Introduction to Plants and Related Organisms:
Human beings are naturally curious about world around them. World of living things is extremely huge in numbers, ancient in history, and compound in diversity. Science of biology has several disciplines, all trying to give us true picture of nature of living things. We are part of the living world. As we see world of life around us through discriminating eyes of biologists.
Diversity of Life on Earth:
There is life around us. There are many organisms in soil, on surface of land, in air, on high mountains, in fresh water and in oceans. Many types of organisms have lately been found in ocean at depths of 4-10 kilometers where it is continually dark and pressure is so high that it would crush us flat. The amplifying glass and microscope can reveal that living things are plentiful in almost any place or thing which we care to study. There are thousands of microorganisms on surface of the leaf, in drop of pond water or saliva and over earth without life. Ours is the planet of life.
Biologists attempt to analytically explain and document different types of life forms and understand the evolutionary history, origins, functioning and reproduction. This is monumental task. Only approx 1.7 million of these have been recognized and in brief explained. Organisms that now lived on earth are descendants of species which lived in the past. History of life on earth is at least 3.5 billion (3,500,000,000) years old. During this long period several types of organisms have developed and most turned into extinct. Human beings are evenly part of the evolutionary drama of life on earth. Human species, homo-sapiens, developed fairly recently, only approx 300,000 years ago. All species which live today symbolize only fraction of total number of species which earth had supported ever since life initiated. Scientists evaluate that approx 99% of species are extinct. There were approx five major and many minor episodes of extinction when millions of type of organisms gone. Several but not all of these are established as fossils today. Last major extinction happened some 65 million years ago when dinosaurs and several types of plants became extinct.
Diversity of life on earth is not limited to great number of species or abundance of individual species. There is great variety in size and form of organisms. The bacterium may be simply one thousandth of millimeter in length while a 30 metre tall palm tree is 30 million times longer that bacterium. Living things exhibit diversity in construction, from single celled organisms to those made up of trillions of cells. Few multicellular organisms are plain in structure whereas others exhibit high degree of discrimination. The green alga called as Ulva situated in the seas seems to be no more than thin and flat green structure. The distinctive flowering plant, in contrast, is discriminated in stems, roots, flowers branches, leaves, and other organs.
There is variety also in ways in which organisms get their food. Carbon is the all significant element in life of living things. Organic compounds of carbon give energy and materials for metabolism, development and reproduction. Green plants are autotrophs. Autotrophs get their carbon from inorganic carbon dioxide in procedure of photosynthesis. Heterotrophs are other organisms that carbon from pre-synthesized organic compounds. Animals are heterotrophs as their carbon source comes from plants or animals that they feed on. Parasites are heterotrophs which occupy living hosts and suck up carbon compounds. Several bacteria are parasites and fungi. Saprotrophs are as well heterotrophs as they get their carbon source from organic remains in the surroundings. Several fungi and bacteria are saprotrophs. Whatsoever means of getting the carbon, every species is able to elaborate simple carbon compounds in thousands of different compounds. This variety in chemical composition can be utilized to recognize numerous plant groups. Several insects selectively feed on some plants as their ability to use chemicals present in such plants for food, defense or reproduction.
Living things have variety in the longevity. Few live for less than week while some plants are known to live for more than 5000 years. Life is varied in way living things reproduce their types, from simple fission to the variety of asexual methods and sexual means which need complex, sex organs. In animal kingdom organism illustrate habitats of organisms. Life can be discovered in snow, in dry deserts, in moist rain forests and in fresh and salt waters. Few bacteria live in very acidic and hot liquids reaching more than 100oC. All the diversity may at first confuse us. Once start to comprehend underlying order and principles and factors which direct diversity, living world becomes meaningful scene of great antiquity and loveliness. As of great diversity in numbers no one individual can ever study or even see all living species in time. This is one explanation why biologists attempted to specialize in examining selected groups of organisms. Therefore, zoologists study animals, botanists study plants and microbiologists study the microbes.
Features of Living Things:
Botany is stated as scientific study of plant. Many of us can identify the living thing from nonliving object and can recognize when living organism has died and has turn out to be nonliving. The problem with definition of life is that for each statement we will discover some exclusion. If "grow. If life is distinguished by reproduction so can fire in forest reproduce itself. Yet, there should be features which are common to most living things which distinguish millions of organisms from non-living world. The clear exception is unusual way on which living things are prearranged at all levels - chemical, subcellular, organ and organism levels. This organization appears to go against law of physics that randomness or decay rather than organization characterizes universe. On closer scrutiny it is discovered that highly ordered structures of living things are possible only at cost of energy got from sun. Sun illustrates increasing "entropy" or randomness. When sun and all living things are taken together, entropy is certainly increasing in universe.
The characteristics are common to most organisms which are given below. Common characteristics assist differentiate living from nonliving things.
1) Life processes are on the basis of organic chemicals in which carbon atom plays primary role. Other atoms, mainly oxygen, hydrogen, and nitrogen join with carbon to produce such common organic chemicals as carbohydrates lipids, proteins and nucleic acids.
