Solved Exercise, Bio-12, Ch-26


(i) Water is slower to heat and ______ than air. (cool)

(ii) The distribution of life in lakes depends on access to ______, and to place for attachment. (light)

(iii) Ecosystem on land is also known as ______ ecosystem. (terrestrial)

(iv) Ecosystem in water is also called as ______ ecosystem. (aquatic)


(i) The soil or terrestrial ecosystem have some adaptations for animals and plants:

(a)  Supporting tissues

(b)  Retention of food

(c)  Temperature

(d) Nutrient

EXPLANATION: Both plants and animals have evolved supporting tissues like vascular bundles (xylem-phloem) in plants and skeleton in animals to support them on land against the force of gravity.

(ii) Most plants fit only into a few ecosystems. Which type of plants seems in ecosystem of green land?

(a)  Trees

(b)  Shrubs

(c)  Perennial herbs

(d) Annual weeds

EXPLANATION: Trees are the most fitting type of plants for green land ecosystems due to their height, woody structure, and significant canopy cover. They play a key role in shaping the ecosystem, providing habitat, shade, and contributing to overall biodiversity.

(iii) In which type of ecosystem, the smallest fraction of nutrients present in soil:

(a)  Savanna

(b)  Tundra

(c)  Grassland

(d) Desert

EXPLANATION: In tundra ecosystems, the smallest fraction of nutrients is present in the soil. Tundra regions are characterized by extremely cold temperatures, permafrost (frozen soil), and a short growing season. The cold conditions limit the decomposition of organic matter, leading to slow nutrient cycling.

(iv) What biome will be richest soil with nutrients and can be converted into agriculture:

(a)  Deciduous forest

(b)  Tropical rain forest

(c)  Grass land

(d) Coniferous forest

EXPLANATION: Characterized by dense cover of grasses which decompose and contribute organic matter (humus) to the soil, enriching it in nutrients. Rainfall is relatively low, preventing nutrient leaching (washing away nutrients).

(v) Which of the biomes has been increased in area by human activities:

(a)  Savanna

(b)  Grassland

(c)  Desert

(d) Coniferous

EXPLANATION: Deserts have been the biome most directly increased in area due to human activities like deforestation. The process by which fertile land degrades into desert is called desertification.


Productivity of an Ecosystem:

“The productivity of an ecosystem refers to the rate at which biomass, the total amount of living matter in an ecosystem, is produced.” It essentially signifies the efficiency of an ecosystem in capturing and storing energy from the sun through photosynthesis, and converting it into organic matter.

Adaptations for Terrestrial Ecosystem:

Plants and animals shifting from water to land developed various types of adaptations for land habitat e.g.,

Supporting tissues: Both plants and animals have evolved supporting tissues like:

(1) Vascular bundles (xylem-phloem) in plants and

(2) Skeleton in animals to support them on land against the force of gravity.

Conservation of water:

(1) Plants and animals evolved various methods to conserve water in their body e.g. homeostasis.

(2) The mechanism of temperature regulation was developed by land plants and animals by developing bark and skin respectively.

Three zones in lake ecosystem are:

(1) Littoral Zone (Near Shore)

(2) Limnetic Zone

(3) Profundal Zone

There are approximately six to twelve major biomes recognized by different organizations. Five of them are:

(1) Tundra

(2) Desert

(3) Temperate forest

(4) Tropical rain forest

(5) Grassland

Some major ecosystems on land in Pakistan are:

(1) Temperate deciduous Forests

(2) Coniferous alpine and boreal Forests

(3) Desert ecosystem

(4) Grasslands ecosystem

(5) Tundra ecosystem


The four major requirements for life on Earth are:

Nutrients: These are the building blocks for an organism’s cells, used for growth, repair, and energy. They include elements like carbon, nitrogen, phosphorus, and others.

Energy: A source of energy is required for all living processes, such as metabolism and movement. The main source of energy for most life on Earth is ultimately the sun, used by plants in photosynthesis.

Water: Water is a universal solvent in which biological reactions occur. It helps transport nutrients, regulate temperature, and provides structure to cells.

Suitable Temperature: Life generally exists within a relatively narrow temperature range, as extreme temperatures can damage vital biological molecules like proteins and DNA.

