A scientist investigated the absorption of amino acids by cells lining the small intestine.

He investigated the rate of absorption at different amino acid concentrations.

His results are shown below. 

Use your knowledge of membrane transport to describe and explain the shape of the curve shown above.

Explain the differences between facilitated diffusion and active transport. 

Sugar molecules enter cells via transport proteins.

Explain why transport proteins are needed to move sugar molecules into cells.

Individuals with a diet high in glucose can develop high blood pressure.

This is due to glucose affecting the volume of blood in the body. 

Use your knowledge of cell transport to explain how an increase in blood glucose levels could affect blood volume.

Most cells are surrounded by a cell-surface membrane responsible for controlling the movement of substances into and out of the cell. 

Describe the arrangement of phospholipids in the cell-surface membrane of animal cells. 

Explain why the structure of the cell-surface membrane is described as fluid and mosaic.

Molecules and ions can move into or out of cells using diffusion or osmosis.

Describe two similarities between these methods of transport.

A scientist investigated the absorption of a monoglyceride by cells in the digestive system. 

He investigated the rate of absorption at different monoglyceride concentrations. 

His results are shown below. 

Use the graph to explain how this monoglyceride is transported into cells within the digestive system.

Water and inorganic ions are constantly transported into and out of eukaryotic cells. 

Describe the similarities and differences between the processes by which water and inorganic ions enter eukaryotic cells.

Euglena viridis is a unicellular algae found in freshwater pools, ponds, and streams.

These algal cells contain structural components known as contractile vacuoles which collect water from the cytoplasm and contract to expel water from the cell.

Explain why Euglena viridis would expel water when placed in a freshwater environment.

A scientist compared the frequency of vacuole contractions when Euglena viridis was placed in freshwater and salt solution.

He counted the number of contractions within 5 minutes.

In freshwater, he counted 30 contractions and in salt solution, he counted 10 contractions.

Apart from time, describe two variables he should have controlled in his investigation.

Explain why there were less contractions when the cells were placed in salt solution.

The diagram below shows some apparatus used to investigate the water potential of carrot tissue. 

The carrot pieces were placed in glucose solutions of different concentrations and left for three hours. 

Their results are shown below. 

The student produced the different concentrations of glucose solutions using one concentrated glucose solution.

Name the method they used to produce these solutions of varying concentrations.

Describe two variables they should have controlled to obtain valid results.

Explain the result for the carrot piece in the 0.6 mmol dm^-3 solution.

Explain how the student could use their results to identify the water potential of the carrot tissue.

Root hair cells absorb nitrate ions from the surrounding soil.

These ions cross the cell surface membrane via the process of active transport. 

Explain why nitrate ions cross cell-surface membranes of root hair cells by active transport and not by simple diffusion. 

A student placed a sample of root hair cells and a sample of red blood cells into a beaker of distilled water. 

Explain why the red blood cells burst, but the root hair cells did not.

The student then placed some red blood cells into a salt solution. 

Explain why the cells did not burst. 

Red blood cells pick up oxygen as they pass through the lungs. 

Explain how oxygen enters red blood cells. 

Students investigated the effect of surface area on the rate of diffusion.

The students used agar cubes containing the alkali sodium hydroxide and the indicator phenolphthalein.

They placed agar cubes of different sizes into a solution of hydrochloric acid.

Phenolphthalein changes from pink to colourless in acidic conditions. 

Their results are shown in the table below.

Calculate the surface area to volume ratio for the cube with 5 mm sides.

Describe two variables that should be controlled in this investigation.

The students observed and recorded the time it took for the cubes to turn colourless.

Explain why this is not a reliable method for measuring the rate of diffusion.

Suggest how this could be measured more accurately. 

Describe the relationship between surface area to volume ratio and the rate of diffusion. 

Use calculations to support your answer.

Some substances can cross the cell-surface membrane of a cell by simple diffusion or osmosis. 

Define osmosis.

Cell-surface membranes are made up of phospholipid molecules.

State and explain how one property of phospholipids helps to provide stability to the cell-surface membrane.

Endocytosis and exocytosis are processes that move large molecules into and out of cells. 

Describe the similarities and differences between endocytosis and exocytosis.

Lactose intolerance is a condition in which the enzyme lactase is not produced, meaning that the body cannot digest the disaccharide lactose. 

Individuals that suffer with lactose intolerance cannot break down lactose and so it remains in the lumen of the intestine.

The presence of lactose decreases the volume of water that is absorbed into the blood, causing diarrhoea. 

Explain why undigested lactose in the intestine causes diarrhoea.

Applying salt to a wound can help prevent bacterial infection.

Using your understanding of water potential, propose how the application of salt to a wound could eliminate bacteria.

Cells in the small intestine are highly specialised to allow for rapid absorption of nutrients from food.

Describe and explain two features of cells that would increase the rate of absorption. 

Living organisms are made up of cells containing a cell-surface membrane. 

This membrane controls the movement of substances into and out of the cells. 

Explain why oxygen and carbon dioxide are able to enter cells via simple diffusion.

Cell-surface membranes contain carrier proteins embedded throughout phospholipid molecules. 

Describe the function of these carrier proteins.

Cell-surface membranes are also involved in the process of cell signalling. Explain how.