Cell Biology 1:Membranes, Cytoplasm and Nucleus

Medical Microanatomy 602Edie C. Goldsmith, Ph.D.

M=mitochondriaER=rER N=nucleusNE=nuclear membraneG=golgi apparatusV=vesicles PM=plasma membraneC=neighboring cell IS=intercellular spaceL=lysosome See Table 2.1 pg 24-25 in your textbook

Plasma Membrane8-10 nm thickFunctionSelective barrierCell adhesionCommunicationThree main componentsPhospholipidsProteins (integral or peripheral)Cholesterol

Modified Fluid Mosaic Model

LipidsMembranes usually have 40-60% lipidType variesPhosphatidylcholine, phosphatidylserine, phosphatidylethanolamineglycolipidsPolar and non-polar groupsSaturated vs unsaturatedNot uniform distributionrafts

Lipid RaftsEnriched in Cholesterol and Sphingolipids

Less fluid than typical membrane

Due to the amphipathic nature of lipids - self associateMicellesBilayersLipids can move within a bilayer

CholesterolIntegrates into bilayerAmount in PM can vary by cell type

Modulates membrane fluidity

Membrane Proteins~50% of membrane (w/w)Two general typesIntegralPeripheralProteins are also asymmetrically distributed with respect to the lipid bilayer

Multiple categories of integral membrane proteinsPumps and enzymes (ion pumps)Channels (gap junctions)ReceptorsLinker (integrins)Carrier proteinsStructural ProteinsGlycoproteins and glycolipidsglycocalyx

Freeze-fractureThe interior of the lipid bilayer is weak and readily split by simple cleavage.Membrane proteins will stay with one or the other bilayer forming bumps and leaving pits.

Freeze FractureThe two faces of the cleaved lipid bilayer may be examined using freeze fracture to help determine membrane protein distribution.Imaged by TEM

TEM vs. Freeze FractureTop: Gap junction between plasma membranes of two cells

Middle/bottom: same structure viewed en face using freeze fracture

Transport MechanismsDiffusion - Passive and FacilitatedPassive concentration gradientGases, lipids, lipid soluble molecules (i.e. EtOH) Facilitated concentration dependentRequire protein carrier molecule; does not require energyGlucose and amino acids (hydrophilic)Active transportRequires energy (ATP)Movement against gradientNa+-K+ATPaseChannel ProteinsSmall, water soluble moleculesProteins create pore in membrane; ion selectiveRegulated by membrane potential (voltage-gated), neurotransmitters (ligand-gated), mechanical stress

Transport MechanismsVesicular transportEndocytosisPinocytosisReceptor mediated endocytosisPhagocytosisExocytosis

Receptor-mediated EndocytosisReceptor mediatedLDL, transferrin/Fe, EGFClathrin dependentAssemble on cytoplasmic side of membraneWeave-like networkCoated pitsFuse with endosomesdynamin

PhagocytosisMacrophages and polymorphonuclear leukocytesEngulfment of large particles~ 250 nmBacteria, protozoa, fungi, damaged cellsNon-biological debrisClathrin independent actin dependentReceptor mediatedFc receptors

Pinocytosiscell drinkingClathrin independentSmooth vesicles80-150 nmIntake of fluid and small moleculesUsually fuse with lysosomesBulk transfer (endothelial cells)

ExocytosisMechanism for releasing molecules into extracellular spaceTwo pathwaysConstitutiveAll cellsSecreted immediately; No granulesSecretorySpecialized cellsProteins to be secreted stored in vesicles

CytoplasmAqueous matrix containing organellesAbout total volume of cell

Cytoskeleton

Site of many metabolic pathwaysGlycolysis, protein synthesis

Nucleus3-10 mmUsually largest organelleFunctionGenome, production of ribosomes and mRNAMultiple nucleiNot always in center of cellNot always roundMultiple componentsChromatinNucleolusNuclear envelopNucleoplasm

Reticulocytes lose the nucleus entirely to become mature RBCsMuscle cells have multiple nuclei

ChromatinDNA-protein complexhistonesBasophilic staining of nucleiHeterogenousHeterochromatin - highly condensed (dense appearance)Cell inactiveEuchromatin - dispersed (light appearance)Active cell

Some cells are very inactive, and their nuclei contain mostly heterochromatin.This is a late red blood cell precursor

NucleusThe nucleus shown in this image appears in four parts.

Can cells have multiple nuclei? Does this cell have multiple nuclei?

ChromosomesSee Folder 3.1 pg 80-81

NucleolusNo membraneBasophilicThree components Fibrillar material/region (Pars fibrosa) active transcription of rRNA genes; lots of rRNAGranular material/region (Pars granulosa) ribosomal assemblyFibrillar centers regions of DNA that code for rRNA, RNA Pol I, transcription factorsNucleolar organizer regionNOR

Nucleoli are the site of rRNA synthesisMay vary greatly in size and appearanceActive cells may contain multiple nucleoli

Light microscopy of Primary Oocytes

Nuclear EnvelopeTwo membranes separated by narrow space (perinuclear cisterna)Outer membrane contiguous with rERInner membrane associated with nuclear laminaLamins (A and C) provide stability and association points for chromosomesLamin mutations can affect striated muscle, adipose tissue, nerve/skeletal development

Nuclear Pores70 - 80 nm openingsNumber and distribution varyOnly site of direct molecular exchange with cytoplasm

Freeze fracture demonstrating distribution of nuclear pores

> than 50 subunitsRegulates bidirectional protein translocationIons & < 9 daltons freelyLarge proteins/complexes, RNA, ribosomal subunits require GTPRequire nuclear localization signal (NLS) to get inNuclear export sequence (NES) to get out

Cell CycleCell populationStatic - no divisionStable - sporadicRenewing - regular divisionSlow (smooth muscle) or rapid (dermal fibroblasts)Cell CycleMitosisInterphase

InterphaseThree phasesG1Longest phase (9-12 hours)Two checkpoints restriction point and DNA damage checkpointS7.5-10 hoursDNA damage checkpoint fidelity of replicationG23.5-4.5 hoursCell growth and organelle organizationTwo checkpoints Unreplicated DNA and DNA damage G0/Terminal Differentiation

MitosisFour stages (~ 1hr)Two checkpoints spindle assembly (anaphase) & chromosome segregation (prior to cytokinesis)ProphaseChromosomes condenseLoss of nucleolus and nuclear membraneMetaphaseAssembly of mitotic spindleChromosomes migrate to equatorial planeAnaphaseChromatids separate and migrate toward polesTelophaseNuclear membrane and nucleoli reappear; cytokinesisIPMA

Cell Cycle RegulationEntry into different phases is controlled by cyclins and cyclin-dependent kinases (Cdks)2-protein complexSynthesis oscillates up and down during cell cycle

ApoptosisNormal part of developmentCharacteristicsDNA fragmentationDecrease in cell & nucleus sizeLoss of mitochondrial functionMembrane blebbingApoptotic bodiesNecrosisPathologic processCharacteristicsDamage to plasma membraneSwellingCell lysisInflammatory response

Apoptosis vs Necorsis

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