Clinical Anatomy and Physiology for Veterinary Technicians Chapter 3
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What are the basic cellular functions that define life?
Cells can grow, develop, reproduce, adapt, become influenced by outside stimuli, maintain a stable internal environment, and convert food into usable energy. Each cell carries vital genetic material that governs its own development, metabolism, and specialization.
Describe the series of events that scientists believe led to the formation of the first cells on earth.
The first cells are thought to have evolved in the massive oceans of our primitive Earth about 3 billion years ago. Jolted by the fierce electrical energy from frequent lightning storms and by the intense, unabated radiation from the sun, four molecules (methane gas [CH4], water [H2O], and ammonia [NH3]) which made up the primitive atmosphere were forced to collide and split apart. The first organic molecules, similar to amino acids, are thought to have formed in this tempestuous environment. Clustering into heavy droplets, these molecules are believed to have been washed by driving rains from the atmosphere into the warm, shallow seas below. There, proteins, lipids, and carbohydrates evolved and arranged themselves over time into sophisticated, organized structures…the first cells.
What is the difference between a prokaryote and a eukaryote?
Prokaryotes do not contain nuclei. Eukaryotic cells have a distinct nucleus in which the DNA has combined with protein to form chromosomes. These, in turn, are surrounded by a protective nuclear envelope.
Why are cells called “cells”?
In 1665 Robert Hooke examined the structure of cork. He noticed that the cork was composed of thousands of small "rooms," and he called these rooms cells. The word cell is from the Latin cella meaning “little chamber.” Hooke did not know at the time that the actual cell as we know it had died and that he was examining the honeycomblike structure of the remaining cell walls. Other researchers began to study plant and animal tissues and continued to use the word cell to describe the basic parts of these tissues.
Why aren’t cells the size of watermelons?
Smaller cells have smaller nutritional requirements than large cells but have a proportionately larger surface through which they can absorb the substances they need. Thus smaller cells are able to complete their metabolic functions more rapidly and efficiently than large cells. If cells were the size of watermelons, they would not be able to take in nutrients fast enough to feed themselves and would die. A second limiting factor in cell size is related to the governing capability of the nucleus. A single nucleus can control the metabolic activity of a small cell better than it could a large one. Also, the more active a cell is, the greater its metabolic needs. Therefore it is not surprising that very large cells or cells that are more active, such as cardiac and skeletal muscle cells, have two or more nuclei.
Name three structures that all mammalian cells possess.
The cell membrane, the cytoplasm, and the nucleus
Draw a picture of the lipid bilayer. Which part is hydrophobic and which part is hydrophilic?
The side of the lipid bilayer facing the outside of the cell is hydrophilic and the side facing the inside of the cell is hydrophobic .
What types of protein are found in the cell membrane?
The cell membrane contains structural and globular proteins. Globular proteins include integral and peripheral proteins.
Where are the proteins located, and what are their functions? Add them to your drawing.
Integral and peripheral proteins are types of globular proteins. Some integral proteins are located within the bilayer, spanning it. These form selective passageways and pores that permit only particular substances to enter or exit the cell. Some integral proteins are membrane receptors that act as binding sites on the cell’s surface. Peripheral proteins are bound to the inside and outside surfaces of the cell membrane and sometimes act as enzymes to catalyze specific chemical reactions. They may also be involved in the mechanics of changing the cell’s shape. Glycoproteins, in addition to glycolipids, are the principal components of the “sugar coating” that covers the surface of some cells. This coating is called the glycocalyx.
What is the glycocalyx?
The glycocalyx is a “sugar coating” on the outside of the cell; it is made of glycoprotein and glycolipid molecules. Similar to the stripes on zebras or the fingerprints on human hands, each glycocalyx is unique. It provides improved cell-to-cell adhesion and represents an important biological marker for intercellular recognition and for the interactions between the cell and antibodies and the cell and viruses.
What are CAMs and what do they do?
CAM refers to cell adhesion molecules, which are sticky glycoproteins (part of the glycocalyx) that cover the surfaces of almost all cells in mammals and allow them to bond to extracellular molecules and to each other. These molecules are also important in helping cells move past one another and in signaling circulating cells, such as white blood cells, to areas of inflammation or infection.
What are membrane receptors and what do they do?
