Structure and function of the cell membrane

Question 1

Fluid mosaic model

Answer 1

• is the model for cell membrane structure
• membrane is said to be fluid because the molecules that make it up are constantly changing position
• it is said to be mosaic because it is composed of many different molecules

Question 2

Structure of the phospholipid bilayer

Answer 2

• main structure is composed of phospholipid molecules which are lipid molecules containing a phosphate group
• phospholipids are arranged in two layers (bilayer) with hydrophilic (water-loving) heads on the outside and hydrophobic (water-hating) heads on the inside
• they drift from place to place with their heads and tails moving, keeping the membrane fluid
• the phosphate heads are negatively charged and therefore attracted to water as water is polar
• it has two fatty acid chains

Question 3

Substances embedded in the membrane

Answer 3

• embedded in the phospholipid bilayer of the membrane are cholesterol and protein molecules
• cholesterol: molecules are wedged between the phospholipids, are important for the function integrity and stability of the membrane
• protein molecules: (receptor proteins, channel proteins, carrier proteins and cell identity markers)
• some molecules extend from one side of the membrane to the other, while others are bound to the membrane surface
• only 2% of molecules in the membrane are proteins but as they are very large molecules they make up 55% of the mass
• carbohydrates that are attached to proteins: chemical ID card of the cell. called major histro-compatibility complex (MHC), helps individual cells ID each other, immune system can identify cells that are ‘non-self’ (e.g. invading pathogens, cells from organs transplants as self)

Question 4

Functions of the cell membrane

Answer 4

• acts as a barrier: separates cytoplasm from extracellular fluid around the cell. Isolation of cytoplasm is important as their compositions are different
• regulates the passage of materials e.g. entry of ions and nutrients, the removal of water and the release of secretion
• sensitive to changes: first part of the cell affected by changes in the extracellular fluid. Also has receptors that are sensitive to particular molecules
• helps support the cell: internal part of the cell membrane is attached to microfilaments of the cell’s cytoskeleton, thus giving support to the whole cell. Also connectors between the membranes of adjacent cells, provides support to the whole tissue