chapter 6 cell membrane and movement across the membrane

7/28/2019 Chapter 6 Cell Membrane and Movement Across the Membrane

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2011 Pearson Education, Inc.

Chapter 6 cell

membrane, lipids andmovement across the

membrane


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Key Concepts

Plasma membranes are made up of selectively permeable bilayers

of phospholipids. Phospholipids are amphipathic lipid molecules

they have hydrophobic and hydrophilic regions.

Ions and molecules diffuse spontaneously from regions of higher

concentration to regions of lower concentration. Movement of

water across a plasma membrane is called osmosis.

In cells, membrane proteins are responsible for the passage of

insoluble substances that cant cross the membrane on their own.


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The Importance of Membranes

The plasma membrane, orcell membrane, separates life from

nonlife.

The plasma membrane separates the cells interior from the external

environment. Membranes function to:

Keep damaging materials out of the cell Allow entry of materials needed by the cell

Facilitate the chemical reactions necessary for life


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Lipids: What Is a Lipid?

Lipids are carbon-containing compounds that are found in

organisms and that are largely nonpolar and hydrophobic.

Hydrocarbons are nonpolar molecules that contain only carbon

and hydrogen.

Lipids do not dissolve in water because they have a major

hydrocarbon component called a fatty acid.

A fatty acid is a hydrocarbon chain bonded to a carboxyl

(COOH) functional group.

Fatty acids and isoprene are the key building blocks of lipids.


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Three Types of Lipids Found in Cells

Lipid structure varies widely.

The three most important types of lipids found in cells:

1. Fats are composed of three fatty acids linked to glycerol.

Also called triacylglycerols ortriglycerides

2. Steroids are a family of lipids with a distinctive four-ring

structure.

Cholesterol is an important steroid in mammals.

3. Phospholipids consist of a glycerol linked to a phosphate

group (PO42) and to either two chains of isoprene or two fatty

acids.


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The Structure of Membrane Lipids

Membrane-forming lipids contain both a polar, hydrophilic region

and a nonpolar, hydrophobic region.

Phospholipids are amphipathic:

The head region, consisting of a glycerol, a phosphate, and a

charged group, contains highly polar covalent bonds. The tail region is comprised of two nonpolar fatty acid or

isoprene chains.

When placed in solution, the phospholipid heads interact withwater while the tails do not, allowing these lipids to form

membranes.


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Phospholipids and Water

Phospholipids do not dissolve when they are placed in water.

Water molecules interact with the hydrophilic heads but not with

the hydrophobic tails.

This drives the hydrophobic tails together.

Upon contact with water phospholipids form either:

Micelles

Heads face the water and tails face each other.

Phospholipid bilayers (lipid bilayers)


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Phospholipid Bilayers

Phospholipid bilayers form when two sheets of phospholipid

molecules align. The hydrophilic heads in each layer face a

surrounding solution, while the hydrophobic tails face one another

inside the bilayer.

Phospholipid bilayers form spontaneously, with no outside input of

energy required.


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Selective Permeability of Lipid Bilayers

The permeability of a structure is its tendency to allow a given

substance to pass across it.

Phospholipid bilayers have selective permeability.

Small or nonpolar molecules move across phospholipidbilayers quickly.

Charged or large polar substances cross slowly, if at all.


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Many Factors Affect Membrane Permeability

Many factors influence the behavior of the membrane:

Number of double bonds between the carbons in the

phospholipids hydrophobic tail

Length of the tail

Number of cholesterol molecules in the membrane

Temperature


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Bond Saturation and Membrane Permeability

Double bonds between carbons in a hydrocarbon chain can cause a

kink in the hydrocarbon chain, preventing the close packing of

hydrocarbon tails, and reducing hydrophobic interactions.

Unsaturated hydrocarbon chains have at least one double

bond.

Hydrocarbon chains without double bonds are termed

saturated.

Saturated fats have more chemical energy than unsaturated fats.

Membranes with unsaturated phospholipid tails are much more

permeable than those formed by phospholipids with saturated tails.


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O Aff i i


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Other Factors That Affect Permeability

Hydrophobic interactions become stronger as saturated

hydrocarbon tails increase in length.

Membranes containing phospholipids with longer tails have

reduced permeability.

Adding cholesterol to membranes increases the density of the

hydrophobic section.

Cholesterol decreases membrane permeability.

Membrane fluidity decreases with temperature because moleculesin the bilayer move more slowly.

Decreased membrane fluidity causes decreased permeability.

Fl idi f h M b


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Fluidity of the Membrane

Individual phospholipids can move laterally throughout the lipid

bilayer.

They rarely flip between layers.

How quickly molecules move within and across membranes is a

function of temperature and the structure of the hydrocarbontails in the bilayer.


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S l t M t Li id Bil


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Solute Movement across Lipid Bilayers

Materials can move across the cell membrane in different ways.

Passive transport does not require an input of energy.

Active transport requires energy to move substances across

the membrane.

Small molecules and ions in solution are called solutes, havethermal energy, and are in constant, random motion.

This random movement is called diffusion.

Diffusion is a form of passive transport.

Diff i l C t ti G di t


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Diffusion along a Concentration Gradient

A difference in solute concentrations creates a concentration

gradient.

