CELL ORGANELLES AND NUCLEUS

MBBS1

2011

A. Mwakikunga

LEARNING OBJECTIVES

Identify major cell regions - cytoplasm, nucleus

Name any two cellular activities that take place a nucleus

Briefly describe the structural organization of the nucleus and

nuclear envelope

Mention membrane bound organelles

Discuss the roles of intramembrane system of a cell

Describe structural and functional differences among membrane

bound organelles

Discuss the significance of tags, v-SNAREs and t-SNAREs in protein

sorting and targeting

Mention proteins used to coat transport vesicles

Describe briefly how mitochondria are used in the process of

apoptosis

MAIN REGIONS OF THE CELL

Two main regions of a cell are: nucleus

cytoplasm

Cytoplasm + nucleus = Protoplasm

NUCLEUS

One of most prominent & characteristic

features of eukaryotic cells - stores genetic

material

Site where:

chromosomes are localized (heterochromatin vs

euchromatin)

chromosomes are replicated

DNA is transcribed

Surrounded by a bouble-membrane nuclear

envelope

NUCLEAR ENVELOPE

Is composed of inner

& outer nuclear

membranes separated

by perinuclear space

Inner membrane is on

nuclear lamina

Outer membrane is

continuous with ER

NUCLEAR ENVELOPE CONTD…

Nuclear envelope has

nuclear pores

Passage of substances

between cytoplasm &

nucleoplasm

Nuclear pore has many

structural proteins

forming nuclear pore

complex

MACROMOLECULAR TRANSPORT INTO & OUT OF THE NUCLEUS

Proteins needed in

nucleus are

transported inward

from cytoplasm

RNA molecules &

ribosomal subunits for

protein synthesis in

cytoplasm are

transported outward

TRANSPORT MECHANISM OF PROTEINS FROM CYTOPLASM INTO NUCLEUS

NUCLEAR MATRIX, LAMINA & NUCLEOLUS

80-90% of nuclear mass is chromatin fibers

Remaining forms nuclear matrix

supports chromatin fibers

maintains shape of nucleus

Nuclear lamina - lines inner membrane and

supports nuclear envelope

May provide attachment site for chromatin

Nucleolus - condensed component of

eukaryotic nucleus

manufactures ribosomes

CYTOPLASM

Cytoplasm has organelles & inclusions

Organelles (little organs) include membrane systems & membrane bound compartments & nonmembranous structural components

Inclusions - materials that may or may not be surrounded by a membrane

CYTOPLASM

CYTOPLASM CONTD…

Examples of inclusions:

secretory granules, pigments, neutral fats,

glycogen, stored waste products

Cytoplasmic ground substance used to be

called cytosol; now called cytoplasmic

matrix

INTRACELLULAR MEMBRANES OR ENDOMEMBRANE SYSTEM

Most organelles & inclusions are membrane bound

Membranes form vesicular, tubular & structural

patterns that may be convoluted (SER) or plicated

(inner mitochondria)

They provide surface area for chemical reactions

Spaces enclosed by membranes are

microcompartments in which substrates &

products of chemical reactions are segregated or

concentrated

ENDOMEMBRANE SYSTEM

INTRACELLULAR MEMBRANES OR ENDOMEMBRANE SYSTEM CONT..

These membranes are related through

direct physical continuity or by transfer of

membrane segments as vesicles

Thickness, molecular composition &

metabolic behaviour of a membrane are not

fixed; they may be modified during its life

MEMBRANE BOUND ORGANELLES

Cell (plasma) membrane

Rough - endoplasmic reticulum

Smooth - endoplasmic reticulum

Golgi apparatus

Mitochondria

Lysosomes

Endosomes

Peroxisomes

NONMEMBRANOUS ORGANELLES

Microtubules

Filaments

Centrioles

Ribosomes

CELL MEMBRANE

It is a bilayer of phospholipids sandwiched with glycoproteins and cholesterols

On the surface, glycolipids & glycoproteins form a cell coat or glycocalyx

It has a trilaminar appearance in Transmission Electron Microscopy:

Outer electron - dense layer

Inner electron - dense layer

Intermediate nonstaining layer

CELL MEMBRANE

CELL MEMBRANE

CELL MEMBRANE CONTD…

It is a fluid & dynamic system - not a static

structure (modified fluid mosaic model)

It is a mosaic of protein molecules in a fluid

bilayer of phospholipids

Phospholipids are amphipathic - have both

hydrophilic & hydrophobic regions

FLUID QUALITY OF A MEMBRANE

It is not static; weak bonds hold it together

Phospholipids move along its plane at

about 2µm per sec.

Unsaturated hydrocarbon tails have kinks

that keep molecules from packing closely

together, enhancing membrane fluidity

Cholesterol reduces phospholipid

movement

MEMBRANES AS MOSAICS

It has many different proteins embedded in fluid matrix of phospholipids

These are integral & surface proteins

Membrane proteins are for: Transport - ion pumps, channels

Enzymatic activity

Signal transduction

Cell - cell recognition; receptors

Attachment to cytoskeleton & extracellular matrix (adhesion)

MEMBRANES AS MOSAICS

CELL MEMBRANE AS BOUNDARY & BARRIER

Some substances pass it by diffusion, others by active transport

Endo- & Exocytosis - vesicular transport of substances entering & leaving cell respectively

Forms of endocytosis: phagocytosis & pinocytosis

CELL MEMBRANE AS SITE FOR SPECIFIC FUNCTIONS

Organization and localization of function

Compartmentization eg

Mitochondria

lysosomes

Peroxisomes

ER, Golgi complex

ROUGH - ENDOPLASMIC RETICULUM

Endoplasmic - within plasm; reticulum - network

ER coined because it resembled a reticule - netted

hand-bag popular in late 19th century

It is continuous with nuclear envelope

A series of interconnected membrane-bound

flattened sacs called cisternae

rough ER membranes form large flattened sheets

Has ribosomes - protein synthesis

Ergastoplasm - basophilic region of cytoplasm -

represents rER in secretory cells

ROUGH ENDOPLASMIC RETICULUM CONTD…

Groups of ribosomes form polyribosomes

or polysomes attached to mRNA

Polysomes synthesize proteins for export &

integral proteins of cell membrane

Secretory proteins pass through rER

membrane to its lumen for modification &

storage

Free ribosomes synthesize proteins to be

used within the cell

ROUGH - ENDOPLASMIC RETICULUM

ROUGH ENDOPLASMIC RETICULUM CONTD…

rER involved in protein biosynthesis

rER involved in initial protein processing

Initial glycosylation

Polypeptide folding - Alzheimer’s disease

Assembly of multi-subunit proteins

Quality control

Transtional elements - rER subdomain

Form transition vesicles - shuttle lipids &

proteins to Golgi apparatus.

SMOOTH ENDOPLASMIC RETICULUM

Has short tubules not associated with

ribosomes

Cells with a lot of sER show acidophilia or

may appear empty when viewed with light

microscopy

Well developed in cells synthesizing &

secreting steroids - Leydig cells in testis

Found abundantly in liver cells

SMOOTH - ENDOPLASMIC RETICULUM

Detoxifies drugs & other noxious substances

Hydroxylation - addition of hydroxyl groups to

organic acceptor molecules. This increases

solubility of hydrophobic drugs in water.

It is also involved in:

lipid absorption

Glycogen metabolism

Membrane formation & recycling – phospholipids

Calcium storage - in muscles - sarcoplasmic retic.

