Cell Theory and Cell Types

Principles of Cell Theory

The cell theory is a foundational concept in biology, stating that all living organisms are composed of cells and that all cells arise from pre-existing cells. The theory is based on three main principles:

• All organisms are made up of one or more cells.

• The cell is the basic unit of structure and function in organisms.

• All cells come from pre-existing cells.

Types of Cells

Cells are classified into two main types based on their internal structure:

• Eukaryotic cells: These cells have a membrane-bound nucleus and other organelles. Examples include animal, plant, fungi, and protist cells.

• Prokaryotic cells: These cells lack a membrane-bound nucleus. Their genetic material is contained in a single circular DNA molecule. Examples include bacteria and archaea.

Additional info: Eukaryotic cells are generally larger and more complex than prokaryotic cells.

Cell Structure and Organelles

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 bilayer of phospholipids with embedded proteins.

• Function: Maintains homeostasis, protects the cell, and facilitates communication.

Major Organelles and Their Functions

• Nucleus: Contains the cell's genetic material (DNA) and acts as the control center.

• Cytoplasm: Fluid portion of the cell where organelles are suspended; site of many metabolic reactions.

• Cytoskeleton: Network of protein filaments that provides structural support, maintains cell shape, and assists in movement.

• Mitochondria: Site of cellular respiration; produces ATP from food molecules.

• Chloroplasts: Found in plant cells; site of photosynthesis. Both mitochondria and chloroplasts are thought to have evolved from prokaryotic ancestors.

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

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

• Lysosomes: Contain digestive enzymes to break down waste materials and cellular debris.

• Contractile Vacuole: Helps regulate water balance in some cells.

• Central Vacuole: Large storage organelle in plant cells.

DNA, Genes, and Cell Division

Genetic material is stored as DNA, which encodes instructions for cell function and division.

• DNA: Double-stranded molecule containing genetic information.

• Genes: Segments of DNA that code for proteins.

• Ribosomes: Translate genetic information into proteins.

• Cell Division: Cells reproduce by dividing, ensuring genetic continuity.

Cell Membrane Structure and Function

Phospholipid Bilayer

The cell membrane is primarily composed of a phospholipid bilayer, which provides fluidity and flexibility.

• Hydrophilic heads: Face outward toward water.

• Hydrophobic tails: Face inward, away from water.

• Membrane proteins: Embedded within the bilayer, serving various functions.

Membrane Proteins

Proteins in the membrane perform critical roles in transport, communication, and structural support.

• Enzymes: Catalyze chemical reactions.

• Recognition proteins: Identify the cell to other cells.

• Receptor proteins: Bind to signaling molecules.

• Connection proteins: Anchor the cell membrane to the cytoskeleton or other cells.

• Transport proteins: Move substances across the membrane.

Transport Across Cell Membranes

Diffusion and Osmosis

Cells exchange materials with their environment through various transport mechanisms:

• Diffusion: Movement of molecules from an area of higher concentration to lower concentration.

• Osmosis: Diffusion of water across a semipermeable membrane.

• Passive transport: Does not require energy; includes diffusion and facilitated diffusion.

• Active transport: Requires energy (ATP) to move substances against their concentration gradient.

Osmosis Example: If a red blood cell is placed in a hypotonic solution (lower solute concentration than inside the cell), water enters the cell, which may cause it to burst. In a hypertonic solution (higher solute concentration outside), water leaves the cell, causing it to shrink.

Types of Solutions

• Isotonic: Equal solute concentration inside and outside the cell; no net water movement.

• Hypotonic: Lower solute concentration outside the cell; water enters the cell.

• Hypertonic: Higher solute concentration outside the cell; water leaves the cell.

Facilitated and Active Transport

• Facilitated diffusion: Transport proteins help move substances across the membrane without energy input.

• Active transport: Uses ATP to move substances against their concentration gradient.

Summary Table: Cell Types and Features

Key Terms and Definitions

• Phospholipid: A lipid molecule with a hydrophilic head and hydrophobic tail, forming the cell membrane bilayer.

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

• Organelle: Specialized structure within a cell that performs a specific function.

• Solute: Substance dissolved in a solvent.

• Gradient: Difference in concentration across a space.

• Osmosis: Movement of water across a semipermeable membrane.

• Endocytosis/Exocytosis: Processes by which cells take in or expel large molecules via vesicles.

Additional Info

• Additional info: The cytoskeleton includes microfilaments, intermediate filaments, and microtubules, each contributing to cell shape, movement, and division.

• Additional info: The endomembrane system coordinates the synthesis, modification, and transport of proteins and lipids.