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Biology | Class 12th

CBSE Board Paper 2018-19

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CBSE Board Paper 2018-19 Set - 1

General Instructions:

1. There are total 27 questions and four sections in the question

paper. All questions are compulsory.

2. Section A contains questions number 1 to 5, very short answer

type questions of one mark each.

3. Section B contains questions number 6 to 12, short answer type-

I questions of two marks each.

4. Section C contains questions number 13 to 24, short answer

type-II questions of three marks each.

5. Section D contains question number 25 to 27, long answer type

questions of five marks each.

6. There is no overall choice in the question paper, however, an

internal choice is provided in two questions of one mark, two

questions of two marks, four questions of three marks and all

the three questions of five marks. In these questions, an

examinee is to attempt any one of the two given alternatives.

7. Wherever necessary, the diagram drawn should be neat and

properly labelled.

Time allowed: 3 Hours Max Marks: 70

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1. British geneticist R.C. Punnett developed a graphical representation of a genetic cross called “Punnett Square”. Mention the possible result this representation predicts of the genetic cross carried.

Section A

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2. State the two principal outcomes of the experiments conducted by Louis Pasteur on origin of life.

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3. Name the layer of the atmosphere that is associated with ‘good ozone’.

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OR

Mention the term used to describe a population interaction between an Orchid growing on a forest tree.

4. What are ‘flocs’, formed during secondary treatment of

sewage?

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OR

Write any two places where methanogens can be found.

5. At what stage does the meiosis occur in an organism exhibiting haploidic life cycle and mention the fate of the products thus produced.

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4

Section B

6. You are conducting artificial hybridization on papaya and potato. Which one of them would require the step of emasculation and why? However for both you will use the process of bagging. Justify giving one reason.

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7. How would the gene flow or genetic drift affect the population in which either of them happen to take place?

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8. Differentiate between the roles of B-lymphocytes and T-lymphocytes in generating immune responses.

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OR

Principle of vaccination is based on the property of “memory” of the immune system. Taking one suitable example, justify the statement.

9. Explain the relevance of “Totipotency” and “Somaclones” in raising healthy banana plants from virus infected banana plants.

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10. How is a continuous culture system maintained in bioreactors and why?

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11. List any four ways by which GMO’s have been useful for enhanced crop output.

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Section C

12. Mention four significant services that a healthy forest ecosystem provides.

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OR

Substantiate with the help of one example that in an ecosystem mutualists (i) tend to co-evolve and (ii) are also one of the major causes of biodiversity loss.

13. Pollen banks are playing a very important role in promoting plant breeding programme the world over. How are pollens preserved in the pollen banks? Explain. How are such banks benefitting our farmer? Write any two ways.

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14. Draw a labelled diagram to show interrelationship of four accessory ducts in a human male reproductive system.

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OR

Draw a sectional view of the human ovary showing the different stages of developing follicles, corpus luteum and ovulation.

15. Compare in any three ways the chromosomal theory of inheritance as proposed by Sutton and Bovery with that of experimental results on pea plant presented by Mendel.

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OR

(a) Explain linkage and recombination as put forth by T.H. Morgan based on his observations with Drosophila melanogaster crossing experiment.

(b) Write the basis on which Alfred Sturtevant explained gene mapping.

16. Explain the mechanism of DNA replication with the help of

a replication fork. What role does the enzyme DNA-ligase play in a DNA replication fork?

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OR

Construct and label a transcription unit from which the RNA segment given below has been transcribed. Write the complete name of the enzyme that transcribed this RNA.

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17. (a) Write two differences between Homo erectus and Homo habilis.

(b) Rearrange the following from early to late geologic periods : Carboniferous, Silurian, Jurassic.

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18. Name the group of bacteria involved in setting milk into curd. Explain the process they carry in doing so. Write another beneficial role of such bacteria.

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19. Bee keeping practice is a good income generating industry. Write the different points to be kept in mind for successful bee keeping. Write the scientific name of the most common Indian species used for the purpose.

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20. (a) Match the microbes listed under Column-A with the products mentioned under Column-B.

Column – A Column – B

(H) Penicillium notatum (i) Statin

(I) Trichoderma polysporum (ii) ethanol

(J) Monascuspurpurea (iii) antibiotic

(K) Saccharomyces cerevisiae (iv) Cyclosporin-A

(b) Why does ‘Swiss Cheese’ develop large holes ?

