Cell Transport, Cell Structure, and Metabolism: Study Guide for General Biology

Chapter 6.2: Cell Transport Mechanisms

Types of Transport Across Cell Membranes

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 passive movement 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 high concentration to low concentration.

• Facilitated Diffusion: Passive transport of molecules across membranes via specific transmembrane proteins.

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

Primary vs. Secondary Active Transport:

• Primary Active Transport: Direct use of ATP to transport molecules (e.g., Na+/K+ ATPase).

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

Key Proteins in Transport:

• ATPase Transporters: Enzymes that hydrolyze ATP to drive active transport (e.g., Na+/K+ pump).

• Aquaporins: Channel proteins that facilitate rapid water movement across membranes.

• Carrier Proteins: Proteins that bind and transport specific molecules across the membrane.

Does Passive Transport Require Energy?

• Passive transport (diffusion, osmosis, facilitated diffusion) does not require cellular energy.

• Active transport does require energy, typically in the form of ATP.

Example: The Na+/K+ ATPase pump moves 3 Na+ ions out and 2 K+ ions into the cell per ATP hydrolyzed.

Chapter 7: Cell Structure and Function

Prokaryotes vs. Eukaryotes

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

• Prokaryotes: Cells without a nucleus or membrane-bound organelles (e.g., Bacteria and Archaea).

• Eukaryotes: Cells with a nucleus and membrane-bound organelles (e.g., Plants, Animals, Fungi, Protists).

Comparison Table:

Feature	Prokaryotes	Eukaryotes
Nucleus	No	Yes
Membrane-bound organelles	No	Yes
Cell size	Smaller	Larger
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Organelles and Their Functions

Organelles are specialized structures within eukaryotic cells that perform distinct functions.

• Ribosomes: Synthesize proteins; can be free in cytoplasm or bound to the rough endoplasmic reticulum (RER).

• Endoplasmic Reticulum (ER): Network of membranes involved in protein and lipid synthesis.

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

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

Example: Proteins synthesized by ribosomes on the RER are often destined for secretion or for use in the cell membrane.

Endocytosis and Exocytosis

Cells transport large molecules and particles via vesicular processes.

• Endocytosis: The process by which cells engulf external substances, forming vesicles.

• Exocytosis: The process by which cells expel materials in vesicles to the exterior.

• Types of Endocytosis:

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

  • Pinocytosis: "Cell drinking"; uptake of fluids and small molecules.

  • Receptor-mediated endocytosis: Specific uptake of molecules via receptor binding.

Chapters 9 and 10: Cellular Metabolism

Oxidation and Reduction (Redox Reactions)

Redox reactions are central to cellular metabolism, involving the transfer of electrons between molecules.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Exergonic Reaction: Releases energy.

• Endergonic Reaction: Requires energy input.

Example Equation:

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

Cellular Respiration

Cellular respiration is the process by which cells extract energy from glucose and other organic molecules.

• Aerobic Respiration: Requires oxygen; produces more ATP.

• Anaerobic Respiration: Does not require oxygen; produces less ATP.

• Catabolic Pathways: Break down molecules to release energy.

• Anabolic Pathways: Build complex molecules from simpler ones; require energy.

Stages of Cellular Respiration:

1. Glycolysis

2. Krebs Cycle (Citric Acid Cycle)

3. Electron Transport Chain (ETC) and Oxidative Phosphorylation

ATP Production: Most ATP is produced during oxidative phosphorylation in the mitochondria.

Equation for ATP Synthesis:

$\text{ADP} + \text{P}_i \rightarrow \text{ATP}$

Photosynthesis

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

• Light Reactions: Occur in the thylakoid membranes; convert solar energy to chemical energy (ATP and NADPH).

• Calvin Cycle: Occurs in the stroma; uses ATP and NADPH to fix carbon dioxide into glucose.

Overall Photosynthesis Equation:

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

Purpose: To produce glucose and oxygen, which are essential for cellular respiration and life on Earth.

Example: Plants use photosynthesis to convert sunlight into usable energy, supporting most life forms.

Additional info: Some context and definitions have been expanded for clarity and completeness.