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Cell Structure and Function

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Cell Structure

• In 1655, the English scientist Robert Hooke coined the term “cellulae” for the small box-like structures he saw while examining a thin slice of cork under a microscope.

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Basic Cell StructureBasic Cell Structure

• All cells have the following basic structure: All cells have the following basic structure: • A thin, flexible A thin, flexible plasma membraneplasma membrane

surrounds the entire cell. surrounds the entire cell. • The interior is filled with a semi-fluid The interior is filled with a semi-fluid

material called thematerial called the cytoplasmcytoplasm.. • Also inside are specialized structures Also inside are specialized structures

called organelles and the cell’s called organelles and the cell’s genetic genetic material.material.

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Generalized Eukaryotic Cell

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Visualizing Cells

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Prokaryotic Cells

• Simplest organisms– Cytoplasm is surrounded by plasma membrane and

encased in a rigid cell wall composed of peptidoglycan.– No distinct interior compartments– Some use flagellum for locomotion, threadlike structures

protruding from cell surface

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Eukaryotic Cells

• Characterized by compartmentalization by an endomembrane system, and the presence of membrane-bound organelles.

– central vacuole– vesicles– chromosomes– cytoskeleton– cell walls

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Animal cell anatomyAnimal Cell

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Membrane FunctionMembrane Function• All cells are surrounded All cells are surrounded

by a plasma membrane. by a plasma membrane. • Cell membranes are Cell membranes are

composed of a lipid composed of a lipid bilayer with globular bilayer with globular proteins embedded in the proteins embedded in the bilayer.bilayer.

• On the external surface, On the external surface, carbohydrate groups join carbohydrate groups join with lipids to form with lipids to form glycolipids, and with glycolipids, and with proteins to form proteins to form glycoproteins. These glycoproteins. These function as cell identity function as cell identity markers.markers.

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Fluid Mosaic ModelFluid Mosaic Model• In 1972, S. Singer and G. Nicolson proposed the Fluid In 1972, S. Singer and G. Nicolson proposed the Fluid

Mosaic Model of membrane structureMosaic Model of membrane structureExtracellular fluid

CarbohydrateGlycolipid

Transmembraneproteins

Glycoprotein

Peripheralprotein

Cholesterol

Filaments ofcytoskeleton

Cytoplasm

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PhospholipidsPhospholipids

• GlycerolGlycerol• Two fatty acidsTwo fatty acids• Phosphate groupPhosphate group

Hydrophilicheads

Hydrophobictails

ECF WATER

ICF WATER

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Phospholipid BilayerPhospholipid Bilayer• Mainly 2 layers of phospholipids; the non-polar tails Mainly 2 layers of phospholipids; the non-polar tails

point inward and the polar heads are on the surface.point inward and the polar heads are on the surface.• Contains cholesterol in animal cells.Contains cholesterol in animal cells.• Is fluid, allowing proteins to move around within the Is fluid, allowing proteins to move around within the

bilayer.bilayer.

Polarhydro-philicheads

Nonpolarhydro-phobictails

Polarhydro-philicheads

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Steroid CholesterolSteroid Cholesterol

• Effects on membrane fluidity within Effects on membrane fluidity within the animal cell membranethe animal cell membrane

Cholesterol

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Glycoprotein

Carbohydrate

Microfilamentsof cytoskeleton Cholesterol Peripheral

proteinIntegral

protein

Glycolipid

Membrane ProteinsMembrane Proteins

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

• Peripheral proteins are appendages loosely Peripheral proteins are appendages loosely bound to the surface of the membranebound to the surface of the membrane

Fibers of extracellularmatrix (ECM)

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Integral proteinsIntegral proteins

• Penetrate the hydrophobic core of the Penetrate the hydrophobic core of the lipid bilayerlipid bilayer

• Are often transmembrane proteins, Are often transmembrane proteins, completely spanning the membranecompletely spanning the membrane

EXTRACELLULARSIDE

N-terminus

C-terminus

HelixCYTOPLASMICSIDE

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Functions of Cell MembranesFunctions of Cell Membranes

• Regulate the passage of substance Regulate the passage of substance into and out of cells and between cell into and out of cells and between cell organelles and cytosolorganelles and cytosol

• Detect chemical messengers arriving Detect chemical messengers arriving at the surfaceat the surface

• Link adjacent cells together by Link adjacent cells together by membrane junctionsmembrane junctions

• Anchor cells to the extracellular Anchor cells to the extracellular matrixmatrix

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6 Major Functions Of Membrane 6 Major Functions Of Membrane ProteinsProteins

1. Transport. (left) A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. (right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy ssource to actively pump substances across the membrane

2. Enzymatic activity. A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway.

3. Signal transduction. A membrane protein may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signal) may cause a conformational change in the protein (receptor) that relays the message to the inside of the cell.

