12.22: Bird Reproduction - Biology

12.22: Bird Reproduction - Biology

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Is this pair of birds actually a “couple”?

Yes. Birds do actually pair up each mating season, if not for life. And the male better be prepared to treat his female properly. There is actually an elaborate process in which the female chooses her mate.

Bird Reproduction

Reproduction in birds may be quite complicated and lengthy. Birds reproduce sexually and have separate sexes and internal fertilization, so males and females must mate for fertilization to occur. Mating is generally preceded by courtship. In most species, parents also take care of their eggs and hatchlings.

Courtship and Mating

Courtship is behavior that is intended to attract a mate. It may involve singing specific courtship songs or putting on some type of visual display. For example, a bird may spread out and display its tail feathers or do a ritualized mating “dance.” Typically, males perform the courtship behavior, and females choose a mate from among competing males.

During mating, a male bird presses his cloaca against his mate’s cloaca and passes sperm from his cloaca to hers. After fertilization, eggs pass out of the female’s body, exiting through the opening in the cloaca.

Nesting and Incubation

Eggs are usually laid in a nest. The nest may be little more than a small depression in the ground, or it may be very elaborate, like the weaver bird nest in Figure below. Eggs that are laid on the ground may be camouflaged to look like their surroundings (also shown in Figure below). Otherwise, eggs are usually white or pastel colors such as pale blue or pink.

Variation in Bird Nests. A weaver bird uses grasses to weave an elaborate nest (left). The eggs of a ground-nesting gull are camouflaged to blend in with the nesting materials (right).

After birds lay their eggs, they generally keep the eggs warm with their body heat while the embryos inside continue to develop. This is called incubation, or brooding. In most species, parents stay together for at least the length of the breeding season. In some species, they stay together for life. By staying together, the males as well as females can incubate the eggs and later care for the hatchlings. Birds are the only nonhuman vertebrates with this level of male parental involvement.


Nest of a marsh warbler (Acrocephalus palustris) with baby birds

Ground-nesting birds, such as ducks and chickens, have hatchlings that are able to run around and feed themselves almost as soon as they break through the eggshell. Being on the ground makes them vulnerable to predators, so they need to be relatively mature when they hatch in order to escape. In contrast, birds that nest off the ground—in trees, bushes, or buildings—have hatchlings that are naked and helpless. The parents must protect and feed the immature offspring for weeks or even months. However, this gives the offspring more time to learn from the parents before they leave the nest and go out on their own.

Parental Care

In birds, 90% to 95% of species are monogamous, meaning the male and female remain together for breeding for a few years or until one mate dies. Birds of all types, from parrots to eagles and falcons, are monogamous. Usually, the parents take turns incubating the eggs. Birds usually incubate their eggs after the last one has been laid. In polygamous species, where there is more than one mate, one parent does all of the incubating. The wild turkey is an example of a polygamous bird.

The length and type of parental care varies widely amongst different species of birds. At one extreme, in a group of birds called the magapodes (which are chicken-like birds), parental care ends at hatching. In this case, the newly-hatched chick digs itself out of the nest mound without parental help and can take care of itself right away. These birds are called precocial. Other precocial birds include the domestic chicken and many species of ducks and geese. At the other extreme, many seabirds care for their young for extended periods of time. For example, the chicks of the Great Frigatebird receive intensive parental care for six months, or until they are ready to fly, and then take an additional 14 months of being fed by the parents (Figure below). These birds are the opposite of precocial birds and are called altricial.

In most animals, male parental care is rare. But it is very common in birds. Often both parents share tasks such as defense of territory and nest site, incubation, and the feeding of chicks. Since birds often take great care of their young, some birds have evolved a behavior calledbrood parasitism. This happens when a bird leaves her eggs in another bird’s nest. The host bird often accepts and raises the parasite bird's eggs.

Great Frigatebird adults are known to care for their young for up to 20 months after hatching, the longest in a bird species. Here, a young bird is begging for food.


  • Birds reproduce sexually and have internal fertilization.
  • Mating is generally preceded by courtship.
  • Birds' amniotic eggs have hard shells and are laid in a nest. The eggs are usually incubated until they hatch.
  • Most species have a relatively long period of parental care.


  1. What is courtship? Give an example.
  2. Contrast hatchling maturity in birds that are ground-nesting and those that nest off the ground.

Breeding Biology of Birds: Proceedings of a Symposium on Breeding Behavior and Reproductive Physiology in Birds (1973)

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Pair Formation

To find a mate, birds resort to a series of signals, which can be simple or complex, depending on the species. The signs can be auditory:

Signals can also be visual:

And Behavioral signs such as:

  • Dances
  • Spectacular acrobatic flights
  • Movement of wings and tails
  • The offering of food items and nests and nest sites

Silver Grebes(Podiceps occipitalis) in a nuptial dance.

In general, it is the males who exhibit these signals, and they do so to attract females and provide them with information about their strength, abilities, and health, to demonstrate that they are the most suitable candidates with which to copulate and have strong and healthy offspring with a greater chance of survival.


As of 2011, NPIC stopped creating technical pesticide fact sheets. The old collection of technical fact sheets will remain available in this archive, but they may contain out-of-date material. NPIC no longer has the capacity to consistently update them. To visit our general fact sheets, click here. For up-to-date technical fact sheets, please visit the Environmental Protection Agency’s webpage.

Chemical Class and Type:

  • Chlorpyrifos is a broad-spectrum, chlorinated organophosphate (OP) insecticide, acaricide and nematicide. Chlorpyrifos is the common name for the chemical 0,0-diethyl 0-(3,5,6-trichloro-2-pyridinyl)-phosphorothioate. The Chemical Abstracts Service (CAS) registry number is 2921-88-2. 1

Laboratory Testing: Before pesticides are registered by the U.S. EPA, they must undergo laboratory testing for short-term (acute) and long-term (chronic) health effects. Laboratory animals are purposely given high enough doses to cause toxic effects. These tests help scientists judge how these chemicals might affect humans, domestic animals, and wildlife in cases of overexposure.

