The Cell is the Basic Unit of Life

Section 1: The Cell is the Basic Unit of Life

Facing the wonders of the natural world, human beings are inherently curious. Countless questions such as "how?" and "why is it like this?" continuously attract people to explore, driving scientific progress forward.

The invention of the microscope opened the door to the microcosm for humanity. Through the microscope, people observed many microscopic organisms in a drop of water that are invisible to the naked eye, and observed various cells within animal and plant bodies. With deeper observation, new questions emerged: What is the relationship between cells and organisms? What are the similarities and differences between animal and plant cells? How are new cells produced? What is the relationship between cell activity and the growth and development of organisms? These series of questions attracted further exploration, and over more than a hundred years, theoretical summaries were finally made, forming the foundational cell theory in biology.

The Development of Cell Theory

The main founders of cell theory were two German scientists, Schleiden (M.J. Schleiden, 1804–1881) and Schwann (T. Schwann, 1810–1882) (Figure 1-1). Based on their research published in 1838 and 1839 respectively, subsequent scholars organized and revised their results, summarizing them into the following points:

1. The cell is an organic unit; all plants and animals develop from cells and are composed of cells and their products.
2. The cell is a relatively independent unit, possessing its own life while also contributing to the overall life of multicellular organisms.
3. New cells arise from the division of pre-existing cells.

The content of cell theory, now seemingly obvious, underwent a long and tortuous process of establishment.

Cell theory revealed the unity of animals and plants, thus elucidating the unity of the biological world. Like atomic theory in chemistry, cell theory has significant implications for the development of biology.

Cell theory made people realize that plants and animals share a common structural basis, breaking down long-standing barriers between botany and zoology in thought, and promoting the integration of long-accumulated disciplines such as anatomy, physiology, and embryology on a common foundation. The integration and unity of these disciplines gave birth to biology.

The viewpoint in cell theory that cells are the basic units of life activities made people realize that the mysteries of biological growth, reproduction, development, and various physiological phenomena all need to be explored at the cellular level. Biological research thus progressed from the level of organs and tissues to the cellular level, laying the groundwork for later exploration at the molecular level.

The conclusion in cell theory that new cells are produced by cell division not only explains individual development but also lays the foundation for the establishment of biological evolution theory. New cells come from old cells, which come from even older cells, tracing back through time; the cells of modern organisms are descendants of ancient microbial cells, encapsulating billions of years of genetic inheritance and changes. Each cell, each organism, is a product of history. Included by Engels in the three great discoveries of natural science in the 19th century, cell theory and evolution theory, as cornerstones of biology, imbue biology with a unique charm distinct from other natural sciences.

The Cell as the Fundamental Life System

As Schwann stated: each cell lives relatively independently but also contributes to the overall function of the organism. Single-celled organisms can independently carry out life activities, while multicellular organisms depend on various differentiated cells closely cooperating to collectively perform a series of complex life activities. For example, the withdrawal reflex involves a complex life activity requiring the participation of a series of different cells (Figure 1-2). Indeed, various physiological activities based on cell metabolism in animals and plants, growth and development based on cell proliferation and differentiation, and genetics and variations based on genetic transmission and changes within cells all demonstrate that cells are the fundamental units of life activities, without which life cannot exist.

Why do such small cells possess such powerful functions? Although cells are small, their structures are complex and intricate. Subsequent chapters of this book will show you that cells are complex systems composed of various components working together. Cells are alive; they are life systems.

Within multicellular organisms, cells are components of tissues, tissues are components of organs, and organs are components of individuals. Tissues, organs, and individuals are all whole entities with life activities, thus representing different levels of life systems.

In nature, biological individuals do not exist in isolation but depend on and interact with other individuals of the same and different species, as well as the inorganic environment. Within a certain spatial range, all individuals of the same species form a whole - a population; interactions between different populations form larger wholes - communities; interactions between communities and the inorganic environment form even larger wholes - ecosystems. It can be seen that in nature, from biological individuals to ecosystems, each can be regarded as different levels of life systems (Figure 1-3).

Examining the relationships between different levels of life systems, we find that both structurally and functionally, the cell as a life system belongs to the most fundamental level. The formation, maintenance, and operation of various levels of life systems are based on cells; even the flow of energy and the circulation of matter in ecosystems are no exception. Therefore, it can be said that cells are the fundamental life systems. This book will take you on a journey through this life system, understanding its composition, operation, and the laws of its development and change.