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The functions of the Muscular system
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Structures and Function of the Muscular Systemstructure and function of the muscular systemThe functions of the skeletonThe functions of the family Identify the functions of the basic tools in photoshop tool barsMost of the functions of a eukaryotic cell are controlled by the nucleus, shown in Figure 4-12. The nucleus is filled with a jellylike liquid called the nucleoplasm, which holds the contents of the nucleus and is similar in function to a cell’s cytoplasm. The nucleus houses and protects the cell’s genetic information. The hereditary information that contains the instructions for the structure and function of the organism is coded in the organism’s DNA, which is contained in the nucleus. When a cell is not dividing, the DNA is in the form of a threadlike material called chromatin. When a cell is about to divide, the chromatin condenses to form chromosomes. Chromosomes are structures in the nucleus made of DNA and protein. The nucleus is the site where DNA is transcribed into ribonucleic acid (RNA). RNA moves through nuclear pores to the cytoplasm, where, depending on the type of RNA, it carries out its function. Nuclear Envelope The nucleus is surrounded by a double membrane called the nuclear envelope. The nuclear envelope is made up of two phos- pholipid bilayers. Covering the surface of the nuclear envelope are tiny, protein-lined holes, which are called nuclear pores. The nuclear pores provide passageways for RNA and other materials to enter and leave the nucleus. Nucleolus Most nuclei contain at least one denser area, called the nucleolus (noo-KLEE-uh-luhs). The nucleolus (plural, nucleoli) is the site where DNA is concentrated when it is in the process of making ribosomal RNA. Ribosomes (RIE-buh-SOHMZ) are organelles made of protein and RNA that direct protein synthesis in the cytoplasm. The nucleus of a cell is surrounded by a double membrane called the nuclear envelope. The nucleus stores the cell’s DNA. FIGURE 4-12 Nuclear envelope Nucleolus Nuclear pores DNA (chromatin) Copyright © by Holt, Rinehart and Winston. All rights reserved. 80 CHAPTER 4 MITOCHONDRIA Mitochondria (MIET-oh-KAHN-dree-uh) (singular, mitochondrion) are tiny organelles that transfer energy from organic molecules to adenosine triphosphate (ATP). ATP ultimately powers most of the cell’s chemical reactions. Highly active cells, such as muscle cells, can have hundreds of mitochondria. Cells that are not very active, such as fat-storage cells, have few mitochondria. Like a nucleus, a mitochondrion has an inner and an outer phos- pholipid membrane, as shown in Figure 4-13. The outer membrane separates the mitochondrion from the cytosol. The inner membrane has many folds, called cristae (KRIS-tee). Cristae contain proteins that carry out energy-harvesting chemical reactions. Mitochondrial DNA Mitochondria have their own DNA and can reproduce only by the division of preexisting mitochondria. Scientists think that mito- chondria originated from prokaryotic cells that were incorporated into ancient eukaryotic cells. This symbiotic relationship provided the prokaryotic invaders with a protected place to live and pro- vided the eukaryotic cell with an increased supply of ATP. RIBOSOMES Ribosomes are small, roughly spherical organelles that are respon- sible for building protein. Ribosomes do not have a membrane. They are made of protein and RNA molecules. Ribosome assembly begins in the nucleolus and is completed in the cytoplasm. One large and one small subunit come together to make a functioning ribosome, shown in Figure 4-14. Some ribosomes are free within the cytosol. Others are attached to the rough endoplasmic reticulum.SCIENCE 6 Coordination and Control: Exploring the functions of the nervous system.How did the framers of the Constitution try to limit the power of the national government? Separation of Powers: a key constitutional principle that divides the functions of government among three branches (legislative, executive, and judicial) to prevent any one branch from gaining too much power How did the framers of the Constitution try to keep one branch of the government from dominating the others? A system of checks and balances Checks: allow one branch to block the actions of other branch Congress can pass laws, but the president can check this power through veto. Congress can check the president’s veto power by a two-thirds majority vote in each house. Judicial branch can check the actions of both Congress and the president through its power of judicial review and so declare a law, a treaty, or an executive action unconstitutional. Balances: allow each branch of the government to have some role in the actions and power of the other branches Judges, ambassadors, and cabinet members are appointed only if the president nominates them and the Senate approves the nomination. President can sign treaties, but they only take effect if the Senate approves them. Powers of judicial branch are also balanced by the other branches. -- The Supreme Court can declare laws unconstitutional, but the president chooses federal judges, and the Senate must approve the appointments. Congress can impeach federal judges.