ATP

Enzymes

Signal

Receptor

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Cell-cell recognition. Some glyco-proteins serve as identification tags that are specifically recognized by other cells.

Intercellular joining. Membrane proteins of adjacent cellsmay hook together in various kinds of junctions, such asgap junctions or tight junctions

Attachment to the cytoskeleton and extracellular matrix(ECM). Microfilaments or other elements of thecytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes

4.

5.

6.

Glyco-protein

6 Major Functions Of Membrane Proteins6 Major Functions Of Membrane Proteins

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Outside

Plasmamembrane

InsideTransporter Cell surface

receptorEnzyme

Cell surface identitymarker

Attachment to thecytoskeletonCell adhesion

Functions of Plasma Membrane Proteins

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Membrane TransportMembrane Transport

• The plasma membrane is the boundary that The plasma membrane is the boundary that separates the living cell from its nonliving separates the living cell from its nonliving surroundingssurroundings

• In order to survive, A cell must exchange In order to survive, A cell must exchange materials with its surroundings, a process materials with its surroundings, a process controlled by the plasma membranecontrolled by the plasma membrane

• Materials must enter and leave the cell through Materials must enter and leave the cell through the plasma membrane.the plasma membrane.

• Membrane structure results in selective Membrane structure results in selective permeability, it allows some substances to cross permeability, it allows some substances to cross it more easily than othersit more easily than others

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Membrane TransportMembrane Transport

• The plasma membrane exhibits selective The plasma membrane exhibits selective permeability - It allows some substances permeability - It allows some substances to cross it more easily than othersto cross it more easily than others

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Passive TransportPassive Transport

• Passive transport is diffusion of a Passive transport is diffusion of a substance across a membrane with no substance across a membrane with no energy investmentenergy investment

• 4 types4 types• Simple diffusionSimple diffusion• DialysisDialysis• OsmosisOsmosis• Facilitated diffusionFacilitated diffusion

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Solutions and TransportSolutions and Transport

• Solution – homogeneous mixture of two or more components• Solvent – dissolving medium• Solutes – components in smaller quantities

within a solution• Intracellular fluid – nucleoplasm and

cytosol• Extracellular fluid

• Interstitial fluid – fluid on the exterior of the cell within tissues

• Plasma – fluid component of blood

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DiffusionDiffusion• The net movement of a substance from an area of higher The net movement of a substance from an area of higher

concentration to an area of lower concentration - down a concentration to an area of lower concentration - down a concentration gradientconcentration gradient

• Caused by the constant random motion of all atoms and moleculesCaused by the constant random motion of all atoms and molecules• Movement of individual atoms & molecules is random, but each Movement of individual atoms & molecules is random, but each

substance moves down its own concentration gradient.substance moves down its own concentration gradient.

Lumpof sugar

No net movement at equilibrium

Random movement leads to net movement down a concentration gradient

Water

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Diffusion Across a MembraneDiffusion Across a Membrane• The membrane has pores large enough for the molecules to pass The membrane has pores large enough for the molecules to pass

through.through.• Random movement of the molecules will cause some to pass Random movement of the molecules will cause some to pass

through the pores; this will happen more often on the side with more through the pores; this will happen more often on the side with more molecules. The dye diffuses from where it is more concentrated to molecules. The dye diffuses from where it is more concentrated to where it is less concentrated where it is less concentrated

• This leads to a dynamic equilibrium: The solute molecules continue This leads to a dynamic equilibrium: The solute molecules continue to cross the membrane, but at equal rates in both directions.to cross the membrane, but at equal rates in both directions.

