Cell Structure and Function: Prokaryotic and Eukaryotic Cells, Organelles, and the Cytoskeleton

Cell Structure and Classification

Prokaryotic vs. Eukaryotic Cells

Cells are the fundamental units of life and can be classified into two main types: prokaryotic and eukaryotic cells. These classifications are based on structural and functional differences.

• Prokaryotic cells are small, lack a membrane-bound nucleus, and have a single internal compartment. Their DNA is clustered in a region called the nucleoid. Prokaryotes include bacteria and archaea.

• Eukaryotic cells possess a true, membrane-bound nucleus and many other membrane-bound organelles. Eukaryotes include animals, plants, fungi, and protists.

The plasma membrane is a selective barrier that surrounds all cells, controlling the movement of substances in and out. It is composed of a phospholipid bilayer with embedded proteins, making it selectively permeable.

Cell Walls

Some organisms have an additional outer layer called the cell wall, which provides structural support and protection.

• Bacteria: Cell walls contain peptidoglycan, a complex of carbohydrates and peptides.

• Plants: Cell walls are primarily made of cellulose.

• Fungi: Cell walls contain chitin.

Cytoplasm and Cytosol

The cytoplasm includes all the contents of the cell except the nucleus. The cytosol is the fluid portion of the cytoplasm, containing water, dissolved ions, and molecules.

Ribosomes

Ribosomes are molecular machines found in both prokaryotic and eukaryotic cells. They synthesize proteins by translating messenger RNA (mRNA).

• In eukaryotes, ribosomes can be free in the cytosol or bound to the rough endoplasmic reticulum (RER).

• In prokaryotes, ribosomes are only found free in the cytosol.

Typical Cell Sizes

Cell Type	Size
Most plant and animal cells	10–100 μm
Bacteria	1–10 μm

Microscopy and Cell Visualization

Electron Microscopy

Electron microscopes use beams of electrons to visualize cell structures at very high resolution, allowing observation of organelles and macromolecular complexes.

• Scanning Electron Microscopy (SEM): Provides detailed images of cell surfaces.

• Transmission Electron Microscopy (TEM): Reveals internal cell structures in great detail.

Membranous Organelles in Eukaryotic Cells

Nucleus

The nucleus contains most of the cell's DNA and is surrounded by a double membrane called the nuclear envelope. The nuclear envelope has nuclear pores that regulate transport in and out of the nucleus.

Endomembrane System

The endomembrane system is a network of membranes inside eukaryotic cells, including the endoplasmic reticulum, Golgi apparatus, lysosomes, and vesicles.

• Endoplasmic Reticulum (ER):

  • Rough ER (RER): Studded with ribosomes; synthesizes membrane proteins, secreted proteins, and proteins for organelles.

  • Smooth ER (SER): Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, and detoxifies drugs and poisons.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain hydrolytic enzymes for intracellular digestion of macromolecules.

• Vesicles: Small membrane-bound sacs that transport substances within the cell.

Peroxisomes

Peroxisomes contain enzymes that break down fatty acids and detoxify harmful substances. They produce hydrogen peroxide (H2O2), which is then broken down by the enzyme catalase.

Vacuoles

Vacuoles are large vesicles found in plant and fungal cells. They store nutrients, waste products, and help maintain turgor pressure in plant cells.

Chloroplasts

Chloroplasts are the sites of photosynthesis in plant cells. They contain the pigment chlorophyll, which captures light energy to convert carbon dioxide and water into glucose and oxygen.

Mitochondria

Mitochondria are the powerhouses of the cell, generating ATP through cellular respiration. They have a double membrane and their own DNA.

Endosymbiotic Theory

The endosymbiotic theory proposes that mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells. Evidence includes:

• Both organelles have double membranes.

• Both contain their own DNA and ribosomes.

• Both replicate independently of the cell cycle.

Cytoskeleton and Cell Junctions

Cytoskeleton

The cytoskeleton is a network of protein filaments that provides structural support, maintains cell shape, and facilitates movement.

• Microtubules: Hollow tubes made of tubulin; form tracks for vesicle movement and are involved in cell division.

• Intermediate filaments: Provide mechanical strength; form the nuclear lamina inside the nucleus.

• Microfilaments (actin filaments): Thin filaments involved in cell movement and muscle contraction.

Motor proteins such as kinesin and dynein move along microtubules to transport vesicles and organelles.

Extracellular Matrix (ECM)

The extracellular matrix is a network of proteins and carbohydrates outside animal cells that provides structural support and mediates cell signaling.

• Collagen: Main structural protein in the ECM.

• Proteoglycans: Proteins with attached carbohydrates.

• Fibronectin: Connects ECM components to cell surface receptors (integrins).

Cell Junctions

Specialized structures connect cells to each other and to the ECM, facilitating communication and maintaining tissue integrity.

• Tight junctions: Seal adjacent cells to prevent leakage of extracellular fluid.

• Desmosomes: Anchor cells together, providing mechanical strength.

• Gap junctions: Allow direct communication between animal cells through channels.

• Plasmodesmata: Channels between plant cells that allow transport of substances.

Summary Table: Key Differences Between Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	No	Yes
Membrane-bound organelles	No	Yes
Cell wall composition	Peptidoglycan (bacteria)	Cellulose (plants), chitin (fungi), or absent (animals)
Size	1–10 μm	10–100 μm
Examples	Bacteria, Archaea	Animals, Plants, Fungi, Protists

Key Terms and Definitions

• Selective permeability: The property of a membrane that allows some substances to pass through while blocking others.

• Phospholipid bilayer: The fundamental structure of cell membranes, consisting of two layers of phospholipids.

• ATP (Adenosine triphosphate): The main energy currency of the cell.

• Endocytosis/Exocytosis: Processes by which cells take in (endocytosis) or release (exocytosis) large molecules via vesicles.

Additional info: Some explanations and definitions have been expanded for clarity and completeness based on standard biology textbooks.