Membranes Flashcards
1
Q
Generalized Cells
A
- All cells have some common structures and functions
- Human cells have three basic parts: Plasma membrane, Cytoplasm, Nucleus
2
Q
Plasma Membrane
A
- Lipid bilayer and proteins in constantly changing fluid mosaic
- Players dynamic role in cellular activity
- Separates intercellular fluid (ICF) from extracellular fluid (ECF)
- Cells surrounded by interstitial fluid (IF)
- Plasma membrane allows cells to:
- obtain from IF exactly what it needs, exactly ehrn it is needed
- keep out what it does not need
3
Q
Membrane Lipids
A
- 75% phospholipids:
- phosphate head: polar and hydrophilic
- fatty acid tails: nonpolar and hydrophobic
- 5% glycolipids: Lipids with polar sugar groups on outer membrane surface
- 20% Cholesterol: increase membrane stability
4
Q
Membrane Proteins
A
- Allow communication with environment
- 1/2 mass of plasma membrane
- Most specialized membrane function
- some float freely
- Some tethered to intracellular structures
two types: Integral proteins, peripheral proteins
5
Q
Integral Proteins
A
- Firmly inserted into membrane
- Have hydrophobic and hydrophilic regions
- function as transport proteins, enzymes, or receptors
6
Q
Peripheral Proteins
A
- loosely attached to integral proteins
- Include filaments on the intracellular surface for membrane support
- Function as enzymes; motor proteins for shape change during cell division and muscle contraction; cell-to-cell connections
7
Q
Six Functions of Membrane Proteins
A
1: Transport
2: Receptors for signal transduction
3: Attachment to the cytoskeleton and extracellular matrix
4: Enzymatic activity
5: Intracellular joining
6: Cell-Cell recognition
8
Q
Types of Membrane Transport
A
- Plasma membranes are selectively permeable: some molecules pass through easily; some do not
- Passive Processes: No cellular Energy(ATP) required, substance moves down its concentration gradient
- Osmosis
- Active Processes: Energy(ATP) required, Occurs only in living cell membranes
9
Q
Passive transport
A
- Diffusion
*Facilitieted transport - Osmosis
10
Q
Diffusion
A
- Describes the spread of particles (which can also be atoms or molecules) through random motion from regions of higher concentration to regions of lower concentration
- Diffusion can still occur when there is no concentration gradient (but there will be no net flux)
- Driven by a decrease in Gibbs free energy
11
Q
Macroscopic theory of diffusion
A
- Fick’s first law of diffusion: Net flux is proportional to the spatial gradient of the concentration function
- Proposed in an analogy to Fourier’s law of heat transfer J = -D(dC/dx) Fick’s first law, where:
- J = Diffusion flux, amount of substance per unit area per unit time
- D = Diffusion coefficient, length/time
- C = Concentration, amount of substance per unit volume
- x = position, length
- Fick’s second law of diffusion: The time rate of change in concentration is proportional to the curvature of the concentration function
- Follows from continuity equation and Fick’s first law
- Can be derived from the one-dimensional random walk
- Predicts how diffusion causes the concentration to change with time
(dC/dx)= D(d^2 C/ dx^2) Fick’s second law
12
Q
Liquid diffusion coefficient
A
- Stokes showed that for a spherical particle, the drag force is related to size and solvent viscosity
- Liquid diffusion coefficients from the Stokes-Einstein equation: D = (kb*T)/f = kbT/(6π µr)
13
Q
Permeability vs. Diffusion
A
- permeability(filtration) = The rate of flow of a liquid or gas through a porous material
- Diffusion = The passive movement of molecules or particles along a concentration gradient
- Permeability depends on:
- diffusion coefficient (D)- P increases when D increases
- Membrane thickness (∆x) - P decreases when ∆x increases
- Partition coefficient (K) - P increases when K increases
- P = (KD/∆x)
14
Q
diffusion across the lipid bilayer
A
- Any molecules will eventually diffuse across a protein-free lipid bilayer down a concentration gradient
- The rate of diffusion depends on the size of the molecule and its hydrophobicity
- Small nonpolar molecules (O2, CO2) diffuse rapidly
- Small polar molecules (water, urea) diffuse slowly
- Lipid bilayer is highly impermeable to charged ions
- Membrane transport proteins are needed to allow essential molecules to pass through the lipid bilayer
- Ions, sugars, amino acids, nucleotides, cell metabolites
- Transporters: Bind a specific solute and undergo conformational changes to transfer the solute across the membrane
- Channels: Form aqueous pores that extend across the lipid bilayer, for example: aquaporins
- Much faster transport than transport via proteins
15
Q
Passive process: Diffusion
A
- Collisions cause molecules to move down or with their concentration gradient
- Difference I concentration between two areas
- Speed influenced by molecule size and temperature
- Nonpolar lipid-soluble substances diffuse directly phospholipid bilayer
- Certain lipophilic molecules are transported passively by: binding to protein carriers, Moving through water-filled channels
16
Q
Carrier-Mediated facilitated diffusion
A
- Transmembrane integral proteins are carriers
- Transport specific polar molecules (e.g., sugars and amino acids) too large for channels
- Binding of the substrate causes shape change in the carrier and then passage across the membrane
- Limited by the number of carriers present
- Carriers saturated when all engaged
17
Q
Channel-mediated facilitated diffusion
A
*Aqueous channels formed by transmembrane proteins
* Selectively transport ions or water
* Two types:
- Leakage channels: Always open
- Gated channels: Controlled by chemical or electrical
signals
18
Q
Passive Processes: Osmosis
A
- water concentration varies with the number of solute particles because solute particles displace water molecules
- Osmolarity: Measure of the total concentration of solute particles
- Water moves by osmosis until hydrostatic pressure and osmotic pressure equalize
- When solutions of different osmolarity are separated by a membrane permeable to all molecules, both solutes and water cross the membrane until equilibrium reached
- When solutions of different osmolarity are separated by a membrane impermeable to solute, osmosis occurs until equilibrium reached
- Osmosis causes cells to swell and shrink
- Change in cell volume disrupts cell function, especially in neurons
19
Q
Tonicity
A
- Tonicity: the ability of a solution to alter a cell’s water volume
- Isotonic: solution with the same non-penetrating solute concentration as cytosol
- Hypertonic: Solution with higher non-penetrating solute concentration than the cytosol
- Hypotonic: Solution with lower non-penetrating solute concentration than the cytosol
20
Q
Osmolarity
A
- Osmolarity: Measure of solute concentration
- Osmosis: Movement of water into or out of cells down a concentration gradient
- Sources of intracellular osmolarity:
- Macromolecules: Contribute very little to the osmolarity of the cell (few of them compared to small molecules)
- Charged, which attracts oppositely charged inorganic ions
- Counter ions make a major contribution to osmolarity
- Small organic molecules (sugars, amino acids, nucleotides):
- Both charged small molecules and their counterions contribute to osmolarity
- Osmolarity is mainly due to small organic ions
- Cell must control osmolarity or water will continuously move into the cell by osmosis
- Special case: Red blood cells
- No nucleus
- Plasma membrane with a high permeability to water
- High number of Na+ - K+ pumps for controlling cell volume
- If placed in a hypotonic solution (low solute, high water concentration):
- Water rushes into cell
- Cells burst
- If placed in a hypertonic solution (high solute, low water concentration)
- Water leaves cell
- Cells shrink
