Plastids are the largest cytoplasmic organelles bounded by double membrane. These are found in most of the plant cells and in some photosynthetic protists. These are absent in prokaryotes and in animal cells. Plastids are of three types namely chloroplasts, Chromoplasts and leucoplasts. Chromoplasts are coloured plastids other than green. They are found in coloured parts of plants such as petals of the flower, pericarp of the fruits etc.
Leucoplasts are the colourless plastids. These colourless plastids are involved in the storage of carbohydrates, fats and oils and proteins.
The plastids which store carbohydrates are called amyloplasts. The plastids storing fats and oils are called elaioplasts. The plastids storing protein are called proteinoplasts.
Ribosomes are small subspherical granular organelles, not enclosed by any membrane. They are composed of ribonucleoproteins and they are the site of protein synthesis. They occur in large number. Each ribosome is 150-250A in diameter and consists of two unequal sub units, a larger dome shaped and a smaller ovoid one. The smaller sub unit fits over the larger one like a cap. These two sub units occur separately in the cytoplasm and join to form ribosomes only at the time of protein synthesis.
At the time of protein synthesis many ribosomes line up and join an mRNA chain to synthesise many copies of a particular polypeptide.
Such a string of ribosomes is called polysome. Ribosomes occur in cytoplasmic matrix and in some cell organelles. Accordingly, they are called cytoplasmic ribosomes or organelle ribosomes.
The organelle ribosomes are found in plastids and mitochondria. The cytoplasmic ribosomes may remain free in the cytoplasmic matrix or attached to the surface of the endoplasmic reticulum.
The attached ribosomes generally transfer their proteins to cisternae of endoplasmic reticulum for transport to other parts both inside and outside the cell. Depending upon size or sedimentation coefficient(s), ribosomes are of two types. 70s and 80s. 70s type of ribosomes are found in all prokaryotic cells and 80s type are found in eukaryotic cells. S is Svedberg unit which is a measure of particle size with which the particle sediments in a centrifuge. In eukaryotic cells, synthesis of ribosomes occurs inside the nucleolus.
Ribosomal RNA are synthesized in the nucleolus. The ribosomal proteins are synthesized in the cytoplasm and shift to the nucleolus for the formation of ribosomal sub units by complexing with rRNA. The sub units pass out into the cytoplasm through the nuclear pores. In prokaryotic cells, both ribosomal RNAs and proteins are synthesized in the cytoplasm. Thus the ribosomes act as the protein factories of the cell.
According to various international studies, the world’s most popular color is blue. Though some researchers also suggest that red and green are a close second and third respectively.
Pink is the palliative color. Apparently, it suppresses anger and anxiety due to its calming effect.
The safest car color was determined to be white. Based on studies, it is the most visible color under all conditions except snow. Though surprisingly, lime-yellow is the most visible color on the road.
Researchers says that red and yellow are the most appetizing colors. Having said that, they advice not to paint your kitchen yellow if you’re on a diet.
Researchers says that red and yellow are the most appetizing colors. Having said that, they advice not to paint your kitchen yellow if you’re on a diet.
Research shows that mosquitoes are attracted to dark colors especially blue.
The color yellow can cause nausea, so it is avoided in airplanes. Also, pure bright yellow is believed to be the most irritating color due to its excessive stimulation to the eye.
In the year (1839) Schleiden and Schwann have jointly proposed the “Cell Theory” It states that all living organisms are made up of cells and cells are the structural and functional units of all organisms.
Development of Cell Theory
If we study the step by step development of cell theory we will understand how scientific methodology operates. It includes the following steps
1. observation
2. Hypothesis
3. Formulation of theory
4. modification of theory ( if it warrants). Observations were made by Schleiden (1804 -1881) a German botanist.
He examined a large variety of plants and found that all of them were composed of cells. In 1838 he concluded that cells are the ultimate structural units of all plant tissues. Schwann a German Zoologist studied many types of animals and found that animal cells lack a cell wall and they are covered by a membrane. He also stated that animal cells and plant cells were basically identical but for the cell wall.
He observed that both contain nucleus and a clear substance around it. He defined the cell as a membrane bound nucleus containing structure. He proposed a hypothesis that the bodies of animals and plants are composed of cells and their products. Schleiden and Schwann both together discussed Schwann’s hypothesis and they formulated cell theory.
The important aspects of cell theory are:
1. All living organisms are made up of minute units, the cells which are the smallest entities that can be called living.
