Transport in and out of cells

The digestion and absorption of lipids involves:

• emulsification by bile salts

• micelles.

Explain each of the following.

The role of micelles in the absorption of lipids

Describe and explain the arrangement of phospholipid molecules in the cell-surface membrane.

Pears are a type of fruit.

When pears ripen, the:

• fructose concentration increases in cell vacuoles

• volume of cell vacuoles increases.

Figure 2 shows information about a pear cell’s vacuole membrane.

• Carrier protein X is linked to an ATP hydrolase enzyme.

• Carrier protein Y co-transports H⁺ ions and fructose molecules in opposite directions.

Explain why the volume of the vacuoles in pear cells increases when pears ripen.

Your answer should refer to the role of:

• ATP hydrolase

• carrier protein X and carrier protein Y.

A respiratory inhibitor caused a change in pH of the solution within the vacuole.

Use information in Figure 2 to explain how.

Explain how a change in the expression of the genes coding for the carrier proteins can increase the rate of ripening of pears.

Figure 1 shows the structure of a fatty acid molecule and the structure of a glycerol molecule.

On Figure 1, draw a circle around the part of the fatty acid molecule and the part of the glycerol molecule that is removed to form a bond in a triglyceride molecule.

Name the bond formed between a fatty acid and glycerol in a triglyceride molecule.

Name the reaction involved in forming a bond between a fatty acid and glycerol in a triglyceride molecule.

Table 1 shows information about three fatty acids.

Name the fatty acid shown in Table 1 that is a saturated fatty acid.

The melting point is the temperature at which a solid changes state to be a liquid.

Use Table 1 to describe the relationship between fatty acid structure and fatty acid melting point.

The ratio of saturated to unsaturated fatty acids in a cell-surface membrane determines the extent of the membrane’s fluidity.

Scientists provided a cell culture of mouse phagocytes with liquid broth rich in unsaturated fatty acids.

The scientists observed:

• an increase in the proportion of phospholipids in the phagocytes containing unsaturated fatty acids

• more phagocytosis.

Suggest and explain why there was more phagocytosis.

Describe the processes of facilitated diffusion and active transport.

Facilitated diffusion

What are microvilli?

Vitamin A is a fat-soluble substance.

Micelles are involved in the process of vitamin A absorption.

Describe the process of vitamin A absorption into cells lining the ileum.

Molasses is a solution obtained from sugar beet plants. The sugars present in molasses are sucrose, glucose and fructose.

Give the number of different types of monosaccharides present in molasses.

A student used the biochemical test for reducing sugars on a clear sample of molasses.

Describe the biochemical test for a reducing sugar.

Explain the result expected from the test on the sample of molasses.

Description of biochemical test

Explanation of expected result

‘Free sugar’ is the sugar in food and drinks released when food is crushed or when sugar is added to food at home or by manufacturers.

Scientists recommend that no more than 5% of the energy consumed per day should come from ‘free sugar’.

The mean daily energy requirement for a 10-year-old child is 8100 kJ

The ‘free sugar ’ in one tablespoon of molasses contains 250 kJ of energy.

Calculate the number of tablespoons of molasses required for a 10-year-old child to reach the recommendation for energy consumed in ‘free sugar’ per day.

A scientist used the apparatus in Figure 3 to investigate osmosis.

Use your understanding of osmosis to explain why the air pressure in the tube increased.

The scientist repeated the investigation, but made one change to the molasses. The scientist did not change the volume of molasses at the start of the investigation.

The scientist observed that the air pressure inside the tube increased by 160 kPa compared with 800 kPa in the first investigation.

Suggest the change the scientist made to the molasses to cause this smaller increase in air pressure.

Use the air pressure figures in a calculation to support your answer.

Write an essay on one of the topics below.

Either

Phosphorus-containing substances and their importance in biological systems.

Or

The mechanisms and importance of transport within organisms.

Some hospital patients suffer from diarrhoea caused by infection with the bacterium Clostridium difficile. The C. difficile bacteria release toxins. These toxins cause the diarrhoea.

