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The CellularLevel of

Organization3.1-3.3August 20, 2012

ReviewHow do inorganic compounds differ from organic

compounds?

Which portion of a phospholipid is hydrophillic,and which portion is hydrophobic?

Distinguish among saturated, monounsaturated,and polyunsaturated fats

Chapter 3: The Cellular Levelof Organization3.1 Parts of a Cell3.2 The Plasma Membrane3.3 Transport Across the Plasma Membrane3.4 Cytoplasm3.5 Nucleus3.6 Protein Synthesis3.7 Cell Division3.8 Cellular Diversity3.9 Aging & Cells

The Big IdeaCells and Homeostasis

Cells carry out a multitude of functions thathelp each system contribute to thehomeostasis of the entire body. At the sametime, all cells share key structures andfunctions that support their intense activity.

3.1 Parts of a CellObjectives

Name and describe the three main parts ofa cell

The CellA cell is the basic unit of all living things

Prokaryotic cells are simple cells that are Pro(“before”) karyon (“nucleus”) – they have nonucleus

Most are unicellular bacteria

Eukaryotic cells are complex cells with a nucleusand subcellular structures (organelles)

All fungi, plants, and animals are eukaryotes

A Generalized CellAll eukaryotic cells are composed of three

main parts:Plasma membrane or “plasmalemma”Cytoplasm, a gelatin-like substance, plus

structural fibers and organelles (but not thenucleus)

Nucleus that contains the genetic library ofthe cell

The plasma membrane forms the cell’s outerboundary and separates the cell’s internalenvironment from the outside environment

It is a selectively permeable barrier, allowingthe passage of some things and not others

It plays a role in cellular communication

The cytoplasm contains allthe cellular contentsbetween the plasmamembrane and thenucleus

The cytosol is the fluidportion (mostly water)

Organelles are subcellularstructures embedded inthe cytosol, havingcharacteristic shapes andspecific functions

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The nucleus is a largeorganelle that containsDNA in molecules calledchromosomes Each chromosome

consists of a singlemolecule of DNA andassociated packagingproteins

A chromosome containsthousands of hereditaryunits called genes

3.2 The Plasma MembraneObjectives

Distinguish between cytoplasm and cytosol

Explain the concept of selective permeability

Define the electrochemical gradient anddescribe its components

The plasma membrane is much more than justa “fence” – it is a flexible yet sturdy,“intelligent” semipermeable regulator that:

Covers and protects the cell

Controls what goes in and comes out

Links to other cells

Flies certain “flags” to tell other cells “who”it is

The Fluid Mosaic Modeldescribes the arrangement ofmolecules within themembrane:They resemble a sea of

phospholipids with protein“icebergs” floating in it

The lipids act as a barrierto certain polar substances

The proteins act as“gatekeepers”, allowingpassage of specificmolecules and ions

Structure of the Plasma MembranePhospholipids form a lipid bilayer - cholesterol

and glycolipids (sugar-lipids) also contribute

Integral proteins extend into or through thebilayer

Transmembrane proteins (most integral proteins)span the entire lipid bilayer

Peripheral proteins attach to the inner or outersurface but do not extend through themembrane

The bilayer arrangement occurs because the lipidsare amphipathic, meaning they have both polarand nonpolar parts

The polar part is the phosphate-containinghead which is hydrophilic

The nonpolar parts are the two long fatty acid“tails” which are hydrophobic hydrocarbonchains

Membrane proteins are classified as integral orperipheral according to whether they areembedded in the membrane

Integral proteins extend into or through the lipidbilayer among the fatty acid tails

Many are transmembrane proteins

Peripheral proteins are not as strongly embeddedin the membrane

They are attached to the polar heads of themembrane lipids or to the integral proteins atthe inner or outer surface of the membrane

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Glycoproteins are membraneproteins with a carbohydrategroup attached that protrudeinto the extracellular fluid

The glycocalyx is the entire“sugary coating” surroundingthe membrane

Made up of thecarbohydrate portions ofthe glycolipids andglycoproteins

Functions of Membrane Proteins Ion channels are pores that specific ions can flow throughCarriers/Transporters selectively move substances through

the membraneReceptors serve as cellular recognition sites

Ligands are molecules that bind with a receptor Enzymes catalyze chemical reactions Linkers anchor proteins in the plasma membranes of

neighboring cells to one another or to protein filamentsinside and outside the cell

Cell-identity markers enable cells to:Recognize cells of the same kind during tissue formationRecognize and respond to potentially dangerous foreign

cells

Membrane Fluidity & PermeabilityMembrane lipids and proteins can easily rotate and

move sideways in their own half of the bilayer

Membrane fluidity depends on the number ofdouble bonds in the fatty acid tails of the lipids thatmake up the bilayer and the amount of cholesterolpresent

Membrane fluidity allows interactions to occurwithin the plasma membrane and enables themovement of membrane components responsiblefor cellular processes

Because of the distribution of lipids and the proteinsembedded in it, the membrane allows somesubstances across but not others; this is calledSelective Permeability

Rule of thumb: small, neutrally-charged, lipid-soluble substances can freely pass through

Water is a special case - it is highly polar, yet stillfreely permeable

For those substances that are needed by the cellbut for which the membrane is impenetrable(impermeable), transmembrane proteins act aschannels and transporters

They assist the entrance of certain substancesthat either can’t pass at all (glucose) or forwhich the cell needs to hasten passage (ions)

