Chapter 7 Membrane Structure & Function. Slide 2 of 38 7.1 Plasma Membrane Cell’s barrier to the external world Selectively permeable Allows only.

Chapter 7

Membrane Structure & Function

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7.1 Plasma Membrane

Cell’s barrier to the external world

Selectively permeable Allows only certain molecules through Most molecules CANNOT pass into the cell Small hydrophobic molecules can pass in or out

CO2 and O2

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Plasma Membrane (Page 2)

Main constituents: Phospholipids & Proteins

Membrane proteins Allow hydrophilic molecules into the cell

Phospholipid bilayer Barrier to most things Amphipathic

Hydrophilic & hydrophobic regions Hydrophilic head & hydrophobic tail

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Fluid Mosaic Model

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Fluid Mosaic Model

Mosaic due to membrane proteins

Fluid due to the phospholipid bilayer Unsaturated hydrocarbons = more fluid Higher Temperature = Higher fluidity Cholesterol = temperature buffer

Reduces fluidity at warm temps. Increases fluidity at cold temp.

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How does it get in?

Phospholipid bilayer is the hydrophobic barrier Only small & hydrophobic can cross Hydrophilic molecules are unable to enter Water & other polar molecules = no entry

For most molecules (large and/or polar) membrane proteins allow passage

Peripheral protein – Bound to 1 surface of the membrane Integral proteins – embedded in membrane

Transmembrane proteins – completely span the membrane

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Membrane Proteins

Receptor proteins

Enzymatic proteins

Structural Support Cell-to-cell OR Cell-to-ECM adhesion

Transport proteins Type of transmembrane protein Transport hydrophilic substances across membrane Aquaporins

Transport protein for moving water across membrane

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Membrane Carbohydrates

Function in cell-cell recognition Cell’s ability to distinguish cell types Basis for rejection of foreign cells by the immune system

Blood types result from membrane carbohydrates

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Questions

1. What do small & nonpolar substances require to cross the membrane?

2. What is an amphipathic molecule?

3. Explain why it is called the “fluid mosaic” model of the membrane?

4. What is the function of membrane proteins?

5. What is the function of a membrane carbohydrate?

6. What does adding cholesterol do to alter a membrane?

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Passive Transport

Diffusion – movement from an area of high concentration to an area of low concentration

Moving down a concentration gradient

No energy expenditure for this transport

Osmosis – diffusion of water across a selectively permeable membrane

Osmosis is only done by water (for us) Requires a permeable membrane

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Tonicity

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Facilitated Diffusion

Transport proteins that allow polar molecules & ions to cross the membrane

Passive transport but requires a membrane protein

Two means of operation 1. Channel Proteins - Hydrophilic channels 2. Carrier Proteins – shape change = translocation

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Facilitated Diffusion (Channel Protein)

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Facilitated Diffusion (Carrier Protein)

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Questions?

1. How is facilitated diffusion different from simple diffusion?

2. What type of energy does facilitated diffusion require?

3. What are the 2 versions of facilitated diffusion?

4. How is osmosis different from diffusion?

5. What is a concentration gradient?

6. Passive transport goes down or goes against the concentration gradient?

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Active Transportation

Some transport proteins do facilitated diffusion, other do active transport

Active transport = carrier proteins only why?

Move substances against their concentration gradient Lower concentration Higher concentration

Requires energy or ATP

Maintains separate & different internal environment Establishes or maintains a gradient

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Na+/K+ Pump

Watch Na+ / K+ Animation

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Electrochemical Gradients

Not concentration gradient, but electrochemical gradient Both concentration & charge gradients

Commonly referred to as membrane potential Negative = Inside

Favored movement: Anions Out Cations In

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Electrochemical Gradients

Ions move down their concentration & ionic gradients

Transport protein that establishes a voltage across the membrane = Electrogenic Pump Na+/K+ Pump Proton Pump

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CoTransport (Secondary Active Transport)

Electrogenic pump creates a concentration gradient

THEN Ions move down their gradient, and ACTIVELY transport another substance

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Movement of LARGE molecules

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Animation-05-03.swf

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Phagocytosis & Pinocytosis

Phagocytosis – cell membrane wraps around and takes in a solid particle

Form of endocytosis Called “cell eating”

Pinocytosis – cell membrane wraps around and takes in small amount of liquid (extracellular fluid)

Form of endocytosis Called “cell drinking”

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