The toxins damage the cells lining the ileum, causing them to lose their microvilli. The damage to the cells reduces the absorption of the products of digestion and reduces the absorption of water, resulting in diarrhoea.

Explain why the damage to the cells lining the ileum reduces absorption of the products of digestion and why this reduces absorption of water.

Not all patients in hospital with C. difficile develop diarrhoea.

Scientists measured the anti-toxin antibody concentration in hospital patients with and without C. difficile infection.

They measured the anti-toxin antibody concentration four times:

• on admission to hospital (day 0)

• on day 3

• on day 6

• on the day the patient left the hospital.

Figure 4 shows the scientists’ results.

The scientists suggest that the anti-toxin antibody could be given to some patients as a form of passive immunity.

Use Figure 4 to suggest how this passive immunity would work and which patients should be offered this anti-toxin antibody.

To be used as passive immunity treatment, the anti-toxin antibody would be injected. If it was given by mouth, it would be digested.

Describe how the anti-toxin antibody would be digested.

Give two features of all prokaryotic cells that are not features of eukaryotic cells.

Many multicellular organisms produce antimicrobial polypeptides (APs) that protect them against prokaryotes. Figure 3 shows how one type of AP acts on the cell-surface membrane of prokaryotes.

This AP has a secondary structure in a helical shape.

Tick () the box to show which type of bond maintains the helical structure of the polypeptide.

The amino acids on one side of each AP helix have hydrophobic properties. The amino acids on the opposite side of each helix have hydrophilic properties. Figure 4 shows this.

Suggest how these properties of the APs allow them to become positioned across the membrane (as shown in Figure 3) and make a channel through which ions can pass.

Figure 5 shows further information about a channel formed in the cell-surface membrane by the APs.

Use Figure 5 to calculate the cross-sectional area of the channel through which ions can pass. Assume the cross-sectional area is circular. Use π = 3.14 in your calculation. Give your answer in nm² and to 1 decimal place.

The APs damage prokaryotic cells but do not damage the eukaryotic cells in the organisms that produce them. Prokaryotic cell membranes do not contain cholesterol.

Assess why the APs do not damage the eukaryotic cells of the organisms that produce them.

Scientists observed these APs on prokaryotes using a transmission electron microscope. They stained the APs using a monoclonal antibody with gold attached to it.

Suggest how these techniques allowed observation of APs on prokaryotes.

Mangrove trees grow near the sea. Sea water surrounds the lower parts of the trees at high tide.

Scientists investigated the rate of transpiration in a mangrove tree. Figure 9 shows the scientists’ results.

Explain the rate of transpiration between 5 am and midday shown in Figure 9.

Use Figure 9 to calculate the percentage increase in the rate of transpiration from 1 pm to 2 pm.

The higher rate of transpiration at high tide shows that the mangrove tree is absorbing water from the sea water surrounding its roots.

Describe an experiment that you could do to investigate whether the mangrove root cells have a lower water potential than sea water.

You are given:

• a piece of fresh mangrove root

• sea water

• access to laboratory equipment.

Describe the structure of DNA.

Name and describe five ways substances can move across the cell-surface membrane into a cell.

Figure 14 shows transmission electron micrographs of two cells, one animal cell and one prokaryotic cell.

Contrast the structure of the two cells visible in the electron micrographs shown in Figure 14.

Figure 1 shows a cell from the lining of the ileum specialised for absorption of products of digestion.

SGLT1 is a carrier protein found in the cell-surface membrane of this cell, it transports glucose and sodium ions (Na⁺) into the cell.

The action of the carrier protein X in Figure 1 is linked to a membrane-bound ATP hydrolase enzyme.

Explain the function of this ATP hydrolase.

The movement of Na⁺ out of the cell allows the absorption of glucose into the cell lining the ileum.

Explain how.

Describe and explain two features you would expect to find in a cell specialised for absorption.

Figure 2 is a diagram of one SGLT1 carrier protein.

Figure 2

Draw phospholipids on Figure 2 to show how the carrier protein, SGLT1, would fit into the cell-surface membrane.

Do not draw more than eight phospholipids.

