Short & Long Question Answers with Solution: Organisms and Populations | Biology Class 12 - NEET PDF Download

Short Answer Type Questions

Q1: Explain how kangaroo rats live in the deserts of North America in the absence of water.
Ans: The kangaroo rat in the North American deserts can fulfill its water requirements by internally oxidizing fat, which generates water as a byproduct. Additionally, it has the ability to reduce the volume of its urine significantly.

Q2: Write four attributes applied in the human population census.
Ans: They are as listed below:

• Sex ratio
• Age distribution
• Natality and mortality
• Population growth

Q3: What is the difference between diapause and hibernation?
Ans: The phenomenon where organisms endure adverse weather conditions during their development is referred to as diapause. Hibernation, on the other hand, is the process by which cold-blooded animals enter a state of winter dormancy, seeking shelter in places like hollow tree trunks, tunnels, caves, and so on, with reduced physiological activity.

Q4: What is a zero population growth rate?
Ans: This occurs when there is a balance in the number of entities in the pre-reproductive age group compared to those in the post-reproductive and reproductive stages.

Q5: How do lizards in the desert maintain a constant temperature?
Ans: Desert lizards can endure extreme heat by using behavioral methods to maintain a relatively stable body temperature. When their body temperature falls below their comfort zone, they bask in the sun to absorb heat, and when it rises, they seek shade.

Q6: Define a tree line.
Ans: This is the specific habitat boundary where trees can thrive. Beyond this point, factors like higher altitudes, colder climates, or insufficient moisture prevent the growth of trees, and if they do grow, they exhibit limited and stunted growth.

Q7: What is Gauss’s competitive exclusion principle?
Ans: As per Gauss's competitive exclusion principle, two closely related species in competition for the same resources cannot coexist because the one with a competitive disadvantage will ultimately be eliminated. Nonetheless, this principle applies solely when resources are limited and not in other situations.

Q8: Name the kind of interaction built by a termite feeding on wood and the other is a protozoan Trichonympha residing in the gut of the termite.
Ans: Termites provide nourishment and a habitat for Trichonympha, a protozoan that resides in their gut and helps in digesting the wood termites consume. If the protozoan is not present, the termite cannot digest wood and consequently dies. This interaction represents mutualism.

Q9: Give examples of defence mechanisms in plants against herbivory.
Ans: Plants develop distinctive physical defense mechanisms, like the thorns on bougainvillea and the spines on cacti. Some compounds produced and stored by plants either directly eliminate insects or deter them from feeding.

Q10: Why do marine entities lack contractile vacuoles as compared to freshwater entities?
Ans: Contractile vacuoles play a role in osmoregulation by assisting freshwater organisms in managing their internal water balance. Freshwater organisms are typically in a hypertonic environment, causing water to constantly enter their cells. Contractile vacuoles collect and expel the excess water, helping to maintain a stable internal environment. However, in marine organisms, where the environment has high salinity, contractile vacuoles are unnecessary.

Long Answer Type Questions

Q1: Is the distribution of organisms affected by light? Explain briefly with suitable examples of animals or plants.
Ans: Plants depend on sunlight for photosynthesis, which in turn influences their distribution. For instance, numerous shrubs and herbs are found in forested areas and have adaptations that allow them to perform photosynthesis in low-light conditions. As a result, they thrive in shaded regions beneath tall, canopied trees. Areas with ample sunlight tend to have larger trees. Some plants have evolved to grow vertically to capture the maximum amount of sunlight. These plants often have smaller leaves compared to individuals of the same species growing in better-lit areas. Keep an eye out for more intriguing concepts in biology.