2. Each cell is made up of protoplasm with a nucleus and bounded by plasma membrane with or without a cell wall.
3. All cells are basically alike in their structure and metabolic activities.
4. Function of an organism is the sum total of activities and interaction of its constituent cells.
Soil water is of paramount importance in the physiology of plants. It occurs in various forms, such as gravitational, capillary, hygroscopic and combined water. Rain is the principal source of water for the soil.
Water which flows down due to the force of gravity is known as gravitational water. The gravitational water is not available to the plants.
However, it is a big soil water reservoir and is trapped out through tube wells. A certain amount of rain water is retained within the intercellular spaces of the soil particles in the form of a capillary network.
It is called capillary water and is used by the plants. Some water molecules form a thin sheet of water around soil particles. It is called hygroscopic water (water of imbibition). The hygroscopic water is also not absorbed by the plants.
The water, which is bound up in chemicals is called combined water or crystalline water. (e.g. MgSo4.7H2O). It is not available to plants. The total water present in the soil is called as field capacity.
Addition of water beyond field capacity causes water logging. It excludes soil air and thus inhibits plant growth. The soils that have poor water holding capacity, cannot afford luxuriant vegetation. In such soils, the plants generally show wilting of their leaves.
The compound microscope which is most commonly used today contains several lenses that magnify the image of a specimen under study. The total magnification of the object is a product of the magnification of the individual lenses; if the objective lens magnifies 100 -fold (a 100x lens, usually employed) and the eye piece magnifies 10- fold, the final magnification recorded by the human eye or on film will be 1000- fold (100 x 10).
The limit of resolution of a light microscope using visible light is about 0.2µm (200nm). No matter how many times the image is magnified, the microscope can never resolve objects that are less than ≈ 0.2µm apart or reveal details smaller than ≈ 0.2 µm in size Samples for light microscopy are usually fixed, sectioned and stained. Specimens for light microscopy are usually fixed with a solution combining alcohol or formaldehyde, compounds that denature most protein and nucleic acids.
Usually the sample is then embedded in paraffin or plastic and cut into thin sections of one of a few micrometers thick using a microtome. Then these sections are stained using appropriate stains.
All living things found on the planet earth are divided into two major groups namely, prokaryotes and Eukaryotes based on the types of cells these organisms possess. Prokaryotic cells lack a well defined nucleus and have a simplified internal organization. Eukaryotic cells have a more complicated internal structure including a well defined, membrane- limited nucleus. Bacteria and Cyanobacteria are prokaryotes. Fungi, plants and animals are eukaryotes.
Prokaryotes
In general, Prokaryotes consist of a single closed compartment containing the cytosol and bounded by the plasma membrane. Although bacterial cells do not have a well defined nucleus, the genetic material, DNA, is condensed into the central region of the cell.
In all prokaryotic cells, most of or all the genetic information resides in a single circular DNA molecule, in the central region of the cell. This region is often referred to as incipient nucleus or nucleoid. In addition, most ribosomes, the cell’s protein synthesizing centres are found in the DNAfree region of the cell.
Some bacteria also have an invagination of the cell membrane called a mesosome, which is associated with synthesis of DNA and secretion of proteins. Thus we can not say that bacterial cells are completely devoid of internal organization. Bacterial cells possess a cell wall which lies adjacent to the external side of the plasma membrane. The cell wall is composed of layers of peptidoglycan, a complex of proteins and oligosaccharides. It protects the cell and maintain its shape. Some bacteria (eg E.coli) have a thin cell wall and an unusual outermembrane separated from the cell wall by the periplasmic space.
Such bacteria are not stained by Gram staining technique and thus are classified as Gram- negative bacteria. Other bacteria (eg. Bacillus polymyxa) that have a thicker cell wall without an outer membrane take the Gram stain and thus are classified as Gram positive bacteria.
Eukaryotes
Eukaryotes comprise all members of Plant Kingdom, Fungi and Animal Kingdoms, including the unicellular fungus Yeast, and protozoans. Eukaryotic cells, like prorkaryotic cells are surrounded by a plasma membrane. However, unlike prokaryotic cells, most eukaryotic cells contain internal membrane bound organelles.
Each type of organelle plays a unique role in the growth and metabolism of the cell, and each contains a set of enzymes that catalyze requisite chemical reactions. The largest organelle in a eukaryotic cell is generally the nucleus, which houses most of the cellular DNA. The DNA of eukaryotic cells is distributed among 1 to about 50 long linear structures called chromosomes. The number and size of the chromosomes are the same in all cells of an organism but vary among different species of organisms.
