Cell Structure, Function, and Diversity: General Biology Study Notes

Cell Structure, Function, and Diversity

Evolution and Cell Theory

Understanding the principles of evolution and cell theory is foundational to biology. These concepts explain how life changes over time and the basic unit of all living organisms.

• Evolution: The change in characteristics of a population over generations, driven by natural selection.

• Cell Theory: States that all living things are composed of one or more cells, and cells are the basic unit of life. All cells arise from pre-existing cells.

• Phylogenetics: The study of evolutionary relationships among species.

Macromolecules in Cells

Cells are composed of four major types of macromolecules, each with distinct functions and properties.

• Carbohydrates: Provide energy and structural support.

• Proteins: Perform a wide range of functions including catalysis, transport, and structural roles.

• Lipids: Form membranes and store energy.

• Nucleic acids: Store and transmit genetic information (DNA and RNA).

Chemical Bonds in Biological Molecules

Chemical bonds determine the structure and function of biological molecules.

• Nonpolar covalent bonds: Atoms share electrons equally; no charge (e.g., hydrogen and methane).

• Polar covalent bonds: Atoms share electrons unequally; partial charges (e.g., ammonia and water).

• Ionic bonds: Atoms have full charges due to electron transfer (e.g., sodium chloride).

Protein Structure

Proteins are polymers of amino acids and have complex structures that determine their function.

• Primary structure: Linear sequence of amino acids determined by the gene (DNA sequence). $\text{No hydrogen bonds}$

• Secondary structure: Folding due to hydrogen bonding between backbone atoms; includes $\alpha$-helix and $\beta$-pleated sheet.

• Tertiary structure: 3D shape stabilized by interactions among side chains; determines protein function.

• Quaternary structure: Multiple polypeptide chains assemble into a functional protein.

Energy and Metabolism

All cells require energy and carbon sources to survive and build macromolecules. ATP is the universal energy currency.

• ATP: Adenosine triphosphate, the main energy carrier in cells.

• Cells use carbon and energy: To build macromolecules and fuel cellular processes.

Energy Sources in Cells

Type	Energy Source	Carbon Source	Examples
Chemolithotroph	Inorganic molecules	CO2	Bacteria, archaea
Photoautotroph	Light	CO2	Bacteria, plants, protists
Chemoheterotroph	Organic molecules	Organic molecules	Bacteria, archaea, fungi, animals
Photoheterotroph	Light	Organic molecules	Bacteria

Three Domains of Life

Life is classified into three domains based on cellular characteristics.

• Bacteria: Prokaryotic, unicellular, no nucleus.

• Archaea: Prokaryotic, unicellular, no nucleus, unique membrane lipids.

• Eukarya: Eukaryotic, cells have a membrane-bound nucleus and organelles.

Common Components & Characteristics of All Cells

Despite diversity, all cells share certain structural components.

• DNA: Genetic material.

• Ribosomes: Site of protein synthesis.

• Plasma membrane: Selective barrier.

• Cytoplasm: Internal fluid containing organelles and molecules.

• Proteins: Carry out cellular functions.

Metabolic Diversity in Bacteria & Archaea

Bacteria and archaea exhibit diverse metabolic pathways to obtain energy and carbon.

• Phototrophs: Use light as energy source.

• Chemotrophs: Use chemicals as energy source.

• Autotrophs: Use CO2 as carbon source.

• Heterotrophs: Use organic molecules as carbon source.

• Archaea: Often survive in extreme environments with unique metabolic pathways.

Structure of Bacterial and Archaeal Cells

Bacteria and archaea have distinct cell structures.

• Bacteria: Cell wall (peptidoglycan), plasma membrane, cytoplasm, circular chromosome, plasmids, ribosomes.

• Archaea: Cell wall (no peptidoglycan), unique lipids, plasma membrane, cytoplasm, circular chromosome, plasmids, ribosomes.

Eukaryotic Cell & Organelles

Eukaryotic cells contain membrane-bound organelles that compartmentalize functions.

• Nucleus: Stores DNA, regulates transport via nuclear pores.

• Endomembrane system: RER → Golgi → lysosomes/plasma membrane; involved in protein/lipid transport.

• Ribosomes: Protein synthesis (free or RER-attached).

• Lysosomes: Digest macromolecules.

• Mitochondria: ATP production.

• Chloroplasts: Photosynthesis in plants.

Transport Into/Out of Nucleus

Transport through the nuclear envelope is highly regulated.

• Regulation: Only molecules with nuclear localization signals can pass through nuclear pores.

Plant vs Animal Cells

Plant and animal cells have key structural differences.

• Plant cells: Cell wall, chloroplasts, large central vacuole.

• Animal cells: No cell wall, no chloroplasts, smaller vacuoles.

Protein Targeting to RER

Proteins with specific signal sequences are directed to the rough endoplasmic reticulum (RER) for synthesis and folding.

• Signal sequences: Direct proteins to RER for proper folding and processing.

Cytoskeleton & Motor Proteins

The cytoskeleton provides structural support and enables movement within cells.

• Microfilaments (actin filaments): Maintain shape, cell movement, division.

• Intermediate filaments: Provide structural support, anchor organelles.

• Microtubules: Maintain shape, resist compression, facilitate organelle/vesicle transport, chromosome movement.

Motor Proteins

• Myosin: Moves along actin filaments (e.g., cytokinesis, cytoplasmic streaming).

• Kinesin: Moves cargo along microtubules toward plus-end (e.g., Golgi → plasma membrane).

• Dynein: Moves cargo along microtubules toward minus-end (e.g., plasma membrane → endosomes); also powers cilia/flagella movement.

Cilia & Flagella

• Made of microtubules in a 9+2 arrangement.

• Dynein arms "walk" along microtubules, causing bending and movement.

Origin of Eukaryotic Cells

The origin of eukaryotic cells is explained by the endomembrane system and endosymbiotic theory.

• Endomembrane system: Formed by invagination of the plasma membrane.

• Endosymbiotic theory: Mitochondria and chloroplasts originated from prokaryotes; evidence includes double membranes and their own DNA.

Summary Table: Prokaryotic vs Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Organelles	Absent	Present
Cell Wall	Present (peptidoglycan in bacteria)	Present in plants/fungi (cellulose/chitin)
Size	Smaller	Larger

Additional info: Some details were expanded for clarity and completeness, including definitions, examples, and summary tables for comparison.