Chapter 6: The inside story: molecules to cells.
After completing this chapter, you should be able to:
* Compare eukaryotes to prokaryotes
* Describe the basic chemical composition of cells
* Describe the functions of the parts of a cell
* Draw and label all parts of a plant cell
* Know the function of the mitochondrion
* Explain the function of the chloroplast
* Understand how Golgi apparatus function
* List other organelles in the plant cell
primary cell wall
secondary cell wall
compartmentalization of enzymes
endoplasmic reticulum (ER)
rough endoplasmic reticulum
smooth endoplasmic reticulum
Molecules of Life
Cells all have a plasma membrane, which encloses an aqueous solution called the cytoplasm. In the watery world enclosed by this membrane, the mechanics of life occur: building large molecules from small molecules, making smaller molecules by tearing down larger ones, and always moving substances around. Constantly in motion, the cellular "factory" knows exactly where each "nut" and "bolt" goes.
Within this aqueous medium are even smaller structures, organelles (or, little organs) of various shapes and sizes. Each type has a specific job in the overall function of the cell.
Under a microscope, cyclosis (movement of the cytoplasm and many of the organelles) can be observed in the living cell; some go in one direction and others go in another. It resembles a network of freeways, with cars going in many directions. These movements are apparently not random, but very highly organized.
Chemistry is the center of cell structure and function. Certain chemical and physical laws govern how molecules are assembled from simple atoms, how the bonds are broken, and how they are reformed in new molecules.
All matter is made from various combinations of the chemical elements, substances which cannot be broken down by ordinary chemical means. The earth's crust contains 92 naturally occurring elements, and several others have been made artificially. The basic particle of an element is the atom, and atoms are combined in various ways to form molecules, the building blocks of life.
Atoms are composed of three primary subparticles: proton, neutron, and electron. The nucleus of the atom contains the protons and neutrons. Electrons move in specific orbits around the nucleus. The proton has a specific mass and positive charge; neutrons also have mass but no charge. Electrons have essentially no mass but have a net negative charge.
Various combinations of protons, neutrons, and electrons produce the chemical elements, and the structure of the elements is determined by number of protons, which is the atomic number. The atomic weight is the sum of protons and neutrons. The simplest chemical element is hydrogen, with only one proton. Helium has two protons, carbon has six, and oxygen has eight. Each chemical element has its own symbol, often but not always indicated by the first letter. Some chemicals' symbols are derived from Greek or Latin terms. C is the symbol for carbon; H is the symbol for hydrogen, and He is the symbol for helium. But K, the symbol for potassium, is derived from the word kalium. The symbol P is reserved for phosphorus, Na (natirum) stands for sodium, and Fe (ferrum) for iron (see Table 6-1).
Basic Cell Structure
Although plant cells and animal cells are very much alike, plant cells have a wall that becomes more or less rigid. Animal's cells lack this cell wall. This is not to say that the plasma membrane surrounding an animal cell is "naked." Large molecules called glycoproteins (carbohydrate protein) occur on the outside of the membrane and serve as a recognition surface. These glycoproteins form characteristic threedimensional surfaces that allow cell sliding past each other to recognize similar surface features and then cling together. Such recognition may be less important in plant cells because the cellulose wall fixes the cells in place; there is little movement. Even in some plant cells, however, protein layers on the exterior surface of the cell wall are important in recognition. Plant cells are grouped together in tissues by virtue of their proximity at the time of division. The plane of division determines whether a group of plant cells will divide in only one plane to form a chain, in two planes to form a sheet, or in all three planes to form a cube.
If cell divisions were strictly random in respect to plane of division, a sphere of cells would result. Such is the case in tumors, which can develop in both plants and animals.
The turgor pressure is the water pressure within each cell. Cells are literally blown up, and all push against each other. These cells are inflated by water. A cellulose molecule may be as many as 2000 glucose molecules, a 6-carbon sugar (hexose). The glucose units are connected end to end, forming a macromolecule. Cellulose molecules may be as many as 2000 glucose molecules in length, and these long molecules are bundled together in packages to form a microfibril. Groups of microfibrils are wound together much like a steel cable to form a macrofibril. The macrofibrils are a major component of the cell wall and are held together with other kinds of macromolecules, including hemicelluloses and pectic compounds. These substances glue the entire structure together in a sheet of fibers. The first microfibrils of the primary wall form a network with a predominately transverse pattern. When turgor causes the cell to expand and the wall increases in surface area, the microfibrils become more parallel to the longitudinal axis of the cell. The overall effect is a crosshatched appearance of the various layers. Two adjacent plant cells are held together by the middle lamella, composed primarily of cementing pectic substances, and the primary cell wall on each side of the middle lamella. In many plants a secondary cell wall may be laid down at a later date, adding strength and rigidity to the tissue, particularly if lignin is present. Tree trunks, for example, have cells with very thick secondary cell walls.
