Cell Biology Test 3

UJ Dr. Jensen: Ch. 11-14

What is the historical development of membrane structure concepts?

Gorter and Grendel (1925) discovered that cell membranes are lipid bilayers by extracting lipids from red blood cells and showing they covered twice the surface area of the cells. Later, Singer and Nicholson (1972) proposed the Fluid Mosaic Model, describing membranes as dynamic structures with proteins floating within a lipid bilayer.

What is the Fluid Mosaic Model?

The Fluid Mosaic Model describes membranes as flexible, fluid structures where proteins move laterally within a bilayer of phospholipids. It emphasizes that membranes are not rigid and that proteins and lipids can diffuse within the plane of the membrane.

What molecules are involved in the Fluid Mosaic Model?

Phospholipids form the bilayer; glycolipids and glycoproteins are involved in cell recognition and signaling; integral proteins span the bilayer; and peripheral proteins are attached to membrane surfaces for support or communication.

What is membrane asymmetry?

Membrane asymmetry means the two sides (leaflets) of the lipid bilayer differ in composition and function. Lipids and proteins are distributed unequally, maintained by enzymes such as flippases, floppases, and scramblases.

What experiments demonstrated lateral movement of membrane proteins?

Fluorescence Recovery After Photobleaching (FRAP) and cell fusion experiments demonstrated that proteins and lipids can move laterally within the bilayer, confirming membrane fluidity.

How does a cell regulate membrane fluidity?

Cells regulate membrane fluidity by changing fatty acid composition and cholesterol content. Unsaturated fatty acids increase fluidity, while cholesterol stabilizes membranes by preventing tight packing of lipids.

What are the functions of membrane proteins?

hey function in transport, enzymatic activity, signal transduction, cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix.

What is the structure and role of membrane channels?

Membrane channels are transmembrane proteins forming hydrophilic pores that allow passage of specific ions or molecules. Many are gated and respond to stimuli such as voltage, ligands, or mechanical stress.

What is the cell cortex and what are its key molecules?

The cell cortex is a network of actin filaments beneath the plasma membrane that provides structural support and facilitates movement. Key molecules include spectrin (especially in red blood cells), proteoglycans, and the glycocalyx.

What are the three categories of transport across membranes?

Simple diffusion (passive movement without proteins), facilitated diffusion (via transport proteins, no energy required), and active transport (requires energy to move substances against gradients).

How does facilitated diffusion differ from simple diffusion?

Simple diffusion is a linear, passive process for small, nonpolar molecules, while facilitated diffusion involves transport proteins, shows saturation kinetics, and allows larger or polar molecules to cross the membrane.

What factors affect membrane permeability and diffusion rate?

Molecule size, polarity, and charge influence permeability. Smaller, nonpolar molecules diffuse faster, while charged or large molecules need transporters. Temperature and lipid composition also affect diffusion.

What is the difference between active and passive transport?

Passive transport moves substances down their concentration gradient without energy input. Active transport requires energy, typically ATP, to move substances against their gradient.

How do channels differ from carriers?

Channels form open pores allowing rapid ion flow, while carriers bind molecules and change shape to move them across the membrane, making them slower but more selective.

How is transport of charged species more complex than uncharged ones?

Charged molecule movement depends on both concentration and electrical gradients, forming an electrochemical gradient that influences direction and rate of transport.

What is the difference between ATP-driven and coupled transport?

ATP-driven pumps use ATP hydrolysis directly (e.g., Na⁺/K⁺-ATPase), while coupled transport uses the energy from one molecule’s downhill movement to drive another’s uphill movement, as in symports and antiports.

What is the sodium-potassium pump and its function?

The Na⁺/K⁺ pump uses ATP to export 3 Na⁺ ions and import 2 K⁺ ions, maintaining resting membrane potential and osmotic stability in animal cells.

How does sodium cotransport work in animal cells?

The sodium gradient, maintained by the Na⁺/K⁺ pump, drives glucose or amino acid uptake through symport mechanisms. In bacteria, a proton (H⁺) gradient serves the same purpose.

What happens during an action potential?

Voltage-gated Na⁺ channels open first, allowing depolarization, followed by voltage-gated K⁺ channels opening for repolarization. The cycle transmits electrical signals along neurons.

What happens during glycolysis?

Glycolysis occurs in the cytoplasm, breaking glucose (6C) into two pyruvate (3C) molecules. It produces a net of 2 ATP and 2 NADH. It is regulated by phosphofructokinase and includes intermediates such as fructose-1,6-bisphosphate and 3-phosphoglycerate.

Why does fermentation of pyruvate occur?

When oxygen is absent, fermentation regenerates NAD⁺ for glycolysis. In animals, pyruvate converts to lactate; in yeast, pyruvate becomes ethanol and CO₂.

What are key metabolic terms to know?

Catabolism: energy-releasing breakdown of molecules; Anabolism: energy-requiring synthesis; Endergonic: requires energy; Exergonic: releases energy; Obligate aerobes need O₂, obligate anaerobes die in O₂, and facultative anaerobes can do both.

What happens in the Krebs cycle and preparatory reactions?