Membrane receptors are integral proteins and glycoproteins that act as binding sites on the cell surface. Some of them play a vital role in cell-to-cell recognition, a process called contact signaling. This is particularly important during cell-mediated immune responses and assists bacteria and viruses in finding preferred “target” cells. Membrane receptors are also involved in a process called chemical signaling. Hormones, neurotransmitters, and other chemical messengers called ligands bind to specific binding sites on cell surfaces. Once bound to the cell membrane, ligands can bring about a change in the cell’s activity. Some ligands act as enzymes to activate or inactivate a particular cellular activity.
How are cilia and flagella different?
- Cilia occur in large numbers on the exposed surface of some cells. They are shorter than flagella and measure only about 10 μm long. They move synchronously, one after the other, creating waves of motion that propel fluid, mucus, and debris across the cell surface. Cilia are best known for their important functions (1) in the upper respiratory tract, where they propel bacteria and mucus away from the lungs, and (2) in the oviduct, where their beating motion pulls the ovulated egg away from the ovary and into the opening of the oviduct.
- Flagella generally occur singly and are significantly longer than cilia. They are typically attached to individual cells and propel the cell forward by undulating. Flagella move cells through fluid, whereas cilia move fluid across cell surfaces. The tail of a sperm cell is an example of a flagellum.
Which are found more commonly in mammalian cells: cilia or flagella?
What are the four principal components of cytoplasm?
Cytosol, cytoskeleton, organelles, and inclusions
What is cytosol and what kind of molecules are found in it?
Cytosol is the fluid of the cell It is a viscous, semitransparent liquid composed of dissolved electrolytes, amino acids, and simple sugars. Proteins are also suspended in the cytosol and give it its thick, jellylike consistency.
What is the cytoskeleton and what is its function?
The cytoskeleton is a three-dimensional frame for the cells that is neither rigid nor permanent. It is a flexible, fibrous structure that changes in accordance with the activities of the cell. The cytoskeleton gives support and shape to the cell and enables it to move, provides direction for metabolic activity, and anchors the organelles.
How many types of fibers make up the cytoskeleton? Can you name them? How do they function differently?
- Three different types of fibers compose the cytoskeleton, all of which are made of protein. The fibers are microtubules, intermediate fibers, and microfilaments.
- Microtubules form secure “cables” to which mitochondria, lysosomes, and secretory granules attach. Proteins that act as “motors” move the attached organelles along the microtubule from one location in the cell to another. Because microtubules act as the “railroad tracks” for organelle travel, they can be easily disassembled and then reassembled to form new paths or take a new direction.
- Intermediate fibers are woven, ropelike fibers that possess high tensile strength and are able to resist pulling forces on the cell by acting as internal guy wires. These fibers are the toughest and most permanent element of the cytoskeleton.
- Microfilaments play a key role in the cell’s ability to change shape, break apart during cell division, and form outpouchings and involutions. In most cells, microfilaments are assembled where and when needed.
How does each of these organelles function within the cell?
- The mitochondrion is known as the “powerhouse” of the cell because it produces 95% of the energy that fuels the cell. In the mitochondria, large nutrient molecules, such as glucose, are processed and broken down into smaller ones that can be used intracellularly to fuel most metabolic processes. The mitochondrion is also where cell respiration takes place: oxygen is consumed, and carbon dioxide is excreted. Numerous biochemical reactions (e.g., amino acid and fatty acid catabolism, respiratory electron transport, oxidative phosphorylation, and the oxidative reactions of the citric acid cycle) occur in the mitochondria.
- The ribosome is an important site for protein synthesis. Soluble protein intended for intracellular use is manufactured on ribosomes that are evenly distributed throughout the cytoskeleton. Protein intended for use in the plasma membrane or meant for cellular export is synthesized on ribosomes attached to the endoplasmic reticulum.
- Rough endoplasmic reticulum(ER) is involved in the production of protein, which is assembled by the ribosomes that stud the ER during protein synthesis (giving it its "rough" appearance). Newly manufactured molecules of protein are moved internally into passageways known as cisternae, Latin for "reservoirs." Here the proteins are modified before being moved on to the Golgi apparatus for further modification and packaging.
- Smooth ER, which is connected to rough ER, is active in the synthesis and storage of lipids, particularly phospholipids and steroids, and is therefore seen in large quantities in gland cells. In liver cells, smooth ER may also function to eliminate drugs and break down glycogen into glucose.
- The Golgi apparatus acts as a modification, packaging, and distribution center for molecules destined for either secretion or intracellular use. It also functions in polysaccharide synthesis and in the coupling of polysaccharides to proteins (glycoproteins) on the cell surface.