Elongation in some plant cells reaches many centimeters. The cotton fiber, for example, often reaches lengths of 4 to 6 cm. The size of certain cells is determined by the species in question, and the genetics will ultimately determine how large a cell can become. Generally, both plant and animal cells are microscopic.
Multicellular plants communicate by tiny passages that connect the cells through one plasma membrane, the primary cell wall, the middle lamella, the adjacent primary cell wall, and finally the adjacent plasma membrane. These connections occur in spots in the wall called primary pit fields, and each strand is called a plasmodesma. Although too small for the interchange of larger organelles, cytoplasmic connections apparently allow for the transfer of chemicals from one cell to another.
The wall itself is extremely permeable to all kinds of substances. Cross-linking of the molecules still allows water and various kinds of solutes to penetrate the wall; the barrier, determining what gets in and out of the cell, is the plasma membrane itself, just as in animal cells.
Prokaryotic and Eukaryotic Cells
Within the living world there are basically two types of cells: Those that are prokaryotic, and those that are eukaryotic. Prokaryotic means "before the nucleus"; therefore, prokaryotic cells are those that do not have a well-defined nucleus. All other cells in the world, both plant and animal, do contain a nucleus and are therefore referred to as eukaryotic. Prokaryotic is the more primitive cell. Probably the prokaryotic cells that exist today are similar to the very first cells on earth. Prokaryotic cells first made their appearance in the oceans some 3.5 billion years ago and today are represented by only two types of organisms, bacteria and cyanobacteria. Now, as then, all prokaryotic organisms were single celled. Some chains, groups, and colonies exist, but no truly multicellular organisms with cellular differential occur. Prokaryotic cells are comparatively simple and small, rarely more than 1 or 2 [micro]m in diameter. They consist of a rigid wall surrounding a plasma membrane that holds the components of the cytoplasm. They do contain ribosomes, but for the most part organic molecules are simply free in the cytoplasm. Packaging and partitioning in these cells is not nearly so dramatic as it is in the eukaryotic cells. The hereditary material within the cells, DNA, consists of a single long molecular thread. It is circular, but it is not organized into any sort of regular chromosome as occurs in the nucleus of eukaryotic cells.
Primary Cell Wall
In newly forming cells, the wall that first surrounded the plasma membrane is referred to as the primary cell wall. Initially, the primary cell wall is relatively plastic, gradually becoming more rigid as the cell ages and enlarges. It finally stretches more and more, synthesizing new cell wall material, until the cell has reached its ultimate size. Certain cells grow more in certain directions than in others, leading to the elongation process. Elongation in some plant cells reaches many centimeters. The cotton fiber, for example, often reaches lengths of 4 to 6 cm. The size of certain cells is determined by the species in question, and genetics will ultimately determine how large a cell can become. Generally, both plant and animal cells are microscopic.
Secondary Cell Wall
Many plant cells develop a secondary cell wall, which is laid down between the primary cell wall and the plasma membrane and may become much thicker than the primary wall. The secondary cell wall adds strength and rigidity to the cell. In some cases, for example the flax fiber, the secondary cell wall gradually fills the space inside the cell by the cytoplasm, and the lumen or living space inside the cell becomes very small indeed. The same is true for many other types of fibers. By the time the secondary cell wall is complete, some plant cells die, and the cytoplasm is simply absorbed.
Cell membranes are approximately half phospholipid and half protein. The three-layered structure of cell membranes consists of a double layer of phospholipids, with the insoluble phosphate portion in the center and the water-soluble phosphate portion oriented toward the outside in each direction. Proteins are inserted on each side of the lipid layer; some of the protein molecules extend across the lipid layer and protrude out the other side; others are a component of only one side of the membrane. The proteins are laterally mobile in the double layer. In a biological membrane seen through an electron microscope, the fixation process causes the protein layers to appear as dense lines, but the phospholipid layers are transparent. Thus, one sees two black lines with a space in between. This typical structure occurs in all biological membranes and is referred to as the unit membrane. This does not mean that all membranes are exactly the same. They have different permeability characteristics, and just because a substance can get across a chloroplast membrane does not mean that it can get across a mitochondrial membrane. Membrane selectivity suggests that each kind of membrane has subtle molecular characteristics that allow it to function in its own conditions. Scientist currently picture the biological membrane as a fluid mosaic in which large protein molecules float in a sea of lipids.
