1. Cells - the fundamental unit of life

CELL CONCEPT

Cells: The fundamental units of life

Unity and Diversity of Cells

Prokaryotic vs Eukaryotic Cells

Instructor: Dr. Victor Okika

CELLS: THE FUNDAMENTAL UNITS OF LIFE

All living organisms are composed of cells, which are the basic units of life. Cells are membrane-enclosed structures filled with a concentrated aqueous solution of chemicals, capable of self-replication through growth and division, distinguishing them from non-living entities.

Types of Organisms:

  • Unicellular organisms: These consist of solitary cells such as bacteria and protozoans, which perform all necessary life functions within a single cell.

  • Multicellular organisms: These are comprised of groups of cells that work together to perform distinct and specialized functions. Examples include fungi, plants, and animals. Higher organisms develop from a single fertilized egg, known as a zygote, with the process of differentiation allowing cells to specialize for particular functions, facilitating communication, and promoting cooperation in maintaining life.

CELL BIOLOGY

Cell biology refers to the comprehensive study of cells, focusing on their structure, function, and behavior. It addresses profound questions about life including its origins, development, diversity, and evolution, and explores the mechanisms underlying cellular processes, genetic information, and intercellular communication.

LIVING ORGANISMS

Organisms may vary as either unicellular or multicellular. Common examples include:

  • Unicellular: Cosmarium, Spirogyra, Escherichia coli, Paramecium, Chlorella

  • Multicellular: Starfish, Volvox, Nostoc, Chara

UNITY AND DIVERSITY OF CELLS

Approximately 100 million species exist on Earth, exhibiting significant differences in form and function (e.g., dolphins, rose plants, monkeys). Cells vary in form and function; for example, animal cells differ from plant cells, with plant cells containing chloroplasts and a rigid cell wall, and cells within multicellular organisms can differ in appearance, activity, and purpose. Despite physical differences, all cells share fundamental biochemical processes such as cellular respiration and metabolism as well as basic components including DNA, RNA, and proteins.

CELL SIZE AND SHAPE

Cell characteristics include variations in size, shape, and function, each tailored to its specific role in the organism.Examples:

  • Bacterial cells may be about 2µm in length; the smallest width of a human hair is approximately 25 times larger.

  • Plant cells are generally immobile with fixed shapes due to rigid cell walls, while macrophages can change shape, allowing them to engulf foreign particles such as pathogens.Common shapes observed are spherical, polygonal, disc-like, cuboidal, columnar, spindle-like, and fiber-like, reflecting their functions (e.g., muscle cells being elongated for contraction).

CELL SIZES

Cell sizes can vary significantly, typically ranging between 0.2µm to 20µm.

  • Smallest cell: Mycoplasma gallisepticum (approximately 0.1µm)

  • Bacteria: Escherichia coli (1-2µm)

  • Protozoa: Amoeba proteus (220–760µm)

  • Generally, plant cells (up to 100µm) are larger than animal cells (10-30µm).

IMPORTANCE OF CELL SIZE

Cells must interact with their environment; as size increases beyond a certain limit, the surface area to volume ratio decreases, which can impede transport processes across the membrane. Therefore, cells must divide to maintain favorable surface area to volume ratios that enable efficient nutrient uptake and waste elimination.

CELL REPLICATION

Cells reproduce via the duplication of genetic material and cellular components, resulting in the formation of two daughter cells. The central dogma of molecular biology (DNA ➔ RNA ➔ Protein) underpins these replication processes, where nucleotide sequences dictate protein assembly, impacting cell function. It is important to note that only living cells have the ability to replicate; viruses, being acellular, do not possess this capability.

EVOLUTION OF CELLS

Mutations that occur during DNA replication can lead to variations among cells; natural selection subsequently influences which variations persist within populations. Additionally, sexual reproduction facilitates genetic recombination, which further affects heredity patterns and drives evolution over time, allowing species to adapt to changing environments.

