Organisms exchange substances with their environment

Explain the advantage for larger animals of having a specialised system that facilitates oxygen uptake. (2 marks)

• Larger organisms have a smaller surface area : volume ratio

• Overcomes long diffusion pathway

Suggest how the environmental conditions have resulted in adaptations of systems using Model A rather than Model B. (2 marks)

• Water has lower oxygen concentration than air

• So system on outside gives large surface area in contact with water

Mammals such as a mouse and a horse are able to maintain a constant body temperature. Use your knowledge of surface area to volume ratio to explain the higher metabolic rate of a mouse compared to a horse. (4 marks)

• The mouse is smaller so larger surface area to volume ratio

• More heat loss

• Faster rate of respiration releases heat

Tubifex worms are small, thin animals that live in water. They have no specialised gas exchange or circulatory system.

The figure shows a tubifex worm.

Using the information provided, explain how two features of the body of the tubifex worm allow efficient gas exchange. (2 marks)

• Thin so short diffusion pathway

• Small so large surface area to volume ratio

Abdominal pumping takes place during vigorous activity in insects. This causes regular squeezing of tubes of the gas exchange system, A scientist investigated the effect of abdominal pumping on the pressure in the tubes and the volume of carbon dioxide released by the insect. Her results are shown.

Describe and explain these results. (3 marks)

• Abdominal pumping in tubes is linked to carbon dioxide release

• Abdominal pumping raises the pressure in the body

• Carbon dioxide is pushed out of the body

The ends of the tracheoles connect directly with the insect’s muscle tissue. When flying, water is absorbed into muscle tissue. Removal of water from the tracheoles increases the rate of diffusion of oxygen between the tracheoles and muscle tissue. Suggest one reason why. (1 mark)

Water is denser than air so when it is removed oxygen can diffuse faster through air than water

A scientist used grasshoppers to investigate the effect of composition of air on breathing rate in insects. He changed the composition of air they breathed in by varying the concentrations of oxygen and carbon dioxide. The scientist collected 20 mature grasshoppers from a meadow. He placed the grasshoppers in a small chamber where he could adjust and control the composition of air surrounding them. The small chamber restricted the movement of the grasshoppers. His results for three of the grasshoppers are shown in the table below in the form in which he presented them.

Use all the data to describe the effect of concentration of carbon dioxide on the breathing rate of grasshoppers

• Breathing rate increases when carbon dioxide more than 0.1%

• Breathing rate of grasshoppers increase as concentration of carbon dioxide increases

• Breathing rate of grasshoppers actually lowest when there is no CO₂

The estimate does not provide a reliable value for the mean breathing rate of all insect species in the meadow. Other than being an estimate, suggest and explain three reasons why this value would not be reliable. (3 marks)

• Only 3 grasshoppers used, small sample size

• Grasshoppers not the only species

• Movement not restricted, rate of respiration increases

Explain how the counter current mechanism in fish gills ensures the maximum amount of the oxygen passes into the blood flowing through the gills. (3 marks)

• Water and blood flow in opposite directions

• Blood always passing water with a higher oxygen concentration

• Diffusion gradient maintained throughout length of gill

Describe the processes involved in the absorption and transport of digested lipid molecules from the ileum into lymph vessels. (5 marks)

• Micelles contain bile salts and fatty acids

  • making them more soluble in water

• Bring fatty acids to the cells lining the epithelium

• Fatty acids are absorbed by diffusion

• Triglycerides reformed in cells

• Vesicles move to cell membrane

Explain the advantages of lipid droplet and micelle formation. (3 marks)

• Droplets increase surface areas for lipase

• So faster digestion of lipids

• Micelles carry fatty acids and glycerol through membrane to intestinal epithelial cell

Name structure Q in the diagram above and suggest how it is involved in the absorption of lipids.

• Golgi apparatus

• Modifies / processes triglycerides

• Combines triglycerides with proteins

• Packaged for exocytosis OR Forms vesicles

Cells lining the ileum of mammals absorb the monosaccharide glucose by co-transport with sodium ions. Explain how.

• Sodium ions actively transported from ileum cell to blood

• Maintains / forms diffusion gradient for sodium to enter cells from gut and with it, glucose

• Glucose enters by facilitated diffusion with sodium ions

Describe the role of enzymes in the digestion of proteins in a mammal. (4 marks)

• Hydrolysis of peptide bonds

• Endopeptidase produces shorter polypeptides

• Exopeptidase produces dipeptides

• Dipeptidase produces single amino acids

At Q on the diagram above there is a small increase in pressure and in rate of blood flow in the aorta. Explain how this happens and its importance. (2 marks)

• Elastic recoil of the aorta tissue

• Smooths blood flow

Suggest two ways the student could improve the quality of his scientific drawing of the blood vessels in this dissection. (2 marks)

• Only use single lines/ do not use sketching lines

• Add labels

• Add magnification/scale bar

• Draw all parts to same scale/relative size

• Do not use shading/hatching

Tissue fluid is formed from blood at the arteriole end of a capillary bed. Explain how water from tissue fluid is returned to the circulatory system. (4 marks)

• Plasma proteins remain

• Creates water potential gradient OR Reduces water potential (of blood)

• Water moves (to blood) by osmosis

• Returns (to blood) by lymphatic system

Binding of one molecule of oxygen to haemoglobin makes it easier for a second oxygen molecule to bind. Explain why. (2 marks)

• Binding of first oxygen changes tertiary structure of haemoglobin

• Creates another binding site

Explain how blood in a vein in the leg is returned to the heart. (6 marks)

• Muscles contract and press on vein wall squeezing blood along vein

• Valves prevent backflow of blood

• Systole pumps blood through arteries into veins

• Recoil of heart muscle during diastole

• Draws blood from veins into atria

• Wide lumen ensure little friction

The rise and fall in blood pressure in the aorta is greater than in the small arteries. Suggest why. (3 marks)

• Pressure is higher as it is linked directly to the heart

• It has more elastic tissue

  • so greater stretch and recoil

The mass flow hypothesis is used to explain the movement of substances through phloem. Use your understanding of the mass flow hypothesis to explain how pressure is generated inside this phloem tube. (3 marks)

• Sucrose actively transported into phloem

• Lowering water potential

• Water moves into phloem by osmosis from xylem)

Phloem pressure is reduced during the hottest part of the day. Use information in the graph above along with your understanding of transpiration and mass flow to explain why. (3 marks)

• High rate of transpiration/evaporation

• Water lost through stomata

• Less water movement from xylem to phloem

Describe the cohesion-tension theory of water transport in the xylem. (5 marks)

• Water lost from leaf because of transpiration / evaporation of water

• Lowers water potential of mesophyll leaf cells

• Water pulled up xylem creating tension

• Water molecules cohere ‘stick’ together by hydrogen bonds;

  • forming continuous water column

• Adhesion of water molecules to walls of xylem

Describe the mass flow hypothesis for the mechanism of translocation in plants. (4 marks)

• In source sugars actively transported into phloem

  • By companion cells

• Lowers water potential of sieve cell / tube and water enters by osmosis

• Increase in pressure causes mass movement (towards sink / root)

• Sugars used / converted in root for respiration for storage.