Cell Membranes and Compartments
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.
- 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
- Robert Hooke (1635-1703)
- used early microscopes to view cork tree bark
- was the first to use the term CELL
- Robert Brown 1825
- identified nuclei in plant cells
- Theodor Schwann (1810 - 1882)
- together with Matthias Schleiden (plants) developed the cell theory in 1839
- All organisms consist of one or more cells
- The cell is the basic unit of structure for all cells
- All cells arise only from preexisting cells
Plasma Membrane Images
The cell membrane (plasma membrane or plasmalemma) encloses or covers all cell types and is 7 nanometers (7 x 109 M) thick. Begin by some different ways of looking microscopically at membranes.
Membrane - Light Micrograph
Membrane - Fluorescent Micrograph
Yeast - Candida albicans
R T Watson, S Shigematsu, S H Chiang, S Mora, M Kanzaki, I G Macara, A R Saltiel, J E Pessin Lipid raft microdomain compartmentalization of TC10 is required for insulin signaling and GLUT4 translocation. J. Cell Biol.: 2001, 154(4);829-40 PMID:11502760
Membrane - Scanning Electron Micrograph
- thickness 3.5 nm
Membrane - Transmission Electron Micrograph
Links: Block-Face Scanning Electron Microscopy to Reconstruct Three-Dimensional Tissue Nanostructure Denk W, Horstmann H (2004) Serial Block-Face Scanning Electron Microscopy to Reconstruct Three-Dimensional Tissue Nanostructure. PLoS Biol 2(11): e329
- Physical Compartments
- membrane bound
- Nucleus, Cytoplasm, Organelles
- cell nomenclature based upon presence or absence of these compartments (eukaryotic, prokaryotic)
- Functional Compartments
- spatial localization
- activation and inactivation
Major Cellular Compartments
- Nucleus (nuclear) - contains a single organelle compartment
- Cytoplasm (cytoplasmic) - contains many organelle compartments
- How many organelles?
- How much space within the cell do they occupy?
- Are all the cells the same?
Take a typical mammalian liver cell....
Compartments are Dynamic
Movies showing flexibility of membranes and their changing shape and size.
- 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 Organelles
- Membrane bound structures
- Endoplasmic reticulum, golgi apparatus, mitochondria, lysosomes, peroxisomes, vesicles
- 3 filament systems
- Cytoplasmic “structures”
- DNA -> mRNA -> Protein
- Receptors, signaling, metabolism, structural
- Viruses, bacteria, prionsl
- Functional compartments
- occur in nucleus, cytoplasm, in organelles and outside organelles
- signaling, metabolic reactions, processing genetic information, cytoskeleton dynamics, vesicle dynamics
- Form compartments
- Allow “specialization”
- Metabolic and biochemical
- Localization of function
- Regulation of transport
- Detection of signals
- Cell-cell communication
- Cell Identity
Plasma Membrane Image
Cell (Plasma) encloses or covers or cell types.
Cell (Plasma) and Organelle Membranes
- phospholipids, proteins and cholesterol
- first compartment formed
- prokaryotes (bacteria) just this 1 compartment
- eukaryotic cells many different compartments
Amphipathic - having both hydrophilic and hydrophobic properties. (phospholipids, cholesterol, glycolipids).
- A liposome (lipid vesicle) is a small aqueous compartment surrounded by a lipid bilayer.
- A micelle is a small compartment surrounded by a single lipid layer.
- 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
- 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
- Links: MCB - Freeze fracturing can separate the two phospholipid leaflets that form every cellular membrane | MCB - Solubilization of integral membrane proteins by nonionic detergents
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.”
- saturated lipids and cholesterol form liquid-ordered domains
|Protein Layer-Lipid-Protein Island Model|
- 20-30% of the genome encodes membrane proteins PMID 9568909
- 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”
Two major protein transmembrane structures
- α-helical - ubiquitously distributed
- β-barrel - outer membranes of Gram-negative bacteria, chloroplasts, and mitochondria
Membrane Protein Functions
- transport channels
- enzyme reactions
- cytoskeleton link
- cell adhesion
- cell identity
- 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)
- distribution of glycoproteins have functional effects (motility, migration, channel distribution, signalling) PMID 24040379
- 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
- bacterial membranes (except for Mycoplasma and some methylotrophic bacteria) have no sterols, they lack the enzymes required for sterol biosynthesis.
(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
- fusion of 2 cells
- membrane domains (polarized cells)
- epithelia - apical, basal and lateral domains
(cytonemes and tunneling nanotubes, TNTs) - New membrane structures identified that can facilitate transfer of cellular signals and components over large distances (hundreds of microns) representing the longest direct connections between cells in vitro and in vivo.
