Muscle System | Quizalize

2 ovaries → follicles → oocyte Follicles: made of 1 oocyte + theca cells and granulosa cells Theca cells - make testosterone Granulosa cells - make estradiol Oocytes: fixed amount per lifetime (~400) Ovulation - when oocyte leaves the follicle Fertilization: Connects ovaries to uterus Open-system: fallopian tubes are open to peritoneal space → easy to get infection! NOTE: Endometriosis: when endometrial cells are found in places other than uterus Retrograde menstruation: shedding of endometrium → goes up to fallopian tube (instead of down to cervix → vagina) → enters peritoneal space → travels Endometrium: hormone-sensitive mucosal lining of the endometrium Non-pregnant menstrual cycle: bleeding = shedding of endometrium Pregnancy: endometrium doesn’t shed → implantation occurs here Myometrium: smooth muscle layer of uterus → contracts during period to shed endometrium → cramps Continuous with cervix Dilates during menses and during birth

The cytoskeleton is a network of thin tubes and filaments that crisscrosses the cytosol. The tubes and filaments give shape to the cell from the inside in the same way that tent poles support the shape of a tent. The cytoskeleton also acts as a system of internal tracks, shown in Figure 4-18, on which items move around inside the cell. The cytoskeleton’s functions are based on several struc- tural elements. Three of these are microtubules, microfilaments, and intermediate filaments, shown and described in Table 4-2. Microtubules Microtubules are hollow tubes made of a protein called tubulin. Each tubulin molecule consists of two slightly different subunits. Microtubules radiate outward from a central point called the centrosome near the nucleus. Microtubules hold organelles in place, maintain a cell’s shape, and act as tracks that guide organelles and molecules as they move within the cell. Microfilaments Finer than microtubules, microfilaments are long threads of the beadlike protein actin and are linked end to end and wrapped around each other like two strands of a rope. Microfilaments con- tribute to cell movement, including the crawling of white blood cells and the contraction of muscle cells. Intermediate Filaments Intermediate filaments are rods that anchor the nucleus and some other organelles to their places in the cell. They maintain the inter- nal shape of the nucleus. Hair-follicle cells produce large quantities of intermediate filament proteins. These proteins make up most of the hair shaft. 84 CHAPTER 4 TABLE 4-2 The Structure of the Cytoskeleton Property Microtubules Microfilaments Intermediate filaments Structure hollow tubes made of two strands of intertwined protein fibers coiled into coiled protein protein cables Protein subunits tubulin, with two subunits: å actin one of several types of and ∫ tubulin fibrous proteins Main function maintenance of cell shape; cell maintenance and changing of maintenance of cell shape; motility (in cilia and flagella); cell shape; muscle contraction; anchor nucleus and other chromosome movement; movement of cytoplasm; cell organelles; maintenance of organelle movement motility; cell division shape of nucleus Shape Microtubules provide a path for organelles and molecules as they move throughout the cell. FIGURE 4-18 Microtubules Nucleus Endoplasmic reticulum Mitochondrion Ribosomes Copyright © by Holt, Rinehart and Winston. All rights reserved. Copyright © by Holt, Rinehart and Winston. All rights reserved. CELL STRUCTURE AND FUNCTION 85 1. Explain how the fluid mosaic model describes the plasma membrane. 2. List three cellular functions that occur in the nucleus. 3. Describe the organelles that are found in a eukaryotic cell. 4. Identify two characteristics that make mitochon- dria different from other organelles. 5. Contrast three types of cytoskeletal fibers. CRITICAL THINKING 6. Relating Concepts If a cell has a high energy requirement, would you expect the cell to have many mitochondria or few mitochondria? Why? 7. Analyzing Information How do scientists think that mitochondria originated? Why? 8. Analyzing Statements It is not completely accurate to say that organelles are floating freely in the cytosol. Why not? SECTION 3 REVIEW During cell division, centrioles organize microtubules that pull the chromosomes (orange) apart. The centrioles are at the center of rays of microtubules, which have been stained green with a fluorescent dye. FIGURE 4-20 Cilia and Flagella Cilia (SIL-ee-uh) and flagella (fluh-JEL-uh) are hairlike structures that extend from the surface of the cell, where they assist in movement. Cilia are short and are present in large numbers on certain cells, whereas flagella are longer and are far less numerous on the cells where they occur. Cilia and flagella have a membrane on their outer surface and an internal structure of nine pairs of micro- tubules around two central tubules, as Figure 4-19 shows. Cilia on cells in the inner ear vibrate and help detect sound. Cilia cover the surfaces of many protists and “row” the protists through water like thousands of oars. On other protists, cilia sweep water and food particles into a mouthlike opening. Many kinds of protists use flagella to propel themselves, as do human sperm cells. Centrioles Centrioles consist of two short cylinders of microtubules at right angles to each other and are situated in the cytoplasm near the nuclear envelope. Centrioles occur in animal cells, where they organize the microtubules of the cytoskeleton during cell division, as shown in Figure 4-20. Plant cells lack centrioles. Basal bodies have the same structure that centrioles do. Basal bodies are found at the base of cilia and flagella and appear to organize the devel- opment of cilia and flagella.

