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SECTION Cell Biology
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CHAPTER
CELL STRUCTURE AND FUNCTIONS Major Concepts: 1.1
Techniques used in Cell Biology (2 Periods)
1.2
Cell wall and Plasma Membrane (2 Periods)
1.3
Cytoplasm and Organelles (10 Periods)
1.4
Prokaryotic and Eukaryotic Cells (2 Periods)
Number of allotted teaching periods: 16
You are quite familiar with the word cell . A cell is the smallest unit of living matter. According to German biologist Rudolf Virchow (pronunciation: Firkoh) every cell comes from a pre-existing cell . By the middle of the nineteenth century, biologists clearly recognized that all living things are composed of cells. This is known as cell theory. The cell theory is one of the unifying concepts of biology. A cell is also the functional unit of the organism. Cells can take in nutrients, break them down to release energy, and get rid of wastes. They can reproduce, react to stimuli, and maintain internal environment different from their surroundings. This chapter will help you to become familiar with the structure of cells and how they work.
1.1 TECHNIQUES USED IN CELL BIOLOGY How can you study the structure and functions of cell and its organelles? To know the structure and functions etc. of cell and cell organelles now different techniques are used. The techniques that will be discussed here in brief are: cell fractionation, centrifugation, differential staining, microdissection, tissue culture, chromatography, electrophoresis, and spectrophotometry.
BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS
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Cell Fractionation A common approach for studying functions of a cell is to isolate a particular cell organelle from other cell components and try to make it perform its normal functions in a test tube. Generally cells are broken apart as gently as possible. A common procedure is to grind up i.e. to homogenize cells in a suitable medium (with correct pH, ionic composition and temperature). This is done with a homogenizer (food mixer). The mixture is then centrifuged. Centrifugation Centrifugation is the process to separate substances on the basis of their densities. It is done by the machine called centrifuge (fig. 1.1) This machine can spin the tubes. Contents are kept in tubes that are much like the test tubes. Spinning the tubes exerts a centrifugal force on the contents. As the number of revolutions per minute increase so does the centrifugal force (measured as G, which is equal to the force of gravity) increase.
Fig: 1.1 A Centrifuge
Differential Staining Most biological structures are transparent. To differentiate between different structures some methods must be used. The most common method is staining. Certain stains when used in low concentrations are non-toxic to living tissues and can therefore be used on living material. These are called vital stains e.g. methylene blue. When only one stain, such as borax carmine is used it is called single staining. When two stains, one that will stain nucleus e.g. haematoxylin and other that will stain cytoplasm e.g. eosin are used, the process is called double staining or differential staining. Microdissections When dissection is made under microscope it is called microdissection. It is done to remove tumour or granules from delicate tissue or cells like, brain, heart and nerve cells. These days the image is seen on large TV screen or monitor while dissecting.
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BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS
Tissue Culture Cell cultures are a suspension of cells in a liquid medium. Tissue cultures are small pieces of plant or animal organ grown in liquid or on solid medium. In plant tissue the root and shoot tips are taken and cultured in a suitable medium to see any infection. Phloem tissue of plants is removed and placed in a germ free medium containing adequate food supply, mineral salts and growth substances. The cell will develop into a new plant, which will flower and produce seeds. Animal tissue culture is usually set up by growing individual cells to form a single layer of cells over the surface of a glass container. Tissue cultured cells are used to see any abnormality in the cell e.g. cancer etc. Chromatography Chromatography is a procedure through which various substances in a mixture are separated from each other and identified. Separation involves the use of two phases, one of which is stationary and the other is mobile. Separation depends upon the differential movement of the test substances between two phases. Can you find out names of seven types of chromatography? You may consult books or search on the internet. Paper chromatography is a simple and most widely used technique.
Fig: 1.2 Chromatography Chamber
Electrophoresis It is a laboratory procedure that separates molecules according to their size, shape, molecular weight and surface charge whether (+) or (–). Macromolecules such as nucleic acids or proteins can be separated in a mixture. Often the gel is sandwiched between glass or plastic plates to form a viscous slab (fig. 1.3). The two ends of the slabs are suspended in two salt solutions that are connected by electrodes to a power source. When voltage is applied to the apparatus, the molecules present in the gel migrate through the electric field according to their individual charge and they move away from
BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS
Spectrophotometry
Upper buffersolution
Movement of proteins
one another in the gel. The negative charged molecule will move towards the positive pole and the molecule having positive charge will move towards the negative pole. Later on the molecules can be pin pointed by staining the gel.
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Electrode Glass tube or plates containing gel Gel Lower buffer solution
Power supply Fig: 1.4 Spectrophotometer
Electrode Fig: 1.3 Gel Electrophoresis
A spectrophotometer (fig. 1.4) is an instrument that measures the amount of light that passes through the sample and from this it can be calculated how much light was absorbed. The amount of light absorbed at each wavelength is plotted in a graph and the result is what we call the absorption spectrum (fig. 4.6). The procedure is called spectrophotometry. It can be used to determine the wavelengths of light that take part in photosynthesis. It is also used to know the turbidity or cloudness. The more cells e.g. microorganisms are in suspension the greater will be turbidity. Resolution and Magnification in Microscopy Most animal cells and plant cells are between 10 µm and 30 µm. µ is the Greek letter mu. The unit of microscopic measurement is micrometre (preffered to micron). The correct symbol is µm (preferred to µ) i.e. the µm is the abbreviation for micrometre (American spelling: micrometer). (1 µm = 1/10000 mm). When two objects are closer together than about 100 micrometres, the two light beams fall on the same detector cells at the rear of the eyes. When the two dots are farther apart than 100 micrometres, the two beams fall on different cells, only then our eyes resolve them i.e. tell that they are two objects and not one. Resolution is the minimum distance that two points can be separated and be distinguished as two separate points by an optical instrument.
