Cell Structure and Function: A Tour of the Cell

Prokaryotic vs. Eukaryotic Cells

Cells are classified into two main types based on their internal organization.

• Prokaryotic Cells:

  • Lack a nucleus; DNA is located in the nucleoid region.

  • Have external appendages such as flagella (for movement) and pili (for attachment).

  • Cell wall provides structural support.

  • Examples: Bacteria and Archaea.

• Eukaryotic Cells:

  • Contain a true nucleus enclosed by a nuclear envelope.

  • Possess membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).

  • Include animal and plant cells.

Cell Organelles and Internal Structures

Eukaryotic cells contain specialized organelles that perform distinct functions.

• Nucleus: Stores genetic material and coordinates cellular activities.

• Endomembrane System: Includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and vesicles. Responsible for synthesis, modification, and transport of proteins and lipids.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins.

  • Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

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

• Lysosomes: Contain digestive enzymes for breaking down macromolecules and cellular debris.

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

• Mitochondria: Site of cellular respiration; produces ATP.

• Chloroplasts: Found in plant cells; site of photosynthesis.

• Cytoskeleton: Network of protein filaments (microtubules, microfilaments) that provide structural support and facilitate movement.

• Centrosomes and Centrioles: Organize microtubules and are important in cell division.

• Extracellular Matrix: Complex network outside animal cells that provides structural and biochemical support.

• Intercellular Junctions: Structures that connect adjacent cells, facilitating communication and adhesion.

Cell Processes and Functions

Cells carry out various processes essential for life.

• Phagocytosis: The process by which cells engulf large particles or other cells.

• Autophagy: The degradation and recycling of cellular components.

• Transport Mechanisms:

  • Passive Transport: Movement of substances down their concentration gradient without energy input.

  • Active Transport: Movement of substances against their concentration gradient, requiring energy (ATP).

Integration of Organelles and Higher Levels of Organization

Cells are organized into tissues, organs, and organ systems, allowing for complex functions in multicellular organisms.

• Tissues: Groups of similar cells performing a specific function.

• Organs: Structures composed of multiple tissue types working together.

• Organ Systems: Groups of organs that carry out major body functions.

Key Terms and Phrases

Understanding the following terms is essential for mastering cell biology:

Term	Definition
Anatomy	Study of the structure of organisms and their parts
Autophagy	Process by which cells degrade and recycle components
Bacteria	Single-celled prokaryotic organisms
Cell Membrane	Selective barrier surrounding the cell
Cell Wall	Rigid structure outside the cell membrane in plants, fungi, and bacteria
Chloroplast	Organelle in plants responsible for photosynthesis
Cytoskeleton	Network of protein filaments providing cell structure and movement
Endoplasmic Reticulum	Organelle involved in protein and lipid synthesis
Golgi Apparatus	Organelle that modifies and packages proteins and lipids
Lysosome	Organelle containing digestive enzymes
Mitochondria	Organelle responsible for cellular respiration and energy production
Nucleus	Membrane-bound organelle containing genetic material
Ribosome	Complex that synthesizes proteins
Surface-to-Volume Ratio	Ratio affecting cell efficiency and size

Formulas and Equations

• Surface Area of a Sphere: $A = 4\pi r^2$

• Volume of a Sphere: $V = \frac{4}{3}\pi r^3$

• Surface Area to Volume Ratio: $\text{Ratio} = \frac{A}{V}$

Example: Surface Area to Volume Ratio

As a cell increases in size, its volume grows faster than its surface area, which limits the rate at which materials can enter or leave the cell. This is why most cells are small.

Comparison Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cell	Eukaryotic Cell
Nucleus	Absent	Present
Membrane-bound Organelles	Absent	Present
Cell Wall	Present (most)	Present (plants, fungi)
Size	Smaller (1-10 μm)	Larger (10-100 μm)
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Additional info: Some context and definitions were expanded for clarity and completeness based on standard General Biology curriculum.