Cells, Organelles, & Biomolecules

Introduction

This study guide covers the fundamental components of cells, including their internal structures (organelles), the cytoskeleton, and the biomolecules that make up cellular machinery. Understanding these topics is essential for grasping how cells function and interact in living organisms.

Word Roots and Scientific Vocabulary

Common Prefixes and Roots in Cell Biology

• cyto-: Refers to a cell (e.g., cytoplasm, cytoskeleton).

• extra-: Outside (e.g., extracellular).

• intra-: Inside or within (e.g., intracellular).

• inter-: Between or across (e.g., intercellular).

• endo-: Inside or interior (e.g., endoplasmic).

• exo-: Exit or outside (e.g., exocytosis).

Understanding these roots helps decode scientific terminology in biology.

Cytosol and Cytoplasm

Definitions and Components

• Cytosol: The gel-like substance within the cell membrane, excluding organelles. It contains water, dissolved ions, small molecules, and proteins.

• Cytoplasm: The entire contents within the cell membrane, including the cytosol, organelles, and the cytoskeleton.

• Cytoskeleton: A network of protein filaments that provides structural support, facilitates movement, and organizes cellular components.

The Cytoskeleton

Types of Cytoskeletal Elements

• Intermediate Filaments: Composed of various proteins; maintain cell shape and provide mechanical strength.

• Microtubules: Hollow tubes made of tubulin; serve as tracks for moving vesicles, organelles, and chromosomes within the cell.

• Microfilaments: Thin filaments made of actin; form a layer beneath the cell membrane, enabling cell movement and division.

Filament means "thread"—these structures are thread-like proteins that form the cell's internal framework.

Proteins: Structure and Function

Monomers and Polymers

• Monomer: The basic building block of proteins is the amino acid.

• Peptide Bond: The covalent bond linking amino acids together.

• Peptide: A polymer of fewer than 50 amino acids.

• Polypeptide: A polymer of more than 50 amino acids.

• Protein: One or more polypeptides folded into a functional shape.

Protein Shapes and Functions

• Globular Proteins: Spherical, soluble proteins that function as enzymes (e.g., Na+-K+ ATPase), hormones (e.g., insulin), and membrane channels.

• Fibrous Proteins: Long, insoluble proteins providing structural support (e.g., collagen, actin, myosin, keratin).

Levels of Protein Structure

Primary Structure

• The unique sequence of amino acids in a polypeptide chain.

• Example: gly-pro-ser-asp-phe-val-tyr-cys

Secondary Structure

• Local folding of the polypeptide chain into structures stabilized by hydrogen bonds.

• Alpha (α) Helix: A coiled structure stabilized by hydrogen bonds.

• Beta (β) Pleated Sheet: A series of folds in the same plane, resembling an accordion.

• Hydrogen bonds form between the carbonyl oxygen of one amino acid and the amino hydrogen of another.

Tertiary Structure

• The overall three-dimensional shape of a single polypeptide chain.

• Stabilized by interactions among side chains (R groups):

  • Hydrogen bonds

  • Ionic bonds

  • Hydrophobic interactions and van der Waals forces

  • Disulfide bridges (covalent bonds between cysteine residues)

Quaternary Structure

• Structure formed by two or more polypeptide chains (subunits) assembling into a functional protein.

• Examples: insulin, collagen, hemoglobin.

Key Chemical Bonds in Proteins

Types of Bonds

• Peptide Bond: Covalent bond linking amino acids in the primary structure.

• Hydrogen Bond: Important in secondary and tertiary structures.

• Ionic Bond: Attraction between oppositely charged side chains.

• Disulfide Bridge: Covalent bond between sulfur atoms of two cysteine residues; stabilizes tertiary and quaternary structures.

Disulfide Bridge Representation

• The correct structural formula for a disulfide bridge is: S—S—

Summary Table: Levels of Protein Structure

Example: Protein Structure in Action

• Hemoglobin: A quaternary protein composed of four polypeptide subunits, each with its own tertiary structure, working together to transport oxygen in the blood.

• Collagen: A fibrous protein with a triple-helix quaternary structure, providing strength to connective tissues.

Additional info:

• Understanding protein structure is crucial for studying enzyme function, cell signaling, and genetic diseases.

• Protein misfolding can lead to diseases such as Alzheimer's and cystic fibrosis.