement of their
atoms).
 Continues until an equilibrium is reached (no
gradient).
 Dynamic equilibrium – particles move freely
and are evenly distributed.
Osmosis
 Diffusion of
water across a
selectively
permeable
membrane.
 Occurs until
water is balanced
on both sides of
the membrane.
Cell Concentrations
 Hypertonic solutions – more
dissolved solute. (less water)
 Hypotonic solutions – less dissolved
solute. (more water)
 Isotonic solutions – the same
dissolved solute.
 QUESTION: What happens to the cell
in each situation?
Osmoregulation
 Osmoregulation: control of
water balance
 Hypertonic: higher
concentration of solutes
 Hypotonic: lower
concentration of solutes
 Isotonic: equal
concentrations of solutes
 Cells with Walls:
 Turgid (very firm)
 Flaccid (limp)
 Plasmolysis: plasma
membrane pulls away from
cell wall
Overcoming Osmosis
 Contractile vacuoles – expel
excess water from bacterial
cells that live in water.
 Turgor pressure – water
pressure in a plant cell. Loss
of turgor pressure causes
wilting (plasmolysis).
Cellular Transport
 Passive transport – (also known as
passive diffusion) no energy is
needed to move particles.
 Facilitated diffusion – embedded
proteins act as tunnels allowing
particles to “fall” through.
Requires the use of transport
proteins
Ion channels: specialized
transport proteins
 Many ions are not soluble in lipids
 To enter the cell, they need to go through a protein
“tunnel” to get into the cell
 Examples: Na+, K+, Ca+2, Cl-
 These protein “tunnels” have “gates” that open or close
to allow ions into the cell or to leave the cell
 Again, this depends on the concentration gradient
 Stimuli in the cell determine when the gates open or
close
Cellular Transport
 Active transport – energy is
needed to move particles.
 Carrier proteins –
embedded proteins
change shape to open
and close passages
across the membrane.
 This system allows the
cell to move substances
from a lower
concentration to a higher
concentration
Example: Sodium-potassium pump
 The sodium-potassium pump is one of the active
transport mechanisms used in the conduction of a
nerve impulse.
 How it works: (open book to pg. 135)
Three Na+ ions (inside the cell) bind to a protein in the
cell membrane
2. You must use energy to move the Na+ ions out of the
cell so an ATP molecule is used (energy molecule) to
change the shape of the carrier protein
3. With a phosphate is bound to the carrier protein it has
“space” for two K+ to bind to the protein
1.
Sodium-potassium pump
4. When the two K+ bind to the carrier protein, the
protein again changes shape by releasing the
phosphate and allows the K+ to enter the cell

NOTE: Another driving force for the pump is an
attempt to maintain a balanced electric charge


You lose 3+ so it’s easier to add + into the cell
SHOWS HOW YOU CAN COUPLE TRANSPORTS
TO SAVE ENERGY
Sodium-potassium pump
ENDOCYTOSIS VS EXOCYTOSIS
 There are two other ways to move substances into and
out of the cell:
 Endocytosis: the cell ingests external substances
(macromolecules, external fluid, other cells)
 The cell membrane engulfs the substance and forms a
vesicle
 The substance inside the vesicle is kept separate from
the rest of the cell by the phospholipid bilayer of the
vesicle
 These substances can be transported to the lysosome for
digestion or other membrane-bound organelles for
other functions
ENDOCYTOSIS – CONT.
 Types of endocytosis
 Pinocytosis: this creates a vesicle that is transporting
fluids
 Phagocytosis: creates a vesicle that transports large
particles or other cells
 Example: Your immune system creates a type of
phagocyte (cell that digests foreign bacteria) called a
macrophage that helps to fight off bacterial infections
 Receptor-mediated endocytosis: ligands (molecules that
bind to a specific receptor site) induce endocytosis
EXOCYTOSIS
 Exocytosis: when a substance is released from the cell by
binding a vesicle to the plasma membrane
 This process is basically the reverse of endocytosis
 This process is used for
 Elimination of large molecules from the cell (they are
large enough that they would damage the cell
membrane if allowed to leave through the plasma
membrane)
 Elimination of toxins that need to be kept separate
from cell interior
 Many endocrine cells use this method to release
hormones
WATER POTENTIAL
 On the AP Exam, you will have to understand a couple
of formulas that deal with water potential:
Ψ = ΨP + ΨS
 Ψ = Free energy associated with water potential
 ΨP = Pressure potential (force from water pressure)
 ΨS = Potential dependent on the solute concentration
(how many particles of material are in solution
WATER POTENTIAL
 Water always moves from an area of high water
potential to low water potential (osmosis)
 In an open beaker (atmospheric pressure only) of PURE
water, the water potential is zero (Ψ = 0)


No difference is solute concentration
No external pressure (gravity, turgor pressure, etc)
 Increasing the ΨP (pressure potential), increases the
water potential (Ψ > 0)

The water wants to “move” to an area of lower pressure
(potential)
WATER POTENTIAL
 Increasing the ΨS (solute potential), lowers the water
potential
 If you put more particles in solution, you make the
solution hypertonic

The water wants to enter the system to equalize the water
potential (Ψ < 0)
 The total water potential results from a combination of
water pressure and solute concentration
SOLUTE POTENTIAL
 Solute potential (ΨS) has its own formula
 ΨS = -iCRT
 i = ionization constant (different based on the
material used for the solute)
 C = the molar concentration (molarity = moles/L)
 R = pressure constant (0.0831 liters*bars/mole*K)
 T = temperature in Kelvin (K = ̊C + 273)
 Bar = 1 atm (at sea level)
EXAMPLE PROBLEM
 A sample of 0.15M sucrose at atmospheric pressure
(ΨP = 0) and 25 ̊C has what water potential? Since
sucrose dose not break apart into ions (i = 1).
SOLUTION
 Ψ=?
 Ψ = ΨP +ΨS
 ΨP = 0
 ΨS = -iCRT
 i=1
 C = 0.15M
 R = 0.0831
liters*bars/mole*K
 T = 25 ̊C + 273 = 298K
 Ψ = o +ΨS
 ΨS = -iCRT
 ΨS = - (1)(0.15M)(0.0831)(298K)
 ΨS = -3.7 bars
 Ψ = o +ΨS
 Ψ = o -3.7bars
 Ψ = -3.7bars