some number is maintained in the cells of the new individual.
Human cells vary in size, shape, and function.
Our cells function interdependently to main- tain homeostasis.
Cell Structure—the major parts of a cell are the cell membrane, nucleus (except mature RBCs), cytoplasm, and cell organelles
1. Cell membrane—the selectively permeable bound- ary of the cell (see Fig. 3–1).
• Phospholipids permit diffusion of lipid-soluble materials.
• Cholesterol provides stability.
• Proteins form channels, transporters, “self ” anti- gens, and receptor sites for hormones or other signaling molecules.
2. Nucleus—the control center of the cell; has a double-layer membrane.
• Nucleolus—forms ribosomal RNA.
• Chromosomes—made of DNA and protein;
DNA is the genetic code for the structure and functioning of the cell. A gene is a segment of DNA that is the code for one protein. Human cells have 46 chromosomes, and their genetic information is called the genome.
3. Cytoplasm—a watery solution of minerals, gases, and organic molecules; contains the cell organelles;
site for many chemical reactions.
4. Cell organelles—intracellular structures with spe- cific functions (see Table 3–1 and Fig. 3–2).
Cellular Transport Mechanisms—the proces- ses by which cells take in or secrete or excrete materials through the selectively permeable cell membrane (see Fig. 3–3 and Table 3–2).
1. Diffusion—movement of molecules from an area of greater concentration to an area of lesser con- centration; occurs because molecules have free energy: They are constantly in motion. Oxygen and carbon dioxide are exchanged by diffusion in the lungs and tissues.
2. Osmosis—the diffusion of water. Water diffuses to an area of less water, that is, to an area of more dis- solved material. The small intestine absorbs water from digested food by osmosis. Isotonic, hyper- tonic, and hypotonic (see Box 3–1).
3. Facilitated diffusion—transporters (carrier enzymes) that are part of the cell membrane permit cells to take in materials that would not diffuse by
themselves. Most cells take in glucose by facilitated diffusion.
4. Active transport—a cell uses ATP to move sub- stances from an area of lesser concentration to an area of greater concentration. Nerve cells and mus- cle cells have sodium pumps to return Na⫹ions to the exterior of the cells; this prevents spontaneous impulses. Cells of the small intestine absorb glu- cose and amino acids from digested food by active transport.
5. Filtration—pressure forces water and dissolved materials through a membrane from an area of higher pressure to an area of lower pressure. Tissue fluid is formed by filtration: Blood pressure forces plasma and dissolved nutrients out of capillaries and into tissues. Blood pressure in the kidney cap- illaries creates filtration, which is the first step in the formation of urine.
6. Phagocytosis—(a form of endocytosis) a moving cell engulfs something; white blood cells phagocy- tize bacteria to destroy them.
7. Pinocytosis—(a form of endocytosis) a stationary cell engulfs small molecules; kidney tubule cells reabsorb small proteins by pinocytosis.
The Genetic Code and Protein Synthesis (see Fig. 3–4 and Table 3–3)
1. DNA and the genetic code
• DNA is a double helix with complementary base pairing: A–T and G–C.
• The sequence of bases in the DNA is the genetic code for proteins.
• The triplet code: three bases (a codon) is the code for one amino acid.
• A gene consists of all the triplets that code for a single protein.
2. RNA and protein synthesis
• Transcription—mRNA is formed as a comple- mentary copy of the sequence of bases in a gene (DNA).
• mRNA moves from the nucleus to the ribosomes in the cytoplasm.
• tRNA molecules (in the cytoplasm) have anti- codons for the triplets on the mRNA.
• Translation—tRNA molecules bring amino acids to their proper triplets on the mRNA.
• Ribosomes contain enzymes to form peptide bonds between the amino acids.
3. Expression of the genetic code 64 Cells
STUDY OUTLINE
• DNA → RNA → proteins (structural proteins and enzymes that catalyze reactions) →heredi- tary characteristics.
• A genetic disease is a “mistake” in the DNA, which is copied by mRNA and results in a mal- functioning protein.
Cell Division
1. Mitosis—one cell with the diploid number of chro- mosomes divides once to form two cells, each with the diploid number of chromosomes (46 for humans).
