3.5 The Cell-Surface Membrane Flashcards
What is the cell-surface membrane?
The plasma membrane that surrounds the cells and forms the boundary between the cell cytoplasm and the environment.
What are plasma membranes?
All membranes around and within cells that have the same basic structure.
What structure do phospholipids form?
A bilayer sheet
Why is the phospholipid bilayer sheet important?
One phospholipid layer has its hydrophilic heads pointing inwards, the other layer of phospholipids has its hydrophilic heads pointing outwards. The hydrophobic tails of both phospholipids point towards the centre of the membrane.
What are the three functions of phospholipids in the membrane?
To allow lipid-soluble substances to enter and leave the cell.
To prevent water-soluble substances entering and leaving the cell.
To make the membrane flexible.
What are the two ways proteins are embedded in the phospholipid bilayer?
Extrinsic proteins and intrinsic proteins.
What are extrinsic proteins?
They occur on the surface or partly embedded in the bilayer, never completely across it. They give mechanical support.
What do intrinsic proteins do?
They completely span the phospholipid bilayer from one side to the other. Some act as carriers to transport water-soluble material, some are enzymes.
What are the six functions of proteins in the membrane?
To provide structural support.
To act as carriers transporting water-soluble substances across the membrane.
To allow active transport across the membrane by forming channels.
To form recognition sites by identifying cells.
To help cells adhere together.
To act as receptors.
What is the name if the arrangement of the various molecules in the cell-surface membrane?
The fluid-mosaic model.
What is the fluid part of the fluid-mosaic model?
The individual phospholipid molecules can move relative to one another. This gives the membrane a flexible structure that is constantly changing in shape.
What is the mosaic part of the fluid-mosaic structure?
The proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern.
