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1
Q

Membrane Selectivity

A
  • For a cell to function efficiently it must be able to control its interactions with
    the external environment.
  • In other words, it needs to regulate what is allowed to go in and out of the
    cell.
  • The plasma membrane is the boundary that separates the living cell from
    its surroundings.
  • The plasma membrane exhibits selective permeability, allowing some
    substances to cross it more easily than others.
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2
Q

fluid mosaics

A

cellular membranes are fluid mosaics of lipids, proteins and carbohydrates

  • Lipid portion: phospholipids, cholesterol, lipid portion of glycolipids
  • Protein portion: both integral and peripheral proteins (and protein parts of glycoproteins).
  • Carbohydrate portion: “glyco” (sugar) portion of glycolipids and glycoproteins.
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3
Q

amphipathic

A

Phospholipids are the most abundant lipid. They are amphipathic molecules, containing hydrophobic
and hydrophilic regions.
- hydrophilic head; hydrophobic tail

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4
Q

The Fluidity of Membranes

A
  • Phospholipids in the plasma membrane can move
    within the bilayer.
  • Most of the lipids, and some proteins, drift laterally
    and can flex and rotate. (lateral mvmnt refers to adjacent phsopholipids switching positions)
  • Rarely, a lipid may flip-flop transversely across
    the membrane.
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5
Q

Experimental evidence for the fluidity of membranes

A 
1. Mouse and human cells
with different membrane
proteins are fused to make
a heterocaryon.
2. Antibodies against each kind of membrane protein are added to the fused cell.
3. The two kinds of antibodies are coupled
to different fluorescent molecules
(rhodamine: red, against human protein; fluorescein: green, mouse).
4. At t=0, the two colors
are on opposite sides of the cell.
5. However, after forty minutes, the two
 proteins have redistributed themselves
 so that the colors are intermixed.
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6
Q

definition of saturated

A
  • Unsaturated hydrocarbons are hydrocarbons that have double or triple covalent bonds between adjacent carbon atoms. The term “unsaturated” means more hydrogen atoms may be added to the hydrocarbon to make it saturated.
  • The hydrocarbon tails of unsaturated phosophlipids are kinked. That’s what the double bond of unsaturation does to the shape of the molecule.
  • Saturated hydrocarbons are molecules with only single bonds.
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7
Q

The fluidity of membranes can be influenced by its composition.

A

Membranes rich in unsaturated fatty acids are more fluid
than those rich in saturated fatty acids.
- Saturated hydrocarbon
tails pack together – membrane is viscous
- Unsaturated tails prevent packing. – membrane is more fluid

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8
Q

Cholesterol within the animal

cell membrane

A
  • The steroid cholesterol is wedged between phospholipid molecules in the plasma membranes of animal cells
  • At relatively high temperatures—at 37°C, the
    body temperature of humans, for example—cholesterol makes
    the membrane less fluid by restraining phospholipid movement.
  • However, because cholesterol also hinders the close packing of phospholipids, at low temperatures it
    hinders solidification by
    inhibiting tight packing
  • Thus, cholesterol can be thought of as a
    “fluidity buffer” for the membrane, resisting changes in membrane fluidity that can be caused by changes in temperature.
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9
Q

peripheral proteins

A

Peripheral proteins are not embedded in the lipid bilayer at all; they are appendages loosely
bound to the surface of the membrane, often to exposed parts
of integral proteins

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10
Q

integral proteins

A
  • penetrate the hydrophobic core of the lipid bilayer
  • Integral proteins that span
    the membrane are called
    transmembrane proteins.
  • The hydrophobic regions of
    an integral protein consist of
    one or more stretches of
    nonpolar amino acids, often
    coiled into ALPHA helices.
  • (Just like phospholipids,
    transmembrane proteins are
    amphipathic.)
  • N-terminus outside the cell; C-terminus inside (C for cytoplasmic side)
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11
Q

Six major functions of membrane proteins

A
  • transport
  • enzymatic activity
  • signal transduction
  • cell-cell recognition
    intercellular joining
  • Attachment to the cytoskeleton and ECM
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12
Q

membrane protein function: transport

A
  • movement of molecules across the membrane
  • Channel protein:
    allows molecules to
    diffuse down their
    concentration gradients
    (does not require energy).
    Example: ion channels.
  • Pump protein:
    moves molecules
    against their
    concentration gradients
    (requires energy).
    Example: Proton pump in lysosomes.
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13
Q

membrane protein function: enzymatic activity

A 
- Protein is oriented so that
 its active site is pointing
toward either the intracellular
or extracellular space.
- Often a team of proteins is
coupled to carry out a series
of reactions; ex:  adenylyl cyclase
converts ATP to cAMP.
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14
Q

