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 an organism to avoid periods of adverse conditions. It is a physiological state where growth and development are temporarily halted. Many species of zooplankton and insects exhibit diapause to survive harsh climatic periods, such as extreme cold or drought, often occurring during their larval or egg stages.

Hibernation: Also known as 'winter sleep,' hibernation is a state of prolonged inactivity and significantly reduced metabolic rate that animals enter to survive cold winter months. During hibernation, body temperature, heart rate, and respiration decrease drastically. Animals like bats, squirrels, and other rodents hibernate to conserve energy when food is scarce and temperatures are low.

In essence, diapause is about pausing development, while hibernation is about entering a dormant, low-energy state.

Solution: A marine fish is unlikely to survive if placed in a freshwater aquarium. This is due to the fundamental differences in osmoregulation required by marine and freshwater environments:

Marine fish live in saltwater, which has a much higher salt concentration (is hypertonic) than their body fluids. Consequently, they tend to lose water to their surroundings and gain salts. To survive, they actively drink water and excrete excess salts through specialized cells in their gills and kidneys.

Freshwater, on the other hand, is very dilute (is hypotonic) compared to the body fluids of a marine fish. If a marine fish is placed in freshwater, water will rapidly enter its body through osmosis, and salts will diffuse out. The fish's body is not equipped to handle this massive influx of water and loss of salts. Without the ability to efficiently excrete excess water and conserve salts, the fish's cells would swell, disrupting bodily functions and ultimately leading to death.

Solution: Phenotypic adaptation refers to the observable physical or biochemical characteristics of an organism that have evolved through natural selection to enhance its survival and reproduction in its specific environment. These adaptations are expressed through the organism's phenotype (its physical traits and functions) and can involve changes in morphology (structure), physiology (function), or behavior.

Example 1: Desert plants often exhibit phenotypic adaptations to minimize water loss. They may have thick, waxy cuticles on their leaves to reduce evaporation and sunken stomata (pores) that open only at night to conserve water.

Example 2: Elephants have large ears with a rich blood supply. These ears help dissipate excess body heat, acting as thermoregulators in hot climates, which is a physiological adaptation.

Solution: Some microbes, particularly certain types of Archaebacteria known as thermophiles, are uniquely adapted to survive in extremely high-temperature environments, such as hot springs and deep-sea hydrothermal vents, where temperatures can exceed 100°C.

Their survival is attributed to specialized biochemical adaptations. These organisms possess unique enzymes that are structurally stable and functional at high temperatures. Unlike enzymes from mesophilic organisms (which thrive at moderate temperatures), these thermo-resistant enzymes do not denature (lose their shape and function) when exposed to extreme heat. This allows essential metabolic processes to continue even under such harsh conditions.

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 individuals have characteristics like age, sex, and genetic makeup, populations exhibit 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 of population per unit of time. It determines the increase in population size.
• Death Rate (Mortality): This refers to the number of deaths occurring per unit of population per unit of time. It determines the decrease in population size.
• Sex Ratio: This is the proportion of males and females in a population, often expressed as the number of females per thousand males or vice versa.
• Age Distribution: This describes the proportion of individuals in different age groups (e.g., pre-reproductive, reproductive, post-reproductive) within a population. Age structure significantly influences population growth potential.
• Population Density: This is the number of individuals of a population per unit area or volume at a given time.

These attributes help ecologists understand and predict population dynamics and their interactions with the environment.