Cell Biology Midterm 2 Review Guide: Membranes, Transport, Endomembrane System, Metabolism, and Photosynthesis

Chapter 7: Membranes as Selective Barriers to the Cell

Structure and Function of Biological Membranes

Biological membranes are essential for maintaining cellular integrity and regulating the movement of substances into and out of the cell. Their structure and composition determine their selective permeability and functional properties.

• Fluid Mosaic Model: Describes the membrane as a dynamic structure composed of a lipid bilayer with embedded proteins. Lipids and proteins can move laterally within the layer, contributing to membrane fluidity.

• Phospholipid Properties: Membranes contain phospholipids with hydrophilic heads and hydrophobic tails. Longer fatty acid chains and saturated fatty acids decrease fluidity, while shorter chains and unsaturated fatty acids increase fluidity.

• Cholesterol: Modulates membrane fluidity by stabilizing the bilayer and preventing extremes of fluidity.

• SDS-Page Gel Electrophoresis: Used to analyze membrane proteins by separating them based on size. Thin Layer Chromatography (TLC) is used for lipid separation and analysis.

Example: The plasma membrane of animal cells contains cholesterol, which helps maintain optimal fluidity across temperature changes.

Chapter 8: Membrane Transport

Mechanisms of Transport Across Membranes

Cells transport molecules across membranes using various mechanisms, which can be passive or active depending on energy requirements and the nature of the molecules.

• Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2) directly through the lipid bilayer down their concentration gradient.

• Facilitated Diffusion: Transport of polar or charged molecules via specific membrane proteins (channels or carriers) without energy input.

• Primary and Secondary Active Transport: Movement of molecules against their concentration gradient using energy, either directly from ATP hydrolysis (primary) or indirectly via coupling to another gradient (secondary).

• Transporter Proteins: Include channels, carriers, and pumps that mediate specific transport processes.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Kinetic Properties: Facilitated diffusion exhibits saturation kinetics, described by Michaelis-Menten equation: $v = \frac{V_{max}[S]}{K_m + [S]}$

• General Properties: Transport is influenced by concentration gradients, membrane permeability, and the presence of specific transport proteins.

• Hypertonic, Hypotonic, and Isotonic Solutions: Describe the relative solute concentrations outside versus inside the cell, affecting water movement and cell volume.

Example: Glucose uptake in red blood cells occurs via facilitated diffusion through GLUT transporters.

Chapter 12: The Endomembrane System and Peroxisomes

Organization and Function of the Endomembrane System

The endomembrane system is a network of organelles involved in the synthesis, modification, and transport of cellular materials. Peroxisomes are specialized for oxidative reactions.

• General Functions: Includes the rough and smooth endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and vesicles. The rough ER is involved in protein synthesis and modification, while the smooth ER is involved in lipid synthesis and detoxification.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Endocytosis and Exocytosis: Endocytosis brings materials into the cell via vesicles; exocytosis expels materials out of the cell.

• Protein Trafficking: Proteins are sorted and delivered to their destinations via vesicular transport, often involving specific signal sequences.

• Lysosomes: Contain hydrolytic enzymes for degradation of macromolecules; defects can lead to storage diseases.

• Peroxisomes: Carry out oxidation reactions, including breakdown of fatty acids and detoxification of hydrogen peroxide via catalase.

Example: The secretion of insulin by pancreatic beta cells involves synthesis in the ER, modification in the Golgi, and release via exocytosis.

Chapter 9: Chemotropic Energy Metabolism I

Metabolism and ATP Production

Cells obtain energy by metabolizing nutrients, primarily through catabolic pathways that generate ATP, the universal energy currency.

• Metabolism: The sum of all chemical reactions in the cell, including catabolism (breakdown) and anabolism (synthesis).

• ATP: Adenosine triphosphate stores and transfers energy for cellular processes. Hydrolysis of ATP releases energy.

• Properties of ATP: High-energy phosphate bonds make ATP an efficient energy carrier.

• Glycolysis: The breakdown of glucose to pyruvate, producing ATP and NADH. Occurs in the cytoplasm.

• Regulation: Key enzymes regulate glycolysis and gluconeogenesis, such as phosphofructokinase and fructose-1,6-bisphosphatase.

• Equation for ATP Hydrolysis: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Example: Muscle cells use glycolysis to rapidly generate ATP during intense exercise.

Chapter 10: Chemotropic Energy Metabolism II

Mitochondrial Metabolism and the TCA Cycle

Mitochondria are the site of aerobic respiration, where pyruvate and fatty acids are oxidized to produce ATP via the TCA cycle and electron transport chain (ETC).

• Mitochondrial Structure: Includes outer and inner membranes, intermembrane space, and matrix. The inner membrane houses the ETC and ATP synthase.

• Pyruvate Dehydrogenase Complex (PDC): Converts pyruvate to acetyl-CoA, linking glycolysis to the TCA cycle.

• Fatty Acid Oxidation: Beta-oxidation of fatty acids generates acetyl-CoA for entry into the TCA cycle.

• TCA Cycle: Series of reactions that oxidize acetyl-CoA to CO2, generating NADH and FADH2 for the ETC. $\text{Acetyl-CoA} + 3\text{NAD}^+ + \text{FAD} + \text{GDP} + \text{P}_i + 2\text{H}_2\text{O} \rightarrow 2\text{CO}_2 + 3\text{NADH} + 3\text{H}^+ + \text{FADH}_2 + \text{GTP} + \text{CoA}$

• Electron Transport Chain (ETC): Transfers electrons from NADH and FADH2 to oxygen, generating a proton gradient used to synthesize ATP.

• Oxidative Phosphorylation: Coupling of electron transport to ATP synthesis via ATP synthase.

Example: The ETC in mitochondria produces the majority of ATP in aerobic cells.

Chapter 11: Photosynthesis

Light Energy Conversion in Chloroplasts

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy, producing organic molecules and oxygen.

• Definition: Photosynthesis uses light energy to convert CO2 and H2O into glucose and O2.

• Light Reactions: Occur in photosystems I (PSI) and II (PSII), where light energy is used to generate ATP and NADPH.

• Chlorophyll: Pigment molecules (chlorophyll a and b) absorb light at specific wavelengths. PSI contains chlorophyll P700, PSII contains chlorophyll P680.

• Oxygen Generation: PSII splits water molecules, releasing oxygen as a byproduct.

• Calvin Cycle: Uses ATP and NADPH to fix CO2 into carbohydrates. $6\text{CO}_2 + 12\text{NADPH} + 18\text{ATP} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 12\text{NADP}^+ + 18\text{ADP} + 18\text{P}_i$

Example: Photosynthesis in green plants is responsible for producing the oxygen we breathe and the food we eat.

Table: Comparison of Membrane Transport Mechanisms

Transport Type	Energy Requirement	Direction	Example
Simple Diffusion	No	Down gradient	O2 movement
Facilitated Diffusion	No	Down gradient	Glucose via GLUT transporter
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Secondary Active Transport	Indirect (gradient)	Against gradient	Sodium-glucose symporter

Additional info: Some details, such as specific enzyme names and equations, were inferred from standard cell biology curriculum to provide a complete and self-contained study guide.