Cells as Units of LifeCells as Units of LifeChapter 3

Cell TheoryCell TheoryCells represent the basic structural and

functional unit of life.Important unifying concept in biology.All organisms are composed of one or more cells.All tissues & organs are composed of cells.

There is no life without cells!

Cell TheoryCell TheoryCell theory states that all living organisms are

composed of cells.Cells come from preexisting cells.

Prokaryotic vs. Eukaryotic Prokaryotic vs. Eukaryotic CellsCells

All cells:Have DNAUse the same genetic codeSynthesize proteinsUse ATP in similar ways

This implies common ancestry.

Prokaryotic vs. Eukaryotic Prokaryotic vs. Eukaryotic CellsCells

Prokaryotic cells – no nucleus or other membrane-bound organelles.Kingdom ArchaebacteriaKingdom Eubacteria

Eukaryotic cells – do have nucleus and membrane-bound organelles.Kingdom ProtistaKingdom FungiKingdom PlantaeKingdom Animalia

Components of Eukaryotic Components of Eukaryotic CellsCells

The plasma membrane surrounds the cell.

The nucleus is the largest organelle.Double layered

nuclear envelope.

Cell Model

Components of Eukaryotic Components of Eukaryotic CellsCells

Cytoplasm refers to the cellular material between the cell membrane and nuclear envelope.

Organelles such as the mitochondria, Golgi complex, centrioles, and endoplasmic reticulum are found in the cytoplasm.

Plasma MembranePlasma MembranePlasma membrane

structure is described using the fluid mosaic model.

http://www.youtube.com/watch?v=ULR79TiUj80

Plasma MembranePlasma MembraneTwo layers of

phospholipid molecules oriented with hydrophilic heads toward the outside and hydrophobic tails inside.Fluid-like – flexible

Plasma MembranePlasma MembraneThe nonpolar nature of the hydrophobic ends in the

interior of the membrane prohibit polar substances from crossing the membrane.

Glycoproteins embedded in the membrane function in the transport of molecules across the membrane.

NucleusNucleusThe nuclear envelope

contains pores to allow molecules to move between nucleus & cytoplasm.

Chromosomes are contained in the nucleus.Chromatin refers to

loosely condensed DNA & proteins.

NucleusNucleusNucleoli are specialized parts of certain

chromosomes that carry multiple copies of the DNA used to synthesize ribosomal RNA.This rRNA combines with protein to from the two

subunits of ribosomes.Ribosomes leave the nucleus through pores in the

nuclear envelope.

Endoplasmic ReticulumEndoplasmic ReticulumThe nuclear envelope joins with a cytoplasmic

membranous system – the endoplasmic reticulum (ER).Rough endoplasmic reticulum (RER) is

covered with ribosomes. Smooth (SER) is not.

Endoplasmic ReticulumEndoplasmic ReticulumRibosomes on the RER synthesize proteins

that enter the ER that will either be incorporated into the plasma membrane, exported from the cell, or they may be bound for lysosomes.

Lipids and phospholipids are synthesized in the SER.

Golgi ComplexGolgi ComplexThe Golgi complex is a stack of membranous

vesicles where storage, modification, and packaging of protein products occurs.

Assembling & Secreting Assembling & Secreting ProteinsProteins

LysosomesLysosomesLysosomes contain

enzymes (proteins) that can breakdown foreign material like bacteria or worn out cellular components.Contents of lysosome

would kill cell if membrane ruptured.

May pour enzymes into food vacuoles.

Mitochondria Mitochondria Mitochondria are the powerhouses of cells –

they contain enzymes that carry out the energy-yielding steps of aerobic metabolism. ATP is produced here.Composed of a double membrane – the inner

membrane is folded into cristae.Mitochondria are self-replicating, containing their

own circular DNA molecule.

CytoskeletonCytoskeletonEukaryotic cells have a

cytoskeleton that provides support and often locomotion and movement of organelles.Composed of

microfilaments, microtubules, and intermediate filaments.

CytoskeletonCytoskeletonMicrofilaments are made of the proteins actin

and myosin and function in a cell’s ability to contract as seen in muscle cells.Actin microfilaments move molecules and organelles

through the cytoplasm.

