Pre-Medicine Program - Cell Compartments and Membranes
- 1 Cell Membranes and Compartments
- 1.1 About Today's Class
- 1.2 Introduction
- 1.3 Objectives
- 1.4 History
- 1.5 Plasma Membrane Images
- 1.6 Compartments
- 1.7 Major Cellular Compartments
- 1.8 Membrane Functions
- 1.9 Membrane Components
- 1.10 Phospholipids
- 1.11 Membrane Proteins
- 1.12 Membrane Glycoproteins
- 1.13 Membrane Cholesterol
- 1.14 Bacterial Membranes
- 1.15 Specializations
- 1.16 References
- 1.17 Videos
- 1.18 Membrane Terms
Cell Membranes and Compartments
About Today's Class
- You will also have a summary of today's class in your course handout.
- Clicking on the images, and opening them, will show more information about the topic.
- For example Eukaryotic Cell Physical Compartments
Pre-Med specific information with the images is in a collapsible table (expand them for your class information)
|Pre-Medicine Program - Membranes and Compartments|
| This is an example of a collapsible table. These tables are on this class page and with the images you open separately.
Note that these images are used in other classes and may have additional information not required for today's class.
At the bottom of each table is a link back to the lecture page.
You can contact Dr Mark Hill by email (firstname.lastname@example.org) if you have specific questions about today's class.
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.
- Medications act on cells or tissues and can mimic, block or activate metabolic or signalling pathways.
- Membranes form compartments and are involved in signalling
- Insulin, glucagon, hormone and neurotransmitter receptors are embedded in the plasma membrane.
- Nutrients - Cells transport nutrients across cell membranes into specific compartments for use.
- Glucose transporter embedded in the plasma membrane.
- Waste - Cells transport waste across cell membranes into extracellular specific compartments for excretion.
- Understand the concept of separate intracellular spaces
- Understand the structure of membranes
- 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 (animals) together with Matthias Schleiden (plants) 1839 developed the cell theory
- 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
|Membrane Research History|
| For background information only (not assessable).
Plasma Membrane Images
The cell membrane (plasma membrane or plasmalemma) encloses or covers all cell types and is 7 microns thick (1000 times smaller than the RBC).
Begin by looking at some of the different ways of seeing microscopically membranes.
Membrane - Light Micrograph
Membrane - Scanning Electron Micrograph
Membrane - Transmission Electron Micrograph
- Physical Compartments - membrane bound - Nucleus, Cytoplasm, Organelles
- Cell nomenclature based upon presence or absence of these compartments (eukaryotic, prokaryotic)
- Functional Compartments - can occur within cytoplasm or membrane bound organelles - spatial localization, targeting, activation and inactivation, signaling
- Compartments are Dynamic - Membranes are flexible, changing cell shape and size. (Membrane Fluidity)
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.... Liver Structure
- Form compartments
- Allow “specialization”
- Metabolic and biochemical
- Localization of function
- Regulation of transport
- Detection of signals
- Cell-cell communication
- Cell Identity
- first compartment formed
- prokaryotes (bacteria) just this 1 compartment
- eukaryotic cells many different compartments
- Contains - phospholipids, proteins and cholesterol
- Proteins can be embedded in the inner phospholipid layer, outer phospholipid layer or span both layers (20-30% of the genome encodes membrane proteins PMID 9568909) Some proteins are folded such that they span the membrane in a series of “loops”
- Some membrane protein functions: transport channels, enzyme reactions, cytoskeleton link, cell adhesion and cell identity
Membrane Receptors - Insulin, glucagon and glucose transporter.
Membrane Insulin Receptors
|Muscle, adipose and other tissues.||Liver|
- 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)
- 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 distribution
- Functions: control membrane protein activity and "raft” formation, fine tuning of membrane lipid composition, organization/dynamics, function
- 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
Adhesion - junctions allow cells to attach to each other and the non-cellular material that they are embedded within (extracellular matrix)
- There are several different types that span the membrane and link to the cell skeleton.
- Adhesion types: Desmosomes ( = macula adherens), Adherens Junctions ( = zonula adherens), Septate Junctions, Tight Junctions, Gap Junctions
- Cells will have some or all of these junction types
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 - Membrane phospholipid impermeable to ions, protein channels permit rapid ion flux
- 75 + different ion channels, opening/closing, “gating” of ions
- 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
Cell Death - most cells die by a programmed cell death (Apoptosis)
- membrane "blebbing" encloses cellular component fragments - prevents "leaking" material into tissues
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
- Links: Time-lapse movie of human HeLa cells undergoing apoptosis PMID: 18073771 | Example of early apoptotic blebbing PMID:16129889
Molecular Cell Biology 4th ed., Lodish, Harvey; Berk, Arnold; Zipursky, S. Lawrence; Matsudaira, Paul; Baltimore, David; Darnell, James E., New York: W. H. Freeman & Co., 1999. The Cell- A Molecular Approach Movies
Here are some general introductory videos to help understand concepts of cell membranes and compartments.
A general introduction to cell membranes.
Active and Passive Transport
Having difficulty with these concepts? Here is a very simplified (high school level) description of cell structure.
- 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.
- 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.
- functional compartment - a specialized region formed within a cell which is not limited by a membrane, compared to a "structural compartment".
- glucagon receptor - G-protein coupled type receptor found in plasma membrane activated by glucagon. Activation increases blood glucose levels, opposite of insulin.
- glucose transporter - GLUT4 is the insulin-regulated glucose transporter found in plasma membrane in adipose tissues and skeletal and cardiac muscle responsible for insulin-regulated glucose transport into the cell.
- 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).
- insulin receptor - tyrosine kinase type receptor found in plasma membrane activated by insulin, IGF-I, IGF-II. Activation has many effects including increasing expression of high affinity glucose transporter (Glut4).
- lipids - 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.
- receptor - usually protein most often found on the surface of a cell (plasma membrane) that receives signals originating externally from the cell.
- structural compartment - a specialized region formed within a cell which is limited by a membrane, compared to a "functional compartment".
- 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.
Dr Mark Hill 2015, UNSW Cell Biology - UNSW CRICOS Provider Code No. 00098G