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SAI INTERNATIONAL SCHOOL
CLASS-XI
LESSON NOTES
Cell Membranes and Transport

4.1 Fluid mosaic membranes

Fluid mosaic model • at low temperatures, cholesterol increases the
• ‘fluid’ refers to the movement of phospholipids while fluidity of the membrane preventing it from being
‘mosaic’ refers to the scattered proteins (and too rigid, this is because it prevents close
glycoproteins) in the phospholipid bilayer packing of phospholipid tails
• at high temperatures, cholesterol decreases
1) Phospholipids the fluidity of membrane and stabilises the cell

2, 3) Glycolipids and glycoproteins
Lipid and protein molecules on the outer surfaces of cell
membrane have carbohydrate chains attached to them
forming glycolipids and glycoproteins

These carbohydrate chains projecting out like antennae:
• stabilise the membrane structure by forming hydrogen
bonds with water molecules surrounding the cell
• glycocalyx – sugary cell coating formed by
carbohydrate chains
• act as receptor molecules:
 signalling receptors – recognise messenger
molecules like hormones and neurotransmitters
 endocytosis – bind to molecule to be engulfed by
Image: https://courses.lumenlearning.com/
membrane
• phospholipids are arranged so that hydrophobic, • act as cell markers/antigens allowing cell-cell
non- polar tails do not face water. Water is on both recognition
the intracellular and extracellular sides
• therefore, tails point inwards, and hydrophilic 4) Proteins
heads face the aqueous medium

• proteins that are found • can be present inside or
Image: https://www.chegg.com/ embedded within the outside of the cell
membrane membrane i.e.,
Membrane fluidity intracellular, and
Membrane fluidity refers to the viscosity of the lipid 2) saturation of fatty acid tails are bent and fit
bilayer of a cell membrane. acid – the more together more loosely
unsaturated they 3) cholesterol
Membrane fluidity is affected by: are, the more fluid • regulates the fluidity of
the membrane.
1) tail length – longer the tail, the less fluid the This is as membrane
membrane unsaturated fatty