Membrane Structure and Function

Transport Across Cellular Membranes

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

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

Types of Membrane Transport

• Simple Diffusion: Movement of molecules directly through the membrane down their concentration gradient. No energy required.

• Osmosis: Diffusion of water across a selectively permeable membrane. Water moves from areas of high water concentration (low solute) to low water concentration (high solute).

• Facilitated Diffusion: Movement of molecules down their concentration gradient via specific transport proteins (channels or carriers). No energy required.

• Active Transport: Movement of molecules against their concentration gradient using energy (usually from ATP). Performed by pumps (e.g., Na+/K+ ATPase).

• Co-transport (Secondary Active Transport): Uses the movement of one molecule down its gradient to drive the movement of another molecule against its gradient (e.g., Na+-glucose symporter).

Channels vs. Pumps

• Channels: Allow passive movement of substances down their concentration gradient. No energy required.

• Pumps: Use energy (often from ATP) to move substances against their concentration gradient.

Table: Comparison of Channels, Pumps, and Co-transporters

Key Misconceptions and Clarifications

• Osmosis is not the opposite of diffusion: Osmosis is the diffusion of water.

• Each substance moves independently: Each follows its own gradient, except for some charged ions (not covered here).

• Pumps and channels are not the same: Pumps require energy; channels do not.

• Pumps and co-transporters differ: Pumps use ATP; co-transporters use the energy from another molecule's movement.

• Pumps are always active: They do not "turn on" only when gradients are out of range.

An Introduction to Metabolism

Spontaneity and Free Energy (ΔG)

Metabolic reactions are governed by changes in free energy (ΔG), which determines whether a reaction is spontaneous.

• Spontaneous Reactions: Occur without input of energy; have negative ΔG.

• Non-spontaneous Reactions: Require energy input; have positive ΔG.

• Exergonic vs. Endergonic: Exergonic reactions release energy (ΔG < 0); Endergonic reactions absorb energy (ΔG > 0).

• Exothermic vs. Exergonic: Exothermic refers to heat release; exergonic refers to free energy release (not always heat).

Equation for Free Energy Change:

• Where ΔH = change in enthalpy (heat), T = temperature (Kelvin), ΔS = change in entropy (disorder).

Coupling Reactions

• ATP Hydrolysis: The breakdown of ATP to ADP and inorganic phosphate (Pi) is highly exergonic and is used to drive endergonic reactions in cells.

• Coupling Mechanism: ATP often transfers a phosphate group to a reactant, making the overall process exergonic.

Equation for ATP Hydrolysis:

Activation Energy and Catalysis

• Activation Energy (Ea): The initial energy input required to start a reaction.

• Enzymes: Biological catalysts that lower activation energy, increasing reaction rate without being consumed.

• Enzymes do not change ΔG: They only affect the rate, not the spontaneity, of a reaction.

Diagram Description: Energy vs. Progress of Reaction

• Exergonic reaction: Reactants start higher than products; ΔG is negative.

• Endergonic reaction: Products are higher than reactants; ΔG is positive.

• With enzyme: Activation energy "hump" is lower.

Key Misconceptions and Clarifications

• Positive ΔG is not spontaneous: Only negative ΔG reactions are spontaneous.

• All reactions require activation energy: Even exergonic reactions need a "push" to start.

• Spontaneous ≠ Fast: High activation energy can make spontaneous reactions slow.

• Exergonic ≠ Exothermic: Exergonic is about free energy, not just heat.

• Enzymes do not make non-spontaneous reactions spontaneous: They only lower activation energy.

Enzyme Structure and Function

Enzyme Properties

Enzymes are proteins that catalyze biochemical reactions by lowering activation energy. Their function depends on their three-dimensional structure.

• Specificity: The shape of the enzyme determines which substrates it can bind and catalyze.

• Not Reactants or Products: Enzymes are not consumed or produced in the reaction; they are reusable.

• Environmental Sensitivity: Enzyme activity depends on conditions such as pH and temperature. Enzymes work best in the conditions in which they evolved.

Key Misconceptions and Clarifications

• Enzyme shape matters: Determines binding and catalysis.

• Enzymes are not used up: They can be used repeatedly.

• Enzymes have optimal conditions: Activity decreases if conditions deviate from the optimum.

Cellular Respiration

Overview of Cellular Respiration

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 pyruvate, producing ATP and NADH.

• Transition Reaction: Converts pyruvate to acetyl-CoA, producing NADH and CO2.

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

• Oxidative Phosphorylation: Includes the electron transport chain and chemiosmosis; uses NADH and FADH2 to produce ATP and water.

Inputs and Outputs

• Overall Equation:

• Oxygen's Role: Final electron acceptor in the electron transport chain; forms water.

• CO2 Production: Carbon from glucose is released as CO2 during the citric acid cycle and transition reaction.

Key Misconceptions and Clarifications

• Decomposition after death: Carbon returns to the ecosystem via decomposers, not the organism's own respiration.

• Source of oxygen in CO2: Comes from glucose, not from inhaled O2.

• Oxygen gas forms water: The O2 we breathe becomes part of H2O, not CO2.

• Only the electron transport chain directly requires O2: Other stages depend on it indirectly.

• If one stage is blocked, subsequent stages stop: Each stage depends on the products of the previous one.

• High levels of a molecule may indicate a problem with the next stage: Accumulation can result from a downstream block.

• Organisms lose mass via respiration: Carbon and oxygen are lost as CO2 during cellular respiration.

Table: Stages of Cellular Respiration

Applications and Experimental Predictions

• Understanding these processes allows prediction of how changes (e.g., blocking a stage, altering inputs) affect ATP production and organismal biomass.

Additional info: This guide is based on common learning objectives and misconceptions for introductory cell biology, focusing on membrane transport, enzyme function, and cellular respiration. For further study, review lecture slides, discussion problems, and homework questions as suggested by your instructors.