C. General Cellular Physiology (Life of Eukaryotic Cells)
1. Cell Division and Cell Cycle
Definition and General Characteristics
The fundamental characteristic of the cell as the unit of life is the ability to divide, which results in the production of two daughter cells from one mother cell. The regulation of cell division is a process of remarkable complexity in eukaryotes. For simpler unicellular prokaryotic or eukaryotic organisms, the regulation is typically straightforward, often requiring only sufficient nutrients and a favorable environment (e.g., temperature) to trigger growth and multiplication.
In contrast, for multicellular organisms, regulation is significantly more complicated. Cells only divide when they receive the appropriate signals indicating that more cells are needed for the overall benefit of the organism. For example, Adipocytes respond to nutrient excess; myoblast division is stimulated by exercise; and an increase in red blood cells is induced by conditions like exercising at altitude (low oxygen levels). Furthermore, eukaryotic cell division is generally slower than in prokaryotic cells, typically requiring 12–24 hours on average.
Mechanisms - The Cell Cycle
The cell cycle is a precisely organized and highly regulated process that functions like a cellular clock, with several phases repeated during every division.
Phases:
- Interphase: The longest phase, dedicated to cell growth, with few visible morphological changes. It is subdivided into three stages: the G1 phase (active growth and preparation for DNA synthesis), the S phase (DNA replication, or synthesis), and the G2 phase (post-replication growth and preparation for mitosis).
- Mitosis: A short phase characterized by dramatic morphological changes, including nuclear disassembly, chromosome condensation and movement, and the partitioning of the genome, ultimately leading to division. It is subdivided into five stages: Prophase (nuclear disassembly and chromosome condensation), Metaphase (chromosome alignment at the metaphase plate), Anaphase (chromatid sorting and separation), and Telophase (nucleus reformation).
- Cytokinesis is the physical separation of the two daughter cells.
Checkpoints:
These are vital regulation controls situated at various phases of the cycle. At a checkpoint, the cell analyzes the state of internal processes (e.g., integrity of DNA) and only proceeds to the next phase if everything is functioning correctly.
Non-dividing Cells:
Cells have the capacity to exit the cell cycle and cease dividing, entering the G0 phase, where they are considered quiescent cells. In complex organisms, only a small percentage of cells are dividing at any given time, as they only proliferate when necessary and upon receiving specific signals.
2. Cell Differentiation
Definition and General Characteristics
Eukaryotic cells exhibit a remarkable ability for specialization through differentiation. Cells undergo differentiation to develop specific structures and molecular profiles, allowing them to perform specific functions with high efficiency. This specialization is the source of the tremendous versatility observed in eukaryotic life.
Differentiation often entails dramatic changes at the morphological level. Classic examples include the highly characteristic biconcave disc shape of mammalian red blood cells and the vastly different morphology of Purkinje neurons. Differentiation is invariably accompanied by significant changes at the molecular level. For example, red blood cells require the synthesis of large amounts of hemoglobin, while neurons must express ion channels.
Mechanisms
The underlying mechanism for differentiation is specific gene expression. All cells start with the same genome (DNA); however, the regulated expression of a specific subset of genes within that genome allows for the generation of vastly different cell types, each capable of taking on specialized physiological functions.
3. Cell Associations: Multicellular Organisms
Definition and General Characteristics
Eukaryotic cells commonly associate and communicate to form groups that coordinate and cooperate in their function. This capacity is the foundation for the biological success of forming complex multicellular organisms. Developmental biology is the study of the complex journey from a single cell to the formation of a multicellular organism.
Groups of cells coordinate their functions to form tissues. Groups of tissues, in turn, associate and coordinate to form organs. Finally, groups of organs associate to form the complete organism.
Mechanisms
Cell associations are facilitated by specific surface molecules known as Cell Adhesion Molecules (CAMs). Cell communication is achieved through intricate cell signaling molecules and cell signaling pathways. This entire process of association and communication is regulated by specific gene expression at the genomic level, which dictates cellular behavior in both space and time during development and homeostasis.
4. Cell Death
Definition and General Characteristics
In multicellular organisms, the elimination of specific cells is a frequent and necessary occurrence. Significant numbers of eukaryotic cells in multicellular organisms proactively commit suicide for the overall benefit of the organism. This process is an essential part of development and homeostasis. Cell death is a process regulated at the genomic level. Its deregulation can lead to a variety of pathologies, including developmental malformations, cancer, degenerative diseases, and autoimmune diseases.
Mechanisms
The most recognized and famous process of programmed cell death is called apoptosis. Apoptosis is characterized by a well-defined set of morphological changes. These changes include membrane blebbing, followed by the shrinking of the cell and its nucleus, and ultimately, fragmentation into apoptotic bodies. Crucially, these cell fragments are then removed by macrophages without causing any inflammation or damage to the surrounding organism, ensuring a "clean removal" of the compromised cell. Multiple genes and proteins are involved in the complex process of regulating and executing the apoptotic pathway.