The total DNA ( the genetic information) in the chromosomes of an organism is referred to as its genome. In addition to the nucleus, several other organelles are present in nearly all eukaryotic cells, the mitochondria in which the cell’s energy metabolism is carried out, the rough and smooth endoplasmic reticula, a network of membranes in which proteins and lipids are synthesized and peroxysomes, in which fatty acids and amino acids are degraded.
Chloroplasts, the site of photosynthesis are found only in plants and some single celled organisms. Both plant cells and some single celled eukaryotes contain one or more vacuoles, large, fluid – filled organelles in which nutrients and waste compounds are stored and some degradative reactions occur. The cytosol of eukaryotic cells contains an array of fibrous proteins collectively called the cytoskeleton. Cytosol is the soluble part of the cytoplasm. It is located between the cell organelles. The plant cell has a rigid cell wall composed of cellulose and other polymers. The cell wall contributes to the strength and rigidity of plant cell.
Stars are born, develop, flourish, and die over periods of millions or billions of years. The mass of the star will often determine its end as stars change and die in different ways. Their deaths leave behind different phenomena, all of which, from white dwarfs to black holes, fascinate astronomers.
STAR BIRTH Most stars are born in enormous star nurseries, known as nebulae. A nebula consists of giant clouds of gas, mostly hydrogen and helium, and dust. Gravitational pull in denser parts of the nebula, sometimes caused by nearby stars or explosions, sees matter clumping together, increasing in mass and temperature, and generating its own gravity.
PROTOSTARS As a large clump of gas shrinks and grows hotter, it is labelled a protostar. This is a potential star in the making. Eventually, most protostars get dense and hot enough to trigger off nuclear reactions deep inside them and the stars start to shine. This initial burst of energy blows away dust and gas surrounding these early stars.
MAIN SEQUENCE The main sequence is a long period, usually lasting billions of years, in which a star generates energy through nuclear fusion, turning hydrogen into helium in its core. For stars up to three times the mass of our Sun, the main sequence comprises about 90 per cent of their life span. The Sun has been in its main sequence for more than 4 billion years.
GIANTS AND SUPERGIANTS When a star of the Sun’s size has used up most of the hydrogen fuel in its core it swells up to become a red giant. When more massive stars, eight or more times greater in mass than our Sun, start to swell, they become supergiants, Betelgeuse (also known as Alpha Orionis) is a red supergiant. It is so huge that if it replaced the Sun in our Solar System, its outer atmosphere would extend past the asteroid belt.
FADE AWAY Some protostars do not have enough mass to trigger nuclear reactions and become stars. Instead, they generate smaller amounts of energy through continuing contraction. These failed stars are known as brown dwarfs. They will radiate their remaining heat out into space, slowly fading until they have no energy left, at which point they are known as black dwarfs.
PLANETARY NEBULA After completing their main sequence, stars that are a similar size to the Sun start to collapse, increasing in density and temperature. The stars then swell to enormous size, before throwing off their outer layers as giant clouds of gas. The clouds cool to form a planetary nebula, surrounding the star’s remains, known as a white dwarf. Since the discovery of the Dumbbell Nebula in 1764, more than 3,000 planetary nebulae have been observed.
WHITE DWARFS A white dwarf may have run out of hydrogen or helium fuel to burn in its core, but it will continue to shine for many millions of years. White dwarfs can range in colour from hot white through to cool red. Scientists estimate that a typical white dwarf is so dense that a teaspoonful of its matter would weigh about 5 tonnes. As with a brown dwarf, white dwarfs fade over time to become black dwarfs.
SUPERNOVA! As some massive stars die, their cores contract sharply and temperatures rise by millions of degrees. The core absorbs more and more energy before erupting in a gigantic explosion. In the first 10 seconds of a typical supernova explosion, 100 times more energy is produced than the Sun will generate during its entire lifetime.
NEUTRON STAR Some massive star deaths result in the core collapsing in on itself to form a neutron star – the smallest, densest stars known in the Universe. Neutron stars may be under 20 km (12 miles) in diameter yet contain the same mass as the Sun. Rapidly spinning neutron stars send out radio waves that we can pick up on Earth. These stars are known as pulsars.
BLACK HOLE Some stars collapse even further, into a dense point called a singularity, The space immediately around a singularity is called a black hole. It is so dense and the pull of gravity so strong that nothing, not even light, can escape from it. Astronomers cannot observe a black hole directly, only its effects on nearby objects, such as the pull of gas into the hole, which can release powerful X-rays.
