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Unit 19

CELL ORGANIZATION

LEARNING OBJECTIVES

1. Gain an appreciation for the small size of cells and the structures contained within the cell. 2. Learn about some of the ways one can study the parts of a cell. 3. Learn the makeup of the plasma membrane and the functions of protein molecules embedded in or on the membrane. 4. Be able to compare the structure and functions of the mitochondrion and the chloroplast. 5. Be able to relate the nucleus, rough and smooth endoplasmic reticulum, ribosome, Golgi complex, and lysosome to the production and transport of proteins. 6. Obtain a general concept of the exoskeleton of the cell.

Introduction

In two previous units (Cells and Energy and Photosynthesis), we have examined two important organelles of the cell—mitochondrion and chloroplast. But there are many other cell organelles, which are responsible for or function in a wide variety of activities—making and modifying proteins, storing food, maintaining the cell structure, cell division, regulating the materials that enter and leave the cell, digesting materials within the cell, serving as transport passageways, and last but not least, giving directions to the cell for its various activities. Realizing that many cells are from 10 to 100 microns (1 u = 1/1000 millimeter, mm), it is amazing that they can contain the mechanisms to carry out all these activities. If you were setting up a factory to carry on some of the listed activities, how much working area would be required? In this unit we will consider all organelles, but not go into great detail about the chloroplast and mitochondrion; you will be expected to review them from the other units.

Cell size in eukaryotic cells (cells with a nucleus) is about 10-100 microns in diameter. Cell organelles are those groups of molecules, often in the form of membranes that carry on a specific function(s) in the cell. Recall the chloroplast produces energy-rich molecules (ATP and NADPH) and a simple carbohydrate (PGAL). The cell organelles must be able to fit within the cell; therefore, they must be less than 10 microns in the smallest cells. The nucleus, the largest cell organelle is about 5 microns; the mitochondrion about 0.5 – 5 microns, ribosome about 0.02 micron, and the plasma membrane (cell membrane) about 0.009 microns (9 nanometers or 90 Angstroms) thick.

How can we study structures that small? The light microscope (figure 19-1) uses light waves in the range of 380-760 nanometers (nm); this is the range of visible light.

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The light microscope has a resolution power of about ½ the wavelength of light. The object to be seen must be able to stop or bend the light waves. Resolution power is the minimum distance two points must be separated so that they can be distinguished as two points.

Figure 19-1. Binocular, compound, light microscope

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The transmission electron microscope, TEM (figure 19-2) uses electrons rather than photons to view an object. It is effective at a minimum of about 5 A (Angstrom). In using the electron microscope one takes very thin sections of a subject and then coats them with a metal ion. A disadvantage is that you cannot examine living material; an advantage is that you can examine very small objects. Another type of electron microscope is the scanning electron microscope, SEM. It allows you to look at whole objects, but they must be larger than those viewed under TEM and they also are fixed, non-living.

Figure 19-2. Transmission electron microscope.

Examining cell and cell organelles can be done by methods other than just microscopy. Cells can be fractionated by grinding or breaking apart the cells by using a type of mortar and pestle or sonication (using sound waves). The cells parts are then spun down in a centrifuge at a specific number of gravities. Larger particles will come down first and then sequentially smaller and smaller particles. By this method, it is possible to obtain a relatively pure pellet of a desired cell organelle. Enzyme studies can be used to identify structures or products. It is known that certain enzymes will hydrolyze (digest) certain organic molecules and thus, enzymes can be added and then a stain is used that is specific for an end product (type of molecule). One can identify certain

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organelles if the organelle contains a certain molecule. Radioactive tracers are also used. A radioactive molecule or element is placed in a solution with living cells and after a specific time, slides are made and the cell or tissue is examined for the location of the radioactive chemical. An example of this would be to used radioactive phosphorus, which we know is taken up by nucleic acid (DNA and RNA). In this way, it is possible to located nucleic acid in the cell. PLASMA MEMBRANE

The plasma membrane (fig. 19-3) borders the cell. It is about 90 A units thick. It consists of a double layer of lipid (phospholipids) with protein molecules dispersed unevenly within the membrane. It is said to be a lipid bilayer with a protein mosaic.

As can be observed in figure 19-3, the proteins embedded in the membrane serve many different purposes. Some are carrier molecules that help transport materials into and out of the cell; some are identifying molecules so that the hormones know where to stop and the immune system knows that the cell is its; and some serve as passage ways to allow water and other materials to pass into the cell. The membrane is said to be selectively permeable; this means it controls most things that enter or leave the cell.

Figure 19-3. Diagram of a section of a plasma membrane (A) and some representative proteins located in the membrane (B).

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NUCLEUS

The nucleus (fig. 19-4) possesses a double membrane with 100nm pores extending through both membranes. The pores regulate the passage of macromolecules into and out of the nucleus; this consist of proteins moving in and RNA and protein-RNA complexes formed in the nucleus moving out. The pores are not empty but contain proteins that act as molecular channels. The nucleolus is dense body within the nucleus that is composed of RNA and protein. It functions in the synthesis of ribosomes. It contains nucleolar organizers, which are regions of specific chromosomes with multiple copies of genes for ribosome synthesis. The nucleus contains most of the cell’s DNA; this DNA codes for most of the cells activities and for organelle proteins.

Figure 19-4. Nucleus showing nucleolus and chromatin. MITOCHONDRION

The mitochondrion (fig. 19-5) possesses a double membrane with the inner membrane invaginated to form cristae. Granules project from the inner surface of the inner membrane and are called intramitochondrial granules. The inner membrane encloses the matrix; a fluid solution. The mitochondria produce ATP and obtain the energy to do so from organic molecules by electron transport. They contain DNA, RNA, and ribosomes. Thus, they can produce some of their own proteins, but most of their proteins are synthesized outside the mitochondrion as a result of messages from the nuclear DNA. Based upon the possession of DNA and RNA and ribosomes, which are more similar to bacterial ribosomes than to the ribosomes in the cytoplasm of the cell, many scientists believe that the mitochondrion originated from a bacterium that was engulfed by another cell.

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Figure 19-5. Diagrammatic representation of a mitochondrion (A) and an electron micrograph of several mitochondria (B) CHLOROPLAST

The chloroplast (fig. 19-6) possesses a double membrane with the inner membrane connected to many stacks of double membranes called thylakoids or discs. The thylakoids are arranged in stacks like a column of coins and they are called grana (single-granum). The fluid outside the thylakoids is called stroma. The chloroplast, like the mitochondrion, contains DNA, RNA, and ribosomes. They too can produce some of their own proteins, but the nuclear DNA codes for most of their proteins. (The chloroplast codes for about 10% of its own proteins.) As with the mitochondrion, many scientists believe that the chloroplast originated by a cell engulfing a photosynthetic bacterium.

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Figure 19-6. Diagram of a section of a chloroplast (A) and an electron micrograph of a section of a chloroplast showing two starch grains. ENDOPLASMIC RETICULUM

There are two types of endoplasmic reticulum—rough and smooth. The rough endoplasmic reticulum (fig. 19-7) has ribosomes embedded in its membranes; thus, the rough appearance.

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Figure 19-7. Electron micrograph (A) and diagrammatic representation (B) of the endoplasmic reticulum.

