Cell Structure and Function

Overview of Cell Types

Cells are the fundamental units of life, and all living organisms are composed of one or more cells. Cells can be classified into two main types: prokaryotic and eukaryotic cells, each with distinct structural features and evolutionary origins.

• Prokaryotes: Simple cells lacking a true nucleus and membrane-bound organelles. They represent some of the earliest forms of life.

• Bacteria and Archaea are the two domains of prokaryotes.

• Eukaryotes: Complex cells with a true nucleus and various organelles. Eukaryotes include plants, animals, fungi, and protists.

Example: A typical bacterium (prokaryote) is much simpler than a plant or animal cell (eukaryote), which contains many specialized structures.

Prokaryotic Cell Structure

Prokaryotic cells are characterized by the absence of a nucleus and most organelles. Their genetic material is located in a region called the nucleoid.

• Nucleoid: Region containing a single, circular chromosome. DNA is anchored to the cell membrane.

• Plasmids: Small, circular DNA molecules independent of the chromosome. They often carry genes for antibiotic resistance or other nonessential functions.

• Cell Wall: Provides shape and protection. Composition varies between groups:

  • Bacteria: Made of peptidoglycan. Gram-positive bacteria have thick peptidoglycan layers; Gram-negative bacteria have an outer membrane and thinner peptidoglycan.

  • Archaea: Cell walls made of pseudopeptidoglycan or other unique polymers.

• Flagella: Helical protein structures used for motility.

• Fimbriae & Pili: Short appendages for attachment. Pili can form conjugation tubes for gene transfer (mainly in Gram-negative bacteria).

Example: Escherichia coli uses flagella to swim and pili to exchange genetic material.

Eukaryotic Cell Structure

Eukaryotic cells possess a true nucleus and a variety of membrane-bound organelles, each with specialized functions.

• Nucleus: Surrounded by a double membrane (nuclear envelope) with nuclear pores for transport. Contains chromatin (DNA and proteins) and a distinct region called the nucleolus (site of ribosome assembly).

• Chromatin: DNA packaged with proteins. Exists as euchromatin (loosely packed, active) and heterochromatin (tightly packed, inactive).

• Ribosomes: Complexes of rRNA and proteins. Sites of protein synthesis. Found free in the cytosol or bound to the rough endoplasmic reticulum (ER).

• Endoplasmic Reticulum (ER):

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

  • Smooth ER: Lacks ribosomes; synthesizes lipids and carbohydrates.

• Golgi Apparatus: Stacks of flattened membranes (cisternae) that modify, sort, and package proteins and lipids for transport.

• Lysosomes: Contain digestive enzymes for breaking down macromolecules, old organelles, and foreign substances.

• Vacuoles:

  • Animal cells: Small vacuoles for storage and transport.

  • Plant cells: Large central vacuole for storage, waste disposal, and maintaining turgor pressure.

• Cell Wall:

  • Plants: Made of cellulose.

  • Fungi: Made of chitin.

  • Bacteria: Made of peptidoglycan.

  • Archaea: Made of pseudopeptidoglycan.

• Cytoskeleton: Network of protein filaments (microtubules, microfilaments, intermediate filaments) that provide structural support, shape, and aid in movement.

• Cilia and Flagella: Complex 9+2 arrangement of microtubules, surrounded by plasma membrane, used for movement.

Example: Plant cells have a rigid cell wall and large central vacuole, while animal cells lack a cell wall and have smaller vacuoles.

Endosymbiotic Theory and Evolution of Eukaryotes

The origin of eukaryotic cells is explained by two major theories: membrane theory and endosymbiotic theory.

• Membrane Theory: Internal membranes formed by invagination of the plasma membrane, leading to the development of the endomembrane system.

• Endosymbiotic Theory: Describes how mitochondria and chloroplasts originated from free-living bacteria engulfed by ancestral eukaryotes, forming mutualistic relationships.

• Mitochondria: Double-membrane organelles responsible for cellular respiration and energy production.

• Chloroplasts: Double-membrane organelles in plants and algae, site of photosynthesis. Inner membrane forms stacks of thylakoids (grana).

• Primary Endosymbiosis: Direct engulfment of a prokaryote by a eukaryote.

• Secondary Endosymbiosis: Eukaryote engulfs another eukaryote that already contains an endosymbiont.

Example: Mitochondria in animal cells and chloroplasts in plant cells both have their own DNA, supporting the endosymbiotic origin.

Surface Area and Volume Relationships

Cell size is limited by the surface area-to-volume ratio, which affects the ability to exchange materials with the environment.

• Formula for Surface Area of a Cube:

• Formula for Volume of a Cube:

• Surface Area-to-Volume Ratio:

• Cells must be large enough to contain necessary components but small enough to efficiently exchange materials.

Example: Smaller cells have a higher surface area-to-volume ratio, facilitating faster exchange of nutrients and waste.

Comparison of Cell Types and Structures

The following table summarizes key differences in cell wall composition among various organisms:

Cell Junctions and the Extracellular Matrix

Cells interact with each other and their environment through specialized structures.

• Cell Junctions:

  • Tight Junctions: Prevent leakage between cells (mainly in animals).

  • Anchoring Junctions: Provide mechanical stability.

  • Gap Junctions: Allow communication between animal cells.

  • Plasmodesmata: Channels for communication between plant cells.

• Extracellular Matrix (ECM): Network outside the plasma membrane that provides structural support and regulates cell behavior. Major component is collagen.

Example: The ECM helps hold animal cells together in tissues and transmits signals to the cytoskeleton.

Summary Table: Major Eukaryotic Organelles and Functions

Diagrams and Labeling (Study Practice)

• Be able to draw and label a typical bacterium, plant cell, and animal cell, including all major structures discussed above.

Additional info: Some content was inferred and expanded for clarity and completeness, including definitions, examples, and summary tables.