Cell Structure and Function: Prokaryotic and Eukaryotic Cells

Cell Theory and Basic Cell Features

Introduction to Cell Theory

The cell theory is a fundamental concept in biology that describes the properties of cells, the basic unit of life. It provides the foundation for understanding the structure and function of all living organisms.

• All living organisms are made of cells: Every organism, from bacteria to plants and animals, is composed of one or more cells.

• Cell is the basic unit of life: Cells carry out all essential life processes.

• All cells arise from pre-existing cells: New cells are produced by the division of existing cells (omnis cellula e cellula).

Basic Features Shared by All Cells

• Plasma membrane: A selective barrier that surrounds the cell, controlling the movement of substances in and out.

• Cytosol: The semifluid portion of the cytoplasm where cellular processes occur.

• Chromosomes: Structures that carry genetic information in the form of DNA.

• Ribosomes: Complexes that synthesize proteins by translating genetic information.

Prokaryotic vs. Eukaryotic Cells

Prokaryotic Cells

Prokaryotic cells are simpler and smaller than eukaryotic cells. They lack membrane-bound organelles and a nucleus.

• No membrane-bound nucleus: Genetic material is located in the nucleoid region.

• No membrane-bound organelles: Cellular functions occur in the cytoplasm or at the plasma membrane.

• Generally smaller in size: Typical diameter is 0.1–5 μm.

• Domains: Bacteria and Archaea.

Example: Cyanobacterium (TEM image shows nucleoid, ribosomes, plasma membrane, and cell wall).

Eukaryotic Cells

Eukaryotic cells are more complex, with internal compartments and specialized functions.

• Membrane-bound nucleus: Contains genetic material organized into chromosomes.

• Membrane-bound organelles: Includes mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, etc.

• Generally larger in size: Typical diameter is 10–100 μm.

• Cytoskeleton: Network of fibers for support and movement.

• Domain: Eukarya (protists, fungi, plants, animals).

Example: Human, yeast, and plant cells (TEM images show nucleus, organelles, and cytoskeleton).

Cellular Organelles and Their Functions

Ribosomes

Ribosomes are the site of protein synthesis in all cells. They are composed of rRNA and protein and are not membrane-bound.

• Free ribosomes: Located in the cytosol; synthesize proteins for use within the cell.

• Bound ribosomes: Attached to the endoplasmic reticulum (ER); synthesize proteins for secretion or for use in membranes.

Structure: Large and small subunits assembled in the nucleolus.

Endoplasmic Reticulum (ER)

The ER is a network of membranous tubules and sacs involved in biosynthesis and transport.

• Rough ER: Studded with ribosomes; synthesizes proteins and adds carbohydrates to proteins to make glycoproteins.

• Smooth ER: Lacks ribosomes; involved in lipid synthesis, calcium storage, carbohydrate metabolism, and detoxification of drugs and poisons.

• Cisternae: Flattened membrane sacs within the ER.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Functions: Protein modification, polysaccharide synthesis, sorting and packaging into vesicles.

• Structure: Flattened stacks of membranous sacs (cisternae) with two faces: cis (receiving) and trans (shipping).

Lysosomes

Lysosomes are digestive compartments containing hydrolytic enzymes.

• Function: Breakdown of macromolecules, phagocytosis, and autophagy (recycling cellular components).

Peroxisomes

Peroxisomes are specialized metabolic compartments that break down fatty acids and detoxify harmful substances.

• Functions: Breakdown of fatty acids, detoxification of alcohol in the liver.

• Enzymes: Remove hydrogen from substrates and transfer it to oxygen, producing hydrogen peroxide ($2RH + 2H_2O_2 \rightarrow 2H_2O + O_2$).

Mitochondria

Mitochondria are the powerhouse of the cell, responsible for cellular respiration and ATP synthesis.

• Structure: Double membrane (outer and inner), with inner membrane folded into cristae.

• Matrix: Contains enzymes, mitochondrial DNA (mtDNA), and ribosomes.

• Function: Cellular respiration ($C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ATP$).

Chloroplasts

Chloroplasts are organelles found in plants and algae that capture light energy for photosynthesis.

• Structure: Inner and outer membranes, thylakoid membranes containing chlorophyll.

• Function: Photosynthesis ($6CO_2 + 6H_2O + light \rightarrow C_6H_{12}O_6 + 6O_2$).

Cytoskeleton

Functions and Structure

The cytoskeleton is a dynamic network of fibers essential for cellular function, structure, and movement.

• Organizes cell activities: Provides structural support and maintains cell shape.

• Motility: Enables movement of the cell and its organelles.

• Anchors organelles: Positions organelles within the cytoplasm.

• Mechanical support: Resists mechanical stress and deformation.

• Transport: Facilitates movement of materials within the cell.

Components: Microtubules, microfilaments, and intermediate filaments.

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent (nucleoid region)	Present (membrane-bound)
Organelles	Absent	Present (mitochondria, ER, Golgi, etc.)
Size	Smaller (0.1–5 μm)	Larger (10–100 μm)
Domains	Bacteria, Archaea	Eukarya

Additional info: The notes have been expanded to include definitions, examples, and equations for key processes such as cellular respiration and photosynthesis. The table provides a clear comparison between prokaryotic and eukaryotic cells.