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

Chapter 3: 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 emphasize the universality and continuity of life.

Classification of Cells

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

• Prokaryotic Cells: Simpler structure, lack a true nucleus, DNA is located in a nucleoid region, no membrane-bound organelles, typically have a rigid cell wall. Examples: Bacteria, blue-green algae.

• Eukaryotic Cells: More complex, possess a true nucleus surrounded by a nuclear membrane, contain membrane-bound organelles, cytoplasm with cytosol, and specialized structures. Examples: Animals, plants.

All cells are surrounded by a plasma membrane that encloses their internal contents.

Structure Reflects Function

Although all cells share basic functions such as gathering raw materials, excreting wastes, synthesizing macromolecules, and reproducing, their structures are specialized for their roles:

• Muscle cells: Abundant organelles for energy production.

• Nerve cells: Long and thin to transmit signals over distances.

• Red blood cells: Round and flexible to move through capillaries.

Each eukaryotic cell type is specialized for its function.

Cell Size and Surface Area

Cells remain small to maximize efficiency. The metabolic activity of a cell is proportional to its volume, but all materials must cross the plasma membrane, whose surface area increases more slowly than volume as a cell grows. Thus, smaller cells are more efficient at exchanging materials with their environment.

• Microvilli: Microscopic projections that increase surface area, commonly found in the digestive tract and kidney tubules.

An eightfold increase in cell volume results in only a fourfold increase in surface area, emphasizing the importance of small cell size.

Microscopes

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

• Light Microscope: Magnifies up to 1000x, used for general cell observation.

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

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

Internal Cell Structures and Functions

The Nucleus

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

• Nuclear membrane: Double phospholipid layer that encloses DNA.

• Nucleolus: Dense region where ribosomal components are formed.

• Nuclear pores: Allow selective movement of materials (e.g., ribosomes, proteins, RNA) in and out of the nucleus, but are too small for DNA to exit.

Ribosomes

Ribosomes are the sites of protein synthesis. They are assembled from RNA and proteins in the nucleolus, then transported to the cytoplasm where they may float freely or attach to the endoplasmic reticulum.

• Function: Assemble amino acids into specific protein chains (primary structure).

Endomembrane System

The endomembrane system includes several interconnected organelles responsible for synthesis, packaging, and transport of cellular materials:

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes, involved in protein synthesis and initial folding.

• 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.

• Specialized Vesicles: Include lysosomes, peroxisomes, secretory and endocytotic vesicles.

Vesicles and Their Types

Vesicles are membrane-bound sacs that transport and store substances within a cell.

• Secretory vesicles: Export products out of the cell.

• Endocytotic vesicles: Bring external materials into the cell.

• Peroxisomes: Contain enzymes to destroy toxic wastes.

• Lysosomes: Contain digestive enzymes, fuse with endocytotic vesicles to digest bacteria and debris, perform housekeeping tasks.

Mitochondria

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

• Structure: Double membrane (outer smooth, inner folded into cristae).

• Function: Enzymes on the inner membrane catalyze reactions to release energy.

• Unique features: Contain their own DNA and ribosomes.

• Energy production equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

Cytoskeleton

The cytoskeleton provides structural support, maintains cell shape, and anchors organelles. It consists of:

• Microtubules: Hollow tubes for support and transport.

• Microfilaments: Thin solid fibers for movement and shape.

Cilia and Flagella

Both cilia and flagella are made of microtubules and are involved in movement.

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

• Flagella: Long, few in number, move the entire cell (e.g., sperm cells).

Centrioles

Centrioles are short, rod-like microtubular structures located near the nucleus. They are essential for cell division, participating in the alignment and separation of 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 by ribosomes
Smooth ER	Macromolecule synthesis (other than proteins)
Golgi apparatus	Refines, packages, and ships macromolecular products
Lysosome	Digests damaged organelles and debris
Peroxisome	Destroys cellular toxic waste
Mitochondrion	Produces energy (ATP) for the cell
Cytoskeleton	Structural framework of the cell
Centrioles	Involved in cell division
Secretory vesicle	Membrane-bound shipping container

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.