STRUCTURE OF PROTEINS:

Definition:

Proteins are polymers of amino acids, the compounds containing carbon, nitrogen, oxygen and hydrogen.

The number of amino acids varies from a few to 3000 or even more in different proteins.

Abundance:

Proteins are the most abundant organic compounds in cells. They are over 50% of total dry weight of the cell.

Amino Acids:

About 170 types of amino acids are present in cells and tissues.

Of these, about 25 types are involved in the formation of proteins. However, most of the proteins are made of 20 types of amino acids.

Structure of Amino Acid:

All the amino acids have an amino group (-NH₂) and a carboxyl group (-COOH) attached to the same carbon (alpha carbon). The general formula of a amino acid is a follows:

Formation of Protein Molecule by Peptide Linkage:

Amino acids are linked to form polypeptide. The polypeptides may assemble to form proteins.

The amino group of one amino acid reacts with the carboxyl group of another amino acid and a water molecule is released. Here OH is released from carboxyl group of one amino acid while H is released from amino group of next amino acid.

The linkage between C of carboxyl group of one amino acid and N of amino group of next amino acid is called peptide bond

Example: 

Glycine and alanine combine to form glycylalanine.

Glycylalanine has two amino acids and is called dipeptide.

A dipeptide has an amino group at one end and a carboxyl group at the other end.

So both reactive sites are available for the formation of further peptide bonds. As a result tripeptides, tetrapeptides, pentapeptides and polypeptides etc can be produced.

Structure of Proteins:

Each protein has specific properties which are due to:

(i) Number of amino acids,

(ii) Kinds of amino acids.

(iii) Specific sequence of amino acids and.

(iv) The shape of protein molecule.

As a result protein structure is formed which is organized at four levels.

(1) Primary Structure of Protein:

It includes the number, kind and sequence of amino acids in a protein molecule.

Size of protein Molecule:

The size of a protein molecule is determined by the type of amino acids and the number of amino acids in that protein molecule.

Sequence of Amino Acids:

There are over 10,000 proteins in the human body. These are formed by the specific arrangements of 20 types of amino acids.

The sequence of amino acids is determined by the order of nucleotides in the DNA.

(2) Secondary Structure:

It is the coiling of primary polypeptide chains.

The chains in a protein molecule are not flat. They usually coil into α helix, or into β-pleated sheet.

(i) α-Helix:

It is most common structure in which the basic polypeptide chain is spirally arranged.

It is very uniform geometric structure with 3.6 amino acids in each turn of the helix.

The helical structure is kept by the formation of hydrogen bonds among amino acid molecules in spiral turns.

(ii) β-Pleated Sheets:

These are formed by the fold backs of the polypeptide (e.g. silk).

(3) Tertiary Structure:

In the tertiary structure, a polypeptide chain bends and folds upon itself forming a globular shape.

Structural Maintenance:

It is maintained by three types of bonds:

(a) Ionic bonds,                   (b) Hydrogen bonds and                   (c) Disulfide (-S-S-) bonds

EVOLUTION OF SEED HABIT:

Dominancy of Seed Plants Over the Non-seed Plants:

A review of the kingdom Plantae indicates that the seed-pants (spermatophytes) predominate over non-seed vascular plants.

Development of Seed Habits:

One of the most significant events in the history of land plants was the development of seed habit.

Change in Reproductive Structure:

It was an important change in the reproductive system of the vascular plants which occurred approximately 390 million years ago.

First Seed Appear in Late Devonian:

First complete seeds appeared approximately 365 million years ago during late Devonian times.

Seed is Fertilized Ovule:

Technically a seed may be defined as a fertilized ovule.

Ovule is Immature Seed:

Integument Indeshiscent:

An ovule is an integumented indehiscent (not discharge or ripe) megasporangium. 

Integuments are specialized protective coverings around megasporangium which vary in number.

Spermatophytes:

All seed-producing plants are called spermatophytes.

Various steps involved in the evolution of seed habit are as follows.

Steps Involved in Evolution of Seed Habit:

Summary:

(i) Evolution of Heterospory.

