UNSW Cell Biology

2008 ANAT3231 Lecture 03 - Compartments and Membranes

Introduction

A major difference between eukayotes and prokaryotes is the presence of physical compartments (membrane bound) within the cell. These compartments allow the separation/specialization of processes within the cell.

There also exist within each of these physical compartments, functional compartments where specific processes may occur or are restricted too.

This lecture is an introduction to compartments within the cell and membranes. The key components are: cell compartments, membrane structure, membrane models, membrane specializations.

Cell Membrane Cartoon

Lab: Lab 02 Tissue Culture

Objectives

Understand the concept of separate intracellular spaces
Understand the structure of membranes
Brief understanding of history of membrane models
Understand the difference between physical and functional compartments
Brief understanding of membrane specializations

Lecture Audio

The University has a system for automated recording of lectures called Lectopia (or iLecture).

I will be adding the link to each iLecture Audio following the Lecture (MH - so you will need to check back to this current page or Lectopia page).

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Links: Lectopia Login Page | Cell Biology Podcast Page | Current Course Outline 2008

Compartments

Physical Compartments

membrane bound
Nucleus, Cytoplasm, Organelles
cell nomenclature based upon presence or absence of these compartments (eukaryotic, prokaryotic)

Functional Compartments

spatial localization
targeting
activation and inactivation
signaling

Links: Prokaryote sizes and structures

Major Cellular Compartments

Nuclear
Cytoplasmic

Organelle Number/Volume

How many organelles?
How much space within the cell do they occupy?
Are all the cells the same?

Take a typical mammalian liver cell....

Compartment	Number/cell	Percentage Total Volume
Cytosol	1	54
Mitochondria	1700	22
Rough Endoplasmic Reticulum	1	9
Smooth Endoplasmic Reticulum		6
Peroxisomes	400	1
Lysosomes	300	1
Nucleus	1	6
Typical liver cell and not all organelles (Golgi?)/vesicles are shown (Table : modified from MBoC)

Compartments are Dynamic

Movies showing flexibility of membranes and their changing shape and size.

Nuclear Compartment

Nuclear matrix - consisting of Intermediate filaments (lamins)
Nucleoli (functional compartment - localised transcription DNA of RNA genes)
Chromosomes (DNA and associated proteins)

(MH - you will not see chromosomes in interphase nuclei only during mitosis, more in the Nucleus Lecture)

Cytoplasmic Compartment

Cytoplasmic Organelles

Membrane bound structures
Endoplasmic reticulum, golgi apparatus, mitochondria, lysosomes, peroxisomes, vesicles

Cytoskeleton

3 filament systems

Cytoplasmic “structures”

Ribosomes
DNA -> mRNA -> Protein
Proteins
Receptors, signaling, metabolism, structural
Viruses, bacteria, prionsl

Membrane Functions

Form compartments
Allow “specialization”
Metabolic and biochemical
Localization of function
Regulation of transport
Detection of signals
Cell-cell communication
Cell Identity

Plasma Membrane

Encloses or covers or cell types

Images of plasma membrane

Denk W, Horstmann H (2004) Serial Block-Face Scanning Electron Microscopy to Reconstruct Three-Dimensional Tissue Nanostructure. PLoS Biol 2(11): e329 Links: Movie - Serial Electron Microscope sections | Serial Block-Face Scanning Electron Microscopy to Reconstruct Three-Dimensional Tissue Nanostructure

Membrane Components

phospholipids, proteins and cholesterol
first compartment formed
prokaryotes (bacteria) just this 1 compartment
eukaryotic cells many different compartments

Phospholipids

Links: The Cell - Fluid mosaic model of membrane structure MBOC - Image 3 views of plasma membrane

Phospholipid Orientation

A liposome (lipid vesicle) is a small aqueous compartment surrounded by a lipid bilayer.

Movie: Detergent Effects Membrane

Membranes History

1890 Charles Overton

selective permeation of membranes
non-polar pass through (lipid soluble)
polar refractory
lipids present as a coat

1905 Irving Langmuir

lipids faced with heads towards water away from organic solvents

1925 Gorter and Grendel

monolayer of lipid isolated from rbc
twice (2x) surface area of cell (bilayer)

1930-40 Danielle-Davson

Proteins coat a bilayer with polar “pores”

1960s Robertson

Modification with glycoprotein on one side, therefore asymmetric

Membranes History 2

1972 Singer and Nicholson

proteins “floating” within lipid bilayer like a “liquid” surface

1975 Unwin and Henderson

integral membrane proteins
both hydrophobic and hydrophilic
alternating -phobic and -philic represent trans-membrane loops
glycoprotein carbohydrate groups on outer surface

Model of the Plasma Membrane

Membranes Recent History

1997 Simons

cholesterol to form rafts that move within the fluid bilayer
“Membrane Rafts”
“A new aspect of cell membrane structure is presented, based on the dynamic clustering of sphingolipids and cholesterol to form rafts that move within the fluid bilayer. It is proposed that these rafts function as platforms for the attachment of proteins when membranes are moved around inside the cell and during signal transduction.”
• Simons K, Ikonen E. Nature 1997 Jun 5;387(6633):569-72

A Intracellular space or cytosol
B Extracellular space or vesicle/Golgi apparatus lumen
1. Non-raft membrane
2. Lipid raft
3. Lipid raft associated transmembrane protein
4. Non-raft membrane protein
5. Glycosylation modifications (on glycoproteins and glycolipids)
6. GPI-anchored protein
7. Cholesterol
8. Glycolipid

