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

Chapter 7: Membranes as Selective Barriers to the Cell

Membrane Structure

The cell membrane is a dynamic structure that serves as a selective barrier, regulating the movement of substances into and out of the cell. Its composition and organization are critical for cellular function.

• Fluid Mosaic Model: Describes the membrane as a bilayer of phospholipids with embedded proteins, allowing lateral movement and flexibility.

• Phospholipid Properties: Membranes contain both saturated and unsaturated fatty acids. Longer chain fatty acids and saturated fats lead to higher melting points and less fluidity, while unsaturated fats increase fluidity.

• Cholesterol: Modulates membrane fluidity by preventing tight packing of phospholipids.

• SDS-PAGE: A laboratory technique used to analyze membrane proteins by separating them based on size.

• TLC (Thin Layer Chromatography): Used for lipid analysis and profiling.

Example: The presence of cholesterol in animal cell membranes helps maintain membrane integrity at varying temperatures.

Chapter 8: Membrane Transport

Transport Mechanisms

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

• Simple Diffusion: Movement of small, nonpolar molecules directly through the lipid bilayer, driven by concentration gradients.

• Facilitated Diffusion: Transport of molecules via specific membrane proteins (channels or carriers), still driven by concentration gradients but allowing passage of larger or polar molecules.

• Primary and Secondary Active Transport: Require energy (often ATP) to move substances against their concentration gradients. Secondary active transport uses the energy stored in gradients of other ions.

• Transporter Proteins: Include channels, carriers, and pumps, each with specific roles in membrane transport.

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

• Kinetic Properties: Facilitated diffusion shows saturation kinetics, unlike simple diffusion.

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

• Hypertonic, Hypotonic, Isotonic Solutions: Describe the relative concentrations of solutes outside versus inside the cell, affecting water movement.

Example: Glucose transport into cells via GLUT transporters is an example of facilitated diffusion.

Chapter 12: The Endomembrane System and Peroxisomes

Endomembrane System Components

The endomembrane system is a network of organelles involved in the synthesis, modification, and transport of cellular materials.

• Rough Endoplasmic Reticulum (ER): Studded with ribosomes, site of protein synthesis and initial modification.

• Smooth Endoplasmic Reticulum: Lacks ribosomes, involved in lipid synthesis and detoxification.

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

• Endocytosis: Uptake of external materials via vesicle formation; includes receptor-mediated, clathrin-coated, and other forms.

• Exocytosis: Secretion of materials from the cell via vesicle fusion with the plasma membrane.

• Protein Trafficking: Proteins are sorted and delivered to specific destinations within the cell, often via vesicles.

• Lysosomes: Organelles containing digestive enzymes that break down macromolecules and cellular debris.

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

Example: The Golgi apparatus modifies glycoproteins before they are secreted from the cell.

Chapter 9: Chemotropic Energy Metabolism I

Metabolism and ATP

Cells obtain energy through metabolic pathways that convert nutrients into usable chemical energy, primarily in the form of ATP.

• Catabolism: Breakdown of molecules to release energy.

• Anabolism: Synthesis of complex molecules from simpler ones, requiring energy.

• ATP (Adenosine Triphosphate): The primary energy currency of the cell, used to power various cellular processes.

• Hydrolysis of ATP: Releases energy by converting ATP to ADP and inorganic phosphate.

• Enzymes: Biological catalysts that accelerate metabolic reactions.

• Regulation: Enzymes are regulated by activators, inhibitors, and feedback mechanisms.

Example: Muscle contraction is powered by the hydrolysis of ATP.

Chapter 10: Chemotropic Energy Metabolism II

Mitochondrial Structure and Function

Mitochondria are the site of aerobic respiration, where nutrients are oxidized to produce ATP.

• Structure: Mitochondria have an outer membrane, inner membrane, intermembrane space, and matrix.

• Pyruvate Dehydrogenase (PDH): Converts pyruvate to Acetyl-CoA, linking glycolysis to the TCA cycle.

• Beta-Oxidation: Fatty acids are broken down to generate Acetyl-CoA for the TCA cycle.

• TCA Cycle (Krebs Cycle): Series of reactions that oxidize Acetyl-CoA to CO2, generating NADH and FADH2 for the electron transport chain.

• 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 and ATP synthesis via the proton gradient.

Example: The ETC is located in the inner mitochondrial membrane and is essential for aerobic ATP production.

Chapter 11: Photosynthesis

Photosynthetic Processes

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), generating ATP and NADPH.

• Chlorophyll: Pigment molecules (e.g., P680 in PSII, P700 in PSI) absorb light at specific wavelengths.

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

• Calvin Cycle: Uses ATP and NADPH to fix CO2 into carbohydrates.

Example: The Calvin cycle occurs in the stroma of chloroplasts and synthesizes glucose from CO2.

Key Equations and Concepts

• ATP Hydrolysis: $ATP + H_2O \rightarrow ADP + P_i + \text{energy}$

• General Reaction for Photosynthesis: $6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$

• TCA Cycle Summary: $Acetyl{-}CoA + 3NAD^+ + FAD + GDP + P_i + 2H_2O \rightarrow 2CO_2 + 3NADH + FADH_2 + GTP + CoA$

Comparison Table: Membrane Transport Mechanisms

Transport Type	Energy Requirement	Example	Saturation Kinetics
Simple Diffusion	No	O2 movement across membrane	No
Facilitated Diffusion	No	Glucose via GLUT transporter	Yes
Active Transport	Yes (ATP or ion gradient)	Na+/K+ pump	Yes

Additional info: Some details, such as the specific roles of enzymes and regulatory mechanisms, were expanded for clarity and completeness.