Cell Structure and Function: Foundations of Life

Cell Structure and Function: Foundations of Life

Introduction to Cells and the Scale of Life

Cells are the fundamental units of life, forming the basis of all living organisms. Understanding their structure and function is essential for studying biology at the molecular, cellular, and organismal levels.

• Definition of a Cell: The smallest unit of life that can carry out all life processes.

• Cell Size: Most cells range from 1 to 100 microns (μm) in diameter.

• Scale of Biological Structures: Structures range from atoms (0.1 nm) to human height (meters). Most plant and animal cells, nuclei, mitochondria, and bacteria fall within the 1–100 μm range.

• Example: A frog egg is about 1 mm, while a typical bacterium is about 1 μm.

Types of Cells: Prokaryotes vs. Eukaryotes

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

• Prokaryotic Cells:

  • Lack a true, membrane-bound nucleus.

  • DNA is located in a region called the nucleoid.

  • Do not have membrane-bound organelles.

  • Examples: Bacteria and Archaea.

• Eukaryotic Cells:

  • Have a nucleus enclosed by a double membrane.

  • Contain membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus).

  • DNA is linear and organized into chromosomes.

  • Can be unicellular or multicellular (e.g., plants, animals, fungi, protists).

Comparing Prokaryotic and Eukaryotic Cells

The following table summarizes the key differences between prokaryotic and eukaryotic cells:

Feature	Prokaryotes	Eukaryotes
Nucleus	No (nucleoid region)	Yes (membrane-bound)
DNA	Circular	Linear
Organelles	Absent	Present (membrane-bound)
Cell Size	Small (1–10 μm)	Larger (10–100 μm)
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Cellular Compartmentalization

Compartmentalization refers to the presence of membrane-bound organelles in eukaryotic cells, allowing for specialized functions and increased efficiency.

• Organelles: Structures such as the nucleus, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, mitochondria, and chloroplasts.

• Advantages: Enables separation of incompatible chemical reactions, concentration of reactants, and regulation of cellular processes.

• Example: Lysosomes contain hydrolytic enzymes for digestion, while mitochondria generate ATP through cellular respiration.

Membrane Structure and Function

The plasma membrane is a selectively permeable barrier that regulates the movement of substances into and out of the cell.

• Phospholipid Bilayer: Composed of hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Selective Permeability: Allows certain molecules to pass while restricting others, maintaining distinct internal environments.

• Proteins: Embedded in the membrane, they function as channels, carriers, receptors, and enzymes.

Transport Across the Membrane

Cells use various mechanisms to transport substances across their membranes, including passive and active processes.

• Passive Transport: Movement of molecules down their concentration gradient (e.g., diffusion, facilitated diffusion, osmosis).

• Active Transport: Requires energy (usually ATP) to move molecules against their concentration gradient.

• Bulk Transport: Involves the movement of large molecules or particles via vesicles (endocytosis and exocytosis).

Bulk Transport Mechanisms

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell. Important for secretion of hormones, neurotransmitters, and waste products.

• Endocytosis: The plasma membrane engulfs material, forming a vesicle that brings substances into the cell. Types include:

  • Phagocytosis: "Cell eating"; uptake of large particles or cells.

  • Pinocytosis: "Cell drinking"; uptake of extracellular fluid and dissolved solutes.

  • Receptor-Mediated Endocytosis: Specific molecules are taken in after binding to receptors on the cell surface.

Endomembrane System

The endomembrane system is a group of interconnected organelles that work together to modify, package, and transport lipids and proteins.

• Components: Nuclear envelope, endoplasmic reticulum (rough and smooth), Golgi apparatus, lysosomes, vesicles, and plasma membrane.

• Pathway Example: Protein synthesis begins in the rough ER, continues through transport vesicles to the Golgi apparatus for modification and sorting, and is then sent to the plasma membrane or other destinations.

Mitochondria and Chloroplasts: Energy Organelles

Mitochondria and chloroplasts are specialized organelles involved in energy conversion. They are believed to have originated from free-living prokaryotes through endosymbiosis.

• Mitochondria: Site of cellular respiration; converts glucose and oxygen into ATP, the cell's energy currency.

• Chloroplasts: Site of photosynthesis in plants and algae; converts light energy, water, and carbon dioxide into glucose and oxygen.

• Unique Features:

  • Both have double membranes.

  • Contain their own circular DNA and ribosomes.

  • Reproduce independently within the cell by binary fission.

Endosymbiotic Theory

The endosymbiotic theory explains the evolutionary origin of mitochondria and chloroplasts as descendants of ancient prokaryotic cells engulfed by ancestral eukaryotes.

• Supporting Evidence:

  • Double membranes surrounding mitochondria and chloroplasts.

  • Presence of their own ribosomes and circular DNA, similar to bacteria.

  • Reproduction by binary fission, like prokaryotes.

  • Genetic and biochemical similarities to certain bacteria (e.g., proteobacteria for mitochondria, cyanobacteria for chloroplasts).

• Implications: These organelles are semi-autonomous and play crucial roles in energy metabolism.

Summary Table: Key Features of Mitochondria and Chloroplasts

Feature	Mitochondria	Chloroplasts
Function	Cellular respiration (ATP production)	Photosynthesis (glucose production)
Membranes	Double	Double (sometimes triple in some algae)
DNA	Circular, prokaryote-like	Circular, prokaryote-like
Ribosomes	Prokaryote-like	Prokaryote-like
Reproduction	Binary fission	Binary fission

Key Terms and Definitions

• Organelle: Specialized subunit within a cell with a specific function, usually membrane-bound in eukaryotes.

• Phagocytosis: The process by which a cell engulfs large particles or other cells.

• Pinocytosis: The process by which a cell takes in extracellular fluid and dissolved solutes.

• Receptor-Mediated Endocytosis: Uptake of specific molecules based on receptor-ligand interactions.

• Endosymbiosis: A symbiotic relationship in which one organism lives inside the cell or body of another organism.

Example Exam Question

• Question: Which of the following statements contributes to the argument that mitochondria and chloroplasts evolved from prokaryotic endosymbionts?

  • They have double membranes.

  • They have their own ribosomes.

  • They have their own circular DNA.

  • Their matrix and stromal spaces contain soluble proteins.

Additional info: Some details, such as the specific order of the endomembrane system pathway and the types of endocytosis, were expanded for clarity and completeness based on standard biology curricula.