Cell Transport, Signaling Pathways, and Metabolism: Key Concepts for Biology Students

For Na⁺ transport into a cell ([Na⁺]out 150 mM, [Na⁺]in 15 mM), what increases spontaneity?

Decreasing the ratio of [Na⁺]in to [Na⁺]out

Which statement about the Na⁺/K⁺ pump is true?

It transports Na⁺ out and K⁺ in

Which form of the heterotrimeric G-protein complex is inactive?

GDP-bound αβγ trimer

Which transporter type mediates chloride shift in RBCs?*

Antiporter*

A nonpolar molecule with fused rings (three hexanes + one pentane) is best classified as?

Steroid

What property lets K⁺ channels select K⁺ over Na⁺?

Backbone carbonyl oxygens perfectly coordinate K⁺

Which transporter classification fits ion exchange in chloride shift?

Antiporter

What type of transport does NOT require ATP?

Passive transport

Secondary active transport requires what condition?

One solute must move down its gradient

A dimeric receptor becomes active through what process?

Cross-phosphorylation by tyrosine kinases

What mutation in the EGF/Ras pathway can lead to uncontrolled cell growth?

Permanent dimerization of the receptor

Which Ras mutation leads to cancer-like uncontrolled growth?

Elimination of intrinsic GTPase activity

Ligand binding to a receptor tyrosine kinase causes what?

Phosphorylation of the receptor, then phosphorylation of the next protein

Which subunit of a heterotrimeric G protein binds GDP/GTP?

α subunit

How is the G-protein pathway terminated?

Intrinsic GTPase activity

Which statement about heterotrimeric G-proteins is true?

They have α, β, γ subunits and are membrane-anchored

Ras is an example of what?

A GTP-dependent protein

What happens when a G-protein is activated?

α subunit dissociates from βγ

What does active G-protein signaling increase?

cAMP

Which of the following does cAMP do?

Acts as a second messenger and activates PKA

Which describes the similarity between DAG, cAMP, and IP₃?

All are second messengers

Which enzyme produces cAMP?

Adenylate cyclase

Which mutation would not terminate a signaling cascade?

Irreversible binding of the primary messenger

Which statement about RTK activation is true?

Ligand binding + cross-phosphorylation activate the receptor

Insulin receptor possesses what enzymatic activity?

Tyrosine kinase activity

Which step is essential for heterotrimeric G-protein activation?

Hormone binding triggers GDP → GTP exchange on the α subunit

What is a consequence of high energy charge?

Lack of glucose breakdown + increased F1,6-BPase activity

What is an example of reciprocal regulation?

F2,6-BP regulating PFK and F1,6-BPase

In the presence of glucagon, the cell will:

Activate gluconeogenesis

Cholera toxin affects G-protein signaling by:

Irreversibly binding G-protein, forcing GTP binding and inhibiting GTPase activity

Which statement about secondary messengers is true?

They amplify the signal inside the cell

Lipases are responsible for breaking down:

Lipids

What is catabolism?

The degradation of nutrients to salvage components and generate energy

What is the best description of metabolism's "futile cycle"?

Breaking glucose to pyruvate and resynthesizing glucose results in net loss of 4 ATP

The oxidizing agent in glycolysis is:

NAD⁺

Why must lactate be produced under anaerobic conditions?

To regenerate NAD⁺

Lactate dehydrogenase catalyzes what?

Oxidation of NADH to NAD⁺

Which statement about the need for NAD⁺ in glycolysis is true?

Glycolysis requires regeneration of NAD⁺ to continue

Which term describes breaking down molecules for energy?

Catabolism

Which compound can phosphorylate ADP with ΔG° = -3.0 kcal/mol?

Acetyl phosphate

Which energy-yield statement about glycolysis is FALSE?

Glycolysis requires NADH to go forward (it requires NAD⁺)

Which statement about ATP hydrolysis is NOT a factor in high phosphoryl transfer potential?

Increase in resonance stabilization in the reactants versus products

Which contribute to ATP's large -ΔG?

Reduced charge repulsion and increased entropy

Which enzyme classification uses NADH to reduce another molecule?

Dehydrogenase

What is the common storage form of glucose in animals?

Glycogen

Which statement about fatty acid storage is true?

Triacylglycerols are the main storage form

Which statement about membrane adaptability is correct?

More saturated at high temperatures, more unsaturated at low temperatures

Which step is the committed step of glycolysis?

Phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate

Why is the F6P → F1,6BP step the committed step?

