fluid mosaic model

structure of a cell surface membrane

• consists of a bilayer of phospholipids with different proteins attached to it or embedded with the bilayer

• carbohydrate chains attached to the phospholipids or proteins are found on the side of the membrane in contact with the external environment of the cell

phospholipids

• phospholipids are primarily responsible for the physical properties of the cell membranes

• a molecule consists of

  • two fatty acid chains linked to two of the three carbons of the glycerol molecule

  • phosphate group is attached to the last carbon of the glycerol molecule

    • the phosphate group is electrically charged, making it hydrophilic

    • the fatty acid chains are hydrophobic

• hence phospholipid molecules are amphipathic molecules due to their distinctive hydrophobic regions

phospholipid → arrangement

• the hydrophilic heads are in contact with the aqueous environment

• the hydrophobic tails are shielded from the aqueous environment

fluid mosaic → definition

• fluid refers to the phospholipid and embedded proteins moving freely and laterally within the phospholipid bilayer

• mosaic refers to the protein and cholesterol (animals only) molecules that are randomly embedded and scattered among the phospholipid molecules; resulting in a mosaic arrangement of the components in the cell membrane

different components of the cell membrane

1. channel proteins

2. carrier proteins

3. enzymes

4. receptors

5. glycoproteins

6. glycolipids

7. cholesterol

channel proteins → functions

allow the passage of specific ions and molecules across the membrane

carrier proteins → function

allow the passage of specific ions and molecules across the membrane

enzymes → functions

catalyse reactions

receptors → functions

have specific binding sites, allowing chemical messengers (e.g. hormones) to bind to the protein, resulting in the relay of message to the inside of the cell

glycoproteins → function

• play a role in cell-cell recognition and cell adhesion

• consist of carbohydrate chains covalently bonded to proteins

• found on the side of the membrane facing the exterior of the cell

glycolipids → function

• involved in cell recognition

• consists of carbohydrates molecules covalently bonded to phospholipid molecules

• carbohydrate molecules are attached to the side of the membrane facing the exterior of the cell

cholesterol → function

plays a role in reducing fluidity of the cell membrane

membrane fluidity

• increased at higher temperatures

• affected by the composition of the phospholipid bilayer

  1. unsaturated fatty acids

  2. saturated fatty acids

  3. cholesterol

unsaturated fatty acids → function in membrane fluidity

presence of unsaturated fatty acids in the phospholipids increases membrane fluidity

saturated fatty acids → function in membrane fluidity

presence of saturated fatty acids decreases membrane fluidity

cholesterol → function in membrane fluidity

• increases membrane fluidity at low temperatures

• decreases membrane fluidity at high temperatures

transport across membrane

• cell membranes are partially permeable, allowing some molecules and ions to pass through but not others

• molecules can cross the cell membrane by different ways, depending on their concentration across the membrane

  1. simple diffusion

  2. facilitated diffusion

  3. active transport

simple diffusion

• small molecules and lipid soluble substances are able to pass directly through the hydrophobic core of the cell surface membrane by simple diffusion

• transport proteins are not involved in moving the molecules across the membrane

• a passive process, there is no utilisation of energy from the hydrolysis of ATP

• oxygen, carbon dioxide, nitrogen and lipids are able to cross the hydrophobic core of the cell membrane by simple diffusion

facilitated diffusion

• transport proteins (carrier proteins and channel proteins) are involved in the transport of hydrophilic molecules and ions across the hydrophobic bilayer

• passive process → molecules or ions move down a concentration gradient, entering/leaving the cell with the help of transport proteins embedded in the phospholipid bilayer

• most transport proteins are very specific → transport some substances but not others

channel proteins → facilitated diffusion

• provide a hydrophilic passage to allow specific hydrophilic molecules (water, amino acids, glucose and glycerol) and ions to move across the hydrophobic core of the cell membrane

• e.g.: proteins channels called aquaporins present in the cell membrane to facilitate the movement of water molecules across the membrane into/out of the cell

carrier proteins → facilitated diffusion

• alternates between two shapes, moving the molecule/ion across the membrane with a change in shape

• change in shape is triggered by the binding and release of the molecule at its binding site

• the molecule to be transported is shielded from the hydrophobic core of the cell membrane

• carrier proteins involved in facilitated diffusion move molecules across the cell membrane down a concentration gradient

• no input of energy from the hydrolysis of ATP is involved

active transport

• movement of molecules/ions from a region of lower concentration to a region of higher concentration across the cell membrane

• energy from the hydrolysis of ATP is required

• cells that constantly transport molecules by active transport requires a lot of energy → large number of mitochondria

• carrier proteins are also required for the process