(ii) Retention and germination of megaspore within the megasporangium.

(iii) Development of protective layers around megasporangium.

(iv) Reduction to a single functional megaspore per sporangium.

(v) Development of an embryo sac within the sporangium.

(vi) Modification or distal end of megasporangium for pollen capture.

In Detail:

(i) Evolution of Heterospory:

• Primitive vascular land plants produced one kind of spores, a condition called homospory.

• All groups of land plants up to pteridophytes are homosporous.

• During the early phase of evolution, some plant groups started producing two different types of spores.

• The microspores produced inside microsporangia germinated to form male gametophyte or microgametophyte.

• The megaspores germinated to form female gametophyte or megagametophyte.

(ii) Retention and Germination of Megaspore within the Megasporangium:

• During the usual reproductive cycle in the heterosporous vascular land plants, the megaspores used to be shed and dispersed soon after their formation in order to germinate into the female gametophyte.

• However, in some plants (e.g. Selaginella) the megaspore was not allowed to escape from megasporangium immediately after its formation.

• In other, the megaspore(s) was/were permanently retained within the megasporangium.

• Here, within the confines of the megasporangium wall, the megaspore germinated to form egg containing female gametophyte.

(iii) Development of Protective Layers Around Megasporangium.

(iv) Reduction to a Single Functional Megaspore per Sporangium.

• Each megaspore mother cell within a megasporangium used to produce four functional megaspores.

• These megaspores germinated to produce four variable female gametophytes.

• There was a competition for space and food among the four gametophytes.

• Soon the early vascular plants adopted a new strategy i.e., only one megaspore was selected for further development into a healthy female gametophyte while the remaining three were aborted.

(v) Development of an Embryo Sac within the Sporangium.

• The single healthy megaspore retained with the megasporangium germinated to form an egg containing female gametophyte called an embryo sac.

(vi) Modification or Distal end of Megasporangium for Pollen Capture.

• When most of the structural and functional changes leading to the development of seed habit were completed.

• Another important modification took place in the megasporangium which was now integumented, indehiscent and permanently attached to the sporophyte.

• The distal end of the megasporangium became modified for capturing pollen (microspore containing male gametophyte).

• Pollen after being trapped in the distal cavity of the megasporangium produced pollen tube which carried male gametes deep into the embryo sac to fertilize the egg, forming a zygote, which forms an embryo.

• The megasporangium (ovule) after fertilization is transformed into a seed, the integuments becoming the second coats (testa).

• The seed offers maximum degree of protection to a developing embryo under the unfavorable terrestrial environment.

• The development and evolution of seed habit was a great success and giant leap which ultimately enable plants to colonize land permanently. 

• Vascular system absent
• Gametophyte dominant
• Sporophyte attached to gametophyte
• Homosporous.

 Differentiation between Acoelomates, Pseudocoelomates and coelomates.

ACOELOMATES:

Ø  In phylum Platyhelminthes there is no body cavity or coelom, and the mesoderm form a loose, cellular tissue called mesenchyma or parenchyma which fills the space between the ectoderm and endoderm.

Ø  It forms a packing around the internal organs of the animals to support and protect them.

Ø  Such animals are called acoelomates

Ø  In acoelomates the gut is sac-type and there is no special transport system.

Ø  Only excretory system is developed for the transport of excretory products.

This system consists of flame cells, excretory ducts and excretory pores.

Ø  However the nervous system is well developed.

PSEUDOCOELOMATES:

Ø  In Aschelminthes the space between the body wall and the digestive tube is called pseudocoelom (false body cavity)

Ø  Pseudocoelom is not homologous to true coelom because.

Ø  It is not lined by coelomic epithelium.

Ø  It has no relation with the reproductive and excretory organs.

Ø  It develops from the blastocoel of the embryo and it is bonded externally by the muscles and internally by the cuticle of the intestine.

Ø  The animals having pseudocoelom are called pseudocoelomates.

COELOMATES:

Ø  Coelom is cavity present between the body wall and the alimentary canal and is lined by mesoderm.

