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
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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
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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
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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
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5
Q

Integral Proteins

A
  • Firmly inserted into membrane
  • Have hydrophobic and hydrophilic regions
  • function as transport proteins, enzymes, or receptors
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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
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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

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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
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9
Q

Passive transport

A
  • Diffusion
    *Facilitieted transport
  • Osmosis
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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
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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
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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)
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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)
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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
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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
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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
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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

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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
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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
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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
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21
Q

Passive and active transport

A
  • Passive transport (facilitated diffusion) allows solutes to pass the membrane inactively
  • Uncharged molecule: Transport must be driven by concentration gradient
  • Charged molecule: Transport drive by concentration gradient and electrical gradient
  • Mostplasmamembraneshaveanelectricalpotential difference, inside is negative with respect to the outside
  • Entryofpositivelychargedionsfavored
  • Active transport: Transporter-mediated solute transport against an electrochemical gradient
  • Requires energy input
22
Q

Membrane transport and channels

A
  • Membrane channels allow passive transmembrane movement
  • Membrane transporters can create large differences in intra- and extracellular concentrations
23
Q

transporter-mediated solute movement

A
  • Uniporters: Mediate the movement of a single solute from one side of the membrane to the other at a rate determined by their Vmax and Km
  • Coupled transporters: Transfer of one solute depends on the transport of a second
  • Symporters (co-transporters): Simultaneously transport a second solute in the same direction
    0 Antiporters (exchangers): Transfers a second solute in the opposite direction
24
Q

Membrane transport: active processes

A
  • Two types of active processes: Active transport, Vesicular transport
  • Both require ATP to move solutes across a living plasma membrane because
  • Solute too large for channels
  • Solute not lipid soluble
  • Solute not able to move down a concentration gradient
25
Q

Active membrane transport

A
  • process by which a transporter transfers absolute across the lipid bilayer resembling an enzyme-substrate reaction
  • each transporter has one or more binding sites for its solute
  • cells link transporters to an energy source in three ways: coupled transport, ATP-driven pumps, Light-driven pumps
26
Q

Active transport

A
  • requires carrier proteins
  • bind specifical;y and reversibly with substrate
  • Moves solutes against a concentration gradient
  • requires energy
  • Two types:
  • Primary active transport: NEED ATP HYDROLYSIS
  • Secondary active transport: INDIRECTLY from ionic gradient created by primary active transport
27
Q

Primary active transport

A
  • energy from hydrolysis of ATP causes shape change in transport protein that “pumps” solutes(ions) across the membrane
  • Sodium-potassium pump:
  • carrier called Na, K ATPase
  • located in the plasma membrane
  • involved in primary and secondary active transport of nutrients and ions
28
Q

Sodium-potassium pump

A
  • Na and K channels allow slow leakage down a concentration gradient
  • Na-K pump works as antiporter
29
Q

Secondary active transport

A
  • depends on ion gradient created by primary active transport
  • energy stored in ionic gradients used indirectly to drive transport of other solues
  • Cotransport- always transports more than one substance at a time
  • symport system: substance transported in same direction
  • Antiport system: substances transported in opposite direction
30
Q

Ion gradient active transport

A
  • the transfer of two solutes allows the coupled transporters to harvest energy stored in the electrochemical gradient
31
Q

ABC Transport

A
  • Cancer:
  • multidrug resistance protein
  • Able to pump hydrophobic drugs out of the cytosol
  • Overexpressed in cancer cells
  • Malaria:
  • p.falciparum overexpress the ABC transporter that pumps out chloroquine
  • Cystic fibrosis:
  • Caused by a mutation in the gene encoding CTFR
  • CFTR functions as a Cl- -channel in epithelial cells
  • Irregular ion concentrations in the extracellular fluid
32
Q

Vascular Transport

A
  • Transport of large particles, macromolecules, and fluids across the membrane in membranous sacs called vesicles
  • Requires cellular energy (e.g., ATP)
  • Functions:
  • Exocytosis—transport out of cell
  • Endocytosis—transport into cell
    • Phagocytosis, pinocytosis, receptor-mediated
      endocytosis
  • Transcytosis—transport into, across, and then out
    of cell
  • Vesicular trafficking—transport from one area or organelle in cell to another
33
Q

