Textbook Notes (369,072)
Canada (162,367)
York University (12,903)
Biology (1,015)
BIOL 1000 (389)
Chapter 5

BIOL 1000 Chapter 5 - Cell Membranes and Signalling.docx

10 Pages

Course Code
BIOL 1000
Jennifer Steeves

This preview shows pages 1,2 and half of page 3. Sign up to view the full 10 pages of the document.
Cell Membranes and Signalling 5.1 – An Overview of the Structure of a Membrane • The Fluid Mosaic Model – The membrane consists of a fluid lipid bilayer in which proteins are embedded and float freely • Membranes are asymmetrical; the two halves of a membrane are not the same.  The membrane proteins found on one half of the bilayer are structurally and functionally distinct from those of the other half Membrane Functions 1. Barrier to water-soluble (polar) molecules, charged molecules, and large molecules  Regulates transport in and out of organelle of cell 2. Separate inside from outside  Difference of cell vs. external environment maintained  Difference of inside organelle vs. cytosol environment maintained 3. Maintain ion gradients  For energy conservation and signalling 4. Involved in Signalling  E.g. relay signals from outside to inside of the cell 5.2 – The Lipid Fabric of a Membrane • The Lipid Bilayer forms the structural framework of membranes and serves as a barrier preventing the passage of most water-soluble molecules • Many different types of lipids compose a membrane • The structural basis of a membrane is a fluid phospholipid bilayer – most abundant group  Polar Regions (heads) lie at the surfaces of the bilayer  Non-Polar Regions (tails) associate together in the interior • Saturated Fatty Acids (SFA) – Contain the maximum # of hydrogen atoms; linear molecules • Unsaturated Fatty Acids (UFA) – Contain 1 or more double bonds; causes the fatty acid to kink • Desaturases – Enzyme that adjusts the fatty acid composition of membrane lipids to maintain proper fluidity • Membrane formation and repair is spontaneous in an aqueous solution  Can be a lipid micelle or a lipid bilayer • The Membrane is Not static/rigid; lots of movement occurs  Lateral Diffusion  Flexion  Rotation  Flip-Flop (rarely occurs) – catalyzed by special proteins • Increased UFAs = Increased Fluidity (opposite is also true) • Increase Temperature = Increase Fluidity (opposite is also true) • Cholesterol – Associated with animal cells only; inserted between phospholipids  Influences membrane fluidity in BOTH Low and High temperatures 5.3 – Membrane Proteins • The number of proteins and types of proteins really depends on the function of the membrane (the function of the cell itself) • 25% to 75% of membrane by weight (depends on the membrane and function of cell)  E.g. myelin (nerve cells) – 25% protein by weight  Inner mitochondrion membrane – 75% protein by weight (ETC – Cellular Respiration) • This helps make up the fluid mosaic model – all the different parts of the membrane; proteins, fats, etc. • Proteins embedded in the phospholipid bilayer carry out most membrane functions  Transport of selected hydrophilic substances  Enzymatic activity and function  Signal transduction  Attachment/Recognition  Glycoproteins – carbohydrate tags that allow membranes to know they are the same and let them join together Categories 1. Integral (transmembrane) Proteins • Inserted into the membrane • Must interact with polar and non-polar region of the membrane (amphipathic) • Must have non polar region for interaction with the hydrophobic region of the membrane • Difficult to remove • Transmembrane Proteins  Most common integral proteins  Cross entire membrane numerous times(from extracellular to intracellular)  Dominated by nonpolar amino acids 2. Peripheral Proteins • Attached to one hydrophilic side (either intracellular or extracellular) • Primarily found on the cytosolic side • Attached via non-covalent bonds (easy to detach - important) • Can attach via integral proteins, or lipid anchor, or directly with lipid molecules 5.4 – Passive Membrane Transport • Passive Transport depends on Diffusion  The Net movement of molecules form a region of higher concentration to a region of lower concentration  It does not require cells to expend energy Types of Passive Transport 1. Simple diffusion • Diffusion directly across the membrane on the concentration gradient. • No transporter involved • No energy required • Concentration gradient, molecular size and lipid solubility determine the rate 2. Facilitated Diffusion • Passive transport that is facilitated by transport proteins  Without the transport protein, diffusion is either very slow or non-existent • Hydrophilic Channel Proteins  Always Open – but often specific for a certain molecule  Diffusion rate dependant on concentration gradient AND the number of channel proteins  E.g. Aquaporins water channels – facilitates water movements across the membrane • Gated Channels  Switch back and forth between open and closed state  When open, ions can flow through. When closed, they cannot  Activated (opened) by different means (e.g. voltage, ligand)  Ligand – molecule that will bind to the receptor  Activation method is channel dependant  E.g. voltage gated ion channels – seen very often in the nervous system  E.g. Nerve impulse to muscle – • Carrier Proteins  Carries specific molecule across the membrane  Molecule binds to area in the transporter  Causes conformational change  Conformational change leads to opening towards the other side 3. Osmosis • Diffusion of water based on solute concentration outside the cell, relative to the inside cell • Water moves by passive diffusion (directly through the membrane) and facilitated diffusion (via aquaporins) – important in Kidney Function • Water can move: into the cell; out of the cell; no movement at all • Tonicity  Measure of osmotic pressure (water pressure on the membrane)  It is always relative (in comparison) to another solution  Knowing the tonicity of a solution permits one to determine the direction of water flow (osmosis direction) • Hypertonic – High relative to the compared solution • Hypotonic – Low relative to the compared solution • Isotonic – Compared solutions have the same concentration • Osmosis is a matter of Life/Death  Slugs sprinkled with salt lose water by osmosis  Keratin in human skin prevents us from being vulnerable to salt 5.5 – Active Membrane Transport • Active Transport – Moves substances against their concentration gradients  Requires cells to expend energy  Depends on membrane proteins  Specific for certain substances  Becomes saturated at high concentrations of the transported substance • Primary Transport Pumps – Directly use ATP as their energy source  Moves/pumps cations against their gradient to maintain or create a stronger gradient  Typically specific to a certain cation  Undergoes conformational changes • Secondary Transport Pumps – Use favourable concentration gradients of positively charged ions as their energy source for transport  Set up by primary transport pumps  Symport – The transported substance moves in the same direction as the concentration gradient used as the energy source – Cotransport + +  E.g. Na /Glucose Symport: binding of Na causes conformation change that increases binding affinity for glucose  Na is transferred with the gradient and glucose against it  Antiport – The transported substance moves in the direction opposite to the concentration gradient used as the energy source – Exchange Diffusion 5.6 – Exocytosis and Endocytosis • Large molecules and particles are move
More Less
Unlock Document

Only pages 1,2 and half of page 3 are available for preview. Some parts have been intentionally blurred.

Unlock Document
You're Reading a Preview

Unlock to view full version

Unlock Document

Log In


Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.