Molecular Structure -

Physical / Chemical Properties:

  • Chlorpyrifos is a colorless to white crystalline solid. 1,2 Chlorpyrifos has a mild mercaptan (thiol) odor, similar to the smell of sulfur compounds found in rotten eggs, onions, garlic and skunks. 2,3
  • Vapor pressure 4 : 1.87 x 10 -5 mmHg at 25 °C
  • Octanol-Water Partition Coefficient (Kow) 2 : 4.70
  • Henry's constant may be determined by estimation or experimentally derived. Reported values include: 4.2 x 10 -6 atm·m 3 /mol at 25 °C and 6.7 x 10 -6 atm·m 3 /mol, depending on the technique used. 2,4
  • Molecular weight 1 : 350.6 g/mol
  • Solubility (water) 2 : 0.0014 g/L (1.4 mg/L) at 25 °C
  • Soil Sorption Coefficient (Koc) 5 : 360 to 31,000 depending on soil type and environmental conditions.
  • Chlorpyrifos is used on agricultural food and feed crops, cattle ear tags, golf course turf, industrial plants and vehicles, non-structural wood treatments including processed wood products, fence posts and utility poles, and to control public health pests such as mosquitoes and fire ants. Chlorpyrifos is registered for indoor residential use only in the form of containerized baits. 1 Uses for individual products containing chlorpyrifos vary widely. Always read and follow the label when applying pesticide products.
  • Chlorpyrifos is a non-systemic insecticide designed to be effective by direct contact, ingestion, and inhalation. 2
  • Signal words for products containing chlorpyrifos may range from Caution to Danger. The signal word reflects the combined toxicity of the active ingredient and other ingredients in the product. See the pesticide label on the product and refer to the NPIC fact sheets on Signal Words and Inert or "Other" Ingredients.
  • To find a list of products containing chlorpyrifos which are registered in your state, visit the website select your state then click on the link for "State Products."

Mode of Action:

Target Organisms

  • Chlorpyrifos is a broad-spectrum insecticide which kills insects upon contact by affecting the normal function of the nervous system. 4 Chlorpyrifos affects the nervous system by inhibiting the breakdown of acetylcholine (ACh), a neurotransmitter. 5 When insects are exposed, chlorpyrifos binds to the active site of the cholinesterase (ChE) enzyme, which prevents breakdown of ACh in the synaptic cleft. 6 The resulting accumulation of ACh in the synaptic cleft causes overstimulation of the neuronal cells, which leads to neurotoxicity and eventually death. 6,7
  • Chlorpyrifos shares a common mechanism of toxicity with other organophosphate insecticides such as malathion and parathion, thus, chlorpyrifos would not be effective against organophosphate-resistant insect populations.

Non-target Organisms

  • The mode of action of chlorpyrifos is similar for target and non-target organisms. 8
  • Acetylcholine is found throughout the mammalian nervous system, including at cholinergic synapses in the central nervous system, the junction of post-ganglionic parasympathetic neurons in exocrine glands and smooth and cardiac muscles, at pre- and post-ganglionic neurons in the autonomic nervous system, at neuromuscular junctions of the somatic nervous system, and on the surface of red blood cells. 8,9
  • Chlorpyrifos affects ChE levels differently in various systems throughout the body. Scientists have observed plasma and red blood cell ChE inhibition in experimental animals at doses lower than those required to cause ChE inhibition in the brain. 5
  • The physiological functions of the neuropathy target esterase (NTE) enzyme were studied in genetically altered mice, which lacked the NTE enzyme. The results demonstrated that NTE plays an essential role in placental development, blood vessel development and protein synthesis in the central nervous system. 10 Chlorpyrifos can inhibit NTE by binding to the active site of the enzyme. Inhibition of the NTE enzyme results in loss of myelin and degeneration of axon fibers of the peripheral and central nerves. 8,9
  • Chlorpyrifos can cause permanent inhibition of the ChE or NTE enzymes, a process known as aging. Cleavage of an alkyl group from the chlorpyrifos residue produces a negative charge at the active site of the enzyme. This causes an unbreakable bond to form between the phosphorous atom on chlorpyrifos and the active site of the ChE or NTE enzyme. 9,10
  • Chlorpyrifos also interacts with other enzymes, such as carboxylesterases and A-esterases. The functional role of these enzymes is not well understood, although they occur in many mammalian systems. 6

Acute Toxicity:

  • Chlorpyrifos is moderately toxic to mice and rats. 1 The oral LD50 for mice is 60 mg/kg for rats it ranges from 95 to 270 mg/ kg. 1,2,11 See the text boxes on Toxicity Classification and LD50/LC50.
  • Chlorpyrifos is slightly toxic to rabbits. The acute oral LD50 in rabbits ranges from 1000 to 2000 mg/kg. 2
  • Chlorpyrifos is moderately toxic to guinea pigs and sheep. The acute oral LD50 in guinea pigs ranges from 500 to 504 mg/ kg and 800 mg/kg in sheep. 2,11
  • Chlorpyrifos is highly toxic to chickens. The acute oral LD50 in chickens ranges from 32 mg/kg to 102 mg/kg. 2,11

LD50/LC50: A common measure of acute toxicity is the lethal dose (LD50) or lethal concentration (LC50) that causes death (resulting from a single or limited exposure) in 50 percent of the treated animals. LD50 is generally expressed as the dose in milligrams (mg) of chemical per kilogram (kg) of body weight. LC50 is often expressed as mg of chemical per volume (e.g., liter (L)) of medium (i.e., air or water) the organism is exposed to. Chemicals are considered highly toxic when the LD50/LC50 is small and practically non-toxic when the value is large. However, the LD50/LC50 does not reflect any effects from long-term exposure (i.e., cancer, birth defects or reproductive toxicity) that may occur at levels below those that cause death.


  • Chlorpyrifos absorbs more easily through rat skin than human skin. Chlorpyrifos is not readily absorbed through human skin. 22,23
  • Skin-applied chlorpyrifos has low toxicity based on animal studies. The acute dermal LD50 for rabbits is >5,000 mg/kg and >2,000 mg/kg for rats, 2 although an acute dermal LD50 of 202 mg/kg has also been reported for rats. 1
    High ToxicityModerate ToxicityLow ToxicityVery Low Toxicity
    Acute Oral LD50Up to and including 50 mg/kg
    (&le 50 mg/kg)
    Greater than 50 through 500 mg/kg
    (>50-500 mg/kg)
    Greater than 500 through 5000 mg/kg
    (>500-5000 mg/kg)
    Greater than 5000 mg/kg
    (>5000 mg/kg)
    Inhalation LC50Up to and including 0.05 mg/L
    (&le0.05 mg/L)
    Greater than 0.05 through 0.5 mg/L
    (>0.05-0.5 mg/L)
    Greater than 0.5 through 2.0 mg/L
    (>0.5-2.0 mg/L)
    Greater than 2.0 mg/L
    (>2.0 mg/L)
    Dermal LD50Up to and including 200 mg/kg
    (&le200 mg/kg)
    Greater than 200 through 2000 mg/kg
    (>200-2000 mg/kg)
    Greater than 2000 through 5000 mg/kg
    (>2000-5000 mg/kg)
    Greater than 5000 mg/kg
    (>5000 mg/kg)
    Primary Eye IrritationCorrosive (irreversible destruction of ocular tissue) or corneal involvement or irritation persisting for more than 21 daysCorneal involvement or other eye irritation clearing in 8 - 21 daysCorneal involvement or other eye irritation clearing in 7 days or lessMinimal effects clearing in less than 24 hours
    Primary Skin IrritationCorrosive (tissue destruction into the dermis and/or scarring)Severe irritation at 72 hours (severe erythema or edema)Moderate irritation at 72 hours (moderate erythema)Mild or slight irritation at 72 hours (no irritation or erythema)
    The highlighted boxes reflect the values in the "Acute Toxicity" section of this fact sheet. Modeled after the U.S. Environmental Protection Agency, Office of Pesticide Programs, Label Review Manual, Chapter 7: Precautionary Labeling.
  • Chlorpyrifos is a mild skin and moderate eye irritant based on rabbit studies. Chlorpyrifos is not a skin sensitizer according to results of tests on guinea pigs. 2