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21. Describe the formation of recombinant DNA by the action of EcoRI.

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OR

Describe the process of amplification of “gene of interest” using PCR technique.

22. Two children, A and B aged 4 and 5 years respectively visited a hospital with a similar genetic disorder. The girl A was provided enzyme-replacement therapy and was advised to revisit periodically for further treatment. The girl, B was, however, given atherapy that did not require revisit for further treatment.

(a) Name the ailments the two girls were suffering from? (b) Why did the treatment provided to girl A required

repeated visits? (c) How was the girl B cured permanently?

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23. List six advantages of “ex-situ” approach to conservation of biodiversity.

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24. While on a visit to a pond in the city-neighbourhood, the visitors were delighted to find large expanse of water covered with colourful algal mass.

(a) As a student of biology, do you agree with their delight ? Give reasons in support of your answer.

(b) Explain the cause of such algal growth.

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Section D

25. (a) Explain one application of each one of the following :

(A) Amniocentesis

(B) Lactational amenorrhea

(C) ZIFT

(b) Prepare a poster for the school programme depicting the objectives of : “Reproductive and Child Health Care Programme”.

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OR

(a) Explain any two ways by which apomictic seed can develop.

(b) List one advantage and one disadvantage of a apomictic crop.

(c) Why do farmers find production of hybrid seeds costly?

26. Differentiate between incomplete dominance and co-dominance. Substantiate your answer with one example of each.

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OR

(a) Write the contributions of the following scientists in deciphering the geneticcode.

Georce Gamow; Har Gobind Khorana; Marshall Nirenberg; Severo Ochoa

(b) State the importance of a Genetic code in protein biosynthesis.

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27. (a) What is “population” according to you as a biology student ?

(b) “The size of a population for any species is not a static parameter.” Justify the statement with specific reference to fluctuations in the population density of a region in a given period of time.

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OR

(a) What is hydrarch succession ?

(b) Compare the pioneer species and climax communities of hydrarch and xerarch succession respectively.

(c) List the factors upon which the type of invading pioneer species depend in secondary hydrarch succession. Why is the rate of this succession faster than that of primary succession?

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1. Punnett square is a square representation to understand the Mendel’s cross.

It predicts all the genotypes (combination of alleles) and phenotype (the traits which can be observed) of offspring from a particular genetic cross.

Explanation:

In this cross, the homozygous (gene with same alleles) Parent

generation TT and tt split into gametes T, T and t, t respectively and this result into the formation of Heterozygous (gene with different alleles) F1 generation. The F1 generation further splits into T, t gametes and when two members of F1 generation were crossed, it resulted into F2 generation which contains 2 homozygous plants i.e. TT and tt and 2 heterozygous plants i.e. Tt.

Solutions (Set-1)

Section A (Solutions)

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Also it can be seen that a recessive trait expresses itself only when two heterozygous plants are crossed.

2. The two principal outcomes of the experiments conducted by Louis Pasteur on origin of life –

a) Life comes from pre-existing life

b) In the pre-sterilized flask, new life doesn’t come from the killed yeast but in the other flask exposed to air, new organisms occur from the killed yeast. Thus, dismissing the concept of spontaneous generation.

3. Stratosphere

Explanation:

Ozone gas is of two types: good ozone and bad ozone.

Bad ozone is formed in the lower atmosphere i.e. troposphere. It is harmful for plants and animals.

Good ozone is found in the upper atmosphere i.e. stratosphere. It protects from harmful ultraviolet rays from the sun.

Commensalism

Explanation:

The interspecific interaction between Orchid and a forest tree is commensalism. Orchid grows as an epiphyte on the branch of a tree and benefits by having a habitat. In this situation, though the Orchid is benefited the tree is neither benefits nor is harmed. Such an interaction is called commensalism.

OR

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4. Flocs are the masses of bacteria associated with fungal filaments to form mesh like structures.

Explanation:

During the secondary treatment of sewage, the primary effluent is passed into the large aeration tanks where it is continuously agitated mechanically and air is pumped into it. This leads to the vigorous growth of useful aerobic microbes into flocs.

Methanogens are found in cattle dung and in the rumen of cattle. They are also found in anaerobic sludge, biogas plants, marshy areas and flooded rice fields.