Substances must pass across biological membranes to get into or out of a cell. These membranes tend to be very permeable to water and certain gases, including oxygen and carbon dioxide. Other kinds of molecules may have difficulty traversing the membrane because of size or polarity. Ions and polar molecules tend to move through the protein portion of the membrane. Many of these proteins are involved in the process called active transport (from ATP), which is used to move substances across membranes against a concentration gradient.
Organelles and Other Inclusions
There is some disagreement among cell biologists concerning the definition of an organelle. For the purpose of this book, an organelle is a distinct entity within the cell that performs a particular function as a compartmentalization of enzymes. Thus, we include ribosomes and vacuoles as organelles; even though some scientists would classify only membrane-bound structures as organelles.
Typical organelles of a photosynthetic plant cell include the nucleus, vacuole, plastids, mitochondria, ribosomes, Golgi apparatus, lysosomes, glyoxysomes, and peroxisomes. A number of other cell inclusions, including microtubules, are important.
The nucleus is a fairly conspicuous organelle within the plant cells. During cyclosis it can be observed to remain in a relatively static position, attached by strands of membranes that form a network to suspend it in space; other organelles seem to slide by. In the process of cell division, the nucleus undergoes dramatic changes as the hereditary material, DNA, is replicated and partitioned to daughter cells.
A typical young plant cell is approximately 30 to 40 [micro]m in diameter, whereas the nucleus itself is about 10 [micro]m in diameter. It is enclosed by a double membrane system that makes up the so-called nuclear envelope. Viewed through the electron microscope, the nuclear envelope is seen to contain relatively large holes, referred to as nuclear pores, through which certain kinds of small and large molecules may pass. Many dramatic changes take place in the nucleus. A dense region in the nucleus is called the nucleolus. Some cells contain only one; others two or three, and others have literally hundreds of nucleoli. They appear to function as the synthesis of rRNA. During interphase, the so-called resting stage of cell division, the nucleoli can be observed in great detail, but as the cell begins the division process, they usually disappear at about the same time the nuclear envelope disappears. Since the nucleus contains the genetic information, it directs the framework of activity for the entire cell--when and how to divide.
The mitochondrion is the organelle responsible for the process of aerobic (uses oxygen) respiration. It is capable of converting sugars into C[O.sub.2] and [H.sub.2]O and releasing energy in the process. Energy from these molecules is produced as ATP, which is the main energy source for the cell. Mitochondria tend to be far more numerous than chloroplast, with perhaps as many as a thousand per cell. They may be oblong, oval, or round and approximately 1 [micro]m in diameter, about the same size as a bacterial cell (see Figure 6-1). Their structure consists of an outer membrane and an inner membrane that is involuted to form the cristae.
[FIGURE 6-1 OMITTED]
The involutions give a tremendous increase to the surface area of the cristae and provide a surface on which the enzymes of respiration occur. Plant cells that carry out a great deal of respiration and are required for producing a tremendous amount of ATP energy tend to have many mitochondria. Other cells that function primarily to provide some service other than respiration may have very few. It is important to remember that all cells carry on respiration, although some may do so under anaerobic (no free oxygen) conditions.
Other than the nucleus and vacuole, the plastids constitute the most conspicuous organelles of a plant cell. A double membrane, just as the nucleus and mitochondria, bound all plastids, and the internal structure is a system of membranes separated by a fairly homogeneous ground substance of membranes separated by a fairly homogeneous ground substance called the stroma.
There are three types of plastids. Chromoplasts are pigment organelles, as the name implies, but are specialized to synthesize and store carotenoid pigments (red, orange, and yellow) instead of chlorophyll. In the process of fruit ripening and in other pigmented tissue, they accumulate large quantities of carotenoids to give the characteristic color to the tissue.
Leucoplasts are nonpigmented plastids but contain enzymes responsible for the synthesis of starch. Large starch gains may accumulate in plastids, as in a potato tuber.
Chloroplast is the green plastids associated with the entire photosynthetic process, and they represent the functional unit in the transfer of light energy into the chemical energy of sugar production (see Figure 6-2). All plastids begin as nonpigmented protoplastids and then differentiate into one of the three basic types. Chromoplasts may begin as chloroplasts but lose chlorophyll and accumulate carotenoids to become chromoplasts during fruit ripening and other processes. Leucoplasts may be transformed into chloroplasts when exposed to light. They still retain the ability to store starch, as do all chloroplasts when exposed to light. Sometimes chloroplast, following several hours of sunshine, will accumulate several large starch grains as products of photosynthesis, which distort the internal membrane structure. As opposed to prokaryotic cells in which pigment molecules are attached to peripheral membranes of the cell, the chloroplast represents highly organized arrangements of the chlorophyll, and the other pigment molecules are arranged in specific double membrane layers called thylakoids. Stacks of thylakoids constitute a granum (plural, grana). The matrix inbetween the grana is called the stroma, and the grana and stroma together make up the body of the chloroplast.