- The lysosome’s principal responsibilities are the breakdown of nutrient molecules into usable smaller units and the digestion of intracellular debris. Lysosomes may also release their enzymes outside the cell to assist with the breakdown of extracellular material. In addition, lysosomal digestion is responsible for decreasing the size of body tissues (for example, shrinkage of the uterus after parturition and atrophy of muscles in paralyzed animals).
- Peroxisomes are commonly found in liver and kidney cells and are important in the detoxification of various molecules. Peroxisomes contain enzymes that use oxygen to detoxify a number of harmful substances, including alcohol and formaldehyde. They also assist in the removal of free radicals, which are normal products of cellular metabolism but can be harmful to the cell in large quantities because they interfere with the structures of proteins, lipids, and nucleic acids.
Why do inclusions vary in appearance? What function do they perform?
The appearance of inclusions varies depending on what they contain and whether or not they have an envelope. They store substances the cell eventually uses.
What role does the centriole play in the formation of cilia and flagella?
Centrioles form the bases of cilia and flagella and in this role are known as basal bodies.
How are centrioles structurally similar to cilia and flagella?
Centrioles are structurally similar to cilia and flagella because all consist of microtubules. Centrioles are small hollow cylinders composed of microtubules. Cilia and flagella are composed of nine pairs of microtubules that encircle a central pair of microtubules.
Why is the nucleus considered the “CEO of operations”?
The nucleus is considered the CEO of the cell because its primary functions are to maintain the hereditary information of the species and to control cellular activities through protein synthesis. The hereditary information (DNA) the nucleus contains enables the cell to divide and produce an identical daughter cell. By controlling cellular activities, the nucleus controls the well-being of the cell.
Can a cell that does not contain a nucleus live as long as a cell that does contain one? Why or why not?
No, because the cell cannot repair itself, divide, or make proteins or enzymes without a nucleus.
Describe the nuclear envelope. How is it different from the cell membrane?
The nuclear envelope is composed of two lipid bilayers, unlike the cell membrane, which is composed of one. The outer layer of the nuclear envelope is continuous with the ER and is studded with ribosomes. Over 10% of the nuclear surface consists of nuclear pores—places where the two layers of the nuclear envelope have fused to form a channel that spans its entire thickness. Although the nuclear envelope is similar in structure and composition to the cell membrane, passage of molecules into the nucleus is less selective because the nuclear pores are relatively large (0.1 nm in diameter).
How do histones play a role in gene regulation?
A single strand of DNA winds around eight histone molecules forming a granule called a nucleosome. The nucleosomes are held together by short strands of DNA called linker DNA. Not only do the histone proteins help keep the DNA strand organized and untangled, they also expose small sections of the DNA (genes) to the outside nucleoplasm. By changing shape, the histones can expose different genes at different times. The exposed genes determine what proteins will be made by the cell. In this way, histones play an important role in regulating gene expression (gene regulation). DNA contains all the important instructions required for synthesis of thousands of different proteins, but not all of them are made. Only a small percentage of the possible thousands of proteins are actually manufactured. Histones help determine which segments of DNA will be expressed and therefore which proteins will be made.
What is the significance of the nucleolus? What happens in that region of the nucleus?
The nucleoli are the places in the nucleus where ribosomal subunits are made. These subunits are exported separately from the nucleus and assembled in the cytoplasm to form functional ribosomes. In addition, nucleoli contain the DNA that governs the synthesis of ribosomal RNA (rRNA).
Where is most of the water in animals found?
Most of the water in animals is found inside the cell and is called intracellular fluid.
What is diffusion? Is it an active or a passive membrane process?
Diffusion can be defined as the process of moving down the concentration gradient from an area of high concentration to a region of low concentration. Diffusion is a passive membrane process.
What molecules are more likely to diffuse into a cell? What three principles are involved?
- Water, oxygen, and carbon dioxide are more likely to diffuse into a cell. The three principles involved are:
- 1. Molecular size: Very small molecules like water (H2O) may pass through cellular membrane pores (approximately 0.8 nm in diameter), but larger molecules like glucose cannot.
- 2. Lipid solubility: Lipid-soluble molecules (e.g., alcohol and steroids) and dissolved gases (e.g., oxygen [O2] and carbon dioxide [CO2]) can pass through the lipid bilayer with ease, whereas other molecules may not.
- 3. Molecular charge: Ions are small in size, but their charge prevents easy passage through the membrane pores. Specialized pores called channels selectively allow certain ions to pass through but not others.
How is facilitated diffusion different from simple diffusion? What is the limiting factor in the rate of facilitated diffusion?
