Cell Membrane and its Transport Mechanisms

Question 1

Lipid bilayer is rigid, not fluid. True or false?

Answer 1

False.
Phospholipids are not held together by chemical bonds hence are constantly moving, largely accounting for cell membrane fluidity.

Question 2

What do the cholesterol molecules that are tucked between phospholipid molecules do?

Answer 2

They prevent fatty acid chains from packing together and crystallizing; thus stablizing phospholipids’ position and hence maintaining the fluidity.
Note though that the more cholesterol in the plasma membrane, the less the membrane fluidity.

Question 3

True or False
a) Integral proteins are also called transmembrane proteins.
b) Integral proteins do not have hydrophilic and hydrophobic regions.
c) Peripheral proteins are found more commonly on the outer than the inner surface.

Answer 3

a) True
b) False
c) False

Question 4

Short carbohydrate chains protrude from the outer surface of cell membrane bound primarily to membrane proteins (glycoproteins) and to a lesser extent to lipids (glycolipids) and the coating they form is called ___________________.

Answer 4

glycocalyx

The glycocalyx, also known as the pericellular matrix and sometime cell coat, is a glycoprotein and glycolipid covering that surrounds the cell membranes of bacteria, epithelial cells, and other cells.

Question 5

State the functions of membrane proteins. (7)
SOME HINTS: channels, carrier, docking-marker acceptors, enzymes, CAMs

Answer 5

1. Some transmembrane proteins form water-filled pathways or channels through the lipid bilayer. (either leak or gated channels)
2. Some transmembrane proteins are carrier molecules or transport molecules.
3. Proteins located on the inner membrane surface serve as docking-marker acceptors.
4. Some proteins located on either the inner or the outer cell surface function as membrane-bound enzymes that control specific chemical reactions.
5. Many proteins on the outer surface are receptors.
6. Some membrane proteins are cell adhesion markers.
7. Some proteins on the outer membrane surface, especially in conjunction with carbohydrates (as glycoproteins) are important in the cells’ ability to recognize “self” (cells of the same type).

Question 6

__________________ are widely used in the management of high blood pressure and abnormal heart rhythms.

A. Na+ channels blockers
B. Mg2+ channels blockers
C. K+ channels blockers
D. Ba2+ channels blockers
E. Ca2+ channels blockers

Answer 6

Choice E

Question 7

In light of the function that proteins located on the inner membrane surface serve as docking-marker acceptors, fill in the following blanks:
Stimulatory signals trigger fusion of ____(1)____ membrane with the inner surface of the cell membrane. ____(1)____ opens up and empties its content to the outside by ____(2)____.

Answer 7

(1) secretory vesicle
(2) exocytosis

Question 8

State five factors that affect the movement of materials across the cell membrane.

Answer 8

1. Concentration gradient
2. Permeability of the membrane
3. Surface area of the membrane
4. The molecular weight of a substance
5. The distance through which diffusion is to take place

Extra note: Permeability is influenced by the substance’s permeability to the phospholipid bilayer or by the number of channels available for that substance.

Question 9

Differentiate between the two subtypes of diffusion across the cell membrane.

Answer 9

Simple diffusion is reliant simply on the kinetic energy of diffusing molecules that will enable them traverse membrane openings or intermolecular spaces without interaction with carrier proteins in the membrane, whereas facilitated diffusion requires interaction of a carrier protein.

Question 10

What are two pathways through which simple diffusion can occur?

Answer 10

1. through the interstices of the lipid bilayer if the diffusing substance is lipid-soluble
2. through watery channels that penetrate all the way through some of the large transport proteins

Question 11

What are the factors that determine the rate of diffusion? (6)

Answer 11

1. the amount of substance available
2. the kinetic energy of diffusing molecules/ions
3. the number and sizes of membrane openings through which molecules/ions can diffuse (permeability of membrane)
4. surface area of membrane
5. molecular weight of substance
6. distance through which diffusion takes place

Question 12

Membrane proteins that facilitate the transport of water through the cell membrane are generally called _________.

Answer 12

aquaporins

Question 13

What are the five factors influencing the rate of net diffusion of a substance?

Answer 13

1. The magnitude or steepness of the concentration gradient (direct proportionality with rate of net diffusion)
2. The surface area of the cell membrane across which diffusion is taking place (direct proportionality with rate of net diffusion)
3. The lipid solubility of the substance - the greater the lipid solubility of a substance the more rapidly the substance can diffuse through the CM’s lipid bilayer down its concentration gradient.
4. The molecular weight of the substance (indirect proportionality with rate of diffusion)
5. The distance through which diffusion must take place (indirect proportionality with rate of diffusion)

Question 14

About a million water molecules can pass through an aquaporin channel in a second. TRUE or FALSE.

Answer 14

FALSE.
About a billion water molecules can pass through an aquaporin channel in a second.

Question 15

Briefly state and describe three classes of carrier membrane proteins.

Answer 15

1. Uniport carriers mediate transport of a single solute.
2. Symport carriers bind two dissimilar substrates and transport them together across a membrane.
3. Antiport carriers exchange one solute for another across a membrane.

