Chapter 3 - Objectives

• Cell Theory

• Plasma Membrane

• Membrane Transport

• Energy Production

Cell Theory

Fundamental Principles of Cell Theory

Cell theory is a foundational concept in biology and anatomy, stating that all living organisms are composed of cells and cell products. This theory underpins our understanding of the structure and function of the human body.

• All organisms are made of cells: Cells are the basic structural and functional units of life.

• Cells arise from pre-existing cells: New cells are produced by the division of existing cells.

• Cells carry out all vital functions: Metabolism, growth, and reproduction occur within cells.

• Cell products: Substances produced by cells (e.g., hormones, enzymes) are essential for organismal function.

Example: Human tissues such as muscle, nerve, and epithelial tissue are all composed of specialized cells.

Types of Human Cells

The human body contains over 200 different cell types, each specialized for particular functions.

• Fibroblasts: Cells that connect body parts or help in tissue repair.

• Erythrocytes (Red blood cells): Cells that transport oxygen in the blood.

• Epithelial cells: Cells that cover and line organs and body surfaces.

• Skeletal muscle cells: Cells that move body parts.

• Smooth muscle cells: Cells that move organs and control involuntary movements.

• Fat cells: Cells that store energy in the form of fat.

• Macrophages: Cells that fight disease by engulfing pathogens.

• Nerve cells: Cells that gather information and control body functions.

• Reproductive cells (Sperm and egg cells): Cells involved in reproduction.

Additional info: Each cell type has a unique structure that supports its specific function in the body.

Cell Size and Structure

Human cells typically range from 10 to 15 micrometers (μm) in diameter. The size of a cell is determined by its surface area-to-volume ratio, which affects the efficiency of nutrient uptake and waste removal.

• Surface area-to-volume ratio: As a cell grows, its volume increases faster than its surface area, limiting the size a cell can attain.

• Phospholipid bilayer: Most cells and their organelles are bounded by a double layer of phospholipids, forming the cell membrane.

Plasma Membrane

Structure and Composition

The plasma membrane is a dynamic boundary that separates the cell from its external environment and regulates the movement of substances in and out of the cell.

• Phospholipid bilayer: The fundamental structure of the membrane, consisting of hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Fluid mosaic model: Describes the plasma membrane as a flexible, fluid structure with proteins embedded within or attached to the bilayer.

• Membrane proteins: Serve various functions, including transport, signaling, and structural support.

• Peripheral proteins: Located on the membrane surface, often involved in signaling or structural roles.

• Receptors: Membrane-bound proteins that bind to specific ligands (polar or nonpolar) and initiate cellular responses.

Example: Steroid hormone receptors bind nonpolar ligands and trigger protein synthesis within the cell.

Specialized Membrane Structures

• Microvilli: Extensions of the cell membrane that increase surface area for absorption.

• Cilia and flagella: Specialized extensions that aid in movement of the cell or substances across the cell surface.

Membrane Transport

Types of Membrane Transport

Cells exchange materials with their environment through various transport mechanisms, which can be passive or active.

• Passive transport: Does not require energy; substances move down their concentration gradients.

• Active transport: Requires energy (ATP); substances move against their concentration gradients.

Passive Transport Mechanisms

• Simple diffusion: Movement of fat-soluble molecules directly through the phospholipid bilayer.

• Facilitated diffusion: Movement of polar or lipid-insoluble molecules via channel or carrier proteins.

• Osmosis: Diffusion of water through a selectively permeable membrane, often via aquaporin channels.

Factors Affecting Diffusion

• Concentration gradient

• Membrane permeability

• Surface area

• Temperature

Osmosis and Tonicity

Osmosis is the movement of water from areas of lower solute concentration to areas of higher solute concentration. Tonicity describes the effect of a solution on cell volume.

Additional info: Tonicity is important in medical settings, such as intravenous fluid administration.

Active Transport Mechanisms

• Primary active transport: Direct use of ATP to move substances (e.g., Na+/K+ pump).

• Secondary active transport: Uses energy from the movement of one substance down its gradient to move another substance against its gradient (e.g., Na+-glucose cotransport).

Equation:

Additional info: Saturation, competition, and specificity are important considerations for carrier-mediated transport.

Energy Production

Cellular Respiration Overview

Cells produce energy primarily through the process of cellular respiration, which converts nutrients into ATP, the cell's energy currency.

• Mitochondria: Organelles responsible for ATP production.

• ATP (Adenosine Triphosphate): The main energy carrier in cells.

Metabolism

• Anabolism: Building larger molecules from smaller ones (requires energy).

• Catabolism: Breaking down larger molecules into smaller ones (releases energy).

Equation:

Enzymes

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy.

• Not changed or destroyed during reactions

• Increase reaction rate

• Essential for life processes

Equation:

Stages of Cellular Respiration

• Glycolysis: Occurs in the cytoplasm; breaks down glucose into pyruvate.

• Acetyl CoA formation: Pyruvate is converted to Acetyl CoA in mitochondria.

• Citric Acid Cycle (Krebs Cycle): Acetyl CoA is oxidized, producing NADH and FADH2.

• Electron Transport Chain: NADH and FADH2 donate electrons, driving ATP synthesis.

Equation for aerobic respiration:

Aerobic vs. Anaerobic Respiration

• Aerobic respiration: Requires oxygen; produces up to 36-38 ATP per glucose.

• Anaerobic respiration: Occurs when oxygen is absent; produces 2 ATP and lactic acid.

Interconversion of Nutrients

Cells can use carbohydrates, fats, and proteins for energy. Excess carbohydrates can be converted to fat, and proteins are used for energy mainly during starvation.

• Carbohydrates: Primary energy source.

• Fats: Secondary energy source; stored for long-term energy.

• Proteins: Used for energy when carbohydrates and fats are insufficient.

Additional info: The ability to interconvert nutrients is crucial for metabolic flexibility and survival.