Facilitated diffusion requires the assistance of an integral protein or carrier protein located in the bilayer. The limiting factor in the rate of facilitated diffusion is the number of available carrier proteins.
What effect does a hypotonic solution have on a cell? What passive membrane process causes this effect?
If the extracellular fluid is hypotonic, the inside of the cell is more concentrated than the outside. In this scenario, water flows into the cell and causes it to swell and possibly burst. This effect is due to the process known as osmosis.
What is the relationship between hydrostatic pressure and filtration?
Filtration is based on a pressure gradient. Liquids may be pushed through a membrane if the pressure on one side of the membrane is greater than that on the other side. The force that pushes a liquid is called hydrostatic pressure.
What is another name for hydrostatic pressure in the body?
When is a membrane process considered “active”?
The movement of molecules and substances across the cell membrane is considered active when the cell is required to use energy (ATP). They cannot move through the plasma membrane passively.
How do electrolytes enter the cell?
Electrolytes enter cells via active transport without the assistance of a concentration gradient.
What is the difference between a symport and an antiport system?
Many active transport systems move more than one substance at a time. If all the substances are moved in the same direction, the system is called a symport system. However, if some substances are moved in one direction and others moved in the opposite direction, the system is called an antiport system.
Describe how sodium and potassium enter and exit the cell.
Because of the concentration gradient of sodium (Na) and potassium (K), potassium tends to diffuse out of the cell and sodium diffuses in. To maintain appropriate levels of intracellular potassium and extracellular sodium, the cell must pump potassium into the cell and move out sodium. Because diffusion is ongoing, the active transport system must work continuously. The rate of transport depends on the concentration of sodium ions in the cell. ATP is provided by cellular respiration and, with the assistance of the enzyme ATPase, is broken down for use as energy on the inner surface of the cell membrane. The pump can cycle several times using just one molecule of ATP, so that for every molecule of ATP, two K ions are moved intracellularly and three Na ions are moved extracellularly.
Describe the three types of endocytosis.
The three types of endocytosis are phagocytosis, pinocytosis, and receptor-mediated endocytosis.
What is the difference between excretion and secretion? These are both examples of what?
Excretion is the movement of waste products from the intracellular to the extracellular environment, and secretion is the movement of manufactured molecules from the intracellular to the extracellular environment. Both are examples of exocytosis.
What are the principal ions involved in maintaining a cell’s resting membrane potential?
Sodium and potassium
Is there normally a higher concentration of sodium inside or outside of the cell? Where is there a higher concentration of potassium?
Sodium is 10 to 20 times higher outside the cell than it is inside. Potassium is 10 to 20 times higher inside the cell than outside.
What are the two major periods of the life cycle of the cell?
The two major periods of the cell’s life cycle are: interphase, when the cell is growing, maturing, and differentiating; and mitotic phase, when the cell is actively dividing.
Is interphase a time when the cell is resting? Why or why not?
No, because it is carrying out metabolic activities during interphase. Before each cell can divide, a perfect copy of the DNA must be created to pass on to the daughter cells. This replication occurs during interphase.
What are the four stages of the mitotic phase?
Prophase, metaphase, anaphase, and telophase
What happens in each of these stages?
- 1. Prophase: Chromatin coils and condenses to form barlike chromosomes that are visible with light microscopy. These chromosomes are composed of two identical chromatids linked together at a constriction in their middle known as the centromere or kinetochore. The cytoplasm becomes more viscous as microtubules from the cytoskeleton are disassembled and the cell becomes round. Two pairs of centrioles form anchors on which new microtubules are constructed, and as the microtubules lengthen, they push the centrioles farther and farther apart. In this way a mitotic spindle is formed that provides the structure and machinery necessary to separate the chromosomes. Because transcription and protein synthesis cannot occur while the DNA is tightly coiled, the appearance of chromosomes marks the cessation of normal synthetic processes. Prophase is thought to conclude with the disintegration of the nuclear envelope.
- 2. Metaphase: Chromosomes line up in the exact center of the spindle, known as the equator. The chromosomes are evenly spread apart and form what is called the metaphase plate midway between the poles of the cell. The centromere of each chromosome is attached to a single spindle fiber.
- 3. Anaphase: The centromeres of the chromosomes split apart and each chromatid becomes its own chromosome. The spindle fiber separates, each spindle segment shortens, and the twin chromosomes are pulled away from each another. The chromosomes take on a V shape as they are dragged at their midpoint toward the centrioles at opposite ends of the cell. The cell becomes elongated, and the cytoplasm begins to constrict along the plane of the metaphase plate. Although anaphase is the shortest phase of mitosis and usually lasts only a few minutes, its importance is clear in light of the devastating consequences if an error were to occur in chromosome separation. In anaphase the advantage of separating compact bodies of chromosomes, rather than long thin threads of chromatin, is particularly obvious.
