In mammals, some cell signalling molecules are steroid (lipid) hormones. These hormones are transported in the bloodstream to reach capillary networks. At a capillary network, hormones pass out of the blood into tissue fluid. (a) Fig. 3.1 is a diagram of a capillary network. blood from artery tissue fluid X Fig. 3.1 body cells blood to vein

Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

1. [3] accept from either column for mark and assume blood if not stated any three from blood: has red blood cells; more, white blood cells / leucocytes / named e.g. neutrophils, lymphocytes, monocytes or fewer macrophages; has platelets; has, more protein / large proteins or has, plasma proteins / named example e.g albumin / fibrinogen / globulin these are large proteins; higher, concentration / AW, oxygen A more oxygenated; higher, concentration / AW, glucose / amino acids / fatty acids; lower, concentration / AW, carbon dioxide A urea, in liver / muscles I waste; higher pressure; tissue fluid: has no red blood cells; fewer white blood cells / leucocytes / named e.g. neutrophils, lymphocytes, monocytes or more macrophages; has no platelets; has, fewer/no large, proteins or has no, plasma proteins / named example e.g. albumin / fibrinogen / globulin only allow fewer plasma proteins if clear the proteins present are small enough to leave blood; lower, concentration / AW, oxygen A less oxygenated; lower, concentration / AW, glucose / amino acids / fatty acids; higher, concentration / AW, carbon dioxide A urea, in liver / muscles I waste; lower pressure;
2. [1] lymph; A lymphatic fluid
3. [2] any two from: 1 can cross /AW, hydrophobic core (of bilayer) / phospholipid bilayer or can pass through gaps between, fatty acid tails / phospholipids ; 2 is, hydrophobic / lipid-soluble / non-polar / not ionic / not charged or not, hydrophilic / water-soluble / polar / ionic / charged; 3 small size ;
4. [1] Reject if hormone S or receptor R described as an antigen or enzyme complementary (shape) to / only fits into / specificity for (binding to), R / receptor;
5. [1] nuclear pore; A nuclear pores
6. [2] B = transcription; C = translation;
7. [1] (80S) ribosome ; I 70S
8. [4] any four from 1 (formation of), secondary / 2° / second level, structure with, alpha-helix / x-helix/ beta-pleated sheet(s) / ẞ-pleated sheet(s); A ẞ-conformation for ẞ-pleated sheet I incorrect detail of bonds 2 folding/coiling, to form tertiary / 3° / third level, structure ; A ref. to 3D structure as alternative to, folding / coiling A secondary structure, folds / coils, to form tertiary structure R alpha helix / beta-pleated sheet, coils / folds 3 two of hydrogen bonds / ionic or electrostatic bonds / disulfide bonds or bridges / hydrophobic interactions ; I peptide bonds as part of a list that contains two correct bond types these must be in context of tertiary structure 4 (formation of), quaternary / 4° / fourth level structure ; A description e.g. polypeptides held together by, interactions / hydrogen bonds / ionic bonds / disulfide bonds R if peptide bonds included 5 (globular protein) has hydrophilic, amino acids / R-groups / side chains, to, outside / AW ora hydrophobic, amino acids / R-groups / side chains, towards centre / AW 6 example of other post-translational modification ; e.g. removal, methionine / met addition sugar group / glycosylation addition phosphate group / phosphorylation addition prosthetic group