Glossary
Adhesion Proteins
Membrane proteins that help cells stick together, forming junctions and tissues.
Example:
In your skin, adhesion proteins act like molecular Velcro, tightly binding skin cells together to form a protective barrier.
Cell Surface Markers
Membrane proteins (often glycoproteins) that act as identification tags, allowing cells to recognize each other.
Example:
Your immune system uses cell surface markers to distinguish your own healthy cells from foreign invaders like bacteria or viruses.
Channel Proteins
A type of transport protein that forms a hydrophilic pore through the membrane, allowing specific ions or small molecules to pass passively.
Example:
Nerve impulses rely on rapid movement of ions through channel proteins, which open and close like tiny gates to allow sodium or potassium to flow.
Fluid Mosaic Model
A model describing the plasma membrane as a dynamic, flexible structure where proteins are embedded within a fluid phospholipid bilayer.
Example:
The fluid mosaic model explains why membrane proteins can move laterally, much like icebergs floating and shifting in a lipid 'sea'.
Glycolipids
Lipids in the cell membrane with attached carbohydrate chains, involved in cell-to-cell recognition and adhesion.
Example:
Blood types (A, B, AB, O) are determined by specific glycolipids on the surface of red blood cells, acting as unique cellular fingerprints.
Glycoproteins
Proteins embedded in the cell membrane with attached carbohydrate chains, crucial for cell recognition and signaling.
Example:
Your immune cells use glycoproteins as 'ID tags' to recognize friendly cells versus invading pathogens, preventing autoimmune attacks.
Hydrophilic Heads
The phosphate-containing, water-attracting regions of phospholipids that face the aqueous environments inside and outside the cell.
Example:
When you see a cell membrane, the outer surfaces are lined with hydrophilic heads, eager to interact with the watery cytoplasm and extracellular fluid.
Hydrophobic Tails
The fatty acid chains of phospholipids that are water-repelling and face inward, forming the core of the membrane.
Example:
The oily, nonpolar hydrophobic tails of the phospholipids huddle together in the middle of the membrane, avoiding the surrounding water like oil in a salad dressing.
Phospholipid Bilayer
The fundamental structure of the plasma membrane, consisting of two layers of phospholipids arranged tail-to-tail.
Example:
Imagine a cell membrane as a double-layered fence, where the phospholipid bilayer forms the main barrier, controlling what enters and exits the cell.
Polar or Charged Molecules
Molecules that have an uneven distribution of charge or carry a net electrical charge, which prevents them from easily passing through the hydrophobic core of the membrane.
Example:
Glucose and sodium ions (Na+) are polar or charged molecules that cannot simply diffuse across the membrane and require the help of transport proteins.
Receptor Proteins
Membrane proteins that bind to specific signaling molecules (like hormones) outside the cell, triggering a response inside.
Example:
When insulin binds to a receptor protein on a muscle cell, it signals the cell to take up glucose from the blood, lowering blood sugar levels.
Selective Permeability
The property of the cell membrane that allows some substances to pass through more easily than others, regulating what enters and leaves the cell.
Example:
The cell membrane's selective permeability is why water can easily cross, but large proteins or charged ions require specific channels or pumps.
Small, Nonpolar Molecules
Molecules that are small in size and lack a charge or significant polarity, allowing them to easily diffuse directly across the lipid bilayer.
Example:
Oxygen (O2) and carbon dioxide (CO2) are small, nonpolar molecules that effortlessly pass through lung cell membranes during gas exchange.
Transport Proteins
Membrane proteins that facilitate the movement of specific substances across the cell membrane, either passively or actively.
Example:
To get glucose into a cell, specialized transport proteins act as molecular ferries, carrying the sugar across the membrane.