Copyright Pearson Prentice Hall Homeostasis & Membrane Transport

Copyright Pearson Prentice Hall

Homeostasis & Membrane Transport


Cell BoundariesAll cells are surrounded by a thin, flexible barrier known as the cell membrane. Many cells also produce a strong supporting layer around the membrane known as a cell wall.

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The Plasma The Plasma MembraneMembrane -

Gateway to the CellGateway to the Cellcopyright cmassengale

Outside of cell

Cell membrane

Inside of cell (cytoplasm) Protein

channel

Proteins

Lipid bilayer

Carbohydrate chains

Cell Membrane

The cell membrane regulates what enters and leaves the cell and also provides protection and support.


Cell Walls


Cell WallCell walls are found in plants, algae, fungi,

and many prokaryotes.

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Cell MembraneCell Membrane

The cell membrane is flexibleflexible and allows a unicellular organism to move

copyright cmassengale

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HomeostasisHomeostasis

•Balanced internal condition of cells

•Also called equilibrium•Maintained by plasma

membrane controlling what enters & leaves the cell


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Functions of Plasma Functions of Plasma MembraneMembrane

Protective barrierProtective barrier

Regulate transport in & out of Regulate transport in & out of cell cell (selectively permeable)(selectively permeable)

Allow cell recognitionAllow cell recognition

Provide anchoring sites for Provide anchoring sites for filaments filaments of cytoskeletonof cytoskeleton


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Functions of Plasma Functions of Plasma MembraneMembrane

Provide a binding site for Provide a binding site for enzymesenzymes

Interlocking surfaces bind cells Interlocking surfaces bind cells together (junctions)together (junctions)

Contains the cytoplasm (fluid in Contains the cytoplasm (fluid in cell) cell)


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Structure of the Structure of the Cell MembraneCell Membrane


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PhospholipidsPhospholipids

CholesterolCholesterol

Proteins(peripheral and integral)Carbohydrates (glucose)

Membrane ComponentsMembrane Components


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PhospholipidsPhospholipids

Make up the cell membrane

Contains 2 fatty acid chains that are nonpolarHead is polar & contains a –PO4 group & glycerol copyright cmassengale

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FLUIDFLUID- because individual phospholipids and - because individual phospholipids and proteins can move around freely within the proteins can move around freely within the layer, like it’s a liquid.layer, like it’s a liquid.

MOSAICMOSAIC- because of the pattern produced by - because of the pattern produced by the scattered protein molecules when the the scattered protein molecules when the membrane is viewed from abovemembrane is viewed from above..

FLUID MOSAIC MODELFLUID MOSAIC MODEL


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Polar heads are hydrophilichydrophilic “water loving”Nonpolar tails are hydrophobichydrophobic “water fearing”


Makes membrane “Selective” in what crosses


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Hydrophobic molecules pass easily; hydrophilic DO NOT

The cell membrane is made of 2 layers of phospholipidphospholipids called the lipid bilayerbilayer


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SolubilitySolubility•Materials

that are soluble in lipids can pass through the cell membrane easily


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Small molecules and larger hydrophobic molecules move through easily.e.g. O2, CO2, H2O

Semipermeable Semipermeable MembraneMembrane


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Ions, hydrophilic molecules larger than water, and large molecules such as proteins do not move through the membrane on their own.

Semipermeable Semipermeable MembraneMembrane


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Types of Transport Types of Transport Across Cell Across Cell MembranesMembranes



Measuring Concentration

A solution is a mixture of two or more substances.

The substances dissolved in the solution are called solutes.

The concentration of a solution is the mass of solute in a given volume of solution, or mass/volume.


Diffusion

Particles in a solution tend to move from an area where they are more concentrated to an area where they are less concentrated.

This process is called diffusion. When the concentration of the solute is

the same throughout a system, the system has reached equilibrium.

