3 Physiology: Cell Membrane

Question 1

The cell membrane is the boundary between?

Answer 1

an ordered intracellular environment (the intracellular fluid; ICF) and a less-ordered extracellular fluid (ECF) or disordered external environment

Question 2

If the membrane was a simple static structure that merely prevented the admixture of these two environments, most cells would?

Answer 2

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

Question 3

The energy required for maintenance of internal order is derived from?

Answer 3

energy-rich molecules obtained from the ECF

Question 4

The stored energy-rich molecules are derived from?

Answer 4

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.

Question 5

What is The Cell Membrane?

Answer 5

is a Dynamic, Heterogeneous, Fluid Structure

Question 6

Describe a dynamic structure

Answer 6

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

Question 7

An example of change in a dynamic cell membrane

Answer 7

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

Question 8

What are two functions of the cell membrane?

Answer 8

allow passage of (some) molecules

they change shape

Question 8

An example of change in a dynamic cell membrane

Answer 8

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

Question 9

The functions of the cell membrane are made possible by the actions of what?

Answer 9

of distinctly different molecules in and associated with the membrane, i.e., the cell membrane is a heterogeneous structure.

Question 10

Common component molecules of cell membranes include?

Answer 10

phospholipids
sphingolipids
cholesterol
proteins and glycoproteins

Question 11

Describe how the cell membrane is a fluid structure?

Answer 11

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

Question 12

the most common molecules in the cell membrane are?

Answer 12

Phospholipids

Question 13

What is a “fluid mosaic” (the cell membrane is often referred to as this)?

Answer 13

a structure composed of a large number of different molecules (a mosaic) which move within the membrane (fluid).

Question 14

How are Phospholipids arranged?

Answer 14

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.

Question 15

What is also found in both monolayers of the membrane?

Answer 15

Cholesterol

Question 16

What are found in the outer monolayer of the membrane?

Answer 16

Sphingolipids

Question 17

Some proteins/glycoproteins are embedded where? And these such molecules are called what?

Answer 17

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.

Question 18

List the functions of the cell membrane

Answer 18

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.

Question 19

Diffusion results from what?

Answer 19

from a greater frequency of collisions between molecules where they exist at a higher concentration than where they exist at a lower one.

Question 20

diffusion is a manifestation of the laws of?

Answer 20

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.

Question 21

A number of small, nonpolar molecules are able to easily what?

Answer 21

diffuse through the phospholipid bilayer of the cell membranes.
Paramount amongst these are the gases O2 and CO2.

Question 22

Other molecules which share the combined hydrophilic/hydrophobic character of phospholipids and cholesterol can do what?

Answer 22

also pass through the membrane with relative ease.

Such molecules include fatty acids and steroid hormones (which are derivatives of cholesterol).

Question 23

Larger polar/hydrophilic molecules, on the other hand, CAN or CANNOT diffuse through the phospholipid bilayer?

Answer 23

Cannot

Question 24

Numerous polar molecules do, however, diffuse through cell membranes by way of?

Answer 24

transmembrane proteins.

Question 25

What allows for the passage of an ion such as Na+ or K+?

Answer 25

these proteins form a water-containing pore or channel

Question 26

Ion channels are characterized as?

Answer 26

specific (i.e., they usually allow only one or two ions to diffuse “down” its/their concentration gradient/s.)

Question 27

What may influence the rate at which ions move through protein channels?

Answer 27

electrical forces as well as diffusion because ions are charged

Question 28

What are Leak Channels?

Answer 28

channels that are always open

Question 29

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)

Answer 29

Ligand-gated
Voltage-gated
Mechanically-gated

Question 30

What is a Ligand-gated channel?

Answer 30

a ligand (molecule) binds to the channel (or a neighboring protein), which results in channel opening

Question 31

What is a Voltage-gated channel?

Answer 31

a change in the electrical potential difference across the membrane (measured in volts) results in channel opening

Question 32

What is a Mechanically-gated channel?

Answer 32

force/pressure/tension exerted on the cell membrane results in channel opening

Question 33

transmembrane proteins may bind a what to gain access to the other side of the membrane?

