Cell Biology - Unit III

What are the main structural components of a neuron?

Dendrites (receive signals), cell body (integration), axon (transmits signals), nerve terminals (release neurotransmitters).

What generates an action potential?

Opening of voltage-gated Na⁺ channels → rapid Na⁺ influx → membrane depolarization.

What happens after Na⁺ channels open?

They inactivate, and voltage-gated K⁺ channels open → repolarization.

Why are voltage-gated cation channels considered evolutionarily related?

They share structural similarities and likely evolved from a common ancestral channel protein.

What triggers neurotransmitter release at the nerve terminal?

Ca²⁺ influx through voltage-gated Ca²⁺ channels.

How does Ca²⁺ cause neurotransmitter release?

It triggers fusion of synaptic vesicles with the plasma membrane → exocytosis.

What is a chemical synapse?

A junction where neurotransmitters transmit signals between cells.

Sequence of synaptic transmission?

1. Action potential arrives

2. Ca²⁺ enters terminal

3. Vesicles release neurotransmitter

4. Neurotransmitter binds postsynaptic receptors

5. Ion channels open → electrical signal

What do transmitter-gated ion channels do?

Convert chemical signals into electrical signals.

What happens when transmitter-gated ion channels open?

Convert chemical signals into electrical signals.

What happens when these channels open?

Ion flow changes membrane potential of the postsynaptic cell.

Examples of excitatory neurotransmitters?

Acetylcholine, glutamate

What ions do excitatory receptors allow?

Na⁺ and Ca²⁺ influx → depolarization.

Examples of inhibitory neurotransmitters?

GABA, glycine.

What ions do inhibitory receptors allow?

Cl⁻ influx → hyperpolarization → prevents action potential.

What type of receptor is the acetylcholine receptor?

Transmitter-gated cation channel (excitatory).

What is the result of acetylcholine binding at NMJ?

Muscle cell depolarization → contraction.

How do barbiturates/tranquilizers work?

Enhance GABA-gated Cl⁻ channel activity → increased inhibition.

How does Prozac act?

Blocks serotonin reuptake → increases signaling.

What is chemiosmotic coupling?

Proton gradient drives ATP synthesis.

What generates the proton gradient?

Electron transport chain (ETC).

Where does glycolysis occur?

Cytoplasm.

What does glycolysis produce?

ATP + NADH.

What does the TCA cycle produce?

NADH and FADH₂.

Where does oxidative phosphorylation occur?

Inner mitochondrial membrane

Key compartments of mitochondria?

Outer membrane, inner membrane (cristae), intermembrane space, matrix.

Where is most metabolic machinery located?

Matrix and inner membrane.

What is the purpose of the ETC?

Transfer electrons to O₂ and generate proton gradient.

Final electron acceptor of ETC?

O₂ → forms H₂O.

Complex I function in ETC?

Accepts electrons from NADH; pumps H⁺.

Complex II function in ETC?

Accepts electrons from FADH₂ (no proton pumping).

Complex III function in ETC?

Transfers electrons to cytochrome c; pumps H⁺.

Complex IV function in ETC?

Transfers electrons to O₂; pumps H⁺.

What is ubiquinone (CoQ)?

Lipid-soluble electron carrier within membrane. Mobile electron carriers

What is cytochrome c?

Peripheral protein that transfers electrons between complexes.

Path of electrons from NADH?

NADH → Complex I → CoQ → Complex III → cytochrome c → Complex IV → O₂.

Which complexes pump protons?

Complex I, III, IV.

What is created by proton pumping?

Electrochemical proton gradient.

Redox potential: What does a more negative E₀′ mean?

Better electron donor.

What does a more positive E₀′ mean?

Better electron acceptor.

What enzyme synthesizes ATP?

ATP synthase (F₀F₁ complex).

What drives ATP synthase?

Proton-motive force.

What is the binding change model?

Conformational changes in ATP synthase produce ATP.

Chemiosmotic stages: Stage 1?

Generate proton gradient.

Chemiosmotic stages: Stage 2?

ATP synthesis via ATP synthase.

What is the ultimate source of energy for most life?

Photosynthesis

Inputs and outputs of photosynthesis?

CO₂ + H₂O + light → O₂ + organic molecules.

Main parts of chloroplast?

Outer membrane, inner membrane, stroma, thylakoid.

Where do light reactions occur?

Thylakoid membrane.

What happens in light reactions?

• Water split → O₂

• ATP produced

• NADPH produced

Two photosystems?

Photosystem II (PSII), Photosystem I (PSI).

Function of PSII?

