A-LevelBiologyCell structure9700/23 · May/June 2018 · Q212 Marks

Adipose tissue, which is composed of cells known as adipocytes, stores large quantities of triglycerides and functions as an energy storage tissue. Fig. 2.1 is a photomicrograph of adipose tissue. [Figure 2.1]

(a)Adipocytes can be very large in size compared to other body cells. This is due to a large lipid droplet within the cell. The largest adipocyte in Fig. 2.1 has a mean diameter of 35 µm. A person with good eyesight can see cells of 0.05mm or greater diameter without a magnifying glass or any other optical aid. State whether the person can see this adipocyte without any optical aid. Show your working to justify your answer.[1]
(b)Only some of the organelles within the adipocyte can be seen using a high quality light microscope set at the highest magnification. Organelles such as rough endoplasmic reticulum, smooth endoplasmic reticulum and ribosomes are only visible using an electron microscope. Explain why these organelles are not visible using a light microscope.[2]
(c)(i)Name the type of bond broken by active ATGL to produce fatty acids and glycerol.[1]
(c)(ii)Name and outline the process by which the fatty acids shown in Fig. 2.2 exit the cell.[3]
(c)(iii)Fig. 2.2 is an example of cell signalling within the body. With reference to Fig. 2.2, outline the process of cell signalling.[4]
(d)The fatty acids released from adipocytes are transported in blood plasma and are taken up by cells. Although most cell types can metabolise fatty acids to synthesise ATP in the presence of oxygen, red blood cells cannot do this. Suggest why red blood cells cannot metabolise fatty acids to synthesise ATP.[1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

[1] too small to see
35 µm is 0.035 mm (smaller than 0.05 mm)
or
0.05 mm is 50 µm (larger than 35 µm);
[2] in context of light microscope
two from:
1 resolution / resolving power, poor / not high enough / AW ;
2 ability to distinguish between two points not high enough / AW ;
3 ref. to limit of resolution ;
e.g. organelles smaller than, 0.2 µm / 200 nm, not visible A organelles smaller than 200 nm (accept range 100–300 nm)
4 organelle too small to interfere with light waves ; A wavelength of light too long
I organelles are small
[1] ester;
[3] facilitated diffusion ;
plus two from:
(diffusion) through (membrane), transport / channel / carrier, protein; A pore I transmembrane / integral, protein
high to low concentration or down, concentration / diffusion, gradient ;
passive; A no, ATP / (metabolic) energy, required
if active transport incorrectly stated allow ecf
through (membrane), transport/carrier, protein; R channel protein
low to high concentration / against a concentration gradient / requires ATP / requires (metabolic) energy ;
[4] four from:
1 ref. to hormones, released into / arrive at adipocyte in, blood stream ; A released by another, cell / tissue (elsewhere in the body)
2 hormone is, (chemical) signal / signalling molecule / messenger molecule ;
3 (adipocyte is) target cell; A (adipose is) target tissue
4 (hormone) binds to / AW, receptor; in context of on cell surface membrane of adipocyte
5 ref.to receptor (shape) complementary to hormone ; A specific receptor for hormone
6 (binding) triggers / stimulates, reactions within the cell ; A cascade of reactions I binding causes a reaction to occur
7 cell signalling results in a response, qualified; e.g. triglyceride breakdown fatty acids (and glycerol) produced fatty acids leave, the cell / via transport protein
[1] any one valid suggestion:
e.g. do not have mitochondria / mitochondria required (to produce ATP) ;
do not have nucleus (so no mRNA produced);
no mRNA for enzyme synthesis ;
(required) enzymes not present ;
impermeable to fatty acids / AW ;
have lost/do not have, the required organelles ;