The endoplasmic reticulum (EN-doh-PLAZ-mik ri-TIK-yuh-luhm), abbre- viated ER, is a system of membranous tubes and sacs, called cisternae (sis-TUHR-nee). The ER functions primarily as an intracellu- lar highway, a path along which molecules move from one part of the cell to another. The amount of ER inside a cell fluctuates, depending on the cell’s activity. There are two types of ER: rough and smooth. The two types of ER are thought to be continuous. Rough Endoplasmic Reticulum The rough endoplasmic reticulum is a system of interconnected, flattened sacs covered with ribosomes, as shown in Figure 4-15. The rough ER produces phospholipids and proteins. Certain types of proteins are made on the rough ER’s ribosomes. These proteins are later exported from the cell or inserted into one of the cell’s own membranes. For example, ribosomes on the rough ER make digestive enzymes, which accumulate inside the endoplasmic retic- ulum. Little sacs or vesicles then pinch off from the ends of the rough ER and store the digestive enzymes until they are released from the cell. Rough ER is most abundant in cells that produce large amounts of protein for export, such as cells in digestive glands and antibody-producing cells. Smooth Endoplasmic Reticulum The smooth ER lacks ribosomes and thus has a smooth appear- ance. Most cells contain very little smooth ER. Smooth ER builds lipids such as cholesterol. In the ovaries and testes, smooth ER produces the steroid hormones estrogen and testosterone. In skeletal and heart muscle cells, smooth ER releases calcium, which stimulates contraction. Smooth ER is also abundant in liver and kidney cells, where it helps detoxify drugs and poisons. Long-term abuse of alcohol and other drugs causes these cells to produce more smooth ER. Increased amounts of smooth ER in liver cells is one of the factors that can lead to drug tolerance. As Figure 4-15 shows, rough ER and smooth ER form an interconnected network. Copyright © by Holt, Rinehart and Winston. All rights reserved. reticulum from the Latin rete, meaning “net”; reticulum means “little net” Word Roots and Origins The endoplasmic reticulum (ER) serves as a site of synthesis for proteins, lipids, and other materials. The dark lines in the photo represent the membranes of the ER, and the narrow lighter areas between the dark lines show the channels and spaces (cisternae) inside the ER. FIGURE 4-15 Smooth ER Ribosomes Rough ER Cisternae 82 CHAPTER 4 GOLGI APPARATUS The Golgi apparatus, shown in Figure 4-16, is another system of flattened, membranous sacs. The sacs nearest the nucleus receive vesicles from the ER containing newly made proteins or lipids. Vesicles travel from one part of the Golgi apparatus to the next and transport substances as they go. The stacked membranes modify the vesicle contents as they move along. The proteins get “address labels” that direct them to various other parts of the cell. During this modification, the Golgi apparatus can add carbohydrate labels to proteins or alter new lipids in various ways. VESICLES Cells contain several types of vesicles, which perform various roles. Vesicles are small, spherically shaped sacs that are surrounded by a single membrane and that are classified by their contents. Vesicles often migrate to and merge with the plasma membrane. As they do, they release their contents to the outside of the cell. Lysosomes Lysosomes (LIE-suh-SOHMZ) are vesicles that bud from the Golgi appa- ratus and that contain digestive enzymes. These enzymes can break down large molecules, such as proteins, nucleic acids, car- bohydrates, and phospholipids. In the liver, lysosomes break down glycogen in order to release glucose into the bloodstream. Certain white blood cells use lysosomes to break down bacteria. Within a cell, lysosomes digest worn-out organelles in a process called autophagy (aw-TAHF-uh-jee). Lysosomes are also responsible for breaking down cells when it is time for the cells to die. The digestion of damaged or extra cells by the enzymes of their own lysosomes is called autolysis (aw-TAHL-uh-sis). Lysosomes play a very important role in maintaining an organism’s health by destroying cells that are no longer functioning properly. Copyright © by Holt, Rinehart and Winston. All rights reserved. The Golgi apparatus modifies many cellular products and prepares them for export. FIGURE 4-16 CELL STRUCTURE AND FUNCTION 83 Peroxisomes Peroxisomes are similar to lysosomes but contain different enzymes and are not produced by the Golgi apparatus. Peroxisomes are abundant in liver and kidney cells, where they neutralize free radicals (oxygen ions that can damage cells) and detoxify alcohol and other drugs. Peroxisomes are named for the hydrogen peroxide, H2O2, they produce when breaking down alco- hol and killing bacteria. Peroxisomes also break down fatty acids, which the mitochondria can then use as an energy source. Other Vesicles Specialized peroxisomes, called glyoxysomes, can be found in the seeds of some plants. They break down stored fats to provide energy for the developing plant embryo. Some cells engulf material by surrounding it with plasma membrane. The resulting pocket buds off to become a vesicle inside the cell. This vesicle is called an endosome. Lysosomes fuse with endosomes and digest the engulfed material. Food vacuoles are vesicles that store nutrients for a cell. Contractile vacuoles are vesicles that can contract and dispose of excess water inside a cell. Protein Synthesis One of the major functions of a cell is the production of protein. The path some proteins take from synthesis to export can be seen in Figure 4-17. In step , proteins are assembled by ribosomes on the rough ER. Then, in step , vesicles transport proteins to the Golgi apparatus. In step , the Golgi modifies proteins and pack- ages them in new vesicles. In step , vesicles release proteins that have destinations outside the cell. In step , vesicles containing enzymes remain inside the cell as lysosomes, peroxisomes, endo- somes, or other types of vesicles. 5 4 3 2 1 Copyright © by Holt, Rinehart and Winston. All rights reserved. Proteins are assembled by ribosomes on the rough ER. Vesicles carry proteins from the rough ER to the Golgi apparatus. Proteins are modified in the Golgi apparatus and enter new vesicles. Some vesicles release their proteins outside the cell. Other vesicles remain in the cell and become lysosomes and other vesicles. Nucleus

