Chapter 5A: Membrane Structure & Function 1. Membrane Structure 2. Diffusion 3. Membrane Transport

1. Membrane Structure

Plasma Membrane All cells are enclosed by a plasma membrane • a selectively permeable barrier to “outside”

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What are Membranes made of? Mainly phospholipids, with some cholesterol (in animal cell membranes) and a variety of membrane proteins. “unsaturated”

“saturated”

Phospholipids form a Lipid Bilayer • all biological membranes are a lipid bilayer

Polar heads face “out”

Hydrophobic tails face “in”

Phospholipid Bilayer with some Cholesterol and Proteins

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The Lipid Bilayer is Fluid Phospholipids, cholesterol, membrane proteins can move freely in the bilayer • consistency of the bilayer is like a viscous oil • degree of “fluidity” depends on: 1) temperature 2) types of “fatty tails” (saturated vs unsaturated) 3) amount of cholesterol

2. Diffusion

Diffusion

Diffusion is the spontaneous movement of a substance from higher to lower concentration • molecules dissolved in liquid move randomly • over time the net effect is equal dispersion of the molecules (provided there is no barrier) • aka “moving down concentration gradient”

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Osmosis is the Diffusion of Water equal concentration of solute

higher lower concentration concentration of solute of solute

solute molecule

H2O

selectively permeable membrane

• water undergoes a net flow from high to low concentration • has powerful effects if a barrier is semipermeable

water molecule

solute molecule with cluster of water molecules

net flow of water

• large molecules can’t diffuse, so water keeps diffusing in

Osmosis can cause cells to shrink, swell

in isotonic solution

in hypertonic solution

in hypotonic solution

(equal conc. of solutes)

(more solutes)

(less solutes)

Water will diffuse to where it is less concentrated!

3. Membrane Transport

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“Small-scale” Transport Cells accomplish membrane transport on a “small scale” (molecule by molecule) in 3 basic ways: 1) passive transport (simple diffusion) • diffusion directly through the membrane bilayer

2) facilitated diffusion • diffusion with the help of specific membrane proteins

3) active transport • movement from low to high concentration • requires special membrane proteins and energy

Passive Transport across a Membrane • small, uncharged molecules can diffuse across a lipid bilayer (membrane) without “help”

e.g. O2 CO2 H2O

Facilitated Diffusion via Protein Channels • allow the diffusion of small polar or charged molecules small molecule

protein channel

• each channel is specific for particular ion or molecule • creates a pore that allows only 1 type of molecule to pass

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Facilitated Diffusion with the help of Transport Proteins

Specific transport proteins bind & transfer specific molecules from high to low concentration • transport proteins change shape upon binding of the molecule and release it on other side of membrane

Active Transport

Substances can be moved from low to high concentration across membranes via specific protein pumps (requires a source of energy such as ATP)

“Large-scale” Transport Cells accomplish membrane transport on a “large scale” (in bulk) in 2 basic ways: 1) exocytosis • release of material packaged in membrane vesicles to the outside of a cell

2) endocytosis • ingestion of large objects or large amounts of material by enclosing within a membrane vesicle:

• PINOCYTOSIS • PHAGOCYTOSIS • RECEPTOR-MEDIATED ENDOCYTOSIS

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Exocytosis Fluid outside cell

Vesicle Protein

Cytoplasm

A general process for “releasing” material from a cell (e.g., release of neurotransmitters).

Endocytosis

Vesicle forming

pinocytosis

A general process for “ingesting” material. • ingestion of fluid = PINOCYTOSIS (“cell drinking”) • ingestion of large particles = PHAGOCYTOSIS

Phagocytosis (“cell eating”) • a type of endocytosis

• how many single-celled organisms feed (e.g., amoeba) • how cells of the immune system destroy invaders

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Receptor-mediated Endocytosis Phospholipid outer layer

LDL particle

Vesicle Cholesterol Protein

Plasma membrane

Receptor protein

Cytoplasm

How specific substances are ingested. • receptors bind specific substances, capture in vesicles • e.g., the ingestion of LDL cholesterol complexes

Key Terms for Chapter 5A (5.1-9) • plasma membrane, lipid bilayer • diffusion, osmosis • isotonic, hypertonic, hypotonic • passive transport (simple diffusion) • facilitated diffusion, active transport • transport proteins, protein channels, pumps • exocytosis, endocytosis, receptor-mediated end. • vesicle, pinocytosis, phagocytosis

Relevant Review Questions: 1, 3, 5, 6, 8

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