- 4. Telophase: Begins when chromosomal movement stops. The chromosomes, having reached the poles, begin to unravel, elongate, and return to a diffuse threadlike form (chromatin). A nuclear envelope appears around each new set of chromosomes, and nucleoli appear in each nucleus. The microtubules that made up the spindle in the earlier phases of mitosis disassemble, and a ring of peripheral microfilaments begins to squeeze the cell into two parts. Ultimately, the cell pinches itself in half, dividing the cytoplasm and forming two completely separate daughter cells. The process of cytoplasmic division is called cytokinesis and marks the end of telophase.
Why is it important for chromatin to coil and form discrete chromosomes before cell division?
Transcription and protein synthesis cannot occur while the DNA is tightly coiled.
What three factors play a role in the control of cell division?
1. Normal cells stop dividing when they come into contact with surrounding cells. This phenomenon is called contact inhibition. 2. Growth-inhibiting substances may be released from cells when their numbers reach a certain point. 3. A number of checkpoints are reached during cell division when the cell reassesses the division process. These checkpoints occur during the G1 and G2 phases of interphase.
What is the genetic basis of cellular differentiation?
The position of genes in chromosomes determines the genetic basis of cellular differentiation. Some genes may be located on a region of the chromosome that is available for transcription, whereas other genes may be located inside the molecule and cannot be reached by transcription molecules. We say that one gene is “turned on” while the other gene is “turned off.” Genes can be turned off permanently or temporarily. Chromosomes are dynamic in their ability to twist, so that a gene that was once inaccessible on the inside can be moved to the outside of the molecule for use. Differentiation involves the temporary or permanent inhibition of genes that may be active in other cells.
Of all the thousands of different proteins that a cell could make, how many does it actually produce? Why?
Of approximately 100,000 proteins that a cell could make, it makes only a few hundred, because cells have different functions. The number of proteins made depends on the function of the cell.
Where does protein synthesis begin?
Protein synthesis begins in the nucleus with transcription and ends in the cytoplasm with translation.
What is a nucleotide and how is it structured?
Nucleotides are the building blocks or units of DNA and RNA molecules and are composed of three subunits: a nitrogenous base, a five-carbon sugar, and a phosphate group. DNA and RNA nucleotides are linked to form a “backbone” of alternating sugar and phosphate groups. The nitrogenous bases project out of this backbone.
Compare and contrast the structures of DNA and RNA.
In DNA, the sugar is deoxyribose, and in RNA, the sugar is ribose. In DNA, these bases are weakly bonded to nitrogenous bases on an opposing strand. In this way, DNA forms a double-stranded molecule, the basic structure of which is analogous to a twisted ladder in which the vertical poles are composed of alternating sugar and phosphate groups and the horizontal rungs are paired nitrogenous bases. DNA’s molecular structure is therefore called the double helix. RNA, however, is a single-stranded molecule that has no opposing strand. The single strand of RNA is similar in structure to each of the strands found in DNA.
What are the nucleotides found in DNA? In RNA?
The nucleotides found in DNA are adenine (A), cytosine (C), guanine (G), and thymine (T). The nucleotides found in RNA are adenine (A), cytosine (C), guanine (G), and uracil (U).
What is the term for mRNA formation?
Messenger RNA (mRNA) formation is known as transcription.
What are codons and what role do they play in transcription?
A codon is a set of three adjacent nucleotides in an mRNA molecule that specifies the incorporation of an amino acid into a polypeptide chain or that signals the end of polypeptide synthesis.
Can you describe the events that occur in translation?
A ribosome binds to the beginning of the mRNA strand. Transfer RNA (tRNA) molecules move near the ribosome. The tRNA anticodon is paired with the appropriate codon on the mRNA molecule. An amino acid carried by the tRNA molecule is released and linked to the neighboring amino acid. The ribosome continues to move along the mRNA molecule until all of the codons have been paired. As the developing chain of amino acids lengthens, it coils and folds into the structure of a functional protein. When translation is complete, the new protein is released and later modified. The ribosome, tRNA, and mRNA are free to repeat the process and form more of the same type of protein.
At what point in the cell cycle does DNA replication occur?
DNA replicates during the synthetic (S) phase of interphase.
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