Gradients Across the Plasma MembraneA concentration gradient is the difference in the

concentration of a chemical from one place to another,like inside and outside the plasma membrane

An electrical gradient is the difference in electricalcharges between two regionsThis is called a membrane potential because it occurs

across a plasma membraneTypically the inner surface is more negatively charged

and the outer surface is more positively chargedThe combined influence of the concentration and

electrical gradients on the movement of a particular ionis referred to as its electrochemical gradient

3.3 Transport Across the PlasmaMembraneObjectives

Describe the processes that transportsubstances across the plasma membrane

Transport ProcessesPassive Processes

Involve substances moving across the cellmembranes without the input of any energy - theyare said to move “with” or “down” theirconcentration gradient

Active ProcessesInvolve the use of energy, primarily from the

breakdown of ATP, to move a substance againstits gradient

Passive Processes

Diffusion of solutesDiffusion of water (called osmosis)

Facilitated diffusion (requires a specificchannel or a carrier molecule, but noenergy is used)

Active ProcessesVarious types of transporters are used, and

energy is required

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Passive ProcessesDiffusion is the passive spread of particles through

random motion, from areas of high concentration toareas of low concentration

It is affected by the amount of substance and thesteepness of the concentration gradient

Temperature

Surface Area

Mass of the Diffusing Substance

Diffusion Distance

In simple diffusion, substances move freely through the lipidbilayer of the plasma membrane without the help of membranetransport proteins

Nonpolar, hydrophobic molecules such as oxygen, carbondioxide, nitrogen gases, fatty acids, steroids, fat-soluble vitamins(A, D, E, K)Small, uncharged molecules such as water, urea, and smallalcoholsIs important for the exchange of oxygen and carbon dioxideand is also the route for absorption and excretion of some cells

In Channel-Mediated Facilitated Diffusion, a solute movesdown its concentration gradient across the lipid bilayerthrough a membranePassage of potassium ions through a gated K+

Channel In Carrier-Mediated Facilitated Diffusion, a carrier moves

a solute down its concentration gradient across theplasma membranePassage of glucose across the cell membrane

Osmosis is the net movement of waterthrough a selectively permeable membranefrom an area of high water concentration toone of lower water concentration

Water can pass through plasma membrane in 2ways:

Through lipid bilayer by simple diffusion

Through aquaporins (integral membraneproteins)

In the third tube, the force generated by themovement of water from the left to the rightside is called osmotic pressure

In the body, tonicity refers to theconcentration of salt solutions in the bloodand elsewhere

Since semipermeable membranes separatethese fluid compartments, osmosis of wateris free to occur between any fluid space andanother

In an isotonic solution a cell will maintain itsnormal shape and volumeThe concentration of solutes that cannot cross

the plasma membrane are the same on bothsides of the plasma membrane

A hypotonic solution has a is a lowerconcentration of solutes than inside the cellWater will enter the cell faster than they leave,

causing the cells to swell and eventually burstA hypertonic solution has a higher concentration

of solutes than inside the cellWater will move outside the cell faster than

they enter, causing the cells to shrink

Active ProcessesSolutes can also be actively transported across

a plasma membrane against theirconcentration gradient ([low] to [high]) byusing energy (usually in the form of ATP)

In Primary Active Transport, energy derived fromhydrolysis of ATP changes the shape of a carrierprotein, which “pumps” a substance across aplasma membrane against its concentrationgradient

The sodium-potassium pump is found in all cells

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In Secondary Active Transport, the energystored in a Na+ or H+ concentration gradientis used to drive other substances across themembrane against their own concentrationgradients

Indirectly uses energy obtained by thehydrolysis of ATP

Antiporters carry two substances across themembrane in opposite directions

Symporters carry two substances across themembrane in the same direction

Transport in VesiclesVesicle - a small spherical sac formed by budding

off from a membrane In endocytosis, materials move into a cell in a

vesicle formed from the plasma membraneReceptor-mediated endocytosis is a highly

selective process where cells will take upspecific ligands

In phagocytosis, the cell engulfs large solidparticles, such as worn-out cells, whole bacteria,or virusesOnly phagocytes can carry out phagocytosis

MacrophagesNeutrophils

In bulk-phase endocytosis (pinocytosis), tinydrops of extracellular fluid are taken upNo receptor proteins are involvedAll solutes dissolved in the extracellular fluid

are brought into the cell

In exocytosis vesicles fuse with the plasmamembrane, releasing their contents into theextracellular fluidSecretory cells that liberate digestive enzymes,

hormones, mucus, or other secretionsNerve cells that release substances called

neurotransmitters In transcytosis, a combination of endocytosis and

exocytosis is used to move a substance into,across, and out of a cellVesicles will undergo endocytosis on one side of

a cell, move across the cell, and then undergoexocytosis on the opposite side

Clinical ConnectionsDigitalis Increases Ca2+ in Heart Muscle Cells

Often given to patients with heart failureSlows the action of the sodium-potassium pump,

which lets more Na+ accumulate inside the heartmuscle cells

Results in a decreased Na+ concentration gradientacross the plasma membrane, causing the Na+/Ca2+

antiporters to slow downMore Ca2+ remains inside the heart muscle cells,

increasing the force of their contractions, thusstrengthening the force of the heartbeat

Phagocytosis and Microbes

The process of phagocytosis is a vital defensemechanism

Macrophages dispose of invading microbes andaged worn-out cells

Neutrophils also help to rid the body ofinvading microbes

Pus is a mixture of dead neutrophils,macrophages, tissue cells, and fluid in an infectedwound