Transport (SL IB Biology)

The effect of humidity on the transpiration stream within the xylem of an oak tree was investigated, the results are displayed in the graphs below.

Describe how the trends shown in graph A compare with the trends shown in graph B.

Outline how the changes in humidity lead to the changes in water tension measured in the xylem at the top of the tree.

During the investigation, the scientist also measured the diameter of the tree trunk, the results can be seen in the table below:

Explain how this data and the data in the graphs from part a) support the cohesion-tension theory.

The scientist who carried out the investigation in part a) concluded from his data that water moves through the xylem through the cohesion tension mechanism.

Evaluate his conclusion based on the validity of the evidence provided by the results of this investigation.

The microscope image below shows a cross section of part of a stem of a herbaceous (non-woody) plant.

Image courtesy of Berkshire Community College Bioscience Image Library. Adapted and redistributed under a Creative Commons CC0 1.0 Universal Public Domain Dedication license under conditions found at https://creativecommons.org/publicdomain/zero/1.0/deed.en 

Identify tissue types X and Y in the image.

Valves are important structures found in the heart and veins.

Explain how valves work.

The graph below shows the rate of blood flow in various blood vessels in the body. It also shows the total cross-sectional area of the vessels.

Suggest why the rate of blood flow increases from the venules to the vena cava despite a large decrease in the total cross-sectional area.                                                               

The relative thickness of layers in the wall of an artery and a vein are shown in the table below.

Explain the difference in the thickness of elastic tissue for the artery versus the vein.

The external diameter of a vein was measured at 1.5 mm. 

Calculate the diameter of the lumen of the vein.

Explain how the structure of capillaries enable them to function efficiently as exchange surfaces.

Plant sterols and plant stanols are found in a range of food, including vegetables,  cereals, seeds and nuts. Plant sterols and stanols have a similar structure to cholesterol and reduce the absorption of cholesterol in the small intestine, so more cholesterol is lost in the faeces. This helps to lower the levels of cholesterol in the blood and in turn, reduce the risk of coronary heart disease (CHD).

A group of scientists wanted to investigate the effects of eating plant sterols and stanols on the risk of CHD. The scientists randomly divided healthy volunteers into two groups. Every day for eight weeks, one group was given plant sterols and stanols to eat. The other group acted as a control.

Each week, the scientists measured the diameter of the lumen of the main artery in the arm of the volunteers.

State how the control group should have been treated.

The results of the experiment in part (a) are shown in the table below.

A student reading the results concluded that there was sufficient evidence to assume a causal relationship between plant sterols and stanols, and a reduced risk of CHD.

Evaluate the student’s conclusion.

Describe how blood returns to the heart from the legs.

The diagram below shows a transverse section (TS) of a plant stem.

Identify the structure found at J and state its function.

In xylem vessels, hydrogen bonds form between polar water molecules.

Describe the role that hydrogen bonding plays in the cohesion-tension theory of water transport in the xylem of plants.

Explain a property of water that allows it to flow as mass transport in the transpiration stream.