AP Biology Notes: Structures of Life

Five Things to Know about Structures of Life

  1. Cells are the basic structural and functional units of all known living organisms. There are two major types: prokaryotic cells and eukaryotic cells.
  2. Prokaryotic cells have no real nucleus, have circular DNA, and reproduce using binary fission. Prokaryotes include archaea and bacteria. These cells are small and move via flagella.
  3. Eukaryotic cells have a nucleus, linear DNA, highly structured cell membranes, and organelles, and they reproduce via mitosis and meiosis. Eukaryotes include protists, fungi, plants, and animals. These cells are large and move via a variety of methods including flagella and cilia.
  4. Cell membranes are made up of a lipid bilayer that includes a hydrophobic and a hydrophilic region. Specific structures embedded within the membrane help to facilitate transport.
  5. The cell membrane is selectively permeable, meaning it allows certain things through while keeping others out. Water diffuses across the membrane from areas of lesser to greater solute concentration (osmosis). While certain things can cross the membrane in the processes of diffusion or facilitated diffusion, which do not require energy, others require the expenditure of energy for active transport against the concentration gradient.

Key Topics–Structures of Life

Remember that the AP Biology exam tests you on the depth of your knowledge, not just your ability to recall facts. While we have provided brief definitions here, you will need to know these terms in even more depth for the AP Biology exam.

Prokaryotes versus Eukaryotes

  • Prokaryote: Unicellular organism lacking organelles, specifically a nucleus
  • Eukaryote: Organism consisting of one or more cells with genetic material in membrane-bound nuclei


  • Nucleus: An organelle that regulates cell functions and contains the genetic material of the cell
  • Cytoplasm: The living matter of a cell, located between the cell membrane and the nucleus
  • Ribosome: An organelle in the cytoplasm that contains RNA; serves as the site of protein synthesis
  • Endoplasmic reticulum: A network of membrane-enclosed spaces connected with the nuclear membrane; transports materials through the cell; can be smooth or rough
  • Golgi apparatus: Membranous organelles involved in the storage and modification of secretory products
  • Lysosome: An organelle that contains enzymes that aid in intracellular digestion
  • Mitochondria: Cytoplasmic organelles that serve as sites of respiration; rod-shaped bodies in the cytoplasm known to be the center of cellular respiration
  • Cristae: Inward folds of the mitochondrial membrane
  • Chloroplast: A plastid containing chlorophyll
  • Vacuole: A space in the cytoplasm of a cell that contains fluid

Membrane Traffic

  • Plasma membrane: The cell membrane
  • Lipid: An organic compound that contains hydrocarbons and includes fats, oils, waxes, and steroids
  • Hydrophobic: Repelling water, “water fearing”
  • Phospholipids: Phosphorus-containing lipids composed of two fatty acids and a phosphate group modified with simple organic molecules
  • Hydrophilic: Having an affinity for water, “water loving”
  • Protein: An organic compound that is composed of many amino acids; contains C, H, O, and N
  • Carbohydrate: An organic compound to which hydrogen and oxygen are attached; the hydrogen and oxygen are in a 2:1 ratio; examples include sugars, starches, and cellulose
  • Sterols: Polycyclic compounds (lipids), such as cholesterol, that play an important role in lipid metabolism

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