GOLGI APPARATUS

1898 - described by Camillo Golgi in nerve

cells

System of membranes parallel to each

other

Very active in secretory cells - exocytosis

It doesn’t stain with H & E; exhibits a clear

area surrounded by ergastoplasm as in

osteoblast

GOLGI APPARATUS

Has 2 faces:

forming face or cis-Golgi closest to rER

maturing face or trans-Golgi closest to cell

membrane

Transport vesicles carry newly synthesized

protein from rER to forming face of Golgi A

Coated vesicles - clathrin, COP1, COPII proteins

Condensing vacuoles pinch off from

maturing face & modified into secretory

vacuoles

GOLGI APPARATUS

GOLGI APPARATUS

Acts as a post office for the cell: Packing & shipping

Modifying

Sorting - length of transmembrane domains determine

sorting within Golgi

Recycles proteins (retrograde) & ER-specific protein

retrieval - tags eg KDEL (Lys-Asp-Glu-Leu)

Delivering enzymes to lysosomes

Handles distribution of newly synthesized

membranes & their proteins

Synthesizes polysaccharide & emulsify lipid

GOLGI APPARATUS

SNARE HYPOTHESIS

SNARE for SNAP receptor

This hypothesis connects coated vesicles

& target membranes

The molecular components that facilitate

sorting and targeting of vesicles include 2

families of SNARE proteins: v-SNAREs & t-

SNAREs

these allow a vesicle to recognize a target

organelle

MITOCHONDRIA

Power plants of cells - Krebs Cycle & ETS

Have DNA & RNA; exhibit cell division &

make own structural proteins

Depend on DNA & free ribosomes of cell for

other proteins

Present in all cells except for RBCs &

terminal keratinocytes

Use inner membrane to create functional

compartments

MITOCHONDRIA

Change its structure related to function

2 configuratons in TEM: orthodox &

condensed

Orthodox configuration

cristae prominet, matrix compartment large, low

level of oxidative phosphorylation

Condensed configuration

cristae not easily recognized, matrix

concentrated & reduced, high level of oxidative

phosphorylation

MITOCHONDRIA

MITOCHONDRIA

MITOCHONDRIA

Very crucial in apoptosis - cytoskeletal

disintegration, metabolic derangement,

genomic fragmentation

Intrinsic pathway

p53, Bid, Bak, bax

Cytochrome c

Apaf1 + procaspase 9 → caspase 3 → death

LYSOSOMES

Discovered in 1953 by Chriatian de Duve

Suicide bags with hydrolytic enzymes; single

membrane

Isolate digestive enzymes from the rest of the cell

Develop from late endosomes

Organelles with full complement of acid hydrolases, BUT

not yet engaged in digestive activity

Mature to form fully active lysosomes after acid

hydrolases from TGN are combined with extra &

intracellular material from early endosomes.

LYSOSOMES CONTD…

Primary lysosomes - newly formed

Secondary lysosomes

phagosomes or heterophagic - substance

extracellular origin

autophagic - with materials of intracellular

origin

Tertiary lysosome or residual body - is

debris filled vacuole of broken down

contents of sec. lysosome

IN SUMMARY - ENDOSOMES

Vesicles formed as a result of phagocytosis

They receive lysosomal enzymes from

clathrin-coated transported vesicles

Late endosomes - with inactive enzymes,

mature to form lysosomes

The pathway followed by endosomes within

cell depends on their contents eg It may fuse with lysosome to form phagosome

It may fuse with another domain or region of cell

membrane to release its product from cell (transcytosis)

TARGETTING SOLUBLE LYSOSOMAL PROTEINS TO ENDOSOMES AND LYSOSOMES

It is a model for protein sorting in the TGN

Mannose-6-phosphate is used to target soluble

lysosomal proteins to endosomes and lysosomes

lysosomal enzyme synthesised & carbohydrate added

mannose is phosphorylated

mannose-6-phosphate binds to receptor on the enzyme

tagged enzyme is parked in transport vesicles

During late endosome stage, pH is lowered, enzyme

dissociates from receptor as endosome matures into a

lysosome

PEROXISOMES (MICROBODIES)

0.5µm in diameter; single membrane

Not derived from ER

Therefore NOT part of endomembrane syst

Have oxidative enzymes eg catalase & other peroxidases - break down hydrogen peroxide (H2O2)

H2O2 is toxic

Catalase regulates H2O2 so protecting cell

Numerous in liver and kidney cells

Take part in oxidation of fatty acids