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

Chapter 7: Membranes as Selective Barriers to the Cell

Structure and Function of Membranes

Cell membranes act as selective barriers, controlling the movement of substances in and out of the cell. Their structure and composition are crucial for maintaining cellular integrity and function.

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

• Phospholipid Properties: Membranes contain phospholipids with hydrophilic heads and hydrophobic tails. Longer fatty acid chains and saturated fats lead to higher melting points and less fluidity.

• Cholesterol: Modulates fluidity and stability of the membrane.

• SDS-Page: Used for protein analysis; separates proteins by size for identification.

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

Additional info: Membrane proteins can be integral or peripheral, each serving specific functions such as transport, signaling, or structural support.

Chapter 8: Membrane Transport

Mechanisms of Transport Across Membranes

Cells transport molecules across membranes using various mechanisms, depending on the properties of the molecules and the requirements of the cell.

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

• Facilitated Diffusion: Transport of molecules via membrane proteins (channels or carriers), allowing passage of ions and polar molecules.

• Primary and Secondary Active Transport: Use of energy (usually ATP) to move substances against their concentration gradients. Secondary active transport uses the energy from the movement of another molecule.

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

• Kinetics: Rates of transport depend on concentration gradients and properties of the transport proteins.

• General Properties: Includes permeability, specificity, and regulation.

• Hypertonic, Hypotonic, and Isotonic Solutions: Describe the relative concentrations of solutes inside and outside 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

Organization and Function of Endomembrane Organelles

The endomembrane system includes organelles involved in synthesis, modification, and transport of cellular materials. Peroxisomes are specialized for detoxification and lipid metabolism.

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

• Exocytosis: Secretion of molecules via vesicles that fuse with the plasma membrane.

• Endocytosis: Uptake of molecules via vesicle formation from the plasma membrane.

• Protein Trafficking: Proteins are sorted and transported to their destinations via vesicles (e.g., COPI, COPII, clathrin-coated vesicles).

• Lysosomes: Organelles containing hydrolytic enzymes for degradation of cellular waste.

• Peroxisomes: Contain enzymes for oxidation of fatty acids and detoxification of harmful substances.

• Enzyme Regulation: Enzymes such as catalase convert hydrogen peroxide to water, protecting cells from oxidative damage.

Additional info: Defects in lysosomal enzymes can lead to storage diseases, such as Tay-Sachs disease.

Chapter 9: Chemotropic Energy Metabolism I

Metabolism and ATP Production

Cells obtain energy through metabolic pathways that convert nutrients into ATP, the universal energy currency.

• Catabolism: Breakdown of molecules to release energy.

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

• ATP: Adenosine triphosphate, stores and transfers energy within cells.

• Glycolysis: The process by which glucose is converted to pyruvate, producing ATP and NADH.

• Regulation: Glycolysis is regulated by key enzymes and feedback mechanisms.

• Fermentation: Occurs in the absence of oxygen, producing lactate or ethanol and regenerating NAD+.

• Gluconeogenesis: Synthesis of glucose from non-carbohydrate precursors.

Equation:

$\text{Glucose} + 2\,\text{ADP} + 2\,\text{NAD}^+ + 2\,\text{P}_i \rightarrow 2\,\text{Pyruvate} + 2\,\text{ATP} + 2\,\text{NADH} + 2\,\text{H}_2\text{O}$

Example: Muscle cells use fermentation to produce ATP during intense exercise when oxygen is limited.

Chapter 10: Chemotropic Energy Metabolism II

Mitochondrial Metabolism and the TCA Cycle

Mitochondria are the site of aerobic respiration, where the TCA cycle and electron transport chain generate ATP from nutrients.

• Mitochondrial Structure: Includes outer and inner membranes, intermembrane space, and matrix.

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

• Fatty Acid Oxidation: Fatty acids are broken down to acetyl-CoA, which enters the TCA cycle.

• TCA 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, producing ATP via oxidative phosphorylation.

• Regulatory Enzymes: Control the rate of metabolic pathways.

Equation:

$\text{Acetyl-CoA} + 3\,\text{NAD}^+ + \text{FAD} + \text{ADP} + \text{P}_i \rightarrow 2\,\text{CO}_2 + 3\,\text{NADH} + \text{FADH}_2 + \text{ATP}$

Example: The ETC couples electron transfer to ATP synthesis via the proton gradient across the inner mitochondrial membrane.

Chapter 11: Photosynthesis

Light Energy Conversion in Plants

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

• Definition: Photosynthesis uses light energy to synthesize organic molecules from CO2 and H2O.

• Light Reactions: Occur in photosystems I and II (PSI and PSII), generating ATP and NADPH.

• Chlorophyll: Pigment that absorbs light; PSI contains chlorophyll P700, PSII contains chlorophyll P680.

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

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

Equation:

$6\,\text{CO}_2 + 6\,\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2$

Example: The Calvin cycle incorporates CO2 into glucose, which can be used for energy or stored as starch.

Table: Comparison of Membrane Transport Mechanisms

Transport Type	Energy Requirement	Example	Direction
Simple Diffusion	No	O2 movement	Down gradient
Facilitated Diffusion	No	Glucose via GLUT	Down gradient
Active Transport	Yes (ATP)	Na+/K+ pump	Against gradient
Osmosis	No	Water movement	Down gradient

Additional info: Secondary active transport uses the energy from one molecule moving down its gradient to drive another molecule against its gradient.