A multicellular organism increases in size as it grows. This growth mainly occurs by which basic biological process?

Introduction / Context:

Growth in multicellular organisms, such as plants, animals, and humans, is a fundamental biological process. When an organism grows, it becomes larger, develops new tissues, and sometimes changes shape. Understanding how this growth happens at the cellular level is essential for topics ranging from development and regeneration to cancer biology. This question checks whether students know that growth is driven by cell division and increase in cell number, rather than cells simply becoming bigger without division or being added from outside.

Given Data / Assumptions:

• The organism under discussion is multicellular, not unicellular.
• We are asked which process is responsible for growth.
• Options mention cell addition, explosion, implosion, division, and fusion.
• We assume normal growth as seen in healthy organisms.

Concept / Approach:

In multicellular organisms, growth occurs mainly through mitotic cell division. In mitosis, one parent cell divides into two genetically identical daughter cells. As cells divide repeatedly, the total number of cells in tissues and organs increases, which leads to an increase in overall size. Cells may also enlarge to some extent, but long term growth depends on repeated divisions. There is no natural process in which new cells are simply added from outside the body, nor do cells grow by exploding or imploding. Therefore, the correct explanation for growth is cell division with an increase in cell number.

Step-by-Step Solution:

Step 1: Recall that multicellular organisms are made of many cells organised into tissues and organs. Step 2: Recognise that growth in size is associated with an increase in the number of cells and sometimes cell size. Step 3: Identify mitotic cell division as the process that produces new cells in body tissues. Step 4: Compare this with unrealistic or destructive processes like explosion or implosion, which would damage cells. Step 5: Choose the option that states cell division and increase in cell number as the main mechanism of growth.

Verification / Alternative check:

Developmental biology explanations describe how a fertilised egg, which is a single cell, undergoes repeated mitotic divisions to form a multicellular embryo and eventually an entire organism. Charts of the cell cycle show how cells pass through phases leading to division. Tissue growth after injury, such as healing of a cut, is also used as an example of cell division driven increase in cell number. None of these descriptions involve cells exploding, imploding, or being added from outside, which confirms that growth is due to cell division.

Why Other Options Are Wrong:

Option A, Cell addition from outside the body, is incorrect because cells do not come from external sources; they arise from pre existing cells. Option B, Cell explosion, would destroy cells rather than promote growth. Option C, Cell implosion, similarly describes cell death or collapse, not growth. Option E, Random fusion of neighbouring cells, would actually reduce cell number and is not a normal growth mechanism in most tissues.

Common Pitfalls:

Some learners may focus only on the increase in size and overlook the increase in cell number, assuming existing cells just become larger without dividing. Others might not fully understand what division means at the cellular level and confuse it with breaking apart or exploding. To avoid these errors, students should remember the simple rule that all new cells arise from existing cells by division and that multicellular growth is essentially the result of many rounds of mitotic division.

Final Answer:

A multicellular organism grows mainly through cell division and increase in cell number.