Extended responses

Question 1

Explain how the heart muscle and the heart valves maintain a one-way flow of blood from the left atrium to the aorta(5 marks)

Answer 1

1. Atrium has higher pressure than ventricle due to filling/ contraction. This causes the atrioventricular valves to open
2. Ventricle now has higher pressure than atrium (due to filling/ contraction) This causes atrioventricular valves to close
3. Ventricle has higher pressure than aorta causing semilunar valve to open ;
4. This leads to a higher pressure the aorta than the ventricle (as heart relaxes) causing semilunar valve to close ;
5. ( Muscle / atrial / ventricular ) contraction causes increase in pressure

Question 2

Explain how tissue fluid is formed and how it may be returned to the circulatory system. (6 marks )

Answer 2

1. Hydrostatic pressure of blood is high at arterial end;
2. Fluid & soluble molecules pass out;
   3 Proteins & large molecules remain behind
3. This lowers the water potential;
4. Water moves back into venous end of capillary by osmosis;
   6 Lymph system collects any excess tissue fluid which returns to blood and returns this tissue fluid to the veins;

Question 3

Describe how you could make a temporary mount of a piece of plant tissue to observe the position of starch grains in the cells when using an optical ( light ) microscope . (4 marks )

Answer 3

1. Add drop of water to glass slide;
2. Obtain thin section of plant tissue and place on slide ;
   3 Stain with iodine in potassium iodide .
   Non 4 Lower cover slip using mounted needle

Question 4

Describe and explain how cell fractionation and ultracentrifugation can be used to isolate mitochondria from a suspension of animal cells . (5 marks )

Answer 4

1. Cell homogenisation to break open cells ;
   2 . Filter to remove large debris / whole cells ;
2. Use isotonic solution to prevent damage to mitochondria /organelles ;
   4 . Keep cold to reduce damage by enzymes / use buffer to prevent enzyme denaturation ;
3. Centrifuge at lower speed to separate nuclei / cell fragments / heavy organelles ;
4. Re - spin (supernatant / after nuclei / pellet removed ) at higher speed to get mitochondria at bottom .

Question 5

Describe the principles and the limitations of using a transmission electron microscope to investigate cell structure. (5 marks)

Answer 5

Principles

1. Electrons pass through thin specimen;
   2 Denser parts absorb more electrons
2. So denser parts appear darker;
3. Electrons have short wavelength so give high resolution;

Limitations:

5. Cannot look at living material / Must be in a vacuum
6. Specimen must be very thin;
7. Artefacts present ;
8. Complex staining method / complex / long preparation time;
   9 Image not in 3D / only images produced

Question 6

Scientists use optical microscopes and transmission electron microscopes (TEMs ) to investigate cell structure . Explain the advantages and the limitations of using a TEM to investigate cell structure . (5 marks )

Answer 6

Advantages

1. Small objects can be seen ;
   2 TEM has high resolution as wavelength of electrons shorter

Limitations :

3 Cannot look at living cells as cells must be in a vacuum / must cut section / thin specimen ;
4. Preparation may create artefact
5 Does not produce colour image ;

Question 7

Many different substances enter and leave a cell by crossing its cell surface membrane . Describe how substances can cross a cell surface membrane .(5 marks )

Answer 7

1. Simple / facilitated diffusion from high to low concentration
2. Small / non-polar / lipid-soluble molecules pass via phospholipids / bilayer
   OR
   Large / polar / water-soluble molecules go through proteins;
3. Water moves by osmosis from high water potential to low water potential;
   4.Active transport against concentration gradient ; 5. Active transport/facilitated diffusion involves proteins/carriers;
4. Active transport requires energy /ATP ;
5. Ref . to Na^ + /glucose co-transport

Question 8

Oxygen and chloride ions came diffuse across cell-surface membrane protein . The diffusion of chloride ions involves a membrane protein . The of oxygen does not involve a membrane protein . Explain why the diffusion of chloride ions involves a membrane protein and the diffusion of oxygen does not

Answer 8

1. Chloride ions water soluble/ charged / polar
2. Cannot cross lipid bilayer
3. Chloride ions transported by facilitated diffusion using channel/carrier protein
4. Oxygen not charged / non polar
5. Oxygen soluble in /can diffuse across bilayer

Question 9

The epithelial cells that line the small intestine are adapted for the absorption of glucose . Explain how. (6m)

