Biological Membranes Flashcards
Fluid Mosaic Model
accounts for presence of lipids, proteins, and carbohydrates in a dynamic semisolid plasma membrane surrounding cells
Phospholipids
move rapidly in the plane of the membrane through simple diffusion
Lipid Rafts
- collections of similar lipids with or without associated proteins that serve as attachment points for other biomolecules
- travel slowly within the plane of the membrane
Flipases
enzymes that assist in the flipping/movement of lipids between the membrane layers
What are the different components of a membrane?
- lipids (fatty acids, triacylglycerols, glycerophospholipids, sphingolipids, cholesterol, steroids)
- Proteins
- Carbohydrates
Lipids
- most plentiful in the cells membrane
- most are phospholipids and few are free fatty acids
Triacylglycerols
- storage lipids involved in metabolic processes
- contain 3 fatty acid chains esterified to a glycerol backbone
Unsaturated Fatty Acids
- “healthier”
- one or more double bonds
- exist in liquid form at room temperature
Saturated Fatty Acids
- main component of animal fat
- exist as solids at room temperature
- “less healthy”
- decrease membrane fluidity
Glycerophospholipids
- replace one fatty acid with a phosphate group which is often linked to other hydrophobic groups
- used for membrane synthesis
- can produce a hydrophilic surface layer of lipoproteins like VLDL
Sphingolipids
- contain a hydrophilic region and two fatty acid derived hydrophobic tails
- classes: ceramide, spingomyelins, cerebrosides, gangliosides
Cholesterol
- regulates membrane fluidity
- necessary for synthesis of steroids
- contains hydrophilic and hydrophobic region
- increases fluidity at low temperatures and decreases fluidity at high temperatures
What are the functions of proteins in a cell membrane?
- transporters
- cell adhesion molecules
- enzymes
Transmembrane Proteins
- have one or more hydrophobic domains
- function as receptors or channels
Embedded Proteins
- either part of a catalytic complex or involved in cellular communication
- usually linked to nearby enzymes
Peripheral Proteins
involved in signaling or are recognition molecules on the extracellular surface
Carbohydrates
- usually attached to proteins on extracellular surface
- generally hydrophilic and can form protective glycoprotein coat
- function in signaling and recognition
Cell Adhesion Molecules
- typically compose cell-cell junctions
- proteins that allow cells to recognize each other and contribute to cell differentiation and development
Gap Junctions
- allow direct cell-cell communication
- found in small bunches
- made up of connexons which are formed by alignment and interaction of pores composed of 6 molecules of connexin
- permit movement of water and some solutes (ions) directly between cells
- essentially like a tunnel that connects two cells
Tight Junctions
- not used in intercellular transport
- form a water tight seal and prevent paracellular transport of water and solutes
- found in epithelial cells
- function as physical link between cells
- limit permeability enough to create a transepithelial voltage difference
Desmosomes
- anchor cytoskeletons of adjacent cells together via interactions between transmembrane proteins associated with intermediate filaments
- ions and water can flow between cell gaps
Where do tight junctions occur in the human body?
- bladder
- intestines
- kidney
Where do desmosomes occur in the human body?
- skin
- intestines
Where do gap junctions occur in the human body?
cells/tissue that spread action potential or cells that use electrical coupling
- cardiac muscle
- neurons
Passive Transport
- spontaneous processes that don’t require energy (- delta G)
- includes: diffusion, facilitated diffusion, osmosis
- primary thermodynamic motivator is an increase in entropy (delta S)
Simple Diffusion
involves movement of small, nonpolar, lipid-soluble molecules down their concentration gradient
Osmosis
- movement of water through aquaporins from an area of low solute concentration to a high solute concentration
- referred to when a specific solute is impermeable
What is the osmolarity of red blood cells?
300 mOsm / L
Hypotonic
- internal solute concentration > external solute concentration
- leads to swelling of cell
Hypertonic
- internal solute concentration < external solute concentration
- leads to cell shriveling
Isotonic
solutions inside and outside of cell are equimolar
Osmotic Pressure
- colligative property [physical property of solutions that depends on the concentration of dissolved particles and not their chemical identity]
- the pressure applied to a pure solvent to prevent osmosis and is used to express the concentration of the solution
Facilitated Diffusion
- uses transport proteins to move impermeable solutes across the cell membrane
- transports large, polar, charged molecules across the membrane
Active Transport
- non-spontaneous processes that require energy (+ delta G)
- results in net movement of solute against its concentration gradient
- includes: primary active, secondary active
Primary Active Transport
- uses ATP directly or use of transmembrane ATPase
- Ex. Sodium/Potassium pump
Secondary Active Transport
- no direct coupling to ATP hydrolysis
- uses an electrochemical gradient – generated by active transport – as an energy source to move molecules against their gradient, and thus does not directly require a chemical source of energy such as ATP
- can transfer molecules in a symport or antiport direction
Endocytosis
- cell membrane invaginates and engulfs material to bring into cell
- initiated by substance binding to specific receptors then carried out by vesicle-coating proteins (ex. clathrin)
- includes: pinocytosis, phagocytosis
Pinocytosis
endocytosis of fluids and dissolved particles
Phagocytosis
endocytosis of large solids like bacteria
Exocytosis
- secretory vesicles fuse with membrane and release material into extracellular fluid
- important in neurotransmitter release and intercellular signaling
Membrane Potential
different in electrical potential across cell membranes
Leak Channels
allow ions to passively diffuse down their concentration gradient through the cell
What type of leak channel occurs most in neurons?
K+ leak channels
Sodium/Potassium Pump
- primary active transport
- pumps 3 Na+ out of the cell and 2 K+ into the cell
- maintains low concentration of sodium ions and high concentration of potassium ions intracellularly
Nernst Equation
used to determine the membrane potential from the intra- and extracellular concentrations of various ions
Goldman-Hodgkin-Katz Voltage Equation
flows from the Nernst Equation but takes into account the relative contribution of each major ion to the membrane potential
How do Mitochondria produce ATP?
oxidative phosphorylation
Outer Mitochondrial Membrane
- highly permeable to ions and small proteins because has many large pores
- small intermembrane space between outer and inner membranes
Inner Mitochondrial Membrane
- more restricted permeability
- membrane lacks cholesterol
- contains cristae (infoldings of the membrane) which increases surface area for integral proteins associated with the ETC and ATP synthesis
- encloses the mitochondrial matrix
- contains a high level of cardiolipin
What important process occurs in the mitochondrial matrix?
Citric Acid Cycle
Does a pH gradient exist between the cytoplasm and intermembrane space in the mitochondria?
No – this is because the outer membrane has such a high permeability to biomolecules
