Metabolism refers to the entire set of reactions within the cell. Most reactions can be classified as either anabolic or catabolic. Anabolic reactions use stored energy to build more complex molecules from simpler ones. Protein synthesis is an example. Catabolic reactions break down complex molecules to simpler ones, releasing energy in the process that may be harvested and stored by the cell. Glucose breakdown is an example.

The energy transfer in each type of reaction almost always involves the interconversion of ATP and ADP (adenosine diphosphate). Energy is released when ATP loses a phosphate to become ADP, while energy is required to make ATP from ADP and phosphate. ATP can also be con-

Energy is released when ATP is broken down, and required when it is formed.

hormones molecules released by one cell to influence another transcription factor protein that increases the rate of transcription of a gene glucose sugar

The nucleus of this liver cell, magnified nearly 3,000 times, has been stained to show some of its components: DNA (dark purple), nucleolus (burgundy). Mitochondria (red), and cytoplasm (pink), are visible outside the nucleus.

verted to AMP by the loss of two phosphates. This reaction, which releases even more energy, is used in replication of DNA and synthesis of RNA (transcription).

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