Net diffusion Net diffusion Equilibrium

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Diffusion Across a MembraneDiffusion Across a Membrane• Two different solutes are separated by a membrane that is Two different solutes are separated by a membrane that is

permeable to both permeable to both • Each solute diffuses down its own concentration gradient.Each solute diffuses down its own concentration gradient.• There will be a net diffusion of the purple molecules toward the left, There will be a net diffusion of the purple molecules toward the left,

even though the total solute concentration was initially greater on even though the total solute concentration was initially greater on the left sidethe left side

Net diffusion

Net diffusion

Net diffusion

Net diffusion Equilibrium

Equilibrium

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The Permeability of the Lipid BilayerThe Permeability of the Lipid Bilayer• Permeability FactorsPermeability Factors

• Lipid solubilityLipid solubility• SizeSize• ChargeCharge• Presence of channels and transportersPresence of channels and transporters

• Hydrophobic molecules are lipid soluble and can Hydrophobic molecules are lipid soluble and can pass through the membrane rapidlypass through the membrane rapidly

• Polar molecules do not cross the membrane Polar molecules do not cross the membrane rapidlyrapidly

• Transport proteins allow passage of hydrophilic Transport proteins allow passage of hydrophilic substances across the membranesubstances across the membrane

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Passive Transport ProcessesPassive Transport Processes• 3 special types of diffusion

that involve movement of materials across a semipermeable membrane

• Dialysis/selective diffusion of solutes

• Lipid-soluble materials• Small molecules that

can pass through membrane pores unassisted

• Facilitated diffusion - substances require a protein carrier for passive transport

• Osmosis – simple diffusion of water

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OsmosisOsmosis• Diffusion of the solvent across a Diffusion of the solvent across a

semipermeable membrane.semipermeable membrane.• In living systems the solvent is In living systems the solvent is

always water, so biologists always water, so biologists generally define osmosis as the generally define osmosis as the diffusion of water across a diffusion of water across a semipermeable membrane:semipermeable membrane:

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Lowerconcentrationof solute (sugar)

Higherconcentrationof sugar

Same concentrationof sugar

Selectivelypermeable mem-brane: sugar mole-cules cannot passthrough pores, butwater molecules can

More free watermolecules (higher

concentration)

Water moleculescluster around sugar molecules

Fewer free watermolecules (lowerconcentration)

Water moves from an area of higher free water concentration to an area of lower free water concentration

Osmosis

OsmosisOsmosis

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Osmotic PressureOsmotic Pressure

• Osmotic pressure of a solution is the Osmotic pressure of a solution is the pressure needed to keep it in equilibrium pressure needed to keep it in equilibrium with pure H20.with pure H20.

• The higher the concentration of solutes in The higher the concentration of solutes in a solution, the higher its osmotic pressure.a solution, the higher its osmotic pressure.

• Tonicity is the ability of a solution to cause Tonicity is the ability of a solution to cause a cell to gain or lose water – based on the a cell to gain or lose water – based on the concentration of solutesconcentration of solutes

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TonicityTonicity• If 2 solutions have equal [solutes], they are called If 2 solutions have equal [solutes], they are called

isotonicisotonic• If one has a higher [solute], and lower [solvent], is If one has a higher [solute], and lower [solvent], is

hypertonichypertonic• The one with a lower [solute], and higher [solvent], is The one with a lower [solute], and higher [solvent], is

hypotonichypotonic

Hypotonic solution Isotonic solution Hypertonic solution

H2O H2O H2O H2O

Lysed Normal Shriveled

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Water Balance In Cells With WallsWater Balance In Cells With Walls

Plant cell. Plant cells are turgid (firm) and generally healthiest ina hypotonic environ-ment, where theuptake of water iseventually balancedby the elastic wallpushing back on thecell.

(b)

H2OH2OH2OH2O

Turgid (normal) Flaccid Plasmolyzed

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My definition of OsmosisMy definition of Osmosis

• Osmosis is the diffusion of water Osmosis is the diffusion of water across a semi-permeable membrane across a semi-permeable membrane from a hypotonic solution to a from a hypotonic solution to a hypertonic solutionhypertonic solution

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Facilitated Diffusion Facilitated Diffusion • Diffusion of solutes through a semipermeable membrane with the Diffusion of solutes through a semipermeable membrane with the

help of special transport proteins i.e. large polar molecules and ions help of special transport proteins i.e. large polar molecules and ions that cannot pass through phospholipid bilayer.that cannot pass through phospholipid bilayer.

• Two types of transport proteins can help ions and large polar Two types of transport proteins can help ions and large polar molecules diffuse through cell membranes:molecules diffuse through cell membranes:• Channel proteins – provide a narrow channel for the substance to pass Channel proteins – provide a narrow channel for the substance to pass

through.through.• Carrier proteins – physically bind to the substance on one side of Carrier proteins – physically bind to the substance on one side of

membrane and release it on the other.membrane and release it on the other.