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BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS
One way to increase resolution, is to increase magnification to make small object seen larger. The increase in the apparent size of an object is called magnification. Robert Hooke was able to see small cell by magnifying i.e. enlarging the size, so the cells appeared larger than 100Âľm limit imposed by the structure of human eye. Graticule and Micrometre Graticule is a photographically produced gird, cemented between two glasses. In order to use the graticule for measurement it must be calibrated so that we know what each square represents when a particular object is used. Measurement of microscopic objects is called micrometry. This can be done using specially designed scales or micrometre. There are two types of micrometres: Ocular graticule or micrometer and stage micrometre. Ocular Micrometre Fig: 1.5 Graticule
Ocular micrometre is also known as eyepiece micrometre (fig. 1.6). It is a circular glass piece. It can be put between the two lenses of the eyepiece. A scale showing 100 arbitrary divisions, have been photographically produced, cemented between two glasses.
Fig: 1.6 Ocular Micrometre or Graticule
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As this micrometre is put between the two lenses of the ocular so, it is called ocular micrometre. The micrometre acts as ruler and its scale is used for direct measurement of the object. Stage Micrometre As this scale is placed on the stage of the microscope, so it is called stage micrometre. This is a plane slide. On the centre for the slide, a scale has been produced photographically or engraving. This scale is usually 1 mm having 100 divisions. 1 mm
=
100 divisions
100 division
=
1000 micrometres
1 division
=
20 Angstrom
=
1000 = 10 micrometres (0.01 mm) 100 1 Nanometre.
Fig: 1.7 Stage Micrometre, Total Length is 1mm
The resolution of an electron microscope is about 0.5 nm in practice, compared with 200 nm for light microscope. Infact, the most powerful modern electron microscope can distinguish objects as small as 0.2 nanometre (abbreviated nm, 1 1nm = 1,000,000 mm), a thousand fold improvements over the light microscope.
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BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS
1.2 CELL WALL AND PLASMA MEMBRANE The plasma membrane is the outer living boundary of the cell. Many cells have an extracellular component that is formed exterior to the membrane, which is called cell wall. Cell Wall The cell wall is present in plant cells, prokaryotes and fungi. Plant cell walls differ in chemical composition from those of the prokaryotes and fungi. Do you know the differences? We will discuss here only plant cell wall. The cell wall is secreted by the cell. The structure, thickness and chemical nature of the cell wall varies with type of cell and its function. The plant cell wall consists of three main layers, primary cell wall, middle lamella and secondary cell wall. Primary cell wall is a true wall and develops in newly growing cell i.e. during cell division. Each cell produces a primary cell wall. The young growing cells, storage cell and photosynthesising cells of leaves have primary cell wall. The cell wall surrounds the protoplast and plasma membrane. The primary cell wall is thin i.e. 1-3 mm in thickness.
BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS
Not in most plant cell
Fig: 1.8 Electron Microscopic Structure of an Animal Cell
Fig: 1.9 Electron Microscopic Structure of a Plant Cell
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Fig: 1.10 Plant Cell Wall
The primary cell wall consists of cellulose microfibrils, running through the matrix of other polysaccharides. The microfibrils show a crisscross (fig. 1.11) arrangement. The primary cell wall is adapted to growth. The wall stretches plastically i.e. irreversibly. The cell wall is porous and allows free passage of water and dissolved material. Secondary cell wall is formed between the primary cell wall and plasma membrane. It is found in most of the cells specially that provide support for the plant. The protoplast secretes extra layers of cellulose on the inner surface of the primary cell wall i.e. outer surface of the plasma membrane. Its microfibrils also show crisscross arrangement (fig. 1.11). The secondary cell wall develops only when the cell has reached maximum size i.e. completes its growth.The secondary cell wall consists of cellulose, hemicelluloses and lignin. Lignin cements and anchors cellulose fibres together. Lignin is a complex polymer, not a polysaccharide. Lignin is much rigid than cellulose and cannot be easily compressed and resists changes in form. It provides the cell wall extra tensile and compressional strength. The secondary cell wall provides mechanical support to the cell and those to the plant as it is present in xylem and Fig: 1.11 Crisscross Arrangement of Microfibrils sclerenchyma.
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Middle Lamella is present between adjacent primary cell walls of two cells. It is the first layer to be formed. It is formed of sticky, gel-like magnesium and calcium salts (pectates) of proteins. The middle lamella holds neighbouring cell walls together. Q. Why cell wall is not present in animal cell? Plasma Membrane Every cell is bounded by plasma membrane. The membrane is also called cell membrane or cell surface membrane. It gives shape and mechanical support to the cell. Chemically cell membrane consists of proteins 60-80%, lipids 20-40% and small quantity of carbohydrates. The most common lipids are phospholipids. Membranes also contain glycolipids, cholesterols and glycoproteins. The membrane is about 7nm thick. Fluid Mosaic Model of Plasma Membrane In 1972 Sanger and Nicholson put forward the fluid mosaic model of membrane structure. The model proposes that the membrane is a phospholipids bilayer in which protein molecules are either partially or wholly embedded. The protein are scattered throughout the membrane in an irregular pattern that can vary from membrane to membrane. The mosaic distribution of protein is supported especially by electron micrograph of freeze fractured membrane (see Glossary).
Fig: 1.12 Fluid Mosaic Model of Plasma Membrane