Chapter 5: The Working Cell

Overview

This chapter explores the structure and function of cell membranes, the mechanisms of transport across membranes, and the role of energy in cellular processes. Understanding these concepts is fundamental to cell biology and the study of life at the molecular level.

Membrane Structure and Function

Fluid Mosaic Model

The fluid mosaic model describes the structure of cell membranes as a mosaic of lipids and proteins that are free to move laterally within the layer. This dynamic arrangement allows membranes to be flexible and selectively permeable.

• Lipid bilayer: Composed mainly of phospholipids with hydrophilic heads and hydrophobic tails.

• Membrane proteins: Serve as channels, carriers, enzymes, and receptors.

• Selectively permeable: Only certain substances can pass through the membrane.

Membrane Proteins

• Transport proteins: Facilitate movement of substances across the membrane.

• Enzymes: Catalyze reactions at the membrane surface.

• Receptors: Receive and transmit signals.

Transport Across Membranes

Passive Transport

Passive transport is the movement of substances across a membrane without the use of cellular energy (ATP). It relies on concentration gradients.

• Diffusion: Movement of molecules from an area of higher concentration to lower concentration.

• Facilitated diffusion: Diffusion of molecules assisted by transport proteins.

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

Active Transport

Active transport requires energy (usually ATP) to move substances against their concentration gradient.

• Active site: Region on a protein where substrate binds.

• ATP: Provides energy for transport.

• Examples: Sodium-potassium pump, proton pump.

Additional info: The above equation is a general rate law for bimolecular reactions, included for context.

Bulk Transport

• Endocytosis: Process by which cells take in large molecules or particles by engulfing them.

• Exocytosis: Process by which cells expel materials in vesicles.

Energy in Cellular Processes

Types of Energy

• Kinetic energy: Energy of motion (e.g., muscle contraction).

• Potential energy: Stored energy (e.g., chemical bonds).

• Chemical energy: Energy stored in molecules (e.g., glucose).

• Heat, light: Other forms of energy relevant to biological systems.

Energy Coupling and ATP

Cells use energy coupling to link exergonic (energy-releasing) reactions to endergonic (energy-consuming) reactions. ATP (adenosine triphosphate) is the primary energy carrier in cells.

• ATP hydrolysis: Releases energy for cellular work.

• ADP + Pi: ATP is regenerated from ADP and inorganic phosphate.

Enzymes and Cellular Metabolism

Enzyme Structure and Function

Enzymes are biological catalysts that speed up chemical reactions without being consumed. They have specific active sites where substrates bind.

• Substrate: The molecule upon which an enzyme acts.

• Product: The result of the enzymatic reaction.

• Enzyme inhibitors: Molecules that decrease enzyme activity (competitive and noncompetitive).

Enzyme Helpers

• Cofactors: Inorganic ions required for enzyme activity (e.g., Mg2+).

• Coenzymes: Organic molecules (often vitamins) that assist enzymes.

Key Vocabulary

Short Answer Practice Questions

1. Explain three different examples of potential and three examples of kinetic energy in your body.

2. Where does potential energy originally come from in your body?

3. Describe the structure of an enzyme and its function. Use an example and include active site, substrate, product, enzyme helper, and enzyme inhibitor.

4. Describe the structure and function of a plasma membrane as a whole and explain the structure and function of each of its four main components.

5. Compare and contrast diffusion, osmosis, facilitated diffusion, active transport, endocytosis, and exocytosis. Include circumstances, concentration gradient, transport proteins, and energy usage.

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