CHAPTER 5CHAPTER 5THE STRUCTURE AND FUNCTION THE STRUCTURE AND FUNCTION

OF MACROMOLECULESOF MACROMOLECULES

MEMBRANE STUCTURE MEMBRANE STUCTURE AND FUNCTIONAND FUNCTION

How things get into and out of the cellHow things get into and out of the cell

ObjectivesObjectives

• Explain how an equilibrium is established as a Explain how an equilibrium is established as a result of diffusion.result of diffusion.

• Distinguish between diffusion and osmosis.Distinguish between diffusion and osmosis.

• Explain how substances cross the cell membrane Explain how substances cross the cell membrane through facilitated diffusion.through facilitated diffusion.

• Explain how ion channels assist the diffusion of Explain how ion channels assist the diffusion of ions across the cell membrane.ions across the cell membrane.

The plasma membrane separates the living cell from its nonliving surroundings.

This thin barrier, 8 nm thick, controls traffic into and out of the cell.

Like other membranes, the plasma membrane is selectively permeable إختياريا ُم�نِف�ذ , allowing some substances to cross more easily than others.

The most abundant lipids The most abundant lipids cell cell membrane are are phospholipidsphospholipids..

Phospholipids and most other membrane constituents are Phospholipids and most other membrane constituents are amphipathic molecules.amphipathic molecules.

Amphipathic molecules have both hydrophobic regions and Amphipathic molecules have both hydrophobic regions and hydrophilic regions.hydrophilic regions.

The The phospholipids and proteins in membranesphospholipids and proteins in membranes create a unique create a unique physical environment, described by the physical environment, described by the fluid mosaic modelfluid mosaic model..

A membrane is a fluid structure with proteins embedded or attached to A membrane is a fluid structure with proteins embedded or attached to a double layer of phospholipids.a double layer of phospholipids.

Section A: Membrane Structure Section A: Membrane Structure ((Introduction)Introduction)

Fig. 8.1b

The molecules in the bilayer ُمزدوجة are arranged as طبقةhydrophobic fatty acid tails are sheltered ُمحمية from water while the hydrophilic phosphate groups interact with water.

Membrane proteins are amphipathic, with hydrophobichydrophobic and hydrophilichydrophilic regions.

If at the surface, the hydrophilic regions would be in contact with water.

In this fluid mosaic model, the hydrophilic regions of proteins and phospholipids are in contact with water and the hydrophobic regions are in a nonaqueous ُمائ ُمائ ال ييال environment.

• A membrane is a collage A membrane is a collage ع� of different proteins of different proteins تَـَج�م�عتَـَج�مembedded embedded م�س�#َغ #َغ�م�سُم�ـن .in the fluid matrix of the lipid bilayer.in the fluid matrix of the lipid bilayer ُم�ـن

Membranes are mosaics of structure and functionMembranes are mosaics of structure and function

To work properly with To work properly with permeability, permeability, membrane must be membrane must be fluid, about as fluid fluid, about as fluid as oil.as oil.

Where they contact the core, Where they contact the core, they they have have hydrophobichydrophobic regions regions with with nonpolar amino acids, often nonpolar amino acids, often coiled coiled into alpha helices.into alpha helices.

Where they are in contact with the Where they are in contact with the aqueous environment, they have aqueous environment, they have hydrophilichydrophilic regions of amino acids. regions of amino acids.

A)- The plasma membrane has a unique collection A)- The plasma membrane has a unique collection ز �ـ ُممي ز تَجمع �ـ ُممي of of تَجمع proteins. proteins.

• There are two populations of There are two populations of MEMBRANE PROTEINSMEMBRANE PROTEINS..

1.1. Peripheral proteinsPeripheral proteins ييطرفطرف are not embedded in the lipid bilayer at all. are not embedded in the lipid bilayer at all. Instead, they are loosely bounded to the surface of the protein, often Instead, they are loosely bounded to the surface of the protein, often connected to the other population of membrane proteins.connected to the other population of membrane proteins.

2.2. Integral proteinsIntegral proteins ُمج� penetrate the hydrophobic core of the lipid penetrate the hydrophobic core of the lipid ُم�نَد�ُمجُم�نَدbilayer, often completely spanning the membrane (bilayer, often completely spanning the membrane (a a transmembranetransmembrane proteinprotein).).

Integral Integral proteinsproteins

Peripheral Peripheral proteinsproteins

Hydrophilic Hydrophilic regionregion

Hydrophobic Hydrophobic regionregion

The proteins in the plasma membrane may The proteins in the plasma membrane may provide a variety of major cell functions.provide a variety of major cell functions.

