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The Plasma Membrane -

Gateway to the Cell

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Photograph of a Cell Membrane

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Functions of Plasma Membrane

Protective barrier (flexible)

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

Allow cell recognition

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Functions of Plasma Membrane

Provide a binding site for enzymes

Provide anchoring sites for cytoskeleton

Contains the cytoplasm (fluid in cell)

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Structure of the Cell Membrane

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Phospholipids

Cholesterol

Proteins(peripheral and integral)Carbohydrates (glucose)

Membrane Components

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PhospholipidsMake up the cell membraneContains 2 fatty acid chains that are nonpolar (non magnetic)

Head is polar (magnetic) & contains a –PO4

(Phosphate)

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FLUID- because individual phospholipids and proteins can move side-to-side within the layer, like it’s a liquid.

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

FLUID MOSAIC MODEL

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Polar heads are hydrophilic “water loving”

Nonpolar tails are hydrophobic “water fearing”

Cell Membrane

Makes membrane “Selective” in what crosses

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

Semipermeable Membrane

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Large molecules such as proteins do not movethrough the membrane on their own.

Semipermeable Membrane

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Types of Transport Across Cell Membranes

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Simple Diffusion

• Requires NOenergy

• Molecules move from area of HIGH to LOWconcentration

Solvent/Solute/Solution

• Solvent is a substance that dissolves a solute to make a solution

• Water is the substance that dissolves sugar to make Sugar-Water

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Diffusion of Liquids

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Diffusion through a Membrane

Cell membrane

Solute moves DOWN concentration gradient (HIGH to LOW)

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Osmosis

• Diffusion of wateracross a membrane

• Moves from HIGH water potential(low solute What the water is in) toLOW water potential (high solute)

Diffusion across a membrane

Semipermeable membrane

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Diffusion of H2O Across A Membrane

High H2O potentialLow solute concentration

Low H2O potentialHigh solute concentration

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Cell in Isotonic Solution

CELL

(Salt Water) 10% NaCL90% H2O

(Salt Water)10% NaCL90% H2O

What is the direction of water movement?

The cell is at _______________.equilibrium

ENVIRONMENT

NO NET MOVEMENT

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Cell in Hypotonic Solution

CELL

(Salt Water) 10% NaCL90% H2O

(Salt Water)20% NaCL80% H2O

What is the direction of water movement?

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Cell in Hypertonic Solution

CELL

(Salt Water) 15% NaCL85% H2O

(Salt water)5% NaCL95% H2O

What is the direction of water movement?

ENVIRONMENT

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Isotonic Solution

NO NET MOVEMENT OF

H2O (equal amounts entering & leaving)

Hypotonic Solution

“Blow” –up

Think Hypo=Blow

Hypertonic Solution

Cell shrinks

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Osmosis in Red Blood Cells

Isotonic Hypotonic Hypertonic

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Three Forms of Transport Across the Membrane

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Passive Transport

1. Simple Diffusion

Doesn’t require energy

Moves high to lowconcentration Example: Oxygen or water diffusing into a cell and carbon dioxide diffusing out.

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Passive Transport

2. Facilitated diffusion

Doesn’t require energy

Uses transport proteinsto move high to lowconcentration

Examples: Glucose or amino acids moving from blood into a cell.

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Types of Transport Proteins

• 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 DiffusionMolecules will randomly move through the pores in Channel Proteins.

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Facilitated Diffusion

• Some Carrier proteins do not extend through the membrane.

• They bond and drag moleculesthrough the lipid bilayer and release them on the opposite side.

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Active Transport

Requires energy or ATP

Moves materials from LOW to HIGHconcentration

AGAINSTconcentration gradient

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Active 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 Pump

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

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Pinocytosis- “Cell drinking”

Most common form of endocytosis.Takes in dissolved molecules as a vesicle.

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Endocytosis – Phagocytosis

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

Called “Cell Eating”

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Molecules are moved out of the cell by vesicles that fusewith the plasma membrane.

Exocytosis- moving things out.

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Exocytosis

Inside Cell Cell environment

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Three Forms of Transport Across the Membrane

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Extra slides …

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ExocytosisExocytic vesicle immediately after fusion with plasma membrane.

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

one of three forms of endocytosis.

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Pinocytosis

• Cell forms an invagination

• Materials dissolve in water to be brought into cell

• Called “Cell Drinking”

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Example of Pinocytosispinocytic vesicles forming mature transport vesicle

Transport across a capillary cell (blue).

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Receptor-Mediated Endocytosis

Some integral proteins have receptorson their surface to recognize & take in hormones, cholesterol, etc.

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Receptor-Mediated Endocytosis

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Phagocytosis About to Occur

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Phagocytosis- Capture of a YeastCell (yellow) by Membrane Extensions of an Immune System Cell(blue)

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Cells in Solutions

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DIFFUSION

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

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Cytolysis & Plasmolysis

Cytolysis Plasmolysis

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hypotonic hypertonic isotonic

hypertonic isotonic hypotonic

What Happens to Blood Cells?

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Proteins Are Critical to Membrane Function

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Carrier Proteins

• Other carrier proteins change shapeto move materials across the cell membrane