Question 16

Briefly explain receptor-mediated endocytosis.

Answer 16

Specific receptors on the cell surface bind tightly to the extracellular macromolecule that it recognizes (i.e. ligand). The plasma membrane region containing the receptor-ligand complex buds inwards and pinches off into the cytoplasm.

Question 17

Name one transmembrane protein that mediates cell-cell adhesion in animals.

Answer 17

cadherin

Question 18

If the pressure in the compartment into which water is flowing is raised to the equivalent of the osmotic pressure, movement of water will stop. This pressure is often called ____________________ pressure.

Answer 18

hydrostatic (‘water-stopping’)

Question 19

For ions and charged molecules, the electrical potential across the membrane also becomes critically important during passive transport. True or false?

Answer 19

True.

Together, gradients in concentration and electric potential across the cell membrane constitute the electrochemical gradient that governs passive transport mechanisms.

Question 20

What is the function of integrins on the surface of the cell membrane?

Answer 20

These are cell adhesion molecules (CAM) that span the cell membrane and link the extracellular matrix to the actin cytoskeleton.

Question 21

Why can’t polar molecules and charged ions diffuse freely across cell membranes (i.e. simple diffusion)?

Answer 21

This is due to the hydrophobic nature of the lipids that make up the lipid bilayers.

Polar molecules and charged ions are already dissolved in water at the time of diffusion.

Question 22

State the two types of gated transmembrane channels.

Answer 22

Voltage-gated channels: they are gated by alterations in membrane potentials e.g. Na+ channels
Ligand-gated channels: these channels open or close when they bind a ligand e.g. acetylcholine receptor

NB: Conformational changes associated with channel opening may also be regulated by membrane stretch e.g., via a link to the cytoskeleton.

Question 23

The transport of a solute in the direction of its increasing electrochemical potential is called ___________________.

Answer 23

secondary active transport

Question 24

When does secondary active transport happen?

Answer 24

When a gradient of one substrate, usually an ion, drives uphill transport of a co-substrate.

Secondary active transport is coupled to the facilitated diffusion of a second solute (usually an ion) in the direction of its decreasing electrochemical potential.

Question 25

Which class of carrier membrane proteins is involved in secondary active transport?

Answer 25

Symport (cotransporter)

Question 26

a) What is active transport?
b) In which direction does molecules move through the concentration gradient?

Answer 26

a) A type of carrier mediated transport that depends on the expenditure of cellular energy in the form of ATP hydrolysis.
b) From lower to higher concentrations against the concentration gradient.

Question 27

Explain the mechanism of active transport.

Answer 27

• The carrier protein contains ATPase, enzyme which catalyzes the breakdown of ATP to ADP, P and energy.
• The substance to be moved binds to the inner surface of the carrier protein and the energy from ATP is used to phosphorylate the carrier protein.
• This induces a change in the configuration which moves the substance through the protein to the other side of the membrane.
• Active transport systems are also termed “pumps”. Some move only one substance eg. H+ pump, others move more than one substance eg. Na+/K+ pump.

phosphorylate: to cause an organic compound to to take up or combine with phosphoric acid or a phosphorus-containing group

Question 28

What is the Na+/K+ ATPase pump?

Answer 28

The Na+/K+ ATPase is an antiport that moves 3 Na+ out of the cell in exchange for 2 K+ ions it moves into the cell for each ATP molecule. It is an enzyme and it is anchored in the plasma membrane by the cytoskeleton.

Question 29

a) What are the functions of the Na+/K+ ATPase?
b) The more active the pump, the more ADP is made available for ATP synthesis by ______________________.
c) What increases the activity of the pump?
d) What inhibits its activity?

Answer 29

a) helps maintain resting potential, avail transport and regulate cellular volume. It also functions as signal transducer for several pathways.
b) oxidative phosphorylation
c) G-actin, thyroid hormone, aldosterone, insulin and intracellular sodium concentration
d) dopamine hormone

Question 30

Differentiate between desmosomes (maculae adherens) and tight junctions.

Answer 30

Desmosomes are junctions that anchor together two separate cells via intercellular filaments such that they resist being pulled apart, whereas tight junctions bind adjacent cells firmly at points of direct contact, sealing off the passageway between two cells [paracellular space].

Question 31

(a) What is endocytosis?
(b) What happens in endocytosis?
(c) Endocytosis includes ___________________ and _____________________.

Answer 31

(a) Endocytosis is a cellular process in which substances are brought into the cell.
(b) The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a vesicle containing the ingested material.
(c) pinocytosis and phagocytosis.

Question 32

What type of cellular transport do tight junctions restrict?

Answer 32

paracellular transport

Question 33

a) Which transport mechanism across cell membrane is suitable for taking in molecules that are far too big to pass through the channels in the membranes?
b) If the molecule is [I] solid, the process mentioned in a) is called and if the molecule is [II] liquid, the process mentioned in a) is called?

Answer 33

a) Endocytosis
b) [I] phagocytosis [II] pinocytosis

Question 34

Gap junctions are communication junctions made up of ____(1)____ which form tunnels that permit movement of charge-carrying ions and small molecules between two adjacent cells. ____(1)____ are made up of 6 subunits called ____(2)____ that are arranged in a hollow tube-like structure.

