Membrane Transport, Cellular Energy, and Respiration

Comprehensive practice flashcards covering membrane structure, transport mechanisms, bioenergetics, enzyme kinetics, and all stages of aerobic and anaerobic cellular respiration.

According to the fluid mosaic model, what do the terms "fluid" and "mosaic" describe about membrane structure?

"Fluid" refers to the flexible phospholipid bilayer, while "mosaic" describes the diverse protein molecules suspended within the membrane.

How are phospholipids arranged in the plasma membrane with respect to water?

They form a bilayer where the hydrophilic heads face outward toward the watery extracellular and intracellular environments, and the hydrophobic tails face inward shielded from water.

In the provided diagram of the plasma membrane, what are the primary structural components depicted?

The phospholipid bilayer, integral proteins, peripheral proteins, glycoproteins, glycolipids, and cholesterol.

What is diffusion, and what physical force drives this process?

Diffusion is the tendency of particles to spread out into available space down their concentration gradient, driven by thermal energy (the constant, random motion of molecules).

What characterizes dynamic equilibrium during membrane diffusion?

Molecules continue to cross the membrane back and forth at equal rates, resulting in no net change in solute concentration on either side.

How do multiple different solutes behave when diffusing across the same membrane simultaneously?

Each type of solute diffuses independently down its own concentration gradient to reach dynamic equilibrium.

What is osmosis, and what rule helps predict the net direction of water movement?

Osmosis is the diffusion of water across a selectively permeable membrane down its concentration gradient (from higher free water concentration to lower free water concentration), following the rule: "water follows solute" toward the side with higher solute concentration.

If a $0.5\%$ sucrose solution is separated from a $2\%$ sucrose solution by a membrane permeable only to water, what will be the net movement of water?

Water will move net from the $0.5\%$ sucrose solution toward the $2\%$ sucrose solution until the solute concentrations on both sides are roughly equal.

How do animal and plant cells behave differently when placed into a hypotonic solution?

Animal cells gain water, swell, and may lyse (burst), whereas plant cells swell until the cell wall exerts turgor pressure, maintaining a turgid state which is ideal for plant cells.

What occurs when animal and plant cells are placed into a hypertonic solution?

Water moves out of the cells: animal cells shrivel and undergo crenation, while plant cells shrivel and their plasma membrane pulls away from the cell wall in a process called plasmolysis.

What is facilitated diffusion, and why is it categorized as passive transport?

Facilitated diffusion is the assisted passage of polar or charged molecules across a membrane via specific transport proteins down a concentration gradient without any expenditure of cellular energy.

What are aquaporins, and which human cells rely on them heavily for rapid fluid movement?

Aquaporins are transport protein channels specialized for the rapid passage of water molecules; they are especially abundant in red blood cells and kidney cells.

In the 1992 Johns Hopkins experiment shown in the graph, how did researchers verify the function of aquaporins?

Frog eggs injected with the candidate protein mRNA were immersed in a hypotonic solution; they rapidly took up water and burst within minutes compared to control eggs.

What distinguishes active transport from passive transport in terms of energy and concentration gradients?

Active transport requires cellular energy (usually $\text{ATP}$) to pump a solute against its concentration gradient toward the more concentrated side, whereas passive transport requires no energy and moves solutes down their gradient.

How does the sodium-potassium ($\text{Na}^+/\text{K}^+$) pump execute active transport?

Cytoplasmic solutes bind the transport protein, which is phosphorylated by the hydrolysis of $\text{ATP}$, inducing a conformational shape change that translocates and releases the solute across the membrane before resetting.

What are the three main types of endocytosis shown in the diagram and their specific mechanisms?

1) Phagocytosis ("cell eating"): engulfment of large particles via pseudopodia into food vacuoles; 2) Pinocytosis ("cell drinking"): non-specific engulfment of extracellular fluid droplets; 3) Receptor-mediated endocytosis: uptake of specific target molecules bound to receptors in coated pits.