AquaporinsAquaporins ((channel proteinschannel proteins): are transport proteins that ): are transport proteins that function by having a function by having a hydrophilic channelhydrophilic channel that facilitate the that facilitate the passage of passage of glucoseglucose and and waterwater molecules through the molecules through the membrane in certain cells. Without aquaporins, only a tiny membrane in certain cells. Without aquaporins, only a tiny fraction of water molecules would pass through the cell fraction of water molecules would pass through the cell membrane. membrane.

Carrier proteinCarrier protein ( (glucose transporterglucose transporter): in the plasma ): in the plasma membrane of red blood cells transports membrane of red blood cells transports glucoseglucose across the across the membrane 50,000 time faster than glucose can pass through membrane 50,000 time faster than glucose can pass through on its own.on its own.

Nonpolar moleculesNonpolar molecules, such as , such as hydrocarbons, COhydrocarbons, CO22, and O, and O22, ,

are are hydrophobichydrophobic and can therefore dissolve in the lipid and can therefore dissolve in the lipid bilayer of the membrane and cross it easily, without the aid of bilayer of the membrane and cross it easily, without the aid of membrane proteins.membrane proteins.

Thus, Thus, the selective permeabilitythe selective permeability of a membrane depends of a membrane depends on both the discriminating barrier of theon both the discriminating barrier of the lipid bilayerlipid bilayer and the and the specificspecific transport proteinstransport proteins built into the membranebuilt into the membrane

A steady traffic A steady traffic المنتطم المنتطم العبور of small molecules and ions moves of small molecules and ions moves العبورacross the plasma membrane in both directions.across the plasma membrane in both directions.

For example, sugars, amino acids, and other nutrients enter a muscle For example, sugars, amino acids, and other nutrients enter a muscle cell and metabolic waste products leave it.cell and metabolic waste products leave it.

The cell absorbs OThe cell absorbs O22 and expels CO and expels CO22..

It also regulates concentrations of inorganic ions, like NaIt also regulates concentrations of inorganic ions, like Na++, K, K++, Ca, Ca2+2+, , and Cland Cl--, by passing them across the membrane., by passing them across the membrane.

However, substances do not move across the barrier However, substances do not move across the barrier indiscriminately indiscriminately عشوائياعشوائيا as membrane is selectively permeable. as membrane is selectively permeable.

Hydrophobic molecules, like hydrocarbons, COHydrophobic molecules, like hydrocarbons, CO22, and O, and O22, can , can dissolve in the lipid bilayer and cross easily as described in the dissolve in the lipid bilayer and cross easily as described in the previous slide.previous slide.

Ions and polar molecules like HIons and polar molecules like H22O and glucose pass through O and glucose pass through channel proteins channel proteins as described in the previous slide.as described in the previous slide.

Thus membrane proteins assist and regulate Thus membrane proteins assist and regulate ينظم و the the يساعَدtransport of ions and polar molecules.transport of ions and polar molecules.

Functions of cell membrane (Functions of cell membrane (Plasma membranePlasma membrane))

1- Selective permeability1- Selective permeability اإلختيارية النِفاذية

COCO22

OO22

OO22

COCO22

NucleusNucleusNucleusNucleus

Selective Selective PermeabilityPermeability

The cell is able to take uptake up يتـنـتـق particular molecules and exclude exclude تـتَجنبتـتَجنب others

No ENERGYNo ENERGY is required to move substances across membrane is required to move substances across membrane (water, lipids, and other lipid soluble substances).(water, lipids, and other lipid soluble substances).

Types of Passive transport:Types of Passive transport:

A.A. Diffusion Diffusion

B.B. OsmosisOsmosis

C.C. Facilitated DiffusionFacilitated Diffusion

Involves the movement of molecules across the cell membrane without an input of energy by the cell.

2- Passive transport 2- Passive transport السلب ياإلنتقال

RatharRathar, , thethe CONCENTRATION GRADIENTCONCENTRATION GRADIENT represents potential represents potential energy and drives diffusionenergy and drives diffusion

A)- Diffusion:A)- Diffusion: اإلنتشاراإلنتشار

Is the tendency Is the tendency إستعَدادإستعَداد of molecules of any substance to spread out of molecules of any substance to spread out لإلنتشارلإلنتشار in in the available space randomly the available space randomly عشوائياعشوائيا..

• For example, a permeable membrane For example, a permeable membrane ُمنِفز ُمنِفز غشاء separating a solution with separating a solution with غشاءsugarsugar moleculesmolecules from from pure waterpure water, sugar molecules will cross the barrier , sugar molecules will cross the barrier randomly.randomly.