Ø  The mesoderm splits into outer parietal layer (somatic) which under lines the body wall and the visceral layer (splanchnic) which covers the alimentary canal and the cavity between them is the true coelom.

Ø  It is filled with fluid called coelomic fluid.

Ø  The animals which posses coelom or true body cavity are called coelomates e.g. animals from annelids to chordates.

Ø  In coelomates gut attains more complexity and neuro-sensory system is well developed along with excretory system, circulatory system, respiratory and reproductive system.

 Test used to detect lipids?

Following tests are used to detect lipids:

(i)    Sudan-III Test: 

(ii)    Emulsion Test:

What are lipids ?

Lipids are the organic compounds made up of carbon, hydrogen and oxygen, are characterized by their solubility in organic solvents such as ether, alcohol and their insolubility in water.

What is the lymphatic system, write down its structure and function?

LYMPHATIC SYSTEM:

Definition:

(That system which consists of tissue fluid its transport and regulates the substances in body)

This system is responsible for the transport and returning of materials from the tissues of the body to the blood.

The system comprises of:

• Lymph capillaries,
• Lymph vessels,
• Lymph nodes, and
• Lymphoid masses,
• Lymph-the fluid which flows in the system.

(1) Lymph Capillaries:

Lymph capillaries end blindly in the body tissues, whore pressure from the accumulation of interstitial fluid or extracellular fluid forces the fluid into the lymph capillaries.

(2) Lymph and Lymph Vessels:

When this fluid enters the lymph capillaries, it is called lymph.

The lymph vessels empty in veins;

So lymph is a fluid in transit between interstitial fluid and the blood.

The intercellular spaces in the walls of lymph vessels are larger than those of the capillaries of blood vascular system.

So large molecules, from the interstitial fluid cal also enter the lymph capillaries.

Thoracic Lymph Vessel and Subclavin Vein:

Lymph capillaries join to form larger and larger lymph vessels; and ultimately from thoracic lymph duct-which opens into subclavian vein.

Direction of Flow:

The flow of lymph is always towards the thoracic duct.

Lacteals:

In the intestine, the branches of lymph capillaries, within villi, are called lacteals.

 The flow of lymph is maintained by:

(i) Activity of skeletal muscles,

(ii) Movement of viscera (internal organs)

(iii) Breathing movements.

(iv) Valves, which prevent back flow of lymph.

(3) Lymph Nodes:

Along the pathway, the lymph vessels, have, at certain points, masses of connective tissue where lymphocytes are present, these are lymph nodes. Several afferent lymph vessels enter a lymph node, which is drained by a single, efferent lymph vessel.

Location:

Lymph nodes are present in the:

(i) Neck Region.

(ii) Axilla (armpit) and.

(iii) Groin (genitalia) of humans.

(4) Lymphoid Masses:

In addition, several lymphoid masses are present in the:

(i) Walls of the digestive tract,

(ii) In the Mucosa and Sub-mucosa

The larger masses are:

(i) Spleen

(ii) Thymus

(iii) Tonsils (tissue mass in mouth) and

(iv) Adenoids (throat tissue) are all lymphoid masses.

These produce lymphocytes.

(5) Functions:

These are several functions performed by the lymphatic system.

(i) Regulation of Blood Volume:

In an average person, about three litres more fluid leaves the blood capillaries than is reabsorbed by them each day.

It returns this excess fluid and its dissolved proteins and other substances to the blood.

(ii) Absorption of Lipids:

The lacteals of villi absorb large fat globules, which are released by interstitial cells after the products of digestion of fats are absorbed.

(iii) Defence Mechanism:

The lymphatic system helps defend the body against foreign invaders.

Lymph nodes have lymphocytes and macrophages that destroy the bacteria and viruses.

The painful swelling of lymph nodes in certain diseases (mumps is an extreme example) is largely a result of the accumulation of dead lymphocytes and macrophages.

(iv) Filtering Blood Destroyed aged RBC and Invaders:

Just as the lymph nodes filter lymph, the spleen filters blood, exposing it to macrophages and lymphocytes that destroy foreign particles and aged red blood cells.