[FIGURE 6-2 OMITTED]
Chloroplasts tend to be elliptical and 5 to 10 [micro]m in diameter. In a green plant cell there might be 20 to 100 chloroplasts. During cyclosis they move freely throughout the cytoplasm. In carrying on the process of photosynthesis, they respond directly to the energy from the sun by orienting themselves perpendicular to the rays of the light. In case the light energy becomes too great, they have the capability of moving away from the sun and orienting themselves at an oblique angle so that less light hits them.
Endoplasmic Reticulum and Ribosomes
The process of synthesis can occur only in the presence of ribosomes found on a series of interlacing membranes that traverse the cytoplasm and form the framework, on which certain important functions are performed, including protein synthesis. This membrane system, the endoplasmic reticulum (ER), provides the scaffolding to which the ribosomes are attached to the ER. ER may have a group of ribosomes, much like buttons attached to a piece of cloth, in what is called rough endoplasmic reticulum; or it may consist of a membrane with ribosomes, in which case it is referred to as smooth endoplasmic reticulum. The ribosomes themselves appear as dark round dots on the endoplasmic reticulum at low magnifications, but as the magnification increases, it becomes apparent that they consist of two parts--a small spherical body and a large concave body. This organelle is about 15 nm in diameter and therefore of much smaller dimensions than the other organelles already described. The ribosome is made of rRNA and protein. In this structure, the amino acids are aligned in proper order for incorporation into the protein. In any given cell, there might be many thousands of ribosomes. Thus, even though the process of protein synthesis may at first seem relatively slow, it is possible to make many molecules in a short period of time because each ribosome may be involved in the synthesis process.
The vacuole begins as a very small organelle that eventually increases in size until, in a mature plant, it dominates the entire cell, as shown in Figure 6-3. As a matter of fact, the cytoplasm may be stretched to the outer limits adjacent to the cell wall. In some cases the nucleus is displaced into or adjacent to the cell wall. In some cases the nucleus is displaced into a "corner" of the cell, and the vacuole actually occupies most of the space within the cell.
Vacuolar sap is mostly water and much less viscous than is the cytoplasm proper. It is probably best to think in terms of the vacuolar as the storage area of the cell, a place where nutrients and various solutes are maintained until they are needed in general metabolism or stored as water material. For the most part, macromolecules are not part of the vacuolar system but are maintained within other organelles or directly in the cytoplasm. The membrane that surrounds the vacuole is the tonoplast. The tonoplast selectively acts to determine what gets in and out of the vacuole. There are many different kinds of relatively small molecules within the vacuolar sap, including the ions and small molecules such as sugar and amino acids.
[FIGURE 6-3 OMITTED]
Located throughout the cytoplasm is a group of organelles collectively called the Golgi apparatus, as shown in Figure 6-4. They appear as flattened membranes, much like a stack of pancakes. At the edges of these flattened membranes one can observe small pieces of membranes called vesicles being pinched off from the periphery of the "pancake." The vesicles contain the macromolecules used in construction of both the membranes and primary cell wall. As the cell grows under the influence of turgor pressure against the plasma membrane, the membrane must enlarge and be strengthened by the deposition of new material. This packing function is performed by the Golgi apparatus, ensuring that as the interior expands the expanded membrane and wall will be able to take the additional stress imposed.
[FIGURE 6-4 OMITTED]
Sometimes other small organelles are found in specific plant tissues. Lysosomes, glyoxysomes, and peroxisomes are organelles bounded by a single membrane and containing a package of enzymes for a specific task. Lysosomes contain acid hydrolytic enzymes capable of breaking down proteins and certain other macromolecules. Glyoxysomes are found primarily in fatty seeds such as cotton and peanut and they provide enzymes for the conversion of fats to carbohydrates during the germination process. Peroxisomes provide a compartment of enzymes important in the glycolic acid metabolism associated with photosynthesis.
Microtubules are relatively small structures found in all eukaryotic cells and characterized by a tubular or spaghettilike appearance. For the most part, they are found in the cytoplasm, but they may also be a part of cilia and flagella, whiplike projections on the surface of motile cells.
