Cell Membrane:Cell Membrane:Structure & FunctionStructure & Function

Ms. LevensailorMs. Levensailor

Membrane StructureMembrane Structure

Staple ingredients Staple ingredients of membranes are:of membranes are: LipidsLipids ProteinsProteins

Phospholipids are Phospholipids are most commonmost common AmphipathicAmphipathic

molecule: it has molecule: it has both a hydrophilic both a hydrophilic and hydrophobic and hydrophobic regionregion

Membranes are fluidMembranes are fluid

Membranes are held together by Membranes are held together by hydrophobichydrophobic interactions (much interactions (much weaker than covalent bonds).weaker than covalent bonds).

They must be fluid to work properlyThey must be fluid to work properly When solid permeability changes & When solid permeability changes &

enzymatic proteins become inactive.enzymatic proteins become inactive.

Membranes are mosaics of Membranes are mosaics of structure & functionstructure & function

A membrane is a collage of different A membrane is a collage of different proteins embedded in the fluid proteins embedded in the fluid matrix of the lipid bilayer.matrix of the lipid bilayer.

Membranes are mosaics of Membranes are mosaics of structure & functionstructure & function

Lipid bilayer is the main fabric of the Lipid bilayer is the main fabric of the membrane.membrane.

Proteins determine most of the Proteins determine most of the membrane’s functions.membrane’s functions.

Membrane CarbohydratesMembrane Carbohydrates

Important in cell to cell recognition:Important in cell to cell recognition: A cell’s ability to distinguish between 1 A cell’s ability to distinguish between 1

type of neighboring cell from another.type of neighboring cell from another. i.e. identifying foreign cells (immune i.e. identifying foreign cells (immune

system)system)

Recognize other cells by keying in on Recognize other cells by keying in on surface molecules, carbohydrates, on surface molecules, carbohydrates, on the plasma membrane.the plasma membrane.

Some Functions of the Some Functions of the membrane proteinsmembrane proteins

Selective PermeabilitySelective Permeability

Substances do not cross the barrier Substances do not cross the barrier indiscriminately!indiscriminately!

Hydrophobic nonpolar molecules Hydrophobic nonpolar molecules (hydrocarbons, carbon dioxide, & (hydrocarbons, carbon dioxide, & oxygen) cross with ease.oxygen) cross with ease.

Hydrophilic polar molecules (ions) Hydrophilic polar molecules (ions) are impeded by the hydrophobic core are impeded by the hydrophobic core of the membrane.of the membrane.

Transport ProteinsTransport Proteins

Proteins built into the membraneProteins built into the membrane Allow passage of hydrophilic substances.Allow passage of hydrophilic substances. Can be so specific that they only allow 1 Can be so specific that they only allow 1

specific substance to move through.specific substance to move through. i.e. glucose carried in blood to the human i.e. glucose carried in blood to the human

liver enters liver cells rapidly through a liver enters liver cells rapidly through a specific transport protein.specific transport protein.

It is so selective it rejects fructose It is so selective it rejects fructose (structural isomer of glucose).(structural isomer of glucose).

Passive transportPassive transport

Molecules have an intrinsic kinetic Molecules have an intrinsic kinetic energy called thermal motion (heat).energy called thermal motion (heat).

A result of this is A result of this is diffusion:diffusion: Tendency of molecules of any substance to Tendency of molecules of any substance to

spread out into available space.spread out into available space. A substance will diffuse from areas of high A substance will diffuse from areas of high

concentration to areas of low concentration.concentration to areas of low concentration. Passive transport does not require Passive transport does not require

energy!energy!

Passive TransportPassive Transport

Diffusion of one soluteDiffusion of one solute

Osmosis Osmosis

The passive transport of water.The passive transport of water. Hypertonic: solution with a higher Hypertonic: solution with a higher

concentration of solute.concentration of solute. Hypotonic: solution with a lower Hypotonic: solution with a lower

solute concentration.solute concentration. Isotonic: solutions of equal solute Isotonic: solutions of equal solute

concentration.concentration.

OsmosisOsmosis

Cell survival depends on water Cell survival depends on water uptake & lossuptake & loss

Cells w/out walls (animal cells)Cells w/out walls (animal cells) If cell loses water it will shrivelIf cell loses water it will shrivel If cell takes up too much water it will burstIf cell takes up too much water it will burst

Cell w/ wall (plants & fungi)Cell w/ wall (plants & fungi) If cell takes in water the wall exerts pressure If cell takes in water the wall exerts pressure

back and prevents further uptake.back and prevents further uptake. Isotonic solutions will make the plant limp.Isotonic solutions will make the plant limp. If a cell loses water it will shrivel and the plasma If a cell loses water it will shrivel and the plasma

membrane pulls away from the wall membrane pulls away from the wall (plasmolysis).(plasmolysis).

PlasmolysisPlasmolysis

Hypotonic Hypertonic

OsmosisOsmosis

Active TransportActive Transport

The pumping of solutes against their The pumping of solutes against their gradients.gradients. Across the plasma membrane from the side Across the plasma membrane from the side

where they are less concentrated to the side where they are less concentrated to the side where they are more concentrate.where they are more concentrate.

Like moving “up hill” & requires work!Like moving “up hill” & requires work! ATP supplies the energy for active transport.ATP supplies the energy for active transport. i.e. sodium-potassium pump (online activity)i.e. sodium-potassium pump (online activity)

Active TransportActive Transport

Exocytosis & EndocytosisExocytosis & Endocytosis

Transport large molecules (proteins Transport large molecules (proteins & polysaccharides).& polysaccharides).

Exocytosis: cell secretes Exocytosis: cell secretes macromolecules by the fusion of macromolecules by the fusion of vesicles with the plasma membrane.vesicles with the plasma membrane.

Endocytosis: cell takes in Endocytosis: cell takes in macromolecules by forming new macromolecules by forming new vesicles from the plasma membrane.vesicles from the plasma membrane.

Exocytosis & EndocytosisExocytosis & Endocytosis