Chapter 4: A Tour of the Cell

Cell Theory

The cell theory is a fundamental concept in biology that describes the properties of cells, the basic units of life.

• All living organisms are made of one or more cells.

• The cell is the basic structural and functional unit of life.

• All cells arise from pre-existing cells.

Microscopy

Microscopes are essential tools for studying cells and their structures. Different types of microscopes provide varying levels of magnification and detail.

• Light Microscope: Uses visible light; suitable for observing live or stained cells (up to 1000× magnification).

• Transmission Electron Microscope (TEM): Passes electrons through specimens for internal details (up to 100,000×).

• Scanning Electron Microscope (SEM): Scans surfaces to produce 3D images (up to 650,000×).

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on the presence of a nucleus and membrane-bound organelles.

• Prokaryotic cells lack membrane-bound organelles and a nucleus.

• Eukaryotic cells contain a nucleus and membrane-bound organelles.

Major Eukaryotic Organelles

Eukaryotic cells contain specialized organelles that perform distinct functions.

Endosymbiosis Theory

The endosymbiosis theory explains the origin of mitochondria and chloroplasts as formerly free-living prokaryotes engulfed by ancestral eukaryotic cells. Evidence includes their own DNA and ribosomes, double membranes, and reproduction similar to bacteria.

Chapter 5: Membrane Transport

Fluid Mosaic Model

The plasma membrane is described by the fluid mosaic model, consisting of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. It is flexible (fluid) and selectively permeable, controlling what enters and leaves the cell.

Types of Membrane Transport

Cells use various mechanisms to move substances across membranes, either passively or actively.

Osmosis and Tonicity

Osmosis is the movement of water across a selectively permeable membrane. Tonicity describes the effect of solute concentration on cell volume.

• Isotonic: Equal solute inside/outside; no net water movement.

• Hypotonic: Lower solute outside; water enters cell → cell swells.

• Hypertonic: Higher solute outside; water leaves cell → cell shrinks.

• Osmoregulation: Control of water balance to prevent bursting or shrinking.

Electrochemical Gradients & Pumps

Active transport can create electrochemical gradients across membranes, essential for cell function.

• Na+/K+ pump: Uses ATP to move 3 Na+ out and 2 K+ in, creating an electrical gradient.

Bulk Transport

Bulk transport moves large particles or volumes across the membrane.

• Endocytosis: Cell takes in large molecules (phagocytosis, pinocytosis, receptor-mediated).

• Exocytosis: Vesicles fuse with the membrane to release materials.

Cell Signaling

Purpose of Cell Signaling

Cell signaling allows cells to coordinate growth, metabolism, defense, and reproduction by communicating with each other.

Types of Signaling

Cells use different mechanisms to send and receive signals.

• Direct contact: Via gap junctions (animals) or plasmodesmata (plants).

• Paracrine: Local signaling through ligands.

• Synaptic: Neurotransmitters across synapses.

• Endocrine: Long-distance via hormones in bloodstream.

Three Stages of Cell Signaling

Cell signaling typically involves three main stages:

1. Reception: Ligand binds to receptor (e.g., G protein-coupled or ion channel receptor).

2. Transduction: Relay and amplification of signal via proteins, phosphorylation cascades, or second messengers.

3. Response: Activation of enzymes, gene expression, or other cellular changes.

Second Messengers

Second messengers are small, nonprotein molecules that relay signals within the cell to amplify responses.

• Examples: cAMP, Ca2+

Apoptosis (Programmed Cell Death)

Apoptosis is a controlled process to remove damaged or unnecessary cells, regulated by signaling pathways. It is important for development (e.g., finger separation in embryos) and tissue maintenance.

Signaling in Single-Celled Organisms

• Yeast: Communicate by releasing a signaling molecule called mating factor.

• Bacteria: Signaling is called quorum sensing. Population density is the key factor for signaling, utilizing molecules called autoinducers.

Practice Review Questions

1. List three parts of the cell theory.

2. Compare prokaryotic and eukaryotic cells using three key features.

3. Describe the structure and function of mitochondria.

4. Explain the difference between simple and facilitated diffusion.

5. Describe what happens to a red blood cell in a hypotonic solution.

6. Define the sodium-potassium pump and explain its importance.

7. List the three types of endocytosis and their functions.

8. Name and describe the three main stages of cell signaling.

9. What is a second messenger? Provide two examples.