BSC1010C: General Biology 1 - Study Notes

BSC1010C: General Biology 1

CELL STRUCTURE (Chapter 4 B2E)

MODULE 4 - PART 3

LEARNING OBJECTIVES PART 3

  • Describe the extracellular matrix
  • List examples of the ways that plant cells and animal cells communicate with adjacent cells
  • Summarize the roles of:
    • Tight junctions
    • Desmosomes
    • Gap junctions
    • Plasmodesmata

CELL-CELL INTERACTIONS

  • Unicellular organisms must contend with constant shifts in environmental conditions.
  • Cells in multicellular organisms must communicate and cooperate with each other, forming an interdependent community of cells.

THE CELL SURFACE

  • The plasma membrane is composed of a phospholipid bilayer that is studded with proteins.
    • Proteins within the membrane may be classified as either integral or peripheral.
  • Membrane proteins have several key functions:
    • Regulate transport: Facilitate the movement of substances in and out of the cell, creating a distinct internal environment.
    • Attach to cytoskeletal elements on the interior surface of the bilayer.
    • Attach to complex extracellular structures.

STRUCTURE AND FUNCTION OF AN EXTRACELLULAR LAYER

  • Most cells possess a protective layer or wall that forms just beyond the plasma membrane, known as the extracellular matrix (ECM):
    • Helps define cell shape.
    • Attaches cells to one another.
    • Acts as a first line of defense against external threats.

Prokaryotic Cell Wall Structures

  • Prokaryotic cells, such as bacteria and archaea, exhibit notably different cell wall structures:
    • Bacteria: Cell walls are primarily made of polysaccharide peptidoglycan polymers connected by peptide bonds.
    • Archaea: Lack peptidoglycan; instead, their cell walls form a dense coat of proteins known as S-layer.

Eukaryotic Extracellular Layers

  • In eukaryotes, extracellular layers maintain a fundamental organization:
    • In animal cells, the juice consists of a fiber composite comprising:
    • A cross-linked network of long filaments that resist tension.
    • A stiff ground substance that provides protection against compression.

Properties of Fiber Composites

  • Fiber composites resist both tension and compression:
    • Concrete (acting as ground substance) resists compression.
    • Steel rods (acting as fibers) resist tension.

THE PRIMARY CELL WALL IN PLANTS

  • Most plant cells are encased by a cell wall:
    • Newly secreted primary cell wall is a fiber composite made of:
    • Long strands of polysaccharide cellulose bundled into crisscrossed microfibrils.
    • Between microfibrils, gelatinous polysaccharides like pectin help keep the cell wall moist.
  • The primary cell wall serves essential functions:
    • Defines the shape of the plant cell.
    • Counteracts turgor pressure caused by water intake via osmosis.
  • Turgor pressure is especially important in young, growing plant cells:
    • Proteins called expansins are secreted, disrupting microfibril cross-linking, allowing the cell’s growth.

Secondary Cell Walls in Plants

  • Mature plant cells secrete a secondary cell wall between the plasma membrane and the primary cell wall:
    • Secondary cell wall structures correlate with specific cell functions:
    • Leaf cells contain waxes.
    • Wood-forming cells contain lignin.

Cell Wall Layers

  • Plant cells typically secrete two cell wall layers:
    • The primary cell wall rich in pectin.

THE EXTRACELLULAR MATRIX IN ANIMALS

  • Most animal cells secrete a fiber composite known as the extracellular matrix (ECM):
    • Provides structural support.
    • Mainly composed of collagen, which forms a network of collagen fibrils consisting of:
    • Groups of collagen triple helixes coalescing together.
    • The ECM ground substance contains proteoglycans:
    • Composed of proteins attached to multiple polysaccharides.
    • Responsible for the rubber-like consistency of cartilage.

Variability in ECM Composition

  • The amount and composition of ECM vary between different tissue types:
    • Tissues are groups of similar cells that function together.
    • Example: Elastin protein in lung tissues allows for stretching.
  • Membrane proteins called integrins connect ECM to the plasma membrane:
    • Integrins bind to cross-linking proteins such as laminins.
    • Integrins also anchor the cytoskeleton to the ECM.

CELL–CELL CONNECTION AND COMMUNICATION

Importance of Cell–Cell Attachments

  • Direct physical connections between cells are fundamental to multicellularity and maintaining tissue structure and function.
  • Cell–cell attachments in multicellular organisms include materials and structures that bind cells together:
    • Especially significant in epithelial tissues (e.g., lining surfaces).
    • The diversity of structures for holding cells together varies among organisms.

Indirect Cell–Cell Attachments in Plants

  • In plants, the extracellular space between adjacent cells is filled with:
    • A central layer known as the middle lamella.
    • The middle lamella glues plant cells together and is continuous with the primary cell walls composed of gelatinous pectins.

Plasmodesmata in Plant Cells

  • Plasmodesmata are perforations in the cell walls of plants that facilitate cytosol flow between cells:
    • Plasma membranes lining plasmodesmata ensure continuous cytoplasm between adjacent cells.
    • Allow the passage of water, small solutes, and certain proteins and RNA molecules, functioning as an intercellular junction.

Corridors in Plant Tissues

  • Plant tissues are divided into two main corridors:
    • Symplast: Shared cytoplasm of connected cells.
    • Apoplast: Extracellular space surrounding cells.

Animal Cell Adhesion and Communication

  • Animal cells communicate and adhere through various specialized structures:

CELL-TO-CELL CONNECTIONS IN ANIMALS

Tight Junctions

  • Tight junctions are formed by cell–cell attachments composed of membrane proteins in adjacent animal cells:
    • These proteins align to bind with one another, creating a watertight seal between cells.
    • Commonly found in epithelial tissues, tight junctions are dynamic and variable:
    • Can loosen to permit transport.
    • Can adjust to environmental changes.

Desmosomes

  • Desmosomes provide strong cell–cell attachments common in epithelial and muscle cells:
    • Comprised of linking proteins and cytosolic anchoring proteins.
    • Cytoskeletal intermediate filaments reinforce desmosomes:
    • Attach to intracellular anchoring proteins, enhancing cell durability.

Cadherins and Specificity

  • Cadherins are major classes of adhesion proteins identified in desmosomes:
    • Each cadherin type binds only to cadherins of the same type, allowing cells of the same type to specifically attach:
    • For example, N-cadherin in nervous cells and E-cadherin in epithelial cells.

Gap Junctions

  • Gap junctions connect adjacent animal cells through protein channels:
    • Allow small molecules to flow between cells, facilitating coordination of activities.
    • Serve as communication portals enabling rapid movement of regulatory ions and small molecules, enhancing signaling and functional responses.

CONNECTIONS BETWEEN ADJACENT CELLS

  • Adjacent animal and plant cells communicate directly through specialized structures that manage adhesion and exchanges between cells, ensuring cooperative functionality across multicellular organisms.

Note: All content is sourced from Pearson Education, Inc. and is part of the General Biology 1 course material, covering various aspects of cell structure, extracellular matrices, and intercellular communication.