Cell Structure and Function: Foundations of General Biology

Cell Structure & Function

Introduction to Cell Structure and Function

Understanding the structure and function of cells is fundamental to the study of biology. Cells are the basic units of life, and their organization, components, and processes form the foundation for all living organisms.

Atomic Structure and Elements in Biology

Atomic Number, Electrons, and Protons

Atoms are the smallest units of matter that retain the properties of an element. The structure of an atom is crucial for understanding how biological molecules interact.

• Atomic Number: The number of protons in the nucleus of an atom, which defines the element.

• Electrons: Negatively charged particles that orbit the nucleus in energy levels (shells).

• Neutrons: Neutral particles in the nucleus; the number of neutrons can vary, resulting in isotopes.

• Valence Electrons: Electrons in the outermost shell, important for chemical bonding.

Example: For an atom with 8 total electrons (such as oxygen):

• Number of protons = 8 (since atoms are electrically neutral)

• Number of neutrons (non-isotope) = 8 (for oxygen-16)

• Atomic number = 8

• Valence electrons = 6 (since the first shell holds 2, the second holds up to 8; oxygen has 6 in the second shell)

Key Formula:

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

$\text{Number of Neutrons} = \text{Mass Number} - \text{Atomic Number}$

Chemical Reactions in Biology

Types of Reactions

Chemical reactions are essential for life, enabling the synthesis and breakdown of biological molecules.

• Dehydration Synthesis: A reaction where two molecules are joined by removing a water molecule. This process forms polymers such as proteins, nucleic acids, and polysaccharides.

• Hydrolysis: A reaction where a molecule is split into two by the addition of water. This process breaks down polymers into monomers.

Example:

• Dehydration synthesis of a polypeptide: Amino acids are joined to form a protein, releasing water.

• Hydrolysis of a polypeptide: A protein is broken down into amino acids by adding water.

General Equation for Dehydration Synthesis:

$\text{Monomer}_1 + \text{Monomer}_2 \rightarrow \text{Polymer} + \text{H}_2\text{O}$

General Equation for Hydrolysis:

$\text{Polymer} + \text{H}_2\text{O} \rightarrow \text{Monomer}_1 + \text{Monomer}_2$

Cell Theory

Foundations of Cell Theory

The cell theory is a fundamental concept in biology, describing the properties of cells.

• All living organisms are composed of one or more cells.

• The cell is the basic unit of structure and function in living things.

• All cells arise from pre-existing cells.

Historical Note: The cell theory was proposed by Matthias Schleiden and Theodor Schwann in the 19th century.

Classification of Cells: Prokaryotes and Eukaryotes

Prokaryotic vs. Eukaryotic Cells

Cells are classified into two main types based on their structural characteristics: prokaryotic and eukaryotic.

• Prokaryotic Cells: Simpler cells without a nucleus or membrane-bound organelles. Includes Bacteria and Archaea.

• Eukaryotic Cells: More complex cells with a nucleus and membrane-bound organelles. Includes plants, animals, fungi, and protists.

Key Differences:

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Membrane-bound Organelles	Absent	Present
DNA Location	Nucleoid region	Nucleus
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Prokaryotic Cell Structure

Components of Prokaryotic Cells

Prokaryotic cells have a simpler structure but share several features with eukaryotic cells.

• Plasma Membrane: A phospholipid bilayer that separates the cell from its environment.

• Cytoplasm: Jelly-like substance where cellular components are suspended.

• DNA: Genetic material located in the nucleoid region.

• Ribosomes: Sites of protein synthesis, composed of RNA and proteins. Not membrane-bound.

• Cytoskeleton: Provides structural support (less complex than in eukaryotes).

Additional info: Some prokaryotes have cell walls, capsules, and flagella for movement.

Archaea: Unique Prokaryotes

Characteristics of Archaea

Archaea are a domain of prokaryotic organisms with unique features distinguishing them from bacteria and eukaryotes.

• Many are extremophiles, living in extreme environments (e.g., high temperature, salinity, or acidity).

• Some are methanogenic, producing methane as a metabolic byproduct.

• Archaea have genes and metabolic pathways that are distinct from bacteria and share similarities with eukaryotes, especially in information processing and protein synthesis.

• They possess unique membrane lipids and cell wall components.

Additional info: Archaea play important roles in global nutrient cycles and biotechnology.

Ribosomes: Protein Synthesis Machinery

Structure and Function of Ribosomes

Ribosomes are essential organelles found in all cells, responsible for synthesizing proteins by translating messenger RNA (mRNA).

• Composed of ribosomal RNA (rRNA) and proteins.

• In prokaryotes, ribosomes are found free in the cytoplasm.

• In eukaryotes, ribosomes can be free or attached to the endoplasmic reticulum.

• Ribosomes are not surrounded by a membrane and are not considered membrane-bound organelles.

Key Formula:

$\text{Protein Synthesis:} \ \text{mRNA} \rightarrow \text{Polypeptide (Protein)}$

Summary Table: Key Features of Cell Types

Feature	Prokaryotes	Eukaryotes
Nucleus	No	Yes
Membrane-bound Organelles	No	Yes
Cell Wall	Yes (most)	Some (plants, fungi)
Ribosomes	Yes (70S)	Yes (80S)
DNA Form	Circular	Linear
Examples	Bacteria, Archaea	Animals, Plants, Fungi, Protists