• Pachytene is the third stage of meiosis prophase-1 (a five-stage process). It is preceded by the zygotene stage, where synapsis, i.e. pairing of homologous chromosomes take place.
• In pachytene, exchange of genetic material between two homologous chromosomes known as crossing over takes place between non-sister chromatids of the homologous chromosomes.

• Recombination or crossing over takes place at this stage. In this process, there is an exchange of genetic material between non-sister chromatids of homologous chromosomes. There is rearrangement and reshuffling of the genetic material of two parental homologous chromosomes. It is an enzyme-mediated process. The enzyme involved in crossing over is known as recombinase.

  Recombination nodules appear and homologous chromosomes are linked to each other at the site of crossing over.

  Recombination is accompanied by the formation of chiasmata, which appear in the next stage called diplotene.

Object:

Demonstration of Relation between Transpiration and Absorption

Requirements:

A bottle with a side, tube, cork, a plant, oil, balance, etc.

Experiment:

Here, the apparatus must be completely filled up with water and air-tight. In the side marked tube of the apparatus some oil drops are being added so that the water may not evaporate. The cork is fitted in the wide mouth of the bottle in the way that the aerial part of a complete plant remains outside of it and the root system or the plant remains within the bottle.

Now the apparatus is weighed on a pan balance to note its weight and thereafter it is kept for few hours in such a place where sufficient transpiration from the aerial parts of the plant takes place. In the end the apparatus is weighed once again and the difference from that of the original weight may be calculated.

Observation:

This way the quantity of transpired water is known in grams. The oil drops present on the surface of water in the marked side tube travel somewhat downward and we know the actual quantity of absorbed water in ml by the roots. By comparison, it is noted that the quantity of absorbed water is always greater than the transpired water because some of the water is used in other metabolic activities.

• This tissue which has a free surface faces either a body fluid or the outside environment and thus provides a covering or a lining for some part of the body is called epithelial tissues.
• The cells are compactly packed with little intercellular matrix.
• There are two types of epithelial tissues
• simple epithelium
• compound epithelium.
• Simple epithelium is composed of a single layer of cells and functions as a lining for body cavities, ducts etc.
• Compound epithelium consists of two or more cell layers and has protective function as it does in our skin.

A cardiac conduction system is a group of specialized cardiac muscle cells in the walls of the heart that send signals to the heart muscle causing it to contract. The main components of the cardiac conduction system are the SA node, AV node, the bundle of His, bundle branches, and Purkinje fibers.

The sequence of electrical events during one full contraction of the heart muscle:

• An excitation signal (an action potential) is created by the sinoatrial (SA) node.
• The wave of excitation spreads across the atria, causing them to contract.
• Upon reaching the atrioventricular (AV) node, the signal is delayed.
• It is then conducted into the bundle of His, down the interventricular septum.
• The bundle of His and the Purkinje fibers spread the wave impulses along the ventricles, causing them to contract.

Components of the Cardiac Conduction System:

1. Sinoatrial Node: The sinoatrial (SA) node is a collection of specialized cells (pacemaker cells), and is located in the upper wall of the right atrium, at the junction where the superior vena cava enters. These pacemaker cells can spontaneously generate electrical impulses. The wave of excitation created by the SA node spreads via gap junctions across both atria, resulting in atrial contraction (atrial systole) – with blood moving from the atria into the ventricles.
2. Atrioventricular Node (AV Node) - The AV Node receives the signal from the SA Node with the help of Bachmann's Bundle. It takes a 0.1s delay for the signal to transmit from the AV node to the Bundle of His.
3. Atrioventricular Bundle:  The atrioventricular bundle (bundle of His) is a continuation of the specialized tissue of the AV node, and serves to transmit the electrical impulse from the AV node to the Purkinje fibers of the ventricles. It descends down the membranous part of the interventricular septum, before dividing into two main bundles: Right bundle branch – conducts the impulse to the Purkinje fibres of the right ventricle. Left bundle branch – conducts the impulse to the Purkinje fibers of the left ventricle.
4. The Purkinje fibers (sub-endocardial plexus of conduction cells) are a network of specialized cells. They are abundant with glycogen and have extensive gap junctions. These cells are located in the subendocardial surface of the ventricular walls and are able to rapidly transmit cardiac action potentials from the atrioventricular bundle to the myocardium of the ventricles. This rapid conduction allows coordinated ventricular contraction (ventricular systole) and blood is moved from the right and left ventricles to the pulmonary artery and aorta respectively.