It compartmentalizes the cytoplasm and aids in the transport of proteins. Smooth endoplasmic reticulum is involved with lipid synthesis and detoxification of drugs. Gonads are rich in smooth e.r. which enables them to synthesize steroid hormones (estrogen, progesterone, and testosterone). Liver cells are rich in smooth e.r. which enables them to detoxify drugs. Both serve in compartmentalizing the cytoplasm and in transport of proteins and lipids. Endoplasmic reticulum serves in some cells for the conduction of electrical impulses and the storage of Ca++. GOLGI COMPLEX

The Golgi complex (fig. 19-8) is a series of double membranes located near the nucleus. It modifies and packages proteins for shipment out of the cell. The membranes are numerous in cells that secrete substances. Proteins and lipids synthesized in the rough and smooth endoplasmic reticulum membranes are transported to the Golgi where they are modified as they pass though the channels. A common modification is the addition of a carbohydrate molecule to a protein or lipid forming a glycoprotein or glycolipid respectively. The formed molecules collect in flattened stacked membranes, cisternae. The membranes pinch of small vesicles that may contain digestive enzymes or hormones or they may be containers of proteins that will become part of the cell membrane.

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Figure 19-8. Showing the Golgi apparatus and the movement of the lysosome from the Golgi to the cell membrane. LYSOSOME

The lysosome (fig. 19-8) is a single membrane sac. It contains digestive enzymes that were made by the rough e.r., and then sent to the Golgi where they were packaged and pinched off as a lysosome. The lysosome moves to the cell membrane of a cell surrounding a duct and dumps it enzymes into the duct. The enzymes are then carried to the digestive tract. Some cells, such as Amoeba and white blood cells utilized lysosomes and their enzymes for phagocytosis. Phagocytosis (cell-eating) is the process of the cell engulfing and literally digesting the engulfed material. Some lysosomes function in autophagy, the digestion and recycling of the cells own materials. PEROXISOME

The peroxisome is a single membrane structure; it is a type of microbody that forms by incorporating lipids and proteins within the cytoplasm. It contains enzymes that detoxify hydrogen breaking it down to oxygen and water. Hydrogen peroxide, H2O2 is toxic to the cell. These microbodies are numerous in the liver. VACUOLE

A vacuole is a single membrane structure that has various functions. Many serve as storage for starch, water, lipids, wastes, etc. Some serve as digestive vacuoles. Many plant cells have a large central vacuole that is filled with fluid. In some single-celled organisms, there is a contractile vacuole(s) that serves to pump water out of the cell. RIBOSOME

Ribosomes (fig. 19-9) are either free in the cytoplasm or bound to the rough endoplasmic reticulum. They are composed of two subunits, which are synthesized in the nucleolus and move through the pores of the nuclear membrane to the cytoplasm.

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The bound ribosomes are largely involved in synthesizing proteins for export. The free floating ribosomes in the cytoplasm are largely involved with synthesis of proteins for the cell’s use.

Figure 19-9. Ribosome showing the two subunits and the groove through which mRNA passes. CENTRIOLES Centrioles (fig. 19-10) occur in pairs usually arranged perpendicularly to each other. They are composed of a circular arrangement of nine triplets of microtubules. Centrioles help in the assembly of microtubules, which influence cell shape and serve as spindle fibers to help move the chromosomes during cell division. There function will be covered in greater detail in the unit “Cell Division”.

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Figure 19-10. Shows a pair of centrioles arranged perpendicular to one another. CYTOSKELETON

Microtubules (fig. 19-11) are about 25nm in diameter. They are hollow tubes composed of 13 protein filaments. They are found in cilia, centrioles, sperm tails, and spindle fibers. They are relatively unstable, continually polymerizing and depolymerizing. The half life of a microtubule is about 10 minutes with a spindle fiber’s being even shorter about 20 seconds.

Intermediate filaments (fig. 19-11) are about 8-10nm in diameter. They are the longest lasting of any of the elements of the cytoskeleton. They help give shape to the cell and help to maintain the position of organelles within the cell. The intermediate filaments in nerve cells are called neurofilaments.

Microfilaments (fig. 19-11) are about 7nm in diameter. Each filament is composed of two chains of globular proteins wound around each other. They contain actin, one of the proteins found in muscle cells. They aid in movement and division of cells.

