3 Physiology: Cell Membrane Flashcards
The cell membrane is the boundary between?
an ordered intracellular environment (the intracellular fluid; ICF) and a less-ordered extracellular fluid (ECF) or disordered external environment
If the membrane was a simple static structure that merely prevented the admixture of these two environments, most cells would?
Quickly die
Death of the cell may be delayed if energy-rich molecules are stored within the cell, and in extraordinary cases, cell death may be delayed almost indefinitely if the cell drastically curtails its energy requirements, i.e., if it lowers its metabolic rate, as is seen in spore- forming bacteria
HOWEVER CELL MEMBRANE IS NOT A STATIC STRUCTURE
The energy required for maintenance of internal order is derived from?
energy-rich molecules obtained from the ECF
The stored energy-rich molecules are derived from?
molecules obtained from the environment, and the bacterium must respond to information from the environment in order to initiate sporulation (and later, to reverse it).
In many cases, it is the membrane which receives this information.
What is The Cell Membrane?
is a Dynamic, Heterogeneous, Fluid Structure
Describe a dynamic structure
unlike a static one, undergoes change.
Over the course of a cell’s lifespan, its membrane continuously exhibits changes in structure which reflect changing emphasis of function
An example of change in a dynamic cell membrane
the membranes of numerous cell types exhibit an increase in the number of glycoprotein molecules which allow for the passage of glucose through the membrane subsequent to a meal
An example of change in a dynamic cell membrane
phagocytosis, wherein a cell membrane undergoes considerable changes in shape as it surrounds and engulfs a molecule or cell which has made contact with the membrane
The functions of the cell membrane are made possible by the actions of what?
of distinctly different molecules in and associated with the membrane, i.e., the cell membrane is a heterogeneous structure.
Common component molecules of cell membranes include?
phospholipids
sphingolipids
cholesterol
proteins and glycoproteins
Describe how the cell membrane is a fluid structure?
Molecules (especially proteins and glycoproteins) can be experimentally labeled with various markers such as fluorescent dyes.
Microscopic examination of cells reveals movement of the dyes, which therefore indicates the movement of the proteins and glycoproteins in the membrane
the most common molecules in the cell membrane are?
Phospholipids
What is a “fluid mosaic” (the cell membrane is often referred to as this)?
a structure composed of a large number of different molecules (a mosaic) which move within the membrane (fluid).
How are Phospholipids arranged?
arranged in a bilayer, with polar (hydrophilic) components in contact with extracellular and intracellular fluid, and nonpolar (hydrophobic) components making up the bulk of the thickness of the membrane.
What is also found in both monolayers of the membrane?
Cholesterol
What are found in the outer monolayer of the membrane?
Sphingolipids
Some proteins/glycoproteins are embedded where? And these such molecules are called what?
in only one of the two monolayers and therefore are in contact with either the extracellular fluid or the intracellular fluid, while other proteins/glycoproteins span the entire membrane and are in contact with the extracellular and intracellular fluids.
Such molecules are termed TRANSMEMBRANE PROTEINS.
List the functions of the cell membrane
a) It regulates the passage of substances into and out of the cell by the processes of diffusion, facilitated diffusion and active transport (and by other processes such as exocytosis, endocytosis and phagocytosis).
b) It receives information in physical and/or chemical form from the ECF/extracellular environment
c) It catalyzes chemical reactions, i.e., it possesses enzymes
d) It serves structural functions, such as attachment and movement
e) Organelles are subcellular membrane-bound compartments. Organelle
membranes also regulate passage, receive information, catalyze reactions and serve structural function.
Diffusion results from what?
from a greater frequency of collisions between molecules where they exist at a higher concentration than where they exist at a lower one.
diffusion is a manifestation of the laws of?
thermodynamics in that a high concentration of molecules in one place and a low concentration in another is an example of order, while an equal concentration of molecules in two places is an example of disorder.
A number of small, nonpolar molecules are able to easily what?
diffuse through the phospholipid bilayer of the cell membranes.
Paramount amongst these are the gases O2 and CO2.
Other molecules which share the combined hydrophilic/hydrophobic character of phospholipids and cholesterol can do what?
also pass through the membrane with relative ease.
Such molecules include fatty acids and steroid hormones (which are derivatives of cholesterol).
Larger polar/hydrophilic molecules, on the other hand, CAN or CANNOT diffuse through the phospholipid bilayer?
Cannot
Numerous polar molecules do, however, diffuse through cell membranes by way of?
transmembrane proteins.
What allows for the passage of an ion such as Na+ or K+?
these proteins form a water-containing pore or channel
Ion channels are characterized as?
specific (i.e., they usually allow only one or two ions to diffuse “down” its/their concentration gradient/s.)
What may influence the rate at which ions move through protein channels?
electrical forces as well as diffusion because ions are charged
What are Leak Channels?
channels that are always open
Others channels open only in response to specific physical or chemical stimuli, and are named with respect to the stimulus. These are: (3 of them)
Ligand-gated
Voltage-gated
Mechanically-gated
What is a Ligand-gated channel?
a ligand (molecule) binds to the channel (or a neighboring protein), which results in channel opening
What is a Voltage-gated channel?
a change in the electrical potential difference across the membrane (measured in volts) results in channel opening
What is a Mechanically-gated channel?
force/pressure/tension exerted on the cell membrane results in channel opening
transmembrane proteins may bind a what to gain access to the other side of the membrane?
bind a hydrophilic molecule on one side of the membrane.