membrane protein function: signal transduction

A 
Membrane protein (receptor) binds
a specific signaling molecule (ligand)
and undergoes a conformational
change that affects the binding of
an intracellular protein.
Example: neurotransmitter receptors.
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15
Q

membrane protein function: cell-cell recognition

A
  • Some glycoproteins act
    as identification tags specifically
    recognized by other cells.
  • This type of
    cell-cell binding is usually short-lived
    compared to intercellular joining
  • Membrane carbohydrates may be covalently bonded to lipids
    (forming glycolipids) or more commonly to proteins
    (forming glycoproteins)
  • ex: blood cells can have A and/or B type antigens.
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16
Q

membrane protein function: intercellular joining

A

Membrane proteins of adjacent
cells may bind together.
Example:
Tight junctions.

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17
Q

membrane protein function: attachment to cytoskeleton and ECM

A 
- Cytoskeletal components
non-covalently bond to membrane
proteins,  a function
that helps maintain cell shape and
stabilizes the location of certain
membrane proteins -- ex: integrins couple ECM to actin
-  Proteins that can
bind to ECM molecules can coordinate
extracellular and intracellular changes
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18
Q

sidedness of membranes

A

Membranes have a “sidedness”. The asymmetrical distribution of proteins, lipids, and
associated carbohydrates in the plasma membrane is determined when the
membrane is built by the ER and Golgi apparatus.

19
Q

passive transport

A
  • Substances diffuse down their concentration
    gradient, the region along which the density of a
    chemical substance increases or decreases.
  • No work must be done to move substances down
    the concentration gradient.
  • The diffusion of a substance across a biological
    membrane is passive transport because no
    energy is expended by the cell to make it happen.
20
Q

diffusion

A
  • Diffusion is the tendency for molecules to spread out evenly
    into the available space.
  • Although each molecule moves randomly, diffusion of a
    population of molecules may be directional.
  • At dynamic equilibrium, as many molecules cross the
    membrane in one direction as in the other.
  • Two different solutes will move independently of the other.
21
Q

osmosis

A
  • Although it may be odd to consider, water can also
    have a concentration. Very concentrated solutions
    (high solute concentrations) would have a lower
    water concentration than very dilute solutions (low
    solute concentrations).
  • Osmosis is the diffusion of water across a
    selectively permeable membrane.
  • Water diffuses across a membrane from the region
    of lower solute concentration to the region of
    higher solute concentration.
22
Q

hypoosmotic

A

having a lower water concentration relative to another solution

23
Q

tonicity

A
  • Tonicity is the ability of a surrounding solution to
    cause a cell to gain or lose water.
  • Note that tonicity is similar to osmolarity but refers
    specifically to cells.
  • Isotonic solution: Solute concentration is the same
    as that inside the cell; no net water movement
    across the plasma membrane.
  • Hypertonic solution: Solute concentration is
    greater than that inside the cell; cell loses water.
  • Hypotonic solution: Solute concentration is less
    than that inside the cell; cell gains water.
24
Q

animal cell under conditions of different tonicity

A
  • hypotonic: water moves in –> lysed
  • iso: normal
  • hyper: water out –> shriveled
25
Q

plant cell under conditions of different tonicity

A
  • hypo: swells until the wall opposes uptake; the cell is now turgid (firm).
  • iso: there is no net movement of water into the cell; the cell becomes flaccid (limp)
  • hyper: plant cells lose water. The membrane pulls away from the cell wall; the plant wilts. The effect is called plasmolysis.
26
Q

facilitated diffusion

A
  • In facilitated diffusion, transport proteins “help along” the passive
    movement of molecules across the plasma membrane by providing a
    pathway across the membrane. Molecules move “downhill” with their
    concentration gradients.
  • Transport proteins include channel proteins and carrier proteins (channel-mediated, carrier- / transporter-mediated diffusion)
27
Q

active transport

A 
- in active transport, proteins move solutes “uphill” against their concentration
gradients.
- some form of energy is
required, usually in the
form of ATP.
28
Q

the two types of transport proteins

A

channel proteins and carrier proteins.
- Channel proteins simply
provide corridors that allow specific molecules or ions to cross
the membrane
- Carrier proteins, such as the glucose transporter mentioned earlier, seem to undergo a subtle change in shape that
somehow translocates the solute-binding site across the
membrane. Such a change in shape may be
triggered by the binding and release of the transported molecule

29
Q

Na-K pump

A
  • ratio: 3 Na out, 2 K in
  • a specific case of active transport
    1. Cytoplasmic Na+ binds to the sodium-potassium pump. The affinity for Na+ is high when the protein has this shape.
    2. Na+ binding stimulates phosphorylation by ATP
    3. Phosphorylation leads to a change in protein shape, reducing its affinity for Na+, which is released outside
    4. The new shape has a high affinity for K+, which binds on the extracellular side and triggers release of the phosphate group
    5. Loss of the phosphate group restores the protein’s original shape, which has a lower affinity for K+
    6. K+ is released; affinity for Na+ is high again, and the cycle repeats.
30
Q

phosphorylation

A

A biochemical process that involves the addition of phosphate to an organic compound.

