4 CELLS
CHAPTER OUTLINE
The World of Cells (p. 72)
4.1 Cells (p. 72; Figs. 4.1, 4.2, 4.3; Table 4.1)
A. The human body is made up of cells, most all of which are too small to be seen with the naked eye.
B. The Cell Theory
1. Robert Hooke first described cells in 1665 and coined the term that eventually came to be “cells.”
2. Modern cell theory includes three principles, as follows:
a. All living organisms are composed of cells.
b. Nothing smaller than a cell is considered to be alive.
c. Cells arise only from preexisting cells, and all living organisms have descended from the earliest cells.
C. Most Cells Are Very Small.
1. Cells vary in size, most of which are 5 to 20 micrometers in diameter.
D. Why Aren’t Cells Larger?
1. Cells are small because larger cells do not function efficiently.
2. Cell size is limited by two factors: (1) surface area-to-volume relationships that make distribution of materials throughout a large cell difficult, and (2) the volume of cytoplasm the nucleus can control.
E. An Overview of Cell Structure
1. Cells are filled with a semifluid matrix, called cytoplasm, surrounded by a membrane that controls the permeability of the cell.
F. Visualizing Cells
1. Cells can be made visible by the use of microscopes.
2. Certain cell structures can be made more visible by staining specific molecules in a cell.
4.2 The Plasma Membrane (p. 76)
A. All cells are surrounded by a plasma membrane composed of a bilayer of phospholipids, according to the fluid mosaic model.
B. The polar ends of these phospholipids interact with the fluid interior and exterior environments of the cell, while the nonpolar fatty acid chains form the interior of the membrane.
C. The phospholipids are arranged in a bilayer, with nonpolar tails extending to the inside.
D. Proteins Within the Membrane
1. Floating within the lipid bilayer are a variety of membrane proteins. Some of these proteins function to transport materials across the membrane (transmembrane proteins), and others serve as identification markers in cell communication (cell surface proteins).
E. Membrane Defects Can Cause Disease.
1. Cystic fibrosis is a deadly genetic disorder caused by a defective transmembrane protein that does not properly admit chloride ions into the body's cells.
Kinds of Cells (p. 78)
4.3 Prokaryotic Cells (p. 78; Figs. 4.4, 4.5)
A. Cells can be divided into prokaryotic cells or eukaryotic cells based on whether or not the cytoplasm of the cells is divided into compartments by internal membranes.
B. Prokaryotic cells lack internal membranes and are evolutionarily more primitive.
C. Prokaryotes include the bacteria and the archaea. Prokaryotic cells contain ribosomes and DNA but no internal membrane-bounded organelles. Most prokaryotic cells are surrounded by a cell wall.
4.4 Eukaryotic Cells (p. 80; Figs. 4.6, 4.7)
A. Eukaryotic cells are more complex and can compartmentalize different chemical reactions into membrane-bounded interior compartments and a variety of organelles.
B. Eukaryotic cells, as their name implies, have a true nucleus, while prokaryotes lack one.
C. Plant cells also have storage vacuoles and protective cell walls.
Tour of a Eukaryotic Cell (p. 83)
4.5 The Nucleus: The Cell’s Control Center (p. 83; Fig. 4.8)
A. The nucleus is the command and control center of the eukaryotic cell, directing all of its activities.
B. Nuclear Membrane
1. The nucleus is enclosed by a double membrane, the nuclear envelope, which contains pores to regulate the passage of materials into and out of the nucleus.
C. Chromosomes
1. The DNA of eukaryotes is divided into segments associated with protein, forming chromosomes.
2. When the cell is not undergoing division, the DNA is uncoiled into threadlike strands called chromatin.
D. Nucleolus
1. A darker staining area inside the nucleus is the nucleolus, which contains information for the construction of ribosomes that are needed for protein synthesis.
4.6 The Endomembrane System (p. 84)
A. The endomembrane system is a tightly packed mass of membranes that divides the cell into compartments, channels the transport of materials, and provides surfaces on which enzymes act.
B. Endoplasmic Reticulum: The Transportation System
1. The endoplasmic reticulum serves to manufacture and package carbohydrates and lipids into vesicles and readies proteins for export from the cell.
2. Rough ER has ribosomes associated with it; smooth ER does not.
C. The Golgi Complex: The Delivery System
1. The Golgi complex collects, packages, and redistributes molecules within the cell and packages them for excretion from the cell.
