CELL BIOLOGYPart II: Cell Membrane and Cellular Transport

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CELLULAR MEMBRANECell membrane

Also called plasma membraneSeparates cell from its surroundings

8 nm thickNeed 8,000 to equal thickness of a piece of paper

Supports the cellProtects the cell Controls what enters and exits the cell

Selectively permeable Different membranes with different functions differ in their chemical composition and structure

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CELL MEMBRANEMostly made of proteins and lipids

Proteins are used for transportLipids are mostly phospholipids

Creates a bilayerMolecular arrangement shelters the hydrophobic tails

from water while exposing the hydrophilic heads to water

Some lipids are cholesterol moleculesHelp strengthen the cell membrane

Also contains carbohydratesUsed for cell to cell recognition

Carbohydrates can attach to lipids creating glycolipids or attach to proteins creating glycoproteins

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FLUID MOSAIC MODELFluid mosaic model refers to the nature of cell membranesFluid structure with a ‘mosaic’

of various molecules embedded in or attached to the bilayer of phospholipids. Mosaic - a pattern made of numerous small pieces fitted together

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MEMBRANES ARE FLUIDMembranes are not molecules locked rigidly in place The lipids and proteins can shift

laterallyThis happens frequently and rapidlyPhospholipids switch places 107 times per second

Proteins are larger and move more slowly but some do shift their positions

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TEMPERATURE AND MEMBRANES

Membranes must be fluid to work properlyThey are usually about as fluid as

salad oilTemperature can affect fluidity

Fluidity will decrease as temperature decreases until phospholipids become a closely packed solid

Cholesterol molecules buffer the effect of temperature changesRestrains the movement of phospholipids but also prevents them from packing together too tightly.

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MEMBRANE PROTEINS AND THEIR FUNCTIONS

A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayerProteins determine the membrane’s function

Different types of cells contain different types of membrane proteins.

More than 50 different types of proteins in just RBC!

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TWO TYPES OF MEMBRANE PROTEINS

Membrane proteins are associated with a cell membrane.

They are very specific to what they do with which molecules they react.

Integral proteinsEmbedded in hydrophobic core of bilayer

Many are transmembrane proteinsSpan entire membrane

Others are only partially embedded in core

Peripheral proteinsNot embedded in lipid bilayer at all

Loosely bound to surface of membrane

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FUNCTIONS OF MEMBRANE PROTEINS

Transport – a protein that spans the membrane may provide a channel across the membrane that is selective for the transport of certain molecules. Channel proteins are involved in passive transport. Carrier proteins may be involved in passive or active transport; some require energy, some do not.

Enzymatic activity – a protein built into the membrane that is an enzyme with its active site exposed to substances outside the cell. Several enzymes in a membrane may be grouped together to take part in a metabolic pathway.

Signal transduction – a membrane protein may act as a receptor and have a binding site that fits a chemical messenger (ligand) like a hormone. The protein can relay the message to the inside of the cell without the chemical messenger actually having to enter the cell.

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FUNCTIONS OF MEMBRANE PROTEINS

Cell to cell recognition – some glycoproteins serve as identification tags that are specifically recognized by membrane proteins of other cells

Intercellular joining – membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions.

Attachment to the cytoskeleton and extracellular matrix – elements of the cytoskeleton may be bound to membrane proteins to help maintain cell shape and stabilize the location of certain membrane proteins.

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CELL – CELL RECOGNITIONCell’s ability to distinguish one type of neighboring cell from anotherNeeded for organism to function

Ex: sorting of cells into tissues and organs in an animal embryo

Basis for rejection of foreign cells by immune system (including those of transplanted organs)A, B, AB, and O blood types are designated by the carbohydrates on the surface of the RBC

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SELECTIVE PERMEABILITY

Cells maintain homeostasis by having selectively permeable cell membranes

Cells regulate transport of materials across the membrane in both directions

Nutrients enter the cellWastes leave the cell

Large molecules require vesiclesSmall, hydrophobic (nonpolar) molecules can pass through the membrane rapidly.

Ex: hydrocarbons

Small, polar molecules require help to cross the membrane - transport proteins

Water, sugar

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TRANSPORT ACROSS THE CELL MEMBRANETwo types of movement occur across a cell membrane: Passive Transport

Simple Diffusion Dialysis and Osmosis

Facilitated Diffusion Channel proteins Carrier proteins

Active TransportProtein pumps

Type of carrier proteinEndocytosis and exocytosis

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PASSIVE TRANSPORT Passive Transport

Does not require energyMoves molecules down their

concentration gradient from an area of high concentration to an area of low concentration

Includes:(Simple) DiffusionFacilitated Diffusion

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PASSIVE TRANSPORTDiffusion

Substance will diffuse down its concentration gradient until dynamic equilibrium is reached.

