Page 13 - Q & A STPM 2022 Biology

Biology STPM Chapter 3 Membrane Structure and Transport

Section C Essay Questions

Question 11
(a) With the aid of a large labelled diagram, describe the structure of a cell
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membrane. [8]
(b) Explain the function of the various types of membrane proteins. [7]
(15 marks) Semester

Answer : 1
(a) – The model for cell membrane is the fluid-mosaic model by Singer
and Nicholson in 1972.
– The cell membrane is a dynamic structure that consists of a
phospholipid bilayer with protein molecules that float on or within
the membrane.
– The non-polar long hydrocarbon tails are hydrophobic and face
inwards and are attracted to one another by hydrophobic interactions
and van der Waals forces.
– The polar phospholipid heads are hydrophilic, face outwards and are
attracted to the water in the cell and the outer aqueous surrounding.
– The protein molecules are scattered between phospholipid molecules
forming a mosaic pattern.
– Extinsic proteins are on the outer and inner surfaces of the membrane.
– Intrinsic proteins are proteins that are partially embedded or
penetrate the membrane (integral or transmembrane protein).
– The phospholipid membrane is fluid because the phospholipid
molecules and some protein molecules can move laterally or change
places.
– Unsaturated fatty acids have double bonds which form kinks. These
prevent close packing of the phospholipid molecules, thus increase
the membrane fluidity.
– Short, branched carbohydrate chains bind to proteins to form
glycoproteins and those that bind to lipids form glycolipids. These
are important for cell recognition.
– There are cholesterol molecules between some phospholipid
molecules.
– These molecules help to stabilise the membrane structure and
regulate membrane fluidity.
– The membrane is supported by interacellular protein filaments at the
inner surface, which act as a cytoskeleton.

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