F1,6BP can enter no other pathway

Why is glucose → G6P not the committed step?

G6P can enter other pathways

What do the steps between glucose and triose phosphates accomplish?

None of the other answers is correct

How many ATP are used in the investment phase?

2

How many ATP are produced in the payoff phase?

4

Net ATP yield per glucose in glycolysis?

2

Why do gross vs net ATP differ?

Investment uses 2 ATP while payoff produces 4 ATP

Which enzyme produces ATP during glycolysis?

Phosphoglycerate kinase

Which glycolytic enzymes perform substrate-level phosphorylation?

Phosphoglycerate kinase; pyruvate kinase

Which enzyme converts DHAP ↔ GAP?

Triose phosphate isomerase (isomerase)

Which enzyme converts PEP → pyruvate?

Pyruvate kinase

What does glyceraldehyde-3-phosphate dehydrogenase produce?

1,3-BPG and NADH

What is the second high-energy intermediate of glycolysis?

1,3-BPG

Which statement about glycolysis is FALSE?

Without NADH, glycolysis cannot go forward (it needs NAD⁺)

What reaction generates NADH in glycolysis?

GAP → 1,3-BPG

Which compound most readily donates phosphate (among glycolytic intermediates)?

1,3-BPG

Which enzyme requires ATP as a substrate?

Phosphofructokinase

Which step produces ATP?

1,3-BPG → 3-PG

Which condition keeps glycolysis active despite high ATP?

Loss-of-function mutations in allosteric sites of PFK and pyruvate kinase

When is glycolysis increased under insulin signaling?

When the bifunctional enzyme is dephosphorylated

What does high energy charge cause?

Reduced glycolysis; increased gluconeogenesis

Which step of glycolysis generates NADH?

Glyceraldehyde-3-phosphate dehydrogenase

Why is lactate produced during anaerobic glycolysis?

To regenerate NAD⁺

Which molecule is limiting during anaerobic glycolysis?

NAD⁺

Which enzyme uses ATP to phosphorylate another molecule?

Kinase

Which of the following describes aldolase?

Lyase that splits F1,6BP into GAP and DHAP

Which statements about glycolysis regulation are true?

Hexokinase, PFK, and pyruvate kinase are major control points

Which molecule acts as a positive allosteric effector for PFK-1?

ADP

Which statement about PFK-1 and ADP is false?

ADP is a competitive inhibitor (it is an activator)

Which glycolytic reaction is highly exergonic?

PEP → pyruvate

What happens if pyruvate kinase is inhibited?

Glycolysis slows at the final step

Which enzyme is inhibited by its product?

Hexokinase inhibited by G6P

How many NADH are produced per glucose in glycolysis?

2

Which statement about glycolysis is NOT true?

Substrate-level phosphorylation occurs at hexokinase (it does not)

Which enzyme is the major regulatory enzyme of gluconeogenesis?

Fructose-1,6-bisphosphatase

Which statements regarding glycolysis vs gluconeogenesis are true?

II and III only (II: F2,6-BP is an important effector; III: Gluconeogenesis has one more step)

Which enzyme catalyzes: Pyruvate + ATP → OAA + ADP?

Pyruvate carboxylase

Which enzyme catalyzes: OAA + GTP → PEP + GDP?

PEP carboxykinase

Which enzyme catalyzes: F1,6BP + H₂O → F6P + Pi?

Fructose-1,6-bisphosphatase

Which enzyme catalyzes: G6P + H₂O → Glucose + Pi?

Glucose-6-phosphatase

What is a positive effector of pyruvate carboxylase?

Acetyl-CoA

Which of the following does insulin promote?

Increased glycolysis via dephosphorylation of the bifunctional enzyme

In the presence of glucagon, what pathway is activated?

Gluconeogenesis

What is an example of reciprocal regulation?

F2,6-BP regulating PFK-1 and F1,6-BPase in opposite directions

Which reaction in glycolysis is bypassed in gluconeogenesis using two enzymes?

Pyruvate → PEP

Why is G6P → glucose necessary in gluconeogenesis?

Allows glucose export from liver

Which molecule stimulates F1,6-BPase?

Citrate

Which molecule inhibits F1,6-BPase?

F2,6-BP

Which statement about gluconeogenesis is true?

It is not a simple reversal of glycolysis

Net ATP cost of converting pyruvate → glucose?

6 ATP equivalents

What is required to reverse the pyruvate kinase step?

Pyruvate carboxylase + PEPCK

Which condition increases gluconeogenesis?

High acetyl-CoA (signals energy abundance)