Such binding causes a change in the conformation of the protein, which subsequently allows the molecule to gain access to the other side of the membrane.
The molecule is released when the bond is broken.
Since the molecule cannot diffuse across the membrane unless it binds to and is released by a protein, the protein is said to?
facilitate the diffusion of the molecule, and the process is termed facilitated diffusion.
One well-known example of facilitated diffusion is?
is provided by the glucose transporter (actually there are a number of different glucose transporters).
What is active transport?
is the movement of a substance from an area of lower concentration to an area of higher concentration.
From a thermodynamic point of view, active transport creates?
order (two distinctly different concentrations on either side of a membrane) and therefore is an energy- requiring process.
Transmembrane proteins which perform active transport are often referred to as?
carriers or pumps.
It is important to realize that if energy is utilized to perform active transport (i.e., to create a concentration gradient), then diffusion (i.e., relaxation or dissipation of a concentration gradient) releases energy, and that a concentration gradient is a form of potential energy. This realization aids in the classification of active transport processes
.
What is Primary Active Transport?
the breaking of a chemical bond in an ATP molecule by the carrier/pump releases the energy required to perform active transport.
What is Secondary Active Transport?
by allowing an ion/molecule to diffuse across the membrane, the carrier/pump utilizes the released energy to perform active transport.
Transmembrane proteins transport ions and molecules in what directions across cell membranes?
Both directions
Uniports transport how many substances?
one substance
Symports transport how many substances and in what directions?
transport two or more substances in the same direction
antiports transport how many substances and in what directions?
(exchangers) transport two substances in opposite directions
Examples of Uniports
Na+ leak channel; K+ leak channel; voltage-gated Na+ channel, mechanically-activated Ca++ channel; aquaporin (water pore)
Examples of Symports
Na+-glucose symport;
Na+ -K+-2 Cl- symport
Examples of Exchangers (antiports)
Na+-Ca++ exchanger;
HCO3- - Cl- exchanger
How can ATPases transport substances?
may transport one substance in one direction (although they
are not called uniports), or two substances in opposite directions (although they are not called antiports): H+ ATPase; Ca++ ATPase; Na+-K+ ATPase
The Cell Membrane Receives Information in Physical and/or Chemical Form from the?
ECF/Extracellular Environment
While changes in voltage or force/pressure can directly alter the shape of transmembrane proteins, ligands must bind to?
to the transmembrane protein at a receptor site or at a neighboring protein receptor.
Many, but not all, ligands are?
neurotransmitters and hormones.
Ligands by virtue of their hydrophobic character, can?
diffuse through cell membranes and bind with intracellular receptors.
Some ligands are generated in the intracellular fluid and therefore bind to transmembrane proteins at the internal monolayer.
The best known of these intracellular ligands is a phosphate group obtained from ATP.
Conformational Changes of Transmembrane Proteins Can Be Elicited By?
Chemical Ligands
Electrical and Physical Stimuli
Many ligands bind to receptors which are associated with?
membrane enzymes.
A number of (but not all) well-characterized receptor systems include?
three membrane proteins: the receptor, a G protein, and the enzyme.
binding of a ligand activates the?
the receptor which in turn activates the G protein.
Activation of a stimulatory G protein increases, and activation of an inhibitory G protein decreases, the activity of the membrane enzyme.
receptor systems are often referred to as what? And why?
signal transduction systems because they convert a signal in the form of an extracellular molecule (the ligand) to a signal in the form of an intracellular molecule produced by the enzyme.
Signal transduction systems are also called?
second messenger systems, where the extracellular ligand (e.g., a neurotransmitter, hormone or other messenger molecule) is the first messenger, and the molecule produced by the enzyme is the second messenger.
While there are many different first messenger molecules, there are only a few well-known second messenger molecules (produced by only a few membrane enzymes).
The most common of these are:
1) cAMP, produced by adenylate cyclase
2) cGMP, produced by guanylate cyclase
3) IP3, produced by phospholipase C
cAMP and cGMP activate what?
kinases (enzymes which perform phosphorylation, a major way by which the activity of enzymes and other proteins is regulated)
IP3 acts as a?
as a ligand at organelle (especially endoplasmic reticulum) membrane.
The binding of IP3 to its receptor causes a transmembrane Ca++ port to open, allowing for the diffusion of Ca++ from endoplasmic reticulum to intracellular fluid.
An increase in intracellular[Ca++] has numerous effects, such as enzyme (including kinase) activation and exocytosis.
The Cell Membrane Serves Structural Functions, Such as?
Attachment and Movement
Cell membrane proteins act to?
attach cells to one another and to elements of the extracellular environment
The binding of a ligand to a receptor frequently results in the?
migration of the ligand-receptor complex to a region of membrane which contains the protein clathrin (Figure 8).
Binding to clathrin initiates the complex series of membrane movements termed endocytosis (the entire process is called receptor-mediated endocytosis).
Exocytosis is another complex membrane movement.
As with other proteins, receptors are subject to?
turnover.
In many cases, receptor-mediated endocytosis is followed by internal degradation of the receptor.
In the steady state, degradation is matched by synthesis, such that the density of receptor molecules in the cell membrane remains constant.
However, if binding of receptor by extracellular ligand increases or decreases dramatically, the density of receptors at the membrane may be altered.
Complex membrane movements such as endocytosis and exocytosis require?
chemical bond energy and the enzymes to release it.
What are two functions of the cell membrane?
allow passage of (some) molecules
they change shape
