phospholipids  Asymmetrical lipid molecules with a hydrophilic head and a hydrophobic tail. Lipids with a phosphate group in place of one of the three fatty acid chains. Phospholipids are the building blocks of cellular membranes. Phospholipids have hydrophilic heads (glycerol and phosphate) and hydrophobic tails (the non-polar fatty acids). PICTURE

phosphorylation The chemical attachment of phosphorous to a molecule, usually associated with the storage of energy in the covalent bond that is also formed. Example: attachment of the third phosphate group to ADP in the formation of the higher energy form, ATP. Photophosphorylation is a type of phosphorylation associated with the formation of ATP in the photosynthesis process.

photic zone  The layer of the ocean that is penetrated by sunlight; extends to a depth of about 200 meters.

photoperiodism  The ability of certain plants to sense the relative amounts of light and dark in a 24-hour period; controls the onset of þowering in many plants.

photosynthesis  The process by which plant cells use solar energy to produce ATP. The conversion of unusable sunlight energy into usable chemical energy, associated with the actions of chlorophyll. PICTURE

photosystems  Clusters of several hundred molecules of chlorophyll in a thylakoid in which photosynthesis takes place. Eukaryotes have two types of photosystems: I and II. The series of green photoreceptive pigments involved in the light reactions, which occur in the thylakoids of the chloroplast (in eukaryotes). Energy from light is passed to the electrons as they move through the photosystem pigments. PICTURE 1 | PICTURE 2 | PICTURE 3 | PICTURE 4

phototrophs  Organisms that use sunlight to synthesize organic nutrients as their energy source; e.g., cyanobacteria, algae, and plants.

phototropism  The reaction of plants to light in which the plants bend toward the light. Plant response to light by unequal growth caused by concentration of the plant hormone Indole Acetic Acid (IAA, an auxin) on the darker side of the plant shoot. PICTURE

phycocyanin An accessory pigment found in cyanobacteria and the chloroplasts of red algae.

phycoerythrin An accessory pigment found in cyanobacteria and the chloroplasts of red algae.

phylogeny 1) the study of evolutionary relationships within a monophyletic group. 2) evolutionary hypotheses represented as a dendrog