NOAEL: No Observable Adverse Effect Level

NOEL: No Observed Effect Level

LOAEL: Lowest Observable Adverse Effect Level

LOEL: Lowest Observed Effect Level


  • Chlorpyrifos is considered moderately toxic by inhalation. The 4- to 6-hour LC50 is >0.2 mg/L in rats. 2,11
  • The NOAEL for short- and intermediate-term inhalation is 0.1 mg/kg/day. The NOAEL is based on two separate 90-day studies of rats where researchers observed no effect at the highest vapor concentration tested. 5

Signs of Toxicity - Animals

  • Acute signs of toxicity can appear within minutes of exposure to chlorpyrifos. The signs typically appear at muscarinic receptor sites first, followed by nicotinic receptor sites and finally at central nervous system receptor sites. 9
  • Muscarinic signs from acute exposure to chlorpyrifos include abdominal pain, bronchospasm, constricted pupils, coughing, decreased heart rate, defecation, difficulty breathing, diminished appetite, distress, vomiting and increased tear production, salivation, and urination. Nicotinic signs include muscle tremors that are noted first in the head and then the body, generalized sustained muscle contractions, stiffness, weakness with paresis, and paralysis. Reported signs from extremely high oral doses include an increase in heart rate and constriction of the pupils. Central nervous system signs include diminished appetite, anxiety, restlessness, hyperactivity, depression, clonic-tonic seizures, depressed respiration, and coma. 9
  • Cats have experienced lethal effects from chlorpyrifos at doses of 10 to 40 mg/kg. 9
  • An exposure to chlorpyrifos may result in an intermediate syndrome, in which signs appear more than 24 hours after exposure, and can last several days or even weeks. Signs have been reported to develop within 24 to 72 hours in dogs and cats. It appears that intermediate syndrome involves tolerance to the overstimulation of ACh in muscarinic receptors. This tolerance does not develop at nicotinic receptors, and therefore the syndrome is characterized primarily by nicotinic effects. Signs from intermediate syndrome include: weakness of the neck, front limbs and respiratory muscles, diminished appetite, depression, diarrhea, muscle tremors, unusual posturing and behavior (including cervical ventroflexion), and death. Additional signs may include cranial nerve defects and clonic-tonic convulsions. 9,24
  • When chlorpyrifos was registered for residential use, dermal exposure of cats to chlorpyrifos residues in the home environment was the most commonly reported cause of intermediate syndrome in domestic animals. In such cases, symptoms appeared 3-10 days after exposure to chlorpyrifos. 9 See the NPIC fact sheet on Pets and Pesticide Use.
  • Another phenomenon, Organophosphate Induced Delayed Neuropathy (OPIDN) differs from intermediate syndrome in that the onset of signs may occur weeks after an acute, high-dose exposure to OPs. 25 Cats and chickens exposed to supralethal doses of chlorpyrifos showed signs consistent with delayed neuropathy. In both cases, the animals were treated with atropine to resolve acute cholinergic symptoms. Ataxia, altered movements, and impairment of spatial perception were reported signs of delayed neuropathy. 26,27 OPIDN signs are primarily evident in the hind or pelvic limbs of exposed animals. 24

Signs of Toxicity - Humans

  • Signs and symptoms typically develop within minutes to hours after an acute exposure to chlorpyrifos. Initial signs and symptoms include tearing of the eyes, runny nose, increased saliva and sweat production, nausea, dizziness and headache. Signs of progression include muscle twitching, weakness or tremors, lack of coordination, vomiting, abdominal cramps, diarrhea, and pupil constriction with blurred or darkened vision. 8,28,29 Signs of severe toxicity include increased heart rate, unconsciousness, loss of control of the urine or bowels, convulsions, respiratory depression, and paralysis. 8,28
  • Psychiatric symptoms associated with acute exposure include anxiety, depression, memory loss, confusion, stupor, bizarre behavior, and restlessness. 8,28,29
  • Children may experience different signs and symptoms from exposure to chlorpyrifos than adults, and diagnosis of poisoning in general may be more difficult. 8,29 Commonly reported signs and symptoms in poisonings with children include seizures, flaccid muscle weakness, pupil constriction, excess salivation ,and mental status changes including lethargy and coma. Some of the typical symptoms seen in adults, such as decreased heart rate, muscle twitching, increased tear production, and sweating, are less common in children. 8
  • Single, high-dose exposures to organophosphates in humans can also result in intermediate syndrome. Signs and symptoms typically occur 24-96 hours after exposure. As in animals, the syndrome is characterized by the absence of muscarinic signs. Signs of toxicity result from the inhibition of nicotinic receptors. Signs observed in humans include reduced tendon reflexes, cranial nerve palsies, weakness in the facial, neck, proximal limb muscles, and partial respiratory paralysis. 8
  • Delayed neurological symptoms, beginning 1-4 weeks after exposure, may also result from an acute, high-dose exposure to OPs. 11 As in animals, this prolonged delay in neurological symptoms is referred to as OPIDN and onset depends on the dose and route of exposure. Reports of OPIDN from exposure to chlorpyrifos are limited to acute, high-dose exposures where treatment with therapeutic agents was used to resolve acute cholinergic toxicity. 30 In one case, a 42-year old man intentionally ingested chlorpyrifos in a suicide attempt, and in a second case, a 3-year-old boy accidentally ingested chlorpyrifos. 31,32 It has been suggested that supralethal doses followed with antidotal therapy, rather than low-level, chronic exposures, would be necessary for chlorpyrifos to cause OPIDN in humans. 30
  • OPIDN typically affects the lower extremities and can cause cramping, muscle pain, weakness and paresthesia, which is described as numbness and tingling sensations. In more severe cases, musculoskeletal effects including depression of tendon reflexes in the arms, symmetrical wasting, flaccid weakness, and paralysis of distal muscles (most commonly the legs) have been reported. Signs and symptoms from OPIDN may persist from weeks to years. 8,10,25
  • Always follow label instructions and take steps to minimize exposure. If any exposure occurs, be sure to follow the First Aid instructions on the product label carefully. For additional treatment advice, contact the Poison Control Center at 1-800- 222-1222. If you wish to discuss an incident with the National Pesticide Information Center, please call 1-800-858-7378.