5. Meiosis occurs in an organism exhibiting haploidic cycle after the zygote formation stage. Thus the product formed will be haploid spores which grow into haploid individuals.

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6. Emasculation is the process of removal of stamens from a flower. Between papaya and potato, potato bears bisexual flowers and papaya bears unisexual flowers. Therefore, potato would require emasculation.

But bagging (the process of covering the stigma with a bag of butter paper to protect from undesired pollens) is performed in both as the pollens from other plants can contaminate the stigma.

7. Gene flow is the movement of genes between the interbreeding populations of a species. Gene flow would result in changed frequency of genes in both populations.

The result is a population with an active gene flow. Such population doesn’t obey the Hardy-Weinberg’s principle (states that the genetic variation in a population will remain constant from one generation to the next in the absence of disturbing factors)

8.

S. No. B – lymphocytes T- lymphocytes 1 Origination and

maturation takes place in the bone marrow.

Origination in bone marrow and maturation in the thymus.

2 Differentiate into plasma cell.

Differentiate into plasma cell or memory cell or cytotoxic T cells.

3 Involved in the humoral immunity

Involved in the cell-mediated immunity

Section B (Solutions)

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Principle of vaccination is based on the property of “memory” of the immune system. Taking an example of polio vaccine in which weakened pathogens of polio are introduced into the body orally. The body starts producing anti-bodies against those pathogens which fight against the pathogens during actual infection. The vaccine generates memory B-cells and T-cells which recognize the pathogen quickly on subsequent exposure. The vaccination works similarly in all the cases.

9. Totipotency is the ability of a cell to give rise to a whole new plant which allows the meristematic tissues of a banana plant to grow in a culture medium. The meristematic tissues don’t get infected by virus. The totipotency allows them to form the callus. This way totipotency helps in raising a healthy banana plant from virus infected banana plant.

Somaclones are plants which are morphologically as well genetically similar to the parents and are produced through tissue culture. Hence somaclones give rise to healthy banana plants from virus infected banana plants.

10. A continuous culture system is maintained in a bioreactor by continuously and regularly feeding with culture medium steadily and by providing optimum growth conditions like pH, temperature, vitamins, salts, oxygen. It is also necessary to drain out the used medium from the other side.

In short, the used medium is drained out from one side of the bioreactor and the fresh medium is added from the other side

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Continuous culture system is maintained in bioreactors to maintain the cells in their physiologically most active log/exponential phase.

This type of culturing method produces a larger biomass leading to higher yields of desired protein.

11. Four ways by which GMO’s have been useful for enhanced crop output are –

i. Genetically modified crops are more tolerant to extreme conditions like cold, heat, drought, etc.

ii. Reduction in post-harvest losses.

iii. Some crops are made pest resistant by genetic modification which cuts down the use of pesticides and hence becomes economically and ecologically beneficial.

iv. Helps in increasing the efficiency of mineral usage by plants which maintains the mineral content of the soil and keeps it fertile for longer duration.

12. Significant services that a healthy forest ecosystem provides –

i. Provides habitat to the wildlife

ii. Purifies the air

iii. Prevents soil erosion

iv. Contributes in maintaining the water cycle

v. Maintains the nutrient cycles.

vi. Helps in keeping the soil fertile

Mutualists are the pairs of the organisms that benefit from interactions between them.

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(i) Mutualists tend to co-evolve. Let’s take the example of a flower and its pollinator. The mutualism between them follows co-evolution. The wasp pollinates the fig flower and in return, wasp uses the fruit for laying eggs and as a source of nourishment for the larvae.

(ii) Mutualists are also one of the major causes of biodiversity loss. In case of any change in any of the participants of mutualism, affects the other participant as well. For example, if the population of bees that pollinate the orchid flower reduces, it will cause the reduction in the population of the orchid plants also i.e. When any one of these two species become extinct - the other species associated with it in obligatory way also becomes extinct and leads to biodiversity loss.

13. Pollen banks are playing a very important role in promoting plant breeding programme the world over.

Pollens are preserved in the pollen banks by storing them in the viable condition in a low temperature of -196 degree Celsius with the help of liquid nitrogen this is known as cryopreservation.

Section C (Solutions)

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Pollen banks are benefitting farmers in the following ways:-

•The agricultural biodiversity is preserved for long durations in the form of preserved pollens

•Stored pollens can be used for the production of hybridised crops.

