Cell Structure and Function Help
Cell Structure and Function
Prokaryotic cells lack a membrane-bound nucleus; instead they contain a single strand of nucleic acid. These cells contain few organelles. Arigid or semirigid cell wall surrounding a cell (plasma) membrane gives the cell its shape. Bacteria are prokaryotic single-celled organisms.
Eukaryotic cells contain a true nucleus with multiple chromosomes. They also have several types of specialized membrane-bound organelles. Like prokaryotes, eukaryotic cells have a cell (plasma) membrane. Since all human cells are eukaryotic, most of this chapter will focus on eukaryotic cells and their functions. Organisms composed of eukaryotic cells include protozoa, fungi, algae, plants, invertebrates, and vertebrate animals.
The following cell structures are common to all cells:
Cell (plasma) membrane. Both prokaryotic and eukaryotic cells are bound by a cell (plasma) membrane. The cell membrane is selectively permeable, allowing certain substances to pass through, but not others. Water, alcohol, and gases readily pass through the cell membrane, but ions, large proteins, and carbohydrates do not. Substances can pass through the cell membrane via one of the processes listed below.
Cytoplasm. Cytoplasm is the fluid matrix within a cell. It consists primarily of water and dissolved substances including O2, CO2, cellular wastes (urea), glucose, ions, proteins, and ATP.
Ribosomes. Ribosomes are the protein factories of the cell. They are responsible for translation, building proteins as directed by mRNA synthesized by the DNA in the nucleus.
Cell wall (found in some prokaryotic and eukaryotic cells). A cell wall is present in only eukaryotic plant cells. It functions to maintain cell shape. Some prokaryotic bacteria also have cell walls.
Chromosomes. The chromosomes of a cell contain the DNA. In eukaryotic cells, the chromosomes are located within the nucleus. Every species has a distinct number of chromosomes.
An organelle is any subcellular structure having a specific function. The organelles described below are found only in eukaryotic cells.
Replication, Transcription, and Translation
Replication refers to the process in which DNA makes an identical copy of itself prior to cell division. Transcription refers to making mRNA from the DNA template. The mRNA then leaves the nucleus and joins with a ribosome in the cytoplasm to synthesize a protein in a process called translation. A mutation is an unrepaired mistake in replication. Some occur spontaneously, but many are induced by various substances called mutagens. Most mutations are harmless or unnoticeable, some may be harmful or lethal, other may be beneficial. Mutations play an important role in creating diversity in the genetic makeup of a species.
Mitosis and Meiosis
Mitosis is the process of normal cell division (Figure 3-1). It occurs whenever the body cells need to produce more cells for growth or for replacement and repair. The result of mitosis is two identical daughter cells with the same chromosomal content as the parent cell.
Meiosis is the process of gamete (sex cell) formation. It resembles mitosis in many ways, except that the end result is four daughter cells, each with half the chromosomal content of the parent cell (Figure 3-1). In humans, the somatic (body) cells all have 46 chromosomes or 23 pairs (diploid). After undergoing meiosis, the gametes have only 23 chromosomes (haploid) or half the number of chromosomes.
Cells adjacent to each other or distant from each other must often communicate in order for a body system to function normally. This communication may be accomplished in several ways. Chemical messengers, such as hormones, secreted into the blood, or neurotransmitters, released from a nerve cell, can accelerate or inhibit cellular functioning. Physical contact of one cell with another may trigger contact inhibition, which is frequently expressed as a repression of mitosis. When cells fail to respond to contact inhibition and continue to divide uncontrolled, the condition is called cancer.
Practice problems for these concepts can be found at: