Multicellularity and Size Limitations

Learning Objectives

• Describe how multicellular forms may have evolved from protists
• Evaluate the advantages and disadvantages of multicellularity

The Problem of Size

• All animals need to exchange substances with their environment
• This exchange occurs via diffusion, influenced by surface area, differences in concentration, and distance.
• Small organisms like bacteria have a large surface area to volume ratio, enabling efficient exchange via diffusion.
• Larger organisms, like whales, have a much smaller surface area to volume ratio, making diffusion inefficient over large distances.
• This limits the maximum size single-celled organisms can reach.
• Organisms larger than this size limit are multicellular.

Cell Size and Microscopy

• A diagram displays various sizes of biological components, ranging from atoms to macroscopic organisms, alongside the corresponding microscopy techniques
• Different types of microscopes have varying magnification capabilities to view different sizes of biological specimens from atoms to simple cells to organisms.

Surface Area to Volume Ratio

• The surface area to volume ratio decreases as a cell grows larger
• This is critical for cell function, as exchange of substances relies on the cell's surface.

Solving the SA:V Problem

• Strategies to overcome the limitations of diffusion in larger organisms:
  • Avoidance
  • Geometric solutions (increase surface area through folding or branching)
  • Decrease the effective cell volume
  • Increase rate of supply by increasing concentration of nutrients
  • Optimize nutrient transport within or between cells
  • Improve efficiency to reduce demand by specialization of cells and division of labour within and between cells

Evolution of Multicellularity

• Multicellularity evolved multiple times in eukaryotes
• There are three main theories: Symbiotic, Syncytial, and Colonial
• Symbiotic Theory: involves different species interacting
• Syncytial Theory: occurs in multinucleated cells
• Colonial Theory: one species involved

Examples of Multicellular Development (Volvocine Algae)

• Detailed descriptions of various stages of multicellular colonial algae, including Chlamydomonas, Gonium, Pandorina, Eudorina, Pleodorina, and Volvox.
• Characteristics of each stage, such as colony size, cell specialization, type of reproduction (isogamy, anisogamy, or oogamy), and other features

Advantages of Multicellularity

• Increased size of the organism
• Cell specialization
• Increased surface area to volume ratio

Problems of Multicellularity

• Interdependence of cells for survival
• Complexity of coordinated function

Summary of Evolutionary Changes

• Unicellular to colonial life
• Increase in the number of cells in colonies
• Change in shape of colonies
• Increased interdependence among vegetative cells
• Increased division and specialization among cells
• Transitions in reproduction from isogamy to anisogamy to oogamy
• Fewer female gametes in later stages

Types of Reproduction

• Isogamy: Involves the fusion of similar-sized gametes
• Anisogamy: Involves the fusion of dissimilar-sized gametes
• Oogamy: Involves the fusion of a large, non-motile egg with a small, motile sperm

Summary of Multicellularity

• Key stages in multicellularity evolution are highlighted, with timelines of development, showing growth from simple single-celled organisms into complex multicellular colonies with intercellular cooperation.
• Key changes include modulations in cell number, cell attachments, organismal polarity, partial inversions, basal body rotation, and incomplete cytokinesis in a variety of multicellular algae.

Explore the evolution of multicellular organisms from protists and assess the pros and cons of multicellularity. Understand the relationship between organism size, surface area, and diffusion efficiency critical to biological exchange processes.