Introduction to Cell Biology: Structure and Function

Prokaryotic vs. Eukaryotic Cells

All organisms can be classified into two broad categories based on their cellular structure: prokaryotic or eukaryotic.

Prokaryotic Cells

Prokaryotes belong to the domains Bacteria and Archaea. These are small, simple, single-celled organisms. A defining characteristic of prokaryotes is that they lack a nucleus. Instead, their circular chromosomes are located in an unprotected region of the cytoplasm called the nucleoid.

Many prokaryotes possess additional structures for protection and movement:

• Cell Wall: A rigid structure outside the membrane. In bacteria, this wall is specifically made of a carbohydrate called peptidoglycan.
• Capsule: A jelly-like outer coating found in many prokaryotes.
• Flagella: Tail-like appendages used for locomotion.

While Bacteria and Archaea are structurally similar, they are genetically and biochemically very different. Archaea are famous for their ability to thrive in extreme environments, such as high-temperature deep-sea vents or highly acidic waters.

Eukaryotic Cells

Eukaryotes belong to the domain Eukarya and can be either unicellular or multicellular. They are significantly larger and more complex than prokaryotes. The hallmark of a eukaryotic cell is the presence of a nucleus that encloses linear chromosomes. Furthermore, eukaryotes contain various membrane-bound organelles—specialized compartments that allow different chemical reactions to occur in isolation.

Specialized Eukaryotic Cells: Plants vs. Animals

Within the domain Eukarya, there are distinct differences between the cells of different kingdoms.

Exam Alert: Mitochondria Confusion If a student sees a cell with a nucleus and mitochondria, they might assume it is an animal cell. However, mitochondria are found in nearly all eukaryotic cells, including plants. To distinguish a plant cell from an animal cell, look for a cell wall or chloroplasts.

The Endomembrane System and Organelles

Eukaryotic cells utilize an internal system of membranes known as the endomembrane system to compartmentalize processes and increase surface area for chemical reactions.

The Nucleus and Protein Synthesis

The nucleus is the control center, housing the DNA organized into chromatin. It is enclosed by a double-membrane called the nuclear envelope, which features nuclear pores to allow material to move in and out. Inside the nucleus, the nucleolus serves as the site where ribosomes are synthesized.

Proteins are synthesized either by free ribosomes (which stay in the cytosol to make proteins for internal use) or bound ribosomes (which are attached to the Endoplasmic Reticulum (ER)).

• Rough ER: Studded with ribosomes, it modifies proteins destined for secretion, the cell membrane, or lysosomes.
• Smooth ER: Lacks ribosomes and is responsible for synthesizing lipids (like steroids) and detoxifying the cell.

Transport and Processing

Once proteins are made in the ER, they are packaged into transport vesicles and sent to the Golgi apparatus. The Golgi acts as a shipping and receiving center; it receives vesicles at its cis face, modifies the contents, and sorts them before sending them out through its trans face.

Digestion and Storage

• Lysosomes: Vesicles containing hydrolytic enzymes that digest macromolecules or recycle worn-out organelles.
• Vacuoles: Large vesicles used for storage. Plant cells specifically use a large central vacuole to store water and nutrients.

Energy Conversion: Mitochondria and Chloroplasts

Two specialized organelles are responsible for the energy needs of eukaryotic cells, likely originating from ancient prokaryotes through a process called endosymbiosis.

Mitochondria (Cellular Respiration)

Found in almost all eukaryotes, mitochondria perform aerobic cellular respiration to produce ATP. The general reaction is: $\text{sugar} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water} + \text{ATP}$

Chloroplasts (Photosynthesis)

Found only in plants and some protists, chloroplasts convert solar energy into chemical energy (sugar). The general reaction is: $\text{light} + \text{carbon dioxide} + \text{water} \rightarrow \text{carbohydrate} + \text{oxygen}$

The Endosymbiotic Theory Evidence suggests that mitochondria and chloroplasts were once free-living prokaryotes that were engulfed by a larger cell. Supporting evidence includes the fact that both organelles have their own circular DNA, their own ribosomes, and a double membrane.

Support and Communication: Cytoskeleton and Junctions

To maintain structure and facilitate movement, cells use a cytoskeleton—a network of fibers including microtubules (thickest), intermediate filaments, and microfilaments (thinnest). This network allows organelles to "walk" to their destinations and helps the cell maintain its shape.

Cells also interact through specialized structures:

• In Plants: Plasmodesmata are channels through cell walls that allow water and materials to pass freely between adjacent cells.
• In Animals:
  • Tight junctions create seals to prevent fluid leakage.
  • Desmosomes act like rivets to fasten cells into strong sheets (e.g., muscle tissue).
  • Gap junctions create pores that allow ions and small molecules to communicate between cells.