Biological Organization and Classification

Domains of Life

The three domains of life represent the broadest classification of living organisms based on cellular structure and genetics.

• Bacteria: Prokaryotic, unicellular organisms without a nucleus.

• Archaea: Prokaryotic, unicellular organisms, often found in extreme environments; genetically distinct from bacteria.

• Eukarya: Organisms with eukaryotic cells, including animals, plants, fungi, and protists.

Hierarchical Organization of Life

Life is organized in a hierarchical fashion from the simplest to the most complex levels.

• Hierarchy (from least to most inclusive):

  • molecule

  • organelle

  • cell

  • tissue

  • organ

  • organ system

  • organism

  • population

  • community

  • ecosystem

Chemistry of Life

Organic Compounds

Organic compounds are molecules primarily composed of carbon atoms bonded with other elements.

• Key Properties:

  • Synthesized by living cells

  • Always contain carbon

Acids, Bases, and pH

The pH scale measures the concentration of hydrogen ions in a solution.

• Acidic solutions: pH < 7, higher concentration of H+ ions

• Basic solutions: pH > 7, lower concentration of H+ ions

• Neutral solution: pH = 7

• Example: A solution with pH 12 is basic and has a lower concentration of H+ than a solution with pH 7.

Phospholipids

Phospholipids are major components of cell membranes, consisting of a glycerol backbone, two fatty acids, and a phosphate group.

• Structure: One glycerol molecule linked to one phosphate group and two fatty acids.

• Function: Form bilayers that make up cell membranes.

Chemical Bonds

Atoms form chemical bonds to achieve stability.

• Covalent bond: Forms when two atoms share electrons.

• Ionic bond: Forms when electrons are transferred from one atom to another.

Carbon Chemistry and Macromolecules

Carbon Properties

Carbon is the backbone of organic molecules due to its ability to form four covalent bonds.

• Versatility: Can bond with up to four other atoms, allowing for complex structures.

• Forms: Chains, rings, and branched skeletons.

Lipids

Lipids are hydrophobic molecules, including fats, oils, and phospholipids.

• Fats: Composed of fatty acids and glycerol; have nonpolar hydrocarbon chains.

• Phospholipids: Have polar phosphate heads and nonpolar fatty acid tails.

Hydrophobicity

• Hydrophobic molecules: Repel water due to nonpolar characteristics.

• Hydrophilic molecules: Attract water due to polar characteristics.

Atoms, Elements, and Chemical Reactions

Atomic Structure

Atoms consist of protons, neutrons, and electrons.

• Atomic number: Number of protons in the nucleus.

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

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons.

• Example: Carbon-12 and Carbon-14

Chemical Reactions

Chemical reactions involve the rearrangement of atoms to form new substances.

• Reactants: Substances that start a reaction

• Products: Substances formed by the reaction

• Example equation:

Water and Solutions

Properties of Water

Water is a polar molecule, allowing it to dissolve many substances and support life processes.

• Hydrogen bonds: Weak attractions between water molecules

• Solvent: Water dissolves polar and ionic substances

pH and Buffers

pH measures the acidity or basicity of a solution.

• Buffer: Substance that minimizes changes in pH

Macromolecules: Carbohydrates, Proteins, Nucleic Acids, Lipids

Carbohydrates

Carbohydrates are energy-rich organic compounds made of carbon, hydrogen, and oxygen.

• Monosaccharides: Simple sugars (e.g., glucose)

• Disaccharides: Two monosaccharides joined by dehydration synthesis

• Polysaccharides: Long chains of monosaccharides (e.g., starch, cellulose, glycogen)

Proteins

Proteins are polymers of amino acids, essential for structure and function in cells.

• Amino acids: Building blocks of proteins; distinguished by R groups

• Peptide bonds: Link amino acids together

Nucleic Acids

Nucleic acids (DNA and RNA) store and transmit genetic information.

• Nucleotides: Monomers of nucleic acids, composed of a sugar, phosphate group, and nitrogenous base

• Examples: DNA, RNA

Lipids

Lipids are hydrophobic molecules, including fats, phospholipids, and steroids.

• Function: Energy storage, membrane structure, signaling

Cell Structure and Function

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on their structure.

• Prokaryotic cells: Lack a nucleus and membrane-bound organelles (e.g., bacteria, archaea)

• Eukaryotic cells: Have a nucleus and membrane-bound organelles (e.g., plants, animals, fungi, protists)

Cell Membrane Structure

The cell membrane is a phospholipid bilayer with embedded proteins.

• Phospholipid heads: Hydrophilic, face outward

• Phospholipid tails: Hydrophobic, face inward

Organelles and Their Functions

• Mitochondria: Site of cellular respiration, ATP production

• Nucleus: Contains genetic material, controls cell activities

• Ribosomes: Protein synthesis

• Endoplasmic reticulum (ER): Protein and lipid synthesis

• Golgi apparatus: Modifies, sorts, and packages proteins

• Lysosomes: Digestion of cellular waste

• Chloroplasts: Photosynthesis in plant cells

Transport Across Membranes

Passive and Active Transport

Cells transport substances across membranes by passive or active mechanisms.

• Passive transport: Movement down a concentration gradient, no energy required (e.g., diffusion, osmosis)

• Active transport: Movement against a concentration gradient, requires energy (ATP)

Osmosis and Tonicity

Osmosis is the diffusion of water across a selectively permeable membrane.

• Isotonic: No net movement of water

• Hypotonic: Water enters the cell

• Hypertonic: Water leaves the cell

Metabolism and Energy

Enzymes

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy.

• Activation energy: Energy required to start a reaction

• Enzyme-substrate complex: Temporary association between enzyme and substrate

Metabolic Pathways

Metabolic pathways are series of chemical reactions in cells.

• Catabolic pathways: Break down molecules, release energy

• Anabolic pathways: Build molecules, require energy

Cellular Respiration and Fermentation

Cellular Respiration

Cellular respiration is the process by which cells extract energy from glucose.

• Stages:

  • Glycolysis (cytosol)

  • Pyruvate oxidation

  • Citric acid cycle

  • Oxidative phosphorylation (electron transport chain)

• Equation:

Fermentation

Fermentation allows cells to produce ATP in the absence of oxygen.

• Example: Yeast cells metabolize glucose to produce ethanol and ATP anaerobically.

Endosymbiosis Theory

The endosymbiosis theory explains the origin of mitochondria and chloroplasts in eukaryotic cells.

• Evidence:

  • Mitochondria and chloroplasts have circular DNA

  • They reproduce by splitting, similar to prokaryotes

  • They have ribosomes similar to prokaryotes

Additional info:

• Some questions reference specific chemical groups (e.g., hydroxyl, carboxyl, amino) and their roles in organic molecules.

• Dehydration synthesis is a process that joins monomers by removing water.

• Hydrolysis breaks polymers into monomers by adding water.