Figure 2 shows the SGLT1 polypeptide with NH₂ at one end and COOH at the other end.

Describe how amino acids join to form a polypeptide so there is always NH₂ at one end and COOH at the other end.

You may use a diagram in your answer.

Space for diagram:

Anthocyanins are coloured pigments found in the cell vacuole of some plant cells. Anthocyanins cannot move across undamaged cell membranes.

A student investigated how to extract anthocyanins from blueberries.

She mixed 10 g of crushed, fresh blueberries with 100 cm³ of extraction solvent for 1 hour.

She investigated three different extraction solvents:

• E – Ethanol, water and acid

• F – Ethanol and water

• G – Water

When making up extraction solvent E, the student used a volume ratio of 70:30:1 ethanol:water:acid.

Tick () one box that shows the most appropriate volumes she would use to make up

The student kept constant:

• the mass of fresh blueberries

• the volume of extraction solvent

• the time for the mixture to stand.

Name two other variables the student should have kept constant during this investigation.

After 1 hour, the student filtered the samples. She placed the filtrate in a colorimeter and measured the light absorbance.

Her results are shown in Figure 4.

Use your knowledge of membrane structure to explain the results in Figure 4.

A different student did this investigation. He did not have a colorimeter.

Describe a method this student could use to prepare colour standards and use them to give data for the total anthocyanin extracted.

Table 2 shows information about two types of medicine.

A journalist studied the data in Table 2. She made the following suggestion.

‘If an adult takes the maximum number of tablets allowed for either of the medicines, then the person would have more than the RDA of sodium.’

The RDA (recommended daily allowance) of sodium for an adult human is 2.4 g per day.

Is the journalist’s statement true for both of the medicines in Table 2 ?

Use suitable calculations to support your answer.

Show your working.

Doctors investigated the link between high sodium concentrations in medicines and hypertension (high blood pressure).

They analysed medical records of patients. 1 292 337 of these patients had taken medicines containing high sodium concentrations. Each of these patients was paired with a patient from a control group.

Give two factors that should have been the same for each pair of patients and one factor that should have been different.

Doctors found:

• 4.73% of the patients who had taken medicines containing high sodium concentrations suffered from hypertension

• there were 7.18 times fewer control patients with hypertension.

Calculate how many of the control patients had hypertension.

Show your working.

A high concentration of sodium in the blood can affect blood volume and cause hypertension.

Use your knowledge of water potential to suggest how high sodium concentrations in the medicines taken could affect blood volume.

The water potential of leaf cells is affected by the water content of the soil.

Scientists grew sunflower plants. They supplied different plants with different volumes of water.

After two days, they determined the water potential in the leaf cells by using an instrument that gave a voltage reading.

The scientists generated a calibration curve to convert the voltage readings to water potential.

Figure 8 shows their calibration curve.

The scientists needed solutions of known water potential to generate their calibration curve.

Table 5 shows how to make a sodium chloride solution with a water potential of

Complete Table 5 by giving all headings, units and volumes required to make 20 cm³ of this sodium chloride solution.

Table 6 shows some of the concentrations of sodium chloride solution the scientists used and the water potential of each solution.

There is a linear relationship between the water potential and the concentration of sodium chloride solution.

Use the data in Table 6 to calculate the concentration of sodium chloride solution with a water potential of 3.41MPa

In addition to determining the water potential in the leaf cells, the scientists measured the growth of the leaves.

They recorded leaf growth as a percentage increase of the original leaf area.

Their results are shown in Figure 9.

One leaf with an original area of 60cm² gave a voltage reading of 7 µV

Use Figure 8 (on page 28) and Figure 9 to calculate by how much this leaf increased in area. Give your answer in cm²

Sunflowers are not xerophytic plants. The scientists repeated the experiment with xerophytic plants.

Suggest and explain one way the leaf growth of xerophytic plants would be different from the leaf growth of sunflowers in Figure 9.

Use your knowledge of gas exchange in leaves to explain why plants grown in soil with very little water grow only slowly.

