Many proteins are actually assemblies of multiple polypeptide chains. The quaternary structure refers to the number and arrangement of the protein subunits with respect to one another. Examples of proteins with quaternary structure include hemoglobin, DNA polymerase, and ion channels.

The most exciting aspect of chemistry deals with isolating thousands of compounds, small and big, from living organisms, determining their structure and if possible synthesising them.

If one were to make a list of biomolecules, such a list would have thousands of organic compounds including amino acids, sugars, etc. For reasons that are given in topic Metabolic basis for living, we can call these biomolecules as metabolites. In animal tissues, one notices the presence of all such categories of compounds shown in Figure 1. These are called primary metabolites. However, when one analyses plant, fungal and microbial cells, one would see thousands of compounds other than these called primary metabolites, e.g. alkaloides, flavonoides, rubber, essential oils, antibiotics, coloured pigments, scents, gums, spices. These are called secondary metabolites

Eukaryotic cells contain many membrane-bound organelles. An organelle is an organized and specialized structure within a living cell. The organelles include the nucleus, ribosomes, endoplasmic reticulum, Golgi apparatus, vacuoles, lysosomes, mitochondria, and, in plants, chloroplasts. Mitochondria are double- membrane-bound structures. The membrane of a mitochondrion is divided into the inner and outer membranes, distinctly divided into two aqueous compartments – outer and inner compartments.

Centromere is a constriction present on the chromosomes where the chromatids are held together.

Chromosomes are divided into four types based on the position of the centromere.

(i) Metacentric chromosome

The chromosomes in which the centromere is present in the middle and divides the chromosome into two equal arms is known as a metacentric chromosome. During anaphase, they appear V-Shaped.

(ii) Sub-metacentric chromosome

The chromosome in which the centromere is slightly away from the middle region is known as a sub-metacentric chromosome. In this, one arm is slightly longer than the other. During anaphase, they appear L-Shaped.
(iii) Acrocentric chromosome

The chromosome in which the centromere is located close to one of the terminal ends is known as an acrocentric chromosome. In this, one arm is extremely long and the other is extremely short. During anaphase, they appear J-Shaped.

(iv) Telocentric chromosome

The chromosome in which the centromere is located at one of the terminal ends is known as a telocentric chromosome. During anaphase, they appear i-Shaped.

Singer and Nicolson proposed the fluid mosaic model of plasma membrane. 
The plasma membrane is composed of a lipid bilayer.
The lipid molecules are arranged in such a way that their polar head are towards the outer side and the hydrophobic tails are on the inner side.
This arrangement of lipid molecules ensures that the non-polar tail of hydrocarbons does not come in contact with the aquatic environment present.
The proteins are of two types. Integral proteins which are either partially or totally embedded in the lipid bilayer and peripheral proteins which are present on the surface of the membrane.
The quasi fluid nature of lipid enables the lateral movements of proteins within the lipid bilayer.
The fluid nature of the membrane enables functions such as cell growth, secretion, endocytosis, cell division and formation of the intercellular junction.

Characteristics of Kingdom Monera:

(i) Kingdom Monera are one-celled without an organized nucleus. Examples: bacteria and blue-green "algae."

(ii) They are found in almost every imaginable habitat; in air, soil, and water and in extreme temperatures and harsh chemical environments.

(iii) There are no membrane bound "organelles".

(iv) Reproduction by asexual method.

(v) Monerans show either autotrophic or heterotrophic nutrition.

(vi) Cell wall may be present or absent.

Characteristics of Kingdom Protista:

1. This group includes many kinds of unicellular eukaryotic organisms such as unicellular algae, protozoans and unicellular fungi. 
2. They are found where there is moisture.
3. They are single celled, but many live in a colonial setting (more or less independently). 
4. Many are unicellular, but some are multicellular (Algae and fungus-like protists).
5.  Some can live in very harsh environments.
6. Reproduction can be sexual (conjugation), asexual (binary fission), or a combination of both.
7. Some of these organisms use appendages, such as hair-like cilia (e.g. Paramecium) or whip-like flagellum (e.g. Euglena).
8. Their mode of nutrition can be autotrophic or heterotrophic.

Actually, golgi apparatus is specialised portion of smooth endoplasmic reticulum meant for cell secretion. it consists of flattened sacs or cisternae, vesicles and large secretary vacuoles.cisternae occur in small stacks .a golgi stack has a convex via face and a concave trans face.cis face is formative it receives vesicles from endoplasmic reticulum which join to form new cisternae of golgi body that's why it is associated with endoplasmic reticulum.

In the process of DNA replication, the DNA makes multiple copies of itself. It is a biological polymerization which proceeds in the sequence of initiation, elongation, and termination. The whole process takes place with the help of enzymes where DNA-dependent DNA polymerase being the chief enzyme.

The evidence suggests that land plants evolved from a line of filamentous green algae that invaded land about 410 million years ago during the Silurian period of the Paleozoic era. They are considered to be an ancestral to plants because both they both reproduce chlorophyll a/b, both have cell walls made of cellulose and in conjugation DNA is being shared between one cell to other without actually reproducing.

Bone

It is highly rigid connective tissue that forms the skeleton of vertebrates. They are the type of tissues which comprises of blood vessels and cells. Overall a newborn baby will have around 300 bones and is reduced to 206. The main function of bones are:

1. Serve as a storage of minerals.
2. Provide the structural support.
3. Protect the internal organs of the body.

Cartilage

It is thin, fibrous, flexible connective tissue, which is mainly found in the external ear, larynx, respiratory tract, and the articulating surface of the joints. These cartilages lack blood vessels, therefore the growth and development of these tissues are slower compared to the other tissues. Altogether there are three different types of cartilages and it includes:

1. Hyaline cartilage: It serves as a shock absorber and allows the smooth movement of the bones at joints. They are mainly found in the nose, respiratory tract, and joints
2. Fibrocartilage is found in the knee, and it is tough and inflexible.
3. Elastic cartilage is found in ear, epiglottis, and larynx. It is the most flexible cartilage.

Xylem and phloem are known as complex tissues as they are made up of more than one type of cells. These cells work in a coordinated manner, as a unit, to perform the various functions of the xylem and phloem.

Xylem helps in conducting water and minerals. It also provides mechanical support to plants. It is made up of the following components:

• Tracheids (xylem vessels and xylem tracheids)

• Xylem parenchyma

• Xylem fibres

Tracheids are elongated, thick-walled dead cells with tapering ends. Vessels are long, tubular, and cylindrical structures formed from the vessel members, with each having lignified walls and large central cavities. Both tracheids and vessels lack protoplasm. Xylem fibres consist of thick walls with an almost insignificant lumen. They help in providing mechanical support to the plant. Xylem parenchyma is made up of thin-walled parenchymatous cells that help in the storage of food materials and in the radial conduction of water.

Phloem helps in conducting food materials. It is composed of:

• Sieve tube elements

• Companion cells

• Phloem parenchyma

• Phloem fibres

Sieve tube elements are tube-like elongated structures associated with companion cells. The end walls of sieve tube elements are perforated to form the sieve plate. Sieve tube elements are living cells containing cytoplasm and nucleus. Companion cells are parenchymatous in nature. They help in maintaining the pressure gradient in the sieve tube elements. Phloem parenchyma helps in the storage of food and is made up of long tapering cells, with a dense cytoplasm. Phloem fibres are made up of elongated sclerenchymatous cells with thick cell walls.

Aestivation is the mode of arrangement of sepals or petals in a floral bud with respect to the other members of the same whorl.
There are four main types of aestivation. They are as follows:

1. Valvate aestivation: Sepals or petals in a whorl just touch one another. They do not overlap one another.

It is observed in <i>Calotropis.</i>

2. Twisted aestivation: One margin of the appendage overlaps the margin of the next appendage.

Such type of aestivation is seen in lady’s finger, china rose and cotton.

3. Imbricate aestivation:Margins of sepals or petals overlap one another but not in any particular direction.

It is seen in Gulmohar and <i>Cassia</i>.
4. Vexillary aestivation: It is also known as Papilionaceous type of aestivation.

There are five petals. The largest petal (called standard) overlaps the two
lateral petals (called wings) which further overlap the two smallest anterior petals (called keel).

Pea belongs to Fabaceae family. The flowers of the family Fabaceae have five sepals that are often fused and five clawed petals, upper posterior petal or the vexillum, two lateral free petals or wings, and two anterior petals or the keel.

Racemose Inflorescence

1. The formation of flowers is indefinite.

2. The main axis does not terminate in a flower and continuous to grow and forms flowers laterally.

3. The floral axis is monopodial.

Cymose Inflorescence

1. The formation of flowers is definite,

2. The main axis and its branches terminate in a flower.

3. The floral axis is multipodial

Placentation :- the arrangement of the ovules within the ovary is known as placentation.
placentation are of different types:-
1) Marginal
2) Axile
3) parietal
4) free central
5) basal

Marginal placentation :- In this, the placenta forms a ridge along the ventral suture of the ovary and ovules are borne on the ridge.
e.g --> pea

Axile placentation :- In this , the placenta is Axile and ovules are attached to it .
e g- China rose , tomato , lemon

Parietal placentation :- in this , the ovules develop on inner wall of the ovary or on peripheral part.
e.g -> mustard , Argemone

Free central placentation :- In this , the ovules are borne on central axis and septa are absent.
e.g-> Dianthus and Primrose.

basal placentation :- in this , placenta develop at base of ovary & single ovule is attached to it.

e g -> sunflower , marigold.

The root tip can be divided into three zones: a zone of cell division, a zone of elongation, and a zone of maturation. The zone of cell division is closest to the root tip and is made up of the actively-dividing cells of the root meristem, which contains the undifferentiated cells of the germinating plant.

Keys are pairs of two contrasting characters. Selection of one character leads to rejection of another character while identifying a particular species or genera or family and so on.

For example presence or absence of hair on body can give a clue if an animal belongs to mammalian or not.

The nervous tissue has two outstanding properties : excitability and conductivity.
(a) Excitability : It is the ability of the nerve cells and fibres to enter into an active state called the state of excitation in response to a stimulus. Excitation arises at the receptors on account of various stimuli such as light, temperature, chemical, electrical or pressure which constantly act on the organisms.
(b) Conductivity : The excitation does not remain at
the site of its origin. It is transmitted along nerve fibres. The transmission of excitation in a particular direction is called conductivity

Spermatogenesis is a concerted sequence of events during maturation of spermatogonia into spermatozoa. The process involves differential gene-expression and cell-cell interplay regulated by the key endocrine stimuli, i.e., follicle-stimulating hormone (FSH) and luteinizing hormone (LH)-stimulated testosterone. In women, this hormone stimulates the growth of ovarian follicles in the ovary before the release of an egg from one follicle at ovulation. It also increases oestradiol production. In men, follicle-stimulating hormone acts on the Sertoli cells of the testes to stimulate sperm production (spermatogenesis).