Chronic Toxicity:


  • The most sensitive endpoint in rats, mice and dogs chronically exposed to chlorpyrifos is inhibition of ChE in the plasma, red blood cells and brain. Dogs showed increased liver weights at doses of 3 mg/kg/day. Rats exposed to 7-10 mg/kg/day displayed fluctuations in body weight as well as adverse effects on the eyes, adrenal glands and liver chemistry. Mice appear to be less sensitive to chronic oral exposures of chlorpyrifos, with decreases in body weights and increases in tissue abnormalities occurring at doses of 45-48 mg/kg/day. 5
  • In rats, age-related differences in sensitivity to chronic chlorpyrifos exposure do not appear to be as significant as the agerelated sensitivity differences observed in rats exposed to acute doses of chlorpyrifos. 19
  • The chronic dermal NOAEL and the long-term inhalation NOAEL are 0.03 mg/kg/day based on five chronic toxicity studies reported in dogs and rats. These studies demonstrated adverse effects including plasma and red blood cell ChE inhibition at 0.22 to 0.30 mg/kg/day. 5
  • Both sexes of Fischer 344 rats exposed orally to 1 mg/kg/day of technical grade chlorpyrifos for 2 years had significantly reduced plasma and red blood cell cholinesterase levels. 33
  • Chronic, low level exposures to organophosphates may lead to the development of a tolerance to the effects of ChE inhibition in exposed animals. Though the exact mechanism of tolerance development has not been identified, it is possible that changes in postsynaptic receptors may mitigate some of the anticholinesterase effects. 34 When a tolerance to anticholinesterase compounds has developed, animals may appear more resistant to the effects of ChE inhibition, and signs of toxicity may be decreased or disappear entirely. Some experimental animals have also shown the ability to handle higher doses of organophosphates than unexposed animals. 34,35


  • A panel of scientists reviewed the available research on chlorpyrifos and its potential to affect human health. The researchers concluded that the current literature does not show that chronic chlorpyrifos exposure causes adverse effects on human health beyond cholinesterase inhibition. The group suggested that further research be conducted on workers in chlorpyrifos manufacturing, as they are likely to be exposed with more frequency and possibly at higher levels than the general population. The group suggested that further research should focus on the potential for chlorpyrifos to cause peripheral neuropathy and cognitive and affective disorders. 36
  • An occupational study was conducted to evaluate the potential for chronic, low-level exposure to chlorpyrifos to affect the central nervous system. Investigators used a prospective cohort study design involving one group of chlorpyrifos-manufacturing workers and a control group. The chlorpyrifos-exposed workers had significantly higher levels of a chlorpyrifos urinary metabolite, 3,5,6-trichloro-2-pyridinol (TCP), and had lower average BuChE levels. There was no significant difference in neurological symptoms or signs between the two groups, nor was there clinical evidence of adverse effects on the central nervous system at baseline or at the 1-year follow-up evaluation. 37 See the text box on Exposure.

Exposure: Effects of chlorpyrifos on human health and the environment depend on how much chlorpyrifos is present and the length and frequency of exposure. Effects also depend on the health of a person and/or certain environmental factors.

Endocrine Disruption:

  • Chlorpyrifos is included in the 2007 draft list of initial chemicals for screening under the U.S. EPA Endocrine Disruptor Screening Program (EDSP). The list of chemicals was generated based on exposure potential, not based on whether the pesticide is a known or likely potential cause of endocrine effects. 39
  • No data were found regarding possible effects of chlorpyrifos on endocrine systems.



  • Chlorpyrifos did not induce treatment-related tumors or carcinogenicity in two chronic rat and two chronic mouse studies. 5
  • Rats exposed to chlorpyrifos for two years at 0.05, 0.10, 1.0 and 10.0 mg/kg/day did not show any carcinogenic effects. 33
  • Scientists observed no genotoxic effects from chlorpyrifos in a range of in vitro and in vivo studies. 2
  • According to the Agency for Toxic Substances and Disease Registry (ATSDR), animal studies do not indicate that chlorpyrifos causes cancer. 7


  • In 1993, chlorpyrifos was classified in Group E, evidence of non-carcinogenicity for humans, by the U.S. EPA. 40 See the text box on Cancer.
  • No human data were found regarding carcinogenic effects of chlorpyrifos.

Cancer: Government agencies in the United States and abroad have developed programs to evaluate the potential for a chemical to cause cancer. Testing guidelines and classification systems vary. To learn more about the meaning of various cancer classification descriptors listed in this fact sheet, please visit the appropriate reference, or call NPIC.

Reproductive or Teratogenic Effects:


  • Researchers have reported behavioral effects from chlorpyrifos in studies with rats, including developmental delays in coordination, reflexes, and locomotor activity. 41,42 Researchers have also noted altered expressions of social behavior 18 and impaired spatial learning in exposed animals. 43 Gender differences in behavioral effects appear to be dependent on the age of the rat at the time of chlorpyrifos exposure. 18
  • Several studies have shown an increased sensitivity and susceptibility to adverse biochemical and behavioral effects in developing rats exposed either pre- or post-natally to chlorpyrifos when compared to adults. 19,20
  • Researchers observed structural changes in brain development of female offspring of rats exposed to chlorpyrifos at 1 mg/ kg/day, the lowest dose administered. In the dams, researchers observed inhibition of ChE in plasma and red blood cells at the same dose. The male and female pups of the exposed dams were exposed to 5 mg/kg/day and exhibited decreased body weight, decreased body weight gain, decreased food consumption, reductions in the number of viable offspring, developmental delays, decreased brain weight and morphological changes in the brain. 5
  • Reproductive and developmental effects from chlorpyrifos exposure have been observed at varying developmental stages in rats, mice and rabbits. 5
  • Age-related differences in neurotoxic effects independent of ChE inhibition have been observed in numerous developmental studies with rats, rabbits and mice exposed to chlorpyrifos. Neurotoxic effects observed include: programmed cell death, altered neuronal development, altered gene transcription and cell differentiation, impaired synthesis of DNA, RNA, and proteins, adverse effects on cell reproduction, and changes in brain development. 18,41,44
  • Some studies have observed neurodevelopmental effects at exposure levels below those causing AChE inhibition, but the mechanism for these effects is uncertain. Researchers have proposed that chlorpyrifos, rather than the oxon or other metabolites, may play a role in developmental neurotoxicity. Due to the relationship between low-level exposures to chlorpyrifos and some observed neurodevelopmental effects, as well as the environmental relevance of low-level exposures, researchers have concluded that further studies are needed to characterize the mechanisms of this potential effect. 45
  • In subacute reproductive studies with mallard ducks (Anas platyrhynchos), scientists observed reduced egg production, thinning of eggshells, reduced number of young, and death when hens were fed chlorpyrifos in their diets at concentrations of 60, 100, and 125 ppm. At the highest dose tested, researchers observed an 84% reduction in the number of eggs and 89% reduction in the number of young. 4


  • A prospective cohort study evaluated the relationship between chlorpyrifos levels in both umbilical cord plasma and mother's plasma at the time of birth, and impacts on neurological and behavioral development of children exposed prenatally. The study included 254 children and assessed cognitive and motor development at 12, 24, and 36 months. Researchers found that children and mothers with detected chlorpyrifos levels at or above 6.17 pg/g plasma were significantly more likely to experience adverse effects, including developmental delays and disorders, attention problems, and attention-deficit/ hyperactivity disorder at three years of age compared to children and/or mothers with levels lower than 6.17 pg/g. 46