Concentration gradient – a region along which the density of a chemical substance increases or decreases

Dynamic equilibrium - occurs when both solutions have equal concentration and as many molecules are crossing the membrane in one direction as in the other

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PASSIVE TRANSPORTDialysis – the passive movement of particles across a semi-permeable membrane from an area of high concentration to an area of low concentration

This occurs with nonpolar and very small uncharged polar molecules.

Osmosis – the passive movement of water across a semi-permeable membrane from an area of high concentration to an area of low concentrationWater can diffuse through the cell membrane but since it is polar this will happen very slowly – too slowly to support life. Facilitated diffusion through aquaporins allows water to

move quickly across a cell membrane.

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PASSIVE TRANSPORTFacilitated diffusion

Process used by many polar molecules and ions to cross a membrane.

Uses transport proteinsStill being studiedMost transport proteins are very specific; transport some substances but not others.

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PASSIVE TRANSPORTFacilitated diffusion involves:

Protein channelsProvide a corridor that allows a specific molecule or ion to cross the membrane AquaporinsIon channels

Move ions such as Na+, Gated channels

Type of ion channelOpen/close in response to electrical or chemical

stimulus

Carrier proteinsChange shape in process of transporting molecules across membraneExample: glucose

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ACTIVE TRANSPORTActive Transport

Requires energy supplied by ATPMoves molecules against their

concentration gradient from an area of low concentration to an area of high concentration

Include:Protein pumps (type of carrier proteins)

Example: sodium-potassium pumpPumps sodium out of the cell and potassium into the cell

Example: proton pump used during chemiosmosisEndocytosisExocytosis

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TRANSPORT PROTEINSTransport proteins (2 types)

Allows polar substances to pass through the membrane quickly

Specific for the substance it movesExample: glucose enters RBC by specific carrier proteins

Channel proteins (protein channels)Have hydrophilic channel that can be used as a tunnel through the membrane

Example: Aquaporins Facilitate the passage of water

Carrier proteins (passive or active)Hold onto substances and change shape in a way that shuttles them across the membrane

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TRANSPORT ACROSS THE CELL MEMBRANETwo types of movement occur across a cell membrane: Passive TransportPassive Transport

Simple DiffusionSimple Diffusion Dialysis and OsmosisDialysis and Osmosis

Facilitated Diffusion Channel proteins Carrier proteins

Active TransportProtein pumps Endocytosis and exocytosis

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PASSIVE TRANSPORTDialysis 0

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PASSIVE TRANSPORT0

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OsmosisWater diffuses across

the membrane from the region of higher water concentration to that of lower water concentration until the solute concentrations on both sides of the membrane are equal.

Water moving into a cell by osmosis creates pressure which is called osmotic pressure

SOLUTIONSSolutes

Solids that are dissolved in a liquidSolvents

Liquids that dissolve the solidsUsually present in greater amountsWater is considered the universal

solvent

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TYPES OF SOLUTIONSThe concentration of all molecules dissolved in a solution (the solutes) is called the osmotic concentration of the solution.

Three terms are used to compare the osmotic concentrations of two solutions: IsotonicHypertonicHypotonic

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TYPES OF SOLUTIONSIsotonic

Solutions with equal solute concentrationsEqual osmotic concentration

If a cell is immersed in an isotonic environment there will be no net movement of water across the plasma membrane.Water continues to flow across the membrane, but at the same rate in both directions. Dynamic equilibrium

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TYPES OF SOLUTIONSAn animal cell fares

best in an isotonic environment

Plant cells need osmotic pressure to help maintain structure so they become flaccid (limp) in isotonic conditions.

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TYPES OF SOLUTIONSHypertonic

Solution with more solutes, less waterGreater osmotic concentration

If a cell is placed in a hypertonic solution the net movement of water will be out of the cell and the cell will shrink because osmosis moves water from an area where there is more water to an area where there is less water. Example: increase in salinity

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Animal cells placed in a hypertonic solution will crenate (shrink/shrivel) as water moves out of the cell

Plant cells, with cell walls, will become plasmolyzed (shrunken/shriveled, cell membrane pulls away from the cell wall) due to lack of osmotic pressure.