14. The accessory ducts in a human male reproductive system are:

•rete testis

•vasa efferentia

•epididymis

•vas deferens

Note: You may draw and label just the four ducts too and draw

the diagram like this:

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•The process of formation of a mature female gamete is called oogenesis.

•The ovum/egg/oocyte is arrested during the first meiotic division as a primary oocyte.

•The primary follicles get surrounded by more layers of granulosa cells and a new theca and called secondary follicles.

•The secondary follicle soon transforms into a tertiary follicle which is characterised by a fluid filled cavity called antrum.

•At this stage that the primary oocyte within the tertiary follicle grows in size and completes its first meiotic division forming the secondary oocyte.

•The tertiary follicle further changes, matures and forms the Graafian follicle.

•The secondary oocyte forms a new membrane called zona pellucida surrounding it.

•The Graafian follicle now ruptures to release the secondary oocyte from the ovary. This process is known as ovulation.

15. i. Mendel deduced that genes come in pairs and are inherited in distinct units, one from each parent and tracked the segregation of genes from the parents and their expression in

OR

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the offspring as dominant or recessive. But he didn’t find out how characteristics are sorted and combined on cellular level.

Sutton and Bovery suggested that chromosomes bear the hereditary material.

ii. Mendel knew nothing of the chromosomes and meiosis. Sutton and Boveri in order to explain the law of independent assortment suggested that different genes were in different chromosomes completely independent of one another.

iii. Sutton and Boveri argued that the pairing and segregation of a pair of chromosomes will lead to the segregation of a pair of factors they carry. Sutton united the knowledge of chromosomal segregation with Mendel’s principles and called it “Chromosomal theory of inheritance”.

(a) Linkage is the term used to describe the physical association of genes on a chromosome.

Recombination is used to describe the generation of non-parental gene combinations.

•T.H. Morgan and his colleagues used Drosophila melanogaster to study linkage. Morgan hybridised yellow-bodied, white-eyed females to brown-bodied, red-eyed males and intercrossed their F1 progeny.

•He observed that the two genes did not segregate independently of each other and the F2 ratio deviated very significantly from the 9:3:3:1 ratio (expected when the two genes are independent). They knew that the genes were located on the X chromosome.

•They observed that when the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations were much higher than the non-parental type.

OR

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•Morgan and his group also found that even when genes were grouped on the same chromosome, some genes were very tightly linked (showed very low recombination), while others were loosely linked (showed higher recombination).

(b) Alfred Sturtevant used the frequency of recombination

between gene pairs on the same chromosome as a measure of the distance between genes and ‘mapped’ their position on the chromosome.

16.

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Mechanism of DNA replication:

i. Initiation –

•There is a particular region called origin of replication. This is the point where the replication originates.

•The DNA strands are unwound to form a replication fork. This is created by polymerases enzyme which is an opening in the DNA strand.

ii. Elongation –

•As the strands are separated, the polymerase enzymes start synthesizing the complementary sequence in each of the strands.

•The parental strands will act as a template

•Elongation is unidirectional i.e. DNA is always polymerized only in the 5′ to 3′ direction.

•In one strand (the template 3‘→5‘) it is continuous, hence called continuous replication while on the other strand (the template 5‘→3‘) it is discontinuous replication. They occur as fragments called Okazaki fragments.

•DNA ligase joins these fragments later on.

iii. Termination –

•The termination occurs in different ways in different organisms.

•In E.coli like organisms, chromosomes are circular. And this happens when the two replication forks between the two terminals meet each other.

DNA - Ligase is the enzyme which glues the discontinuous DNA strands (OKAZAKI FRAGMENTS)

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RNA molecule doesn’t contain Thymine (T). Hence the question is wrong.

Assuming the correct sequence was the given, the question would be solved like this:

The correct RNA segment should be

5’ A U G C A U G C A U G C 3’

3’ U A C G U A C G U A C G 5’

The process of copying genetic information from strand of DNA into RNA is known as transcription.

The transcription unit for the above mentioned RNA molecules would be like this:

The enzyme that transcribes this RNA is DNA-dependant RNA

polymerase.

17. (a)

(b) Silurian, Carboniferous, Jurassic

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18. Lactobacillus bacteria are involved in setting milk to curd.

Process-

•The sugar in the milk i.e. lactose is acted upon and utilised by these bacteria.

