Cellular Basis of Life: Structure and Function of Cells

Topic 3: Cellular Basis of Life

Introduction to Cells

The cell is the fundamental structural and functional unit of life, as described by Cell Theory. All living organisms are composed of cells, which carry out essential life processes. Cells vary in size, structure, and function, but share several key characteristics.

• Cell Theory:

  • All living things are composed of cells.

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

  • All cells arise from pre-existing cells.

• Basic Characteristics of All Cells:

  • Plasma membrane: Encloses the cell, controlling movement of substances in and out.

  • Cytoplasm: Semi-fluid substance inside the cell.

  • Chromosomes: Carry genetic information (DNA).

  • Ribosomes: Sites of protein synthesis.

Types of Cells

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

Prokaryotic Cells

• Characteristics:

  • No nucleus; DNA is located in an unbound region called the nucleoid.

  • No membrane-bound organelles.

  • Cytoplasm bound by the plasma membrane.

• Examples: Bacteria, Archaea

Eukaryotic Cells

• Characteristics:

  • DNA is contained within a nucleus bounded by a nuclear envelope.

  • Membrane-bound organelles present.

  • Cytoplasm is the region between the plasma membrane and nucleus.

  • Generally larger than prokaryotic cells.

• Examples: Animal cells, Plant cells, Fungal cells

Structures of the Eukaryotic Cell

Structures of the Cell Surface

• Plasma Membrane

  • Structure: Phospholipid bilayer with embedded proteins and carbohydrates.

  • Function: Separates intracellular fluid from extracellular fluid; controls entry and exit of substances.

• Cell Wall (plants and fungi)

  • Structure: Polysaccharide or protein matrix (cellulose in plants, chitin in fungi).

  • Function: Protection and structural support.

• Cilia and Flagella (mostly in animals)

  • Structure: Protein filaments (microtubules) arranged in specific patterns.

  • Function: Movement of the cell or movement of substances past the cell.

• Cell Junctions

  • Tight junctions: Form impermeable connections between adjacent cells.

  • Desmosomes: Anchor cells together, providing mechanical strength.

  • Gap junctions: Channels for communication and exchange of ions and molecules between adjacent cells.

  • Plasmodesmata (plants): Channels connecting cytoplasm of adjacent plant cells.

Structures of the Nucleus

• Nuclear Envelope: Double-layered membrane surrounding the nucleus.

• Chromosomes: DNA organized into structures that carry genetic information.

• Nucleolus: Site of ribosome synthesis.

• Transcription: Process of copying genetic information from DNA to RNA.

Structures of the Cytoplasm

• Cytosol: Watery solution of dissolved proteins, salts, sugars, and other solutes.

• Cytoplasmic Matrix: Structures organized within the cell that perform specific functions.

• Ribosomes: Sites of protein synthesis; can be free in cytosol or bound to endoplasmic reticulum.

Endoplasmic Reticulum (ER)

• Rough ER:

  • Structure: Network of branching membranous sacs with ribosomes attached.

  • Function: Synthesis of proteins for secretion or membrane insertion.

• Smooth ER:

  • Structure: Network of branching membranous sacs without ribosomes.

  • Function: Synthesis of lipids, metabolism of carbohydrates, detoxification of drugs and poisons.

Golgi Apparatus

• Structure: Flattened membranous sacs (cisternae).

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

Vesicles

• Structure: Membranous sacs.

• Function: Transport of materials within the cell (exocytosis, endocytosis, phagocytosis, pinocytosis, vesicular trafficking).

Vacuoles

• Structure: Large membranous sacs similar to vesicles.

• Function: Storage of substances (food, water, waste, organic compounds).

Lysosomes

• Structure: Membranous sacs containing hydrolytic enzymes.

• Function: Digestion of macromolecules, recycling of cellular components.

Peroxisomes

• Structure: Membranous sacs containing enzymes.

• Function: Breakdown of fatty acids and detoxification of harmful substances.

Mitochondria

• Structure: Double-membrane organelle containing its own DNA and RNA.

• Function: Synthesis of ATP via aerobic cellular respiration. Equation: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

Chloroplasts (in plants and algae)

• Structure: Double-membrane organelle containing its own DNA and RNA; contains thylakoids and stroma.

• Function: Production of ATP and sugars via photosynthesis. Equation: $6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Cytoskeleton

• Structure: System of protein fibers including microtubules, microfilaments, and intermediate filaments.

• Functions:

  • Gives the cell shape and structural stability.

  • Cell movement.

  • Movement of materials within the cell.

Comparison Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent (DNA in nucleoid)	Present (DNA in nucleus)
Membrane-bound organelles	Absent	Present
Cell size	Generally smaller	Generally larger
Examples	Bacteria, Archaea	Animals, Plants, Fungi, Protists

Example: Animal Cell vs. Plant Cell

• Animal Cell: Contains lysosomes, centrioles, and lacks a cell wall and chloroplasts.

• Plant Cell: Contains cell wall, chloroplasts, large central vacuole, and lacks centrioles and lysosomes.

Additional info: The notes have been expanded to include definitions, examples, and equations for cellular respiration and photosynthesis, as well as a comparison table for prokaryotic and eukaryotic cells for clarity and completeness.