Why does a cell's plasma membrane grow larger during exocytosis but shrink during endocytosis?

Exocytosis fuses secretory vesicles directly into the plasma membrane to expel contents, adding surface area, whereas endocytosis pinches pieces of the plasma membrane inward to create vesicles, removing membrane surface area.

What does the First Law of Thermodynamics state?

Energy is constant in the universe; it can be transferred or transformed from one form to another, but it cannot be created or destroyed.

How do exergonic and endergonic reactions differ regarding potential energy and spontaneity?

Exergonic reactions release energy spontaneously because reactants hold more chemical energy in their bonds than products; endergonic reactions are non-spontaneous and require energy input because products store more chemical energy than reactants.

How does $\text{ATP}$ hydrolysis drive endergonic cellular work?

The exergonic breakdown of $\text{ATP}$ into $\text{ADP}$ and inorganic phosphate is coupled to an endergonic process through phosphorylation (transferring the released phosphate group directly to another molecule).

What is activation energy ($E_a$), and why is it biologically necessary for cell survival?

Activation energy is the initial energy barrier required to contort and weaken chemical bonds so they can break; it prevents essential biomolecules (such as DNA, proteins, and lipids) from spontaneously breaking down.

In the reaction coordinate graph shown, what do variables $a$, $b$, and $c$ designate?

Label $a$ represents the uncatalyzed activation energy barrier, $b$ represents the reduced activation energy barrier achieved by the enzyme, and $c$ represents the net energy difference between reactants and products.

What is the "induced fit" model of enzyme catalysis?

It describes how the active site of an enzyme subtly shifts its conformation around the entering substrate to enfold it tightly, putting strain on specific bonds to catalyze the reaction.

Based on the provided illustration, how do competitive and noncompetitive inhibitors differ in their mode of action?

A competitive inhibitor mimics the substrate and blocks the active site directly, whereas a noncompetitive inhibitor binds to an allosteric site elsewhere on the enzyme, altering its active site shape so the substrate cannot bind.

What is feedback inhibition, and what type of regulatory loop does it exemplify?

It is a negative feedback loop in which the final product of a multi-step metabolic pathway allosterically inhibits an enzyme working earlier in the pathway when the product is abundant.

What is the overall chemical equation for aerobic cellular respiration?

$$\text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2 \rightarrow 6\,\text{CO}_2 + 6\,\text{H}_2\text{O} + \text{ATP} + \text{Heat}$$

Approximately how many $\text{ATP}$ molecules are produced from the complete cellular respiration of one glucose molecule?

Approximately $32\,\text{ATP}$ molecules.

What do the terms oxidation and reduction mean in a redox reaction, and what mnemonic device summarizes them?

Oxidation is the loss of electrons and reduction is the gain of electrons, remembered by the mnemonic "OIL RIG" (Oxidation Is Loss, Reduction Is Gain).

In cellular respiration, which molecule undergoes oxidation and which undergoes reduction?

Glucose is oxidized to carbon dioxide ($\text{CO}_2$) as it loses hydrogen atoms (electrons), while oxygen ($\text{O}_2$) is reduced to water ($\text{H}_2\text{O}$) as it gains hydrogen atoms (electrons).

What is the function of $\text{NAD}^+$ in cellular respiration?

$\text{NAD}^+$ is an organic coenzyme electron carrier that accepts two electrons and one proton to become reduced to $\text{NADH}$, shuttling high-energy electrons to the electron transport chain.

What are the three main stages of cellular respiration and their specific subcellular locations in eukaryotes?

1) Glycolysis in the cytosol; 2) Pyruvate oxidation and the citric acid cycle in the mitochondrial matrix; 3) Oxidative phosphorylation across the inner mitochondrial membrane (cristae).

What is substrate-level phosphorylation, and in which stages of cellular respiration does it take place?

It is the direct enzymatic transfer of a phosphate group from a phosphorylated organic substrate to $\text{ADP}$, occurring in glycolysis and the citric acid cycle.

What are the net inputs and outputs of glycolysis per molecule of glucose?

