Page 23 - Q & A STPM 2022 Biology

Biology STPM Chapter 3 Membrane Structure and Transport

Section C Essay Questions

Question 15
(a) Explain the roles of the cell membrane structures in the transportation of
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substances into the cell.
(b) Explain the role of carbohydrate in the membrane. Semester

Answer : 1
(a) – The cell membrane is semi-permeable.
– The membrane consists of phospholipids and are embedded with
protein molecules.
– The phospholipid bilayer is permeable to steroids, fatty acids and
lipid-soluble molecules as well as very small uncharged molecules
like oxygen, carbon dioxide and water.
– Osmosis involves the diffusion of water molecules through the
semi-permeable membrane from a region of high water potential
to a region of low water potential. Some protein channels called
aquaporins in the membrane increase the rate of diffusion.
– Some proteins form ion channels that follow diffusion of ions such
as K , Ca , Na and Cl down their concentration gradients.
+
–
+
2+
– Some large molecules such as glucose and amino acids cannot move
across the phospholipid bilayer. These molecules move across the
membrane by facilitated diffusion using a protein carrier molecule.
– Carrier proteins / protein pumps are involved in active transport,
whereby energy is required to transport ions or molecules across
the cell membrane against a concentration gradient.
– For example, the sodium-potassium protein pump in the membrane
of a neuron uses energy to pump sodium ions out of the cell and
potassium ions into the cell.
– Endocytosis and exocytosis are active transport processes that
transport material in bulk across the membranes.
– Endocytosis involves the bulk transport of substances into a cell.
– In exocytosis, the vesicle fuses with the cell membrane and the
contents such as enzymes, hormones and mucus in the vesicle are
released out of the cell.

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