Cell Structure and Function: Prokaryotic, Animal, and Plant Cells

Cell Structure and Function

Overview of Cell Types

Cells are the basic units of life and can be classified into two major types: prokaryotic and eukaryotic cells. Each type has distinct structural features and functions that are essential for the survival and operation of living organisms.

• Prokaryotic Cells: Cells without membrane-bound organelles (e.g., nucleus).

• Eukaryotic Cells: Cells with membrane-bound organelles, such as the nucleus and mitochondria.

Prokaryotic Cells

Characteristics and Structure

Prokaryotic cells are simpler in structure and are found in two kingdoms: Bacteria and Archaea. They lack a true nucleus and other membrane-bound organelles.

• Size: Approximately 5 μm (micrometers).

• Key Structures:

  • Plasma membrane: Controls what enters and exits the cell.

  • Cell wall: Provides structure and protection.

  • Nucleoid region: Contains a single circular strand of DNA responsible for heredity and protein production.

  • Cytoplasm: Gel-like matrix where cellular metabolism occurs.

  • Ribosomes: Sites of protein synthesis.

  • Flagella: Enables movement.

  • Capsule: (in some bacteria) Offers additional protection.

  • Mesosome: (in some bacteria) Involved in cellular processes such as respiration.

Example: Escherichia coli is a common prokaryotic cell found in the human gut.

Eukaryotic Cells

Characteristics and Structure

Eukaryotic cells are more complex and contain membrane-bound organelles. They are found in four kingdoms: Protista, Fungi, Plantae, and Animalia.

• Size: Typically larger than prokaryotic cells (about 50 μm).

• Key Structures:

  • Nucleus: Contains DNA and controls cellular activities.

  • Mitochondria: Produces ATP for cellular energy.

  • Endoplasmic reticulum (ER): Synthesizes proteins (rough ER) and lipids (smooth ER).

  • Golgi apparatus: Modifies, sorts, and packages proteins and lipids.

  • Lysosomes: Break down waste and toxins.

  • Cytoskeleton: Provides structural support and aids in cell movement.

  • Centrioles: Involved in cell division (animal cells).

Example: Human cheek cells are eukaryotic and contain a visible nucleus when stained.

Function of Cell Structures

Nucleus

The nucleus is the control center of the cell, housing genetic material and coordinating activities such as growth and reproduction.

• Nuclear envelope: Double membrane that regulates passage of materials.

• Nucleoplasm: Gel-like substance inside the nucleus.

• Nucleolus: Dense region where ribosome assembly begins.

Cytoplasm

The cytoplasm is a semi-fluid matrix that supports organelles and is the site of many metabolic reactions.

Mitochondria

Mitochondria are the "powerhouses" of the cell, generating ATP through cellular respiration.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes proteins for export.

• Smooth ER: Lacks ribosomes; synthesizes lipids and processes toxins.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins and lipids for delivery inside or outside the cell.

Lysosomes

Lysosomes contain digestive enzymes that break down cellular waste and toxins.

Centrioles

Centrioles help organize microtubules during cell division, forming spindle fibers.

Plant Cell Structures

Unique Features of Plant Cells

Plant cells have additional structures that support their functions, such as photosynthesis and storage.

• Chloroplasts: Site of photosynthesis, converting sunlight, water, and CO2 into glucose.

• Cell wall: Made of cellulose, provides rigidity and protection.

• Central vacuole: Large membrane-bound sac that stores water, nutrients, and waste products.

Example: Leaf cells contain many chloroplasts to maximize photosynthesis.

Comparison of Prokaryotic and Eukaryotic Cells

The following table summarizes the main differences between prokaryotic and eukaryotic cells:

Feature	Prokaryotic Cells	Eukaryotic Cells
Organelles	No membrane-bound organelles	Membrane-bound organelles present
Nucleus	Absent (nucleoid region)	Present
Cell Size	~5 μm	~50 μm
Examples	Bacteria, Archaea	Protists, Fungi, Plants, Animals

Key Equations

• Size conversion: $1\ \mu m = 1 \times 10^{-6}\ \text{meters}$

Summary

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

• Prokaryotic cells are simpler and smaller, while eukaryotic cells are more complex and larger.

• Plant cells have unique structures such as chloroplasts, cell walls, and central vacuoles.

• Understanding cell structure is fundamental to studying biology and the functions of living organisms.