Chapter 7 Review Membrane Structure and Function.

Chapter 7 ReviewMembrane Structure and Function

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Selective Permeability The plasma membrane exhibits selective

permeability It allows some substances to cross it more

easily than others

Cellular membranes are fluid mosaics of lipids and proteins

Phospholipids (fluid part) Are the most abundant lipid in the plasma membrane Are amphipathic, containing both hydrophobic and

hydrophilic regions Proteins (mosaic part) can be

Transmembrane (integral) Peripheral

Fluid Mosaic Model


Lipids Phospholipid structure

Consists of a hydrophilic “head” and hydrophobic “tails”

CH2

OPO OOCH2CHCH2

OO

C O C O

Phosphate

Glycerol

Fatty acids

Hyd

roph

obic

tails

Hydrophilichead

Hydrophobictails

–

Hyd

roph

ilic

hea d CH2 Choline

+N(CH3)3


Lipids The structure of phospholipids

Results in a bilayer arrangement found in cell membranes

Hydrophilichead

WATER

WATER

Hydrophobictail


Lipids Saturated fatty acids

Have the maximum number of hydrogen atoms possible

Have no double bonds

Are typically solid at room temperature

Stearic acid


Unsaturated fatty acids

Have one or more double bonds

Are typically liquid at room temperature

cis double bondcauses bending

Oleic acid

Lipids


How fluid? The type of hydrocarbon tails in phospholipids

Affects the fluidity of the plasma membrane

Fluid Viscous

Unsaturated hydrocarbontails with kinks

Saturated hydro-Carbon tails

Membrane fluidity


How fluid? The steroid cholesterol

Has different effects on membrane fluidity at different temperatures

Cholesterol within the animal cell membrane

Cholesterol

Thinking Question

Would you expect an amoeba that lives in a cold climate to have a higher or lower amount of saturated hydrocarbon chains in its phospholipids compared to a species that lives in a warm climate?

Thinking QuestionWould you expect an amoeba that lives in a cold climate to have a higher or lower amount of saturated hydrocarbon chains in its phospholipids compared to a species that lives in a warm climate?

Fluid Viscous

Unsaturated hydrocarbontails with kinks

Saturated hydro-Carbon tails

Membrane fluidity


Fluid-Mosaic Model Membrane proteins (mosaic part) are

dispersed and individually inserted into the phospholipid bilayer

Phospholipidbilayer

Hydrophilic region of protein

Hydrophobic region of protein

Membrane Carbohydrates Cell-cell recognition

Is a cell’s ability to distinguish one type of neighboring cell from another

Short carbohydrates bound to lipids (glycolipids) or proteins (glycoproteins) cover the outer surface of cells These carbohydrates mediate cell-cell recognition

Crossing the Membrane

Cells need to control the exchange of material with their environment

Hydrophobic molecules (non-polar) Are lipid soluble and can pass through the

membrane rapidly Hydrophilic molecules (polar, ionic)

Do not cross the membrane rapidly Need help of transport proteins

Passive vs. Active Transport Passive transport is the movement of a substance across a

membrane with no energy investment Involves the process of simple diffusion Or facilitated diffusion

Active transport requires an energy input


Passive Transport Simple Diffusion

Is the tendency for molecules to move from areas of high concentration to areas of low concentration

Small, hydrophobic molecules can move across the membrane this way

Molecules of dye Membrane (cross section)

Net diffusion Net diffusion Equilibrium

Facilitated Diffusion

In facilitated diffusion Transport proteins speed the movement of

molecules across the plasma membrane


Facilitated Diffusion Transport proteins

Provide corridors that allow a specific molecule or ion to cross the membraneEXTRACELLULARFLUID

Channel proteinSolute

CYTOPLASM

(a)


Special Case of Facilitated Diffusion Osmosis

Is the diffusion of water across a semipermeable membrane Lowerconcentrationof solute = Higherconcentration ofwater

Higherconcentrationof solute = Lower concentration ofwater

Water moves from an area of higher water concentration to an area of lower water concentration

s

Tonicity Tonicity

Is the ability of a solution to cause a cell to gain or lose water If a solution is isotonic

The concentration of solutes is the same as it is inside the cell There will be no net movement of water

If a solution is hypertonic The concentration of solutes is greater than it is inside the cell The cell will lose water

If a solution is hypotonic The concentration of solutes is less than it is inside the cell The cell will gain water

Tonicity Diagram

5% NaCl 5% NaCl 10% NaCl 20% NaCl

IsotonicHypotonic

Hypertonic

Hypotonic

Hypertonic

Water


Osmosis in Cells Water balance in cells without walls

Such as animal or protistHypotonic solution Isotonic solution Hypertonic solution

H2O H2O H2O H2O

Lysed Normal Shriveled/Crenation


Osmosis in Cells Water balance in cells with walls

Such as plant cells

H2OH2OH2OH2O

Turgid (normal) Flaccid Plasmolyzed

Hypotonic solution Isotonic solution Hypertonic solution

Thinking Question

Imagine you are in the hospital recovering from an operation and require intravenous fluids. Why should you be concerned about the concentration of the solution in the IV bags?

Active Transport

Active transport Moves substances against their concentration

gradient from low to high Requires energy, usually in the form of ATP


Review: Passive and Active Transport Compared

Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane.

Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP.

Diffusion. Hydrophobicmolecules and (at a slow rate) very small uncharged polar molecules can diffuse through the lipid bilayer.

Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins,either channel or carrier proteins.

ATP

Movement of Macromolecules Transport of large molecules across the plasma

membrane occurs by exocytosis and endocytosis In exocytosis

Transport vesicles migrate to the plasma membrane, fuse with it, and release their contents

In endocytosis The cell takes in macromolecules by forming new

vesicles from the plasma membrane


EXTRACELLULARFLUID

PseudopodiumCYTOPLASM

“Food” or other particle

Foodvacuole

1 µm

Pseudopodiumof amoeba

Bacterium

Food vacuole

PINOCYTOSIS0.5 µm

Plasmamembrane

Vesicle

Endocytosis Three types of endocytosis

PHAGOCYTOSIS


0.25 µm

RECEPTOR-MEDIATED ENDOCYTOSIS

Receptor

Ligand

Coat protein

Coatedpit

Coatedvesicle

Plasmamembrane

Coatprotein

Key Points of Chapter 7 Cellular membranes are fluid mosaics of lipids and

proteins Membrane structure results in selective permeability Passive transport is diffusion of a substance across a

membrane with no energy investment Active transport uses energy to move solutes against their

gradients Bulk transport across the plasma membrane occurs by

exocytosis and endocytosis

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Chapter 7 Review Membrane Structure and Function.

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