Cell Structure and Function: Prokaryotic and Eukaryotic Cells

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

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	No true nucleus; nucleoid region	True nucleus with nuclear envelope
Organelles	No membrane-bound organelles	Membrane-bound organelles present
Size	Generally smaller (1-10 μm)	Generally larger (10-100 μm)
DNA	Circular DNA	Linear DNA in chromosomes
Examples	Bacteria, Archaea	Animals, plants, fungi, protists

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

Organelle	Main Function
Ribosomes	Protein synthesis
Rough Endoplasmic Reticulum	Protein modification and transport
Smooth Endoplasmic Reticulum	Lipid synthesis and detoxification
Golgi Apparatus	Processing and packaging of proteins/lipids
Lysosomes	Digestion of cellular waste
Mitochondria	ATP production
Chloroplasts	Photosynthesis

Cellular Processes

Cellular Respiration

• Occurs in mitochondria.

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

Equation:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP}$

Photosynthesis

• Occurs in chloroplasts of plants and algae.

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

Equation:

$6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$

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

Feature	Prokaryotic	Eukaryotic	Both
Membrane-bound nucleus	No	Yes
DNA	Yes (circular)	Yes (linear)	Yes
Ribosomes	Yes (smaller)	Yes (larger)	Yes
Cell wall	Yes (most)	Some (plants, fungi)
Organelles	No	Yes

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