• DNA replication forms two sets of chromosomes during interphase.
• Stages of mitosis (see Fig. 3–5 and Table 3–4):
prophase, metaphase, anaphase, and telophase.
Cytokinesis is the division of the cytoplasm fol- lowing telophase.
• Mitosis is essential for growth and for repair and replacement of damaged cells.
• Most adult nerve and muscle cells seem unable to divide; their loss may involve permanent loss of function.
2. Meiosis—one cell with the diploid number of chro- mosomes divides twice to form four cells, each with the haploid number of chromosomes (23 for humans).
• Oogenesis in the ovaries forms egg cells.
• Spermatogenesis in the testes forms sperm cells.
• Fertilization of an egg by a sperm restores the diploid number in the fertilized egg.
REVIEW QUESTIONS
1. State the functions of the organic molecules of cell membranes: cholesterol, proteins, and phos- pholipids. (p. 48)
2. Describe the function of each of these cell organelles: mitochondria, lysosomes, Golgi appa- ratus, ribosomes, proteasomes, and endoplasmic reticulum. (p. 51)
3. Explain why the nucleus is the control center of the cell. (p. 49)
4. What part of the cell membrane is necessary for facilitated diffusion? Describe one way this process is important within the body. (p. 54) 5. What provides the energy for filtration? Describe
one way this process is important within the body.
(p. 54)
6. What provides the energy for diffusion? Describe one way this process is important within the body.
(p. 52)
7. What provides the energy for active transport?
Describe one way this process is important within the body. (p. 54)
8. Define osmosis, and describe one way this process is important within the body. (p. 52–53)
9. Explain the difference between hypertonic and hypotonic, using human cells as a reference point.
(p. 53)
10. In what way are phagocytosis and pinocytosis sim-
ilar? Describe one way each process is important within the body. (p. 56)
11. How many chromosomes does a human cell have?
What are these chromosomes made of ? (p. 56) 12. Name the stage of mitosis in which each of the
following takes place: (p. 63)
a. The two sets of chromosomes are pulled toward opposite poles of the cell
b. The chromosomes become visible as short rods c. A nuclear membrane re-forms around each
complete set of chromosomes
d. The pairs of chromatids line up along the equator of the cell
e. The centrioles organize the spindle fibers f. Cytokinesis takes place after this stage
13. Describe two specific ways mitosis is important within the body. Explain why meiosis is impor- tant. (pp. 60, 62)
14. Compare mitosis and meiosis in terms of: (pp.
60–62)
a. Number of divisions b. Number of cells formed
c. Chromosome number of the cells formed 15. Explain the triplet code of DNA. Name the mol-
ecule that copies the triplet code of DNA. Name the organelle that is the site of protein synthesis.
What other function does this organelle have in protein formation? (pp. 56–58)
1. Antibiotics are drugs used to treat bacterial infec- tions. Some antibiotics disrupt the process of pro- tein synthesis within bacteria. Others block DNA synthesis and cell division by the bacteria. Still oth- ers inhibit cell wall synthesis by the bacteria. If all antibiotics worked equally well against bacteria, would any of those mentioned here be better than the others, from the patient’s perspective? Explain your answer.
2. A new lab instructor wants his students to see liv- ing cells. He puts a drop of his own blood on a glass slide, adds two drops of distilled water “so the cells will be spread out and easier to see,” puts on a cover glass, and places the slide under a microscope on high power. He invites his students to see living red blood cells. The students claim that they can- not see any cells. Explain what has happened. How could this have been prevented?
3. A friend asks you how DNA can be used to identify someone, and why it is called a “DNA fingerprint.”
What simple explanation can you give?
4. A cell has extensive rough ER and Golgi apparatus.
Give a brief explanation of its function. A second cell has microvilli and many mitochondria. Give a brief explanation of its function.
5. A bacterial toxin is found to cause harm by first fit- ting into a receptor on human cell membranes;
once the toxin fits, the cell will be destroyed. A medication is going to be made to stop this toxin, and can work in one of two ways: The drug can block the receptors to prevent the toxin from fit- ting in, or the drug can act as decoy molecules shaped like the receptors. Which one of these might be better, and why?
66 Cells