Levels of Organization in Biology: From Atoms to Organisms

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Levels of Organization

From Atoms to Organisms

Atom, molecule, organelle, cell, tissue, organ, organ system

Inorganic Biomolecules

  • Water
  • Mineral salts

Organic Molecules

  • Carbohydrates
    • Monosaccharides: one molecule; examples: glucose, fructose
    • Disaccharides: two monosaccharides; examples: sucrose, maltose
    • Polysaccharides: many monosaccharides; examples: glycogen, cellulose
  • Proteins: macromolecules composed of small molecules called amino acids; examples: collagen, hemoglobin, antibodies
  • Lipids: examples: fats, phospholipids, cholesterol. Fats break down into fatty acids and glycerol.
  • Nucleic acids: macromolecules composed of nucleotides, two types: DNA and RNA; examples: genes


Simplest living unit. Living things can be unicellular organisms or multicellular organisms. Examples: sperm, enterocyte, muscle cell, erythrocyte, neuron, fibroblast.

Life Functions in Cells

  • Nutrition: cells obtain the matter and energy necessary to perform life functions. Cells take nutrients from the outside. Nutrients are used for energy and for growth and repair structures. Nutrients undergo cellular metabolism: catabolism and anabolism.
    • Catabolism: destructive phase. Breaks nutrients into small molecules; complex organic substances become simple substances and energy.
    • Anabolism: constructive phase. From small molecules to macromolecules; simple substances and energy become complex organic substances.
  • Interaction: communication between cells.
  • Reproduction: a parent cell divides into two or more new cells called daughter cells. In unicellular organisms, this gives rise to an entire organism. In multicellular organisms, this increases the number of cells in the organism.

Prokaryotic Cells

No nucleus, ribosomes are the only organelles, covered by a cell wall. Example: bacteria.

  • Cell membrane: surrounds the cytoplasm
  • Cell wall: surrounds the cell membrane
  • Bacterial capsule: thick outer covering on some bacterial cells
  • Bacterial chromosome: circular DNA molecule that contains genetic material
  • Ribosomes: organelles that carry out protein synthesis
  • Appendices: structures like flagella and fimbriae. They enable bacteria to adhere to a surface.

Eukaryotic Cells

Found in human beings, animals, and plants.

  • Cell membrane: surrounds the cell. Two layers of phospholipids.
  • Cytoplasm: fills the space between the cell membrane and the nucleus; contains hyaloplasm, organelles, and the cytoskeleton.
  • Nucleus: contains the genetic material that controls cellular function; contains the nucleolus, nucleoplasm, and chromatin.

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