D. Lysosomes: Recycling Centers
1. Lysosomes digest worn-out cell parts and can recycle proteins and other materials.
E. Peroxisomes: Chemical Specialty Shops
1. Peroxisomes house enzymes that detoxify harmful molecules and convert fats to carbohydrates.
4.7 Organelles that Contain DNA (p. 86; Figs. 4.9, 4.10, 4.11)
A. Mitochondria and chloroplasts contain DNA, giving evidence for their ancient past as solitary bacteria.
B. Mitochondria: Powerhouses of the Cell
1. Mitochondria house the enzymes needed for the production of energy in a form the cell can use through a process called oxidative metabolism.
C. Chloroplasts: Energy-Capturing Centers
1. Chloroplasts (in plants and algae) harvest light energy and convert it to chemical form.
D. Endosymbiosis
1. Mitochondria and chloroplasts are cell-like organelles that appear to be ancient bacteria that formed endosymbiotic relationships with early eukaryotic cells.
4.8 The Cytoskeleton: Internal Framework of the Cell (p. 88; Figs. 4.12, 4.13, 4.14, 4.15; 4.16, 4.17; Table 4.2)
A. The cytoplasm of eukaryotic cells is organized by a network of protein fibers known as the cytoskeleton.
1. Three kinds of protein fibers comprise the cytoskeleton: microfilaments, microtubules, and intermediate filaments.
B. These fibers help determine the shape of animal cells, provide a place for enzymes to attach so that metabolic reactions can occur more efficiently, move chromosomes during cell division, and anchor organelles.
C. Centrioles
1. One type of cytoskeleton fiber, the microtubule, also plays a role in the structure and function of centrioles, which are used to anchor and move flagella as well as to aid in cell division.
D. Vacuoles: A Central Storage Compartment
1. The interior of plant and most protist cells contains compartments called vacuoles used to store water and other substances.
E. Cell Movement
1. All cell motion is tied to the movement of actin filaments, microtubules, or both.
2. Some cells crawl: the arrangement of actin filaments within the cytoplasm allows cells such as those of the immune system to crawl.
3. Swimming with flagella and cilia: cilia and flagella are composed of groups of microtubules and function in cell motility and movement of materials past stationary cells.
6. Flagella and cilia exhibit a 9+2 arrangement of microtubules.
F. Moving Materials within the Cell
1. Most cells use the endomembrane system as an intracellular highway.
2. For long-distance intracellular transport, however, cells move materials along microtubule tracks. To do this, four components are required: a vesicle, motor molecules, connector molecules, and microtubules.
4.9 Outside the Plasma Membrane (p. 94; Figs. 4.18, 4.19)
A. Cell Walls: Protection and Support
1. Plant, fungi, and many protist cells have protective cell walls outside their plasma membranes.
2. Eukaryotic cell walls are chemically and structurally different than bacterial cell walls.
3. In plant cells, primary walls are laid down while the cell is still growing; secondary cell walls are later deposited inside the primary walls.
B. An Extracellular Matrix Surrounds Animal Cells
1. Animal cells secrete a mixture of glycoproteins into the space around them forming the extracellular matrix.
2. Collagen and elastin are embedded in the glycoproteins.
3. The ECM is attached to the plasma membrane by fibronectin molecules, which attach to integrins.
Transport Across Plasma Membranes (p. 95)
4.10 Diffusion (p. 95; Fig. 4.20)
A. Materials must move in and out of the cell in order for the cell to survive.
B. Movement across the cell membrane occurs in three ways:
1. Water diffuses through the membrane.
2. Particles and liquids are engulfed by the membrane folding around them.
3. Proteins in the membrane allow the passage of certain molecules.
C. Diffusion
1. The random motion of molecules tends to make them disperse from areas in which they are more concentrated to areas of lesser concentration until a uniform distribution is attained.
2. Simple diffusion is based on the random motion of molecules and is the way that small molecules, like gases, enter and exit cells.
4.11 Facilitated Diffusion (p. 97)
A. Cells use an attribute called selective permeability to regulate the passage of specific molecules into or out of the cell.
B. Selective Diffusion
1. Cells can carefully control what enters and exits their membranes by using channel proteins.
C. Facilitated Diffusion
1. Facilitated diffusion is a special case of diffusion that involves passage in either direction through a carrier protein.
2. Carrier proteins are limited in number and can become saturated when there is an excess of the transported substance.
4.12 Osmosis (p. 98; Fig.4.21)
A. Osmosis
1. Movement of water across a membrane is a special case of diffusion called osmosis.
2. When water moves into a cell, there is greater pressure, called osmotic pressure.
3. Water moves from areas with less solute and proportionately more free water, to areas with more solute and less free water.
4.13 Bulk Passage into and Out of the Cell (p. 100; Figs. 4.22, 4.23, 4.24)
A. Endocytosis and Exocytosis
1. Many eukaryotic cells extend their plasma membranes to engulf particles, a process called endocytosis.
2. Particles can be expelled via exocytosis when cytoplasmic vesicles fuse with the plasma membrane, spilling contents to the outside.
