Cells

Describe the structure of the nucleus. (4 marks)

• Nuclear envelope

• Nuclear pores

• Chromosomes/chromatin

• Nucleolus

What are the functions of the nucleus? (2 marks)

• Codes genetic information to code for polypeptides

• Site of DNA replication

Name the structures labelled W to Z in the diagram (2 marks)

W - (cell surface) membrane
X - cell wall
Y - capsule
Z - flagellum

Contrast how an optical microscope and a transmission electron microscope work and contrast the limitations of their use when studying cells. (6 marks)

• TEM use electrons and optical use light

• TEM allows a greater resolution

• So with TEM smaller organelles can be observed OR greater detail in organelles can be observed

• TEM can view only dead specimens and optical can view live specimens

• TEM does not show colour and optical does

• TEM requires thinner specimens

• TEM requires a more complex/time consuming preparation

The figure 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 the figure above. (5 marks)

• Magnification (figures) show A is bigger than B

• A has a nucleus whereas B has free DNA

• A has Golgi body whereas B does not

• A has no cell wall whereas B has a murein/glycoprotein cell wall

• A has larger ribosomes

Identify structures labelled D and E. (2 marks)

D - Thylakoid

E - Starch grain

U. marinum cells ingest bacteria and digest them in the cytoplasm.

Describe the role of one named organelle in digesting these bacteria. (3 marks)

• Lysosomes

• Fuse with vesicle

• Releases hydrolytic enzymes

What type of microscope was used to obtain the image shown in the diagram?

Give one piece of evidence to support your answer. (2 marks)

• Scanning electron microscope

• 3D image

Outline the sequence of events following the production of extracellular proteins that leads to their release from the cell. (3 marks)

• Proteins move to Golgi apparatus where they are processed or modified

• Packaged into vesicles

• Vesicles moved to CSM where they fuse in a process called exocytosis

The epithelial cells of the small intestine have large numbers of mitochondria. Explain how this is an adaption for the function of these cells. (3 marks)

• Mitochondria is site of ATP production

• High respiration rate if there are lots of them for active transport.

• Energy released needed for the absorption of digested foods

Describe how to make a temporary mount of a piece of plant tissue to observe the number of chloroplasts in the cells when using an optical microscope. (4 marks)

• Add a drop of water to the glass slide

• Obtain a thin section of plant tissue and place on slide

• Stain with iodine in potassium iodide

• Lower cover slip using a mounted needle

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

• Use isotonic solution to prevent damage to organelles

• Keep cold to prevent damage by enzymes/use buffer to prevent protein denaturation

• Cell homogenisation to break open cells

• Filter to remove large debris and whole cells

• Centrifuge (at lower speed) to separate nuclei/heavy organelles

• Re-spin supernatant at higher speed to get mitochondria in pellet

Explain why the solution the biologist used was ice-cold, buffered and the same water potential as the liver tissue. (3 marks)

• Ice-cold - reduces enzyme activity to prevent digestion of organelles

• Buffered - Maintains pH so that proteins are not denatured

• Same water potential - Prevents osmosis so no bursting / shrinking of organelles

Before the cell was examined using the electron microscope, it was stained. This stain caused parts of the structure of the cell-surface membrane to appear as two dark lines.

Suggest an explanation for the appearance of the cell-surface membrane as two dark lines. (3 marks)

• Membrane has phospholipid bilayer

• Stain binds to phosphate

• On inside and outside of membrane

Describe binary fission in bacteria. (3 marks)

• Replication of circular DNA

• Replication of plasmids

• Division of cytoplasm to produce daughter cells

Suggest and explain how two environmental variables could be changed to increase the growth rate of these cells. (4 marks)

• Increased concentration of glucose

  • Increased respiration

• Increased temperature

  • Increased enzyme activity

The student carried out several repeats at each ____________ . Explain why the repeats are important. (2 marks)

• Allows anomalies to be identified, which can then be ignored

• Makes the average more reliable

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

Describe the similarities and differences between endocytosis and exocytosis. (3 marks)

• Both processes form vesicles

• Both processes require energy from ATP

• Endocytosis involves substances entering cell while exocytosis involve substances leaving cell

Suggest one explanation for the faster rate of plasmid replication in cells growing in a culture with a high amino acid concentration. (2 marks)

• Amino acids used in protein synthesis

• So more enzymes for DNA replication

Read the following passage.

In laboratory tests, scientists investigated the effects of a new drug called ABZ on stomach tumour cells. They found ABZ stopped mitosis by preventing the formation of spindle fibres. They also found that ABZ affected some healthy cells.

Suggest why preventing the formation of spindle fibres stopped the cell cycle. (2 marks)

• Chromatids cannot separate (on spindle)

• So no anaphase

Name the three phases of mitosis shown by C, D and E on the figure above. Describe the role of the spindle fibres and the behaviour of the chromosomes during each of these phases. (5 marks)

• C = prophase and D = metaphase and E = anaphase

• In prophase, chromosomes condense

• In metaphase, chromosomes at equator/centre of cell

• In anaphase, chromatids (from each pair) pulled to (opposite) poles/ends of cell

• In anaphase, spindle fibres shorten

The image shows one cell the student saw in the onion tissue. The student concluded that the cell in the image above was in the anaphase stage of mitosis.

Was she correct? Give two reasons for your answer. (2 marks)

• Yes - Chromatids are in separate poles of spindle

• V-shape shows that sister chromatids have been pulled apart at their centromeres

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. (1 mark)

Differences in base sequences

Describe the appearance and behaviour of chromosomes during mitosis. (5 marks)

During prophase

• Chromosomes condense

During metaphase

• Chromosomes line up on the equator / centre of the cell

• Chromosomes attached to spindle fibres

During anaphase

• The centromere divides

• Sister chromatids are pulled to opposite poles of the cell

During telophase

• Chromatids uncoil

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

• Method to ensure all cut surfaces of the eight cubes are exposed to the sucrose solution

• Method of controlling temperature

• Method of drying cubes before measuring

• Measure mass of cubes at stated time intervals

The diagram shows one of the test tubes from the 20°C water bath and one from the 65°C water bath.

