Overview

Cell Theory

The cell theory is a fundamental concept in biology, stating that all living organisms are composed of cells, and that the cell is the basic unit of life. Cells arise only from pre-existing cells, and all functions of life occur within cells.

• Key Point 1: All organisms are made of one or more cells.

• Key Point 2: The cell is the basic structural and functional unit of life.

• Key Point 3: Cells arise from pre-existing cells by division.

• Example: Bacteria, plants, and animals all consist of cells, though their complexity varies.

Macromolecules

Types of Macromolecules

Cells contain four major types of macromolecules: proteins, nucleic acids, polysaccharides (carbohydrates), and lipids. Each macromolecule is built from specific monomers and has unique chemical properties and functions.

• Proteins: Polymers of amino acids, joined by peptide bonds. Functions include catalysis, structure, transport, and signaling.

• Nucleic Acids: Polymers of nucleotides (DNA and RNA), joined by phosphodiester bonds. Functions include genetic information storage and transfer.

• Polysaccharides: Polymers of monosaccharides (e.g., glucose), joined by glycosidic bonds. Functions include energy storage and structural support.

• Lipids: Not true polymers; composed of fatty acids and glycerol. Functions include membrane structure, energy storage, and signaling.

Monomers and Bonds

• Proteins: Monomer = amino acid; Bond = peptide bond

• Nucleic Acids: Monomer = nucleotide; Bond = phosphodiester bond

• Polysaccharides: Monomer = monosaccharide; Bond = glycosidic bond

• Lipids: Components = fatty acids, glycerol; Bond = ester bond

Protein Structure

Proteins have four levels of structure, each stabilized by specific interactions:

• Primary Structure: Linear sequence of amino acids.

• Secondary Structure: Local folding into α-helices and β-sheets, stabilized by hydrogen bonds.

• Tertiary Structure: Overall 3D shape, stabilized by hydrophobic interactions, ionic bonds, hydrogen bonds, and disulfide bridges.

• Quaternary Structure: Association of multiple polypeptide chains.

• Example: Hemoglobin is a quaternary protein composed of four polypeptide subunits.

Enzymes

Most enzymes are proteins, though some RNA molecules (ribozymes) also have catalytic activity. Enzymes accelerate biochemical reactions by lowering activation energy.

• Active Site: Region where substrate binds; composed of amino acids that may not be adjacent in the primary sequence.

• Induced-Fit Model: Substrate binding induces conformational changes in the enzyme, optimizing catalysis.

• Enzyme Inhibition: Competitive inhibitors bind the active site; noncompetitive inhibitors bind elsewhere, altering enzyme activity.

• Regulation: Enzyme activity is regulated by substrate-level control, feedback inhibition, allosteric regulation, and covalent modification (reversible or irreversible).

• Example: Feedback inhibition in glycolysis: phosphofructokinase is inhibited by ATP.

Cells and Organelles

Types of Cells

Cells are classified as prokaryotic or eukaryotic. Eukaryotic cells include plant and animal cells, while prokaryotic cells include bacteria and archaea.

• Prokaryotic Cells: Lack membrane-bound organelles; DNA is in a nucleoid region.

• Eukaryotic Cells: Contain membrane-bound organelles, including a nucleus.

• Example: Escherichia coli (prokaryote), human cell (eukaryote).

Major Differences Between Prokaryotic and Eukaryotic Cells

Major Organelles and Cell Structures

• Nucleus: Contains genetic material; site of transcription; membrane-bound.

• Mitochondrion: Site of aerobic respiration; membrane-bound.

• Chloroplast: Site of photosynthesis in plants; membrane-bound.

• Endomembrane System: Includes ER (protein/lipid synthesis), Golgi (modification/sorting), lysosome (digestion), endosome (sorting), peroxisome (detoxification), vacuole (storage).

• Ribosome: Site of protein synthesis; not membrane-bound.

• Cytoskeleton: Provides structural support and motility; not membrane-bound.

• Cell Wall/Extracellular Matrix: Structural support outside plasma membrane.

Similarities between bacterium, mitochondrion, and chloroplast: All contain DNA, ribosomes, and can divide independently; mitochondria and chloroplasts are thought to have evolved from bacteria (endosymbiotic theory).

Composition of Virus

• Viruses: Composed of genetic material (DNA or RNA) enclosed in a protein coat (capsid); some have a lipid envelope.

• Example: Influenza virus has an RNA genome and a lipid envelope.

Membranes and Membrane Transport

Biological Membranes and the Fluid Mosaic Model

Biological membranes are lipid bilayers with embedded proteins, described by the fluid mosaic model. Membranes are dynamic, with lipids and proteins able to move laterally.

• Lipids: Main types are phospholipids, glycolipids, and cholesterol. Lipid composition affects membrane fluidity, characterized by melting temperature (Tm).

• Membrane Asymmetry: Lipid and protein composition differs between inner and outer leaflets.

• Membrane Proteins: Include integral, peripheral, and lipid-anchored proteins; properties include hydrophobic regions and glycosylation.

Membrane Fluidity

• Fluidity: Determined by lipid composition (saturated vs. unsaturated fatty acids) and cholesterol content.

• Tm (Melting Temperature): Temperature at which membrane transitions from solid to fluid; higher unsaturated fatty acid content lowers Tm.

Transport Across Membranes

Cells regulate the movement of solutes across membranes using various mechanisms:

• Simple Diffusion: Passive movement of small, nonpolar molecules (e.g., O2, CO2) down their concentration gradient.

• Facilitated Diffusion: Passive movement of molecules via specific membrane proteins (channels or carriers); no energy required.

• Active Transport: Movement of molecules against their concentration gradient, requiring energy (usually ATP); involves transport proteins (pumps).

• Example: Sodium-potassium pump (Na+/K+ ATPase) actively transports Na+ out and K+ into the cell.

Additional info: Membrane transport is essential for maintaining cellular homeostasis, nutrient uptake, and waste removal.