Scope, Sequence, and Coordination
A Framework for High School Science Education
Based on the National Science Education Standards
Cells: A First-Hand View
Cell Structures that Underlie Cell Functions
The fundamental unit of all organisms is the cell. By the early 1800s it had been determined that all organisms are made up of cells and that the cells carry on the processes characteristic of all living organisms. The complexity of cell structure and function has been made evident with advances in technology. Through the development of optical and electron microscopes and breakthroughs in molecular biology, much information has been discovered concerning the structure and function of cells.
The cell membrane transports protein-regulated substances entering the cell. Regardless of the type of cell, all cells carry out basic activities of life maintenance: absorption and conversion of energy, digestion, biosynthesis, respiration, excretion, secretion, generation of a membrane potential or voltage, response, and reproduction. These activities are maintained by the interaction of specialized structures in the cell called organelles. The transport proteins in the cell membrane, also found in membranes of cell organelles, are highly selective. They can move substances counter to diffusion. Transport proteins that move substances against diffusion gradients require energy, and substances are said to be actively transported into or out of the cell. This regulation of exchanges of substances into and out of a cell is essential to cell maintenance.
Of specific importance in maintaining homeostasis in cells are cell membranes. The cell membrane controls what enters and leaves the cell. This selective permeability depends upon the structural components making up the membrane. These structural components enable cell membranes to identify specific substances the cell needs and to transport them into the cell. In addition, the cell membrane sometimes prevents harmful substances or substances that the cell does not need from entering the cell.
Within the cell exist several "organelles." Organelles are specialized bodies that serve specific life functions of the cell. The organelle that is critical to fully functioning cells is the nucleus. The nucleus contains chromatin, a complex of DNA and proteins necessary for storage and transport of genetic information. Besides the nucleus, there are several other important organelles necessary for cell maintenance, such as the mitochondria, which convert sugar into energy for the cell; ribosomes, which manufacture enzymes, hormones, and structural proteins of the cell; and the Golgi body, which synthesizes carbohydrates and chemically tags proteins for distribution in the cell.
Some organelles are specific to plant cells, for example the plastids. The most important plastid found in plants is the chloroplast, which manufactures glucose by using light energy to split water molecules. Other plastids, such as leucoplasts, serve as storage locations for starch and oil. Microtubules and actinfilaments are components of a complex network of protein filaments called the cytoskeleton. The cytoskeleton maintains the shape of the cell, allows the cell to move, and gives it the ability to provide and maintain directed pathways from one part of the cell to another for its organelles.
Evolutionary aspects of parts of the cell must be considered if structure and function are to be understood. Some organelles such as the mitochondria and chloroplasts are thought to have originally existed as separate bacteria cells. During evolutionary stages these cells were engulfed by large prokaryotic cells, but they still maintained some autonomy.
Microscopy, absorption, nutrition, excretion, secretion, cell membrane
Permeability, diffusion, osmosis, concentration gradients, cell size, toxicity, surface-to-volume ratio
Prokaryote, eukaryote, transport proteins, active transport
Polarity, membrane selectivity, membrane potential
Only cells beget cells, phenomenon of membrane potential, osmosis, the process of diffusion, the process of active transport, surface-to-volume ratio as a limiting factor
Fluid mosaic model, complementarity of structure and function, the cell as the functional unit of life, all organisms are made of cells of various kinds (cell theory), active transport