Membrane Structure and Function

Transport Across Cellular Membranes

Biological membranes are selectively permeable barriers that regulate the movement of substances into and out of cells. Understanding how molecules cross membranes is essential for predicting cellular behavior and function.

• Hydrophobic vs. Hydrophilic Molecules: Hydrophobic (nonpolar) molecules can diffuse through the lipid bilayer, while hydrophilic (polar or charged) molecules require specific transport proteins.

• Size and Permeability: Small, nonpolar molecules (e.g., O2, CO2) cross membranes easily; large or charged molecules (e.g., glucose, ions) need transporters.

• Concentration Gradients: Substances move from areas of high concentration to low concentration (down their gradient) unless energy is used to move them against the gradient.

• Channels vs. Pumps: Channels allow passive movement down a gradient (no energy required). Pumps use energy (usually from ATP) to move substances against their gradient.

• Co-transporters: Move one substance against its gradient by coupling to the movement of another substance down its gradient.

• Diffusion: Random movement of molecules from high to low concentration.

• Osmosis: Diffusion of water across a selectively permeable membrane. Water moves toward higher solute concentration (lower water concentration).

• Facilitated Diffusion: Movement of substances down their gradient via specific transport proteins (no energy required).

• Na+/K+ ATPase Pump: Maintains concentration gradients of sodium and potassium across the plasma membrane, which can be used to drive the transport of other substances.

Example: Glucose is transported into cells by facilitated diffusion using a glucose transporter protein.

Additional info: The Na+/K+ ATPase pump moves 3 Na+ ions out and 2 K+ ions in per ATP hydrolyzed, creating electrochemical gradients essential for nerve impulses and secondary active transport.

Common Misconceptions in Membrane Transport

An Introduction to Metabolism

Spontaneity and Free Energy in Chemical Reactions

Cells carry out a variety of chemical reactions, some of which release energy (exergonic) and some that require energy input (endergonic). The concept of free energy (Gibbs free energy, G) is central to understanding these processes.

• Spontaneous Reactions: Occur without energy input; characterized by a negative change in free energy ().

• Non-spontaneous Reactions: Require energy input; characterized by a positive change in free energy ().

• Exergonic vs. Endergonic: Exergonic reactions release energy (); Endergonic reactions absorb energy ().

• Exothermic vs. Endothermic: Exothermic reactions release heat; endothermic reactions absorb heat. Not all exergonic reactions are exothermic.

• Coupling Reactions: Cells often couple exergonic reactions (e.g., ATP hydrolysis) to drive endergonic processes.

• ATP Hydrolysis: is highly exergonic and commonly used to power cellular work.

• Activation Energy (): The initial energy required to start a reaction. Even spontaneous reactions need activation energy.

Example: The synthesis of glutamine from glutamate and ammonia is endergonic, but can proceed in cells when coupled to ATP hydrolysis.

Common Misconceptions in Chemical Reactions and Enzymes

Chemical Reactions, Catalysis, and Enzymes

Enzyme Structure and Function

Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy required. Their function is highly dependent on their three-dimensional structure.

• Enzyme Specificity: The shape of an enzyme determines which substrates it can bind and what reactions it can catalyze.

• Catalysts: Enzymes are not reactants or products; they are not consumed in the reaction and can be reused.

• Environmental Effects: Enzymes function best under specific conditions (e.g., temperature, pH) that match their evolutionary context.

• Active Site: The region of the enzyme where substrate binding and catalysis occur. Changes to the active site can affect enzyme activity.

Example: The enzyme lactase catalyzes the hydrolysis of lactose into glucose and galactose. If the active site is altered, the enzyme may lose its ability to bind lactose.

Common Misconceptions in Enzyme Function

Cellular Respiration

Overview and Stages

Cellular respiration is the process by which cells extract energy from organic molecules, primarily glucose, to produce ATP. It consists of several stages:

• Glycolysis: Occurs in the cytoplasm; breaks glucose into two pyruvate molecules, producing a small amount of ATP and NADH.

• Transition Reaction (Pyruvate Oxidation): Converts pyruvate to acetyl-CoA, producing NADH and releasing CO2.

• Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondrial matrix; completes the breakdown of acetyl-CoA, generating ATP, NADH, FADH2, and CO2.

• Oxidative Phosphorylation: Includes the electron transport chain and chemiosmosis; uses NADH and FADH2 to generate a large amount of ATP, with O2 as the final electron acceptor.

Example: In aerobic respiration, one molecule of glucose can yield up to 30-32 molecules of ATP.

Common Misconceptions in Cellular Respiration

Key Equations

• Gibbs Free Energy: Where: = change in free energy, = change in enthalpy, = temperature (Kelvin), = change in entropy.

• ATP Hydrolysis:

• Overall Cellular Respiration:

Summary Table: Stages of Cellular Respiration

Additional info: If the electron transport chain is blocked (e.g., by lack of O2), NADH and FADH2 cannot be oxidized, halting the citric acid cycle and glycolysis (unless fermentation occurs).