krit.club logo

Biology - Organisms and their Environment

Grade 9IGCSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

•

The Sun is the principal source of energy input to biological systems, providing light energy for photosynthesis.

•

Energy is transferred between organisms through food chains and food webs; however, energy transfer is inefficient, with approximately 90%90\% of energy lost at each trophic level as heat via respiration, or through egestion and excretion.

•

Trophic levels describe the position of an organism in a food chain: Producers (1st level), Primary Consumers (2nd level), Secondary Consumers (3rd level), and Tertiary Consumers (4th level).

•

The Carbon Cycle maintains the balance of carbon in the atmosphere through processes such as photosynthesis (CO2CO_2 removal), respiration (CO2CO_2 release), decomposition, and combustion of fossil fuels.

•

The Nitrogen Cycle involves the conversion of atmospheric nitrogen (N2N_2) into usable forms like nitrates (NO3−NO_3^-) via nitrogen-fixing bacteria, and the return of N2N_2 to the atmosphere via denitrifying bacteria.

•

Population growth is characterized by four phases: the lag phase (slow growth), exponential/log phase (rapid growth where birth rate > death rate), stationary phase (birth rate = death rate), and the death phase (death rate > birth rate).

•

Human impact on the environment includes the enhanced greenhouse effect caused by increased levels of CO2CO_2 and CH4CH_4 (methane), leading to global warming and climate change.

📐Formulae

6CO2+6H2O→light + chlorophyllC6H12O6+6O26CO_2 + 6H_2O \xrightarrow{\text{light + chlorophyll}} C_6H_{12}O_6 + 6O_2

C6H12O6+6O2→6CO2+6H2O+EnergyC_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}

Efficiency of Energy Transfer=Energy available to next trophic levelEnergy received from previous trophic level×100\text{Efficiency of Energy Transfer} = \frac{\text{Energy available to next trophic level}}{\text{Energy received from previous trophic level}} \times 100

Net Population Change=(Births+Immigration)−(Deaths+Emigration)\text{Net Population Change} = (\text{Births} + \text{Immigration}) - (\text{Deaths} + \text{Emigration})

💡Examples

Problem 1:

A field of clover (producer) captures 20,000 kJ20,000\text{ kJ} of energy from sunlight. A rabbit (primary consumer) eats the clover and incorporates 2,000 kJ2,000\text{ kJ} into its biomass. Calculate the efficiency of energy transfer from the clover to the rabbit.

Solution:

Efficiency=2,000 kJ20,000 kJ×100=10%\text{Efficiency} = \frac{2,000\text{ kJ}}{20,000\text{ kJ}} \times 100 = 10\%

Explanation:

To find the efficiency, divide the energy stored in the consumer by the energy originally available in the producer, then multiply by 100 to get a percentage. This reflects the 10%10\% rule typically observed in ecosystems.

Problem 2:

Identify the process in the nitrogen cycle where bacteria convert NO3−NO_3^- ions back into N2N_2 gas.

Solution:

The process is known as denitrification, performed by denitrifying bacteria.

Explanation:

Denitrifying bacteria thrive in anaerobic conditions (like waterlogged soil) and convert nitrates (NO3−NO_3^-) into nitrogen gas (N2N_2), reducing soil fertility.

Problem 3:

Explain why food chains rarely have more than five trophic levels.

Solution:

Due to the inefficiency of energy transfer (where roughly 90%90\% of energy is lost as heat or waste), the amount of energy remaining after 44 or 55 transfers is too small to support a viable population of higher-level consumers.

Explanation:

Energy loss at each level (e.g., via respiration C6H12O6+6O2→6CO2+6H2OC_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O) limits the total biomass and number of organisms that can exist at higher trophic levels.