- Tubular Bridges for Bronchial Epithelial Cell Migration and Communication Membrane structures
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J F Presley, N B Cole, T A Schroer, K Hirschberg, K J Zaal, J Lippincott-Schwartz ER-to-Golgi transport visualized in living cells. Nature: 1997, 389(6646);81-5 PMID:9288971
- plasma membrane cytoskeleton
- different directly under membranes
- adhesion complexes
- absorbtive and secretory
- synaptic junctions
This range of membrane specializations will be covered in detail in later Lectures.
A series of different types of proteins and cytoskeleton associations forming different classes of adhesion junctions (MH - covered in detail in a lecture 8)
- Desmosomes ( = macula adherens)
- Adherens Junctions ( = zonula adherens)
- Septate Junctions
- Tight Junctions
- Gap Junctions
Covered in Cell Junctions
Three major forms of transport across the membrane
- Passive - Simple diffusion
- Facilitated - transport proteins
- Active - transport proteins for nutrient uptake, secretion, ion balance
- 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
Apoptosis and Membranes
- programmed cell death
- membrane "blebbing" encloses cellular component fragments
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
Below are some example historical research finding related to cell membranes from the JCB Archive and other sources.
1957 The invention of freeze fracture EM and the determination of membrane structure Russell Steere introduces his home-made contraption for freeze fracture electron microscopy (EM), and Daniel Branton uses it to conclude that membranes are bilayers.
1971 Spectrin is peripheral S. Jonathan Singer, Garth Nicolson, and Vincent Marchesi use red cell ghosts to provide strong evidence for the existence of peripheral membrane proteins.
1992 Lipid raft idea is floated Gerrit van Meer and Kai Simons get the first hints of lipid rafts based on lipid sorting experiments.
- Molecular Biology of the Cell
Search Online Textbooks
- "cell compartments" Molecular Biology of the Cell | Molecular Cell Biology | The Cell- A molecular Approach
- "cell membrane" Molecular Biology of the Cell | Molecular Cell Biology | The Cell- A molecular Approach
Vertebrate membrane proteins: structure, function, and insights from biophysical approaches. Müller DJ, Wu N, Palczewski K. Pharmacol Rev. 2008 Mar;60(1):43-78. Epub 2008 Mar 5. Review. PMID: 18321962
- adhesion junction membrane specialization allowing either cell-cell or cell-extracellular matrix adhesion generally in multicellular organisms.
- bilayer having two layers, refers to the 2 lipid layers of a single membrane.
- blebbing a plasma membrane change often associated with apoptosis. The underlying cell cytoskeleton is disrupted leading to a the bubbling of the plasma membrane, which will enclose cytoplasmic and nuclear components.
- cholesterol small steroid metabolite that decreases membrane motility involved in many membrane functions (endocytosis, membrane rafts). Bacterial membranes (except for Mycoplasma and some methylotrophic bacteria) have no sterols, they lack the enzymes required for sterol biosynthesis.
- cytonemes thin, actin-based extensions that project from cells and allow cell-cell communication.
- electron microscopy a microscope technique that uses beams of electrons instead of light to generate high resolution images of cellular components. This technique historically gave the first images of the membrane bilayer structure.
- electron tomography an electron microscopic technique to generate a three dimensional (3-D) image from any electron microscopy specimen.
- exosomes small vesicles that bud from the endosome membrane into its lumen. Following endosome fusion with the plasma membrane, the exosomes are released into the extracellular space. http://jcb.rupress.org/cgi/content/full/172/6/785?
- flippases enzymes that catalyze rapid translocation of phospholipids across the endoplasmic reticulum membrane. Required for balanced growth of both halves of the bilayer.
- flotillin (flotillin-1 and -2) protein that is ubiquitously enriched in detergent resistant membranes (membrane rafts).
- functional compartment a specialized region formed within a cell which is not limited by a membrane, compared to a "structural compartment".
- Gram negative term used to describe bacteria which do not retain the Gram dye when stained. These are bacteria with double membranes, the inner membrane is the cell's plasma membrane (Example: E. coli).
- Gram positive term used to describe bacteria which do retain the Gram dye when stained. These are bacteria with single membranes and thicker cell wall (Example: staphylo-cocci and streptococci).
- lipids the basic molecules forming the lipid bilayer as phospholipids, glycolipids, and steroids. The main lipid components include phosphatidylcholine (~50%), phosphatidylethanolamine (~10%), phosphatidylserine (~15%), sphingolipids (~10%), cholesterol (~10%), and phosphatidylinositol (1%). Medical Microbiology - Plasma (Cytoplasmic) Membranes
- liposome (lipid vesicle) is a small aqueous compartment surrounded by a lipid bilayer.
- membrane cytoskeleton the components of the cell cytoskeleton that directly underly either the cell (plasma) and nuclear membranes.
- micelle is a small compartment surrounded by a single lipid layer.