Q2: Mention the various defensive techniques that lessen the effects of predatory behaviour.
Ans: Plant species have evolved various defense mechanisms to reduce the impact of predation. Some examples include:

• Camouflage or cryptic coloration in certain insect species and frogs to avoid detection by predators.
• The development of specific chemical compounds that make animals like monarch butterflies unpalatable to predators. These chemicals are acquired by consuming noxious weeds during their caterpillar stage.
• Some prey species are poisonous, deterring predators from consuming them.
• Plants have developed both morphological and chemical defenses against herbivores, such as the presence of thorns on bougainvillea.
• Production of various compounds that serve multiple purposes:
  i. Making animals ill if consumed.
  ii. Acting as deterrents to prevent animals from eating them.
  iii. Hindering digestion in potential herbivores.

Q3: Name the element that causes salinity in the soil. Explain the state at which soil turns saline.
Ans: Soil salinity results from improper irrigation practices that lead to an accumulation of salt content in the soil. Salinization is the term used to describe the process of increasing salt levels in the soil. Here are the causes of soil salinity:

• Inadequate irrigation methods that use water with a high salt concentration can lead to the accumulation of salts in the soil due to evaporation.
• The application of fertilizers to crops can contribute to soil salinity, as fertilizers often contain potassium, which can naturally form salt (sylvite).

This salinity has detrimental effects on both vegetation and the soil. The optimal pH range for soil is typically between 2.2 and 9.7, and any pH level above this range can lead to soil deterioration due to salinity.

Q4: Describe the population’s logistic growth model and provide a suitable curve. Explain the reason why this curve is more realistic.
Ans: The logistic growth curve exhibits an S-shaped pattern and consists of three distinct stages:

• Lag-phase: This initial stage is characterized by minimal to no growth. During the lag phase, the underpopulated cells adapt to or stabilize with the growth conditions before they begin to multiply.
• Exponential phase or log phase: This is the middle stage characterized by rapid and geometric growth. Once the small population of cells becomes stable, those that have already started reproducing continue to do so rapidly because there is an abundance of resources like food and other necessities for survival.
• Stationary phase or steady phase: This phase occurs shortly after the available food supply decreases in proportion to the number of cells. Consequently, there is no significant increase in the number of cells during this phase as the number of newly produced cells becomes nearly equal to the number of cells that die off.

Q5: Describe the different kinds of positive interactions that take place between different species.
Ans: There are three main types of interspecific interactions:

• Positive or Beneficial Interaction:
  • Mutualism: In mutualism, both interacting species benefit. For example, lichens represent a mutualistic relationship between a fungus and cyanobacteria. Another example is the mycorrhizal association between plant roots and fungi, where fungi assist in nutrient absorption from the soil while receiving carbohydrates from the plants.
  • Commensalism: In commensalism, one species benefits without affecting the other. For instance, orchids growing on a tree branch as epiphytes or barnacles attaching to the back of a whale.
• Negative Interaction:
  • Scavenging: Scavenging occurs when scavengers feed on dead remains. Examples include fungi and bacteria decomposing the carcasses of animals.
• Neutral Interaction:
  • Protocooperation: Protocooperation is a form of interaction where both species benefit mutually, and they can exist independently of each other.

These interactions play important roles in ecological relationships and the functioning of ecosystems.

Q6: Co-evolution of mutualists is a common aspect of mutualism. Explain this phenomenon, taking the wasp fig as an illustration of an animal-plant connection.
Ans: Animals play a crucial role in assisting plants by pollinating their flowers and aiding in the dispersal of their seeds. It is often suggested that animals should be rewarded for their contributions to plants. According to the study, "Plant-animal interactions typically involve co-evolution between mutualists." This means that the evolution of flowers and their pollinators is often so closely intertwined that a particular flower species can only be pollinated by a specific partner wasp species, creating a mutualistic relationship.
In this mutualism, plants provide rewards or compensation in the form of seed dispersal. For example, female wasps not only use the fruit of a plant as a site to lay their eggs but also utilize developing seeds within the fruit to feed their larvae.
The process involves the wasp pollinating the fig inflorescence while searching for suitable egg-laying sites. In return for pollination, the fig tree provides the wasp with some of its developing seeds as food for the wasp larvae. This mutually beneficial relationship illustrates the intricate and interdependent nature of plant-animal interactions in ecosystems.