Many of water’s biological functions stem from its chemical struc- ture. Recall that in the water molecule, H2O, the hydrogen and oxygen atoms share electrons to form covalent bonds. However, these atoms do not share the electrons equally. The oxygen atom has a greater ability to attract electrons to it because it pulls hydrogen’s electrons towards its nucleus. As a result, as shown in Figure 2-8, the region of the molecule where the oxygen atom is located has a partial negative charge, denoted with a , while the regions of the molecule where each of the two hydrogen atoms are located have partial positive charges, each of which are denoted with a . Thus, even though the total charge on a water molecule is neutral, the charge is unevenly distributed across the water molecule. Because of this uneven distribution of charge, water is called a polar compound. Notice also in Figure 2-8 that the three atoms in a water mole- cule are not arranged in a straight line as you might expect. Rather, the two hydrogen atoms bond with the single oxygen atom at an angle. SECTION 3 OBJECTIVES ● Describe the structure of a water molecule. ● Explain how water’s polar nature affects its ability to dissolve substances. ● Outline the relationship between hydrogen bonding and the different properties of water. ● Identify the roles of solutes and solvents in solutions. ● Differentiate between acids and bases. VOCABULARY polar hydrogen bond cohesion adhesion capillarity solution solute solvent concentration saturated solution aqueous solution hydroxide ion hydronium ion acid base pH scale buffer Copyright © by Holt, Rinehart and Winston. All rights reserved. (a) Electron cloud model (b) Space-filling model H H O The oxygen region of the water molecule is weakly negative, and the hydrogen regions are weakly positive. Notice the different ways to represent water, H2O. You are familiar with the electron cloud model (a). The space- filling model (b) shows the three- dimensional structure of a molecule. FIGURE 2-8 40 CHAPTER 2 Hydrogen bond H H H H H H H H H O O O O O O H H H H H – – – – – – – + + + + + + + + + + + + + + The dotted lines in this figure represent hydrogen bonds. A hydrogen bond is a force of attraction between a hydrogen atom in one molecule and a negatively charged region or atom in a second molecule. FIGURE 2-10 The positive region of a water molecule attracts the negative region of an ionic compound, such as the Cl portion of NaCl. Similarly, the negative region of the water molecule attracts the positive region of the compound—the Na portion of NaCl. As a result, NaCl breaks apart, or dissolves, in water. FIGURE 2-9 CI– Na+ H2O + + – – Solubility of Water The polar nature of water allows it to dissolve polar substances, such as sugars, ionic compounds, and some proteins. Water does not dissolve nonpolar substances, such as oil because a weaker attraction exists between polar and nonpolar molecules than between two polar molecules. Figure 2-9 shows how water dissolves the ionic compound sodium chloride, NaCl. In your body, ions, such as sodium and chloride, are essential to bodily func- tions, such as muscle contraction and transmission of impulses in the nervous system. In fact, dissolved, or dissociated ions, are pre- sent in all of the aqueous solutions found in living things and are important in maintaining normal body functions. HYDROGEN BONDING The polar nature of water also causes water molecules to be attracted to one another. As is shown in Figure 2-10, the positively charged region of one water molecule is attracted to the negatively charged region of another water molecule. This attraction is called a hydrogen bond. A hydrogen bond is the force of attraction between a hydrogen molecule with a partial positive charge and another atom or molecule with a partial or full negative charge. Hydrogen bonds in water exert an attractive force strong enough so that water “clings” to itself and some other substances. Hydrogen bonds form, break, and reform with great frequency. However, at any one time, a great number of water molecules are bonded together. The number of hydrogen bonds that exist depends on the state that water is in. If water is in its solid state all its water molecules are hydrogen bonded and do not break. As water liquifies, more hydrogen bonds are broken than are formed, until an equal number of bonds are formed and broken. Hydrogen bonding accounts for the unique properties of water, some of which we will examine further. These properties include cohesion and adhesion, the ability of water to absorb a relatively large amount of energy as heat, the ability of water to cool surfaces through evaporation, the density of ice, and the ability of water to dissolve many substances.