- phospholipid the basic molecule forming the lipid bilayer of a typical membrane (see also lipid).
- raft (lipid rafts, membrane raft) term used to describe stabilized regions that form within membranes. These rafts "float" within the lipid membrane and are formed by cholesterol altering (stabilizing) the fluidity of the local membrane.
- structural compartment a specialized region formed within a cell which is limited by a membrane, compared to a "functional compartment".
- trogocytosis process of T and B cells capture antigens via membrane fragments of antigen presenting cells (APC).
- vesicle general term given to any membrane enclosing material within the cytoplasm.
- protein-to-lipid ratio the analysis of membranes by separating the 2 main components. For example, bacterial plasma membranes are approximately 3:1, close to those for mitochondrial membranes.
External Links Notice - The dynamic nature of the internet may mean that some of these listed links may no longer function. If the link no longer works search the web with the link text or name.
- American Society Cell Biology
- The Nobel Prize in Physiology or Medicine - Laureates
- Museum of Microscopy
- The WWW Virtual Library of Cell Biology- General Cell Biology
- The Biology Project- Studying Cells
- 2003 Double Helix Celebrations
- Genome Timeline
- A marker for sequential exocytosis The SNARE protein SNAP25, say Takahashi et al., marks the plasma membrane after an initial exocytic event to allow rapid sequential exocytic events.
- A myosin V moves yeast secretory vesicles Secretory vesicles actively move to the site of exocytosis in yeast. Schott et al. find that multiple secretory vesicles often follow the same linear track and frequently enter and cross the bud. This movement requires the activity of the myosin-V heavy chain encoded by the MYO2 gene. When the predicted lever arm of this motor is progressively shortened (with the most extreme example being the 0IQ mutant), the vesicle movements are progressively slowed.
- Rapid cycling of lipid rafts to and from the Golgi Nichols et al. detect rapid cycling of lipid raft markers between the plasma membrane and the Golgi. Through selective photobleaching, they are able to study transport either out from the Golgi to the plasma membrane, or in from the plasma membrane to the Golgi.
- Membrane docking at the immunological synapse requires Rab27a Stinchcombe et al. find that normal membrane docking of lytic granules at the immunological synapse is defective in cells lacking Rab27a. In cells lacking other Rab proteins, polarization of the secretory granules is incomplete.
- Visualizing the location and dynamics of exocytosis Schmoranzer et al. use total internal reflection (TIR) fluorescence microscopy to visualize exocytosis in mammalian cells (e.g., see event on left side of video). The analysis reveals that there are no preferred sites for constitutive exocytosis in this system.
- Visualizing the location and dynamics of exocytosis Toomre et al. use a combination of TIR microscopy (green, labeling molecules close to or at the membrane) and standard fluorescence microscopy (red, for molecules further from the membrane) to visualize trafficking to and fusion with the plasma membrane during exocytosis. Red dots turn yellow then green as they approach the membrane, and then explode in a burst of light as they fuse with the plasma membrane during exocytosis. The transport containers appear to be partially anchored at the membrane before fusion, and can undergo either partial or complete fusion events.
- File:Membrane label and endosomes.mov
- Dynamic Changes in the Spatiotemporal Localization of Rab21 in Live RAW264 Cells during Macropinocytosis
- Ordered Patterns of Cell Shape and Orientational Correlation during Spontaneous Cell Migration
- From Dynamic Live Cell Imaging to 3D Ultrastructure: Novel Integrated Methods for High Pressure Freezing and Correlative Light-Electron Microscopy
- SoftSimu Movie Gallery
- Structure of Fluid Lipid Bilayers
- Changes in cholesterol levels in the plasma membrane modulate cell signaling and regulate cell adhesion and migration on fibronectin
- Neuronal growth cones
2015 Course Content
Lectures: Cell Biology Introduction | Cells Eukaryotes and Prokaryotes | Cell Membranes and Compartments | Cell Nucleus | Cell Export - Exocytosis | Cell Import - Endocytosis | Cytoskeleton Introduction | Cytoskeleton - Microfilaments | Cytoskeleton - Microtubules | Cytoskeleton - Intermediate Filaments | Cell Mitochondria | Cell Junctions | Extracellular Matrix 1 | Extracellular Matrix 2 | Cell Cycle | Cell Division | Cell Death 1 | Cell Death 2 | Signal 1 | Signal 2 | Stem Cells 1 | Stem Cells 2 | Development | 2015 Revision
Laboratories: Introduction to Lab | Microscopy Methods | Preparation/Fixation | Cell Knockout Methods | Cytoskeleton Exercise | Immunochemistry | Project Work | Confocal Microscopy | Tissue Culture | Stem Cells Lab | Microarray Visit
Dr Mark Hill 2015, UNSW Cell Biology - UNSW CRICOS Provider Code No. 00098G