Metabolism

Cells continually carry out chemical reactions. Some break down molecules into smaller substances; others join smaller substances to make larger molecules. Metabolism is the sum of all the chemical reactions in a cell or an organism. These reactions are controlled by enzymes.

Respiration is a metabolic process that transfers energy. Cells use this energy for other processes, including the synthesis of larger molecules from smaller substances.

Two examples of metabolic synthesis are:

• Glucose molecules can be joined to form glycogen, a carbohydrate used for storage in animals.
• One glycerol molecule can be joined to three fatty acid molecules to form a triglyceride, a type of lipid.

Uses of digestion products

Digestion breaks large food molecules into small, soluble molecules that can be absorbed into the bloodstream. The blood carries these products to cells, where they can be used to build molecules or release energy.

Different food molecules produce different digestion products:

• Carbohydrates are broken down by carbohydrases into simple sugars, including glucose.
• Proteins are broken down into amino acids.
• Lipids are broken down into glycerol and fatty acids.

Cells use absorbed simple sugars to build new carbohydrates, amino acids to build new proteins, and glycerol and fatty acids to build new lipids. These new molecules can then be used by the body.

Some absorbed glucose is also used in respiration, which releases energy for cellular activities.

Fates of glucose in plants

Plants use the glucose made by photosynthesis in several ways. Some is used to release energy, some is stored, and some is used to build other substances needed by the plant.

Glucose for energy and storage

In respiration, plant cells break down glucose to release energy for cellular processes.

Glucose can also be converted into starch for storage. Starch is insoluble, so it does not dissolve in the water inside cells. Plants can also convert glucose into fats and oils, which act as energy stores.

Glucose for building substances

Plants use glucose to make cellulose, which strengthens their cell walls.

Glucose is also used to produce amino acids, which are joined together to make proteins. To make amino acids and proteins, plants need nitrogen. They obtain it from nitrate ions absorbed from the soil through their roots.

Deamination and urea formation

Proteins are broken down into amino acids. The body cannot store excess amino acids as amino acids, so the liver breaks them down by a process called deamination.

During deamination, the nitrogen-containing amino group is removed from an amino acid. This produces ammonia, which is toxic. The liver converts the ammonia into urea, which is transported in the blood to the kidneys. The kidneys remove urea from the blood, and it is excreted in urine.