Cytoplasmic microtubules are rather uniform in size and remarkably straight. They are about 23 nm in outside diameter and several micrometers in length. The wall of the microtubules consists of individual linear or spiraling filamentous structures, about 13 subunits. There is a lumen (open area in the center), but occasionally dots or rods are observed in the center portion. Microtubules are apparently composed of a special type of protein called tubulin.
Although the function of microtubules is still not perfectly clear, their orientation and distribution suggest that they form a framework, which somehow shapes the cell and redistributes its contents. We will soon see how cell specialization brings about different shapes and functions, and cell differentiation begins to occur at the same time that numerous microtubules begin to appear. Microtubules may also transport macromolecules, possibly forming channels in the cytoplasm. In plant cells microtubules occur inside the plasma lemma and are oriented tangentially to the cell. It has been suggested that they function in cell wall deposition, and microtubules have been noted to underlie the points where the secondary cell wall is being deposited in spiral or reticulate patterns. Spindle fibers, to be discussed with the cell division process, are proteins composed of microtubules.
1. The basic unit of life is the cell, and essentially all organisms are composed of remarkably similar cells.
2. Water is basic to all life as we know it, and its unique chemical and physical properties are related to hydrogen bonding. All the features of water density, cohesion, adhesion, and heat gain and loss can be attributed to this compound's special composition.
3. Energy is moved through living systems by a number of transfer molecules. These molecules allow for an orderly passage of energy from one chemical compound to another, ensuring that the efficiency of conversion is maintained.
4. The macromolecules of life are primarily carbohydrates, proteins, lipids, and nucleic acids. These large molecules, constructed from simpler molecules, provide the chemical framework of life.
5. In the process of plant metabolism, many plants synthesize so-called secondary compounds important to human needs. Some of these, such as alkaloids, volatile oils, anthocyanins, and tannins, are probably parts of an adaptive strategy in plant-herbivore interaction. Others, such as terpenes, resins, and sterols, are used in the manufacture of various industrial products.
6. Prokaryotic cells have no nucleus, whereas eukaryotic cells do have a distinct nucleus that can undergo division to produce new cells. Plant cells have a distinct cellulose wall; animal cells do not.
7. Membranes are made of lipids and proteins, and substances move through biological membranes according to properties of size and solubility. Membranes provide a large surface area to allow for collisions of molecules responsible for biochemical reactions.
8. Within the green plant cell are found the nucleus, mitochondria, chloroplasts, and endoplasmic reticulum with ribosomes, the Golgi apparatus, and microtubules. Many other smaller organelles are sometimes included. Mature plant cells usually have a single, large vacuole dominating the water relationships within the cell.
Something to Think About
1. What are the differences between eukaryotes and prokaryotes?
2. Name and tell the function of six plant cell parts.
3. What is the function of microtubules in a plant cell?
4. Name some small organelles found in plant tissue.
5. What is the special type of protein found in microtubules?
6. What are membranes made of?
7. What is the chemical framework of life called?
8. Where is the Golgi apparatus found in the plant cell?
9. How do vesicles work?
10. Explain how grana work in chloroplast when the light intensity changes.
Ferguson, T. 2000. Discovery windows on science. New York: Van Nostrand Reinhold.
Turnbull, C. 2006. Plant architecture and its manipulation. Oxford: Blackwell Publishing.
Twenty First Century Science. 2006. Science Educators Group of York, Nuffield Curriculum Centre.
Internet sites represent a vast resource of information. The URLs for Web Sites can change. Using one of the search engines on the Internet, such as Google, Yahoo!, Ask.com, and MSN Live Search, find more information by searching for these words or phrases: plant cell structure, prokaryotic cells, eukaryotic cells, plant primary cell wall, plant secondary cell wall, membranes, organelles, mitochondrion, plastids, chloroplasts, endoplasmic reticulum, ribosomes, Golgi apparatus, and microtubules.
Table 6-1 Selected Chemical Elements of Protons and Neutrons Chemical element Number of protons Number of neutrons Hydrogen (H) 1 0 Helium (He) 2 2 Carbon (C) 6 6 Nitrogen (N) 7 7 Oxygen (O) 8 8 Sodium (Na) 11 12 Chlorine (Cl) 17 18 Calcium (Ca) 20 20 Note: The number of protons does not always equal the number of neutrons. On the other hand, the number of electrons always equals the number of protons-one negative charge for each positive charge.
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|Title Annotation:||PART 2: Form and Structure|
|Publication:||Fundamentals of Plant Science|
|Date:||Jan 1, 2009|
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