Q7: Explain the adaptations of the animal parasites while living in and on the host species with suitable examples.
Ans: Here are some adaptations observed in various organisms:

• Reduction of Unwanted Sense Organs: Certain organisms like mites, fleas, and lice have adapted to have fewer or no unnecessary sensory organs. For instance, some of them may lack wings.
• Suckers for Attachment: Organisms like leeches and tapeworms have developed suckers that enable them to firmly attach to their host.
• Absence of Digestive System: Tapeworms are an example of organisms that have adapted to lack a digestive system.
• Increased Reproductive Capacity: Some organisms, like roundworms, have evolved to have a high reproductive capacity, producing a large number of offspring.

Q8: List and explain the important characteristics of a population.
Ans: These are the four primary characteristics of a population:

• Population Density: Population density measures the size of a population in relation to a specific unit of space. The upper limit of density is determined by factors like metabolic equilibrium, nutritional resources at trophic levels, and energy flow within an ecosystem. Population density can be calculated using the following formula: Population Density = Birth Rate (Natality) - Death Rate (Mortality)
• Birth Rate (Natality): The birth rate, also known as natality, quantifies the number of newborns produced by natural processes within a given timeframe.
• Death Rate (Mortality): The death rate is a measure of the mortality rate within the population, indicating the number of individuals who die within a specific time period.
• Carrying Capacity: Carrying capacity refers to the maximum number of individuals of a species that a given area or ecosystem can support based on its size and productivity. It represents the limit to the population size that an environment can sustain.

Q9:  Describe how the root system, stem, and leaves of xerophytes are specifically adapted.
Ans: Xerophytes, which are plants adapted to arid environments, exhibit several adaptations:

• Root Adaptations:
  • Lengthy and Well-Developed Roots: Many xerophytes have extensive root systems that are heavily branched. These roots are efficient in searching for water deep within the soil.
  • Deep but Shallow Root Systems in Succulent Perennial Xerophytes: Some succulent perennial xerophytes have unique root systems that are both deep and shallow. These roots can capture water from dewdrops and small raindrops, helping the plant thrive in arid conditions.
  • Phylloclades in Woody Xerophytes: Woody xerophytes, like acacia trees, may have stunted, rigid stems. In some cases, the main stem and branches are modified into thick, fleshy, flattened, and green structures known as phylloclades. These structures help reduce water loss through transpiration.
• Leaf Adaptations:
  • Short Leaves: Xerophytes often have shorter leaves, which reduces the surface area exposed to intense solar radiation. This adaptation helps prevent overheating and lowers the rate of transpiration, conserving water.
  • Thick, Fleshy, and Leathery Leaves: The leaves of many xerophytes are thick, fleshy, and leathery in texture. These characteristics allow the leaves to store water, providing a reserve during dry periods and reducing the plant's dependence on frequent water uptake.

These adaptations enable xerophytes to survive and thrive in arid and water-scarce environments by efficiently utilizing available water and minimizing water loss.

Q10: Define brood parasitism and the type of adaptation that has evolved in this phenomenon.
Ans: To prevent host birds from recognizing and expelling foreign eggs from their nests, evolution has led to the development of a strategy where the parasite bird's eggs closely resemble the host bird's eggs in terms of size, color, and other characteristics. An example of this is when a cuckoo bird lays its eggs in a crow's nest. This relationship is considered a parasitic form of interspecific interaction.
In this parasitic interaction, the parasite, represented by the cuckoo bird eggs, relies on the crow's nest for food and shelter. However, the crow is negatively affected because competition arises for limited food and shelter resources between the crow's own eggs and the cuckoo's eggs. Therefore, in a parasitic interspecific interaction, the parasite benefits while the host is harmed, illustrating the imbalanced nature of this relationship.