CytoskeletonCytoskeletonMicrotubules are

larger tubular structures composed of the protein tubulin.Move

chromosomes during cell division.

Part of the structure of cilia & flagella.

CytoskeletonCytoskeleton

Microtubules radiate out from the centrosome – the microtubule organizing center.Located near nucleus.Not membrane bound.Centrioles are found in

the centrosome.Centrioles composed

of 9 triplets of microtubules.

Replicate before cell division.

CytoskeletonCytoskeletonIntermediate fibers fall in between

microfilaments and microtubules in size.There are five biochemically distinct types of

intermediate fibers.

Cilia & FlagellaCilia & FlagellaCilia & flagella are motile extensions of the

cell surface.In many single celled organisms they are a source of

locomotion.In multicellular animals they usually sweep material

past the fixed cell.Nine pairs of microtubules enclose a central pair. At the base is a basal body - identical to a centriole.

PseudopodiaPseudopodiaSome single-celled organisms, migrating cells

in embryos, and white blood cells show ameboid movement.Cytoplasmic streaming through the action of actin

microfilaments extends a pseudopodium outward.Some have specialized pseudopodia with

microtubules that are assembled & disassembled to allow movement.

http://www.youtube.com/watch?v=zTFsn9xt7b0&feature=player_embedded

JunctionsJunctionsTight junctions form

when cell membranes ofadjacent cells fuse.Function as seals.

Adhesion junctions occur under tight junctions. Transmembrane proteins link across a small space and connect to microfilaments.

JunctionsJunctionsDesmosomes act as spot welds and increase

the strength of the tissue.

Hemidesmosomes are found at the base of cells and anchor them to connective tissue.

Gap junctions are canals between cells that provide intercellular communication.

MicrovilliMicrovilliMicrovilli are small

fingerlike projections that have bundles of actin microfilaments.

They serve to increase the surface area of the tissue as in the intestine.

Membrane FunctionMembrane FunctionMembranes surround the outside of the cell

and the organelles inside it.

The plasma membrane acts as a selective gatekeeper.A substance may cross the membrane:

By diffusionBy a mediated transport systemBy endocytosis

Diffusion & OsmosisDiffusion & OsmosisDiffusion is the movement of molecules from

an area of high concentration to an area of low concentration. This tends to equalize the concentration.Down the concentration gradient.Solutes are molecules (e.g. salt) that are found in a

solution.

Diffusion & OsmosisDiffusion & Osmosis

Cell membranes are selectively permeable – water can pass through, but not most solutes.Gases (oxygen &

carbon dioxide), urea, lipid soluble solutes can cross the membrane.

http://www.youtube.com/watch?v=sdiJtDRJQEc&feature=player_embedded#!

Diffusion & OsmosisDiffusion & OsmosisOsmosis - if there is a membrane between two

solutions with unequal concentration of solutes that can not cross the membrane, water will flow toward the side with less water / more solute until the two sides have equal concentrations.

Diffusion & OsmosisDiffusion & Osmosis

Diffusion & OsmosisDiffusion & OsmosisAnimals utilize osmosis to control internal fluid

and solute levels.The blood of marine fishes has 1/3 the salt content of

the water. They are hypoosmotic to seawater.Freshwater fishes have blood that is saltier than the

water. They are hyperosmotic to the water.If the solute concentrations were the same, the two

solutions would be isoosmotic.

Diffusion Through ChannelsDiffusion Through ChannelsCharged substances,

like water and dissolved ions, can’t simply diffuse across the cell membrane.They pass through

channels created by transmembrane proteins.Some channels always

open.Some are gated

channels.

http://www.youtube.com/watch?v=PYP4JYL-KQk&list=FL9N_Px072WuVorSwDfqf-9w&index=87&feature=plpp_video

Diffusion Through ChannelsDiffusion Through Channels

Gated channels require a signal to open or close them.Chemically-gated channels open or close when a

signaling molecule binds to a binding site on the transmembrane protein.

Voltage-gated channels open or close when the ionic charge across the membrane changes.

Carrier Mediated TransportCarrier Mediated TransportSugars & amino acids must be able to enter

cells and waste products must be able to leave.