Endocytosis and transcytosis

A
  • Involve formation of protein-coated vesicles
  • Often receptor-mediated, therefore very selective
  • Some pathogens also hijack for transport into cell
  • Once the vesicle is inside the cell it may
  • Fuse with lysosome
  • Undergo transcytosis
34
Q

Endocytosis

A
  • Receptor-mediated endocytosis
  • allows specific endocytosis and transcytosis
  • Clathrin-coated pits provide the main route for endocytosis and transcytosis
  • different coat proteins:
    • caveolae: captures specific molecules and use transcytosis
    • Coatomer: Function in vascular trafficking
35
Q

Exocytosis

A
  • usaually actiavted by cell-surface signal or change in membrane voltage
  • Substacne enclosed in Secetory vesicle
  • v-SNAREs(“v” = vesicle) on vesicle find
  • t-SNAREs(“t”= target) on membrane and bind
  • function: hormone secretion, neurotransmitter release, mucus secretion, ejection of wastes
36
Q

The Glyconcalux

A
  • “sugar coating” at cell surface
  • every cell type has a different pattern of sugars: which allows the immune system to recognize “self” and “non-self”
37
Q

Cell junctions

A
  • co
38
Q

Tight junction

A
  • Adjacent internal proteins fuse -> form impermeable junction encircling cell
  • Where might this be useful? - endothelial cells, skin
39
Q

Desomosomes

A
  • “Rivets” or “spot-welds” that anchor cells together at plaques: linker proteins between cells connect plaques
  • Where might these be useful? -tendons, ligaments
40
Q

Gap junctions

A
  • transmembrane proteins form pores that allow small molecules to pass from cell to cell
  • for the spread of ions, simple sugars, and other small molecules between cardiac or smooth muscle cells
41
Q

Cell environment interactions

A
  • Cells interact directly or indirectly by responding to extracellular signals
  • always involves glycocalyx
42
Q

Roles of Cell Adhesion Molecules

A
  • Thousands on approximately every cell in the body
  • Anchor to extracellular matrix or each other
  • Assist in the movement of cells past one another
  • Attract WBCs to injured or infected areas
  • Stimulate synthesis or degradation of adhesive membrane junctions
  • Transmit intracellular signals to direct cell migration, proliferation, and specialization
43
Q

Roles of Plasma Membrane Receptors

A
  • Contacts Signaling: toughing and recognition of cells
  • Chemical Signaling: interaction between receptors and ligands to alter the activity of cell proteins
44
Q

Chemical signaling

A
  • Ligand binding -> receptor structural change -> protein alteration
  • Catalytic receptor proteins become activated enzymes
  • Chemically gated channel-linked receptors open and close ion gates -> changes in excitability
  • G protein-linked receptors activate G protein, affecting an ion channel or enzyme, or causing the release of internal second messenger, such as cyclic AMP
45
Q

Membrane proteins

A
  • integral proteins
  • Firmly inserted into the membrane (most are
    transmembrane)
  • Have hydrophobic and hydrophilic regions
  • Can interact with lipid tails and water
  • Function as transport proteins (channels and carriers), enzymes, or receptors
46
Q

Transport

A
  • A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute
  • Some transport proteins hydrolyze ATP as an energy source to actively pump substances across the membrane
47
Q

Signal Transduction

A
  • A membrane protein exposed to the outside of the cell may have a binding site that fits the shape of a specific chemical messenger, such as hormones
  • When bound, the chemical messenger may cause a change in shape in the protein that initiates a chain of chemical reactions in the cell
48
Q

Attachment

A
  • elements of the cytoskeleton and the extracellular matrix may anchor to membrane proteins, which help maintain. cell shape and fix the location of certain membrane proteins
  • Others play a role in cell movement or bind adjacent cells together
49
Q

Enzyme activity

A
  • a membrane protein may be an enzyme with its activation site exposed to substances in the adjacent solution
  • a team of several enzymes in a membrane may catalyze sequential steps of a metabolic pathway as indicated
50
Q

Cell-Cell junctions

A
  • membrane proteins of adjacent cels may be hooked together in various kinds of intracellular junctions
  • some membrane proteins of this group provide temporary binding sites that guide cell migration and other cell-to-cell interactions
51
Q

Recognition

A
  • some glycoproteins serve as identification tags that are specifically recognized by other cells