EXTRACELLULARFLUID

Channel protein Solute

CYTOPLASMCarrier protein

Solute

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Facilitated DiffusionFacilitated Diffusion• SpecificSpecific – each channel or carrier – each channel or carrier

transports certain ions or molecules onlytransports certain ions or molecules only• PassivePassive – direction of net movement is – direction of net movement is

always down the concentration gradientalways down the concentration gradient• SaturatesSaturates – once all transport proteins are – once all transport proteins are

in use, rate of diffusion cannot be in use, rate of diffusion cannot be increased furtherincreased further

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Active TransportActive Transport

• Uses energy (from ATP) to move a Uses energy (from ATP) to move a substance against its natural tendency e.g. substance against its natural tendency e.g. up a concentration gradient.up a concentration gradient.

• Requires the use of carrier proteins Requires the use of carrier proteins (transport proteins that physically bind to (transport proteins that physically bind to the substance being transported).the substance being transported).

• 2 types: 2 types: • Membrane pump (protein-mediated active Membrane pump (protein-mediated active

transport)transport)• Coupled transport (cotransport).Coupled transport (cotransport).

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Membrane PumpMembrane Pump

• A carrier protein uses energy from ATP to A carrier protein uses energy from ATP to move a substance across a membrane, up move a substance across a membrane, up its concentration gradient:its concentration gradient:

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• One type of active transport systemOne type of active transport system

The Sodium-potassium PumpThe Sodium-potassium Pump

2. Na+ binding stimulatesphosphorylation by ATP.

1. Cytoplasmic Na+ binds to the sodium-potassium pump.

6. K+ is released and Na+

sites are receptive again; the cycle repeats.

3. Phosphorylation causes the protein to change its conformation, expelling Na+ to the outside.

4. Extracellular K+ binds to the protein, triggering release of the Phosphate group.

5. Loss of the phosphaterestores the protein’s original conformation.

P

EXTRACELLULARFLUID

Na+

CYTOPLASM

[Na+] low[K+] high

Na+

Na+

Na+

Na+

Na+

P ATP

Na+

Na+

Na+

P

ADP

K+

K+

K+

K+ K+

K+

[Na+] high[K+] low

P i

P i

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Coupled transportCoupled transport• 2 stages: 2 stages:

• Carrier protein uses ATP to move a substance across the Carrier protein uses ATP to move a substance across the membrane against its concentration gradient. Storing energy.membrane against its concentration gradient. Storing energy.

• Coupled transport protein allows the substance to move down its Coupled transport protein allows the substance to move down its concentration gradient using the stored energy to move a concentration gradient using the stored energy to move a second substance up its concentration gradient:second substance up its concentration gradient:

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Review: Passive And Active Transport ComparedReview: Passive And Active Transport Compared

Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane.

Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP.

Diffusion. Hydrophobicmolecules and (at a slow rate) very small uncharged polar molecules can diffuse through the lipid bilayer.

Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins,either channel or carrier proteins.

ATP

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Bulk TransportBulk Transport

• Allows small particles, or groups of Allows small particles, or groups of molecules to enter or leave a cell molecules to enter or leave a cell without actually passing through the without actually passing through the membrane.membrane.

• 2 mechanisms of bulk transport: 2 mechanisms of bulk transport: endocytosis and exocytosis.endocytosis and exocytosis.

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EndocytosisEndocytosis• The plasma membrane envelops small The plasma membrane envelops small

particles or fluid, then seals on itself to particles or fluid, then seals on itself to form a vesicle or vacuole which enters the form a vesicle or vacuole which enters the cell:cell:• PhagocytosisPhagocytosis• PinocytosisPinocytosis• Receptor-Mediated Endocytosis -Receptor-Mediated Endocytosis -

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Three Types Of EndocytosisThree Types Of Endocytosis

EXTRACELLULARFLUID

PseudopodiumCYTOPLASM

“Food” or other particle

Foodvacuole

1 µm

Pseudopodiumof amoeba

Bacterium

Food vacuole

An amoeba engulfing a bacterium viaphagocytosis (TEM).PINOCYTOSIS

Pinocytosis vesiclesforming (arrows) ina cell lining a smallblood vessel (TEM).

0.5 µm

In pinocytosis, the cell “gulps” droplets of extracellular fluid into tinyvesicles. It is not the fluiditself that is needed by the cell, but the molecules dissolved in the droplet. Because any and all included solutes are taken into the cell, pinocytosisis nonspecific in the substances it transports.

