Cell Structure, Function, and History: Study Notes for General Biology

Cell Structure and Function

History of Cell Theory

The development of cell theory was a major milestone in biology, establishing the cell as the fundamental unit of life. Key discoveries and scientists contributed to our understanding of cells over time.

• Cell Theory: All living things are composed of cells; cells are the basic unit of structure and function in living organisms; all cells come from pre-existing cells.

• Key Contributors:

  • Robert Hooke (1665): First observed cells in cork using a microscope.

  • Anton van Leeuwenhoek (1674): Observed living cells ("animalcules") in pond water.

  • Matthias Schleiden (1838): Proposed that plants are made of cells.

  • Theodor Schwann (1839): Proposed that animals are made of cells.

  • Rudolf Virchow (1855): Stated that all cells arise from pre-existing cells.

• Timeline of Discoveries:

  • 1665: Hooke

  • 1674: Leeuwenhoek

  • 1838: Schleiden

  • 1839: Schwann

  • 1855: Virchow

Types of Cells

Cells are classified into two main types based on their structure: prokaryotic and eukaryotic.

• Prokaryotic Cells:

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

  • No membrane-bound organelles.

  • Examples: Bacteria and Archaea.

• Eukaryotic Cells:

  • Have a true nucleus containing DNA.

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

  • Examples: Plants, Animals, Fungi, Protists.

Cell Membrane Structure and Function

Plasma Membrane

The plasma membrane is a selectively permeable barrier that surrounds the cell, controlling the movement of substances in and out.

• Structure:

  • Composed of a phospholipid bilayer with embedded proteins.

  • Phospholipids have hydrophilic heads and hydrophobic tails.

  • Proteins serve as channels, carriers, receptors, and enzymes.

• Functions:

  • Regulates transport of materials.

  • Provides protection and structural support.

  • Facilitates cell communication and signaling.

• Fluid Mosaic Model: Describes the membrane as a dynamic structure with proteins floating in or on the fluid lipid bilayer.

Transport Across Membranes

Cells use various mechanisms to move substances across the plasma membrane.

• Passive Transport:

  • Does not require energy.

  • Includes diffusion, osmosis, and facilitated diffusion.

  • Diffusion: Movement of molecules from high to low concentration.

  • Osmosis: Diffusion of water across a selectively permeable membrane.

  • Facilitated Diffusion: Movement of molecules via transport proteins.

• Active Transport:

  • Requires energy (ATP).

  • Moves substances against their concentration gradient.

  • Example: Sodium-potassium pump.

• Bulk Transport:

  • Endocytosis: Cell takes in materials by engulfing them.

  • Exocytosis: Cell expels materials by fusing vesicles with the membrane.

Cell Organelles and Their Functions

Major Organelles

Eukaryotic cells contain specialized structures called organelles, each with distinct functions.

• Nucleus: Contains genetic material (DNA); controls cell activities.

• Mitochondria: Site of cellular respiration; produces ATP.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins.

  • Smooth ER: Synthesizes lipids; detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Ribosomes: Sites of protein synthesis; found free in cytoplasm or attached to ER.

• Lysosomes: Contain digestive enzymes; break down waste.

• Vacuoles: Storage organelles; large central vacuole in plant cells stores water and nutrients.

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

• Cell Wall: Provides structure and protection; found in plants, fungi, and some prokaryotes.

Comparison of Plant and Animal Cells

Plant and animal cells share many organelles but also have key differences.

Organelle/Feature	Plant Cell	Animal Cell
Cell Wall	Present	Absent
Chloroplasts	Present	Absent
Central Vacuole	Large	Small or absent
Lysosomes	Rare	Common

Endomembrane System

Components and Functions

The endomembrane system is a network of membranes within eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Includes: Nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, and plasma membrane.

• Function: Synthesis and transport of proteins and lipids; detoxification; waste processing.

• Vesicles: Small membrane-bound sacs that transport materials between organelles.

Cellular Transport Mechanisms

Osmosis and Water Balance

Osmosis is crucial for maintaining water balance in cells.

• Hypertonic Solution: Higher solute concentration outside the cell; water moves out, cell shrinks.

• Hypotonic Solution: Lower solute concentration outside; water moves in, cell swells.

• Isotonic Solution: Equal solute concentration; no net water movement.

Equations

• Osmotic Pressure: $ext{Osmotic Pressure} = iMRT$ Where i is the van 't Hoff factor, M is molarity, R is the gas constant, and T is temperature in Kelvin.

Summary Table: Prokaryotic vs. Eukaryotic Cells

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

Additional info:

• Some details, such as the fluid mosaic model and the van 't Hoff equation for osmotic pressure, were inferred to provide academic completeness.

• Tables were reconstructed for clarity and comparison.