AQA A level BIOLOGY 7402/3 Paper 3 Mark scheme June 2023.

AQA A level BIOLOGY 7402/3 Paper 3 Mark scheme June 2023. (There are) stretch-mediated sodium ion channels (in the membrane); 2. (Increased pressure) deforms/changes (sensory neurone/axon) membrane/lamella(e) OR (Increased pressure) deforms/changes sodium ion channels; 3. Sodium ion channels open; 4. Sodium ions diffuse in; 5. Depolarisation (leading to generator potential); 3 max (3 x AO1) 1. 2. 3. or 4. Accept Na+ Max 2 if only reference to sodium and not sodium ions 4. Accept sodium ions move in down a concentration gradient 5. Accept inside becomes positive Question Marking Guidance Mark Comments 01.2 1. An action potential is only generated/produced when threshold is reached OR An action potential is not generated/produced until/unless threshold is reached; 2. When high pressure was applied (to the fingertip), threshold was reached OR High pressure is (an) above threshold (stimulus) OR High pressure generates an action potential; 2 (1 x AO1, 1 x AO3) 1. Ignore any numbers linked to threshold 1. and 2 Accept impulse for action potential 2. Accept references to ‘the first peak’ for high pressure 2. Accept the converse for low pressure Question Marking Guidance Mark Comments 01.3 (The) refractory (period); 1 (AO1) Ignore any reference to absolute or relative Reject refraction period MARK SCHEME – A-LEVEL BIOLOGY – 7402/3 – JUNE 2023 6 Question Marking Guidance Mark Comments 02.1 Accept any two of the following for one mark: Concentration/mass of yeast Concentration/mass of glucose/sugar (solution/s) Concentration of oxygen (in solution) Volume of yeast Volume of glucose/sugar (solution/s) pH (of glucose/sugar solution/s) Species/type of yeast Size/diameter/volume of test tubes 1 (AO3) Ignore number of yeast cells Ignore amount for volume or concentration Question Marking Guidance Mark Comments 02.2 Correct answer of 2.5 x 10–2 = 2 marks;; 4.2 x 10–2 (scale read from the wrong direction) = 1 mark OR 2.5 x 10–3 (scale read as 3.7 mm not 3.7 cm) = 1 mark OR 2.4 x 10–2 (scale read as 36.5) = 1 mark OR Correct answer not in standard form and/or not to 2 significant figures, eg 0.0247/0.025/2.47 x 10–2 = 1 mark OR Answer in incorrect standard form eg 25 x 10–3 or 0.25 x 10–1 = 1 mark OR Evidence of 37 ÷ 1500 or 37 ÷ (25 x 60) = 1 mark 2 (2 x AO2) MARK SCHEME – A-LEVEL BIOLOGY – 7402/3 – JUNE 2023 7 Question Marking Guidance Mark Comments 02.3 1. Maltose is a disaccharide; 2. (Time to be) hydrolysed into glucose OR (Time to) break glycosidic bond(s); 2 (2 x AO2) 1. Accept maltose is made of two glucose/ monosaccharides 2. Accept monosaccharides for glucose Question Marking Guidance Mark Comments 02.4 This method would be less accurate because 1. Colour/endpoint is subjective; The accuracy of this method could be improved by 2. (Have a) colour standard (of the yellow) as a reference for the end point; OR (Have a) colorimeter/absorbance/transmission reading (of the yellow) as a reference for the end point; 2 (2 x AO3) 1. Accept descriptions of the word ‘subjective’ 2. Accept use a colorimeter to time how long it takes to reach a certain value/absorbance/ transmission 2. Ignore any references to coloured filters used in a colorimeter MARK SCHEME – A-LEVEL BIOLOGY – 7402/3 – JUNE 2023 8 Question Marking Guidance Mark Comments 02.5 All 4 correct = 2 marks;; Any 2 or 3 correct = 1 mark 2 (2 x AO1) Accept pyruvic acid for pyruvate For reduced NAD accept NADH, NADH + H+ or NADH2 For reduced NAD reject reduced NADP / NADPH / NADPH2 / NADPH + H+ For acetylcoenzyme A accept acetyl co A MARK SCHEME – A-LEVEL BIOLOGY – 7402/3 – JUNE 2023 9 Question Marking Guidance Mark Comments 03.1 Second box ticked; Answer key: B – statements 1, 2 and 4 1 (AO1) Question Marking Guidance Mark Comments 03.2 A correct answer of 9(.144) x 108 = 2 marks;; Answer not in standard form () = 1 mark OR Answer in incorrect standard form = 1 mark OR 1.016 × 108 (everything else correct except failed to multiply by 9) = 1 mark OR 2.286 × 108 (everything else correct but failed to take the plating of 0.25 cm3 into account) = 1 mark OR 9.432 x 108 (discounted 238 as anomalous, all other calculations correct) = 1 mark