Chapter 6.2: Membrane Transport

Types of Membrane Transport

Cells use various mechanisms to move substances across their membranes. Understanding these processes is essential for grasping how cells maintain homeostasis and interact with their environment.

• Osmosis: The diffusion of water molecules across a selectively permeable membrane from an area of low solute concentration to high solute concentration.

• Diffusion: The passive movement of molecules from an area of higher concentration to an area of lower concentration.

• Facilitated Diffusion: Passive transport of substances across a membrane via specific transmembrane integral proteins.

• Active Transport: The movement of substances against their concentration gradient, requiring energy (usually from ATP).

Primary vs. Secondary Active Transport:

• Primary Active Transport: Direct use of metabolic energy (e.g., ATP hydrolysis) to transport molecules.

• Secondary Active Transport: Uses the energy from the electrochemical gradient created by primary active transport.

Key Proteins in Membrane Transport:

• Na+/K+ ATPase: A primary active transport protein that pumps 3 Na+ ions out and 2 K+ ions into the cell per ATP molecule hydrolyzed.

• Aquaporins: Channel proteins that facilitate the rapid transport of water across cell membranes.

• Carrier Proteins: Proteins that bind specific molecules and change shape to shuttle them across the membrane.

Does Passive Transport Require Energy? No, passive transport (including diffusion and facilitated diffusion) does not require cellular energy.

Identifying Transport Types: Be able to determine the type of transport based on whether energy is required, the direction of movement, and the involvement of specific proteins.

Chapter 7: Cell Structure and Function

Prokaryotes vs. Eukaryotes

Cells are classified as prokaryotic or eukaryotic based on their structural features.

• Prokaryotes: Lack a nucleus and membrane-bound organelles; DNA is located in the nucleoid region. Examples: Bacteria and Archaea.

• Eukaryotes: Have a true nucleus and membrane-bound organelles. Examples: Animals, plants, fungi, and protists.

Comparison Table:

Cell Specialization: In multicellular organisms, different cell types perform specialized functions (e.g., muscle cells, nerve cells).

Organelles and Their Functions

• Ribosomes: Sites of protein synthesis. Free ribosomes synthesize proteins for use within the cell; ribosomes bound to the rough endoplasmic reticulum (RER) synthesize proteins for secretion or for use in membranes.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; involved in protein synthesis and modification.

  • Smooth ER: Lacks ribosomes; involved in lipid synthesis and detoxification.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for storage or transport out of the cell.

• Organelles with DNA: Mitochondria and chloroplasts contain their own DNA and are involved in energy conversion.

Endomembrane System: Includes the nuclear envelope, ER, Golgi apparatus, lysosomes, and vesicles. Responsible for the synthesis, modification, and transport of cellular materials.

Exo- and Endocytosis

• Exocytosis: The process by which cells expel materials in vesicles that fuse with the plasma membrane.

• Endocytosis: The process by which cells take in materials by engulfing them in a vesicle. Types include:

  • Phagocytosis: "Cell eating"; engulfment of large particles.

  • Pinocytosis: "Cell drinking"; uptake of fluids and dissolved substances.

  • Receptor-mediated endocytosis: Uptake of specific molecules based on receptor binding.

Chapters 9 and 10: Cellular Respiration and Photosynthesis

Redox Reactions and Metabolic Pathways

Metabolism involves a series of chemical reactions that are either anabolic (building up) or catabolic (breaking down). Redox reactions are central to energy transfer in cells.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Exergonic Reactions: Release energy (e.g., cellular respiration).

• Endergonic Reactions: Require energy input (e.g., photosynthesis).

Example Equation (Cellular Respiration):

Example Equation (Photosynthesis):

Cellular Respiration

• Purpose: To convert biochemical energy from nutrients into ATP, the cell's energy currency.

• Main Stages:

  1. Glycolysis (cytoplasm): Glucose is broken down into pyruvate.

  2. Citric Acid Cycle (mitochondrial matrix): Completes the breakdown of glucose, producing CO2, NADH, and FADH2.

  3. Electron Transport Chain (inner mitochondrial membrane): Uses electrons from NADH and FADH2 to create a proton gradient that drives ATP synthesis.

• ATP Yield: Up to 38 ATP molecules per glucose (theoretical maximum).

• Oxygen's Role: Final electron acceptor in the electron transport chain, forming water.

Photosynthesis

• Purpose: To convert light energy into chemical energy stored in glucose.

• Main Stages:

  1. Light Reactions: Occur in the thylakoid membranes; convert solar energy to chemical energy (ATP and NADPH), release O2 as a byproduct.

  2. Calvin Cycle (Light-independent Reactions): Occur in the stroma; use ATP and NADPH to fix CO2 into glucose.

• Chloroplast Structure: Contains thylakoids (site of light reactions) and stroma (site of Calvin cycle).

Comparison Table: Cellular Respiration vs. Photosynthesis

Additional info:

• Be able to identify and explain the main steps and purposes of both cellular respiration and photosynthesis.

• Understand the role of redox reactions in energy transfer within cells.

• Recognize the importance of organelles in compartmentalizing cellular functions.