Organic chemistry, 45. See also Chemistry of life
Organic molecules, 554
Organisms, interactions of, 65
Organ rejection, 1019
Organs
Organ systems
Osmotic pressure, 211
Ossification, 925
Osteoarthritis, 925
Osteoblasts, 925
Osteoclasts, 925
Osteocytes, 925
Osteoporosis, 925
Ovaries
Overfishing, 176
Oviparous species, 822
Ovoviviparous species, 822
Ovulation, 993
Oxygen, 35
Ozone, 163
Ozone layer, 175
Pacemaker, 951
Pain receptors, 909
Paleozoic Era, 560–561
Palisade mesophyll, 681
Parasympathetic nervous system, 908
Parathyroid hormone, 985
Parenchyma, 667
Parental care, 824
Parietal lobe, 903
Park ranger, 105
Particulates, 164
Passion flower, 697
Passive immunity, 1020
Passive transport, 209–211
Pathogens, 586, 828, 1010–1011. See also Infectious diseases
Pathologist, 195
Pavlov, Ivan, 843
Pax6 gene, 387
Peer review, 12
Perennial plants, 654
Period, geologic, 543
Periodic Table, 35
Periosteum, 924
Peristalsis, 877
Permafrost, 115
Petals, 696
Petiole, 680
Pharyngeal pouches, 731
Phenotypes, 315
Pheromones, 850
Phipps, James, 1020
Phosphorus cycle, 85
Photoperiod, 713
Photoreceptors, 912–913
Photosynthetic protists, 610–611
Photosystems, 235–236
Phototropism, 712
Phylogenetic systematics, 516–517
Phylogeny, 516
Phylum, 514
Phytochrome, 713–714
Pigments, 230–231
Pikaia, 757
Pili, 581
Pineal gland, 979
Pinocytosis, 213
Pioneer species, 107
Pistil, 697
Pith in plant stem, 675
Planarian, 812
Plankton, 118
Plant breeder, 322
Plant pathologist, 655
Plant reproduction, 696–718
Plants, 634–654. See also Angiosperms