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Simple DiffusionSimple Diffusion

• Requires NONO energy

• Molecules move from area of HIGH HIGH to LOWto LOW concentration


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DIFFUSIONDIFFUSION

Diffusion is a PASSIVEPASSIVE process which means no energy is used to make the molecules move, they have a natural KINETIC ENERGY


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Diffusion of LiquidsDiffusion of Liquids


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Diffusion through a Diffusion through a MembraneMembrane

Cell membrane

Solute moves DOWN concentration gradient (HIGH to LOW)copyright cmassengale


Diffusion Through Cell Boundaries

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OsmosisOsmosis• Diffusion of Diffusion of

waterwater across a across a membranemembrane

• Moves fromMoves from HIGH water HIGH water potentialpotential (low (low solute) tosolute) to LOW LOW water potentialwater potential (high solute)(high solute)

Diffusion across a membrane

Semipermeable

membrane


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Diffusion of HDiffusion of H22O Across O Across A MembraneA Membrane

High H2O potentialLow solute concentration

Low H2O potentialHigh solute concentration

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AquaporinsAquaporins• Water Channels• Protein pores used during

OSMOSISWATERMOLECULES



Osmosis

How Osmosis Works

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Cell in Isotonic SolutionCell in Isotonic Solution

CELLCELL

10% NaCL90% H2O

10% NaCL

90% H2O

What is the direction of water movement?The cell is at _______________.equilibrium

ENVIRONMENTENVIRONMENT

NO NET NO NET MOVEMENMOVEMENTT


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Cell in Hypotonic Cell in Hypotonic SolutionSolution

CELLCELL

10% NaCL90% H2O

20% NaCL

80% H2O

What is the direction of water movement? copyright cmassengale

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Cell in Hypertonic Cell in Hypertonic SolutionSolution

CELLCELL

15% NaCL85% H2O

5% NaCL95% H2O

What is the direction of water movement?

ENVIRONMENTENVIRONMENT



Water tends to diffuse from a highly concentrated region to a less concentrated region.If you compare two solutions, three terms can be used to describe the concentrations:

hypertonic (“above strength”).hypotonic (“below strength”).isotonic (”same strength”)

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Cells in SolutionsCells in Solutions


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Isotonic Solution

NO NET MOVEMENT OF

H2O (equal amounts entering

& leaving)

Hypotonic Solution

CYTOLYSIS

Hypertonic Solution

PLASMOLYSIS


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Cytolysis & PlasmolysisCytolysis & Plasmolysis

Cytolysis Plasmolysis


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Osmosis in Red Blood Osmosis in Red Blood CellsCells

IsotonicIsotonic Hypotonic

Hypertoniccopyright cmassengale


Osmotic Pressure Osmosis exerts a pressure known as

osmotic pressure on the hypertonic side of a selectively permeable membrane.

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hypotonic hypertonic isotonic

hypertonic isotonic hypotonic


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Three Forms of Transport Across the MembraneThree Forms of Transport Across the Membrane


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Passive Passive TransportTransport

Simple DiffusionSimple Diffusion

Doesn’tDoesn’t require require energyenergy

Moves Moves high to lowhigh to low concentrationconcentration Example: Example: OxygenOxygen diffusing into a cell diffusing into a cell and and carbon dioxidecarbon dioxide diffusing outdiffusing out.


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Passive Passive TransportTransport

Facilitated diffusion

Doesn’t require energy

Uses transport proteins to move high to low concentrationExamples: Examples: GlucoseGlucose or or amino acidsamino acids moving moving from blood into a from blood into a cell.cell.


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Proteins Are Critical to Proteins Are Critical to Membrane FunctionMembrane Function


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Types of Transport Types of Transport ProteinsProteins

• Channel proteins are embedded in the cell membrane & have a pore for materials to cross

• Carrier proteins can change shape to move material from one side of the membrane to the other


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Facilitated DiffusionFacilitated Diffusion

Molecules will randomly move Molecules will randomly move through the through the porespores in in Channel Channel ProteinsProteins..