Pyruvate is converted to acetyl-CoA, which enters the Krebs cycle. It generates citrate → isocitrate → α-ketoglutarate → succinyl-CoA → succinate → fumarate → malate → oxaloacetate, producing 6 NADH, 2 FADH₂, 4 CO₂, and 2 ATP per glucose.

How are proteins and fatty acids metabolized?

Proteins are broken into amino acids that enter the Krebs cycle as intermediates. Fatty acids undergo β-oxidation to form acetyl-CoA, which enters the Krebs cycle for ATP production.

What are the main structures of mitochondria and their functions?

The outer membrane is permeable to small molecules; the inner membrane houses the electron transport chain and ATP synthase; and the matrix contains enzymes for the Krebs cycle and mitochondrial DNA.

What happens in the electron transport chain (ETC)?

Electrons move through complexes I–IV, assisted by coenzymes CoQ and cytochrome c. Oxygen is the final electron acceptor, forming water. The process pumps protons to create a gradient for ATP synthesis.

What is chemiosmotic coupling?

Proposed by Peter Mitchell, this theory explains that the proton gradient created by the ETC drives ATP synthesis through ATP synthase, composed of the F₀ channel and F₁ catalytic head.

How are ATP, ADP, and NADH transported in mitochondria?

ADP/ATP translocase swaps ATP (out) for ADP (in), while shuttle systems like the malate-aspartate shuttle move reducing equivalents into the mitochondria.

What are the key structures of the chloroplast?

The outer and inner membranes surround the stroma. Inside are thylakoid membranes that host the light reactions. The thylakoid lumen accumulates protons used to produce ATP.

What pigments are involved in photosynthesis?

Chlorophyll a and b capture light, while carotenoids protect against damage and broaden the absorption range. Pigments are arranged in photosystems I and II.

What happens during noncyclic photophosphorylation?

Water is split to release O₂; electrons flow from PSII → PSI → NADP⁺ forming NADPH. The electron flow powers ATP synthesis through the proton gradient.

What happens during cyclic photophosphorylation?

Electrons from PSI cycle back to the ETC to produce ATP only, without generating NADPH or oxygen. This process helps balance the ATP/NADPH ratio.

What occurs in the Calvin cycle?

CO₂ is fixed to RuBP by Rubisco, forming 3-phosphoglycerate, which becomes glyceraldehyde-3-phosphate. ATP and NADPH from the light reactions power this process; 3 ATP and 2 NADPH are used per CO₂ fixed.

What is photorespiration and why does it occur?

When Rubisco binds O₂ instead of CO₂, phosphoglycolate forms, wasting energy and decreasing photosynthetic efficiency. It’s more common under high O₂ and heat conditions.

What is the C4 (Hatch-Slack) pathway?

C4 plants fix CO₂ into oxaloacetate using PEP carboxylase in mesophyll cells, then transfer it to bundle sheath cells where CO₂ is released for the Calvin cycle. This reduces photorespiration but costs more ATP.

How did respiration and photosynthesis evolve?

Early life relied on anaerobic glycolysis. Photosynthetic organisms evolved to produce oxygen, and aerobic respiration later emerged as a more efficient energy system.

What is cholesterol’s role in regulating membrane fluidity?

Cholesterol stabilizes the membrane by reducing fluidity at high temperatures and preventing tight packing of phospholipids at low temperatures. It acts as a fluidity buffer to maintain consistent membrane flexibility.

What is the total ATP yield from aerobic respiration per glucose molecule?

Aerobic respiration produces about 30–32 ATP per glucose molecule in eukaryotes, depending on which NADH shuttle system (glycerol phosphate or malate-aspartate) is used.

What are the four major complexes of the electron transport chain (ETC) and their roles?

• Complex I (NADH dehydrogenase): Transfers electrons from NADH to CoQ and pumps protons.

• Complex II (succinate dehydrogenase): Accepts electrons from FADH₂ but does not pump protons.

• Complex III (cytochrome bc₁ complex): Transfers electrons from CoQ to cytochrome c and pumps protons.

• Complex IV (cytochrome c oxidase): Transfers electrons to O₂, forming water and pumping protons.

How does Complex II differ from the other ETC complexes?

Complex II (succinate dehydrogenase) connects the Krebs cycle to the ETC by oxidizing succinate to fumarate, using FADH₂ as an electron donor. It’s the only complex that does not pump protons.

What are the components and roles of ATP synthase?

ATP synthase has two parts: F₀, a membrane channel that allows proton flow, and F₁, the catalytic head in the matrix or stroma that rotates to synthesize ATP from ADP and inorganic phosphate.

What enzyme catalyzes CO₂ fixation in the Calvin cycle, and what else can it do?

Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes CO₂ fixation in the Calvin cycle but can also bind O₂, leading to photorespiration when O₂ levels are high.

What is the total energy requirement of the Calvin cycle per G3P molecule produced?

The Calvin cycle requires 9 ATP and 6 NADPH to fix three CO₂ molecules and produce one G3P (glyceraldehyde-3-phosphate).

How do proton gradients differ between mitochondria and chloroplasts?

In mitochondria, protons are pumped into the intermembrane space and flow back into the matrix to make ATP. In chloroplasts, protons accumulate in the thylakoid lumen and flow back into the stroma to drive ATP synthesis.