Chapter summary
While the definition of "living" has been stretched to include viruses and perhaps prions, for animal physiology, the cell is the fundamental unit of life. Combinations of cells create tissues, groups of tissues lead to organs, and groups of organs lead to physiological systems.
Four principles are derived from cell theory:• The cell is the primary structural and functional unit of living organisms.
• Function of organisms depends on activities of cells.
• Cell biochemistry is dictated by cellular organelles.
• Propagation of life depends on cellular activity.
Cells and organelles
The combinations of structures within cells allow for specific functions linked to particular cell types, but common features of nearly all cells include the cell membrane, mitochondria, nucleus, nucleolus, Golgi, ribosomes, rough endoplasmic reticulum, microtubules, microfilaments, and inclusions (lysosomes, peroxisomes, secretory vesicles). The plasma membrane as well as membranes surrounding many internal organelles are highly structured, with an inner and outer leaflet of highly polar "heads" of phospholipids and internal leaflet consisting on the nonpolar tails of the phospholipids. In addition, integral membrane proteins are incorporated in either the inner or outer phospholipid leaflets or are arranged so they span the entire membrane. Many of these proteins function as receptors for hormones and GFs or are involved in signaling related to immune function. This membrane structure is frequently described as a fluid mosaic model. Centrioles are a specialized arrangement of cellular proteins and microtubules that appear in cilia and flagella. They are also critical in cell division as they organize and direct movement and separation of the chromosomes when the cell divides during mitosis.
Macromolecules and cell function
It should be no surprise that cell and physiological function depends on multiple interactions to catabo- Iize various nutrient macromolecules (lipids, proteins, and carbohydrates) to supply the materials to synthesize ATP or to synthesize cellular organelles.
Most cellular proteins are created from the uptake of amino acids or peptides absorbed from the bloodstream. Basic amino acid structure is centered on the α-carbon, an amino group, and a carboxy group. The remaining side group attached to the carbon varies to create the individual amino acids. For example, if this residual or R group is a hydrogen atom, this yields the simple amino acid glycine. There are 20 common amino acids.Simple monomeric sugars include the 6-carbon hexoses such as glucose, galactose, or the 5-carbon ribose. Disaccharides are familiar as lactose or sucrose. In polymeric forms, glucose creates glycogen, which is important as a storage molecule to supply glucose to maintain blood glucose concentrations between meals.
In addition to the phospholipids critical for membranes, other lipids, for example, cholesterol, provide the backbone for synthesis of a myriad of steroid hormones. Lipid stored to generate ATP comes from triglycerides, which are composed of a 3-carbon glycerol molecule with three attached fatty acids.
Nucleic acids are necessary in synthesized DNA and RNA as well as a number of coenzymes necessary for ATP production. The two-ringed base purine gives rise to adenine and guanine, while single-ring pyrimidine is utilized to produce thymine, cytosine, and uracil.
Making and breaking of molecular bonds is at the center of cellular biochemistry. Covalent bonds are critically important, but the presence of hydrogen bonding is extremely important in maintenance of the structural integrity of DNA and higher-level structures of proteins. Chemical reactions in cells are dependent on the actions of enzymes, which allow reactions that would rarely, if ever, happen spontaneously. All enzymes are cellular proteins, and the actions of many require the presence of cofactors or coenzymes.
Movement of critical nutrients across cell membranes often depends on the presence of specific transport proteins located within the membrane.
Carriers that move materials against their concentration gradients require energy expenditure usually either directly or indirectly in the form of ATP hydrolysis. Three broad classes of transporters are recognized: uniport, symport, and antiport. Materials can also enter cells via bulk phase endocytosis, receptor-mediated endocytosis, or, in special cases, phagocytosis.√s θ Review questions and answers are available a online.
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