Plant Bio Chapter 1 PDF

# Chapter One: Cell Structure and Organisation ## The Cell: The cell is the fundamental structural and functional unit of all organisms. The cell theory was first propounded in 1838-39 as the climax of almost two centuries of observations, beginning with the drawings of Robert Hooke in 1665 of the...

# Chapter One: Cell Structure and Organisation ## The Cell: The cell is the fundamental structural and functional unit of all organisms. The cell theory was first propounded in 1838-39 as the climax of almost two centuries of observations, beginning with the drawings of Robert Hooke in 1665 of the cellular structure of a cork. Robert Hooke is regarded as the first man to observe the cell. Robert Hooke propounded the cell theory. ## The Cell Theory The cell theory which postulates that all plant and animal bodies are composed of fundamental structural and physiological units, was formulated by two German scientists, Schleiden and Schwann (1838-­­­­1839). Although Robert Hooke observed the cell compartments, the cell wall and the protoplasmic contents, he thought of the protoplasm only as a nourishing fluid. It is now known (as the cell theory) that: * All cells contain protoplasm and are delimited by a membrane called the plasmalemma, plasma membrane or cell membrane. The cell of plants have a wall in addition to the membrane. The cell of plants have a wall in addition to the membrane. This is called a cell wall. * A controlled exchange of materials between the cell and its environment occurs through the cell walls and plasmalemma. * Cells also contain genetic materials (DNA) that control inheritance as well as the machinery (such as ribosomes) needed to transcribe and translate the genetic code into proteins. * All cells are derived from pre-existing cells, usually by division but sometimes by fusion (e.g. zygotes). ## Variations in Cell Size & Form Cells vary greatly in size and form. The smallest known free-living cells are the mycoplasmas, with a diameter of about 0.1um while the largest are the egg cells, notably the eggs of birds and reptiles. The average cell is between 0.5 - 20µm in diameter. Cells are characterized by a high degree of internal organization, involving division of labour among various organelles. ## Unicellular vs. Multicellular Organisms may be unicellular or multicellular. In the former, the organism is the cell, while in the latter, the cells are specialized for particular functions and aggregate into tissues which in turn form the organs of the body. Thus, cells of multicellular organisms are very diverse. ## Prokaryotic vs. Eukaryotic Cells Two fundamentally different cell types are known: the prokaryote and eukaryote types. Prokaryotic cells lack true nuclei, their genetic materials lie free in the cytoplasm. These can be found among blue-green algae and bacteria. The eukaryotic cells have their genetic materials within a nucleus separated from the cytoplasm by a membrane known as the nuclear membrane. Some of the major differences between the two types of cell are listed in Table-1. | | **Prokaryote** | **Eukaryote** | |-----------------------------|--------------------------------|---------------------------------| | **Size** | Average diameter | May be up to 40µm in diameter | | **Genetic Material** | No nucleolus. No membrane bound nucleus, DNA molecule is circular and single | Nucleus is membrane bound. There is nucleolus and DNA is associated with chromosomes. | | **Organelles** | Organelles are few and not double-membrane bound | Organelles are double membrane-bound. | | **Protein Synthesis Cell Walls** | 70S ribosomes. Cell walls contain polysaccharides and amino acids | 80S ribosomes. Cell walls usually polysaccharides only. | | **Flagella** | Flagella are simple, without microtubules and not membrane bounded. Usually 200µm diameter | Flagella are complex with microtubules and membrane bounded. Usually 200µm diameter | | **Respiration** | Respiration is by mesosomes | Respiration is by mitochondria. | ## The Structure of the Cell The outermost covering of the cell of plants is the cell wall. It is absent in the animal cells. ### Cell Wall The cell wall consists of a non-living structure secreted by the protoplast. It is a strong porous, rather rigid but somewhat elastic wall. The walls are interconnected and provide strength and support to the entire plant body. The cell wall develops after the division of a pre-existing cell. In the early stages of development, the cell wall is composed of granules secreted by the protoplasm. These granules increase in size and number until they coalesce to form a thin layer called the middle lamella. ### Middle Lamella The middle lamella is composed of large layers of pectic substances, mainly calcium peptate which function as the cementitious material that holds the adjoining materials together. The protoplast secretes an additional layer to the middle lamella, consisting of intertwined molecules of celluloses but rather plastic and capable of extension as the cell grows. This is called the primary wall as illustrated in figure 1. In some cells, a second wall, usually of 3 layers develop. These layers are basically of cellulose, but additional materials such as lignin which is a complex material responsible for hardness and decay resistance qualities may be present. **Figure 1:** Diagrammatic representation of plant cells showing the middle lamella and the primary wall. ### Pits and Plasmodesmata The pit is a region in the cell wall at which no secondary wall is deposited. The thinner perforated region in the primary wall is termed primary pit field. Cytoplasmic strands may extend through cell walls in the areas where there are no pits or pit fields just as they do in pitted areas. Such cytoplasmic strands are called plasmodesmata as illustrated in figure 2, and they aid movements of materials as well as transmission of stimuli. The cell walls are quite porous and are easily penetrated by water and various salt materials except where cutin and suberin are present. **Figure 2:** A diagram showing pit and plasmodesmata ### The Protoplasm