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Facilitated DiffusionFacilitated Diffusion

• SomeSome Carrier Carrier proteinsproteins do not do not extend through the extend through the membrane.membrane.

• They They bond and bond and drag moleculesdrag molecules through the lipid through the lipid bilayer and release bilayer and release them on the them on the opposite side.opposite side.



Facilitated Diffusion

Facilitated Diffusion

Protein channel

Glucose molecules

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Carrier ProteinsCarrier Proteins• Other carrier Other carrier

proteins proteins change shapechange shape to move to move materials materials across the cell across the cell membranemembrane



Active TransportSometimes cells move materials in the

opposite direction from which the materials would normally move—that is against a concentration difference. This process is known as active transport.

Active transport requires energy.


Active Transport

Molecular TransportIn active transport, small molecules and

ions are carried across membranes by proteins in the membrane.

Energy use in these systems enables cells to concentrate substances in a particular location, even when diffusion might move them in the opposite direction.


Active TransportMolecule to be carried

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Active TransportActive Transport

Requires energy or ATP

Moves materials from LOW to HIGH concentration

AGAINST concentration gradient


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Active transportActive transport

Examples: Pumping Na+ (sodium ions) out and K+ (potassium ions) in against strong concentration gradients.

Called Na+-K+ Pump


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Sodium-Potassium Sodium-Potassium PumpPump

3 Na+ pumped in for every 2 K+ pumped out; creates a membrane

potential


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Moving the “Big Stuff”Moving the “Big Stuff”

Molecules are Molecules are moved outmoved out of the cell by of the cell by vesiclesvesicles that that fusefuse with the plasma membrane. with the plasma membrane.

ExocytosExocytosisis-

moving things out.

This is how many This is how many hormoneshormones are secreted and how are secreted and how nerve cellsnerve cells communicate with one another communicate with one another.


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ExocytosisExocytosisExocytic Exocytic vesicle vesicle immediately immediately after fusion after fusion with plasma with plasma membrane.membrane.


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ExocytosisExocytosis The opposite of endocytosis is exocytosis. The opposite of endocytosis is exocytosis. Large moleculesLarge molecules that are manufactured in that are manufactured in

the cell are the cell are releasedreleased through the cell through the cell membranemembrane..

Inside Cell Cell environment


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Moving the “Big Moving the “Big Stuff”Stuff”Large molecules move materials into the Large molecules move materials into the

cell by one of cell by one of three forms of endocytosisthree forms of endocytosis.


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PinocytosisPinocytosis

Most Most commoncommon form of endocytosis form of endocytosis. Takes in Takes in dissolveddissolved molecules as a molecules as a vesiclevesicle.


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PinocytosisPinocytosis

• Cell forms an Cell forms an invaginationinvagination

• Materials Materials dissolve in dissolve in waterwater to be to be brought into brought into cellcell

• Called Called “Cell “Cell Drinking”Drinking”


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Example of Example of PinocytosisPinocytosispinocytic vesicles forming mature transport vesicle

Transport across a capillary cell (blue).copyright cmassengale

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Receptor-Mediated EndocytosisReceptor-Mediated Endocytosis

Some Some integral proteinsintegral proteins have have receptorsreceptors on their surface to on their surface to recognize & take in recognize & take in hormones, hormones, cholesterolcholesterol, etc., etc.


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Receptor-Mediated EndocytosisReceptor-Mediated Endocytosis

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Endocytosis – Phagocytosis Endocytosis – Phagocytosis

Used to Used to engulf large particlesengulf large particles such such as food, as food, bacteriabacteria, etc. into vesicles, etc. into vesicles

Called Called “Cell Eating”“Cell Eating”copyright cmassengale

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Phagocytosis About to OccurPhagocytosis About to Occur

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PhagocytoPhagocytosissis - Capture of a Yeast Cell (yellow) by Membrane Extensions of an Immune System Cell (blue)


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Copyright Pearson Prentice Hall Homeostasis & Membrane Transport