AIIMS 1981 Biology Question Paper with Answer and Solution

(D) The correct answer is $(d)$.
$(a)$ Cycadophyta includes fossil gymnosperms such as $Williamsonia$.
$(b)$ Coniferophyta includes fossil gymnosperms such as $Cordaites$.
Since both classes contain fossil representatives,the correct option is $(d)$.

(D) In the family Compositae (also known as Asteraceae),the flowers are epigynous.
In an epigynous flower,the thalamus (receptacle) grows upward and completely encloses the ovary,fusing with its wall.
Consequently,the other floral parts (sepals,petals,and stamens) arise from above the level of the ovary.
Since the ovary is situated below the other floral parts,it is described as an inferior ovary.
Therefore,the flowers are epigynous and the ovary is inferior.

(D) The correct answer is $D$.
$1$. Epipetalous stamens are those where the stamens are attached to the petals.
$2$. Syngenesious stamens are those where the anthers are fused together while the filaments remain free.
$3$. These conditions are characteristic features of the family $Asteraceae$ (also known as $Compositae$).
$4$. In $Solanaceae$, stamens are epipetalous but not syngenesious.
$5$. In $Brassicaceae$, the condition is tetradynamous.
$6$. In $Fabaceae$, the condition is usually diadelphous.

(A) The template theory,also known as the $Lock$ and $Key$ hypothesis,suggests that an enzyme has a specific active site that fits a specific substrate perfectly,much like a key fits a lock.
Competitive inhibition provides the best evidence for this theory.
When a compound that is structurally similar to the substrate (a competitive inhibitor) binds to the active site,it prevents the actual substrate from binding.
This demonstrates that the enzyme's active site is specific to a particular molecular shape,which is the core principle of the template theory.

(A) The cell wall is a non-living,rigid,and freely permeable layer that allows the movement of water and dissolved mineral salts without any selective barrier.
In the root,water and minerals from the soil enter the periplasmic space (the space between the cell wall and the plasma membrane) of the root hairs by passing through the cell wall.
Since the cell wall does not restrict the entry of these substances,it demonstrates its permeable nature.
In contrast,the movement from root hairs to cortical cells or further into the pericycle involves crossing the plasma membrane,which is a semipermeable (selectively permeable) barrier.
Therefore,the passage from soil into the periplasmic space of root hairs best illustrates the permeable nature of the cell wall.

(B) In cyclic photophosphorylation,only $PS-I$ is involved. The electrons are circulated within the $PS-I$ system and the phosphorylation occurs due to cyclic electron flow. $PS-II$ is not involved in this process,and consequently,there is no photolysis of water and no production of $NADPH + H^+$. Therefore,$PS-II$ does not participate in the cyclic flow of electrons.

(B) The correct answer is $B$. Gynaecomastia is a condition characterized by the enlargement of breast tissue in males. This often occurs due to a hormonal imbalance,specifically an increase in estrogen levels or a decrease in testosterone levels,which can lead to the development of mammary glands similar to those in females.

(C) The correct answer is $C$.
The $Tapetum$ is the innermost layer of the microsporangium wall.
Its primary functions include the production of enzymes,hormones like $IAA$ (Indole$-3-$acetic acid),and the secretion of nutrients required for the development of pollen grains.

(B) The process of copying genetic information from one strand of the $DNA$ into $RNA$ is termed as transcription. This process occurs in the nucleus where $DNA$ serves as a template for the synthesis of $mRNA$.

(D) $DNA$ is not directly involved in protein synthesis.
$DNA$ acts as a template for the synthesis of $mRNA$ (transcription),$tRNA$,and $rRNA$.
$DNA$ also acts as a template for its own replication (synthesis of another $DNA$ strand).
However,$DNA$ does not move to the site of protein synthesis (ribosomes) to directly guide the process.
Instead,it transfers its genetic information to $mRNA$ molecules,which then move to the ribosomes to direct the synthesis of proteins.