B. Phagocytosis and Pinocytosis
1. When endocytosis involves larger particles, it is referred to as phagocytosis.
2. Pinocytosis occurs when cells engulf smaller particles.
C. Receptor-Mediated Endocytosis
1. Receptor-mediated endocytosis is a selective transport process, bringing in only those substances that are able to bind to specific receptors.
4.14 Active Transport (p. 102; Table 4.3)
A. Active Transport
1. Active transport is a process that requires both a transport protein and energy to move molecules from an area of lesser concentration to one of greater concentration.
2. One of the most important active transport channels in cells is the sodium-potassium pump.
3. Active transport (sodium-potassium pump) uses channel proteins to move materials against a concentration gradient, and thus requires energy.
4. Active transport occurs as ATP is split, releasing energy, and a phosphate group is added to the channel protein, causing the channel protein to change shape and transport the desired molecule across the membrane.
5. The sodium-potassium pump is used to actively transport sodium ions out of the cell and move potassium ions into the cell.
6. The resulting sodium-potassium gradient is used in the conduction of nerve impulses and in the movement of sugars and amino acids through coupled channels.
7. In coupled transport, sodium ions can move back into cells, but only along with another molecule through coupled channels. In this way, materials such as amino acids or sugars can move into cells against their concentration gradient.
KEY TERMS
· cells (p. 70)
· cell theory (p. 70) This is one of the major themes of biology.
· surface-to-volume ratio (p. 71)
· cytoplasm (p. 71)
· microscope (p. 72)
· resolution (p. 72)
· plasma membrane (p. 74)
· fluid-mosaic model (p. 76) One of the amazing features of a plasma membrane is the ability of the phospholipids to exchange places as needed, such as during endocytosis, and is the reason for the term “fluid” in the name.
· phospholipid (p.76)
· lipid bilayer (p. 76)
· cell surface proteins (p.77)
· transmembrane proteins (p. 77)
· prokaryotic (p.79)
· organelles (p. 80)
· flagellum (p. 79)
· eukaryotic (p.80 )
· plasma membrane (p. 80)
· nucleus (p. 80)
· vesicles (p.80)
· cytoskeleton (p. 88)
· ribosome (p.83 )
· endoplasmic reticulum (p.84)
· lysosomes (p.85 )
· mitochondria (p.86 ) Plants, animals, and other eukaryotes employ these energy powerhouses (students often think plants do not require mitochondria). Mitochondrial function is discussed in depth in Chapter 7.
· Golgi bodies (p.84 )
· chloroplast (p. 81) Chloroplast structure and function is discussed in depth in Chapter 6.
· plasmodesmata (p. 81)
· cell walls (p. 81)
· cystic fibrosis (p.82)
· nucleolus (p. 83)
· endosymbiosis (p. 87)
· microfilaments (p. 88)
· microtubules (p. 88)
· contractile vacuole (p. 89)
· cilia (p. 93)
· diffusion (p. 95)
· selective permeability (p. 95)
· facilitated diffusion (p. 97)
· osmosis (p. 98) Just like in diffusion, where a substance diffuses from an area of greater concentration to one of lesser concentration, in osmosis, water moves from an area where water is more abundant to one in which there is less water.
· endocytosis (p. 100) The opposite of endocytosis is exocytosis.
· active transport (p. 102)
LECTURE SUGGESTIONS AND ENRICHMENT TIPS
1. Why Cells Are Small. Go through a mathematical exercise comparing the various sizes of spherical objects and their surface area-to-volume relationships. Using a golf ball, a tennis ball, and a basketball, have a student volunteer to measure the radius of each object. Then determine the surface area and volume for each ball (surface area is 4πr2, and volume is 4/3 πr3). Review with students why the golf ball would make a more likely candidate for a cell than would a basketball.
2. Why Onions Make You Cry. The following information can be presented to show how enzymes trigger chemical reactions, how cells can compartmentalize different compounds, and/or how organisms have evolved means of defense: A number of plants, including onions and garlic, produce compounds that protect them against grazing predators such as insects, certain bacteria, and animals. In both garlic and onions, a compound stored inside cells interacts with an enzyme stored between cells only when the bulb is chewed or cut. In onions, the compound produced when the enzyme comes in contact with the intercellular chemical is called a lachrymator because it produces tears. What happens is, the lachrymator becomes airborne, combines with the fluids in the eye, and forms sulfuric acid. The formation of sulfuric acid in our eyes is very painful, stings the eyes, and produces tears. Onions are not offensive until they are cut or grazed. The rupture of the cell membrane triggers a chemical reaction that has evolved as a mechanism to protect these plants.