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PHYLUM ARTHROPODA

PHYLUM ARTHROPODA

The phylum Arthropoda is the largest and most varied in the animal kingdom. It includes well over one million described species. This represents approximately three-quarters of all known biological organisms, living or extinct. Countless arthropods remain undescribed, and the actual number of living species could be as high as ten million or more. Some of the more well-known arthropods include insects, crustaceans, and spiders. Arthropods are found in virtually every known marine, freshwater, and terrestrial ecosystem, and vary tremendously in their habitats, life histories, and dietary preferences.

Characteristics of Arthropods

All arthropods possess a stiff exoskeleton composed primarily of chitin . In some species, lipids, proteins, and calcium carbonate may also contribute to the exoskeleton. The external skeleton offers organisms protection as well as support for the body. Its walls provide anchors for the attachment of muscles. The exoskeleton is incapable of growth, and is molted repeatedly during the growth of the animal. This process is called ecdysis. Molting allows for rapid growth until the newly secreted exoskeleton hardens.

Arthropod bodies are divided into segments. However, a number of segments are sometimes fused to form integrated body parts known as tagmata. This process of fusion is called tagmosis. The head, thorax, and abdomen are examples of tagmata. Arthropods also have appendages with joints. In early, primitive anthropods, each body segment was associated with a single pair of appendages. However, in most species some appendages have been modified to form other structures, such as mouthparts, antennae, or reproductive organs. 

Some arthropods have highly developed sense organs. Most species have paired compound eyes , and many also have a number of simpler eyes called ocelli. Arthropods have an open circulatory system that consists of a tube that is the heart and an open hemocoel , the coelom of the animal, in which blood pools. Arthropods also have a complete gut with two openings, the mouth and the anus.

Gas exchange in the phylum occurs in various ways. Some species have gills, while others employ tracheae, or book lungs. The tracheal respiratory system consists of external openings called spiracles that are linked to a system of branched tubules which allow respiratory gases to reach internal tissues. Arthropods are characterized by a brain as well as a nerve ring around the area of the pharynx, in the oral cavity. A double nerve cord extends backwards along the ventral surface of the body, and each body segment is associated with its own ganglion, or mass of nerve cells. In most arthropod species, the sexes are separate. Fertilization usually occurs internally, and most species are egg laying. While some species exhibit direct development, in which eggs hatch as miniature versions of adults, other species pass through an immature larval stage and undergo a dramatic metamorphosis before reaching adult form.

Major Groups of Arthropods

Arthropods are divided into four subphyla. These are the Chelicerata, the Crustacea, the Uniramia, and the Trilobita.

Subphylum Chelicerata

 Subphylum Chelicerata comprises a major group within Phylum Arthropoda, including such animals as the arachnids (e.g., spiders and scopions), the extinct eurypterids, and the extant horseshoe crabs that are considered to be living fossils. These arthropods and their ancestral and extinct forms were and are mainly predators. Chelicerates are now predominently terrestrial animals, with most marine chelicerates, including all of the eurypterids, now extinct.

Their name comes from their chelicerae, pointed appendages that grasp food, that differ from the chewing mandibles of most arthropods. Being unable to ingest solid food, most Chelicerata either drink blood or spit or otherwise inject digestive enzymes into their prey, and feed on the fluidized result. Like all arthropods, chelicerates' bodies and appendages are covered with a tough cuticle primarily composed of chitin and proteins that chemically harden. Because this exoskeleton cannot stretch, chelicerates have to molt in order to grow. Thus, they have to molt the old, and await hardening of the new, during which time they have reduced mobility and are otherwise more defenseless.

Subphylum Crustacea

Members of Subphylum Crustacea (the Crustaceans) comprise a large group of arthropods. The group contains familiar popular marine food animals such as lobsters, crabs, shrimp. While mainly found in salt and freshwater environments, there are also terrestrial Crustacea such as woodlice and isopods. Crustaceans have three distinct body parts: head, thorax, and abdomen (also called a pleon). In some, the head and thorax are fused to form a cephalothorax. They have two pairs of antennae on the head, compound eyes, three pairs of mouthparts and a telson. Crustaceans often have a thick carapace on the top (dorsal) side that makes fossilization more likely; crabs and lobsters, for example, have a thicker exoskeleton containing calcium carbonate that is more readily fossilized.

Subphylum Uniramia

Uniramia (uni - one, ramus - branch, i.e. single-branches) is a group within the arthropods. In the past this group included the Onychophora, which are now considered a separate category. The group is currently used in a narrower sense.

