According to the modern cell theory in biology, which of the following activities would be impossible because it contradicts the principle that all cells arise from pre-existing living cells?

Introduction / Context:
Cell theory is one of the fundamental principles of modern biology. It states that all living organisms are made up of one or more cells, that the cell is the basic structural and functional unit of life, and that all cells arise from pre-existing cells. This question tests whether you can apply that last part of the theory to identify which proposed activity or situation is impossible from a biological point of view. Understanding this helps to distinguish modern scientific thinking from older ideas like spontaneous generation, which claimed life could arise from non-living matter without pre-existing life.

Given Data / Assumptions:

• Modern cell theory says that new cells come only from division of existing living cells.
• We are asked which activity is impossible under this theory.
• Other listed activities (cell division, photosynthesis, multicellularity) are known biological realities.
• We assume normal conditions on Earth, not speculative origins-of-life scenarios.

Concept / Approach:
The key concept is that of biogenesis: life comes from pre-existing life. This is part of modern cell theory and is supported by a long history of experiments, including those of Louis Pasteur. The theory rejects spontaneous generation, the idea that living cells could simply appear from non-living substances such as mud, meat, or pure water. Therefore, any option that suggests fully formed cells coming directly from non-living matter, without parent cells, contradicts cell theory. In contrast, heart cells forming from other heart cells through cell division is a normal process, plants using sunlight for energy is photosynthesis, and eukaryotes forming multicellular organisms is a basic fact of biology. Only spontaneous formation of cells from pure water is impossible under modern cell theory.

Step-by-Step Solution:
Step 1: Recall that cell theory states: all cells arise from pre-existing living cells. Step 2: Examine each option and see whether it involves cells coming from non-living material without parent cells. Step 3: Identify that “cells being formed from pure water” implies spontaneous generation from a non-living substance. Step 4: Recognise that heart cells forming from other heart cells is simply cell division within a tissue. Step 5: Note that plants using sun energy and eukaryotes being part of multicellular organisms do not violate cell theory at all. Step 6: Conclude that the only activity impossible according to cell theory is new cells forming from pure water with no pre-existing cells.

Verification / Alternative check:
Historical experiments, such as those by Pasteur, used sterilised broths and special flasks to show that microorganisms appeared only when contamination from pre-existing life was possible. When all possible contamination was excluded, no new life forms appeared spontaneously, undermining the idea of spontaneous generation. Modern observations with microscopes show cell division as the origin of new cells in tissues, embryos, and unicellular organisms. No credible observations show cells suddenly appearing from pure water alone. This confirms that the option describing cells forming from pure water contradicts cell theory and is therefore impossible.

Why Other Options Are Wrong:
Heart muscle cells forming by division of other heart muscle cells is entirely consistent with cell theory, which describes new cells arising from existing cells in tissues and organs.
Green plants using energy from sunlight to make food by photosynthesis involves energy transformation, not spontaneous cell formation, and fully fits modern biology.
Eukaryotic cells existing together as part of a multicellular organism is a central concept in biology, seen in animals, plants, and many fungi, and does not violate cell theory.

Common Pitfalls:
Students may be distracted by the complexity of processes like photosynthesis or multicellularity and think they might contradict simple rules like cell theory. It is important to focus directly on what cell theory says about the origin of cells: they must come from pre-existing cells by division. Any scenario that involves fully formed cells popping into existence from non-living substances is not allowed. Remembering the historical shift from spontaneous generation to biogenesis helps you quickly identify options that conflict with cell theory in exam questions.

Final Answer:
According to cell theory, the impossible activity is New cells being formed spontaneously from pure water without any pre-existing cells.