Fate in the Body:


  • Chlorpyrifos is absorbed by all routes of exposure. Urinalysis of exposed human volunteers indicates that approximately 70% is absorbed by the oral route, while less than 3% is absorbed through the skin. 23 Exposure to chlorpyrifos by inhalation results in the fastest appearance of symptoms, followed by oral and then dermal routes of exposure. 8
  • Researchers evaluated the absorption of chlorpyrifos by oral and dermal exposure in five human volunteers. Absorption of chlorpyrifos was based on levels of the dialkylphosphate metabolites of chlorpyrifos, diethylphosphate and diethylthiophosphate. Peak urinary metabolite levels were observed at seven hours following oral exposure. For dermal exposure, peak metabolite concentrations were observed at 17 to 24 hours post-exposure. 47
  • In a similar study, maximum absorption levels for oral and dermal chlorpyrifos exposure were determined with six human volunteers. In this study, oral and dermal absorption rates were based on urinary concentrations of TCP, a primary chlorpyrifos metabolite. For oral exposure, peak levels were measured 6 hours after exposure. Maximum urinary TCP levels occurred 24 hours after dermal exposure. 23
  • The chlorine group on chlorpyrifos increases the compound's lipid solubility and half-life in the body, resulting in a more gradual, but persistent, lowering of ChE levels compared to other organophosphorus pesticides. 9


  • Chlorpyrifos is distributed throughout the body following exposure. 9
  • Although some chlorpyrifos may be stored in fat tissue, bioaccumulation is not expected to be significant due to an elimination half-life in humans of less than three days. 11


  • Metabolic bioactivation is necessary for chlorpyrifos to exert cholinesterase inhibition. 6,48 Bioactivation occurs primarily in the liver by cytochrome P450 enzymes (CYP). The CYP2B6 enzyme metabolizes chlorpyrifos to chlorpyrifos-oxon by replacing the sulfur group with oxygen. 48
  • Oxidase enzymes in the liver detoxify chlorpyrifos-oxon through inactivation. B-esterases such as carboxylesterase and BuChE become structurally inhibited after the process of inactivation, whereas the A-esterases such as paraoxonase 1 (PON1) can hydrolyze chlorpyrifos-oxon to TCP and remain functional. 48
  • The activity of PON1 in humans is genetically determined and varies among individuals. A higher level of PON1 appears to be protective against cholinergic effects, as evidenced by research in some animals exposed to organophosphates. Thus, certain individuals may have an increased sensitivity to chlorpyrifos toxicity based on a reduced capacity to detoxify chlorpyrifos-oxon. 5 Rabbits have greater PON1 activity and resistance to toxicity than rats, and birds are more sensitive than mammals in general. Birds have nearly undetectable levels of PON1. 5
  • Chlorpyrifos-oxon is metabolized primarily to TCP in addition to diethylphosphate and diethylthiophosphate. 22
  • Glucuronide and sulfate conjugates of TCP have also been observed in the urine of humans and rodents. 5,49 Chlorpyrifos-oxon is the only metabolite of chlorpyrifos that induces ChE inhibition therefore all other metabolites are considered less toxic. 1,22
  • Subchronic or chronic exposure to TCP at 30 mg/kg/day resulted in altered liver enzyme profiles. At exposures of 100 mg/ kg/day, researchers noted increases in liver and kidney weights. 1


  • Elimination of chlorpyrifos occurs mainly through the kidneys. Chlorpyrifos is excreted in the urine as TCP, diethylphosphate and diethylthiophosphate. 11,22
  • In a study with five human volunteers, Griffin and colleagues reported elimination half-lives for oral and dermal exposure of 15.5 hours and 30.0 hours, respectively. Rates were based on levels of diethylphosphate and diethylthiophosphate in the urine. A total of 93% of the oral dose was recovered as urinary metabolites, while 1% of the dermal dose was recovered. 47
  • In a similar study, Nolan et al observed an elimination half-life of 27 hours following both dermal and oral exposure, based on urinary TCP levels. Nolan and colleagues recovered 70.0% of the oral dose and 1.3% of the dermal dose in the urine as TCP. 23
  • Following oral exposure, rats excreted 90% of ingested chlorpyrifos through the urine and 10% in the feces. 11

Medical Tests and Monitoring:

  • The most common laboratory tests for organophosphate pesticide exposures are ChE inhibition tests which are used to analyze the blood for lowered levels of plasma or red blood cell AChE. These tests may be conducted by hospital laboratories, local clinical laboratories, or other referred laboratories. Other tests for chlorpyrifos exposure are less common and include detection of the parent compound or metabolites in blood or urine. 50
  • The potential for exposure to chlorpyrifos is present in several occupational fields, including agriculture, manufacturing, animal health technicians, pesticide applicators, and others. A baseline analysis of ChE levels in the blood may be mandatory for people who work closely with organophosphates. Following the establishment of a baseline, ChE testing of workers may be conducted to detect cumulative effects from daily exposure before clinical signs are apparent. Monitoring may also be useful to characterize exposures to the workforce as a whole to identify problem areas in the workplace. 51,52
  • Humans and animals may be exposed to metabolites of chlorpyrifos through dietary sources and from background levels found in the environment. The metabolites excreted by humans and animals are in the same family of chemicals as degradates that form when chlorpyrifos is broken down in the environment. Therefore, the presence of metabolites in human urine may indicate direct exposure to metabolites themselves, and doesn't necessarily confirm exposure to chlorpyrifos. 22,53
  • The presence of chlorpyrifos metabolites in the blood or urine does not necessarily indicate that adverse health effects will occur. 22
  • The National Health and Nutrition Examination Survey (NHANES) III study found that 82% of the 993 adults measured had detectable levels of TCP in their urine. The Minnesota Children's Exposure Study found that 92% of the 89 children evaluated had detectable concentrations of TCP in their urine. Similarly, a biomonitoring study in North and South Carolina detected urinary metabolites in 100% of the 416 children evaluated. 5 Amounts of TCP detected in food samples were greater than amounts of the parent chemical, chlorpyrifos, indicating a high background level of TCP in food. High background levels of TCP may contribute to higher detected urinary TCP levels. 45 See the NPIC medical case profile on Biomarkers of Exposure: Organophosphates.

The "half-life" is the time required for half of the compound to break down in the environment.

1 half-life = 50% remaining
2 half-lives = 25% remaining
3 half-lives = 12% remaining
4 half-lives = 6% remaining
5 half-lives = 3% remaining

Half-lives can vary widely based on environmental factors. The amount of chemical remaining after a half-life will always depend on the amount of the chemical originally applied. It should be noted that some chemicals may degrade into compounds of toxicological significance.