Uniramia is one of three subphyla in Arthropoda classification suggested by Sidnie Manton. This classification divided arthropods into a three-phyla polyphyletic group, with phylum Uniramia including the Hexapoda (insects), Myriapoda (centipedes and millipedes) and the Onychophora (velvetworms). The discovery of fossil lobopods, determined to be intermediate between onychophorans and arthropods led to the splintering of the Lobopoda and Onychophora into separate groups. This redefined the Uniramia as strictly "true" arthropods with exoskeletons and jointed appendages. Uniramians have strictly uniramous appendages.

Subphylum Trilobita

The subphylum Trilobita includes only extinct species found in fossil form. The trilobites were a primitive group of marine species that was particularly abundant during the Cambrian (570 million years ago) and Ordovician (505 million years ago) periods. The group became extinct at the end of the Permian (286 million years ago). Trilobites had flattened, oval-shaped bodies. Most were a few inches long, although one species is known to have attained a length of 0.6 meters (2 feet).

 By

Reshma P

VASCULAR BUNDLES

VASCULAR BUNDLES

Plants have two systems for the transportation of substances - using two different types of transport tissue. Xylem transports water and solutes from the roots to the leaves, while phloem transports food from the leaves to the rest of the plant. Transpiration is the process by which water evaporates from the leaves, which results in more water being drawn up from the roots. Plants have adaptations to reduce excessive water loss.

Xylem and phloem

Plants have two transport systems to move food, water and minerals through their roots, stems and leaves. These systems use continuous tubes called xylem and phloem, and together they are known as vascular bundles.

Plant stem

Stem – the xylem and phloem are arranged in bundles near the edge of the stem to resist compression and bending forces.

Plant root

Root - xylem and phloem in the centre of the root to withstand stretching forces.

Xylem

Xylem vessels are involved in the movement of water through a plant - from its roots to its leaves via the stem.

During this process:

1. Water is absorbed from the soil through root hair cells.
2. Water moves by osmosis from root cell to root cell until it reaches the xylem.
3. It is transported through the xylem vessels up the stem to the leaves.
4. It evaporates from the leaves (transpiration).

The xylem tubes are made from dead xylem cells which have the cell walls removed at the end of the cells, forming tubes through which the water and dissolved mineral ions can flow. The rest of the xylem cell has a thick, reinforced cell wall which provides strength.

Phloem

Phloem vessels are involved in translocation. Dissolved sugars, produced during photosynthesis, and other soluble food molecules are moved from the leaves to growing tissues (eg the tips of the roots and shoots) and storage tissues (eg in the roots).

In contrast to xylem, phloem consists of columns of living cells. The cell walls of these cells do not completely break down, but instead form small holes at the ends of the cell. The ends of the cell are referred to as sieve plates. The connection of phloem cells effectively forms a tube which allows dissolved sugars to be transported.

Transpiration

Water on the surface of spongy and palisade cells (inside the leaf) evaporates and then diffuses out of the leaf. This is called transpiration. 

leaf

More water is drawn out of the xylem cells inside the leaf to replace what has been lost. Water molecules have a tendency to stick together – so as water leaves the xylem to enter the leaf, more water is pulled up behind it. This produces a continuous flow of water and dissolved minerals moving up the xylem tube from the roots, up the stem, and into the leaves. This is known as the transpiration stream.

Movement of water through the roots

The movement of water up the xylem means more water must be drawn in through the roots from the soil. To do this, water passes from root cell to root cell by osmosis.

The pathway of water across a root

As water moves into the root hair cell down the concentration gradient, the solution inside the root hair cell becomes more dilute. This means that there is now a concentration gradient between the root hair cell and adjacent root cells, so water moves from the root hair cell and into the adjacent cells by osmosis.
This pattern continues until the water reaches the xylem vessel within the root - where it enters the xylem to replace the water which has been drawn up the stem.

Factors that affect transpiration rate

Light

Transpiration increases in bright light. The stomata open wider to allow more carbon dioxide into the leaf for photosynthesis. More water is therefore able to evaporate. 

Temperature

Transpiration is faster in higher temperatures. Evaporation and diffusion are faster at higher temperatures.

Wind

Transpiration is faster in windy conditions. Water vapour is removed quickly by air movement, speeding up diffusion of more water vapour out of the leaf.

Humidity

Transpiration is slower in humid conditions. Diffusion of water vapour out of the leaf slows down if the leaf is already surrounded by moist air.