Extract electrons from water.

Function of PSI?

Generate NADPH.

Path of electrons in photosynthesis?

H₂O → PSII → ETC → PSI → NADP⁺ → NADPH.

What does chlorophyll absorb best?

Red light (~680–700 nm).

Where are protons pumped in chloroplasts?

Into thylakoid space.

Where is ATP produced in chloroplasts?

Stroma.

Major similarity between mitochondria and chloroplasts?

Both use electron transport chains to create proton gradients that drive ATP synthesis.

Major difference between chloroplasts and mitochondria?

• Mitochondria: energy from chemical fuels

• Chloroplasts: energy from light

An action potential is initiated when voltage-gated ______ channels open, causing rapid membrane ______.

Na⁺ ; depolarization

At the presynaptic terminal, influx of ______ triggers fusion of ______ with the plasma membrane.

Ca²⁺ ; synaptic vesicles

Neurotransmitters are released into the ______ and bind receptors on the ______ cell.

synaptic cleft ; postsynaptic

Excitatory neurotransmitters typically open ______ channels, leading to ______ of the membrane.

Na⁺ (or cation) ; depolarization

Inhibitory neurotransmitters often open ______ channels, causing ______ of the membrane.

Cl⁻ ; hyperpolarization

GABA enhances inhibition by increasing ______ influx, making the neuron less likely to ______.

Cl⁻ ; fire an action potential

At the neuromuscular junction, binding of ______ to its receptor causes muscle cell ______.

acetylcholine ; depolarization

Transmitter-gated ion channels convert ______ signals into ______ signals.

chemical ; electrical

Prozac increases signaling by blocking the ______ of serotonin.

reuptake

Glycolysis occurs in the ______ and produces ______ and ______.

cytoplasm ; ATP ; NADH

The TCA cycle oxidizes ______ to generate high-energy electron carriers.

acetyl-CoA

The majority of ATP in aerobic cells is produced during ______ ______.

oxidative phosphorylation

The inner mitochondrial membrane is folded into ______, which increase surface area for ______.

cristae ; electron transport / ATP production

Electrons from NADH enter the ETC at Complex ______, while electrons from FADH₂ enter at Complex ______.

I ; II

The final electron acceptor in the electron transport chain is ______, forming ______.

oxygen (O₂) ; water (H₂O)

Complexes ______, ______, and ______ pump protons across the inner membrane.

I ; III ; IV

The accumulation of protons in the ______ space creates an electrochemical gradient in mitochondria.

intermembrane

The energy stored in the proton gradient is called the ______ ______ ______.

proton-motive force

Ubiquinone (CoQ) is unique because it is ______-soluble and moves within the ______.

lipid ; membrane

Cytochrome c transfers electrons between Complex ______ and Complex ______.

III ; IV

A more negative redox potential indicates a stronger electron ______.

donor

As electrons move through the ETC, their redox potential becomes more ______.

positive

Oxygen has a very ______ redox potential, making it an excellent electron ______.

positive ; acceptor

ATP synthase consists of two major parts: ______ (membrane) and ______ (catalytic head).

F₀ ; F₁

Protons flow from the ______ space into the ______ through ATP synthase.

intermembrane ; matrix

Rotation within ATP synthase leads to conformational changes described by the ______ ______ model.

binding change

Chemiosmotic coupling links ______ transport to ______ synthesis.

electron ; ATP

Light reactions occur in the ______ membrane, while carbon fixation occurs in the ______.

thylakoid ; stroma

Photosystem ______ splits water to release oxygen.

II

Electrons ultimately reduce ______ to form ______ in photosynthesis

NADP⁺ ; NADPH

The proton gradient in chloroplasts is highest in the ______ space.

thylakoid

Chlorophyll absorbs light most strongly in the ______ wavelength range.

red

The two photosystems function ______ (in series / independently) to transfer electrons.

in series

Both mitochondria and chloroplasts use ______ gradients to drive ATP synthesis.

proton

In mitochondria, energy comes from ______, whereas in chloroplasts it comes from ______.

chemical fuels ; light

In both systems, an electron transport chain creates a ______ gradient across a ______.

proton ; membrane

What is the purpose of photosynthesis?

Convert light energy into chemical energy (ATP + NADPH) and fix carbon into organic molecules.

Where do light vs. dark reactions occur?

• Light reactions → thylakoid membrane

• Calvin cycle → stroma

Key compartments of chloroplast?

Outer membrane, inner membrane, stroma, thylakoid membrane, thylakoid lumen

Where is the proton gradient generated?

Across the thylakoid membrane (into lumen)