12. explain how chemical reactions support growth in living organisms.
13. how do plants and animals use the energy stored during cellular respiration?
14. discuss the significance of matter conservation in biological systems.
15. provide an example of how nutrients are cycled in an ecosystem and how this demonstrates matter conservation.

Question

Accepted Answer

### Question 12
# Brief Explanations:
1. **Anabolic Reactions**: Chemical reactions like photosynthesis (in plants) and protein synthesis build complex molecules from simpler ones. For example, plants use $6CO_2 + 6H_2O \xrightarrow[	ext{Chlorophyll}]{	ext{Sunlight}} C_6H_{12}O_6 + 6O_2$ to make glucose, which is used for growth (cellulose for cell walls, energy storage). In animals, amino acids combine to form proteins for muscle growth.
2. **Energy Conversion**: Reactions like cellular respiration ($C_6H_{12}O_6 + 6O_2
ightarrow 6CO_2 + 6H_2O + ATP$) release energy from nutrients. This ATP powers processes like DNA replication, cell division, and nutrient uptake, all essential for growth.
3. **Nutrient Assimilation**: Chemical reactions break down food (e.g., digestion of carbohydrates into glucose) so nutrients can be absorbed and used to build new cells/tissues. For instance, glucose is converted to glycogen for storage or used in glycolysis to fuel growth - related activities.

# Answer:
Chemical reactions support growth via anabolic reactions (building complex molecules like glucose into cell structures), energy - releasing reactions (e.g., cellular respiration providing ATP for growth - related processes like cell division), and nutrient assimilation (breaking down food into usable nutrients to build new cells/tissues). For example, photosynthesis makes glucose for plant cell wall (cellulose) synthesis, and protein synthesis from amino acids (via chemical reactions) builds animal muscle tissue.

### Question 13
# Brief Explanations:
1. **ATP as Energy Currency**: During cellular respiration, energy is stored in ATP ($	ext{ADP} + 	ext{Pi} + 	ext{Energy}
ightarrow 	ext{ATP}$). Plants and animals use ATP to power various processes.
2. **Plant Energy Use**: Plants use ATP for active transport (e.g., moving mineral ions from roots to leaves), DNA replication and cell division (for growth), and synthesis of organic compounds (like proteins from amino acids, which requires energy). For example, in meristematic cells, ATP fuels cell division for plant growth.
3. **Animal Energy Use**: Animals use ATP for muscle contraction (movement, hunting, escaping), active transport (e.g., absorbing nutrients in the small intestine), protein synthesis (for growth, repair), and nerve impulse transmission. For example, a running animal uses ATP - powered muscle contractions, and a growing animal uses ATP for cell division and tissue repair.

# Answer:
Plants and animals use energy stored as ATP during cellular respiration. Plants use it for active transport (e.g., mineral uptake), cell division (growth), and organic compound synthesis (e.g., protein). Animals use it for muscle contraction (movement), active transport (nutrient absorption), protein synthesis (growth/repair), and nerve impulse transmission. For example, a plant’s root cells use ATP to take up nitrates, and a dog uses ATP to contract leg muscles for running.

### Question 14
# Brief Explanations:
1. **Matter Conservation Principle**: Matter is neither created nor destroyed, only transformed. In biological systems, this means elements (C, N, O, etc.) are recycled.
2. **Cellular Processes**: In cells, matter is conserved during metabolism. For example, in cellular respiration, glucose ($C_6H_{12}O_6$) is broken down, but the carbon, hydrogen, and oxygen atoms are re - arranged into $CO_2$ and $H_2O$, not lost. This allows reuse of atoms for new molecules (e.g., $CO_2$ from respiration is used in photosynthesis to make glucose).
3. **Ecosystem Level**: At the ecosystem level, nutrient cycles (e.g., carbon, nitrogen cycles) rely on matter conservation. Decomposers break down dead organisms, releasing nutrients back into the environment, which are then taken up by producers. Without matter conservation, nutrients would be depleted, and life would not be sustainable. For example, in the carbon cycle, carbon moves from plants (via photosynthesis) to animals (via consumption) and back to the environment (via respiration/decomposition), with the total amount of carbon in the ecosystem remaining constant (conserved).

# Answer:
Matter conservation in biological systems is significant as it enables nutrient recycling. At the cellular level, during metabolism (e.g., respiration, photosynthesis), atoms are re - arranged (not lost) to form new molecules, allowing reuse (e.g., $CO_2$ from respiration is used in photosynthesis). At the ecosystem level, nutrient cycles (e.g., carbon, nitrogen) depend on matter conservation. Decomposers release nutrients from dead organisms, which producers re - use. Without it, nutrients would be exhausted, and life - sustaining processes (growth, reproduction) would fail. For example, carbon in glucose (produced by plants) moves to animals (via food) and back to the environment (via respiration), with total carbon conserved.

### Question 15
# Brief Explanations:
1. **Nitrogen Cycle Example**:
   - **Step 1: Nitrogen Fixation**: Bacteria (e.g., Rhizobium in legume roots) convert atmospheric $N_2$ into ammonia ($NH_3$) or ammonium ions ($NH_4^+$).
   - **Step 2: Nitrification**: Other bacteria convert $NH_4^+$ to nitrites ($NO_2^-$) and then to nitrates ($NO_3^-$).
   - **Step 3: Assimilation**: Plants take up $NO_3^-$ (or $NH_4^+$) and use it to make proteins (incorporating nitrogen into organic molecules). Animals eat plants, assimilating nitrogen into their own proteins.
   - **Step 4: Decomposition/Ammonification**: When plants/animals die, decomposers break down their proteins, releasing $NH_4^+$ back into the soil.
   - **Step 5: Denitrification**: Some bacteria convert $NO_3^-$ back to $N_2$, which returns to the atmosphere.
2. **Matter Conservation in Nitrogen Cycle**: The total amount of nitrogen in the ecosystem (atmosphere, soil, organisms) remains constant. Nitrogen atoms are transformed (e.g., $N_2$ to $NH_3$ to $NO_3^-$ to protein - N and back to $N_2$) but not created or destroyed. For example, the nitrogen in a plant’s protein comes from soil nitrates, which came from atmospheric $N_2$ (via fixation). When the plant dies, decomposers release nitrogen back to the soil, and eventually, it can be converted back to $N_2$, completing the cycle with no net loss or gain of nitrogen.

# Answer:
### Example: Nitrogen Cycle
1. **Nitrogen Fixation**: Rhizobium bacteria in legume roots convert atmospheric $N_2$ to $NH_3$ (or $NH_4^+$).
2. **Nitrification**: Bacteria convert $NH_4^+$ to $NO_2^-$ and then $NO_3^-$.
3. **Assimilation**: Plants absorb $NO_3^-$ (or $NH_4^+$) and use it to make proteins. Animals eat plants, assimilating nitrogen into their proteins.
4. **Decomposition/Ammonification**: Decomposers break down dead plants/animals, releasing $NH_4^+$ into the soil.
5. **Denitrification**: Bacteria convert $NO_3^-$ back to $N_2$, which returns to the atmosphere.

### Matter Conservation Demonstration:
The total nitrogen in the ecosystem (atmosphere, soil, organisms) is conserved. Nitrogen atoms are transformed (e.g., $N_2
ightarrow NH_3
ightarrow NO_3^-
ightarrow$ protein - N $
ightarrow NH_4^+
ightarrow N_2$) but not created or destroyed. For example, nitrogen in a plant’s protein originates from atmospheric $N_2$ (via fixation). When the plant dies, decomposers release nitrogen back to the soil, and it can be cycled back to $N_2$, with no net change in the total amount of nitrogen.

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