The acid-soluble pool represents roughly the cytoplasmic composition whereas the macromolecules from cytoplasm and organelles become the acid-insoluble fraction. Together they represent the entire chemical composition of living tissues or organisms. Al! the compounds which are found in the add-soluble pool, have their molecular weights ranging from approx. 18 to around 800 daltons (Da). 

The acid-insoluble fraction, has only four types of organic compounds i.e., proteins, nucleic acids, polysaccharides and lipid.

Genus <i>Alexandreum</i> of dinoflagellates secretes neurotoxin saxitoxins which are the causative agents of paralytic shellfish poisoning (PSP). Molluscs such as mussels and clams feed on a large number of dinoflagellates, human consumption of those molluscs causes paralysis. 

Madness is a characteristic symptom of rabies which is caused by RNA virus “rabies virus” and is mostly spread through the bite of an infected dog. 

Syphilis is a sexually transmitted disease, caused by gram negative spirochete, <i>Treponema pallidum</i>. 

Plague is caused by gram negative rod shaped bacteria. <i>Yersinia pestis</i> and is characterised by flu-like symptoms. It is transmitted by bites from infected fleas. 

Hydrocolloids are the carbohydrates obtained from different species of algae.

Algin is a hydrocolloid which has good water holding capacity derived from brown algae (Phaeophyceae).

Agar is jelly like substance used in dairy toppings and other instant food products, derived from red algae (Rhodophyceae)

Bacteriophage : Virus infection bacteria are known as bacteriophage. (a) These are obligate parasites that occur in soil, sewage water, fruits.
(b) The phage possesses a tail and a head. .
© The viral DNA is thread like ds (double standed) macromolecule.
(d) There are 4 segments of the tail.
(e) Bacteriophage are of two types : (i) Lytic and (ii) Lysogenic phoges e.g., phage (Lambda), T2 phage etc.

The accumulation of large amounts of cell wall deposits left back by diatoms over billions of years is called diatomaceous earth.

1) CHLOROPHYCEAE also known as Green Algae.
    Major Pigments:- Chlorophyll a, chlorophyll b, Xanthophyll and Carotene.
    Food Stored:- Starch, Protein and Oil [Starch+Protein=Pyrenoid]

2) PHAEOPHYCEAE also know as Brown Algae.
    Major Pigments:- Chlorophyll a, Chlorophyll c, Fucoxanthin [Brown Colour].
    Food Stored:- Mannitol and Laminarin.

3) RHODOPHYCEAE also known as Red Algae.
    Major Pigments:- Chlorophyll a, Chlorophyll d, Phycoerythrin [Red Colour] and Phycocyanin [Blue Colour]
    Food Stored:- Floridean Starch

    
Two types of population growth patterns may occur depending on specific environmental conditions:

• An exponential growth pattern (J curve) occurs in an ideal, unlimited environment
• A logistic growth pattern (S curve) occurs when environmental pressures slow the rate of growth

Exponential Growth

• Exponential population growth will occur in an ideal environment where resources are unlimited
• In such an environment there will be no competition to place limits on a geometric rate of growth
• Initially population growth will be slow as there is a shortage of reproducing individuals that may be widely dispersed
• As population numbers increase the rate of growth similarly increases, resulting in an exponential (J-shaped) curve
• This maximal growth rate for a given population is known as its biotic potential
• Exponential growth can be seen in populations that are very small or in regions that are newly colonised by a species

Logistic Growth

• Logistic population growth will occur when population numbers begin to approach a finite carrying capacity
• The carrying capacity is the maximum number of a species that can be sustainably supported by the environment
• As a population approaches the carrying capacity, environmental resistance occurs, slowing the rate of growth
• This results in a sigmoidal (S-shaped) growth curve that plateaus at the carrying capacity (denoted by κ)
• Logistic growth will eventually be seen in any stable population occupying a fixed geographic space

The four whorls of a complete flower are:

First whorl: Calyx
Second whorl: Corolla
Third whorl: Androecium
Fourth whorl: Gynoecium

The compounds oxidised during the process of respiration are called as respiratory substrates. Carbohydrates, especially glucose, act as a respiratory substrate. Fats, proteins, and organic acids can also act as respiratory substrates depending upon the physiological condition of the body of an organism.

Phylum Arthropoda has almost a million species described to date. It accounts for 80% of known animal species. So phylum Arthropoda is the largest phylum in the animal kingdom. Several factors contributed to the success of arthropods especially the terrestrial arthropods as they adapted to solve the problems of support, stability, desiccation and respiration.

Lichens are the composite or dual organisms representing a symbiotic association between a fungus and alga. The fungal partner is called mycobiont whereas algal partner is called phycobiont. Algae prepares food for fungi and fungi provides shelter and absorbs mineral nutrient and water for its partner. Lichens grow on barren rocks, cooled volcanic lava and icy tundra soils. They can tolerate extreme dessication but not the polluted air.

Some of the characteristic features of viroids are given below-

• Viroids contain only RNA.
• These are known to be smaller in size and infect only the plants.
• These are among the smallest known agents causing infectious disease.
• Viroids are the species of nucleic acid with relatively low molecular weight and a unique structure.
• They reproduce within the host cell which they affect in and cause variations in them causing death.
• Viroids are mainly classified into two families namely Pospiviroidae- nuclear viroids and Avsunviroidae- chloroplastic viroids.
• Viroids are said to move in an intracellular manner, cell to cell through the plasmodesmata, and a long-distance through the phloem.

• Cilia and flagella are hair-like outgrowths of the cell membrane.
• Cilia and flagella help in cell movement.
• Core of cilia and flagella is called the axoneme, possesses a number of microtubules running parallel to the long axis.
• The axoneme usually has nine pairs of doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules. Such an arrangement of axonemal microtubules is referred to as the 9+2 array.
• The central tubules are connected by bridges and is also enclosed by a central sheath, which is connected to one of the tubules of each peripheral doublets by a radial spoke and hence there are nine radial spokes.
• The peripheral doublets are also interconnected by linkers.
• Both the cilium and flagellum emerge from centriole-like structure called the basal bodies.

(A) Phycomycetes- This group of fungi includes members such as Rhizopus, Albugo, etc.

(i) Mode of nutrition

They are obligate parasites on plants or are found on decaying matter such as wood.

(ii) Mode of reproduction

Asexual reproduction takes place through motile zoospores or non-motile aplanospores produced endogenously in sporangium.

Sexual reproduction may be of isogamous, anisogamous, or oogamous type resulting in the formation of thick-walled zygospore.

(B) Ascomycetes- This group of fungi includes members such as Penicillium, Aspergillus, Claviceps, and Neurospora.

(i) Mode of nutrition

They are sporophytic, decomposers, parasitic or coprophilous (growing on dung).

(ii) Mode of reproduction

Asexual reproduction occurs through asexual spores produced exogenously, such as conidia produced on conidiophores.

Sexual reproduction takes place through ascospores produced endogenously in sac-like asci and arranged inside ascocarps.

(C) Basidiomycetes- This group of fungi includes members such as Ustilago, Agaricus and Puccinia.

(i) Mode of nutrition

They grow as decomposers in soil or on logs and tree stumps. They also occur as parasites in plants causing diseases such as rusts and smuts.

(ii) Mode of reproduction

Asexual reproduction takes place commonly through fragmentation. Asexual spores are absent.

*** organs are absent but sexual reproduction takes place through plasmogamy. It involves fusion of two different strains of hyphae. The resulting dikaryon gives rise to a basidium. Four basidiospores are produced inside a basidium.

(D) Deuteromycetes – This group of fungi includes members such as Alternaria, Trichoderma, and Colletotrichum.

(i) Mode of nutrition

Some members are saprophytes while others are parasites. However, a large number act as decomposers of leaf litter.

(ii) Mode of reproduction

Asexual reproduction is the only way of reproduction in deuteromycetes. Which occurs through asexual spores called conidia.

Sexual reproduction is absent in deuteromycetes.

• We fall ill because we live in an area with polluted surroundings or lack of personal hygiene.

• It is very important to adopt certain strategies to stay healthy and prevent diseases.

• One should remain active by exercising daily.