Cell Structure and Function: Prokaryotes vs. Eukaryotes, Organelles, and Genetic Material

Cell Components

Three Domains of Life

All living organisms are classified into three domains based on cellular structure and genetic characteristics:

• Bacteria: Prokaryotic cells

• Archaea: Prokaryotic cells

• Eukarya: Eukaryotic cells, including:

  • Kingdom Plantae: Obtain nutrients by photosynthesis

  • Kingdom Fungi: Absorb nutrients from surroundings

  • Kingdom Animalia: Ingest food

  • Protists: Unicellular eukaryotes

Key Definitions:

• Eukaryotic cells: Contain a nucleus and membrane-bound organelles (e.g., animals, plants, fungi, protists).

• Prokaryotic cells: Lack a nucleus and most organelles (e.g., bacteria, archaea).

Examples:

• Yeast: Unicellular eukaryote

• Bacteria: Prokaryote

• Animals/plants: Multicellular eukaryotes

Cell Structure: Prokaryotes vs. Eukaryotes

All cells share certain features, but differ in complexity and organization.

• Plasma membrane: Selective barrier surrounding all cells

• Cytoplasm: Interior of the cell, containing cytosol and organelles

• Chromosomes: Carry genetic information (DNA)

• Ribosomes: Sites of protein synthesis

Property	Prokaryote	Eukaryote
Location of DNA	Nucleoid (not membrane-bound)	Nucleus (membrane-bound)
Organelles	Few or none	Many, membrane-bound
Size	Smaller	Much larger

Surface Area to Volume Ratio: As a cell grows, its surface area increases less rapidly than its volume, limiting cell size.

Major Eukaryotic Organelles and Their Functions

• Cytoskeleton: Framework of the cell, provides structure and facilitates movement.

• Mitochondria: Site of ATP (energy) production.

• Nucleus: Contains most of the cell's DNA; site of transcription (DNA to mRNA).

• Ribosomes: Synthesize proteins by translating mRNA.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins for membranes or secretion.

  • Smooth ER: Synthesizes lipids, detoxifies drugs, stores calcium ions.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for storage or transport.

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

• Chloroplasts (plants and algae): Site of photosynthesis.

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

Genetic Material in Eukaryotic Cells

Genes are located in:

• Nucleus: Main repository of genetic information.

• Mitochondria: Contain their own DNA, inherited maternally.

• Chloroplasts: Also contain their own DNA (in plants and algae).

Chromatin: DNA and proteins (histones) that package DNA into chromosomes.

Nucleolus: Region within the nucleus where ribosomal RNA is synthesized and ribosome assembly begins.

Protein Synthesis and Trafficking

• Ribosomes translate mRNA into polypeptide chains (proteins).

• Proteins made on free ribosomes function in the cytosol; those made on bound ribosomes (attached to rough ER) are destined for membranes, secretion, or organelles.

• Polypeptides enter the ER, fold into functional shapes, and are transported to the Golgi apparatus for further modification and sorting.

• Vesicles transport proteins to their final destinations (e.g., plasma membrane, lysosomes).

Prokaryotic Cell Features

• Lack membrane-bound organelles.

• DNA is located in the nucleoid region.

• Cell wall provides structural support.

• Some have specialized structures such as flagella (for movement) and pili (for attachment).

Summary Table: Eukaryotic vs. Prokaryotic Cells

Feature	Eukaryotic Cell	Prokaryotic Cell
Nucleus	Present	Absent
Membrane-bound organelles	Present	Absent
DNA location	Nucleus, mitochondria, chloroplasts	Nucleoid
Cell wall	Plants, fungi, some protists	Most
Size	Larger (10–100 μm)	Smaller (0.1–5 μm)

Additional info:

• Histones are proteins that help package DNA into chromatin in eukaryotes.

• Prokaryotic DNA is generally circular, while eukaryotic DNA is linear and organized into chromosomes.

• Both cell types carry out essential life processes such as metabolism, growth, and reproduction.