CBSE Class 12 Biology Chapter 13: Organisms and Populations NCERT Solutions

Solution: Diapause and hibernation are both strategies organisms use to survive unfavorable environmental conditions, but they differ significantly:

Diapause: This is a specific stage of suspended development that allows certain organisms, particularly many species of zooplankton and insects, to survive periods of adverse climatic conditions. During diapause, the organism's development is temporarily halted, enabling it to 'wait out' the unfavorable period and resume growth when conditions improve.

Hibernation: Also known as winter sleep, hibernation is a resting state adopted by some animals, like bats, squirrels, and rodents, to escape the harshness of winter (cold temperatures). During hibernation, animals reduce their metabolic rate, heart rate, and body temperature, often hiding in sheltered locations to conserve energy and survive the cold season.

Solution: It is highly unlikely that a marine fish will survive if placed in a freshwater aquarium. Here's why:

Marine fish are adapted to live in environments with a high salt concentration (hypertonic to their body fluids). Their bodies have evolved mechanisms to cope with this saline environment, primarily by conserving water and excreting excess salts.

Freshwater, on the other hand, is hypotonic compared to the body fluids of a marine fish. When placed in freshwater, water will tend to move into the fish's body cells and tissues via osmosis, down its concentration gradient. The marine fish lacks the physiological adaptations to effectively prevent this excessive influx of water. This continuous osmotic uptake of water can cause the fish's cells to swell, disrupt its internal balance, and ultimately lead to death.

Solution: Phenotypic adaptation refers to the observable physical or physiological characteristics of an organism that have evolved over time to help it survive and reproduce in its specific natural environment. These adaptations are a result of the organism's genetic makeup interacting with environmental pressures.

Examples:

• Desert Plants: Many desert plants have developed adaptations like thick, waxy cuticles on their leaves to reduce water loss through transpiration. They also often possess sunken stomata, which are pores that open inwards, further minimizing evaporation.
• Elephants: Elephants have large, thin ears with a rich network of blood vessels. These ears help them dissipate excess body heat, acting as effective thermoregulators in hot climates.

Solution: The microbes that can survive in habitats with temperatures exceeding 100°C are typically a group of ancient bacteria known as Archaebacteria, specifically those called thermophiles. These organisms thrive in extreme environments like hot water springs and deep-sea hydrothermal vents.

Their ability to withstand such high temperatures is due to specialized adaptations at the molecular level. Thermophiles possess unique enzymes that are highly resistant to heat denaturation. These thermo-stable enzymes can continue to catalyze essential metabolic reactions even under extreme heat conditions, allowing the organism to maintain its life processes without its cellular machinery breaking down.

Solution: A population is defined as a group of individuals of the same species living in a particular geographical area at a specific time and capable of interbreeding. While individual organisms have characteristics like metabolism, growth, and response to stimuli, populations possess certain unique attributes that describe the group as a whole. These population-level attributes include:

Birth Rate (Natality): This refers to the number of births occurring per unit time in a population, relative to its size. It represents the addition of new individuals.

Death Rate (Mortality): This is the number of deaths occurring per unit time in a population, relative to its size. It represents the loss of individuals.

Sex Ratio: This describes the proportion of males and females within a population, often expressed as the number of females per thousand males or vice versa.

Age Distribution: This is the distribution of individuals across different age groups (e.g., pre-reproductive, reproductive, post-reproductive) within a population. It significantly influences the population's growth potential.

Population Density: This is the number of individuals of a population present per unit area or volume at a given time.