Answer 33

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.

Question 34

Since the molecule cannot diffuse across the membrane unless it binds to and is released by a protein, the protein is said to?

Answer 34

facilitate the diffusion of the molecule, and the process is termed facilitated diffusion.

Question 35

One well-known example of facilitated diffusion is?

Answer 35

is provided by the glucose transporter (actually there are a number of different glucose transporters).

Question 36

What is active transport?

Answer 36

is the movement of a substance from an area of lower concentration to an area of higher concentration.

Question 37

From a thermodynamic point of view, active transport creates?

Answer 37

order (two distinctly different concentrations on either side of a membrane) and therefore is an energy- requiring process.

Question 38

Transmembrane proteins which perform active transport are often referred to as?

Answer 38

carriers or pumps.

Question 39

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

Answer 39

.

Question 40

What is Primary Active Transport?

Answer 40

the breaking of a chemical bond in an ATP molecule by the carrier/pump releases the energy required to perform active transport.

Question 41

What is Secondary Active Transport?

Answer 41

by allowing an ion/molecule to diffuse across the membrane, the carrier/pump utilizes the released energy to perform active transport.

Question 42

Transmembrane proteins transport ions and molecules in what directions across cell membranes?

Answer 42

Both directions

Question 43

Uniports transport how many substances?

Answer 43

one substance

Question 44

Symports transport how many substances and in what directions?

Answer 44

transport two or more substances in the same direction

Question 45

antiports transport how many substances and in what directions?

Answer 45

(exchangers) transport two substances in opposite directions

Question 46

Examples of Uniports

Answer 46

Na+ leak channel; 
K+ leak channel; 
voltage-gated Na+ channel, 
mechanically-activated Ca++ channel; 
aquaporin (water pore)

Question 47

Examples of Symports

Answer 47

Na+-glucose symport;

Na+ -K+-2 Cl- symport

Question 48

Examples of Exchangers (antiports)

Answer 48

Na+-Ca++ exchanger;

HCO3- - Cl- exchanger

Question 49

How can ATPases transport substances?

Answer 49

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

Question 50

The Cell Membrane Receives Information in Physical and/or Chemical Form from the?

Answer 50

ECF/Extracellular Environment

Question 51

While changes in voltage or force/pressure can directly alter the shape of transmembrane proteins, ligands must bind to?

Answer 51

to the transmembrane protein at a receptor site or at a neighboring protein receptor.

Question 52

Many, but not all, ligands are?

Answer 52

neurotransmitters and hormones.

Question 53

Ligands by virtue of their hydrophobic character, can?

Answer 53

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.

Question 54

Conformational Changes of Transmembrane Proteins Can Be Elicited By?

Answer 54

Chemical Ligands

Electrical and Physical Stimuli

Question 55

Many ligands bind to receptors which are associated with?

Answer 55

membrane enzymes.

Question 56

A number of (but not all) well-characterized receptor systems include?

Answer 56

three membrane proteins: the receptor, a G protein, and the enzyme.

Question 57

binding of a ligand activates the?

Answer 57

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.

Question 58

receptor systems are often referred to as what? And why?

Answer 58

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.

Question 59

Signal transduction systems are also called?

Answer 59

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.

Question 60

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:

Answer 60

1) cAMP, produced by adenylate cyclase
2) cGMP, produced by guanylate cyclase
3) IP3, produced by phospholipase C

Question 61

cAMP and cGMP activate what?

Answer 61

kinases (enzymes which perform phosphorylation, a major way by which the activity of enzymes and other proteins is regulated)

Question 62

IP3 acts as a?

Answer 62

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.

Question 63

The Cell Membrane Serves Structural Functions, Such as?

Answer 63

Attachment and Movement

Question 64

Cell membrane proteins act to?

Answer 64

attach cells to one another and to elements of the extracellular environment

Question 65

The binding of a ligand to a receptor frequently results in the?

Answer 65

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.

Question 66

As with other proteins, receptors are subject to?

Answer 66

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.

Question 67

Complex membrane movements such as endocytosis and exocytosis require?

Answer 67

chemical bond energy and the enzymes to release it.