Lipid Raft Cartoon

Cytosol surface

Extracellular surface

Membrane Proteins

Proteins can be embedded in the inner phospholipid layer, outer phospholipid layer or span both layers
Some proteins are folded such that they span the membrane in a series of “loops”

Membrane Glycoproteins

Glycoproteins are proteins which have carbohydrate groups (sugars) attached
to produce these proteins go through a very specific cellular pathway of organelles (secretory pathway)
to reach the cell surface where they are either secreted (form part of the extracellular matrix)
or are embedded in the membrane with the carbohydrate grouped on the outside surface (integral membrane protein)

Membrane Cholesterol

Small molecule embedded between the phospholipid molecules and regulates lipid mobility (MH - see rafts)
Cholesterol can be at different concentrations in different regions of plasma membrane
lateral organization of membranes and free volume distribution
may control membrane protein activity and "raft” formation
fine tuning of membrane lipid composition, organization/dynamics, function

Links: MBOC - Cholesterol in a lipid bilayer

Bacterial Membranes

(MH - covered this in previous Lecture)

Bacteria with double membranes (Example: E. coli)

inner membrane is the cell's plasma membrane
Gram Negative do not retain dark blue dye used in gram staining

Bacteria with single membranes (Example: staphylo-cocci and streptococci)

thicker cell walls
Gram Positive because they do retain blue dye
single membrane comparable to inner (plasma) membrane of gram negative bacteria

Membrane Fluidity

Membrane Fluidity Movie 1

fusion of cells

Membrane Fluidity Movie 2

sperm domains Links: MBC - Membrane Fluidity |

Membrane Turnover

Movies: Exocytosis Links: MBC - The secretory pathway of protein synthesis and sorting | MBC - synthesis and release of secretory proteins

Membrane Protein Function

transport channels
enzyme reactions
cytoskeleton link
cell identity
cell adhesion

Membrane Specializations

plasma membrane cytoskeleton
different directly under membranes
adhesion complexes
absorbtive and secretory
synaptic junctions

Adhesion Specializations

A series of different types of proteins and cytoskeleton associations forming different classes of adhesion junctions (MH - covered in detail in a later lecture)

Desmosomes ( = macula adherens)
Adherens Junctions ( = zonula adherens)
Septate Junctions
Tight Junctions
Gap Junctions

Membrane Transport

Three major forms of transport across the membrane

Passive - Simple diffusion
Facilitated - transport proteins
Active - transport proteins for nutrient uptake, secretion, ion balance

Ion Channels

phospholipid impermeable to ions in aqueous solution
protein channels permit rapid ion flux
1960’s structure and function, ionophores (simple ion channels)
common structural motif alpha helix
75 + different ion channels
Allosteric proteins - conformation regulated by different stimuli
opening/closing, “gating” of ions

Ion Channel Types

3 rapid + 1 slow gate (gap junction)

Voltage-gated - propogation of electrical signals along nerve, muscle
Ligand-gated - opened by non-covalent, reversible binding of ligand between nerve cells, nerve-muscle, gland cells
Mechanical-gated - regulated by mechanical deformation
Gap junction - allow ions to flow between adjacent cells open/close in response to Ca2+ and protons

Membrane Transport Disease

Cystic Fibrosis

1989 Collins (US), Tsui and Riordan (Canada)
Chloride channel protein mutation
point mutant, folded improperly, trapped and degraded in ER

Lipid Rafts

Cell plasma membrane

patchy and locally differentiated into domains
some domains seem to arise through the confinement of diffusible membrane proteins
other domains may arise through lipid-lipid interactions
Domains are transient on a biological timescale
both could create local conditions that enhance molecular interactions (e.g. receptor-mediated signaling)

membrane-active sterols

major step in membrane evolution
control membrane permeability by reducing average "fluidity" and free volume

(TRENDS in Cell Biology Vol.11 No.12 Dec 2001 492)

Movies: Lipid Rafts

Membrane Evolution


	Model of Membrane Evoloution (Image: PLOS)

Links: MBOC - Hypothetical schemes for the evolutionary origins of some membrane-enclosed organelles | PLOS - A Structural Analysis of Eukaryotic Membrane Evolution

References

Essential Cell Biology - Ch11 Membrane Structure (p347), Ch12 Membrane Transport (p371)

Molecular Biology of the Cell - Part 3 Chapter 10 (membranes),12,13

Ch 10 Membrane Stucture Movies (CD)

10.1 Fluidity of the Lipid Bilayer
10.3 Membrane Disruption by Detergent
10.4 Membrane Effects in a Red Blood Cell
10.6 FRAP

NLM Online Textbooks

Textbook - Essential Cell Biology

Links: ASCB - Exploring the Cell Booklet - PDF document from American Society for Cell Biology

Web Links

American Society Cell Biology | ASCB - Booklet Exploring the Cell
The Nobel Prize in Physiology or Medicine - Laureates
Museum of Microscopy
The Biology Project- Studying Cells
The WWW Virtual Library of Cell Biology- General Cell Biology
2003 Double Helix Celebrations
Genome Timeline

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2007 Lecture Slides

Below are links to previous year's lecture slides if you wish to browse or download the PDF document for later viewing/printing. (MH - note that content will not match exactly current lecture structure but has been selected as having similar content)

Lecture Slides 1/page (viewing only) (43 pages, 1.4 Mb)
Lecture Slides 4/page (printing) (11 pages, 868 Kb)
Lecture Slide Outline (printing no images) (4 pages, 128 Kb)

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Comments

In 2008 a new way of presenting course content online is being trialled. Please let me know of any difficulties/suggestions or things that work well.

Notice also that in some slides I have added annotations in brackets with my initials (MH - )

Links: Current Course Outline 2008