e water loss from cells. Only a few proteins are found in cell walls, but they serve an important role in regulating the size of the cell. Enzymes initiate reactions that form the structural networks of molecules, help protect plants against fungal invasions by breaking fragments off of the cell walls of the fungi, and degrade the cell walls to induce fruit to ripen and leaves to fall in autumn.
             The most important characteristic of plants is their ability to photosynthesize, i.e., make their food by converting light energy into chemical energy. This process is carried out in specialized organelles called chloroplasts. Chloroplasts are one of three types of plastids, plant cell organelles that are involved in energy storage. The colorless leucoplasts are involved in the synthesis of starch, oils, and proteins. Yellow-to-red colored chromoplasts manufacture carotenoids. The chloroplast is enclosed in a double membrane. Inside, are the stroma, a semi-fluid material that contains dissolved enzymes and makes up most of the chloroplast's volume, and the lamellae, internal membranes with stacks of closed hollow disks called the thylakoids. Light travels as packets of energy called photons and is absorbed in this form by light-absorbing chlorophyll molecules embedded in the thylakoid disks. When the chlorophyll molecules inside the thylakoids absorb the photons, they emit electrons. The free electrons are then taken up by molecules in the stroma to produce ATP. ATP is the chemical energy "currency" of the cell that powers the cell's metabolic activities. In the stroma, the light-independent reactions of photosynthesis occur; low-energy carbon dioxide is transformed into a high-energy compound like glucose.
             The endoplasmic reticulum (ER) is an network of sacs that manufactures, processes, and transports chemical compounds for use inside and outside of the cell. The ER is a continuous membrane with branching tubules and flattened sacs th...