• The sugar molecules will cross the membrane until both solutions have equal The sugar molecules will cross the membrane until both solutions have equal concentrations of the sugar (concentrations of the sugar (dynamic equilibriumdynamic equilibrium الَديناُميك يالتعادل ).).

• A substance will diffuse from where it is more concentrated to where A substance will diffuse from where it is more concentrated to where it is less concentrated, it is less concentrated, down its concentration gradient down its concentration gradient التركيز ُمُـنحَدر ..

dynamic equilibriumdynamic equilibriumLump of sugarLump of sugar

• Differences in concentration of Differences in concentration of dissolved materialsdissolved materials in in two solutionstwo solutions can lead to the movement of ions from one to the other.can lead to the movement of ions from one to the other.

• The solution with the higher concentration of solutes is The solution with the higher concentration of solutes is hypertonichypertonic..

• The solution with the lower concentration of solutes is The solution with the lower concentration of solutes is hypotonichypotonic..

• Solutions with equal solute concentrations are Solutions with equal solute concentrations are isotonicisotonic..

• Osmosis:Osmosis:

Is a passive transport in which water Is a passive transport in which water diffuses across a selectively permeable diffuses across a selectively permeable membrane from the membrane from the hypotonichypotonic solution solution to the to the hypertonichypertonic solution until the solution until the solutions become solutions become isotonicisotonic..

B). OsmosisB). Osmosis األسموزيةاألسموزية :: the passive transport of waterthe passive transport of water

Low conc. of sugar High conc. of sugar

SelectivelSelectively y

permeablpermeable e

membranmembranee

Principal of water Principal of water movementmovement

hypotonichypotonic hypertonichypertonic

IsotonicIsotonicOsmosisOsmosis OsmosisOsmosis

Types of solutions and Types of solutions and Osmosis

• Hypertonic solution:Hypertonic solution: التركيز عالى contains high concentration of solute contains high concentration of solute ُم�ذابُم�ذاب molecules.molecules.

• Hypotonic solution:Hypotonic solution: التركيز ُمنخِفض contains low concentration of solute molecules.contains low concentration of solute molecules.

• Isotonic solution:Isotonic solution: ُمتعادل contains equal concentrations of solute moleculescontains equal concentrations of solute molecules

Biological Membrane

HypotonicHypotonicHypertonicHypertonic

HH22OO

• The cell in a hypertonic The cell in a hypertonic environment will loose water, environment will loose water, shrivel shrivel تنكمشتنكمش, and die., and die.

• A cell in a hypotonic solution will A cell in a hypotonic solution will gain gain تسحبتسحب water, swell, and water, swell, and burst.burst.

• Organisms without rigid walls Organisms without rigid walls have osmotic problems in either have osmotic problems in either a a hypertonichypertonic or or hypotonichypotonic environment and must have environment and must have adaptations for adaptations for osmoregulationosmoregulation to maintain to maintain على على للحِفاظ their their للحِفاظinternal environment.internal environment.

• Example, Example, ParameciumParamecium have a have a specialized organelle (specialized organelle (the the contractile vacuolecontractile vacuole), that ), that functions as a pump to force functions as a pump to force .water out of the cell.water out of the cell يطرديطرد

OsmoregulationOsmoregulation التوازنياألسموز

Behavior of water when a living cell is placed in Behavior of water when a living cell is placed in HypertonicHypertonic, , HypotonicHypotonic or or IsotonicIsotonic Solutions Solutions

C)- Facilitated DiffusionC)- Facilitated Diffusion: : Specific proteins facilitate Specific proteins facilitate هِّـ0ل passive passive تُـَس3transporttransport

• It Helps diffusion of molecules across a membrane when they are It Helps diffusion of molecules across a membrane when they are not soluble not soluble in lipidsin lipids or are or are too largetoo large (e.g. glucose) to pass through pores in membrane (e.g. glucose) to pass through pores in membrane

• Thus, a molecule binds to a carrier protein on one side of the cell membrane. Thus, a molecule binds to a carrier protein on one side of the cell membrane.

• The carrier protein (specific for one type of molecule) then changes its shape The carrier protein (specific for one type of molecule) then changes its shape and transports the molecule and transports the molecule down its concentration gradientdown its concentration gradient to the other to the other side of the membrane.side of the membrane.

• It is the passive movement of molecules down its concentration gradient via a It is the passive movement of molecules down its concentration gradient via a transport protein.transport protein.

• Other transport proteins Other transport proteins translocatetranslocate ينقلينقل the the molecules across the molecules across the membrane as the protein membrane as the protein changes its shape (changes its shape (active active

transporttransport))