Factors that speed up transpiration will also increase the rate of water uptake from the soil. If the loss of water is faster than the rate at which it is being replaced by the roots, then plants can slow down the transpiration rate by closing some of their stomata. This is regulated by guard cells, which lie on either side of a stoma.

Plants can slow down the transpiration rate by closing some of their stomata

If the guard cells are turgid, then they curve forming ‘sausage-shaped’ structures with a hole between them. This is the stoma.
However, if the guard cells are flaccid due to water loss, they shrivel up and come closer together, closing the stoma. This is turn reduces the water loss due to transpiration, and can prevent the plant from wilting.

By

Vinesh S

 

IMMUNOLOGY

IMMUNOLOGY

Anything that causes an immune response is called an antigen. An antigen may be harmless, such as grass pollen, or harmful, such as the flu virus. Disease-causing antigens are called pathogens. The immune system is designed to protect the body from pathogens. 

In humans, the immune system begins to develop in the embryo. The immune system starts with hematopoietic (from Greek, "blood-making") stem cells. These stem cells differentiate into the major players in the immune system (granulocytes, monocytes, and lymphocytes). These stems cells also differentiate into cells in the blood that are not involved in immune function, such as erythrocytes (red blood cells) and megakaryocytes (for blood clotting). Stem cells continue to be produced and differentiate throughout your lifetime. 

Hematopoietic stem cells produce cells in blood and lymph

By the time a baby is born, the immune system is a sophisticated collection of tissues that includes the blood, lymphatic system, thymus, spleen, skin, and mucosa. The immune system is typically divided into two categories--innate and adaptive--although these distinctions are not mutually exclusive. 

Innate immunity

Innate immunity refers to nonspecific defense mechanisms that come into play immediately or within hours of an antigen's appearance in the body. These mechanisms include physical barriers such as skin, chemicals in the blood, and immune system cells that attack foreign cells in the body. The innate immune response is activated by chemical properties of the antigen.

Adaptive immunity

Adaptive immunity refers to antigen-specific immune response. The adaptive immune response is more complex than the innate. The antigen first must be processed and recognized. Once an antigen has been recognized, the adaptive immune system creates an army of immune cells specifically designed to attack that antigen. Adaptive immunity also includes a "memory" that makes future responses against a specific antigen more efficient.

The cellular system

• T-cells differentiate in the thymus, and have a specific receptor for a fragment of antigen..
• Cytotoxic T-cells contain a surface protein called CD8 and destroy pathogen infected cells, cancer cells, and foreign cells (transplanted organs).
• Helper T-cells contain a surface protein called CD4 and regulate both the cellular and humoral immune systems. This regulation reduces autoimmunity.
• Autoimmune disease- self immunity. Some examples include rheumatic fever, rheumatoid arthritis, ulcerative colitis, myasthenia gravis, etc.

Immunological response 

The graph shows a very important feature of the immune response. When first exposed to antigen "A", we begin to make low levels of antibody in about a week However, a second exposure to antigen "A" produces a much faster response, and several orders of magnitude higher levels of antibody. The ability of antibody to bind antigen also increases dramatically in the secondary response. Injecting a new antigen "B" with "A" shows that a memory or prior exposure is required for the accelerated response. The memory of antigen and the stimulated response is the basis for success in vaccination programs.

The Clonal Selection Theory

• The immune systems produces Billions of kinds of B-cells each making one kind of antibody receptor.
• The presence of antigen leads to the proliferation and differentiation of clones that have antibody capable of binding the antigen. In the diagram the "green" antigen binds to the green antibody on a B-cell. The color code means that only this antibody receptor on the cell binds free antigen.
• The "green" helper T-cell must give a stimulatory signal to allow a particular B-cell to be selected. This step allows a regulation or control of the process.
• The antigen driven selection produces memory cells and plasma cells secreting antibody capable of binding the original selecting antigen with high affinity..
• If antigen appears in the organism a second time, then the memory cells are already present at high levels, and produce a more rapid and much stronger immune response. 

 By

Aparna S M

BIOMOLECULES

BIOMOLECULES

Biomolecules are molecules that occur naturally in living organisms. Biomolecules include macromolecules like proteins, carbohydrates, lipids and nucleic acids. It also includes small molecules like primary and secondary metabolites and natural products. Biomolecules consists mainly of carbon and hydrogen with nitrogen, oxygen, sulphur, and phosphorus. Biomolecules are very large molecules of many atoms, that are covalently bound together. 

