Gametogenesis

Gametogenesis

During fertilisation, the acrosome of a mature sperm cell releases enzymes. These enzymes digest a path through the glycoprotein in the layer surrounding the secondary oocyte. Explain how the glycoprotein is broken down by these enzymes.

Addition of water molecules / hydrolysis;Protein to amino acids / break peptide bonds;Carbohydrate to monosaccharides / break gycosidic bonds;Glycoprotein attaches to active site / forms E-S complex;Because it has complementary/matching shape; [reject same shape] max 2

[allow 1 mark for glycoprotein broken down to amino acids/monosaccharides]

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Past exam questionDescribe a blastocyst.

The markschemeHollow balls of cells;Cells at one end form embryo;Accept points from a labelled diagram / diagram accompanying explanation.

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cyst

P u l m o n a r y a r t e r y

F o r a m e n o v a l e

L i v e r

U m b i l i c a l v e i n

P l a c e n t a

U m b i l i c a l a r t e r i e s

L u n g s

R i g h t a t r i u m L e f t a t r i u m

R i g h t v e n t r i c l e L e f t v e n t r i c l e

P l a c e n t a

B o d yt i s s u e s

D u c t u sa r t e r i o s u s

U m b i l i c a lv e i n

U m b i l i c a la r t e r y

Only a small amount of blood passes through the lungs of a fetus. Draw arrows on the Figure to show one pathway which transports carbon dioxide from the body tissues to the placenta and which does not pass through the lungs.

(Arrows) from body tissues up to and into right atrium; [Reject: If arrow points down umbilical vein to placenta]

Into right ventricle and ductus arteriosus, then to umbilical artery and placenta/ into left atrium and left ventricle, then to umbilical artery and placenta; 2

[Reject: [1f arrow passes from lungs][Accept: Arrows alongside structures]