These molecules cross the membrane with the help of transporter proteins.Transporter proteins are specific.Facilitated diffusionActive transport

Facilitated DiffusionFacilitated DiffusionIn facilitated diffusion,

the transporter protein binds to the substrate molecule on one side of the plasma membrane then changes shape to release it on the other side.Takes place in the

direction of the concentration gradient.

Active TransportActive TransportActive transport requires energy (ATP) to

transport molecules in the direction opposite the concentration gradient.

http://www.youtube.com/watch?v=STzOiRqzzL4

EndocytosisEndocytosisEndocytosis is the ingestion of material by cells.

Phagocytosis – cell eating – method of feeding by single-celled organisms.

Pinocytosis – small molecules or ions are enclosed in vesicles called caveolae.

Receptor-mediated endocytosis – method of bringing large molecules into a cell with the help of the protein clathrin.

Endocytosis and ExocytosisEndocytosis and Exocytosis

http://www.youtube.com/watch?v=W6rnhiMxtKU&feature=player_embedded#! http://www.youtube.com/watch?v=U9pvm_4-bHg&feature=player_embedded

ExocytosisExocytosisExocytosis - membranes of a vesicle inside

the cell can fuse with the plasma membrane to discharge the contents of the vesicle outside the cell.

Transcytosis – a substance may be picked up on one side of the cell, transported completely across the cell and discharged on the other side.

Mitosis and Cell DivisionMitosis and Cell DivisionMitosis is the process of nuclear cell division

in nonreproductive, or somatic, cells.A fertilized egg, or zygote, divides by mitosis to

produce a multicellular organism.Damaged cells are replaced by mitosis.

ChromosomesChromosomesIn cells that are not dividing, the DNA is loosely

organized so that individual chromosomes can’t be distinguished – it is now referred to as chromatin.

Before division, chromatin becomes more compact and chromosomes can be recognized.

ChromosomesChromosomesAll nonreproductive cells in a species have the

same number of chromosomes.46 in humans

Half of these chromosomes come from each parent.Result is two sets of chromosomes.

Diploid

Chromosome 1 from Mom and chromosome 1 from Dad are called homologous chromosomes.

The Cell CycleThe Cell CycleCells come from

preexisting cells through the process of cell division.

Cell division – mitosis and cytokinesis – occupy a very small portion of the cell cycle.

The Cell CycleThe Cell CycleInterphase includes:

G1 – growth phase where RNA and functional proteins are synthesized.

S – DNA replication.G2 – growth phase where structural proteins are

made.

Mitosis

Cytokinesis

Chromosome StructureChromosome StructureDuring S phase, each

of the 2 homologues replicates, resulting in identical copies called sister chromatids.

Chromatids remain connected at a linkage site called the centromere.

Cell DivisionCell DivisionThere are two phases of cell division:

Mitosis – nuclear cell divisionProphaseMetaphaseAnaphaseTelophase

Cytokinesis – division of the cytoplasm

Multiple nuclear divisions not accompanied by cytokinesis result in a multinucleate cell.

Cell DivisionCell DivisionProphase –

Chromosomes condense enough to be seen with a light microscope.Spindle forms

between the 2 centrioles.

Spindle fibers attach to kinetochores.

Cell DivisionCell DivisionMetaphase –

Alignment of the chromosomes along center of cell (metaphase plate). Fibers attached to

kinetochores on both sides of each chromosome.

Cell DivisionCell DivisionAnaphase –

Separation of the sister chromatids.

Centromere splits apart – sister chromatids move toward opposite poles.Disassembly of the

tubulin subunits shortens the microtubules.

Cell DivisionCell DivisionTelophase – re-

formation of the nuclei once the chromosomes are at opposite poles.Chromosomes

unwind.

Cell DivisionCell DivisionCytokinesis –

division of the cytoplasm.

Two complete, diploid cells that are identical to the original cell.

CytokinesisCytokinesisDuring cytokinesis

in animal cells, the cell pinches in two.A cleavage furrow

produced by microfilaments deepens until the cell splits.

MitosisMitosis

http://www.youtube.com/watch?v=VGV3fv-uZYI