Problem 8 - Campbell Biology

Question

Two groups of tomatoes were grown under laboratory conditions, one with humus added to the soil and one a control without humus. The leaves of the plants grown without humus were yellowish (less green) compared with those of the plants grown in humus-enriched soil. The best explanation is that (A) the healthy plants used the food in the decomposing leaves of the humus for energy to make chlorophyll. (B) the humus made the soil more loosely packed, so water penetrated more easily to the roots. (C) the humus contained minerals such as magnesium and iron needed for the synthesis of chlorophyll. (D) the heat released by the decomposing leaves of the humus caused more rapid growth and chlorophyll synthesis.

Step-by-Step Solution

Answer

C

Step 1: Understand the context

We have two groups of tomatoes: one grown with humus added to the soil and another grown without humus. The observation is that plants grown without humus have yellowish leaves.

Step 2: Identify key differences

The only difference between the two groups is the presence of humus in the soil. The yellowish leaves indicate a potential issue related to chlorophyll production.

Step 3: Evaluate the options

Let's look at each option to determine which best explains the observed difference: (A) Incorrect. Chlorophyll is not made using energy from decomposing leaves. (B) Incorrect. Water penetration benefits plant health but doesn't directly lead to chlorophyll synthesis. (C) Correct. Magnesium and iron in humus are essential nutrients for chlorophyll production. (D) Incorrect. Heat from decomposing leaves does not significantly affect chlorophyll synthesis.

Step 4: Choose the best explanation

From the above evaluation, option (C) is the best explanation because it directly links the absence of necessary minerals (magnesium and iron) to the yellowish leaves.

Key Concepts

Humus benefits

Humus is an incredibly beneficial component of soil. It is formed from the decomposition of plant and animal matter. Adding humus to soil can improve its quality significantly.

One key benefit is soil structure improvement. Humus helps make the soil more loosely packed. This enhances the soil's ability to retain moisture and allows better air flow to the roots.

Humus also contains important organic matter and nutrients that plants need to thrive. These include carbon, nitrogen, phosphorus, and sulfur.

Organic matter in humus acts as a slow-release fertilizer. This means the nutrients provided to plants are available over a longer period. As a result, plants get a steady supply of essential minerals.

Additionally, humus can buffer the soil against sudden changes in pH. This creates a more stable environment for plants.

• Improves soil structure
• Retains moisture better
• Provides a slow-release supply of nutrients
• Buffers against pH changes

Chlorophyll production

Chlorophyll is the molecule responsible for the green color in plants. It's crucial for photosynthesis, which is the process plants use to make food. Yellowish leaves in plants often indicate a problem with chlorophyll production.

Chlorophyll production relies heavily on the availability of certain minerals. Magnesium and iron, both found in humus, are two of the most important minerals. Magnesium is a central atom in the chlorophyll molecule, while iron plays a critical role in the synthesis process.

When plants lack these essential minerals, they struggle to produce chlorophyll. This deficiency results in yellow leaves and stunted growth.

In our exercise case, the group of tomatoes without humus had yellowish leaves. The absence of humus meant a lack of magnesium and iron, leading to poor chlorophyll production.

• Essential for photosynthesis
• Relies on magnesium and iron
• Deficiency causes yellow leaves

Essential plant minerals

Plants require various minerals to grow healthily and efficiently. These minerals are usually absorbed from the soil through the roots.

Some of the most important minerals for plant growth include:

• Nitrogen: Vital for the growth of leaves and stems.
• Phosphorus: Encourages root growth and blooming.
• Potassium: Improves overall plant health and resistance to diseases.
• Magnesium: Key component of chlorophyll, aids in photosynthesis.
• Iron: Crucial for chlorophyll synthesis and oxygen transport.

Deficiency in any of these essential minerals can lead to various growth problems in plants. For tomatoes, a lack of magnesium or iron can specifically cause issues in producing enough chlorophyll. This results in less vibrant, yellowish leaves.

Incorporating humus into soil can address these deficiencies. By providing a natural source of essential minerals, humus-enriched soil promotes healthier plant growth and better chlorophyll production. This explains why the tomato plants in the exercise with humus were greener and healthier.

• Provides vital nutrients
• Prevents growth problems
• Enhances overall plant health