¶ Cell Structure and Transport Mechanisms
Welcome to the Chapter Review Podcast for Scanlon and Sanders Essentials of Anatomy and Physiology, 6th Edition. Chapter 3. Cells. Human cells vary in size, shape, and function. Our cells function interdependently to maintain homeostasis of the body as a whole. The major parts of a cell are the cell membrane, the nucleus, except mature RBCs,
the cytoplasm and the cell organelles. The cell membrane is the selectively permeable boundary of the cell. Phospholipids permit diffusion of lipid-soluble materials. Cholesterol provides stability. Proteins form channels, transporters, self-antigens, and receptor sites for hormones or other signaling molecules. The nucleus is the control center of the cell. It has a double layer membrane. The nucleolus forms ribosomal RNA. Chromosomes are 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 the total of their genetic information is called the genome. Cytoplasm is a watery solution of minerals, gases, and organic molecules. It contains the cell organelles and is the site for many chemical reactions. Cell organelles are the intracellular structures with specific functions.
Cellular transport mechanisms are the processes by which cells take in or secrete or excrete materials through the selectively permeable cell membrane. diffusion is the movement of molecules from an area of greater concentration to an area of lesser concentration it occurs because molecules have free energy they are constantly in motion oxygen and carbon dioxide are exchanged by diffusion in the lungs and tissues. Osmosis is the diffusion of water. Water diffuses to an area of less water.
that is, to an area of more dissolved material. The small intestine absorbs water from digested food by osmosis. In 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. In active transport, a cell uses ATP to move substances from an area of lesser concentration to an area of greater concentration.
nerve cells and muscle cells have sodium pumps to return sodium ions to the exterior of the cells. This prevents spontaneous impulses. Cells of the small intestine absorb glucose and amino acids from digested food by active transport. In 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 capillaries creates filtration, which is the first step in the formation of urine. In phagocytosis, a form of endocytosis, a moving cell engulfs something. White blood cells phagocytize bacteria to destroy them. In pinocytosis, another form of endocytosis,
A stationary cell engulfs small molecules. Kidney tubule cells reabsorb small proteins by pinocytosis. The genetic code and protein synthesis.
¶ Genetic Code and Protein Synthesis
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 code for the sequence of amino acids in a protein is a triplet code. Three DNA bases, also called a codon, are the code for one amino acid. A gene consists of all the triplets that code for a single protein.
mRNA is formed as a complementary 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 anticodons for the triplets on the mRNA. In translation, tRNA molecules bring amino acids to their proper triplets on the mRNA. ribosomes contain enzymes to form peptide bonds between the amino acids a summary of the expression of the genetic code is as follows DNA is the template for synthesis of RNA
RNA directs protein synthesis. Proteins may be structured or may be enzymes that catalyze reactions. Protein structure and reactions determine hereditary characteristics. A genetic disease is a mistake in the DNA that is copied by mRNA and results in a malfunctioning protein. Cell Division
¶ Cell Division and Life Cycles
In mitosis, one cell with a diploid number of chromosomes divides once to form two cells, each with a diploid number of chromosomes. The human diploid number is 46. When a cell is between divisions, DNA replication forms two sets of chromosomes. This part of the cell cycle may be called the interphase. The stages of mitosis are prophase, metaphase
anaphase, and telophase. Cytokinesis is the division of the cytoplasm following telophase. Mitosis is essential for growth and for repair and replacement of damaged cells. Most adult nerve and muscle cells are not able to divide sufficiently to bring about repair of damaged tissue. Their loss may involve permanent loss of function. Apoptosis is genetically programmed cell death.
which occurs at the end of a cell's lifespan or when a cell is damaged. In meiosis, one cell with a diploid number of chromosomes divides twice to form four cells. each with a haploid number of chromosomes. The human haploid number is 23, found in egg cells and sperm cells. 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