Answer 34

1. connexons
2. connexins

Question 35

Gap junctions enable synchronized contraction hence are especially abundant in ____(1)____ and ____(2)____ muscle.

Answer 35

cardiac, smooth

Question 36

What is exocytosis?

Answer 36

Exocytosis is the fusion of secretory vesicles with the plasma membrane and results in the discharge of vesicle content into the extracellular space and the incorporation of new proteins and lipids into the plasma membrane.

Question 37

State and explain the two types of exocytosis.

Answer 37

1. Constitutive exocytosis: This pathway serves to transfer molecules from the Golgi network to the cell’s outer surface and to eliminate substances to the cell’s exterior. Constitutive exocytosis is performed by all cells and is the default pathway for most membrane proteins.
2. Regulated exocytosis: It is performed only by secretory cells. It serves to release hormones, cytokines, neurotransmitters and other small signaling molecules such as histamine that are stored in secretory cells.

Question 38

Extracellular fluid is made up of two components, which are?

Answer 38

• plasma, the fluid portion of blood
• interstitial fluid which surrounds the cells

Question 39

List the 3 major protein fibres in the interstitial fluid.

Answer 39

1. Collagen
2. Elastin
3. Fibronectin

Question 40

Collagen is the least abundant protein in the body. True or False?

Answer 40

False. Collagen is in fact, the most abundant protein in the body.

Question 41

Why are there bleeding gums in a person with scurvy?

Answer 41

Scurvy is due to Vitamin C deficiency in which collagen fibres are not properly formed.

Question 42

The cell-matrix adhesion molecules and the cell-cell adhesion molecules are extremely important in regulating _____________.
a. shape of cells
b. growth of cells
c. migration of immune cells
d. direction of growth of chromaffin cells axons
e. differentiation of cells

Answer 42

Choice a.

Question 43

Select one of the following concerning integrins that is not correct:
A. are cell adhesion molecules that span the cell membrane
B. serve as a gap junction between the outer membrane surface and its extracellular surroundings
C. connect the inner membrane surface to the intracellular cytoskeletal scaffolding
D. mechanically link the cell’s external environment and intracellular components
E. integrins relay regulatory signals through the cell membrane in either direction

Answer 43

Choice B.

Question 44

Which of the following phospholipids are located in the:
(a) outer leaf of the plasma membrane
(b) inner leaf of the plasma membrane

phophatidyl choline, phosphatidyl ethanolamine, phosphatidyl serine, sphingomyelin

Answer 44

(a) phosphatidyl choline, sphingomyelin
(b) phosphatidyl ethanolamine, phosphatidyl serine

Further reading:
Phosphatidyl choline: It’s a major component of cell membranes. Found in high amounts in lecithin, which is commonly derived from soybeans. Important for cell structure, nerve function, and lipid metabolism.
Phosphatidylethanolamine: Another key player in cell membranes. Involved in membrane curvature, cell signaling, and lipid transport. Essential for brain health.
Phosphatidyl serine: Concentrated in brain tissue. Vital for neurotransmitter release, memory, and cognitive function.
Sphingomyelin: A bit different—it contains a sphingosine backbone instead of glycerol. Found in myelin sheaths around nerve fibers. Critical for nerve signal transmission.

Question 45

Discuss the structure of the Sodium-Potassium ATPase pump.

Answer 45

The Sodium-Potassium pump is a P-type ion pump that consists of two main subunits: alpha and beta. The alpha subunit contains the sites for binding of Na+, K+ and ATP. [Diagram]

Question 46

Describe the mechanism of pinocytosis.

Answer 46

[Pinocytosis is a process by which macromolecules are taken into the cells. It is otherwise called cell drinking.]
✈ Macromolecules (in the form of droplets of fluid) bind to the outer surface of the cell membrane.
✈ The cell membrane evaginates around the droplets and the droplets are engulfed by the membrane.
✈ The evaginated cell membrane and engulfed droplets are converted into vesicles/vacuoles, which are called endosomes.
✈ The endosome travels into the interior of the cell.
✈ A primary lysosome in the cytoplasm fuses with the endosome and forms the secondary lysosome.
✈ Hydrolytic enzymes present in the secondary lysosome are activated resulting in digestion and degradation of the endosomal contents.

Question 47

Describe the mechanism of phagocytosis.

Answer 47

[Phagocytosis is the process by which particles larger than macromolecules are engulfed into the cells. It is also called cell eating. Bacteria, antigens and other foreign bodies are taken inside the cell by means of phagocytosis. It occurs only in a few cells e.g. neutrophils, monocytes, and the tissue macrophages.]
✈ When a pathogen enters the body, the phagocytic cell sends out cytoplasmic extensions (pseudopodia) that surround it.
✈ The pathogen is consequently engulfed and converted into a very large vacuole called the phagosome.
✈ The phagosome travels to the interior of the cell.
✈ A primary lysosome fuses with the phagosome and forms a secondary lysosome.
✈ Hydrolytic enzymes present in the secondary lysosome are activated resulting in digestion and degradation of the phagosomal contents.