Cell Structure and Domains of Life: Prokaryotes vs. Eukaryotes

Cell Structure and Domains of Life

Introduction

Cells are the fundamental units of life, capable of replicating and passing on genetic information. All organisms possess genes that instruct cellular form and function. Evolutionary biology explains how diverse organisms share a common ancestry, and cells are classified based on their structural characteristics.

Domains of Life

Classification Based on Cell Type

• Prokaryotic Cells: Include Bacteria and Archaea

• Eukaryotic Cells: Include Eukarya (plants, animals, fungi, protists)

Common Ancestry

All living cells evolved from a single ancestral cell. The three domains of life are:

• Bacteria: Small, single-celled, lack nucleus, circular chromosome, DNA is circular and found in the nucleoid.

• Archaea: Single-celled, contain cell wall, circular chromosome, unique membrane lipids, extremophiles.

• Eukarya: Protists, fungi, plants, animals; single or multicellular; have linear chromosomes and a nucleus.

Cellular Features

General Features Shared by All Cells

• Plasma membrane encloses the cell

• Contain cytoplasm

• Chromosomes with DNA that carry genes

• Ribosomes to make proteins

• Cytoskeleton (in most cells)

Specific Features of Prokaryotes

• No nucleus

• Hardly any membrane-enclosed organelles

• Single, circular chromosome

• Simple, small structure

• Most have cell wall and flagella for movement

Specific Features of Eukaryotes

• Have a nucleus

• Numerous membrane-bound organelles

• Complex, larger cells

• Multiple linear chromosomes

• DNA is linear and found inside the nucleus

• Have mitochondria, cytoskeleton, and divide by mitosis and cytokinesis

Similarities and Differences Between Prokaryotic and Eukaryotic Cells

Similarities

• Both have plasma membrane, chromosomes, ribosomes, cytoskeleton, and cytoplasm

Differences

• Eukaryotes are larger

• Eukaryotes have a nucleus and a greater diversity of membrane-bound organelles

Membrane-Bound Organelles in Eukaryotic Cells

Nucleus

• Information store of the cell

• Contains DNA and nucleolus (site of ribosome assembly)

• Nuclear membrane regulates exchange of molecules

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes, involved in protein synthesis and packaging

• Smooth ER: Lacks ribosomes, produces lipids, breaks down toxins

Golgi Apparatus

• Modifies, sorts, and ships products from ER

• Uses "bar codes" for compartmentalization

Lysosomes

• Digestive organelles containing enzymes

• Break down waste and cellular debris

Peroxisomes

• Break down lipids and toxins using hydrogen peroxide

Vesicles

• Transport materials within the cell

• Move products from Golgi apparatus to other organelles or plasma membrane

Ribosomes

Structure and Function

• Made of proteins and RNA molecules

• Consist of large and small subunits

• Present in both prokaryotic and eukaryotic cells

• Not membrane-bound

• Facilitate protein synthesis

The Cytoskeleton

Components

• Actin microfilaments: Provide cell shape and movement

• Microtubules: Involved in cell division and transport

• Intermediate filaments: Provide structural support

Cellular Chemistry and Bonding

Atoms and Elements

• Atoms are the smallest units of matter

• Elements are pure substances made of one type of atom

• Atomic number = number of protons

• Electron orbitals and energy shells determine chemical behavior

• Valence electrons are found in the outermost shell and participate in chemical bonding

Chemical Bonds

• Covalent bonds: Atoms share electrons to form molecules

• Nonpolar covalent bonds: Electrons are shared equally

• Polar covalent bonds: Electrons are shared unequally, creating partial charges

• Ionic bonds: Attraction between oppositely charged ions (anions and cations)

• Hydrogen bonds: Weak interactions between a hydrogen atom and a highly electronegative atom (O, N, F)

Electronegativity

• Measure of an atom's ability to attract electrons

• Difference in electronegativity determines bond polarity

Hydrophilic vs. Hydrophobic Molecules

• Hydrophilic: Substances that dissolve in water; usually polar or charged

• Hydrophobic: Substances that do not dissolve in water; usually nonpolar

Table: Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Chromosomes	Single, circular	Multiple, linear
Membrane-bound organelles	Absent	Present
Cell size	Small	Larger
Cell wall	Present (most)	Present (plants, fungi)
Division	Binary fission	Mitosis and cytokinesis

Key Equations

• Atomic Number: $\text{Atomic Number} = \text{Number of Protons}$

• Octet Rule: Atoms are most stable when their valence shell is filled with 8 electrons.

Example: Water Molecule

• Water ($H_2O$) is formed by polar covalent bonds between hydrogen and oxygen.

• Oxygen is more electronegative, creating partial negative and positive charges.

• This allows water to dissolve hydrophilic substances and form hydrogen bonds.

Additional info: Academic context and definitions have been expanded for clarity and completeness.