31
Q

electrogenic pump

A
  • An electrogenic pump is a transport protein that generates
    voltage across a membrane.
  • The sodium-potassium pump is an electrogenic pump of animal cells (recall the ratio)
  • Another electrogenic pump is a proton pump.
  • help store energy that can be used for cellular work
32
Q

membrane potential

A

Voltage is created by differences in the distribution of positive and
negative ions across a membrane. This difference is called a
membrane potential.

33
Q

electrochemical gradient

A

Two combined forces, collectively called the
electrochemical gradient, drive the diffusion of ions
across a membrane:
- A chemical force (the ion’s concentration gradient).
- An electrical force (the effect of the membrane potential
on the ion’s movement).

34
Q

Bulk transport across the plasma membrane

A

occurs by exocytosis and endocytosis
- Small molecules and water enter or leave the cell
through the lipid bilayer or via transport proteins.
- However, large molecules, such as
polysaccharides and proteins, are too large to
cross via transport proteins.
- Instead they cross the membrane in bulk via
vesicles.

35
Q

exocytosis

A

In exocytosis, transport vesicles migrate to the
membrane, fuse with it, and release their contents
outside the cell.

36
Q

endocytosis

A

In endocytosis, the cell takes in macromolecules
by forming vesicles from the plasma membrane.
- Endocytosis is a reversal of exocytosis, involving
different proteins.
- There are three types of endocytosis:
- Phagocytosis (“cellular eating”, not selective)
- Pinocytosis (“cellular drinking”, not selective)
- Receptor-mediated endocytosis (very selective)

37
Q

phagocytosis

A 
- Cell engulfs a particle by extending
a psuedopodium and wrapping it
with its plasma membrane.
- The particle is then moved into the
cytoplasm where it can be digested
by a lysosome.
38
Q

\

A 
- Pinocytosis (cell drinking)
is similar to phagocytosis.
- One important difference is
that it involves specific
proteins that give structure
to the invaginated membrane.
- The invagination is called a
coated pit and the vesicle
is called a coated vesicle.
39
Q

receptor-mediated endocytosis

A
  • involves coated pits, coated vesicles
  • Only specific molecules that bind to receptors trigger the process of endocytosis.
  • Example is cholesterol uptake.
40
Q

cotransport

A
  • A single ATP-powered pump that transports a specific solute can indirectly drive the active transport of several other solutes in a mechanism called cotransport. A substance that has been pumped across a membrane can do work as it moves back across the membrane by diffusion. Another transport protein, a cotransporter separate from the pump, can couple the “downhill” diffusion of this substance to the “uphill” transport of a second substance against its own concentration (or electrochemical) gradient
  • For example, a plant cell uses the gradient of H+ generated by its proton pumps to drive the active transport of sucrose into the cell. This protein can translocate sucrose into the cell against a concentration gradient, but only if the sucrose molecule travels in the company of a H+. The H+ uses the transport protein as an avenue to diffuse down the electrochemical gradient maintained by the proton pump.
  • One type of glucose transporter uses the Na+
    gradient across the
    membrane to drive glucose into a cell against its concentration gradient.
41
Q

uniporter

A
  • an integral membrane protein that transports a single type of substrate species across a cell membrane.
  • may use either facilitated diffusion and transport along a diffusion gradient or transport against one with an active transport process.
  • can be either ion channels or carrier proteins
  • Uniporter carrier proteins work by binding to one molecule of substrate at a time. Uniporter channels open in response to a stimulus and allow the free flow of specific molecules.
42
Q

symporters

A
  • fall under the category of coupled transport
  • The symporter works in the plasma membrane and molecules are transported across the cell membrane at the same time, and is, therefore, a type of cotransporter. - called a symporter, because the molecules will travel in the same direction in relation to each other.
  • Typically, the ion(s) will move down the electrochemical gradient, allowing the other molecule(s) to move against the concentration gradient. The movement of the ion(s) across the membrane is facilitated diffusion, and is coupled with the active transport of the molecule(s).
43
Q

antiporters

A
  • aka exchangers, counter-transporters
  • fall under category of coupled transport
  • they are integral membrane proteins involved in secondary active transport of two or more different molecules or ions across a phospholipid membrane such as the plasma membrane in opposite directions
44
Q

intracellular vs extracellular concentration of substances

A

For example, a plant cell uses the gradient of H generated by
its proton pumps to drive the active transport of amino acids,
sugars, and several other nutrients into the cell

b2 (11 decks)

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