Give brief explanations:

1. Briefly explain the reason why ions such as Na+, K+, and Ca2+ diffuse across plasma membranes more rapidly than would be expected across a lipid membrane.

Why glucose does not cross the cell membrane through open channels?

2. Why do steroid hormones not require signal transduction and second messengers to exert their actions?

3. In the sliding filament theory of contraction, what prevents the filaments from sliding back to their original positions each time a myosin head releases to bind to next actin binding site?

4. The compound tetraethylammonium (TEA) blocks the voltage-gated changes in potassium permeability that occurs during an action potential. After administration of TEA, what changes would you expect in the action potential?

5. Explain briefly why action potential of cardiac muscle is significantly longer than skeletal muscle.

Part B.

1. The intracellular fluid

A. makes up about 20% of the total body fluid

B. makes more than 80% of the total body fluid

C. contains more potassium

D. is equivalent to the plasma

E. all of the above

2. Which of the following affects the permeability constant of a molecule in diffusion?

A. the molecular weight of the diffusing molecule

B. the characteristics of the membrane through which the molecule must diffuse

C. the temperature of the system

D. the size of the molecule

E. all of the above

3. Which one of the following is an incorrect base pair in nucleic acids?

A. A-T

B. C-G

C. G-C

D. A-G

4. The two strands in a molecule of DNA are held together by

A. peptide bonds

B. covalent bonds

C. hydrogen bonds

D. ionic bonds

5. A negative feedback system is one in which

A. a variable is changed in the same direction as the original change that initiated the system

B. the initiating variable is always reduced (made more negative)

C. a variable is changed in the opposite direction to the original change that initiated the system

D. two opposing variables are played against one other

E. the initiating variable is always increased (made more positive)

6. Messenger RNA (mRNA) is

A. used to transfer nucleotide sequences from DNA to tRNA

B. the molecule that determines the sequence of amino acids in a protein

C. used to translate the genetic code

D. used to transcribe the genetic code

7. In DNA replication

A. new molecule is form different from previous

B. only one strand is replicated

C. both strands are replicated such that a new copy is composed of both new strands, and the original molecule is composed of both original strands

D. both strands are replicated such that two molecules result, each having one old strand and one new strand

8. Secondary active transport

A. directly uses ATP for the energy needed to function

B. is a form of diffusion utilizing no energy

C. uses a calcium concentration difference for energy

D. does not use energy

E. uses an ion concentration difference for energy

9. One of the most important substances to be transported into cells by facilitated diffusion is

A. water

B. sodium

C. lipid

D. potassium

E. glucose

10. Information in DNA is carried to the site of protein synthesis by

A. transfer RNA (tRNA)

B. pre-messenger RNA

C. ribosomal RNA (rRNA)

D. messenger RNA (mRNA)

11. Facilitated diffusion differs from active transport in that

A. facilitated diffusion transports against a concentration gradient while active transport does not

B. facilitated diffusion requires expenditure of energy while active transport does not

C. active transport requires expenditure of energy while facilitated diffusion does not

D. facilitated diffusion involves carrier proteins while active transport does not

E. facilitated diffusion works against a concentration gradient while active transport works only with the concentration gradient

12. Amino acids are carried to the ribosome attached to molecules called

A. initiation factors

B. anticodons

C. mRNA

D. tRNA

13. When two chambers are separated by a membrane permeable to water but not solute, if water is free to move in either direction without limitation or pressure, then water will

A. move until the concentration of solute on both sides of the chamber is equal

B. move until there is no water on the dilute side

C. move until there is no water on the concentrated side

D. move until the concentration of water on both sides of the chamber is equal

14.  The two components of extracellular fluid, the interstitial fluid and the blood plasma, are

A. completely continuous

B. separated by the cell walls

C. within the cells of the body

D. separated by the capillary walls

15. An aqueous solution has an osmolarity of 318 mOsm.  If a cell with an intracellular osmolarity of 260 mOsm is placed into this solution, the solution is said to be

A. hypertonic

B. isotonic

C. isoosmotic

D. hypotonic

16. The reason ions such as Na+, K+, and Ca2+ diffuse across plasma membranes more rapidly than would be expected across a lipid membrane is that

A. their negative charge causes them to dissolve in the polar lipids in the membrane

B. ions diffuse through polar lipids more readily than non-charged particles do

C. their positive charge causes them to dissolve in the polar lipids in the membrane

D. there are proteins that form channels through which these ions can more readily pass

 17. Which of these ions are actively transported through the cell membrane to re-establish a resting potential?

A. K and Mg

B.  Na and K

C. Mg and Cl

D. Ca and Cl

18. In what way does the interior surface of the cell membrane of a resting (non-conducting) neuron differ from the external environment?  The interior is:

  A. positively charged and contains less sodium.

  B. negatively charged and contains less sodium.

  C. negatively charged and contains less potassium

  D. positively charged and contains more sodium.

 19. Which of the following is true about the movement of ions across excitable living membranes?

  A.  Ions can move passively across membranes through leakage channels.

  B.  Some ions are prevented from moving down their concentration gradient by ATP-driven pumps.

  C.  Ion gates in the membrane can open in response to electrical potential changes.

  D.  All of these are correct.

20. Propagation of action potential and transmission mean:

  A.  current flow along the cell membrane

  B.  current flow through the synapse

  C.  current flow using saltatory conduction

  D.  all of the above

21. According to the sliding filament mechanism of skeletal muscle contraction, during contraction:

A. The thick filaments stay the same size but the thin filaments shorten.

B. The thin filaments stay the same size but the thick filaments shorten.

C. Both thick and thin filaments shorten.

D. The sarcomeres shorten.

22. Which of the following is an important function of ATP in skeletal muscle?

1. It is required for the dissociation of myosin from actin.
2. It is hydrolyzed by the ATPase that pumps calcium ions into the sarcoplasmic reticulum.
3. It is used for the phosphorylation of myosin.
4. It is required for the attachment of myosin to actin
5. A and B.

23. At the neuromuscular junction,

  A. the release of acetylcholine from the motor neuron in the motor end plate occurs.

  B. acetylcholinesterase inhibits repolarization of the motor end - plate.

  C. It is used for the phosphorylation of myosin.

  D. It is required for the attachment of myosin to actin

  E. A and B.

24. Which of the following statements regarding the action of curare at the neuromuscular junction is true?

  A. Curare is an epinephrine agonist.

  B. Curare is an acetylcholine agonist

  C. Curare inhibits acetylcholinesterase.

  D. Curare binds to acetylcholine receptors

25. Which of the following descriptions of skeletal muscle contraction is correct?

  A. Skeletal muscle contraction requires the influx of extracellular Ca²⁺

  B. Skeletal muscle contraction involves interactions between thick and thin filaments

  C. Skeletal muscle contraction results from cross-bridge cycling,

  D. The site of calcium regulation of skeletal muscle contraction is on the thin filament

  E. All of the above

Clinical problem based question:

Some strains of bacteria or microorganisms have developed toxins that can modify the activity of the alpha subunit of G proteins. I results in disease. For ex. cholera toxin, produced by Vibrio cholerae, causes ADP ribosylation of the stimulatory Gαs subunit of G proteins.  This modification abolishes the GTPase of Gαs, and results in an αs subunit that is always in the "on" or active state. It results in continuous stimulation of adenylyl cyclase (AC). The main cells affected by this are the epithelial cell in gastrointestinal tract.

Knowing this altered activity of AC, explain why patients affected by this toxin experience severe diarrhea and dehydration that may result in death.