•It leads to the oxidation of lactose into lactic acid which leads to partial digestion and coagulation of milk proteins to form curd.

Beneficial role of such bacteria –

They lead to an increase in Vitamin B12 content of curd and consumption of such curd leads to healthy gut.

19. Different points to be kept in mind for successful bee keeping –

•Catching and hiving of group of bees

•Selecting suitable location for keeping beehives.

•Awareness about habits and nature of bees.

•Management of beehives during different seasons.

The scientific name of the most common Indian species used for bee keeping is Apis indica.

20. (a)

H – iii, because Penicillium notatum is an antibiotic

I – iv, because Cyclosporine A is widely produced by submerged fermentation of aerobic fungi identified as Trichoderma polysporum.

J – i, cholesterol lowering statins are produced by the molds Monascuspurpurea

K – ii, Saccharomyces cerevisiae is widely used in ethanol production.

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(b) Swiss cheese develops large holes because the bacteria Propionibacterium shermanii which helps in its formation release a large amount of carbon dioxide which cause large holes.

21. EcoRI identifies its palindromic sequence on both vector DNA and foreign DNA. EcoRI is used in molecular biology to cut foreign DNA and vector DNA to form sticky ends. These sticky ends form hydrogen bonds with their complementary counterparts. The segments with the help of DNA ligases are joined to produce recombinant DNA.

•Multiple copies of the gene of interest is synthesized in vitro using two sets of primers and the enzyme DNA polymerase.

•Primers are small chemically synthesized oligonucleotides that are complementary to the regions of DNA.

•PCR includes three major steps-

a) Denaturation - The process of heating of target DNA at 94 degree Celsius to separate the two strands of DNA.

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b) Annealing - The process of pairing of primers with complimentary base sequences of the two separated strands.

c) Extension - The process of adding complimentary deoxyribonucleotides one by one to the 3/OH ends of primers by the activity of DNA polymerase and as a result new DNA strand is synthesized.

•If the process of replication of DNA is repeated many times, the segment of DNA can be amplified to approximately billion times by the use of a thermostable DNA polymerase isolated from a bacterium, Thermus aquaticus.

•The amplified fragment can be used to ligate with a vector for further cloning.

22. (a) The disease mentioned here is ADA (adenosine deaminase) deficiency which is caused due to mutation in gene that code for ADA enzymes. The deficiency of these enzymes causes destruction of B and T lymphocytes which results in weak immune system.

(b) Girl A must be given Enzyme Replacement Therapy in which lymphocytes are isolated from patient’s body and then cultured in-vitro. In the cultured lymphocytes, the ADA cDNA

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are introduced and then inserted back into patient’s body. But since lymphocytes die after a certain period of time, repeated infusions are given to the patient.

(c) The Girl B must be given the Gene Therapy in which the gene isolated from bone marrow cells producing ADA is introduced into cells at embryonic stages which is a permanent cure.

23. In ex-situ approach to conservation of biodiversity, threatened animals and plants are taken out from their natural habitat and placed in a special setting, like zoo, wildlife sanctuaries, botanical gardens, etc., where they can be protected and given special care.

Advantages of ex-situ approach to conservation of biology –

i. The plants and animals are under supervision all the time due to which in case of any health issue, help can be provided immediately.

ii. The protection, they get in ex-situ approach, prevents them from exploitation.

iii. Gametes of the threatened species can be preserved in viable and fertile condition for long periods using cryopreservation techniques.

iv. Seeds of different genetic strains of commercially important plants can be kept for long periods in seed banks.

v. Makes the study of threatened and endangered species easier.

vi. Modern techniques like in vitro fertilization, tissue culture methods, etc. make the better success rate of reproduction.

24. (a) No, because being a biology student, we are aware of the fact that the colourful algal mass covering the large expanse of

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water is the result of eutrophication. Eutrophication ultimately leads to reduced oxygen content of the water body which leads to the death of the water body and the life it was supporting. These algal mass (algal bloom) causes deterioration of the water quality , increase fish mortality , are (extremely) toxic to humans and animals

(b) Such algal growth is a result of an excess of nutrients (particularly phosphorus and nitrogen) into waters. Such nutrients are released in water due to the use of fertilizers in the areas near the water body and the effluents released by the factories near the water body.

