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