Plasmamembrane

Vesicle

In phagocytosis, a cellengulfs a particle by Wrapping pseudopodia around it and packaging it within a membrane-enclosed sac large enough to be classified as a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes.

PHAGOCYTOSIS

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Process of PhagocytosisProcess of Phagocytosis

460.25 µm

Ligand

Coat protein

Coatedpit

Coatedvesicle

Receptor

A coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs).

Plasmamembrane

Coatprotein

Receptor-mediated endocytosis enables the cell to acquire bulk quantities of specific substances, 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 already clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coat proteins. Extracellular substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules. Notice that there are relatively more bound molecules (purple) inside the vesicle, other molecules (green) are also present. After this ingested material is liberated from the vesicle, the receptors are recycled to the plasma membrane by the same vesicle.

Receptor-mediated EndocytosisReceptor-mediated Endocytosis

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Exocytosis Exocytosis

• The reverse of endocytosisThe reverse of endocytosis• During this process, the membrane of a vesicle During this process, the membrane of a vesicle

fuses with the plasma membrane and its fuses with the plasma membrane and its contents are released outside the cell:contents are released outside the cell:

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Cell Junctions• Long-lasting or permanent connections between

adjacent cells, 3 types of cell junctions:

Tight junctions prevent fluid from moving across a layer of cells

Tight junction

0.5 µm

1 µm

Spacebetweencells

Plasma membranesof adjacent cells

Extracellularmatrix

Gap junction

Tight junctions

0.1 µm

Intermediatefilaments

Desmosome

Gapjunctions

At tight junctions, the membranes ofneighboring cells are very tightly pressedagainst each other, bound together byspecific proteins (purple). Forming continu-ous seals around the cells, tight junctionsprevent leakage of extracellular fluid acrossA layer of epithelial cells.

Desmosomes (also called anchoringjunctions) function like rivets, fastening cellsTogether into strong sheets. IntermediateFilaments made of sturdy keratin proteinsAnchor desmosomes in the cytoplasm.

Gap junctions (also called communicatingjunctions) provide cytoplasmic channels fromone cell to an adjacent cell. Gap junctions consist of special membrane proteins that surround a pore through which ions, sugars,amino acids, and other small molecules maypass. Gap junctions are necessary for commu-nication between cells in many types of tissues,including heart muscle and animal embryos.

TIGHT JUNCTIONS

DESMOSOMES

GAP JUNCTIONS

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Nucleus

NucleusNucleolus

Chromatin

Nuclear envelope:Inner membraneOuter membrane

Nuclear pore

Rough ER

Porecomplex

Surface of nuclear envelope.

Pore complexes (TEM). Nuclear lamina (TEM).

Close-up of nuclearenvelope

Ribosome

1 µm

1 µm0.25 µm

The Nucleus And The Nuclear Envelope• Repository for genetic material called chromatin - DNA and proteins• Nucleolus: holds chromatin and ribosomal subunits - region of intensive

ribosomal RNA synthesis• Nuclear envelope: Surface of nucleus bound by two phospholipid bilayer

membranes - Double membrane with pores• Nucleoplasm: semifluid medium inside the nucleus

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Chromosomes• DNA of eukaryotes is divided into linear

chromosomes.– Exist as strands of chromatin, except

during cell division– Histones associated packaging proteins

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Ribosomes• Ribosomes are RNA-protein complexes composed of two

subunits that join and attach to messenger RNA.– Site of protein synthesis– Assembled in nucleoli

ERRibosomes Cytosol

Free ribosomes

Bound ribosomes

Largesubunit

Smallsubunit

TEM showing ER and ribosomes Diagram of a ribosome

0.5 µm

Endoplasmic reticulum (ER)

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Endomembrane System• Compartmentalizes cell, channeling passage

of molecules through cell’s interior.– Endoplasmic reticulum

Rough ER - studded with ribosomes Smooth ER - few ribosomes

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Rough ER• Rough ER is especially abundant in cells that secrete proteins.

– As a polypeptide is synthesized on a ribosome attached to rough ER, it is threaded into the cisternal space through a pore formed by a protein complex in the ER membrane.

– As it enters the cisternal space, the new protein folds into its native conformation.– Most secretory polypeptides are glycoproteins, proteins to which a carbohydrate is

attached.– Secretory proteins are packaged in transport vesicles that carry them to their next stage.

