Unit II Study Guide: Membrane Transport, Metabolism, Cellular Respiration, and Photosynthesis

Chapter 5: Membrane Transport and Cell Signaling

Membrane Structure and Function

The plasma membrane is a selectively permeable barrier that regulates the movement of substances into and out of the cell. Its structure and properties are essential for maintaining cellular homeostasis and facilitating communication.

• Water Potential and Toxicity: Predict how water moves in plant and animal cells based on water potential and the effect of tonicity (isotonic, hypertonic, hypotonic solutions).

• Protein Association with Membranes: The structure and charge of a protein influence its interaction with the hydrophobic and hydrophilic regions of the membrane.

• Membrane Protein Functions: Proteins in the membrane serve as channels, carriers, receptors, and enzymes, each with specific roles in transport and signaling.

• Diffusion and Concentration Gradients: Molecules move from areas of high to low concentration by diffusion. The rate depends on the molecule's size, charge, and the membrane's properties.

• Predicting Membrane Permeability: The ability of a molecule to cross the membrane depends on its polarity, size, and charge.

• Active Transport: Active transport requires energy (usually ATP) to move substances against their concentration gradient. Examples include the sodium-potassium pump and proton pumps.

• Energy in Membrane Transport: Biological membranes use energy to maintain gradients essential for cellular function.

• Cell Communication: Cells communicate via signaling molecules that bind to receptors, initiating a cascade of cellular responses. This process is essential for coordination and regulation.

• Signal Transduction: The process by which a signal on a cell's surface is converted into a specific cellular response, often involving multiple steps and amplification.

Example:

Osmosis is the diffusion of water across a selectively permeable membrane. In a hypertonic solution, animal cells shrink, while plant cells undergo plasmolysis.

Chapter 6: An Introduction to Metabolism

Energy and Metabolic Pathways

Metabolism encompasses all chemical reactions in a cell, including those that build (anabolic) and break down (catabolic) molecules. Energy transformations are governed by the laws of thermodynamics.

• Energy in Organisms: Organisms obtain energy through photosynthesis (autotrophs) or by consuming other organisms (heterotrophs).

• Anabolic vs. Catabolic Metabolism: Anabolic pathways build complex molecules from simpler ones (require energy), while catabolic pathways break down molecules (release energy).

• Energy Coupling: Cells couple exergonic (energy-releasing) and endergonic (energy-consuming) reactions, often using ATP as an energy intermediary.

• Thermodynamics: The first law states that energy cannot be created or destroyed, only transformed. The second law states that every energy transfer increases the entropy (disorder) of the universe.

• Free Energy (Gibbs Free Energy): Determines whether a reaction is spontaneous. $\Delta G = \Delta H - T\Delta S$

• Enzyme Function: Enzymes are biological catalysts that speed up reactions by lowering activation energy. They are specific to their substrates and can be regulated by inhibitors and activators.

• Enzyme Regulation: Allosteric regulation involves molecules binding to sites other than the active site, altering enzyme activity. Feedback inhibition is a common regulatory mechanism.

Example:

Cellular respiration is a catabolic pathway that releases energy by breaking down glucose and other organic molecules in the presence of oxygen.

Chapter 7: Cellular Respiration and Fermentation

Overview of Cellular Respiration

Cellular respiration is the process by which cells extract energy from glucose to produce ATP. It involves glycolysis, the citric acid cycle, and oxidative phosphorylation.

• Glycolysis: Occurs in the cytoplasm; breaks down glucose into two molecules of pyruvate, producing a net gain of 2 ATP and 2 NADH.

• Pyruvate Oxidation: Pyruvate is transported into the mitochondria and converted to acetyl-CoA, producing NADH and releasing CO2.

• Citric Acid Cycle (Krebs Cycle): Completes the breakdown of glucose, generating ATP, NADH, and FADH2.

• Electron Transport Chain (ETC): Located in the inner mitochondrial membrane; uses electrons from NADH and FADH2 to create a proton gradient that drives ATP synthesis.

• Oxidative Phosphorylation: ATP is produced as protons flow back into the mitochondrial matrix through ATP synthase.

• Fermentation: In the absence of oxygen, cells can undergo fermentation to regenerate NAD+ and produce ATP.

Example:

Lactic acid fermentation in muscle cells produces lactate when oxygen is scarce, allowing glycolysis to continue.

Chapter 8: Photosynthesis

Photosynthetic Processes

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. It consists of light-dependent reactions and the Calvin cycle (light-independent reactions).

• Redox Reactions: Photosynthesis involves the transfer of electrons from water to carbon dioxide, reducing CO2 to glucose and oxidizing water to O2.

• Chloroplast Structure: Photosynthesis occurs in the chloroplasts, primarily within the thylakoid membranes.

• Light Reactions: Capture light energy to produce ATP and NADPH, releasing O2 as a byproduct.

• Electron Transport Chain in Photosynthesis: Electrons move through photosystems II and I, generating a proton gradient for ATP synthesis.

• Calvin Cycle: Uses ATP and NADPH to fix CO2 into organic molecules (glucose). The enzyme RuBisCO catalyzes the first step.

• Energy Flow: Light energy is converted to chemical energy, which is then used to build carbohydrates.

• Oxygen Evolution: Water is split to provide electrons, releasing O2 as a waste product.

Example:

During the light reactions, the splitting of water provides electrons for the electron transport chain and produces oxygen as a byproduct.

Table: Comparison of Cellular Respiration and Photosynthesis

Process	Location	Reactants	Products	Energy Conversion
Cellular Respiration	Mitochondria	Glucose, O2	CO2, H2O, ATP	Chemical energy (glucose) to ATP
Photosynthesis	Chloroplasts	CO2, H2O, Light	Glucose, O2	Light energy to chemical energy (glucose)