Cell membranes Flashcards
What are the general functions of a cell membrane?
- Separates intracellular fluid from extracellular
- Controls entry of nutrients, elimination of waste, and release of products
- Cell recognition and communication, cascades in response to stimuli
- Structural support via cytoskeleton, ECM
What were the names of the early and present model of cell membranes?
Butter sandwich: protein layers sandwiching fat layer
Present day: fluid mosaic model
What are the structures found in cell membranes?
- Lipids (glycolipids, phospholipids, cholesterol, sphingolipids)
- Proteins (integral, peripheral)
- Cytoskeleton and Extracellular matrix
What is the structure of phospholipids?
Glycerol backbone + two non-polar fatty acid chains + polar glycerol-phosphate head
What is the difference between a micelle and a liposome?
micelle: droplet with single layer of phospholipids
lipisomes: larger spheres with bilayer phospholipid walls and aqueous centre; centre sometimes filled with drugs and called immunoliposomes
What are the three types of phospholipids?
phosphatidyl-ethanolamine
phosphatidyl-serine
phosphatidyl-choline
What is a sphingolipid’s structure and function?
structure: Lipid molecule consisting of a fatty acid tail, a sphingosine and a phospholipid/glycolipid head.
function: aggregate in rafts in cell membranes due to slightly longer tails than phospholipids (sphingomyelin), interact with cholesterol
What is cholesterol’s structure and function?
Flat molecule consisting of three cyclohexane and one cyclopentane that wedge between hydrophilic heads to maintain fluidity and semipermeability
What are integral proteins?
Proteins tightly bound to the membrane that cannot be removed without destroying the membrane’s integrity
What are peripheral proteins?
attach to other membrane proteins by noncovalent interactions and can be removed without destroying the membrane’s integrity.
What are transmembrnae proteins? What happens to the intracellular and extracellular loops?
membrane-spanning proteins that extend all the way across the membrane more than once.
Extracellular loops may have carbs attached, intracellular loops may have phosphates attached.
How are transmembrane proteins classified?
Classified into families according to how many transmembrane segments they have. Typically seven, but may be up to twelve.
How many amino acids are in a membrane spanning protein? How do they stay in the membrane?
20-25 non-polar amino acids create strong non-covalent interactions with the lipid tails of phospholipids which hold them tightly in place.
What are lipid anchored proteins? What are ways they can be anchored?
Surface proteins that are anchored to the membrane.
- Can be covalently bound to lipid tails and insert themselves in the bilayer
- Can be held by a GPI anchor
- Can associate with sphingolipids in lipid rafts
- May be anchored to cytoskeleton proteins.
What is a GPI anchor?
lipid + sugar-phosphate chain that holds onto proteins on the external surface of the cell.
What is the difference between montopic, bitopic, and polytopic integral membranes
monotopic: do not span the lipid bilayer
bitopic: span the lipid bilayer once
polytopic: span the lipid bilayer more than once
How are monotopic integral proteins attached to the membrane
- Strongly hydrophobic sections that tightly associate with the lipid portion of the bilayer
- Modified via fatty acid to stay in the bilayer
- Electrostatic or ionic interaction between protein and phospholipid
What is the glycocalyx?
Protective husk on the made of membrane carbohydrates found exclusively on the exterior of the cell (glycoproteins/glycolipids) used in recognition in immune responses.
What are three types of equilibrium found in the body?
Osmotic equilibrium: fluid concentrations are equal on the two sides of the cell membrane
Chemical disequilibrium: uneven distribution of solutes between the ECF and the ICF
Electrical disequilibrium: uneven distribution of ions between ECF and ICF
What is the extracellular matrix?
Membrane proteins and secreted protein found on the extracellular side of cell membranes. Contributes to physical strength of cells, immune recognition and adhesion.
What is dystrophin?
Protein that anchors muscle fibres by connecting actin cytoskeleton to proteins of the ECM.
What is muscular dystrophy?
Dystrophin protein is missing, truncated (short lived), or doesn’t function properly which results in easily damaged muscles, myonecrosis (Death of muscle), and death when cardiac muscles are involved.
What is Duchenne muscular dystrophy and what causes it?
X-linked recessive disorder characterized by muscle degeneration. It is caused by a frameshift mutation in the dystrophin gene which translates a truncated protein.
What are clues to suspect Duchenne Muscular Dystrophy?
- Family history of DMD
- Muscle weakness beginning in pelvic girdle muscles and progresses superiorly
- Enlarged calf muscles from pseudo hypertrophy (Fibrofatty replacement of muscle)
- Mental retardation and developmental delay
What is found during clinical tests to support the diagnosis of DMD?
Serum: Elevated creatine kinase from muscle breakdown. Elevated aldolase.
Western blot: absence of dystrophin protein
Muscle biopsy: necrotic muscle fibres from degeneration
What is the cause of death of DMD?
Dilated cardiomyopathy: dilated left ventricle stretches and becomes weak
Respiratory failure: secondary muscle weakness in diaphragm
What are the ways to approach the treatment of muscular dystrophy.
- Inserting dystrophin minigene into viral carrier which enters the target cell and takes over genetic commands
- Cell transplant of dystrophin producing myoblasts and/or satellite cells to rebuild muscle
- Upregulation of Utrophin to replace dystrophin
- Inhibition of Myostatin (protein that inhibits muscle growth) to stimulate muscle growth.
