Cell Structure and Function

Introduction

Cells are the fundamental units of life, classified into two main types: prokaryotic and eukaryotic cells. Understanding their structures and functions is essential for studying biology at the cellular level.

Prokaryotic Cells

General Features

• Prokaryotic cells lack a membrane-bound nucleus and most other organelles.

• They are generally smaller and simpler than eukaryotic cells.

• Bacteria and Archaea are the two domains of prokaryotes.

Structure of a Bacterial Cell

• Cell wall: Provides structural support and shape.

• Plasma membrane: Controls the movement of substances in and out of the cell.

• Cytoplasm: Gel-like substance where cellular processes occur.

• Ribosomes: Sites of protein synthesis.

• Nucleoid: Region containing the cell's circular DNA.

• Plasmid DNA: Small, circular DNA molecules separate from chromosomal DNA; often carry beneficial genes.

• Flagellum: Tail-like structure used for movement.

• Pili: Hair-like structures for attachment and conjugation.

• Capsule: Outer protective layer (not always present).

Example:

Escherichia coli is a common prokaryotic bacterium found in the intestines of animals.

Eukaryotic Cells

General Features

• Eukaryotic cells have a true nucleus enclosed by a nuclear membrane.

• They contain membrane-bound organelles, such as mitochondria, endoplasmic reticulum, and Golgi apparatus.

• Examples include animal, plant, fungal, and protist cells.

Structure of a Typical Eukaryotic Cell

• Nucleus: Contains genetic material (DNA) and controls cellular activities.

• Nucleolus: Site of ribosome synthesis within the nucleus.

• Ribosomes: Synthesize proteins; found free in cytoplasm or attached to rough endoplasmic reticulum (RER).

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes; involved in protein synthesis and modification.

• Smooth Endoplasmic Reticulum (SER): Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

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

• Mitochondria: Sites of cellular respiration and energy (ATP) production.

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

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Cytoskeleton: Network of protein filaments (microtubules, microfilaments, intermediate filaments) that provide structural support and facilitate movement.

• Centrioles: Involved in cell division in animal cells.

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

• Chloroplasts (in plants and some protists): Sites of photosynthesis.

• Cell wall (in plants, fungi, and some protists): Provides structural support and protection.

Example:

Plant cells contain chloroplasts and a large central vacuole, while animal cells do not.

Comparison of Prokaryotic and Eukaryotic Cells

Cell Organelles and Their Functions

Key Organelles

• Ribosomes: Protein synthesis

• Mitochondria: ATP production via cellular respiration

• Chloroplasts: Photosynthesis (in plants and algae)

• Golgi Apparatus: Processing and packaging of proteins and lipids

• Lysosomes: Digestion of macromolecules

• Endoplasmic Reticulum: RER for protein synthesis; SER for lipid synthesis and detoxification

• Peroxisomes: Breakdown of fatty acids and detoxification

• Cytoskeleton: Structural support, intracellular transport, and cell movement

Organelle Functions Table

Cellular Processes

Cellular Respiration

• Occurs in mitochondria.

• Converts glucose and oxygen into ATP, carbon dioxide, and water.

Equation:

Photosynthesis

• Occurs in chloroplasts of plants and algae.

• Converts light energy, carbon dioxide, and water into glucose and oxygen.

Equation:

Cell Movement Structures

Cilia and Flagella

• Cilia: Short, numerous, hair-like structures for movement or moving substances along the cell surface.

• Flagella: Longer, usually fewer, whip-like structures for cell movement.

• Both are composed of microtubules arranged in a "9+2" pattern.

Microscopy and Cell Observation

• Resolving power of a microscope is the ability to distinguish two close points as separate entities.

• Cells are generally too small to be seen without a microscope due to their minute size (micrometers in diameter).

Endosymbiotic Theory

• Proposes that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence includes their own circular DNA, double membranes, and similarities to certain bacteria.

Example:

Chloroplasts resemble cyanobacteria, and mitochondria resemble aerobic bacteria in structure and genetics.

Summary Table: Prokaryotic vs. Eukaryotic Cell Features

Additional info: Some context and explanations have been expanded for clarity and completeness, including the summary tables and equations for cellular respiration and photosynthesis.