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TYPES OF SOLUTIONS

TYPES OF SOLUTIONSHypotonic

Solution with less solutes, more water

If a cell is placed in a hypotonic solution the net movement of water will be into the cell and the cell will swell and burst because osmosis moves water from an area where there is more water to an area where there is less water

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TYPES OF SOLUTIONS

Animal cells can by lysed (or burst) due to osmotic pressure.

Plant cells can be turgid, or full of water but not lysed due to their cell walls.This is healthy for plants

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TYPES OF SOLUTIONS

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• ANIMAL CELL – fares best in an isotonic environment… will lyse in hypotonic solution or crenate in hypertonic solution.

• PLANT CELL – turgid and generally fares best in a hypotonic environment….tendency for water uptake balanced by the elastic cell wall pushing back on the cell. Become plasmolyzed if placed in hypertonic solution.

• ARROW INDICATES WATER MOVEMENT WHEN CELL IS FIRST PLACED IN THE SOLUTIONS!!!

OSMOREGULATIONOrganisms without cell walls living in hypertonic or hypotonic environments must have adaptations for osmoregulation.

control of water balance. Example: Paramecium live in pond

water which is hypotonic to the cellParamecium doesn’t burst because it is equipped with a contractile vacuole, an organelle that functions as a bilge pump to force water out of the cell as fast as it enters by osmosis.

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OSMOREGULATION0

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TRANSPORT ACROSS THE CELL MEMBRANETwo types of movement occur across a cell membrane: Passive Transport

Simple Diffusion Dialysis and Osmosis

Facilitated Diffusion Channel proteins Carrier proteins

Active TransportActive TransportProtein pumps Protein pumps Endocytosis and exocytosis Endocytosis and exocytosis

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ACTIVE TRANSPORTActive Transport

Requires energy supplied by ATPMoves molecules against their

concentration gradient from an area of low concentration to an area of high concentration

Include:Protein pumps (type of carrier proteins)

Example: sodium-potassium pumpPumps sodium out of the cell and potassium into the cell

Example: proton pump used during chemiosmosisEndocytosisExocytosis

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ACTIVE TRANSPORTSodium-potassium pump

Found in animal cellsUses energy from ATP

Phosphate group is transferred directly to the transport proteinThis causes transport protein to change shape in a manner that moves a solute across the membrane.

The pump oscillates between two conformational states in a pumping cycle that translocates three Na+ ions out of the cell for every two K+ ions pumped into the cell.

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SODIUM – POTASSIUM PUMP

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SODIUM – POTASSIUM PUMP

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ACTIVE TRANSPORTMoving LARGE molecules across the membrane

Examples: proteins, polysaccharides, viruses, – too large to pass through membrane using channels or carrier proteins

Includes exocytosis and endocytosisExocytosis

Vesicle from cytoplasm fuses with membrane and materials are pushed out.

EndocytosisVesicle from extracellular area fuses with membrane and materials are brought in

Three types of endocytosisPhagocytosisPinocytosisReceptor-mediated endocytosis

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ACTIVE TRANSPORTExocytosis

Type of active transportVesicles fuse with membrane to

secrete materials out of the cellA transport vesicle that has budded from the Golgi apparatus moves along microtubules of the cytoskeleton to the cell membrane

When the vesicle membrane and the cell membrane come into contact, the lipid molecules of the two bilayers rearrange themselves so that the two membranes fuse.

The contents of the vesicle than spill to the outside of the cell and the vesicle membrane becomes part of the cell membrane

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EXOCYTOSIS0

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ACTIVE TRANSPORTEndocytosis

Type of active transportVesicle from extracellular area fuses

with membrane and materials are brought inA small area of the cell membrane sinks inward to form a pocket

As the pocket deepens, it pinches in, forming a vesicle containing material that had been outside the cell

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ENDOCYTOSISPhagocytosis

“cellular eating” - particles taken inIn phagocytosis, a cell engulfs a particle by wrapping

pseudopodia around it and packaging it within a membrane-enclosed sac that can be large enough to be considered a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes.

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ENDOCYTOSISPinocytosis

“cellular drinking” – fluid taken inIn pinocytosis, the cell “gulps” droplets of

extracellular fluid into tiny vesicles. It is not the fluid itself that is needed by the cell, but the molecules dissolved in the droplets. Because any and all included solutes are taken into the cell, pinocytosis is nonspecific in the substances it transports.

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RECEPTOR-MEDIATED ENDOCYTOSIS This enables the cell to acquire

bulk quantities of specific substance, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid. The receptor proteins are usually clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coat proteins. The specific substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules.

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Example: cholesterol

TRANSPORT REVIEW0

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