Environmental Fate:

  • Chlorpyrifos is stable in soils with reported half-lives ranging between 7 and 120 days. Studies have found chlorpyrifos in soils for over one year following application. Soil persistence may depend on the formulation, rate of application, soil type, climate and other conditions. 4,11,54 See the text box on Half-life.
  • Chlorpyrifos bound to soil may be broken down by UV light, chemical hydrolysis, dechlorination, and soil microbes. 11,54
  • Chlorpyrifos binds strongly to soils, is relatively immobile, and has low water solubility. In contrast, its degradate TCP adsorbs weakly to soil particles and is moderately mobile and persistent in soils. 4,11
  • The major degradates of chlorpyrifos found in soils are similar to the metabolites created by plants and animals. The degradates are formed by oxidative dealkylation or hydrolysis to diethyl phosphates and TCP. 54
  • In a study of seven aerobic soils ranging in texture from loamy sand to clay, with soil pH values from 5.4 to 7.4, the soil halflife for radiolabeled chlorpyrifos ranged from 11 to 141 days. After 360 days, researchers detected carbon dioxide (27-88%), TCP (up to 22%), and small amounts of 3,5,6-trichloro-2-methoxypyridine (&le8%) in the soil. 4,11
  • In medium-textured soils in field conditions in California, Illinois and Michigan, the half-lives reported for chlorpyrifos ranged from 33 to 56 days. 4
  • Chlorpyrifos is less persistent in soils with a higher pH. 4,11
  • Volatilization of chlorpyrifos from soil is not likely. According to a laboratory volatility study, carbon dioxide appears to be the major volatile degradate of chlorpyrifos. In this study, less than 10% of chlorpyrifos applied to soil volatilized within 30 days after application. 4


  • Chlorpyrifos does not partition easily from soil to water. Therefore, chlorpyrifos found in runoff water is likely a result of soil-bound chlorpyrifos from eroding soil, rather than from dissolved chlorpyrifos. 4
  • Volatilization of chlorpyrifos from water is the most likely route of loss for chlorpyrifos, with volatilization half-lives of 3.5 and 20 days estimated for pond water. 11
  • During midsummer, the photolysis half-life of chlorpyrifos in water is between three and four weeks. 11
  • The rate of hydrolysis for chlorpyrifos increases with temperature and alkalinity. Half-lives ranging from 35 to 78 days have been reported in water with a pH of 7 and a temperature of 25 °C. 11
  • The U.S. EPA conducted an analysis of well-monitoring data from the United States Geological Survey's (USGS) National Water Quality Assessment (NAWQA) Program database and the EPA's Pesticide Ground Water Database. Chlorpyrifos was detected in less than 1% of the more than 3000 wells sampled. The majority of the water concentrations reported were less than 0.01 ppb, with a maximum concentration of 0.65 ppb. Groundwater contamination could be significantly higher in areas treated with a termiticide containing chlorpyrifos, especially if contamination of a well occurs. 5
  • The U.S. EPA also analyzed NAWQA data for surface water contamination. A total of 1530 agricultural streams and 604 urban streams were tested. Of the streams tested, 15% of the agricultural streams and 26% of the urban streams contained chlorpyrifos at concentrations ranging from 0.026 ppb to 0.400 ppb. However, monitoring data were not collected for the watersheds where chlorpyrifos use is pervasive. 5 See the NPIC fact sheet on Pesticides in Drinking Water.
  • Researchers monitored concentrations of chlorpyrifos in outdoor air following ground application of chlorpyrifos in an agricultural setting. Air was sampled for chlorpyrifos and chlorpyrifos-oxon over a four week period during late spring, 24 hours a day, five days per week. Monitoring stations were located within three miles of average daily chlorpyrifos applications of 7.7 pounds per square mile per day. Median air concentrations of chlorpyrifos and chlorpyrifos-oxon were measured at 33 ng/m 3 and 22 ng/m 3 , respectively. 55
  • Chlorpyrifos reacts with photochemically-produced hydroxyl radicals in the atmosphere and degrades to chlorpyrifosoxon. An atmospheric vapor half-life of 4.2 hours has been estimated for this reaction. 56 In one study, researchers estimated an outdoor air residence time of 4 and 11 hours for chlorpyrifos and chlorpyrifos oxon, respectively. However, these calculations are based on approximate hydroxyl radical concentrations in a specific geographical area. 57


  • Chlorpyrifos is not expected to be taken up from soil through the roots of plants. 2
  • Chlorpyrifos was applied to the leaves and fruit of orange and grapefruit trees, and residues and dissipation on the rinds were measured using gas chromatography. Chlorpyrifos residues on fruit rinds were found to dissipate quickly, with initial mean half-lives of 2.8 days in oranges and 3.7 to 6.7 days in grapefruit, at which point residues were at or below 2 ppm. Chlorpyrifos residues were not found above levels of detection (0.03 ppm) in the edible portion (pulp) of citrus fruit tested. 58
  • Though some chlorpyrifos may be taken up by plants through leaf surfaces, much of the applied chlorpyrifos is usually lost from volatilization, and very little is translocated throughout the plant. 54 Chlorpyrifos taken up by plant tissues is primarily metabolized to TCP, which is then stored as glycoside conjugates. 2,54
  • Foliar applied chlorpyrifos on leaf surfaces is lost primarily by volatilization. 54
  • Studies report chlorpyrifos residues remain on plant surfaces for 10 to 14 days after application. 11
  • Although most of the chlorpyrifos applied to plants is lost through volatilization or converted to TCP and sequestered, desulfuration to the chlorpyrifos oxon on plant surfaces has been reported. 54


  • Several studies have reported detections of chlorpyrifos in dust, air, carpets, and on surfaces within indoor environments. 59,60,61
  • Research was conducted as part of the Children's Total Exposure to Persistent Pesticides and Other Persistent Organic Pollutants (CTEPP) study to evaluate the potential exposures of preschool children to chlorpyrifos and TCP in their homes and at day care centers in North Carolina. Monitoring of residues was performed at 129 homes and 13 day care centers, and included indoor and outdoor air, indoor floor dust, duplicate meals, transferable residues and surface wipe samples from floors, food preparation surfaces and children's hands. Urine was also collected from children by caretakers. Chlorpyrifos was detected in the indoor floor dust and indoor air at all locations. Median amounts of TCP were 12 and 29 times higher than those of chlorpyrifos in solid food at homes and daycare centers, respectively. Mean chlorpyrifos levels in homes were 19 ng/m 3 in indoor air, 413 ng/g in indoor floor dust and 0.6 ng/g in solid food. Mean chlorpyrifos levels in day care centers were 8.2 ng/m 3 in indoor air, 237.0 ng/g in indoor floor dust, and 0.2 ng/g in solid food. 61
  • Researchers detected chlorpyrifos in all seven homes tested in a New Jersey study. Concentrations in dust ranged from 0.053 ppm to 15.00 ppm. The highest indoor air concentrations detected were between 151.2 ng/m 3 and 154.2 ng/m 3 . 59 The air samples with detectable levels of chlorpyrifos were correlated with dust samples that contained the highest levels of chlorpyrifos. 59
  • In another study, researchers tested the indoor air and surfaces of ten urban residences in New Jersey. Chlorpyrifos residues were measured in samples of air and from non-target surfaces including plush toys, smooth surfaces, furniture, windowsills and flooring after the homes were treated with a water emulsion crack and crevice formulation containing 0.25 to 0.50% chlorpyrifos. Chlorpyrifos was detected in all homes within the treated areas throughout the two week post-application period. The highest concentrations of chlorpyrifos detected were 816 ng/m 3 in air, 24.6 ng/m 3 on non-target surfaces, and 1949 ng per toy on plush toys. 60 See the NPIC fact sheet on Pesticides in Indoor Air of Homes - Technical.