The viscous colourless matrix, within which all various forms of complex functions of a living cell is carried out is called the protoplasm. Its consistency varies from time to time, from that of raw egg-white, to that of semi solid gelatin. It is not homogeneous because different structures called organelles can be found in it. The most obvious of the organelles is the nucleus. Apart from the cell wall that protects the protoplasm of one cell from the other and form the outside environment in the case of plant cells, a membrane that underlies the cell wall also exists. This membrane is called cell membrane, plasma membrane or plasmalema. ### The Cell Membrane The cell membrane or plasma membrane is a unit membrane system. It is more elastic than fibrous in nature. In the late 1930s, J.F. Danielli and H. Davson put forward a theory of membrane structure. The Danielli-Davson hypothesis proposes that the plasma membrane is made up of three layers: a bimolecular layer of lipid sandwiched between two layers of protein, the lipid molecule being set at right angles to the surface as illustrated in figure 3. **Figure 3:** Diagram of the cell membrane based on the Davson-Danielli hypothesis. ### The Cytoplasm The portion of the cell protoplasm that surrounds the nucleus is called the cytoplasm. Many cellular enzymes are located in the cytoplasm. In young plant cells, the cytoplasm occupies most of the volume of the cell. But as the cells mature, the cytoplasm occurs as thin layer lining the wall and with thin strands of cytoplasm penetrating throughout the cell. The cytoplasm that is detached from the plasma membrane has a membrane covering it. This membrane is termed cytoplasmic membrane. The cytoplasm contain organelles that can be recognised as follows: #### The Endoplasmic Reticulum This is a network of canals which are bounded by membranes. They traverse the entire cytoplasm. There is no definite direction as regards this. The Endoplasmic Reticulum extends from the nuclear membrane to the plasma membrane. The nuclear membrane is double layered, differentiated into inner and outer nuclear membrane. It is believed that it is the outer nuclear membrane that extends into the Endoplasmic Recticulum as illustrated in figure 4. It implies therefore, that the Endoplasmic Recticulum has pores just as the nuclear membrane has pores. The morphology of Endoplasmic Rectieulum (ER), varies from cell to cell and may vary even within the same cell. They also vary in their activities. **Figure 4:** Diagram of a portion of an animal cell. Showing the relationship between the plasmalema and nuclear membrane. The amount of endoplasmic recticulum present in each cell may as well vary considerably. Two kinds of endoplasmic recticulum are present: * **Rough or granular endoplasmic recticulum** * **Smooth or agranular endoplasmic recticulum.** It is possible to find the two types in the same cell, but usually more common to find either type in a cell. The rough nature of the granular endoplasmic recticulum is as a result of certain particles known as ribosomes presents on them. The ribosomes are responsible for protein synthesis. Therefore, when cells have the rough type of endoplasmic recticulum, such cells will be protein synthesizing. The smooth or agranular endoplasmic recticulum are responsible for synthesizing other materials other than protein. ### The Golgi Complex This is another organelle found in the cytoplasm. It is named after the discoverer. It is also a unit membrane system which are arranged in packs of bundles stacked together. Golgi bodies or complex are thought to be responsible for the secretion of lipids, carbohydrates and proteins. Golgi complex is actually regarded as the organelles that store and secrete all cellular secretions. When the ribosomes produce protein, they (proteins) move down along the canals of endoplasmic reticulum, until they get to the golgi complex where they are stored and released when needed. This phenomenon is possible because the canals of the golgi complex are continuous with those of the endoplasmic reticulum. In animal cells, golgi complex play an active role in the transformation of spermatozoa from the spermatid into spermatozoa (a maturation process). ### Mitochondrion The organelle called mitochondrion is found in almost every type of cells, in human body, it is in all cells except the red blood cells. Its shape varies from cells to cells. The number or quantity varies from cell to cell. In Amoeba for example, they are up to half a million, while it may not be more than one in another type of cell. The mitochondrion is bounded by two unit membranes, out of which one is the inner membrane and the other, outer as illustrated in fig. 5. The inner membrane is thrown into folds known as cristae, thus providing increased surface area on which chemical reactions can take place. There are numerous enzymes associated with the cristae, Mitochondrion is also known to contain the nucleic acid -DNA, an indication that mitochondrion probably govern or controls its own mode of inheritance. In function, mitochondrion is responsible for the final oxidation of food to yield energy in the form of ATP (Adenosine Triphosphate). The mitochondrion is therefore popularly referred to as the power house of the cell. **Figure 5:** Diagram of the Mitochondrion ### Lysosomes Lysosomes were not recognised as functional parts of the cell until the 20th century. They are also membrane bound organelles. Although their origin is not clear, they occur as sacs and contain enzymes referred to as hydrolytic enzymes or simply lytic enzymes. They use these enzymes in destroying unwanted cells. The enzymes are well protected by the membrane so that they don't come in contact with the protoplasm else they digest the protoplasm itself. The act of the digestion of the unwanted cells is referred to as AUTOLYSIS. ### Ribosomes These are organelles found in every kind of cell. They are concerned with the synthesis of protein and are spherical in shape. They are made up of two unequal halves or sub-units. Each of the sub-unit contain a