Eukaryotic Cells

Generalized eukaryotic cells and the organelles of eukaryotic cells. Essentials Ch 1 and 2

The Eukaryotic Cell

All cells of the human body are eukaryotic cells

Each contains a nucleus (as you may know RBC do lose their nucleus, but develop from nucleated cells)

Eukaryotic cells are more complex than the other general type of cell; the bacterial cell

A typical eukaryotic cell

No such thing as typical Eukaryotic Cell

The cells of the body are quite different in appearance and function from one tissue type to another

They share many organelles common, but have unique character as to how they utilize the complement of organelles

In discussions of tissues always examine the use each cell puts to its organelles

Scale

Nuclear Text reference

Junqueria p.48 -54 Relevant figures 3-1 - 3-8

Essentials

Nucleus

Nucleus structure Nuclear envelope Nucleolus Nucleoplasm

ChromatinDNANuclear laminaNucleoskeleton

Nuclear Pore

Functions of Nucleus

Repository for the information that is required to run the cell

DNA is the molecule in the nucleus which contains the information

Information containing regions are referred to as genes

Multiple genes reside on linear chromosomes

Functions of Nucleus 2

Nucleus also functions in faithfully copying the DNA information and passing it to the next generation

Nucleus is fundamental in beginning the process of Gene Expression whereby the information is accessed and utilized to produce rRNA, tRNA and Proteins

Flow of Information

Nuclear Envelope

Visible using light microscopeComplex structure under EM

Double lipid bilayer• Inner membrane• Outer membrane

Covered on cytoplasmic surface with polyribosomes

Internal surface nuclear lamina Nuclear cisternae (Nuclear space)

Nucleolus

Site where genes for ribosomal RNA is amplified (hence the intense staining)

Multiple copies of rRNA genes are expressed at High level by RNA Polymerase I

Sub-assembly of small and large ribosomal subunits occurs here

Nucleolar Organizing Regions - Nucleolus 2

Regions within Nucleolus Nucleolar organizer DNA - rRNA genes Associated with this NOR DNA is pars fibrosa

Pars fibrosa is composed of primary transcripts of rRNA genes

Third region is pars granulosa15-20 nm particles which contain the maturing

subassembled ribosomes

Nucleoplasm

Chromatin Euchromatin - diffuse lighter staining Heterochromatin - dense darker staining

Nuclear Laminins Intermediate filaments - nucleoskeleton Have a chromatin binding function Thought to organize chromatin in the to aid in

gene expression

Structure of Chromatin

Chromatin is made up of DNA and primarily histone proteins

Structural organizationLevels of Chromatin packing p252

Nuclear Pore Complex

Formed where there is a fusion of two membranes

Intricate protein structure called NPCDiameter of entire complex 120 nmMass 120 million DaConsists of 100 or more different

polypeptides

Nuclear Pore Complex 2

Structure has an octagonal arrayRings of eight subunitsA central granule is prominentThis central granule called transporter has

a poreTransporter pore is much smaller than the

overall size of the NPC

Nuclear Pore Complex 3

Proteins smaller than 60,000 Da can diffuse through the pores Histone proteins average 20,000 Da and

therefore freely diffuse back into the nucleus

Proteins larger than 60,000 Da require specialized signals and also require docking and ATP energy

Nuclear Pore Complex 4

Nuclear localization signal - an example of how the transporter works for large proteins

8 - 30 amino acids rich in arginine and lysine in a protein that is to be imported back into the nucleus

Cytosolic receptor protein recognizesDocking with NPCTransporter opens like iris diaphragm using ATP

and passes protein into the nucleoplasm

Nuclear Matrix

Component that fills the space between chromatin fibers Composed of proteins, metabolites and ions If we remove the proteins, metabolites and

ions, remainder is nucleoskeleton Nucleoskeleton continous with the nuclear

lamina (Intermediate filaments class of cytoskeletal elements)

Karyotype

The condensed chromosomes of of the dividing cell (metaphase chr) are used in karyotyping individuals

Essentials Fig 8-2 p. 247Junqueria fig 3-7 p. 57 Note the chromosomes are ordered based

on size

G - banding

It is sometimes difficult to tell chromosomes of similar size apart

Giemsa’s stain produces unique banding patterns to individual chromosomes

Fluorescent dyes have also been used and now multiple fluorescent stains are being used to give cytogeneticists even more information

Plasma Membrane

General characteristics 7.5 to 10 nm across, surrounding the cell Composed of Phospholipids, Cholesterol,

other lipids and ProteinsProteins are found either running into or through

the lipid bilayer = integral or intrinsic OR the protein may be associated with either leaflet of the lipid bilayer = peripheral or extrinsic.

All lipids present have amphipathic nature

Plasma Membrane Functions

Membranes are indispensable to lifePrimary function is to form the barrier

between outside an inside the cellMembranes are highly selective

permeability barriersWhat goes out of and comes into the cell

is controlled by specific proteins

Key Membrane Functions

Key Membrane Functions

Create & MaintainGradients

[ion] differences fromone side to the other

Regulate flow ofinformation

Receptors – bindsignal (SignalTransduction)

Convert energy conversion of oneform of energy toanother (ETS)

Fluid Mosaic Model

Overall organization of the PM is described as a fluid mosaic A sea of amphipathic lipids with proteins floating within and adhering to the lipids