What conclusion can be made about the effect of temperature on the damage to beetroot cells? Provide an explanation for your conclusion. (4 marks)

• water at 20°C caused no damage

  • to the cell-surface membrane

• 65°C caused damage to the membrane

• 65°C denatured membrane proteins OR 65°C increased fluidity/permeability of the membrane

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

• Phospholipid bilayer allows movement of nonpolar/lipid-soluble substances

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

• Carrier proteins allow active transport

• Channel/carrier proteins allow facilitated diffusion/co-transport

• Cholesterol affects permeability

Use the graph to explain how this monoglyceride is transported into cells within the digestive system. (3 marks)

• Rate of uptake is proportional to concentration

• This means that monoglycerides move by simple diffusion

• Because they are lipid soluble

A scientist investigated the rate of absorption at different amino acid concentrations by cells lining the small intestine.

Use your knowledge of membrane transport to describe and explain the shape of the curve shown above. (4 marks)

• As the amino acid concentration increases, the rate of uptake increases

• Because as amino acid concentration increases, more amino acids move through carrier proteins

• The rate of uptake levels off at higher amino acid concentrations

• Because all carrier proteins are saturated

The sports drink contains sodium chloride. Sodium chloride increases uptake of glucose in the small intestine. Explain how. (4 marks)

• Sodium ions and glucose absorbed by co-transport

• Sodium ions removed by active transport into blood

• Sodium ions enter epithelial cells by facilitated diffusion taking glucose with them

• Glucose moved by facilitated diffusion into blood

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. (3 marks)

• Lactose lowers the water potential (in the lumen of the intestine)

• This reduces the water potential gradient (between the intestine and blood)

• Less water is reabsorbed into the blood by osmosis so more water is present in faeces

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. (3 marks)

• Salt lowers the water potential of the solution surrounding the bacteria

• The bacterial cytoplasm contains a higher water potential than its surroundings

• Water moves out the bacterial cells by osmosis

• Loss of water from bacterial cells causes dehydration and death of bacteria

Explain the differences between facilitated diffusion and active transport. (3 marks)

• Facilitated diffusion involves channel or carrier proteins while active transport only involves carrier proteins

• Facilitated diffusion is passive while active transport is not passive

• Facilitated diffusion takes place down a concentration gradient while active transport can occur against a concentration gradient

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. (2 marks)

• ATP to ADP + Pi releases energy

• Energy allows ions to be moved against a concentration gradient

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. (3 marks)

• Co-transport

• Uses (hydrolysis of) ATP

• Sodium ion and proton bind to the protein

• Protein changes shape (to move sodium ion and proton across the membrane)

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 the table.

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. (4 marks)

• There is a significant difference with Tenapanor

• There is a less than 0.05 probability that the difference is due to chance

• More salt in gut reduces water potential in gut contents

• So less water absorbed out of gut contents by osmosis

Describe how phagocytosis of a virus leads to presentation of its antigens. (3 marks)

• Phagocyte engulfs pathogen and forms phagosome vesicle

• Lysosomes release lysozymes which hydrolyse the pathogen

• Antigen is presented on cell surface membrane

Suggest one reason why it was necessary to give two doses of the vaccine. (1 mark)

Antibodies go down over time

An antigen in a vaccine leads to the production of antibodies. Describe the part played by B lymphocytes in the process. (4 marks)

• Macrophages present antigens to B lymphocytes

• Antigen binds to receptors on lymphocyte

• B lymphocytes reproduce by mitosis

• Plasma cells secrete antibodies

Although this vaccine is made from antigens from malarial parasites, it does not cause malaria. Explain why this vaccine does not cause malaria. (2 marks)

• Antigens only part of the parasite

• They do not cause the disease

Changes to the protein coat of the influenza virus cause antigenic variability. Explain how antigenic variability has caused some people to become infected more than once with influenza viruses. (2 marks)

• Antibodies previously produced are not effective

• As shape not complementary to the new antigen

Describe how antibodies are produced in the body following a viral infection. (6 marks)

• Virus contains antigen

• Virus engulfed by phagocyte

• Presents antigen to B-cell;

• B-cell becomes activated

• Divides to form clones by mitosis

• Plasma cells produce antibodies specific to antigen

Explain the role of B-lymphocytes and T-lymphocytes in the defence of the body against a virus infection. (6 marks)

• B lymphocytes produce antibodies involved in humoral response

• T lymphocytes involved in cell-mediated immunity

• B lymphocytes bind to antigen which increase in numbers by mitosis

• Produce plasma cells, secrete antibodies which bind to and agglutinate virus

• T_c cells kill virus-infected cells

Antibiotics are not used to treat viral infections, such as HIV. Explain why. (2 marks)

• Antibiotics stop metabolism, and viruses don’t have metabolism

• Viruses hide in cells where antibiotics cannot reach them

• Some antibiotics work against ribosomes, which viruses don’t have

So far, these types of vaccine have not been considered safe to use in a mass vaccination programme.

Suggest why they have not been considered safe. (3 marks)

• Inactive virus may become active

• Attenuated virus may become harmful

• Non-pathogenic virus may mutate and harm cells

Describe how HIV is replicated after it has entered a human cell. (4 marks)

• Reverse transcriptase enzyme uses HIV RNA to make DNA copy

• Used to make HIV capsid proteins

• New virus particles assembled

• Budding off from membrane