Give two pieces of evidence from Figure 1 that this cell was undergoing mitosis. Explain your answers.

When preparing the cells for observation the scientist placed them in a solution that had a slightly higher (less negative) water potential than the cytoplasm. This did not cause the cells to burst but moved the chromosomes further apart in order to reduce the overlapping of the chromosomes when observed with an optical microscope.

Suggest how this procedure moved the chromosomes apart.

The dark stain used on the chromosomes binds more to some areas of the chromosomes than others, giving the chromosomes a striped appearance.

Suggest one way the structure of the chromosome could differ along its length to result in the stain binding more in some areas.

In Figure 2 the chromosomes are arranged in homologous pairs. What is a homologous pair of chromosomes?

Give two ways in which the arrangement of prokaryotic DNA is different from the arrangement of the human DNA in Figure 1.

A student investigated the effect of surface area on osmosis in cubes of potato.

• He cut two cubes of potato tissue, each with sides of 35 mm in length.

• He put one cube into a concentrated sucrose solution.

• He cut the other cube into eight equal-sized smaller cubes and put them into a sucrose solution of the same concentration as the solution used for the large cube.

• He recorded the masses of the cubes at intervals.

His results are shown in Figure 3.

Describe the method the student would have used to obtain the results in Figure 3. Start after all of the cubes of potato have been cut. Also consider variables he should have controlled.

The loss in mass shown in Figure 3 is due to osmosis. The rate of osmosis between 0 and 40 minutes is faster in B (the eight small cubes) than in A (single large cube).

Is the rate of osmosis per mm² per minute different between A and B during this time? Use appropriate calculations to support your answer.

Scientists investigated the role of a protein called CENP-W in mitosis. Their method involved cell fractionation and ultracentrifugation.

The scientists began by lysing (breaking open) cells and organelles using a detergent that dissolves lipids in water.

Suggest how the detergent releases CENP-W from cells.

Explain how ultracentrifugation separates CENP-W from other molecules.

CENP-W is involved in the formation of spindle fibres in mitosis. Spindle fibres are made of molecules of a protein called tubulin.

The scientists treated cells in a culture with small interfering RNA (siRNA). This siRNA causes RNA interference of expression of the CENP-W gene. The scientists took samples of cells at 0, 48 and 72 hours after adding the siRNA. They then used gel electrophoresis to separate CENP-W and tubulin from these samples.

Figure 5 shows the results of the electrophoresis. The size of each band is proportional to the amount of CENP-W or tubulin present.

Suggest how the siRNA produced these results.

In mammals, in the early stages of pregnancy, a developing embryo exchanges substances with its mother via cells in the lining of the uterus. At this stage, there is a high concentration of glycogen in cells lining the uterus.

Describe the structure of glycogen.

During early pregnancy, the glycogen in the cells lining the uterus is an important energy source for the embryo.

Suggest how glycogen acts as a source of energy.

Do not include transport across membranes in your answer.

Suggest and explain two ways the cell-surface membranes of the cells lining the uterus may be adapted to allow rapid transport of nutrients.

In humans, after the gametes join at fertilisation, every cell of the developing embryo undergoes mitotic divisions before the embryo attaches to the uterus lining.

The first cell division takes 24 hours.

The subsequent divisions each take 8 hours.

After 3 days, the embryo has a total volume of 4.2 × 10^–3 mm³.

What is the mean volume of each cell after 3 days? Express your answer in standard form.

Show your working.

Sodium ions from salt (sodium chloride) are absorbed by cells lining the gut. Some of these cells have membranes with a carrier protein called NHE3.

NHE3 actively transports one sodium ion into the cell in exchange for one proton (hydrogen ion) out of the cell.

Use your knowledge of transport across cell membranes to suggest how NHE3 does this.

Scientists investigated the use of a drug called Tenapanor to reduce salt absorption in the gut. Tenapanor inhibits the carrier protein, NHE3.

The scientists fed a diet containing a high concentration of salt to two groups of rats, A and B.

The rats in Group A were not given Tenapanor (0 mg kg^–1).

The rats in Group B were given 3 mg kg^–1 Tenapanor.

