Chapter 4

Chapter 4: CELLS—THE SMALLEST PART OF YOU

Learning Objectives:

• Define a cell and explain the cell theory.

• Compare prokaryotic and eukaryotic cells.

• Explain endosymbiosis and invagination theories.

• Describe plasma membrane structure and function.

• Explain transport mechanisms and their effects on cells.

• Discuss cell junctions and cell identification markers.

• Identify major organelles and their functions.

• Compare plant and animal cells.

I. What Is a Cell?

A. All Organisms Are Made of Cells

• A cell is the smallest unit of life.

• The term “cell” was first used by Robert Hooke in the 1600s.

• Cell Theory:

  1. All living organisms are composed of one or more cells.

  2. Cells are the basic unit of structure and function in organisms.

  3. All cells arise from pre-existing cells.

B. Prokaryotic Cells: Simple Yet Diverse

• Two types of cells: Prokaryotic and Eukaryotic.

• Only eukaryotic cells have a nucleus.

• Prokaryotes exist for ~3.5 billion years, eukaryotes for ~2 billion years.

• Two major groups of prokaryotes:

  1. Bacteria

  2. Archaea

• Four basic features of prokaryotic cells:

  1. Plasma membrane: Encloses the cell.

  2. Cytoplasm: Jelly-like interior.

  3. Ribosomes: Protein synthesis.

  4. DNA: Genetic material.

• Additional structures: Flagella (movement), Pili (attachment), Capsule (protection).

• Multicellular organisms cannot be made of prokaryotic cells due to lack of complexity.

C. Eukaryotic Cells: Specialized Compartments

• Contain organelles for efficiency.

• Size: 10-100 times larger than prokaryotic cells.

• Unicellular & Multicellular: Some eukaryotes (e.g., amoeba) are unicellular.

• Plant vs. Animal Cells:

  • Plants: Cell wall, chloroplasts, central vacuole.

  • Animals: Centrioles.

• Endosymbiosis Theory: Mitochondria and chloroplasts evolved from engulfed bacteria.

  • Evidence: Own DNA, double membrane, self-replication.

• Invagination Theory: Internal membranes formed by infolding of plasma membrane.

II. Cell Membranes Are Gatekeepers

D. Plasma Membrane Structure

• Phospholipid bilayer:

  • Hydrophilic head (water-attracting).

  • Hydrophobic tails (water-repelling).

• Membrane Proteins:

  1. Receptor proteins: Receive signals.

  2. Recognition proteins: Cell identification.

  3. Transport proteins: Move molecules.

  4. Enzymatic proteins: Speed up reactions.

• Cholesterol: Maintains fluidity.

• Fluid Mosaic Model: Membrane is flexible and dynamic.

E. Faulty Membranes & Disease

• Cystic Fibrosis: Defective chloride transport protein leads to mucus buildup.

• Beta-blockers: Reduce blood pressure by blocking adrenaline receptors.

F. Cell Fingerprinting & Immunity

• Membrane markers help immune recognition.

• Organ transplants: Require matching markers to prevent rejection.

• HIV infection: Uses CD4 markers to enter cells.

G. Cell Junctions

III. Molecule Movement Across Membranes

H. Passive Transport: No Energy Required

• Diffusion: Movement from high to low concentration.

• Facilitated Diffusion: Uses a protein channel.

• Osmosis: Water movement across a membrane.

  • Isotonic: No net movement.

  • Hypertonic: Water exits, cell shrinks.

  • Hypotonic: Water enters, cell swells.

• Example of osmosis: Salt on a slug dehydrates it.

I. Active Transport: Energy Required

• Primary Active Transport: Direct ATP use (e.g., sodium-potassium pump).

• Secondary Active Transport: Indirect ATP use (coupled transport).

• Proton pumps in stomach: Move hydrogen ions to aid digestion.

J. Bulk Transport

• Endocytosis: Moving large particles into the cell.

  • Phagocytosis: Cell eating (e.g., white blood cells engulf bacteria).

  • Pinocytosis: Cell drinking.

  • Receptor-Mediated Endocytosis: Specific molecule intake.

• Exocytosis: Moving large particles out of the cell (e.g., hormone secretion).

IV. Eukaryotic Cell Structures & Functions

K. The Nucleus: Control Center

• Enclosed by nuclear membrane.

• Contains chromatin (DNA + proteins).

• Nucleolus: Produces ribosome subunits.

L. Cytoskeleton: Support & Movement

• Functions:

  1. Structural support.

  2. Intracellular transport.

  3. Cell movement.

• Components:

  • Microtubules: Shape, cell division.

  • Intermediate Filaments: Strength.

  • Microfilaments: Movement.

• Cilia: Move fluids (e.g., in respiratory tract).

• Flagella: Propel cells (e.g., sperm).

M. Mitochondria: Energy Powerhouse

• Converts food into ATP.

• Produces CO2 and water.

• More mitochondria = more energy needs (e.g., muscle cells).

• Endosymbiosis evidence: Own DNA, double membrane.

• Inherited from mother because sperm contributes no mitochondria.

N. Cell Adaptation Research

• Cells adjust their composition based on environmental changes.

Conclusion

• Cells are the fundamental unit of life.

• Eukaryotic cells are highly organized with organelles.

• The plasma membrane regulates interactions with the environment.

• Transport mechanisms ensure balance and homeostasis.

• Cellular structures enable growth, communication, and energy production.

This chapter provides a foundation for understanding how cells function, interact, and maintain life.