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 need 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

Option (C) is correct because humus contains minerals necessary for chlorophyll synthesis.

Step 1 - Understand the Problem

Two groups of tomatoes were grown: one with humus and one without humus. The goal is to explain why the leaves of plants grown without humus turned yellowish, while those with humus remained green.

Step 2 - Identify Key Factors

Key factors include the presence of humus, the color change in leaves, and the potential nutrients provided by the humus.

Step 3 - Analyze Each Option

Option (A) suggests food energy from humus. Option (B) suggests easier water penetration. Option (C) refers to minerals like magnesium and iron necessary for chlorophyll. Option (D) attributes it to heat from decomposition.

Step 4 - Evaluate Options

Evaluate how each option relates to chlorophyll synthesis. Option (A) does not directly connect humus to chlorophyll. Option (B) discusses water penetration but not chlorophyll directly. Option (D) links heat to growth, not specifically chlorophyll. Only Option (C) directly states that minerals necessary for chlorophyll are provided by the humus.

Key Concepts

Humus in Soil

Humus is the organic component of soil, formed by the decomposition of leaves and other plant materials. It plays a crucial role in soil fertility and plant health.

Humus improves soil structure by making it more porous, allowing water and air to penetrate easily. This helps the roots of plants to access water and oxygen efficiently. Additionally, it enhances the soil's ability to retain nutrients, ensuring that plants have a steady supply of these essential elements.

In the context of the original exercise, humus in the soil provides an environment rich in nutrients, particularly important minerals needed for chlorophyll synthesis. This explains why the leaves of plants grown in humus-enriched soil were greener compared to those grown without humus. The healthier appearance of the plants with humus is a clear indicator of more efficient nutrient uptake and better overall plant health.

Plant Nutrients

Plants require a variety of nutrients to grow and thrive. These nutrients are generally divided into two categories: macronutrients and micronutrients.

Macronutrients, needed in larger amounts, include nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. Micronutrients, needed in smaller quantities, include iron, manganese, copper, zinc, boron, molybdenum, and chlorine.

Each nutrient has a specific role in plant growth. Nitrogen, for example, is crucial for the formation of proteins and chlorophyll. Phosphorus is vital for energy transfer and photosynthesis. Potassium helps in enzyme activation and water regulation. Magnesium and iron, on the other hand, are key components for the synthesis of chlorophyll, the pigment responsible for the green color of plant leaves.

In the case of the tomato plants in the exercise, the presence of humus enriched the soil with these essential nutrients, particularly magnesium and iron, which are necessary for chlorophyll production. This nutrient enrichment led to the greener and healthier leaves observed in the plants grown with humus.

Magnesium and Iron

Magnesium and iron are vital micronutrients for plants, especially for the synthesis of chlorophyll.

**Magnesium** is a central atom in the chlorophyll molecule. Without adequate magnesium, plants cannot produce enough chlorophyll, which leads to yellowing leaves, a condition known as chlorosis. Magnesium also plays a role in enzyme activation and energy transfer processes within plant cells.

**Iron** is crucial for the formation of chlorophyll as well. It acts as a catalyst in various biochemical reactions within the plant, including the synthesis of chlorophyll. Iron is also involved in energy production and nitrogen fixation.

In the original exercise, the yellowish leaves of the plants grown without humus suggest a deficiency in these essential nutrients. Humus, rich in decomposed organic material, provides an ample supply of magnesium and iron, thereby supporting robust chlorophyll production and resulting in greener leaves. The healthier appearance of plants grown with humus is a clear indication of adequate magnesium and iron, necessary for proper chlorophyll synthesis and overall plant health.