2) Metabolism is the characteristic of life. Organism's energy from food and utilize this energy to manufacture other necessary substances required for development, reproduction and other functions.
3) Living things are composed of cells.
4) Living things nurture. Development can happen by improvement, elongation, increase in dry weight or increase in number of cells.
5) Development and discrimination accompany growth, at least in most multi-cellular organisms.
6) Organisms reproduce their own types. Heredity system is handled by all significant information molecules DNA and RNA.
7) Living things are extremely planned. Cellular organelles, Biochemical reactions, tissues and organs reveal the system of high complexity and organization.
8) Living things reply to different environment stimuli. This reply, called as irritability makes sure that organisms use desirable environmental inputs or avoid those which influence then adversely.
9) Many animals and unicellular organisms move. Generally plants don't move. Though, individual organs of many plants exhibit use movement in reaction to gravity, light and other stimuli.
10) Organisms vary and evolve. Over long periods of time alteration happening in genetic material lead to changes in structure or function which are acted on by selective forces resulting in novel types of organisms.
Origin of Life:
Astronomers think that universe initiated about 15 million (15,000,000,000) years ago. The solar system is thought to be about 5 billion year old, and earth was well formed by 4.5 billion years ago. Time of origin of life can be replied by finding fossil remains of ancient organisms in oldest rocks whose age has been found by physical techniques. Organic chemical remains investigative of life procedures have been situated in 3.8-billion year-old rocks in Greenland. Though, authentic micro fossils and stomatolites are known mainly from rocks which are 3.1 billion years old stromatolites are laminated fossil rocks formed by activity of cyanobacteria. Scientists have same opinion that life existed between 3-3.5 billion years ago and thus we can suppose that life should have initiated a little bit previous to this period.
According to hypothesis over a long period of time inorganic chemicals on surface of the earth were transformed in simple organic chemicals in low pools of water. Organic chemicals in turn aggregated in more complex units. Long polymers of amino acids and nucleotides could have been part of the complex microscopic structures. Skill to replicate them and catalyze various chemical reactions has made the structures most prehistoric living things. As RNA could store and broadcast inherited information and also catalyze few chemical reactions scientists think that RNA rather than DNA might have been genetic material of earliest organisms. There is proof that plain organic compounds could have been making from inorganic matter without activities of living things. Sugars, alcohols, amino acids and nitrogen bases have been mined from interior of meteorites. Organic compounds detected in rock samples brought from moon. Life prospers on earth today in atmosphere rich in oxygen and free from great and many outbursts of energy. Every present forms of life would expire if they were to live in circumstances which existed 4 billion years ago. There was no free reducing due to presence of such reducing substances as hydrogen methane, sulphide and ammonia. Temperature in prebiotic environment was extremely high, maybe about 500oC.
It is presently considered that life originated when temperatures were above 100oC and early organisms perhaps resembled few modern bacteria that life in high temperature environments. Few bacteria of group archacnacteria dwell in temperature between 100 and 140oC. They don't rely on sunlight as source of energy. Instead they use energy from molecular hydrogen and sulphur compounds and join carbon dioxide in organic compounds. They dwell in under anaerobic circumstances. Few of these bacteria live in hot vents at bottom of oceans where earth's crust is created from molten lava and sea floor is spreading, pushing continents apart. Though these bacteria don't have chlorophyll and don't rely on sunlight they are still autotrophic as they can get carbon compounds from inorganic carbon dioxide.
Organization of Cells-Prokaryotes and Eukaryotes:
Every living thing can be clustered into 2 main groups on basis of structure and biochemistry of the cells, mainly on organization of the genetic material. Bacteria and associated organisms are prokaryotes. Prokaryotes (pro=before; karyon=nucleus) have primitive kind of organization where nuclear material, DNA, is not enclosed by nuclear envelope. In its place, DNA happens as circular strand within cell. In eukaryotes (eu=true) DNA is prepared into rod-like chromosomes and chromosomes in turn are surrounded by the nuclear membrane. Together, chromosomes and nuclear envelope and other components like nucleolus comprise the nucleus. The nucleus is feature of algae, fungi, protozoa, plants and animals. All these are eukaryotes. Only few thousand species of bacteria are protaryotes. Rest of all organisms, 10-30 million species, is eukaryotes.
There are several other differences between prokaryotic and eukaryotic organization of cells. Cytoplasm is enclosed by plasma membrane. Nucleus is restricted by double membrane that has pores and extensions into cytoplasm. Membrane extensions comprise endoplasmic reticulum. Ribosomes comprised in protein synthesis are frequently related with endoplasmic reticulum. Dictyosomes or golgi vesicles are membranous structures with the secretory function. The large vacuole is also restricted by the membrane called as tonoplast. Mitochondria and plastids are too membrane-bound organelles. Membrane-bound organelles are lacking in prokaryotes. They do have the cytoplasmic membrane of different chemical composition. Photosynthetic bacteria are strange in having internal lamellae which resemble membranes. One organelle common to eukaryotes is ribosome. Though, ribosomes aren't membrane-bound. Several prokaryotic and eukaryotic cells move by help of flagella. The prokaryotic flagellum is simpler in structure and is made up of protein flagellin. Eukaryotic flagellum is made up of paralled mircrotubules. In transverse sections flagellum is observed to have 9+2 arrangement of microtubules. There are 2 central microtubules enclosed by 9 pairs of microtubules. Microtubules are made up of tubulin protein. Eukaryotic flagellar structure, when present, is stated similar in fungi algae, protozoa and plants and animals. This is the sign that flagellum should have initiated very early during course of evolution and should have been present in common ancestor of all modern eukaryotes.
