Fig. 1.6 Basic subcellular constituents of cells. The table presents the number of various organelles within a typical hepatocyte, as well as their volume within the cell. The figure shows geographic relationships but is not intended to be accurate to scale. *Rough and smooth ER form a single compartment; the Golgi apparatus is organized as a set of discrete stacked cisternae interconnected by transport vesicles. (Adapted from Weibel ER, Stäubli W, Gnägi HR, et al: Correlated morphometric and biochemical studies on the liver cell. I. Morphometric model, stereologic methods, and normal morphometric data for rat liver, J Cell Biol 42:68, 1969.) play a specialized role in the breakdown of very long chain fatty acids, generating hydrogen peroxide in the process.
The contents and position of cellular organelles also are subject to regulation. Endosomal vesicles shuttle internalized material to the appropriate intracellular sites or direct newly synthesized materials to the cell surface or targeted organelle. Movement of both organelles and proteins within the cell and of the cell in its environment is orchestrated by the cytoskeleton. These structural proteins also regulate cellular shape and intracellular organization, requisites for maintaining cell polarity. This is particularly critical in epithelia, in which the top of the cell (apical) and the bottom and side of the cell (basolateral) are often exposed to different environments and have distinct  functions.
Most of the adenosine triphosphate (ATP) that powers cells is made through oxidative phosphorylation in the mitochondria. However, mitochondria also serve as an important source of metabolic intermediates that are needed for anabolic metabolism. They also are sites of synthesis of certain macromolecules (e.g., heme), and contain important sensors of cell damage that can initiate and regulate the process of apoptotic cell death.
Cell growth and maintenance require a constant supply of both energy and the building blocks that are needed for synthesis of macromolecules. In growing and dividing cells, all of these organelles have to be replicated (organellar biogenesis) and correctly apportioned in daughter cells following mitosis. Moreover, because the macromolecules and organelles have finite life spans (mitochondria, e.g., last only about 10 days), mechanisms also must exist that allow for the recognition and degradation of “worn out” cellular components. The final catabolism occurs in  lysosomes.
With this as a primer, we now move on to discuss cellular components and their function in greater detail.
Plasma Membrane: Protection and Nutrient Acquisition
Plasma membranes (and all other organellar membranes) are more than just static lipid sheaths. Rather, they are fluid bilayers of amphipathic phospholipids with hydrophilic head groups that face the aqueous environment and hydrophobic lipid tails that interact with each other to form a barrier to passive diffusion of large or charged molecules (
Fig. 1.7A ). The bilayer is composed of a heterogeneous collection of different phospholipids, which are distributed asymmetrically—for example, certain membrane lipids preferentially associate with extracellular or cytosolic faces. Asymmetric partitioning of phospholipids is important in several cellular processes:
Phosphatidylinositol on the inner membrane leaflet can be phosphorylated, serving as an electrostatic scaffold for intracellular proteins; alternatively, polyphosphoinositides can be hydrolyzed by phospholipase C to generate intracellular second signals such as diacylglycerol and inositol trisphosphate.