considerable amount of RNA and consequently a considerable amount of protein. The ribosomes cluster together into what is known as-polysomes (poly-ribosomes). The RNA associated with a ribosome is so intimately connected with it, such that one cannot get at the RNA without destroying the ribosome itself. ### Plastids Another organelle found in the cytoplasm is the plastid. They are lens shaped bodies and may be clearly visible in the cell without staining. There are three kinds of plastids, each bounded by a double membrane and a cell may contain more than one type. A green plastid is called the chloroplast, plastids of other colours such as yellow, orange etc., are called chromoplast, while the colourless plastids are leucoplasts. The structure of a chloroplast is illustrated in figure 6. **Figure 6:** Diagram of a Chloroplast. ### Cell Inclusions Some non-living structures are present in the cytoplasm and are termed cell inclusions. They include the vacuoles, crystals and stored food. The vacuoles are found in mature plant cells where they occupy the greater portion of the space in the cell. Within the vacuole is a neutral fluid also called the cell sap. The crystals are waste products or excretory products of the protoplast. They are found more in plants and are located primarily in the vacuoles. Stored foods may also occur in cells as starch grains, oil globules aleurone etc. found when excess food is produced by the plant. Less frequently also, there may be tannins, musilages, resins and gums which are all waste products. ### The Nucleus The most prominent of the structures in the protoplasm is the nucleus. The nucleus in the eukaryotic cells are membrane bound. Nucleus is present in every cell at least at a point in their development. Usually there is a nucleus per cell but some cells have more than one. The nucleus is usually oval in shape but in some cases they may be elongated or round and at times they are made into lobes. The nuclear membrane is a unit membrane system with two layers, namely the outer membrane and the inner membrane as illustrated in figure 7. The space between the two membranes is known as the perinuclear space. Tiny pores can be seen on the nuclear membranes. The pores are responsible for transporting micro molecules mostly out of the nucleus but sometimes into the nucleus also. **Figure 7:** Diagram of a nucleus. When the nucleus is stained, it reveals a network of fine threads within the nucleus. It reveals also some'dark patches. Both the fine threads and dark patches are known as chromatin materials.. The nucleus also houses a densely stained body known as the nucleolus. Some organic acids can also be found within the nucleus. The organic acids which are known as nucleic acids are mainly DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid). The RNA and DNA are important in the transformation of genes and other hereditary materials from generation to generation. The fine threads that may be observed in a nucleus are the chromosomes and they are of a definite number in the cell of a particular organism. The chromosomes carry the genes and are so closely associated with the RNA and DNA to the effect that any interruption in the RNA or DNA means an interruption in the genetic composition of the species of the organism. ## Why the Cell is an Organized Body The cell can be considered to be an organized body for the following reasons: 1. All the enzymes in the cell are membrane bound such that they are restricted to their functions. 2. The positioning of the organelles is such that the position of each suits the function it performs. 3. Each organelle is performing a single function such that there is maximum efficiency. 4. No organelle can be removed without the activities of the cell being disturbed. 5. The ability of the cell to carry out the functions of the body is a proof of the versatility of the cell. ## The Diversity of Cells So far, the basic features of cells in general have been considered. This should not be taken to imply that all cells are identical. It is considered necessary to appreciate the striking diversity of cells in both the plant and animal kingdoms. This diversity is seen between different species and within a single species. Thus, the cells of Hydra differ considerably from those of man, despite the fact that they share the same basic features and to some extent perform the same functions. The epithelial cells (of which there are several kinds) possess a shape and form that makes them most suitable for lining the surface of the body and the organs and cavities within it. Glandular cells are responsible for producing some kind of secretion. Such cells have a particularly prominent Golgi body, an indirect evidence implicating the Golgi body in secretion. Leucocytes (white blood cells) generally amoeboid play an important part in defending the body against disease. One of the most specialized of all animal cells is the nerve cell or neurone whose slender arm-like processes transmit electrical impulse through the nervous system. In plants, photosynthetic cells of various shapes and forms packed with Chloroplasts perform the task of building up complex molecules. Parenchyma cells, packed tightly/together, fill up spaces between tissues- One of the most striking specializations is seen in liquefied vessels, tracheids and fibres. These play an important part in strengthening the stems of higher plants and in the case of vessels and tracheids, conducting water and mineral salts from the roots to the leaves. It should therefore be clear that cells are structurally specialized to perform particular tasks. In extreme cases specialization may entail loss of protoplasm or some important constituent of it. But in majority of cases it involves modification of the shape and form of the cell, its basic feature (nucleus mitochondria, endoplasmic reticulum etc.) remaining unchanged (fig. 8). **Figure 8:** A variety of cells. Despite differences in shape and form these cells are all fundamentally similar in possessing nucleus and cytoplasm together with many of the organelles described earlier.