What is osmosis? Osmotic pressure?
Water moves to dilute the more concentrated solution. Once concentrations are equal, net movement of water remains in dynamic equilibrium. Osmotic pressure is the amount of pressure applied to oppose osmosis.
What is osmolarity?
The number of osmotically active particles (molecules + dissociated ions) per liter of solution.
What is osmolality?
The number of osmotically active particles (molecules + dissociated ions) per kg of water
What is the difference between isosmotic, hyperosmotic, and hyposmotic?
isomotic: two solutions contain the same number of solute particles per unit volume
hyperosmotic: when one solution has a higher osmolarity compared to another solution
hyposmotic: when one solution has a lower osmolarity compared to another solution
What slows diffusion?
cold temp, large molecules, far distances, diffusion across a membrane
What solutes can pass or not pass through the bilayer?
Pass: O2, CO2, Lipids, Steroids, nonpolar molecules
Channel diffusion: H2O, Urea
Protein Reliant: ions, large uncharged polar molecules (glucose, proteins, amino acids)
What factors affect the rate of diffusion?
- Permeability across membrane (size/solubility)
- Concentration gradient
- Surface area
- Temperature
- Composition of membrane (ECM, types of phospholipids/sphingolipids, presence of cholesterol)
What is Fick’s Law of diffusion?
The rate of diffusion is proportional to both SA and concentration gradient, and is inversely proportional to membrane thickness.
What is tonicity?
Describes how solution would affect cell volume if the cell were placed in the solution. Depends on relative concentrations of penetrating solutes.
What are 3 ways osmolarity and tonicity differ?
- Osmolarity describes number of particles dissolved and has units measurable by an osmometer. Tonicity has no units as it is a relative term.
- Osmolarity can compare any two solutions and relationship is reciprocal. Tonicity compares a solution and a cell where only the solution is described
- Osmolarity does not tell you about what happens to a cell placed in solution. Tonicity tells you what happens to cell volume when placed in a solution.
What is the difference between penetrating solutes and nonpenetrating solutes?
penetrating: solute particles can cross the cell membrane
nonpenetrating: solute particles that cannot cross the cell membrane
What are the three types of -tonic solutions?
hypotonic: less concentrated with NP solutes than inside of cell; net movement of water inside cell
hypertonic: more concentrated with NP solutes than inside of cell; net movement of water outside cell
isotonic: concentrations of nonpenetrating solutes are the same in the cell and the solution
Why does isosmolarity not reflect isotonicity?
An isosmolar solution has the same concentration of dissolved particles on the inside and outside of the cell, but if the dissolved particles in each side have a different ratio of NP solutes, it will not be isotonic.
What is bulk flow?
Pressure gradient causes fluid to flow from regions of higher pressure to regions of lower pressure
What properties influence a molecule’s movement across the cell membrane and why?
Size and lipid solubility. If the molecule is small and lipid soluble, it will freely cross the bilayer. If it is large and less lipid soluble, they usually do not cross unless the cell has mechanics to allow it.
What is passive transport and active transport?
Passive: does not require energy and relies on concentration/pressure gradient (i.e. simple diffusion, facilitated diffusion)
Active: requires input of energy from outside source
What is Fick’s law of diffusion? What is flux?
Rate of diffusion is proportional to surface area, concentration gradient, and membrane permeability.
Flux is the diffusion rate per unit SA of membrane.
What is mediated transport? What kinds of diffusion does it include and with what proteins?
Transport across the membrane with the help of proteins. Includes facilitated diffusion (e.g. GLUT) or active transport (e.g. Na+/K+/ATPase)
What are the three major roles of structural proteins?
- Hold Tissues together
- Connect membrane to cytoskeleton to maintain shape of cell.
- Attach cells to extracellular matrix by linking cytoskeleton fibres to extracellular collagen and other protein fibres.
What do membrane enzymes do? What about receptor proteins?
Enzymes: Catalyze chemical reactions on the inside or outside of the cells
Receptor Proteins: Binding of a signal ligand triggers an event at the membrane
What is the difference between channel proteins and carrier proteins?
Channel: water filled passageways that link extracellular compartments
Carrier: Bind to substrates and transports them without forming an open channel between two sides of the membrane.
What is aquaporin?
A type of channel protein for transporting water.
What are types of active transport pumps?
- Na+/K+/ ATPase (2K+ in, 3Na+ out)
- H+ ATPase
- H+/K+ ATPase
What are the physiological concentrations of K+, Na+, Cl-, Ca++ OUTSIDE the cell?
K+: 5mM
Na+: 145mM
Cl-: 108mM
Ca++ 1mM
What are the physiological concentrations of K+, Na+, Cl-, and Ca++ INSIDE the cell?
K+: 150mM
Na+: 15mM
Cl-: 5mM
Ca++: 0.0001mM
What is the difference between primary and secondary active transport?
Primary: directly uses ATP as source of energy
Secondary: uses the concenrtation gradient of one prticle to move another against its gradient (e.g. SGLT-protein)
How does the SGLT protein work?
Na+ binds to carrier due to low concentration inside the cell which creates a binding site for glucose. Glucose binding changes carrier form and Na+ is released into cytosol, glucose following.
What are the different types of carrier proteins?
Uniport: Transports one molecule
Symport: Transports two molecules
Antiport: Transports two molecules in different directions
