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The Plasma Membrane and Homeostasis
FLUID MOSAIC MODEL
Homeostasis – Maintaining a BalanceCells 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 MEMBRANEPhospholipids: membrane
fluidityCholesterol: 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 MembraneLipid 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 ProteinsTransport: what can
enter/leave cell.Serve as enzymes Signal transduction
(ie. Hormones)Intercellular joiningCell-cell recognition
(T-cells)ECM attachment
Cellular TransportDiffusion – 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.
OsmosisDiffusion of water
across a selectively permeable membrane.
Occurs until water is balanced on both sides of the membrane.
Cell ConcentrationsHypertonic 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?
OsmoregulationOsmoregulation: control of
water balanceHypertonic: higher
concentration of solutesHypotonic: lower
concentration of solutes Isotonic: equal
concentrations of solutesCells with Walls:
Turgid (very firm)Flaccid (limp)Plasmolysis: plasma
membrane pulls away from cell wall
Overcoming OsmosisContractile 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 proteinsMany ions are not soluble in lipidsTo enter the cell, they need to go through a
protein “tunnel” to get into the cellExamples: Na+, K+, Ca+2, Cl-
These protein “tunnels” have “gates” that open or close to allow ions into the cell or to leave the cellAgain, this depends on the concentration gradient
Stimuli in the cell determine when the gates open or close
Cellular TransportActive 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 pumpThe 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)1. Three Na+ ions (inside the cell) bind to a protein in
the cell membrane2. 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
Sodium-potassium pump4. 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 EXOCYTOSISThere 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 vesicleThe 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 cellsExample: 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
EXOCYTOSISExocytosis: when a substance is released from the cell
by binding a vesicle to the plasma membraneThis process is basically the reverse of endocytosisThis 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 POTENTIALOn 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 potentialΨP = Pressure potential (force from water pressure)
ΨS = Potential dependent on the solute concentration (how many particles of material are in solution
WATER POTENTIALWater 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 POTENTIALIncreasing the ΨS (solute potential), lowers
the water potentialIf 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 POTENTIALSolute potential (ΨS) has its own formula
ΨS = -iCRTi = 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 = KC + 273)
Bar = 1 atm (at sea level)
EXAMPLE PROBLEMA sample of 0.15M sucrose at atmospheric
pressure (ΨP = 0) and 25 KC has what water potential? Since sucrose dose not break apart into ions (i = 1).
SOLUTIONΨ = ?ΨP = 0ΨS = -iCRTi = 1C = 0.15MR = 0.0831
liters*bars/mole*KT = 25 KC + 273 =
298K
Ψ = ΨP +ΨS
Ψ = o +ΨS
ΨS = -iCRTΨS = - (1)(0.15M)(0.0831)(298K)
ΨS = -3.7 bars
Ψ = o +ΨS
Ψ = o -3.7barsΨ = -3.7bars