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Figure 19-11. Showing microtubules, intermediate filaments and microfilaments. CELL WALL The cell wall (fig. 19-12), while not a part of animal cells, is found surrounding plant cells. Cellulose is the main constituent and it is about 0.1 to several microns thick. It does not serve as a barrier to molecular or ionic movement. Some organisms (fungi) have cell walls composed of chitin; this is the same material that serves as the exoskeleton of arthropods. Another important constituent of many cell walls is lignin; it is rigid and is commonly found in walls of plant cells that have a supporting or mechanical function. When cells divide, the primary walls of adjacent cells form a layer called the middle lamella. The primary constituent of the middle lamella is pectin. This is the material that is used in making jams and jellies to provide some cohesiveness in the fruit mixture. It also is a constituent in kaopectate an antidiarrhetic medication. It is a combination of clay particles and pectin.

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Figure 19-12. Cell wall of a plant cell.

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Unit 19

OBJECTIVE QUESTIONS OVER CELL ORGANIZATION 1. Eukaryotic cells (cells with nuclei) are about (A) 10-100mm (B) 10-100 µm

(C) 10-100nm (D) 10-100Å. 2. The largest cell organelle is the (A) centriole (B) mitochondrion (C) nucleus (D) ribosome. 3. The (A) mitochondrion (B) chloroplast (C) both A and B (D) neither A or B

contain(s) DNA, RNA, and ribosomes. 4. The (A) nucleus (B) chloroplast (C) mitochondrion (D) two of the preceding (E) all the preceding contain(s) a double membrane. 5. The cristae of the mitochondrion are similar in function to the (A) thylakoids (B) stroma (C) grana (D) chlorophyll of the chloroplast. 6. The resolution of a good light microscope is about (A) 200 Angstroms (B) 200nm (C) 200 µm (D) 200mm. 7. The principle organelle of the cell involved in proteins synthesis is the (A) nucleus (B) mitochondrion (C) chloroplast (D) ribosome. 8. (A) Rough ER. (B) Smooth E.R. (C) Both A and B (D) Neither A or B is/are

involved in synthesizing lipids (steroids). 9. (A) Bound ribosomes (B) Free floating ribosomes (C) Both A and B

(D) Neither A or B is/are involved in synthesizing proteins for the cell’s use. 10. The largest diameter is found in (A) intermediate filaments (B) microfilaments (C) microtubules. 11. (A) Lysosomes (B) Peroxisomes (C) Both A and B (D) Neither A or B contain digestive enzymes. 12. Ribosomes are synthesized by (A) nucleoli (B) mitochondria (C) chloroplasts (D) two of the preceding (E) all the preceding. 13. (A) Intermediate filaments (B) Microfilaments (C) Microtubules are found in sperm

tails.

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14. The major constituents of a plasma membrane are (A) carbohydrates and lipids (B) carbohydrates and proteins (C) lipids and proteins. 15. You would expect to find a large number of peroxisomes in (A) heart (B) liver (C) muscle (D) bone cells.

DISCUSSION QUESTIONS OVER CELL ORGANIZATION 1. Is there communication between organelles within the cell? Explain 2. What structures and functions do the mitochondrion and chloroplast have in common? 3. What are an advantage and a disadvantage of the cell membrane being selectively

permeable? 4. What is the advantage of cristae in the mitochondrion and thylakoids in the

chloroplast? 5. Trace the birth of a protein; include structures involved. 6. Fetal tissue has considerable difficulty with toxic materials consumed by the mother,

resulting in the fetus often being adversely affected even though the mother may not be. Explain the reason(s) for this.

7. Permitting meat to hang for a period of time under refrigeration results in the meat

being more tender than it would have been if it had not been permitted to age. Explain this phenomenon.

8. A scientist examined 3 different types of cells—pancreatic, ovarian follicular cell, and

muscle cell, and she found that one of these cells was very rich in smooth e.r. and the other two were very rich in rough e.r. Indicate which cell was rich in smooth e.r. and which ones in rough e.r. and give the reasons.

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9. What is the difference in the fate of the end products of bound ribosomes and free-

floating ribosomes? 10. What color light is the best for use in a microbiology laboratory? Explain.