Food Residue

  • The United States Department of Agriculture (USDA) Pesticide Data Program collects data on pesticide residues in foods and compiles an annual report of the findings. The 2007 annual summary reported 9734 samples of fruit and vegetable commodities tested for chlorpyrifos residues. Chlorpyrifos was detected in 339 (3.48%) of these samples. 62
  • Chlorpyrifos residues were found in 18.0% of peaches tested (100 detections), in 15.8% of nectarines tested (89 detections), in 6.8% of broccoli tested (50 detections) and in 5.2% of kale greens (20 detections). Chlorpyrifos residues were also monitored in almonds (46% of samples tested, 166 detections) and corn grain (30% of samples tested, 195 detections). 62
  • Chlorpyrifos was detected at levels exceeding the U.S. EPA tolerance in one sample each of collard greens (353 samples, 10 with detectable residues) and summer squash (742 samples, 5 with detectable residues). In collard greens, residues were detected in one sample at 6.3 ppm (tolerance of 2.0 ppm). In summer squash, residues were detected in one sample at 0.33 ppm (tolerance 0.10 ppm). 62

Ecotoxicity Studies:


  • Chlorpyrifos is very highly toxic to common grackles (Quiscalus quiscula) and ring-necked pheasants (Phasianus colchicus) with an LD50 of 5.62 mg/kg and 8.41 mg/kg, respectively. Chlorpyrifos is highly toxic to common pigeons (Columba livia) and house sparrows (Passer domesticus) with an LD50 of 10 mg/kg. 4
  • Chlorpyrifos is highly toxic to chickens with an oral LD50 ranging from 32-102 mg/kg. 2
  • Chlorpyrifos is moderately toxic to mallard ducks (Anas platyrhynchos) with an acute oral LD50 of 490 mg/kg. 2
  • The American robin (Turdus migratorius) is the most frequently reported avian species killed in field incidents with chlorpyrifos. 4 Currently the acute LD50 for the American robin is unknown.

Fish and Aquatic Life

  • Chlorpyrifos is very highly toxic to aquatic invertebrates, freshwater fish, and other estuarine and marine organisms. 11
  • The 96-hour LC50 is 0.007-0.051 mg/L for rainbow trout (Oncorhynchus mykiss), 0.002-0.010 mg/L for bluegill sunfish (Lepomis macrochirus), and 0.12-0.54 mg/L for fathead minnows (Pimephales promelas). 2
  • The 48-hour LC50 for Daphnia is 1.7 &mug/L. The LC50 for Korean shrimp (Palaemon macrodactylus) is 0.05 &mug/L. 2
  • There is potential for chlorpyrifos to bioaccumulate in the tissues of aquatic species. 1 Residues of chlorpyrifos found in fish tissue included the metabolites TCP and two glucuronide conjugates of TCP. 4 Researchers exposed various fish species to chlorpyrifos continuosly during early development, and calculated bioconcentration values ranging from 58 to 5100. 63

Terrestrial Invertebrates

  • There are data gaps in terrestrial risk assessment due to a lack of quantitative methods available to assess risks posed by dermal and inhalation exposures for wildlife. 4
  • Chlorpyrifos is highly toxic to bees. The honey bee (Apis sp.) oral LD50 is 360 ng/bee. 2 Contact LD50s for honey bees of 59 and 70 ng/bee have been reported. 2,4
  • The 14-day LC50 for worms (Eisenia foetida) is 210 mg/kg chlorpyrifos in soil. 2
  • Foliar residues from spray applications of 0.5 and 1.0 lbs active ingredient/acre demonstrated toxicity to non-target insects for up to 24 hours post-treatment. 4 See the NPIC fact sheet on Wildlife and Pesticides.

Regulatory Guidelines:

  • The acute Reference Dose (RfD) for chlorpyrifos is 5 x 10 -3 mg/kg/day. 1 See the text box on Reference Dose (RfD).

Reference Dose (RfD): The RfD is an estimate of the quantity of chemical that a person could be exposed to every day for the rest of their life with no appreciable risk of adverse health effects. The reference dose is typically measured in milligrams (mg) of chemical per kilogram (kg) of body weight per day.

CBSE Class 12 Biology –Chapter 1 Reproduction in Organism- Study Materials

Find CBSE Class 12th Biology notes for the chapter Reproduction in Organisms. Every concept is followed by the solved and unsolved questions. You can also find the various categories questions like short, very short and long answer type

Find CBSE Class 12th Biology notes for the chapter Reproduction in Organisms. Every concept is followed by the solved and unsolved questions. You can also find the various categories questions like short, very short and long answer type.

Some important terminologies and concepts are given below:

Reproduction-As a biological process in which an organisms gives rise to young ones similar to itself.

Clone-Morphologically and genetically similar individuals is called clone.

Binary Fission-Many single celled organisms reproduce by binary fission, where a cell divides in two halves and each rapidly -grows into adult.

e.g. Amoeba, Paramecium and yeast.

Vegetative Propagation-In plant the formation of vegetative propagation does not evolve two parent, the process Involved in a sexual so is called vegetative propagation.

Monoceious – Homothelics like cucurbits, Bisexual Coconut, Chara.

Dioecious – Heterothelic and Unisexual Papaya, date Palm, marchantia.

Parthenogenesis – When the female gamete undergoes development to form new organisms without fertilization, This Phenomenon is called parthenogenesis e.g. Rotifers, honeybees, lizards and birds (turkey).

Gametogenesis – The process of formation of the two type of gametes male and female. The male gamete is called the aniherozoid or sperm and female gamete is called egg or ovum.

Spermatogenesis – lt is the process of formation of haploid spermatozoa from diploid male germ cells of the testes.

A spermatogonium produces four spermatozoa (sperms).

Oogenesis – It is the process of formation of haploid ova from the gamete’ mother cells (oogania) in the ovany. An oogonium form only one ovum.

Lets discuss some unanswered questions from this chapter

Q1.Explain – in asexual reproduction a part of organism separates and forms a new organism?

Q2.Name the parasite which causes black fever

Q3 .How does reproduction take place by multiple fission?

Q4. What do you understand by the term daughter nuclei?

Q5.Colonies of yeast do not multiply in water but they do multiply in sugar solution. explain

Q6.How does bread mould formation takes place

Q7.Regeneration and its function. Why complex organisms cannot reproduce though regeneration?