CLASSES OF BIOMOLECULES

There are four major classes of biomolecules: 

• Carbohydrates
• Lipids
• Proteins
• Nucleic acids

Carbohydrates

Carbohydrates are good source of energy. Carbohydrates (polysaccharides) are long chains of sugars. Monosaccharides are simple sugars that are composed of 3-7 carbon atoms. They have a free aldehyde or ketone group, which acts as reducing agents and are known as reducing sugars. Disaccharides are made of  two monosaccharides. The bonds shared between two monosaccharides is the glycosidic bonds. Monosaccharides and disaccharides are sweet, crystalline and water soluble substances.Polysaccharides are polymers of monosaccharides. They are unsweet, and complex carbohydrates.They are insoluble in water and are not in crystalline form. 

Example: glucose, fructose, sucrose, maltose, starch, cellulose etc.

Lipids

Lipids are composed of long hydrocarbon chains. Lipid molecules hold a large amount of energy and are energy storage molecules. Lipids are generally esters of fatty acids and are building blocks of biological membranes. Most of the lipids have a polar head and non-polar tail. Fatty acids can be unsaturated and saturated fatty acids.

Lipids present in biological membranes are of three classes based on the type of hydrophilic head present:

• Glycolipids are lipids whose head contains oligosaccharides with 1-15 saccharide residues.  
• Phospholipids contain a positively charged head which are linked to the negatively charged phosphate groups. 
• Sterols, whose head contain a steroid ring. Example steroid.

Example of lipids: oils, fats, phospholipids, glycolipids, etc.

Proteins

Proteins are heteropolymers of stings of amino acids. Amino acids are joined together by the peptide bond which is formed in between the carboxyl group and amino group of successive amino acids. Proteins are formed from 20 different amino acids, depending on the number of amino acids and the sequence of amino acids. 

There are four levels of protein structure:

• Primary structure of Protein - Here protein exist as long chain of amino acids arranged in a particular sequence. They are non-functional proteins.
• Secondary structure of protein - The long chain of proteins are folded and arranged in a helix shape, where the amino acids interact by the formation of hydrogen bonds. This structure is called the pleated sheet. Example: silk fibres. 
• Tertiary structure of protein - Long polypeptide chains become more stabilizes by folding and coiling, by the formation of ionic or hydrophobic bonds or disulphide bridges, this results in the tertiary structure of protein.
• Quaternary  structure of protein - When a protein is an assembly of more than one polypeptide or subunits of its own, this is said to be the quaternary structure of protein. Example: Haemoglobin, insulin.

Nucleic Acids

Nucleic acids are organic compounds with heterocyclic rings. Nucleic acids are made of polymer of nucleotides. Nucleotides consists of nitrogenous base, a pentose sugar and a phosphate group. A nucleoside is made of nitrogenous base attached to a pentose sugar. The nitrogenous bases are adenine, guanine, thyamine, cytosine and uracil. Polymerized nucleotides form DNA and RNA which are genetic material. 

FUNCTIONS OF BIOMOLECULES

Carbohydrates provide the body with source of fuel and energy, it aids in proper functioning of our brain, heart and nervous, digestive and immune system. Deficiency of carbohydrates in the diet causes fatigue, poor mental function.

Each protein in the body has specific functions, some proteins provide structural support, help in body movement, and also defense against germs and infections. Proteins can be antibodies, hormonal, enzymes and contractile proteins.

Lipids, the primary purpose of lipids in body is energy storage. Structural membranes are composed of lipids which forms a barrier and controls flow of material in and out of the cell. Lipid hormones, like sterols, help in mediating communication between cells.

Nucleic Acids are the DNA and RNA, they carry genetic information in the cell. They also help in synthesis of proteins, through the process of translation and transcription.

STRUCTURE OF BIOMOLECULES

Structure of biomolecule is intricate folded, three-dimensional structure that is formed by protein, RNA, and DNA. The structure of these molecules are in different forms, primary, secondary, tertiary and quaternary structure. The scaffold for this is provided by the hydrogen bonds within the molecule.

• Primary structure of a biomolecule is the exact specification of its atomic composition and and the chemical bonds connecting the atoms. 
• Secondary structure of the biomolecule is the three-dimensional form of biopolymers, secondary structure is defined by the hydrogen bonds of the biomolecules. 
• Tertiary structure of the biomolecule is the three-dimensional structure,defined by its atomic coordinates, by the formation of hydrogen, ionic or sulphide bonds. 
• Quaternary structure is the arrangement of multiple folds of complex, in a mutli-subunit complex.

By

Bincy mol