Molecules and ions move randomly when a concentration gradient

exists, but there is a net movementfrom high- concentration

regions to low-concentration regions. Diffusion along a

concentration gradient increases entropy and is thus spontaneous.

Equilibrium is established once the molecules or ions are

randomly distributed throughout a solution.

Molecules are still moving randomly but there is no more net

movement.


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Osmosis

Water moves quickly across lipid bilayers.

The movement of water is a special case of diffusion called

osmosis.

Water moves from regions of lowsolute concentration to regions of

highsolute concentration.

This movement dilutes the higher concentration, thus

equalizing the concentration on both sides of the bilayer.

Osmosis only occurs across a selectively permeable membrane.


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O i d R l ti S l t C t ti


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Osmosis and Relative Solute Concentration

The concentration of a solution outside a cell may differ from the

concentration inside the cell.

An outside solution with a higher concentration is said to be

hypertonic to the inside of a cell.

A solution with a lower concentration is hypotonic to the cell.

If solute concentrations are equal on the outside and inside of a

cell, solutions are isotonic to each other.

Osmosis in Hypertonic Hypotonic and Isotonic Solutions


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Osmosis in Hypertonic, Hypotonic, and Isotonic Solutions

In a hypertonic solution, water will move out of the cell by osmosis

and the cell will shrink.

In a hypotonic solution, water will move into the cell by osmosis

and the cell will swell.

In an isotonic solution, there will be no net water movement and the

cell size will remain the same.


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The Fluid Mosaic Model of Membrane Structure


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The Fluid-Mosaic Model of Membrane Structure

Although phospholipids provide the basic membrane structure,

plasma membranes contain as much protein as phospholipids.

The fluid-mosaic model of membrane structure suggests that some

proteins are inserted into the lipid bilayer, making the membrane a

fluid, dynamic mosaic of phospholipids and proteins.


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Membrane Proteins


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Membrane Proteins

Integral proteins are amphipathic and so can span a membrane,

with segments facing both its interior and exterior surfaces.

Integral proteins that span the membrane are called

transmembrane proteins.

These proteins are involved in the transport of selected ions

and molecules across the plasma membrane.

Transmembrane proteins can therefore affect membrane

permeability.

Peripheral proteins are found only on one side of the membrane.

Often attached to integral proteins


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Membrane Proteins Affect Ions and Molecules


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Membrane Proteins Affect Ions and Molecules

The transmembrane proteins that transport molecules are called

transport proteins. There are three broad classes of transport

proteins, each of which affects membrane permeability:

1. Channels

2. Carrier proteins ortransporters

3. Pumps

Ion Channels and the Electrochemical Gradient


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Ion Channels and the Electrochemical Gradient

Ion channels are specialized membrane proteins.

Ion channels circumvent the plasma membranes

impermeability to small, charged compounds.

When ions build up on one side of a plasma membrane, they

establish both a concentration gradient and a charge gradient,

collectively called the electrochemical gradient.

Ions diffuse through channels down their electrochemical gradients.

This passive transport decreases the charge and concentration

differences between the cells exterior and interior.


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Facilitated Diffusion via Channel Proteins


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Facilitated Diffusion via Channel Proteins

Cells have many different types of channel proteins in their

membranes, each featuring a structure that allows it to admit a

particular type of ion or small molecule.

These channels are responsible forfacilitated diffusion: the

passive transport of substances that would not otherwise cross the

membrane.


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Facilitated Diffusion via Carrier Proteins


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c ed us o v C e o e s

Facilitated diffusion can occur through channels or through carrier

proteins, ortransporters, which change shape during the transport

process.

Facilitated diffusion by transporters occurs only down a

concentration gradient, reducing differences between solutions.

Glucose is a building block for important macromolecules and a

major energy source, but lipid bilayers are only moderately

permeable to glucose.

A glucose transporter named GLUT-1 increases membrane

permeability to glucose.


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Active Transport by Pumps


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p y p

Cells can transport molecules or ions againstan electrochemical

gradient.

This process requires energy in the form ofATP and is called

active transport.

Pumps are membrane proteins that provide active transport of

molecules across the membrane.

For example, the sodium-potassium pump, Na+/K+-ATPase,

uses ATP to transport Na+ and K+ against their concentration

gradients.


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Secondary Active Transport


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In addition to moving materials against their concentration

gradients, pumps set up electrochemical gradients.

These gradients make it possible for cells to engage in secondary

active transport, orcotransport.

The gradientprovides the potential energy required to power

the movement of a different molecule against its particular

gradient.

Summary of Membrane Transport


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There are three mechanisms of membrane transport:

1. Diffusion

2. Facilitated diffusion

3. Active transport

Diffusion and facilitated diffusion are forms of passive transportand thus move materials down their concentration gradient and do

not require an input of energy.

Active transport moves materials against their concentrationgradient and requires energy provided by ATP or an

electrochemical gradient.


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Plasma Membrane and the Intracellular Environment


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The selective permeability of the lipid bilayer and the specificity

of the proteins involved in passive transport and active transport

enable cells to create an internal environment that is muchdifferent from the external one.

chapter 6 cell membrane and movement across the membrane

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