Lets discuss 2 marks long answer questions from this chapter

Q. 1. Draw a labelled diagram of conidia of penicillium.

Q. 2. Diagrammatically represent the asexual reproduction in yeast.

Q. 3. What is fission ? Name the type of fission.

Ans. Fission : It is a type of sexual reproduction in which a fully grown parental organism divides into two or more than two daughters.

Q. 4. Diagrammatically represent the asexual reproduction in Amoeba ?

Q. 5. Write the two significance of vegetative propagation ?

Ans. (i) Characters of the parents plants are preserved. A good variety produced can be propagated by vegetative propagation?

(ii) It is easier and cheaper method of propagation.

Q. 6. When artificial methods of vegetative propagation are utilised ?

Ans (i) For quick production of new plants.

(ii) For combining good qualities of two different varieties.

Q. 7. Write the Zoological name of given animals.

Rhesus monkey, Goat, Guinea, Pig, Ascaris.

Q. 8. Identify each part and write whether it is haploid (n) or diploid (2n) Ovary, Anther, Egg, Pollen, male gamete and Zygote.

Q. 9. If the Chromosome numbers in meiocytes of human beings, rat, elephant, rice, butterfly and onion. 46, 42, 56, 24, 380, and 32 respectively. What will be the chromosome number in gametes of these species.

Ans. Human beings = 23

Lets discuss 3 marks answer questions from this chapter

Q. 1. In extreme summer and winter, certain animals like frogs and hazards abandon active life.

This is popularly called summer sleep and winter sleep respectively.

(i) What are the technical terms for summer sleep and winter sleep?

(ii) State any two changes in the body that occur during the above mentioned dormant stale.

Ans. Technical terms are as follows :

Winter sleep – Hibernation

Summer sleep — Aestivation

(i) Rate of metabolism declines

(ii) Respiration takes place through skin only (Cutaneous respiration).

Q. 2 Show by a series of diagrams the manner of regeneration in a hydra if it is cut into two pieces transversely at the, middle.

Show by a series of diagrams the manner of transverse binary fission in Planaria.

Q. 3. What is vegetative propagation? Give two suitable examples.

Ans. Vegetative propagation in plants is a, type of asexual reproduction in which new individual appears from any vegetative parts of parents plant.

In plants, The units of vegetative propagation such as runner, rhizome, sucker, tuber, offset, bulb are all capable of giving rise to new offsprings.

Such structures are called vegetative propagules.

Examples of vegetative propagation are:

(i) Vegetative propagation by leaves, in Bryophyllum.

(ii) Vegetative propagation by Stem (rhizome) in Ginger, Turmeric etc.

Q. 4. Define the following terms:

(i) Juvenile Phase,

(ii) Reproductive Phrase

(iii) Senescent Phase

Ans. (i) Juvenile Phase- The period of growth and maturity in life when then organisms reproduce sexually. This period of growth is called Juvenil Phase, or Vegetative phase.

(ii) Reproductive Phase-The end of Juvenile Phase is indicated by many of them showing morphological and physiological changes prior to active reproductive behaviour,

(iii) Senscent Phase-The end of reproductive phase is one of the parameters of senescene or old stage. At this stage body metabolism is shown down and ultimately this stage leads to death.

Q. 5. Explain why meiosis and gamete genesis are always Inter linked ?

Ans. Gametogenesis refers to the process of formation of two types of gametes male and female. Gametes are haploid cells. Gametes in all heterogametic species are of two types i.e., male and female. A haploid parent produced gametes by mitosis.

But in majority of organisms, Parent body is diploid. Thus meiosis is required by such diploid. Organisms, to produce haploid gametes. In haploid organisms meiocytes undergo meiosis. At the end of meiosis only one set of chromosome is incorporated into each gametes.

Let’s discuss 5 marks long answer questions from this chapter

Also Read: CBSE Class 12th Solved Question Papers PCB – eBook

Q. 1. Define fission, what are two types of fusion?
Describe multiple .fission irt Amoeba and Binary fission.

Ans. Fission is a type of sexual reproduction in which a fully grown parental organism divides into two or more than two daughters. It is of two type.

(i) Binary fission-A type of asexual reproduction in which the Parental Organisms divides in two daughters cells during favourable conditions.

(ii) Multiple fission-A type of sexual reproduction in which the Parental Organisms Produces many daughter cells simultaneously during unfavourable conditions.

Q. 2. The- unicellular organisms which reproduce by binary fission are causidered immortal Instify.

Ans. Asexual reproduction is common among single-celled organisms. Thus in these organisms cell division is itself a mode of reproduction. In binary fission. The parent body divides into two halves and each rapidly growing into an adult. There is no remains of parents body cell and parent cannot be said to have dead. In fact after binary fission, parent continues living as two daughter- individuals. Hence, the unicellular organisms which undergo binary fission are considered immortal. Example-Amoeba and Paramecium.

There are some small and I marks questions based on this chapter

Q. 1. What is life Span ?

Ans. The period between birth to the natural death of an organisms is known as life span.

Q. 2. Define reproduction.

Ans. Reproduction is a biological process by which an organisms produces another organisms of its own kind.

Q. 3. Name two monoecious plants.

Ans. Cucurbita and Coconuts.

Q.4. Name two dioecious plants.

Ans. Papaya and date palm.

Q. 5. Name one bisexual and one unisexual animal.

Ans. Bisexual – Earthworm. Unisexual – Cockroach.

Q. 6. What are post-fertilisation events?

Ans. The post-fertilisation events included:
1. Formation of diploid zygote.
2. Development of embryo from zygote by the process called embryogenesis.

Q. 7. “Amoeba is immortal”. Explain.

Ans. Amoeba is considered immortal because it does not undergo natural death.

Q. 8. Which is the most critical event in Sexual Reproduction.

Ans. Fertilisation of gametes is the most critical event in Sexual Reproduction.

Q. 9. Give two example which reproduce a sexually by binary fission.

Ans. Bacteria and Amoeba reproduce by binary fission.

Q. 10. What are the Vegetative Propagules.

Ans. In plants, the units of vegetative propagation, such as tuber, bulb, rhizome etc. are called vegetive propagules.

Q. 11. Why offsprings of Oviperous animals are at a greater risk as compared to offsprings of viviparous animals.

Ans. Oviparous animals lay eggs in a safe place in the environment. In an open area, the eggs are not always safe and the offsprings are always at a risk.

Q. 12. How many type of natural vegetative reproduction take place in flowering plants?

Ans. The natural vegetative reproduction takes place by modified tuberous roots having adventitious buds, under ground modified stems, creeping stems, leaves, balbs and turions.

Q. 13. Name the artificial means of vegetative reproduction.

Ans. The artificial means of vegetative reproduction are cutting, layering, grafting and micropropagation.

Watch the video: Gorilla Mating. Mountain Gorilla. BBC Earth (January 2023).