Structure and Function of Cells: Cell Theory, Types, and Organelles

Structure and Function of Cells

Cell Theory

The cell theory is a foundational concept in biology, describing the properties and significance of cells in living organisms.

• All living things are composed of cells and cell products.

• The cell is the smallest unit that exhibits all the characteristics of life.

• All cells arise only from preexisting cells.

These principles remain central to modern biology and guide our understanding of life at the cellular level.

Classification of Cells

Cells are classified based on their internal organization into two main types: prokaryotic and eukaryotic cells.

• Prokaryotic Cells: Simpler structure, lack membrane-bound organelles, DNA is located in a nucleoid region, surrounded by a plasma membrane and often a rigid cell wall. Examples: Bacteria, blue-green algae.

• Eukaryotic Cells: More complex, contain a true nucleus (membrane-bound DNA), cytoplasm with specialized organelles, surrounded by a plasma membrane. Examples: Animals, plants.

Structure reflects function: Cell shape and internal structures are adapted to specific roles (e.g., muscle cells have many mitochondria for energy, nerve cells are long for signal transmission).

Cell Size and Efficiency

Cells remain small to maximize efficiency in exchanging materials with their environment.

• Metabolic activities are proportional to cell volume.

• All materials must cross the plasma membrane; as cells grow, volume increases faster than surface area, limiting exchange efficiency.

• Microvilli are microscopic projections that increase surface area, common in cells of the digestive tract and kidney tubules.

Key Principle: Smaller cells are more efficient at exchanging materials due to a higher surface area-to-volume ratio.

Microscopes in Cell Biology

Microscopes are essential tools for studying cells due to their small size. Three main types are used:

• Light Microscope: Magnifies up to 1000x, suitable for viewing live cells and general structures.

• Transmission Electron Microscope (TEM): Uses electrons to produce detailed 2D images of internal structures, up to 100,000x magnification.

• Scanning Electron Microscope (SEM): Produces 3D images of cell surfaces, also up to 100,000x magnification.

Internal Structures and Functions of Eukaryotic Cells

The Nucleus

The nucleus is the control center of the cell, containing most of the genetic material (DNA).

• Nuclear membrane: Double phospholipid bilayer that encloses the DNA.

• Nucleolus: Dense region where ribosomal components are formed.

• Nuclear pores: Allow selective exchange of materials (e.g., ribosomes, RNA) between nucleus and cytoplasm; DNA remains inside.

Ribosomes

Ribosomes are the sites of protein synthesis, assembling amino acids into polypeptide chains.

• Formed from RNA and proteins in the nucleolus.

• Can be free in the cytoplasm or attached to the endoplasmic reticulum.

Endomembrane System

The endomembrane system includes several interconnected organelles responsible for synthesis, modification, and transport of cellular products.

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes, synthesizes proteins.

• Smooth Endoplasmic Reticulum (SER): Lacks ribosomes, synthesizes lipids and some hormones, packages proteins and lipids for transport.

• Golgi Apparatus: Receives vesicles from ER, refines and sorts proteins and lipids, packages them for delivery.

• Vesicles: Membrane-bound sacs for transport, secretion, and digestion (e.g., lysosomes, peroxisomes).

Types of Vesicles

• Secretory vesicles: Export products from the cell.

• Endocytotic vesicles: Bring external materials into the cell.

• Peroxisomes: Contain enzymes to break down toxic wastes.

• Lysosomes: Contain digestive enzymes for breaking down bacteria, large particles, and cellular debris.

Mitochondria

Mitochondria are the powerhouses of the cell, converting nutrients into ATP through cellular respiration.

• Double membrane: Outer membrane is smooth; inner membrane is folded (cristae) and contains enzymes for energy production.

• Own DNA and ribosomes: Reflects evolutionary origins.

• ATP production: Main site for energy conversion in the cell.

Equation for Cellular Respiration:

$\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ATP}$

Cytoskeleton

The cytoskeleton provides structural support, maintains cell shape, and anchors organelles.

• Microtubules: Hollow tubes for support and transport.

• Microfilaments: Thin fibers for movement and shape changes.

• Connects to glycoproteins in the plasma membrane.

Cilia and Flagella

Both are structures made of microtubules, specialized for movement.

• Cilia: Short, numerous, move materials along cell surfaces (e.g., in the respiratory tract, oviduct).

• Flagella: Long, few in number, propel cells (e.g., sperm cells).

Centrioles

Centrioles are short, rod-like structures near the nucleus, essential for cell division. They help organize and separate genetic material during mitosis and meiosis.

Summary Table: Major Eukaryotic Cell Structures and Functions

Structure	Main Function
Plasma membrane	Controls movement of materials into and out of cell
Nucleus	Information center; contains DNA
Ribosomes	Site of protein synthesis
Rough ER	Protein synthesis and processing
Smooth ER	Lipid and hormone synthesis; packaging
Golgi apparatus	Refines, packages, and ships macromolecules
Lysosome	Digests damaged organelles and debris
Peroxisome	Destroys toxic waste
Mitochondrion	Produces ATP (energy)
Cytoskeleton	Structural framework
Centrioles	Involved in cell division
Cilia/Flagella	Movement

References: Johnson, M.D. (2017). Human biology: Concepts and current issues (8th ed). Pearson Education Inc.; Johnson, M.D. & Long, S (2021). Human biology: Concepts and current issues (9th ed). Pearson Education Inc.