Chapter 3 Part 1: Cells and Their Organelles – Structure and Function

Cells and Their Organelles

Background: Cellular Homeostasis and Disease

Cells maintain homeostasis through regulated interactions with their environment. Disruption of these processes can lead to disease, such as Graves' disease, where the cell membrane overreacts to hormone signals, causing metabolic and physiological problems.

• Homeostasis: The ability of cells to maintain a stable internal environment.

• Example: Graves' disease involves abnormal cell membrane response to thyroid hormone.

• Treatment: May include radioactive pills to destroy thyroid cells or medication to block hormone release.

Cell Theory

The cell theory is a foundational concept in biology, stating the properties and origins of cells.

• Every organism is composed of one or more cells.

• The cell is the smallest unit having properties of life.

• All cells come from pre-existing cells.

Additional info: Cell theory underpins all study of anatomy and physiology, emphasizing the continuity and universality of cellular life.

Why Are Cells So Small?

Cell size is limited by the surface-to-volume ratio, which affects the efficiency of material exchange.

• Surface-to-volume ratio: Higher ratios allow more efficient movement of substances in and out of the cell.

• Equation: $\text{Surface Area} / \text{Volume}$

• Limitation: As cells grow larger, the ratio decreases, slowing exchange and limiting cell size.

• Example: Small cells can quickly exchange nutrients and waste; large cells cannot.

Overview of Cell Types

Prokaryotic Cells

Prokaryotic cells are simple, lacking a nucleus and internal organelles. They are typically single-celled organisms such as bacteria.

• No nucleus, chromosomes, or internal organelles.

• Smaller and faster to reproduce than eukaryotes.

• Specialized for extreme environments (e.g., hot springs).

• All bacteria are prokaryotic.

Eukaryotic Cells

Eukaryotic cells are more complex, containing a nucleus and various organelles that compartmentalize metabolic functions.

• Have a nucleus to house DNA and chromosomes.

• Contain internal organelles for specialized functions.

• Can be single-celled or multi-celled (e.g., plants, animals).

Macromolecule #1: Lipids

Structure and Function of Lipids

Lipids are hydrophobic macromolecules essential for cell membrane structure and energy storage.

• Monomer: Fatty acids

• Polymers: Lipids (fats, phospholipids, waxes)

• Properties: Most lipids do not dissolve in water.

• Phospholipids: Major component of cell membranes, with hydrophilic heads and hydrophobic tails.

Type	Structure	Function
Fats	Glycerol + 3 fatty acids	Energy storage
Phospholipids	Glycerol + 2 fatty acids + phosphate group	Cell membrane structure
Waxes	Long-chain fatty acids	Water repellency (e.g., earwax)

Example: Earwax repels water, protecting the ear canal and supporting homeostasis.

Cell Membrane

Structure and Properties

The cell membrane is a dynamic barrier composed mainly of phospholipids, proteins, and cholesterol.

• Phospholipid bilayer: Two layers with hydrophilic heads facing outward and hydrophobic tails inward.

• Fluid mosaic model: Membrane is flexible and contains embedded proteins for transport and signaling.

Component	Role
Phospholipids	Structural barrier
Proteins	Transport, signaling, structural support
Cholesterol	Membrane fluidity

Transport Across the Membrane

Cells regulate movement of ions and molecules through specialized membrane proteins.

• Ion channels: Allow passive movement of ions down their concentration gradient.

• Ion carriers: Facilitate transport by changing shape.

• Ion pumps: Use ATP to move ions against their gradient.

Equation: $\text{ATP} \rightarrow \text{ADP} + \text{P}_i$ (energy for active transport)

Organelles

Nucleus

The nucleus is the control center of eukaryotic cells, housing genetic material and coordinating cell activities.

• Nuclear membrane: Encloses the nucleus, separating it from cytoplasm.

• Nucleoplasm: Fluid inside the nucleus.

• Nucleolus: Site of ribosome synthesis.

Chromatin

Chromatin consists of DNA and proteins, existing in a loose form for gene expression and condensing into chromosomes during cell division.

• Loose chromatin: Accessible for transcription.

• Condensed chromosomes: Visible during cell division.

Cytoplasm

The cytoplasm is the aqueous environment outside organelles, containing enzymes, hormones, and messenger molecules.

• Function: Site of many metabolic reactions.

Endomembrane System

The endomembrane system is a network of organelles involved in synthesis, modification, and transport of biomolecules.

• Includes: Endoplasmic reticulum (ER), Golgi apparatus, vesicles.

• Function: Assembles lipids and proteins, modifies polypeptides, sorts and ships products.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes and modifies proteins.

• Smooth ER: Lacks ribosomes; synthesizes and modifies lipids and some proteins.

Golgi Bodies

The Golgi apparatus processes and packages proteins and lipids for delivery inside or outside the cell.

• Function: Adds finishing touches, sorts, and ships molecules.

• Vesicles: Transport materials to and from the Golgi.

Vesicles

• Lysosomes: Contain digestive enzymes for recycling cellular debris.

• Peroxisomes: Contain enzymes for detoxification.

Mitochondria

Mitochondria are the cell's powerhouses, generating ATP through aerobic respiration.

• Double-membrane system: Outer and inner membranes create compartments for efficient energy production.

• ATP production: Requires oxygen.

• Equation: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

Cytoskeleton

The cytoskeleton provides structural support, organization, and enables movement within cells.

• Components: Microfilaments, intermediate filaments, microtubules.

• Function: Maintains cell shape, enables organelle movement, and cell mobility.

Cell Connections

Cells are connected by specialized junctions that facilitate communication and structural integrity.

• Adhesion junctions: Cytoskeleton fibers join between cells, adding flexibility.

• Gap junctions: Small channels between neighboring cell membranes allow small molecules to diffuse.

Summary Table: Major Organelles and Their Functions

Organelle	Main Function
Nucleus	Stores genetic material, coordinates cell activities
Rough ER	Protein synthesis and modification
Smooth ER	Lipid synthesis and modification
Golgi Bodies	Processing and packaging of proteins/lipids
Lysosomes	Digestion and recycling of cellular debris
Peroxisomes	Detoxification
Mitochondria	ATP production
Cytoskeleton	Structural support and movement

Key Questions for Review

• How do cells maintain homeostasis?

• Why are cells so small?

• How do cells show all the characteristics of life?

• What happens when cells fail?