Evolution by Endosymbiosis:
Earliest organisms were prokaryotic and didn't have such organelles as true nucleus, mitochondrion, flagellum or plastid. Mutations and genetic recombinations are two mechanisms which make variations among organisms. Natural selection acts on the variations resulting in evolution of more complex structures and organisms. It was once though that mutation and recombination have been liable for internal differentiation of above organelles within prokaryotic ancestral cells. Most biologists now think that different mechanism, that is, symbiolosis, might have played the major role in development of organelles and more advanced groups of organisms.
Symbiosis is living together, in close association with one another, of two or more different organisms. This connection may range from parasitism, in which one organism advantage by deriving nutrients from other, to mutualism, in which both partners advantage from each other. Association between nitrogen-fixing bacteria and roots of leguminous plants is the good instance of mutualism. Endosymbiont Theory postulates that mitochondria and plastids are descendants of once free-living prokaryotes. The prokaryotic ancestor has naked DNA without related protein or nuclear membrane. It also possesses 70S ribosomes typical of all prokaryotes. Condensation of nuclear material and internal discrimination of nuclear membrane would have distorted such a prokaryote into eukaryote. Ribosome would have suffered biochemical changes resulting in 80S kind of eukaryotes. Mitochondria like prokaryotes might have initially entered ancestral eukaryotic cells as invading or engulfed prokaryote. Prokaryote wasn't digested by eukaryote. Neither prokaryote killed host eukaryotic cell. Instead it was stabilized in eukaryotic cell resulting in symbiotic relationship between two. Prokaryote could have carried on more effective respiratory breakdown of nutrients to make energy-rich compounds like ATP. These could have been utilized by eukaryotic cell which in turn provided nutrients essentially for prokaryotic to survive. In course of time prokaryote would have lost its cell wall and distinguished internal membranes therefore evolving in true mitochondrion. It is not surprising thus those mitochondria of all organisms today share numerous characteristics with prokaryotic bacteria. Mitochondria, such as bacteria, are small in size, about 0.2 to 0.5 um. They have circular DNA and 70S ribonsomes. It is exciting to note that few free-living bacteria can synthesize ATP in process amazingly similar to respiratory ATP synthesis in mitochondria. Eukaryotic cells have obtai8ned flagella very early in the evolution. This could have occurred either before or after symbiotic attainment of mitochondria. It is not clear how flagella created. One option is that flagella also originated in symbiotic event where motile prokaryote, not unlike modern spiral bacterium, was stabilized as part of eukaryotic cell. This could have been benefit to eukaryote as it could move around in look for of food. At times after gaining of flagella 9+2 arrangement of microtubules and centrioles have developed Centrioles and associated spindle fibre were essential previous to mitosis and metiosis could develop. Mitochondria, flagella and mitosis and meiosis were present in eukaryotes previous to another significant endosymbiotic event happened -acquisition of chloroplasts. This was time of difference between animal, fungal and plant lines of evolution. Primitive eukaryotes were nonphotosynthetic. Few of these colorless organisms were adapted to getting food from organic medium around them and such saprotrophs developed into fungi. Though early eukaryotes were nonphotosynthetic there were photosynthetic prokaryotes around them. Cyanobacteria have chlorophyll a and developed oxygen during photosynthesis. Endosymbiont theory recommends that symbiotic acquisition of photosynthetic prokaryotes by few ancestral eukaryotes resulted in evolution of chloroplasts. This in order directed to evolution of different algal groups and higher plants, all of which possess chlorphylla a. chloroplasts are approx 1-6 um in length, much like cyanobacterial cells and have a prokaryotic organization of DNA and ribosomes. Recent algae groups possess plastids with chlorophyl a and ultrastructurally and biochemically very alike to recent cyanobacteria.
Green algal and higher plant plastids are distinguished by presence of chlorophyll a and chlorophyll b. The most exciting discovery prepared in recent years has revealed existence of at least 3 genera of prokaryotes that have both chlorophyll a and b. Considerable research is now devoted to study of the organisms called as prochlorophytes. Prochlorophytes are either treated as separate division or as prochlorophyte was progenitor of green plant chloroplasts. Endosymbiont theory is innovative with far-reaching implications for way in which helps to understand life and ourselves. Theory not only pleasingly describes origin of diverse groups of organisms and the organelles but also proposes that behind individuality of every species lies history of symbliotic co-evolution mitochondria in all the cells are associated to mitochondria in green plant, mitochondria which were part of extinct dinosaurs, and eventually to few prokaryotes Descendants of once free-living photosynthic bacteria are now chloroplasts which synthesize food in leaf.
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