Download Science – Class 9 – Ch. 7 – BioGeoChemical cycles 1. What are

Science – Class 9 – Ch. 7 – BioGeoChemical cycles
1. What are biogeochemical cycles?
Biogeochemical cycles are representations of the circulation and recycling of matter in nature. The main
biogeochemical cycles studied in ecology are the water cycle, the carbon cycle and the nitrogen cycle.
2. What is the importance of water, carbon and nitrogen for living organisms?
 Water is the main solvent for living organisms and it is necessary for almost all biochemical reactions,
including as reagent of photosynthesis. Many properties of water are very important for life.
 Carbon is the main chemical element of organic molecules; carbon dioxide is also reagent of
photosynthesis and a product of the energy metabolism of living organisms.
 Nitrogen is a fundamental chemical element of amino acids, the building blocks of proteins that are in
turn the main functional molecules of living organisms; nitrogen is also part of nucleic acid molecules,
which are the basis for reproduction, heredity and protein synthesis.
3. What is the water cycle?
The water cycle represents the circulation and recycling of water in nature.
Liquid water on the planet's
surface is heated by the sun
and turns into water vapor,
which enters the atmosphere.
In the atmosphere, large
volumes of water vapor form
clouds that, when cooled,
precipitate liquid water as rain.
Therefore, water comes back
to the planet surface and the
cycle is complete. During
possible steps of the cycle,
water may still be stored in
subterranean reserves or in
the form of ice in mountains
and oceans, and may also be
used in the metabolism of
living organisms, incorporated
into the body of individuals or
excreted through urine, feces
and sweat.
4. Why is the sun the “motor” of the water cycle?
The sun can be considered the motor of the water cycle because the transformation of liquid water into water
vapor depends on its energy. Therefore, the sun is the energy source that causes water to circulate in nature.
5. What is the carbon cycle?
The carbon cycle represents the circulation and recycling of the chemical element carbon in nature as a result of
the effect of living organisms.
a. Photosynthetic organisms absorb carbon
as carbon dioxide available in the
atmosphere and the carbon atoms
become part of glucose molecules.
b. During the cellular respiration of these
organisms, part of this organic material is
consumed to generate energy and, in this
process, carbon dioxide is returned to the
atmosphere.
c. The other part is incorporated by the
photosynthetic organisms into the
molecules that compose their structure.
d. The carbon atoms incorporated into the
producers are transferred to the next
trophic level and again a portion of them is
released by cellular respiration in
consumers, another part becomes a
component of the consumer's body, and
the last part is excreted as uric acid or urea
(excretions later recycled by decomposer bacteria).
e. Therefore, carbon absorbed by producers via photosynthesis returns to the atmosphere through cellular
respiration along the food chain until reaching the decomposers that also release carbon dioxide in their
energy metabolism.
f. Under special conditions, through a process that takes millions of years, carbon incorporated into
organisms may also constitute fossil fuels stored in deposits under the surface of the planet; as fossil fuels
burn. the carbon atoms return to the atmosphere as carbon dioxide or carbon monoxide.
g. The burning of vegetable fuels, such as wood, also returns carbon to the atmosphere.
6. What is the nitrogen cycle?
The nitrogen cycle represents the circulation and recycling of the
chemical element nitrogen in nature.
a. The nitrogen cycle basically depends on the effect of
specialized bacteria.
b. Bacteria in the soil called nitrogen-fixing bacteria present
in plant roots absorb molecular nitrogen from the air and
release nitrogen in the form of ammonia. The
decomposition of organic material also produces
ammonia.
i. In the soil and roots (mainly of leguminous
plants), a first group of chemosynthetic bacteria
called nitrifying bacteria or nitrosomonas
produces energy by consuming ammonia and
releasing nitrite (NO₂).
ii. The second group of nitrifying bacteria, called
nitrobacteria, uses nitrite in chemosynthesis,
releasing nitrate (NO₃). In the form of nitrate,
nitrogen is then incorporated by plants to be used a component of proteins and nucleic acids, and
the element then follows along the food chain.
iii. Nitrogen returns to the atmosphere through the effect of denitrifying bacteria that use nitrogencontaining compounds from the soil and release nitrogen gas (molecular nitrogen).
7. What are the four processes in which nitrogen cycles through the environment?
Nitrogen cycles through the environment due to the following four processes
a. Nitrogen Fixation
b. Ammonification (Decay)
c. Nitrification
d. Denitrification
Objective Questions
1. What is the role of nitrogen fixing bacteria in the nitrogen cycle?
a. They make nitrogen into a more useful form.
2. What is the recycling of chemical elements throughout an ecosystem called?
a. Biogeochemical Cycle
3. Nutrients move through an ecosystem in
.
a. Biogeochemical cycles
4. Which biogeochemical cycle does NOT include a major path in which the substance cycles through the
atmosphere?
a. Nitrogen cycle
5. An increase in the greenhouse effect causes an increase in
a. Temperature
6. Which of the following is NOT a way that carbon in dead organisms is returned to the carbon cycle?
a. by becoming fossil fuel
b. through combustion
c. through decomposition
d. through photosynthesis
7. This phenomenon occurs when gases function like glass in a greenhouse, allowing visible light to enter, but
trapping heat.
a. solar flares
b. abiotic factors
c. ozone effect
d. greenhouse effect
8. How do living things contribute to the water cycle?
9.
10.
11.
12.
13.
14.
a. Transpiration
What is the greenhouse effect?
a. Gases are trapped in the earth's atmosphere and cool the earth
b. Gases are trapped in the earth's atmosphere and warm the earth
c. The sun is heating up earth's surface
d. The moon is cooling earth's surface
What is the best definition of global warming?
a. it is when the earth is warm on all sides
b. an increase in the average temperature worldwide caused by the greenhouse effect.
c. when people around the world are feeling warm all over
d. is it when the earth is warm because of many fires in the jungle
Which of these gases is NOT a greenhouse gas?
a. Carbon dioxide
b. Nitrous oxide
c. Methane
d. Oxygen
The initial energy that propels the carbon cycle is the sun.
a. True
In the water cycle, how does water leave the atmosphere and come back to the earth's surface?
a. Precipitation
Which of the nutrient cycles requires the nutrient to be fixed before it can be used by producers and consumers?
a. Nitrogen