One hour after treatment, the scientists removed the gut contents of the rats and immediately weighed them.

Their results are shown in Table 2.

The scientists carried out a statistical test to see whether the difference in the means was significant. They calculated a P value of less than 0.05. They concluded that Tenapanor did reduce salt absorption in the gut.

Use all the information provided and your knowledge of water potential to explain how they reached this conclusion.

High absorption of salt from the diet can result in a higher than normal concentration of salt in the blood plasma entering capillaries. This can lead to a build-up of tissue fluid.

Explain how.

2,4-D is a selective herbicide that kills some species of plants but not others. 2,4-D disrupts cell-surface membranes but the extent of disruption differs in different species.

Scientists investigated the effect of 2,4-D on wheat plants (a crop) and on wild oat plants (a weed).

They grew plants of both species in glasshouses. They put plants of each species into one of two groups, W and H, which were treated as follows:

Group W – leaves sprayed with water

Group H – leaves sprayed with a solution of 2,4-D.

After spraying, they cut 40 discs from the leaves of plants in each group and placed them in flasks containing 10 cm³ de-ionised water. After 5 minutes, they calculated the disruption to cell-surface membranes by measuring the concentration of ions released into the water from the leaf discs.

Their results are shown in Table 3.

The lowest significant difference (LSD), is the smallest difference between two means that would be significant at P≤0.05

Give three environmental variables that should be controlled when growing the plants before treatment with the different sprays.

Evaluate the use of 2,4-D as a herbicide on a wheat crop that contains wild oats as a weed. Use all the information provided.

The scientists incubated the flasks containing the leaf discs at 26 C and gently shook the flasks.

Suggest one reason why the scientists ensured the temperature remained constant and one reason why the leaf discs were shaken.

A student used a potometer to measure the movement of water through the shoot of a plant. The potometer is shown in Figure 5. As water is lost from the shoot, it is replaced by water from the capillary tube.

In one experiment, the air bubble moved 7.5 mm in 15 minutes. The diameter of the capillary tube was 1.0 mm.

Calculate the rate of water uptake by the shoot in this experiment.

Give your answer in mm³ per hour. Show your working. (The area of a circle is found using the formula, area = πr²)

The student wanted to determine the rate of water loss per mm² of surface area of the leaves of the shoot in Figure 5.

Outline a method she could have used to find this rate. You should assume that all water loss from the shoot is from the leaves.

The rate of water movement through a shoot in a potometer may not be the same as the rate of water movement through the shoot of a whole plant.

Suggest one reason why.

Aquaporins are channel proteins that allow the diffusion of water across membranes. One type of aquaporin, called PIP1, can also transport carbon dioxide molecules across membranes.

Figure 6 shows the structure of a water molecule and of a carbon dioxide molecule. They are drawn to the same scale.

Suggest two reasons why water molecules and carbon dioxide molecules can both pass through PIP1.

The scientists first produced transgenic poplar trees. These trees all had a length of foreign DNA inserted into them. This DNA led to the production of single-stranded RNA that specifically inhibited expression of the gene for PIP1.

The scientists then measured the difference in the amount of PIP1 in leaves of transgenic poplars and in leaves of wild type poplars without the foreign DNA. The amount of PIP1 in the transgenic poplars was approximately 15% of that in the wild type poplars.

Using this information, what can you conclude about the effect of the foreign DNA in the transgenic poplar trees?

The transgenic poplars still produced some PIP1. Suggest why.

The scientists investigated the importance of PIP1 in the movement of water and carbon dioxide through the tissues of leaves of poplar trees.

They measured the mean rates of movement of carbon dioxide and water through the tissues of leaves of transgenic poplars and through the tissues of leaves of wild type poplars.

Their results are shown in Figure 7.

Figure 7

Using only Figure 7, evaluate the importance of PIP1 in the movement of carbon dioxide and water through leaves of poplar trees.

Section B

one question.

Write an essay on one of the topics below.

EITHER

The importance of nitrogen-containing substances in biological systems.

OR

The importance of diffusion in organisms.