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The actions of the neurotransmitter acetylcholine influence the function of many brain regions. Within these regions, acetylcholine allows you to learn and remember, to regulate your attention and mood, and to control how well you can move. Thus, anything that affects the function of acetylcholine neurons has the potential to affect all of these brain and body functions. In the brains of people with Alzheimer's disease acetylcholine neurons very slowly die. Because of this these people have difficulty paying attention or remembering almost everything. Blocking the actions of acetylcholine can have a wide range of consequences, a fact that was recognized by Homer, voodoo priests, and witches during the darks ages. Various nuts, mushrooms, and plants, such as the infamous tobacco leaf, contain chemicals that can either mimic or antagonize the actions of acetylcholine in the brain; the consequences of which include feelings of happiness, relaxation, and well-being as well as dramatic hallucinations that lead to the adventures of Alice in Wonderland.

As noted earlier and as anticipated by Charles and Francis Darwin it has been argued that plants sense the direction of gravity (gravitropism) by movement of starch granules found in cells called statocytes that contain compartments (organelles) called statoliths. The synthesis of statoliths appears to occur in the plastid (plant organelle) compartments called amyloplasts (Figure 7.1, 1). It has been suggested that this gravitropic signal then leads to movement of plant hormones such as indole-3-acetic acid (auxin) (Figure 7.2), through the phloem opposite to the pull of gravity to promote stem growth. Chloroplasts (Figure 7.1, 2) are cell compartments (plastids or organelles) in which photosynthesis is carried out. The process of photosynthesis, discussed more fully later, is accompanied by the production of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate (Pi) (Figure 7.3). ATP is consumed and converted to ADP and Pi in living systems. The cycle of production and consumption allows ATP to serve as an “energy currency” to pay for the reactions in living systems. Beyond this generally recognized critical function of chloroplasts, it has recently been pointed out that light/dark conditions affect alternative splicing of genes which may be necessary for proper plant responses to varying light conditions. The organelles or plastids which contain the pigments for photosynthesis and the amyloplasts that store starch are only two of many kinds of plastids. Other plastids, leucoplasts for example, hold the enzymes for the synthesis of terpenes, and elaioplasts store fatty acids. Apparently, all plastids are derived from proplastids which are present in the pluripotent apical and root meristem cells. The cell wall (Figure 7.1, 3) is the tough, rigid layer that surrounds cells. It is located on the outside of the flexible cell membrane, thus adding fixed structure. A representation of a portion of the cell wall (as made up of cellulose and peptide cross-linking) is shown below in Figure 7.7. The cells will have different sizes as a function of where they are found (e.g., leaf, stalk, root), but in every case, the cell wall limits the size of the membrane that lies within.