CELL GENOME

A cell's genome encompasses all the genetic information (DNA) required for its function, shaping the characteristics and behaviors of the cell. Variations in cell types arise based on how different cells utilize this genetic information in response to various internal signals and external environmental cues, which can also engage in epigenetic changes, affecting gene expression without altering the DNA sequence.

MICROSCOPY OF CELLS

Cells were first observed using a microscope in the 17th century:

  • Light Microscopes: Employ light to visualize living tissues but may obscure finer details due to resolution limits.

  • Electron Microscopes: Utilize electron beams for significantly higher resolution images of internal cell structures, allowing researchers to delve into cell morphology and organelle function.

DISCOVERY OF CELLS

  • Robert Hooke first discovered cells in 1665 using a microscope, observing cork and coining the term "cells".

  • Antonie van Leeuwenhoek later observed living cells (e.g., bacteria and protozoa) in 1672, expanding our understanding of cellular diversity.

  • Robert Brown identified the nucleus in 1831, leading to the conclusion that all cells possess this structure, further propelling cell theory.

CELL THEORY

The traditional cell theory, proposed by Schleiden and Schwann, posits that:

  1. All organisms are made of cells.

  2. Cells are the basic units of life.

  3. All cells arise from pre-existing cells.Modern interpretations extend to include functional and genetic commonalities among cells, recognizing the intricate relationships and communication that sustain multicellular life.

PROKARYOTIC CELLS

Prokaryotic organisms, which encompass bacteria and archaea, lack a defined nucleus and typically reproduce rapidly by division. They contain tough protective coats (cell walls) and may exist as unicellular entities or in chains. Prokaryotes are broadly divided into two groups:

  • Bacteria: Well-known and diverse, playing essential roles in ecology, including decomposition and nutrient cycling.

  • Archaea: Found in extreme environments, such as hot springs and salt lakes, showcasing adaptations to survive and thrive in harsh conditions.

EUKARYOTIC CELLS

Eukaryotic cells possess a nucleus and membrane-bound organelles, allowing for complex processes to occur in specialized compartments. This group includes both unicellular (e.g., yeast) and multicellular (e.g., humans) forms, where cells are organized into tissues and organs that perform specific functions.

MITOCHONDRIA AND CHLOROPLASTS

  • Mitochondria are the powerhouses of the cell, producing ATP through cellular respiration; they possess their own circular DNA, which allows for semi-autonomous replication.

  • Chloroplasts, found in plants and algae, are essential for photosynthesis, capturing light energy to convert carbon dioxide and water into glucose and oxygen, thus playing a crucial role in energy transfer in ecosystems.

ENDOPLASMIC RETICULUM AND GOLGI BODIES

These organelles are vital for the synthesis, modification, and transport of proteins within cells.

  • Endoplasmic Reticulum (ER): Exists in two forms: rough ER, which is studded with ribosomes for protein synthesis, and smooth ER, which is involved in lipid synthesis and detoxification.

  • Golgi Bodies: Modify, package, and transport molecules produced in the ER, facilitating their delivery to various destinations within or outside the cell.

CELL ORGANELLES

Organelles play specialized roles in cellular function, including:

  • Lysosomes: Contain digestive enzymes for breaking down waste materials and cellular debris.

  • Peroxisomes: Involved in detoxifying harmful substances and metabolizing fatty acids.

  • Various vesicles are involved in facilitating intracellular transport of molecules, enabling homeostasis and communication.

CYTOSKELETON

A dynamic network of protein filaments supports cell shape, intracellular transport, and cell division, composed of three primary components: actin filaments, microtubules, and intermediate filaments. This structure provides mechanical support and plays critical roles in motility and the organization of organelles within the cell.

CONCLUSION

Cells serve as the foundational building blocks of all life forms, exhibiting incredible diversity and complexity, while still sharing fundamental characteristics that unite them as living entities. Understanding cellular structure and function is essential for grasping the principles of biology, medicine, and the interconnectedness of life.