C3plant The majority of photosynthetic plants that
produce, as the initial steps of CO 2 incorporation, a
three-carbon compound, phosphoglyceric acid (PGA),
as the first stable intermediate (CALVIN CYCLE). The
PGA molecules are further phosphorylated (by ATP)
and are reduced by NADPHto form phosphoglyceralde-
hyde (PGAL), which then serves as the starting material
for the synthesis of glucose and fructose, which, when
combined, make sucrose that travels through the plant.
Velvetleaf (Abutilon theophrasti) is an example of a C3
plant.

C4 plant Asmall number of plants that incorpo-
rate CO 2 using a carboxylase for the CO 2 capture,
producing a four-carbon compound (carboxylic acid)
as a stable intermediary in the first step of photosyn-
thesis. C4 plants (e.g., corn) supply CO 2 for the
CALVIN CYCLE.

CADD SeeCOMPUTER-ASSISTED DRUG DESIGN.

cage An aggregate of molecules, generally in the con-
densed phase, that surround the fragments formed by
thermal or photochemical dissociation of a species.

calcitonin Calcitonin is a hormone produced by the
thyroid gland that acts primarily on bone. It inhibits

bone removal by osteoclasts and promotes bone forma-

tion by osteoblasts; lowers blood calcium levels.

calmodulin A Ca2+binding protein involved in mus-

cular contraction.

calorie An energy measurement unit; the amount of

energy required to raise the temperature of 1 g of water

by 1°C. A kilocalorie (1,000 calories) is used in food

science to describe the energy content of food products.

calpain A calcium-activated neutral protease.

Calvin cycle The second major stage in photosyn-

thesis after light reactions—discovered by chemist

Melvin Calvin (1911–97)—whereby carbon molecules

from CO 2 are fixed into sugar (glucose) and mediated

by the enzyme rubisco (ribulose-1-5-biphosphate car-

boxylase). It occurs in the stroma of chloroplasts. The

Calvin cycle is also known as the dark reaction, as

opposed to the first-stage light reactions.

CAM(crassulacean acid metabolism) A metabolic

adaptation of certain plants, particularly xerophytes

(desert loving, e.g., succulents), in arid areas that

allows them to take up CO 2 at night, not during the

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