Variation in lipidsGlycolipids & glycoproteins are restricted

to outer leaflet ; form glycocalyx

Peripheral vs Integral Proteins

PM Transport Processes

Passive transport Simple diffusion - few

molecules Facilitated diffusion -

ion channel or carrier protein

PM Transport Processes 2

Active Transport - require energy Example Na+ K+ ATPase pump Antiport - two ions or molecules move in

opposite directions (Na+/K+ ATPase) Symport- two ions or molecules move in

same direction (Glucose symport) Uniport- single ion or molecule move in a

single direction (Ca+2 ATPase pump)

Cytoplasm or Cytosol

Bounded by PM outside and ending at membrane of various membranous organelles

Aqueous environment full of Enzymes - example glycolytic enzymes Organelles Inclusions Cytoskeletal elements

Organelles in Cytosol

Ribosomes Structure

Made up of rRNA and ribosomal proteinsHas two subunits - large and smallSizes of the two subunits determined by

sedimentation in a density gradientUnits are Svedberg’s or simply designated S

Individual or in Polyribosome complexes Function Translation

Organelles in Cytosol 2

Endoplasmic reticulum Structure- membranous and continous with

outer lipid bilayer of nuclear envelope Two forms

Rough ER - membranous labyrinth which has an inner space referred to as the lumen; typically studded with ribosomes

Smooth ER- tubular membranous network which also has a lumen; typically has no ribosomes

Organelles in Cytosol 3

Rough ER Function Start the process of Membrane-packaged

proteins Works in concert with Golgi Complex Produces

Secretory proteinsIntegral membrane proteinsLysosomal proteins

Organelles in Cytosol 3

Smooth ER Function Steroid hormone synthesis occurs in SER rich

cells (Leydig cells of the testis) Drug detoxification - hepatocytes produces

elaborated SER in response to drugs like phenobarbital

Muscle Contraction and Relaxation involves release of Ca+2 ions from sarcoplasmic ret.

Organelles in cytosol 4

Mitochondria (pl..) Mitochondrion (sing.) Structure - double membrane bound

organelle inner and outer membranesHas its own DNA and ribosomesDNA codes for some of the proteins of ETS and

tRNA’sOther proteins are coded for by nucleusDue to structure its has an intermembrane spaceThe inner most space is the matrix

Mitochondria Functions

Structure helps to impart functionEnzymes of TCA cycle and -oxidation of

fatty acidsETS carrier proteins and ATP synthaseGenetic apparatus that codes for some of

the above functions

Organelles in cytosol 5

Golgi complex Structure - series of disk-shaped cisternae or

saccules arranged in stacks Typical stack has flattened center and

rounded edge rims There is a distinct polarity to the stacks

Cis face is the forming face (RER --> cis-face)Trans face is the exiting face (Golgi -->

destination)

Golgi functions

The Golgi complex processes membrane - packaged proteins synthesized in the RER Processing involves some proteins being

glycosylated All proteins being sorted into groups to be

shipped to a final functional destination Membrane recycling also occurs

Organelles in cytosol 6

Clathrin Coated vesicles Clathrin is a protein with a Triskelion structure Clathrin is involved in receptor mediated

endocytosis Clathrin molecules form a molecular cage

around receptor coated pits and eventually around the vesicle that is brought into the cell

Organelles in cytosol 7

Coatomer vesicles Coats vesicles - NOT IN CAGE FORM Coatomer is a large protein complex formed

by individual coat protein subunits called COP’s

Coatomer proteins mediate continous constitutive protein transport within cells

Represents a default system for bulk flow

Organelles in cytosol 8

Lysosomes Structure - dense membrane bound organelle

with diverse shapes and sizes; identified by presence of acid phosphatase

Function - over 40 hydrolases (pH ~ 5)Digest materials brought into the cellDigest aged organelles marked for destructionApoptosis - cell suicide

Organelles in cytosol 9

Peroxisomes (Microbodies) Structure - membrane bound spherical to

ovoid; identified by presence of catalase Structure often has a crystalline core

consisting of urate oxidase Function - oxidative enzyme complement

Oxidize some fatty acidsDetoxification of substances like ethanol

Organelles in cytosol 10

Centrioles and centrosomesStructure - centrioles are 9 + 0 structures

made up of 9 triplets of microtubules Structure of centrosome is the centrioles +

surrounding matrix of proteinsCentrioles are the primary Microtubule

Organizing Centers (MTOC) of the cellThe centriole is (-) end the ends of MT (+)

Centrosome + MT’s

Inclusions in cytosol

Glycogen granules - polysaccharide storage form of glucose prominent in liver and muscle

Lipid droplets - an energy storage inclusion - lipid contains more energy available than carbohydrates like glucose

Cytoskeleton

Microtubules Straight, hollow tubules 25 nm diameter and

several micrometers long Each tube consists of polymerized and

tubulin which first forms 13 protofilaments, which then role into tube

There is polarity to MT’s - centrioles become minus end with ends out in cell as + or growing end

Microtubles

Functions Maintain cell shape Aid in the transport of macromolecules and

organelles in cytosol Allow for the movement of cilia, flagella and

chromosomes

Cytoskeleton 2

Microfilaments Structure - Polymerized form of actin = F- actin

; double helix 6 nm in diameter Polarity - polymerized and depolymerized at + Abundant in periphery of cell Functions are many

Focal contact for cell with environmentLocomotion of nonmuscle cellsContractile ring in dividing cells

Cytoskeleton 3

Intermediate filamentsStructure - 8 - 11 nm diameter ;

heterogenous group of proteins that make up intermediate filaments, but all have regions of conserved sequence

Function - Provide mechanical strengthsOnce laid down do not change rapidly

Interactions among Organelles

Secretion of proteinsProduction of lysosomesProtein SortingEndocytosis, Exocytosis and PinocytosisSignal transduction via receptor ligand

interaction