Introduction: The Interactive and Connected Cell
In multicellular organisms, cells are not isolated units; they are organized into tissues and organs, communicating and adhering to one another. This lesson explores cell junctions, the specialized structures that connect cells. We will then revisit the plasma membrane in greater detail to understand the sophisticated mechanisms of membrane transport that allow cells to maintain their internal environment and interact with the outside world.
Part 1: Cell Junctions - The Social Life of Cells
1.1 What are Cell Junctions?
Cell junctions are points of contact between adjacent cells or between a cell and the extracellular matrix. They are crucial for maintaining the integrity of tissues, enabling communication between cells, and forming barriers to control the passage of substances.
1.2 Junctions in Animal Tissues
Animal cells have three main types of junctions, each with a distinct function.
Diagram: Types of Cell Junctions in Animal Tissues
- Tight Junctions: Form a continuous seal around cells, preventing the leakage of extracellular fluid across a layer of epithelial cells. They are like the watertight seal in a ship's hull.
- Desmosomes (Anchoring Junctions): Function like rivets, fastening cells together into strong sheets. Intermediate filaments made of keratin anchor desmosomes in the cytoplasm.
- Gap Junctions (Communicating Junctions): Provide cytoplasmic channels between adjacent cells. They consist of membrane proteins that surround a pore through which ions, sugars, amino acids, and other small molecules may pass. They are essential for communication in many tissues, like the heart muscle.
1.3 Junctions in Plant Tissues
Plant cells are encased in rigid cell walls, so their junctions are different. Plasmodesmata are channels that perforate plant cell walls. Through plasmodesmata, water and small solutes (and sometimes proteins and RNA) can pass from cell to cell. They are functionally similar to gap junctions in animal cells.
Part 2: The Plasma Membrane and Transport
2.1 The Fluid Mosaic Model Revisited
The fluid mosaic model describes the plasma membrane as a fluid structure with a "mosaic" of various proteins embedded in or attached to a bilayer of phospholipids. The membrane is selectively permeable, regulating the cell’s molecular traffic. Cholesterol molecules wedged between phospholipids help maintain membrane fluidity at different temperatures.
2.2 Passive Transport: No Energy Required
Passive transport is the diffusion of a substance across a membrane with no energy investment. Substances move down their concentration gradient (from high to low concentration).
- Simple Diffusion: The net movement of small, nonpolar molecules like O₂, CO₂, and lipids directly through the lipid bilayer.
- Facilitated Diffusion: The passage of polar molecules (like glucose) and ions (like Na⁺) with the help of specific transmembrane transport proteins. Channel proteins provide corridors, while carrier proteins change shape to shuttle their cargo across.
- Osmosis: The diffusion of free water across a selectively permeable membrane. Water moves from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration).
2.3 Active Transport: Energy Required
Active transport uses energy (usually in the form of ATP) to move solutes
• Primary Active Transport: Directly uses ATP to power the transport. The sodium-potassium pump is a crucial example, pumping 3 Na⁺ out and 2 K⁺ in, creating both concentration and electrical gradients.
• Secondary Active Transport (Cotransport): Uses the concentration gradient created by a primary active transporter to drive the movement of another substance against its own gradient.
Diagram: Sodium-Potassium Pump (Primary Active Transport)
2.4 Bulk Transport: Vesicles for Large Molecules
Large molecules such as polysaccharides and proteins cross the membrane in bulk via vesicles, which requires energy.
- Exocytosis: The cell secretes certain molecules by the fusion of vesicles with the plasma membrane.
- Endocytosis: The cell takes in molecules by forming new vesicles from the plasma membrane. Includes phagocytosis ("cellular eating"), pinocytosis ("cellular drinking"), and receptor-mediated endocytosis (highly specific).