Some substances, such as macromolecules and nutrients, are too large to pass through the cell membrane by the transport processes you have studied so far. Cells employ two other transport mecha- nisms—endocytosis and exocytosis—to move such substances into or out of cells. Endocytosis and exocytosis are also used to transport large quantities of small molecules into or out of cells at a single time. Both endocytosis and exocytosis require cells to expend energy. Therefore, they are types of active transport. Endocytosis Endocytosis (EN-doh-sie-TOH-sis) is the process by which cells ingest external fluid, macromolecules, and large particles, including other cells. As you can see in Figure 5-7, these external materials are enclosed by a portion of the cell’s membrane, which folds into itself and forms a pouch. The pouch then pinches off from the cell membrane and becomes a membrane-bound organelle called a vesicle. Some of the vesicles fuse with lysosomes, and their con- tents are digested by lysosomal enzymes. Other vesicles that form during endocytosis fuse with other membrane-bound organelles. Two main types of endocytosis are based on the kind of material that is taken into the cell: pinocytosis (PIEN-oh-sie-TOH-sis) involves the transport of solutes or fluids, and phagocytosis (FAG-oh-sie-TOH-sis) is the movement of large particles or whole cells. Many unicellular organisms feed by phagocytosis. In addition, certain cells in animals use phagocytosis to ingest bacteria and viruses that invade the body. These cells, known as phagocytes, allow lysosomes to fuse with the vesicles that contain the ingested bacteria and viruses. Lysosomal enzymes then destroy the bacteria and viruses before they can harm the animal. CYTOSOL EXTERNAL ENVIRONMENT During endocytosis, the cell membrane folds around food or liquid and forms a small pouch. The pouch then pinches off from the cell membrane to become a vesicle. FIGURE 5-7 vesicle from the Latin vesicula, meaning “bladder” or “sac” Word Roots and Origins www.scilinks.org Topic: Endocytosis Keyword: HM60505 mb06se_homs02.qxd 5/18/07 11:03 AM Page 105 106 CHAPTER 5 1. Explain the difference between passive trans- port and active transport. 2. What functions do carrier proteins perform in active transport? 3. What provides the energy that drives the sodium-potassium pump? 4. Explain the difference between pinocytosis and phagocytosis. 5. Describe the steps involved in exocytosis. 6. How do endocytosis and exocytosis differ? How can that difference be seen? CRITICAL THINKING 7. Analyzing Information During intense exercise, potassium tends to accumulate in the fluid surrounding muscle cells. What membrane protein helps muscle cells counteract this tendency? Explain your answer. 8. Evaluating Differences How does the sodium- potassium pump differ from facilitated diffusion? 9. Relating Concepts The vesicles formed during pinocytosis are much smaller than those formed during phagocytosis. Explain. SECTION 2 REVIEW Vesicle Cell membrane EXTERNAL ENVIRONMENT CYTOSOL During exocytosis, a vesicle moves to the cell membrane, fuses with it, and then releases its contents to the outside of the cell. FIGURE 5-8 INSIDE OF CELL Vesicle OUTSIDE OF CELL Exocytosis Exocytosis (EK-soh-sie-TOH-sis) is the process by which a substance is released from the cell through a vesicle that transports the sub- stance to the cell surface and then fuses with the membrane to let the substance out of the cell. This process, illustrated in Figure 5-8, is basically the reverse of endocytosis. During exocytosis, vesi- cles release their contents into the cell’s external environment. Figure 5-8 also shows a photo of a vesicle during exocytosis. Cells may use exocytosis to release large molecules such as pro- teins, waste products, or toxins that would damage the cell if they were released within the cytosol. Recall that proteins are made on ribosomes and packaged into vesicles by the Golgi apparatus. The vesicles then move to the cell membrane and fuse with it, deliver- ing the proteins outside the cell. Cells in the nervous and endocrine systems also use exocytosis to release small molecules that control the activities of other cells.

Related quizzes

Related quizzes