In terrestrial ecosystems, the two most limiting factors are water and temperature:

Water: Rainfall in terrestrial environments is unevenly distributed and can be unpredictable. Limited water can affect plant growth, which then influences all other organisms up the food chain.

Temperature: Temperature swings and extremes are common in terrestrial environments, making it harder for organisms to maintain a stable internal temperature. This can impact metabolic processes and overall survival.

Adaptations in Desert Plants:

Desert plants conserve water in a variety of ways:

(1) Plants are covered with the waterproof waxy coating to prevent evaporation of precious water.

(2) Water is stored in thick stems of cacti and other succulents. Cacti and Euphorbia have fleshy stems in which water is stored for use during the period of drought.

Adaptations in Desert Plants:

Like plants, animals are also specially adapted to survive on little water. for example:

(1) Most deserts appear to be almost completely devoid of animal life during day, because the animals seek relief from the sun and heat in cool underground burrows.

(2) In the dark, when desert cools down, homed lizards, snakes and other reptiles emerge to feed, as do mammals such as kangaroo, rat, and birds such as burrowing owl.

(3) Most of the smaller animals survive without ever drinking at all, getting all the water they need from their food and what produced during cellular respiration in their tissues.

(4) Large animals such as desert bighorn sheep and camel are dependent on permanent water holes during the driest times of the year.

“Hydrospheric ecosystem is a system in water where living and non-living components exchange materials and transfer of energy also takes place within water.” Salt-water ocean and sea are the largest ecosystems on the earth forming about 71% of its surface. Fresh water ecosystems, in contrast, covers less than 1%.

The unique properties of water lend some common features to aquatic ecosystem.

1) Temperature: Water changes its temperature slower than air, so temperature in aquatic ecosystem is more moderate to support life.

2) Absorption of energy: Although water may appear quite transparent, it absorbs a considerable amount of the light energy that sustains life. Even in clearest water, the intensity of light decreases rapidly with depth, so at the depth of 600 feet or more, a little light is left to power photosynthesis.

3) Nutrients: The nutrients in aquatic ecosystem tend to be concentrated near the bottom sediments supporting life where light levels often are too low to support photosynthesis.

4) Abundant water with appropriate temperature: Water is an essential requirement for life. It is available abundantly in aquatic ecosystem to support life. The major factors that determine the quantity and type of life in aquatic ecosystems are energy and nutrient. Appropriate temperature is present in aquatic ecosystem to carry out all metabolic processes.

Fresh Water Lakes:

Fresh water lakes vary tremendously in size, depth, and nutrient content, including distinct life zones and temperature stratification.

Life zones are based on access to light and nutrients:

The distribution of life in lakes depends on access to light, to nutrients and to place for attachment. The lake ecosystem can be divided into three main zones.

Littoral zone (Near-shore): In this zone, the water is shallow, and plants find abundant light, anchorage and adequate nutrients from the bottom sediments. Plants in littoral zone communities are the most diverse; water lilies and entirely submerged vascular plants and algae flourish at the deepest region of the littoral zone. The plants of this zone trap sediments carried by stream, increasing the nutrient content in this region. Living among the anchored plants are microscopic organisms called plankton. These can be divided into two groups.

Phytoplankton (Greek “drifting plants”) these include photosynthetic Protista, bacteria and algae. Zooplankton (Greek “drifting animals”) such as protozoa and tiny crustaceans.

The greatest diversity of animals in the lake is also found in this zone. Littoral invertebrate animals include small crustaceans, insect larvae, snails, flatworms, Hydra, vertebrates include frogs, aquatic snakes and turtles. As the water increases in depth farther from the shore, plants are unable to anchor to the bottom and still collect enough light for photosynthesis. This open water area is divided into two regions: the upper limnetic zone and the lower profundal zone.

Limnetic Zone:

In this zone enough light penetrates to support photosynthesis. Here, phytoplankton includes cyanobacteria (blue green algae) which serve as producers. These are eaten by protozoa and small crustaceans, which in turn are consumed by fishes.

Profundal Zone:

Here, light is insufficient to support photosynthesis. The organisms of this zone are mainly nourished by detritus that falls from the littoral and limnetic zone and by incoming sediment. Decomposers and detritus feeders, such as, snails and certain insect larvae, bacteria, fungi and fishes, inhabit it.

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