• Rough ER is also a membrane factory.– Membrane-bound proteins are synthesized directly into the membrane.– Enzymes in the rough ER also synthesize phospholipids from precursors in the cytosol.– As the ER membrane expands, membrane can be transferred as transport vesicles to other

components of the endomembrane system.

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Smooth ER• The smooth ER is rich in enzymes and plays a role in a variety of metabolic processes.• Enzymes of smooth ER synthesize lipids, including oils, phospholipids, and steroids.• These include the sex hormones of vertebrates and adrenal steroids.• In the smooth ER of the liver, enzymes help detoxify poisons and drugs such as

alcohol and barbiturates.• Smooth ER stores calcium ions.

Muscle cells have a specialized smooth ER that pumps calcium ions from the cytosol and stores them in its cisternal space.

When a nerve impulse stimulates a muscle cell, calcium ions rush from the ER into the cytosol, triggering contraction.

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The Golgi apparatus• The Golgi apparatus is the shipping and receiving center for cell

products.– Many transport vesicles from the ER travel to the Golgi apparatus for

modification of their contents.– The Golgi is a center of manufacturing, warehousing, sorting, and

shipping.– The Golgi apparatus consists of flattened membranous sacs—

cisternae—looking like a stack of pita bread.– The Golgi sorts and packages materials into transport vesicles.

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Functions Of The Golgi Apparatus

TEM of Golgi apparatus

cis face(“receiving” side ofGolgi apparatus)

Vesicles movefrom ER to Golgi Vesicles also

transport certainproteins back to ER

Vesicles coalesce toform new cis Golgi cisternae

Cisternalmaturation:Golgi cisternaemove in a cis-to-transdirection

Vesicles form andleave Golgi, carryingspecific proteins toother locations or tothe plasma mem-brane for secretion

Vesicles transport specificproteins backward to newerGolgi cisternae

Cisternae

trans face(“shipping” side ofGolgi apparatus)

0.1 0 µm16

5

2

3

4

Golgiapparatus

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Membrane Bound Organelles

• Lysosomes – vesicle containing digestive enzymes that break down food/foreign particles

• Vacuoles – food storage and water regulation

• Peroxisomes - contain enzymes that catalyze the removal of electrons and associated hydrogen atoms

(a) Phagocytosis: lysosome digesting food

1 µm

Lysosome containsactive hydrolyticenzymes

Food vacuole fuses with lysosome

Hydrolyticenzymes digestfood particles

Digestion

Food vacuole

Plasma membraneLysosome

Digestiveenzymes

Lysosome

Nucleus

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Mitochondria• Sites of cellular respiration, ATP synthesis• Bound by a double membrane surrounding fluid-filled matrix.• The inner membranes of mitochondria are cristae• The matrix contains enzymes that break down carbohydrates and

the cristae house protein complexes that produce ATP

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Cytoskeleton• The eukaryotic cytoskeleton is a network of

filaments and tubules that extends from the nucleus to the plasma membrane that support cell shape and anchor organelles.

• Protein fibers– Actin filaments

cell movement– Intermediate filaments– Microtubules

centrioles

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Centrioles

• Centrioles are short cylinders with a 9 + 0 pattern of microtubule triplets.

• Centrioles may be involved in microtubule formation and disassembly during cell division and in the organization of cilia and flagella.

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Cilia and Flagella• Contain specialized arrangements of microtubules• Are locomotor appendages of some cells• Cilia and flagella share a common ultrastructure

(a)

(c)

(b)

Outer microtubuledoublet

Dynein arms

CentralmicrotubuleOuter doublets cross-linkingproteins inside

Radialspoke

Plasmamembrane

Microtubules

Plasmamembrane

Basal body

0.5 µm

0.1 µm

0.1 µm

Cross section of basal body

Triplet

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Cilia and Flagella• Cilia (small and numerous) and flagella (large and single)

have a 9 + 2 pattern of microtubules and are involved in cell movement.

• Cilia and flagella move when the microtubule doublets slide past one another.

• Each cilium and flagellum has a basal body at its base.

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(a) Motion of flagella. A flagellum usually undulates, its snakelike motion driving a cell in the same direction as the axis of the flagellum. Propulsion of a human sperm cell is an example of flagellatelocomotion (LM).

1 µm

Direction of swimming

Cilia and Flagella

(b) Motion of cilia. Cilia have a back- and-forth motion that moves the cell in a direction perpendicular to the axis of the cilium. A dense nap of cilia, beating at a rate of about 40 to 60 strokes a second, covers this Colpidium, a freshwater protozoan (SEM).

15 µm