Inputs: $1\,\text{glucose}$, $2\,\text{ATP}$, and $2\,\text{NAD}^+$. Outputs: $2\,\text{pyruvate}$, $4\,\text{ATP}$ (net $2\,\text{ATP}$), $2\,\text{NADH}$, and $2\,\text{H}_2\text{O}$.

What happens during step 4 of glycolysis?

The energized six-carbon intermediate is split into two three-carbon molecules called glyceraldehyde-3-phosphate ($\text{G3P}$).

What three chemical transformations occur during the preparatory reaction (pyruvate oxidation)?

1) Pyruvate loses a carboxyl group ($-\text{COO}^-$) released as $\text{CO}_2$; 2) The resulting two-carbon fragment is oxidized while $\text{NAD}^+$ is reduced to $\text{NADH}$; 3) Coenzyme A joins the fragment to form acetyl $\text{CoA}$.

Which molecules combine in step 1 of the citric acid cycle to form a six-carbon compound?

A two-carbon acetyl group from acetyl $\text{CoA}$ combines with a four-carbon oxaloacetate molecule to produce six-carbon citrate.

What are the cumulative yields of $\text{ATP}$, $\text{CO}_2$, $\text{NADH}$, and $\text{FADH}_2$ per glucose molecule after glycolysis, pyruvate oxidation, and the citric acid cycle?

$4\,\text{ATP}$ ($2$ net from glycolysis, $2$ from citric acid cycle), $6\,\text{CO}_2$ ($2$ from pyruvate oxidation, $4$ from citric acid cycle), $10\,\text{NADH}$ ($2$ from glycolysis, $2$ from pyruvate oxidation, $6$ from citric acid cycle), and $2\,\text{FADH}_2$ (from citric acid cycle).

How do electron transport complexes generate the proton gradient across the inner mitochondrial membrane?

As high-energy electrons from $\text{NADH}$ and $\text{FADH}_2$ are transferred sequentially down the chain, Complexes I, III, and IV use the released energy to actively pump hydrogen ions ($\text{H}^+$) from the matrix into the intermembrane space.

What serves as the terminal electron acceptor in the electron transport chain, and what is the final product formed?

Molecular oxygen ($\text{O}_2$) acts as the final electron acceptor, picking up two electrons and two protons ($\text{H}^+$) to form water ($\text{H}_2\text{O}$).

What is chemiosmosis and how does it power $\text{ATP}$ synthesis?

Chemiosmosis is the generation of $\text{ATP}$ using the potential energy stored in an $\text{H}^+$ concentration gradient; protons diffuse through ATP synthase, spinning its rotor to drive the phosphorylation of $\text{ADP}$ to $\text{ATP}$.

What is the primary biochemical role of fermentation in cells lacking oxygen?

Fermentation provides an anaerobic pathway to oxidize $\text{NADH}$ back into $\text{NAD}^+$, ensuring a continuous supply of $\text{NAD}^+$ so that glycolysis can keep producing $\text{ATP}$.

How do the products of lactic acid fermentation and alcohol fermentation differ?

Lactic acid fermentation reduces pyruvate directly into lactate without releasing gas, while alcohol fermentation converts pyruvate into ethanol and carbon dioxide gas ($\text{CO}_2$).

How do obligate anaerobes differ from facultative anaerobes?

Obligate anaerobes require oxygen-free environments and are killed by atmospheric oxygen, whereas facultative anaerobes can produce $\text{ATP}$ by fermentation or oxidative phosphorylation depending on oxygen availability, preferring oxidative phosphorylation to yield more $\text{ATP}$.

According to the diagram, at what entry points do fats and proteins feed into cellular respiration?

Fats are broken into glycerol (which enters glycolysis as $\text{G3P}$) and fatty acids (which convert to acetyl $\text{CoA}$); proteins are hydrolyzed into amino acids, stripped of amino groups, and enter as intermediates of glycolysis, acetyl $\text{CoA}$, or the citric acid cycle.