BIO 110 Exam 3: Cell Membranes, Transport, and Cellular Respiration Study Guide

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Cell Membranes and Transport

Fluid Mosaic Model

The fluid mosaic model describes the structure of the plasma membrane as a dynamic and flexible layer composed of a phospholipid bilayer with embedded proteins, carbohydrates, and cholesterol. The 'fluid' aspect refers to the lateral movement of lipids and proteins, while 'mosaic' refers to the patchwork of proteins that float in or on the fluid lipid bilayer.

Phospholipid bilayer: Main structural component; hydrophilic heads face outward, hydrophobic tails face inward.
Proteins: Serve as channels, carriers, receptors, and enzymes.
Carbohydrates: Attached to proteins or lipids; involved in cell recognition.
Cholesterol: Maintains membrane fluidity and stability.
Example: Glycoproteins act as cell surface markers for immune recognition.

Selective Permeability

Selective permeability means the plasma membrane allows some substances to cross more easily than others, maintaining homeostasis.

Small, nonpolar molecules (e.g., O2, CO2) pass easily.
Ions and large polar molecules require transport proteins.

Structure of the Plasma Membrane

Phospholipids: Amphipathic molecules with hydrophilic heads (face aqueous environments) and hydrophobic tails (face inward, away from water).
Proteins: Integral (span the membrane) and peripheral (attached to surface).
Carbohydrates: Glycolipids and glycoproteins on the extracellular surface.
Cholesterol: Interspersed among phospholipids, modulating fluidity.

Functions of the Plasma Membrane

Regulates entry and exit of substances.
Facilitates communication via receptors.
Provides structural support and cell recognition.

Transport Across Membranes

Passive Transport

Passive transport is the movement of substances across the membrane without energy input, down their concentration gradient.

Diffusion: Movement of molecules from high to low concentration.
Passive diffusion: Direct movement through the lipid bilayer (e.g., O2).
Facilitated diffusion: Movement via transport proteins (e.g., glucose transporters).
Osmosis: Diffusion of water across a selectively permeable membrane.

Osmosis and Tonicity

Osmosis: Water moves from areas of low solute concentration to high solute concentration.
Hypotonic solution: Lower solute concentration outside; water enters cell, causing swelling.
Hypertonic solution: Higher solute concentration outside; water leaves cell, causing shrinkage.
Isotonic solution: Equal solute concentration; no net water movement.
Direction of water movement: Water moves toward the hypertonic side.

Active Transport

Active transport moves substances against their concentration gradient, requiring energy (usually ATP).

Example: Sodium-potassium pump (Na+/K+ ATPase).

Bulk Transport

Endocytosis: Uptake of materials via vesicle formation.
Exocytosis: Release of materials by vesicle fusion with the membrane.
Phagocytosis: "Cell eating"; engulfment of large particles.

Energy and Enzymes

Kinetic and Potential Energy

Kinetic energy: Energy of motion (e.g., movement of molecules).
Potential energy: Stored energy due to position or structure.
Chemical energy: A form of potential energy stored in chemical bonds (e.g., glucose).
Comparison: Kinetic energy is used to do work; potential energy can be converted to kinetic energy.

Enzymes

Type of molecule: Enzymes are proteins (not lipids, nucleic acids, or carbohydrates).
Active site: Region where substrate binds and reaction occurs.
Lowering activation energy: Enzymes reduce the energy barrier for reactions.
Factors affecting function: Temperature, pH, substrate concentration, presence of inhibitors.

Enzyme Inhibition

Competitive inhibitors: Bind to the active site, blocking substrate access.
Noncompetitive inhibitors: Bind elsewhere, changing enzyme shape and reducing activity.
Comparison: Competitive inhibition can be overcome by increasing substrate; noncompetitive cannot.

Phosphorylation

Phosphorylation is the addition of a phosphate group to a molecule, often regulating enzyme activity and energy transfer (e.g., ATP hydrolysis).

Cellular Respiration

Overview and Equation

Purpose: Harvest energy from glucose to produce ATP.
Overall equation:

Inputs: Glucose, oxygen
End products: Carbon dioxide, water, ATP

Stages of Cellular Respiration

Glycolysis: Occurs in cytoplasm; breaks glucose into 2 pyruvate, net gain of 2 ATP, 2 NADH.
Citric Acid (Krebs) Cycle: Occurs in mitochondrial matrix; produces CO2, 2 ATP, 6 NADH, 2 FADH2 per glucose.
Oxidative Phosphorylation: Occurs in inner mitochondrial membrane; includes electron transport chain and chemiosmosis, produces most ATP.

Electron Transport Chain (ETC) and Chemiosmosis

ETC: Series of protein complexes that transfer electrons from NADH/FADH2 to O2 (final electron acceptor), forming H2O.
Chemiosmosis: H+ gradient drives ATP synthesis via ATP synthase.
ATP synthase: Enzyme that synthesizes ATP as protons flow through it.

Redox Reactions in Respiration

Oxidation: Loss of electrons (e.g., glucose is oxidized).
Reduction: Gain of electrons (e.g., O2 is reduced to H2O).
Electron carriers: NAD+ and FAD shuttle electrons to the ETC.

Aerobic vs. Anaerobic Respiration

Aerobic respiration: Requires O2; produces up to ~32 ATP per glucose.
Anaerobic respiration: Does not require O2; less ATP produced.
Fermentation: Anaerobic process; includes alcohol fermentation (produces ethanol, CO2) and lactic acid fermentation (produces lactate).

Locations of Cellular Respiration Stages

Stage	Location
Glycolysis	Cytoplasm
Citric Acid Cycle	Mitochondrial matrix
Oxidative Phosphorylation	Inner mitochondrial membrane

Glycolysis Details

Net ATP gain: 2 ATP per glucose
End products: 2 pyruvate, 2 NADH, 2 ATP
Considered evolutionarily "old": Occurs in all organisms, does not require oxygen or organelles.

Citric Acid Cycle Details

Inputs: Acetyl-CoA
End products (per glucose): 4 CO2, 6 NADH, 2 FADH2, 2 ATP
Intermediates: Citrate, isocitrate, α-ketoglutarate, succinate, fumarate, malate, oxaloacetate

Oxidative Phosphorylation Details

Location: Inner mitochondrial membrane
ATP synthase: Uses H+ gradient to produce ATP
H+ gradient: Established by ETC pumping protons into intermembrane space

Summary Table: Aerobic vs. Anaerobic Respiration

Feature	Aerobic Respiration	Anaerobic Respiration
Oxygen Required?	Yes	No
ATP Yield (per glucose)	~32	2
End Products	CO2, H2O	Lactic acid or ethanol, CO2
Examples	Most eukaryotes	Yeast, muscle cells (temporarily)

Additional Info

During cell respiration, O2 is reduced to form H2O at the end of the electron transport chain.
Fermentation regenerates NAD+ for glycolysis in the absence of oxygen.