Chapter Summary

1. All living organisms are composed of cells. Cells are modified according to the functions they perform; some live for a few days, while others live for many years.

2. The discovery of cells is associated with the development of the microscope. In 1665, Robert Hooke coined the word cells for boxlike compartments he saw in cork. Leeuwenhoek and Grew reported frequently during the next 50 years on the existence of cells in a variety of tissues.

3. In 1809, Lamarck concluded that all living tissue is composed of cells, and in 1824, Dutrochet reinforced Lamarck's conclusions. In 1833, Brown discovered that all cells contain a nucleus, and shortly thereafter, Schleiden saw a nucleolus within a nucleus. Schleiden and Schwann are credited with developing the cell theory in 1838 to 1839. The theory holds that all living organisms are composed of cells and that cells form a unifying structural basis of organization.

4. In 1858, Virchow contended that every cell comes from a preexisting cell and that there is no spontaneous generation of cells from dust. In 1862, Pasteur experimentally confirmed Virchow's contentions and later proved that fermentation involves activity of yeast cells. In 1897, Buchner found that yeast cells do not need to be alive for fermentation to occur. This led to the discovery of enzymes.

5. Light microscopes can magnify up to 1,500 times. Thinly sliced materials can be viewed with compound microscopes. Opaque objects can be viewed with stereomicroscopes; most magnify up to 30 times.

6. Electron microscopes have electromagnetic lenses and a beam of electrons within a vacuum that achieve magnification. Transmission electron microscopes magnify up to 200,000 or more times. Scanning electron microscopes, which can be used with opaque objects, usually magnify up to 10,000 times.

7. Scanning tunneling microscopes use a minute probe to scan surfaces at a width as narrow as that of two atoms.

8. Eukaryotic cells are the subject of this chapter. Prokaryotic cells, which lack some of the features of eukaryotic cells, are discussed in Chapter 17.

9. Cells are minute, varying in diameter between 10 and 100 micrometers. They number into the billions in larger organisms, such as trees. Plant cells are bounded by walls that surround the living protoplast. The cytoplasm contains a souplike fluid called the cytosol and all cellular components between the plasma membrane and nucleus.

10. A pectic middle lamella is sandwiched between the primary cell walls of adjacent cells. The primary wall and also the secondary cell wall, often added inside the primary wall, are composed of cellulose polymers, with hemicelluloses and glycoproteins. Secondary cell walls contain lignin that strengthens the wall.

11. Living cells are in contact with one another via fine strands of cytoplasm called plasmodesmata, which often extend through minute holes in the walls.

12. A flexible plasma membrane, which is sandwich-like and often forms folds, constitutes the outer boundary of the cytoplasm. It regulates the substances that enter and leave the cell.

13. The nucleus is bounded by a nuclear envelope consisting of two membranes that are perforated by numerous pores. Within the nucleus are a fluid called nucleoplasm, one or more spherical nucleoli, and thin strands of chromatin, which condense and become chromosomes when nuclei divide. Each species of organism has a specific number of chromosomes in each cell.

14. The endoplasmic reticulum is a system of flattened sacs and tubes associated with the storing and transporting of protein and other cell products. Granular particles called ribosomes, which function in protein synthesis, may line the outer surfaces of the endoplasmic reticulum. Ribosomes also occur independently in the cytoplasm.

15. Dictyosomes are structures that appear as stacks of sacs and function as collecting and packaging centers for the cell.

16. Plastids are larger green, orange, red, or colorless organelles. Green plastids, known as chloroplasts, contain enzymes that catalyze reactions of photosynthesis. These reactions take place in the membranes of structures that resemble stacks of coins, called thylakoids, as well as the surrounding matrix, called the stroma. Plastids develop from proplastids, which divide frequently, and also arise from the division of mature plastids.

17. Mitochondria are tiny, numerous organelles that are bounded by two membranes with inner platelike folds called cristae; they are associated with cellular respiration.

18. One or more vacuoles may occupy 90% or more of the volume of a mature cell. Vacuoles are bounded by a vacuolar membrane (tonoplast) and contain a watery fluid called cell sap. Cell sap contains dissolved substances and sometimes water-soluble red or blue anthocyanin pigments.

19. The cytoskeleton, which is involved in the architecture of cells and internal movement, is composed of microtubules and microfilaments. Microfilaments may be responsible for cytoplasmic streaming.

20. Cells that are not dividing are in interphase, which is subdivided into three periods of intense activity that precede mitosis or division of the nucleus. Mitosis is usually accompanied by division of the rest of the cell and takes place in meristems.

21. Mitosis is arbitrarily divided into four phases: (1) prophase, in which the chromosomes and their two-stranded nature become apparent and the nuclear envelope breaks down; (2) metaphase, in which the chromosomes become aligned at the equator of the cell; a spindle composed of spindle fibers is fully developed, with some spindle fibers being attached to the chromosomes at their centromeres; (3) anaphase, in which the sister chromatids of each chromosome (now called daughter chromosomes) separate lengthwise, with each group of daughter chromosomes migrating to opposite poles of the cell; and (4) telophase, in which each group of daughter chromosomes becomes surrounded by a nuclear envelope, thus becoming new nuclei, and a wall dividing the daughter nuclei forms, creating two daughter cells.

22. Animal cells differ from those of higher plants in not having a wall, plastids, or large vacuoles. Also, they have keg-shaped centrioles in pairs just outside the nucleus and pinch in two instead of forming a cell plate when they divide.