Here we are bringing the CBSE Class 12 Biology Solved Question Paper 2018. Solutions have been collated in the form of PDF which students may download to check the right criteria to write perfect exam and score high marks.
CBSE class 12 Biology Solved Question Paper has been solved as per the Marking Scheme and along with the answers, we have given the explanations for the questions. This solved question paper will also provide an idea of important points to mention while answering in CBSE Board Examination.
The solved Biology question paper 2018 given here will offer a variety of problems to solve and give you the practice to code and know what kind of questions might be asked in the board exam 2019.
Subject experts at jagranjosh.com has solved the CBSE Class 12 Biology question paper 2018 to help students know the right solutions to all questions. Each step in a solution is structured in a way to clear the concept involved and give students an idea to frame appropriate answers in CBSE board exams 2019.
Some of the questions and their solutions from the CBSE Class 12 Biology Solved Question Paper 2018, are given below:
Question. (a) Describe any two devices in a flowering plant which prevent both autogamy and geitonogamy.
(b) Explain the events up to double fertilization after the pollen tube enters one of the synergids in an ovule of an angiosperm.
(a) Explain menstrual cycle in human females.
(b) How can the scientific understanding of the menstrual cycle of human females help as a contraceptive measure?
Autogamy: Transfer of pollen grains from anther to the stigma of same flower.
Geitonogamy: Transfer of pollen grains from anther to the stigma of another flower of same plant.
Two devices that prevent both autogamy and geitonogamy are:
(i) Self – incompatibility: This is a genetic mechanism & prevents self pollen from fertilising the ovules by inhibiting pollen germination or pollen tube growth in the pistil.
(ii) Dioecious plants: Male and female flowers are present on different plants, that is each plant is either male or female.
The events seen after the pollen tube enters one of the synergids in an ovule are as follows:
(i) Pollen tube, after reaching the ovary, enters the ovule through the micropyle and thus enters one of the synergids through filiform apparatus.
(ii) After entering one of synergids, the pollen tube releases the two male gametes into the cytoplasm of the synergid.
(iii) One of the male gametes move towards the egg cell and fuses with its nucleus thus results in formation of zygote (diploid cell). This is Syngamy.
(iv) The other male gamete move towards the two polar nuclei located in the central cell and fuses to form triploid primary endosperm nucleus (PEN). This involves fusion of three haploid nuclei & hence termed as triple fusion.
(v) Two types of fusions, syngamy & triple fusion takes place in an embryosac and hence the phenomenon is termed as double fertilisation.
(vi) After fertilisation, PEN becomes the primary endosperm cell (PEC) & develops into endosperm while zygote develops into an embryo.
Ans (a). Menstrual Cycle:
(i) The reproductive cycle in the female primates (e.g. Monkeys, apes and humans) is called menstrual cycle.
(ii) The first menstruation begins at puberty and is called menarche.
(iii) In human females, menstruation is repeated at an average interval of about 28/29 days and the cycle of events starting from one menstruation till the next one is menstrual cycle.
(iv) The phases of menstrual cycle are as follows:
It lasts for 3-5 days. The menstrual flow results due to breakdown of endometrial lining of the uterus and its blood vessels which forms liquid that comes out through vagina. Menstruation only occurs if the released ovum is not fertilised.
Follicular phase :
It lasts for 8-10 days. During this phase, the primary follicles in the ovary grow to become a fully mature Grafian follicle and simultaneously the endometrium of uterus regerates through proliferation. The secretion of LH and FSH increases gradually.
It lasts for 1 day. There is release of ovum.
It lasts for 13 days. There is LH surge. These induces the remaining parts of Grafian follicle to transform as corpus luteum and it’s secretes progesterone.
If fertilisation occurs, endomentrium starts preparing for implantation.
In the absence of fertilisation, corpus luteum degenerates.