Cells: The Working Units of Life

Introduction

Cells are the basic structural and functional units of all living organisms. The study of cells is fundamental to understanding biology, as all life processes occur within cells. This section explores the features that make cells the fundamental units of life, their types, and their evolutionary origins.

5.1 What Features Make Cells the Fundamental Units of Life?

Cell Theory

• Cell theory is the first unifying theory of biology and states:

  • All organisms are composed of cells.

  • Cells are the fundamental units of life.

  • All cells come from preexisting cells.

  • Modern cells evolved from a common ancestor.

• Implications of cell theory:

  • Functions of all cells are similar.

  • Life is continuous.

  • The origin of life was the origin of cells.

The Scale of Life

Most cells are small, which allows for efficient exchange of materials with their environment. However, there are exceptions such as bird eggs, some algae, and certain bacteria.

• Microscopy: Cells and their components are studied using light and electron microscopes, which allow visualization at different scales (from atoms to frog eggs).

Surface Area-to-Volume Ratio

Cells are small because a high surface area-to-volume ratio is essential for efficient exchange of materials.

• Smaller objects have a greater ratio of surface area to volume than larger objects.

• Larger organisms have more cells rather than larger cells.

• Formula for surface area of a sphere:

• Formula for volume of a sphere:

Importance of Surface Area-to-Volume Ratio

• Exchange Efficiency: Smaller cells with a high surface area-to-volume ratio are more efficient at exchanging materials with their environment.

• Limitations on Cell Size: The surface area-to-volume ratio sets a limit on how large a cell can be and still function efficiently.

• Specialized Cell Shapes: Some cells overcome size limitations by developing specialized shapes or structures, such as villi in the small intestine or the branching structure of neurons.

5.2 What Features Characterize Prokaryotic Cells?

Prokaryotic Cell Structure

Prokaryotic cells, which include Bacteria and Archaea, lack membrane-bound organelles and a true nucleus.

• Enclosed by a plasma membrane.

• Genetic material is contained in a region called the nucleoid.

• Cytoplasm consists of cytosol (liquid component) and suspended particles.

• Most prokaryotes have a rigid cell wall outside the plasma membrane, often containing peptidoglycan.

• Some bacteria have a slimy polysaccharide capsule for protection.

• Movement is often achieved by flagella (made of flagellin protein).

• Pili: Hairlike structures that help bacteria adhere to surfaces or exchange DNA.

5.3 What Features Characterize Eukaryotic Cells?

Eukaryotic Cell Structure

Eukaryotic cells are generally larger and more complex than prokaryotic cells, containing membrane-bound organelles.

• DNA is contained within a nucleus.

• Organelles compartmentalize cellular functions, allowing for specialization and multicellularity.

• Examples of organelles: Mitochondria, Chloroplasts (in plants), Endoplasmic Reticulum, Golgi Apparatus, Lysosomes.

Comparing Prokaryotes and Eukaryotes

5.4 What Are the Roles of Extracellular Structures?

Cell Walls and Extracellular Matrix

• Plant cell walls: Composed of cellulose fibers embedded in other polysaccharides and proteins.

• Plasmodesmata: Channels connecting adjacent plant cells, allowing transport and communication.

• Animal cells: Surrounded by an extracellular matrix (ECM) made of fibrous proteins (e.g., collagen), proteoglycans, and other proteins.

• ECM functions:

  • Binds cells together in tissues.

  • Contributes to properties of bone, cartilage, and skin.

  • Regulates movement of materials and cell signaling.

Cell Junctions

• Tight junctions: Seal membranes of adjacent cells.

• Desmosomes: Anchor cells together, strengthened by intermediate filaments.

• Gap junctions: Provide channels for communication between adjacent cells.

5.5 How Did Eukaryotic Cells Originate?

Origin of Eukaryotic Cells

• Eukaryotic cells first appeared about 1.5 billion years ago.

• Compartmentalization was a major evolutionary event.

• Endomembrane system and nucleus: May have originated from inward folds of the plasma membrane in prokaryotes.

Endosymbiotic Theory

• Some organelles (mitochondria and plastids) arose by endosymbiosis—one cell engulfed another cell.

• Evidence for endosymbiosis:

  • Mitochondria and chloroplasts have double membranes.

  • They contain their own DNA (circular, like prokaryotes).

  • They have ribosomes similar to those in prokaryotes.

  • Examples: The single-celled eukaryote Hatena ingests a green alga Nephroselmis, which then acts as a chloroplast.

Summary Table: Key Differences Between Cell Types

Example

• Hatena and Nephroselmis: Demonstrates a modern example of endosymbiosis, supporting the theory of organelle origin.

Additional info: Some details, such as the formulas for surface area and volume, and the summary tables, were expanded for clarity and completeness.