25. (a)

(A) Amniocentesis – It is a pre-natal diagnostic test in which amniotic fluid (the fluid that surrounds the unborn baby inside the uterus) is drawn out from the womb of a pregnant woman using a needle. It is used to test the chromosomal pattern of the cells in the amniotic fluid which helps to determine the sex of the foetus and to detect genetic disorders like Down’s syndrome, anaemia, thalassemia, etc.

Section D (Solutions)

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(B) Lactational amenorrhea – During lactation, the menstruation is absent in the woman who is lactating. This is known as lactational amenorrhea. The chances of conception during this time period are nearly zero.

(C) ZIFT – It is the short form of Zygote Intra Fallopian Transfer. The sperm of the male and ova from the female are fused together in the laboratory. This leads to the formation of a zygote which when is at the 8-blastomere stage, is transferred to the fallopian tube.

(c) A poster including the slogans about the importance of reproductive health care and child health care. Include slogans like "Hum Do Humaare Do" or "Stop Female Infanticide" or "AIDS Prevention."

(a) The viable seeds produced without pollination or fertilization, are called apomitic seeds. It is an asexual method so the offspring is a clone of parent. Example – Asteraceae and grasses.

Ways to develop apomictic seeds –

•In some species, the diploid egg cell is formed without reduction division and hence develops into embryo without fertilization.

•In many varieties, some of the nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac and develop into the embryos.

(b) Advantage of apomictic crop – Reduction in the cost of production of hybrids

OR

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Disadvantage of apomictic crop – Reduction in the genetic diversity from one generation to the other as there is no variation from parent to offspring in asexual reproduction.

(c) Production of hybrid seeds involves emasculation, bagging, raising of seedlings in nursery beds, etc. This is time consuming and not economically beneficial for the farmers. Also the hybrid seeds can’t be used in next season so it adds up to the cost.

26.

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(a) George Gamow was a physicist who argued that if all 20 amino acids have to be coded by 4 bases then the code should be a combination of 3 bases.

Har Gobind Khorana contributed in the development of a chemical method to synthesize the RNA molecules with defined combination of bases.

Marshall Nirenberg developed a cell-free system of protein synthesis which helped the code to be deciphered (decoded).

Severo Ochoa discovered an enzyme (polynucleotide phosphorylase). It was helpful in polymerizing the RNA with defined sequences in a template independent manner.

(b) Importance of a Genetic Code in protein biosynthesis:

•The codon is triplet i.e. a combination of three out of four bases. 4 bases when combined in a combination of 3, it gives

64 possible combinations . Out of these 64 combinations, 61 code for amino acids and rest 3 are stop codons.

•One codon codes for only one amino acid. Therefore, it is unambiguous and specific.

•Some amino acids are coded by more than one codon. Therefore it is degenerate.

•Codons are read without punctuations.

•AUG has dual functions. It codes for Methionine and also acts as initiator codon.

•The code is nearly universal i.e. 1 codon codes for the same amino acids in all the organisms.

27. (a) Population refers to a group of organisms that belong to a species and are living in a well-defined geographical area,

OR

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sharing or competing for similar resources and potentially interbreeding. For example, Stray dogs of Delhi, living together in groups, interbreeding and competing with each other for food and water.

(b)The size of a population for any species is not a stable parameter. It keeps changing with time, depending upon various factors including food availability, predation pressure and adverse weather.

The population density of a region in a given period of time can fluctuate due to the following factors:

•Natality: It means the number of births during a given time period in the population that are added to the initial density.

•Mortality: It is the number of deaths in the population during a given period of time.

•Immigration: It is the number individuals of the same species that have come into the habitat from somewhere else during a time period.

•Emigration: It is the number of individuals of the population who left the habitat and gone elsewhere during the time period under consideration.

(a) Hydrarch Succession is a succession that occurs in wetter areas and the successional series progress from hydrich to the mesic conditions.

(b) The pioneer species are the species that invade a bare area.

A climax community is the final stage of succession, remaining relatively unchanged until destroyed by an event such as fire or human interference.

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(c) The type of invading pioneer species depend in secondary

hydrarch succession depends upon the following factors:

•Condition of the soil

•Availability of water

•Environment

•The seeds and propagules already present

The rate of secondary succession is faster than the primary succession. This is because to establish a biotic community, there must be soil which forms gradually during the primary succession. During secondary succession, the soil is already present.

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