Answer 9

1. Microvilli provide an increased surface area
2. Many mitochondria produce ATP / release or provide energy for active transport
3. Carrier proteins for active transport ;
4. Channel /carrier proteins for facilitated diffusion ; 5 Co-transport of sodium ions and glucose using a symport carrier protein ;
5. Membrane-bound enzymes digest disaccharides to produce glucose

Question 10

The movement of substances across cell membranes is affected by membrane structure. Describe how. (5m)

Answer 10

1. Phospholipid ( bilayer ) allows movement /diffusion of non- polar /lipid - soluble substances ;
2. Phospholipid (bilayer) prevents movement /diffusion of polar / charged /lipid-insoluble substances

OR

(Membrane) proteins allow polar / charged substances to cross the membrane /bilayer

3. Carrier proteins allow active transport;
4. Channel /carrier proteins allow facilitated diffusion /co -transport ;
5. Shape / charge of channel / carrier determines which substances move ;
6. Number of channels / carriers determines how much movement ;
7. Membrane surface area determines how much diffusion /movemer ;
8. Cholesterol affects fluidity /rigidity /permeability ;

Question 11

Explain how the heart muscle and the heart valves maintain a one- way flow of blood from the left atrium to the aorta. ( 5 marks)

Answer 11

1. Atrium has higher pressure than ventricle due to filling / contraction. This causes the atrioventricular valves to open
2. Ventricle now has higher pressure than atrium (due to filling / contraction). This causes atrioventricular valves to close;
   3 Ventricle has higher pressure than aorta causing semilunar valve to open
3. This leads to a higher pressure in the aorta than the ventricle (as heart relaxes) causing semilunar valve to close;
4. ( Muscle / atrial / ventricular) contraction causes increase in pressure;

Question 12

Describe and explain four ways in which the structure of a capillary adapts it for the exchange of substances between blood and the surrounding tissue (4 marks )

Answer 12

1. Permeable capillary membrane
2. Single cell thick walls - reduces diffusion distance
3. Flattened (endothelial) cells - reduces diffusion distance;
   4 Fenestrations - allows large molecules through;
4. Small diameter/ narrow - gives a large surface area to volume / short diffusion distance
5. Narrow lumen - reduces flow rate giving more time for diffusion;
6. Red blood cells in contact with wall / pass singly - gives short diffusion distance / more time for diffusion

Question 13

Explain how tissue fluid is formed and how it may be returned to the circulatory system. (6 marks )

Answer 13

1. Hydrostatic pressure of blood is high at arterial end;
2. Fluid & soluble molecules pass out;
   3 Proteins & large molecules remain behind;
3. This lowers the water potential;
4. Water moves back into venous end of capillary by osmosis;
5. Lymph system collects any excess tissue fluid which returns to blood and returns this tissue fluid to the veins;

Question 14

Describe and explain how the structure of the mammalian breathing system enables uptake of oxygen into the blood . ( 6 marks )

Answer 14

1. alveoli provide a large area
2. Walls of alveoli thin to provide a short diffusion pathway
3. Walls of capillary are thin between the alveoli so provides a short diffusion pathway
4. Walls capillaries/alveoli have flattened cells;
5. Cel membrane permeable to gases
6. Many blood capillaries provide large SA
7. Intercostal muscles &diaphragm muscles used to ventilate lungs to maintain a diffusion gradient

Question 15

Explain how the ventilation mechanism of a fish and the structure of its gills resulta the efficient uptake of oxygen from water ( 6 marks )

Answer 15

• Large SA provided by lamellae which increases diffusion
• thin epithelium which decreases distance between the water in the blood water and -water and blood flow in opposite direction which maintains the concentration gradient
• circulation replaces blood saturated with oxygen
• ventilation replaces water (as oxygen removed)
• large no of capillaries to remove O2
• pressure changes to bring in more water

Question 16

Describe the gross structure of the human gas exchange system and how we breathe in and out . (6 marks )

Answer 16

1. Named structures – trachea, bronchioles alveoli
2. Above structures reamed correct order

OR

Above structures labelled in correct positions on a diagram

3. Breathing in - diaphragm contracts and external intercostal muscles contract ,
4. (Causes ) volume increase and pressure decrease in thoracic cavity (to below atmospheric , resulting in air moving in
5. Breathing out - Diaphragm relaxes and internal intercostal muscles contract
6. (Causes) volume decrease and pressure increase in thoracic cavity (to above atmospheric , resulting in air moving out )