Comprehension (AS Only) (AQA AS Biology): Exam Questions

The milk produced by mammals is nutrient-rich and contains the correct proportions of the different nutrients that offspring need to grow and develop. This milk is produced by milk-producing cells located in mammary glands.
Milk-producing cells synthesise lipids, carbohydrates and proteins by absorbing substances from the blood. These cells have a height to breadth ratio of 1.3: 1.

They are approximately cube-shaped.

Lactose is a disaccharide and is the main carbohydrate in milk. The condensation of glucose and galactose (both monosaccharides) forms lactose. Galactose has the same chemical formula as glucose but has a slightly different structure.

Lactose-synthesis occurs in the Golgi apparatus and is then transported through the cytoplasm in vesicles. The lactose inside these vesicles cannot escape. The diameter of the vesicles increases as they move to the plasma membrane. When they get there, the vesicle membranes fuse with the plasma membrane and release their lactose out of the cell.

Use the information provided above and your own knowledge to answer the following questions.

(i) Milk-producing cells have a height to breadth ratio of 1.3: 1. (line 5). The breadth of a milk-producing cell is 37 um. Calculate the height of this cell.

(ii) Milk-producing cells are approximately cube-shaped (line 6). Describe and explain how the surface area to volume ratio of an epithelial cell from the inner lining of the small intestine might differ from the surface area to volume ratio of a milk-producing cell.

Using your knowledge of water potential, explain why the diameter of lactose-containing vesicles increases as they move through the cytoplasm towards the plasma membrane (lines 11-12).

Why do milk-producing cells need to contain large numbers of mitochondria?

Lactose is released from milk-producing cells via exocytosis (lines 12-14). However, some substances diffuse through the plasma membranes of milk-producing cells.

Explain how substances can pass through the membrane via diffusion.

The human body contains over 200 unique cell types. Further still, numerous human body cell types can be split further into different varieties of cell, within the cell type. For example, we can split white blood cells into several distinct varieties including lymphocytes and macrophages (a type of phagocyte).

Despite many common features present across a range of human body cell types, 5 each type is also specialised (sometimes very highly) to carry out its specific function. Although the majority of cell types contain the same organelles, the number of certain organelles that each cell type contains can differ significantly. A notably large number of mitochondria are present in muscle cells, for example, and hormone-secreting cells in the pancreas have distinctly large amounts of rough endoplasmic reticulum.

Interestingly, the amount of a certain organelle is not always fixed during a cells 'life' and this was observed by scientists looking at the cells in the tails of tadpoles. As a tadpole develops into a frog, its tail is gradually reabsorbed into the main body of the tadpole. The scientists found that the number of lysosomes in the tail cells actually increased as the tadpole matured.

Use the information provided above and your own knowledge to answer the following questions.

A notably large number of mitochondria are present in muscle cells (lines 8-9. Explain why muscle cells are adapted this way.

Wheat stalks are mainly composed of three different organic materials. These are cellulose, hemicellulose and lignin. Molecules of cellulose are joined in chains, which then layer together to create cellulose fibres. Hemicellulose is mostly made up of pentose sugar monomers. Hemicellulose is a small molecule and functions a bit like a glue in that it holds cellulose fibres together. The third organic material, 5 lignin, is also a polymer. Unlike the other two substances, however, it is not a carbohydrate. It provides extra strength by covering the cellulose in the cell wall.

After the harvesting of a wheat crop, the hollow wheat stalks are dried, then cut and formed into bales of straw. Straw has a very low moisture content.

The straw can be used to produce a variety of useful organic substances. For example, it can be used as an ingredient in biofuels, packaging materials and paper manufacturing. A key step in the process is converting the cellulose to glucose. Although enzymes can be used for this process, a complication with this is that the lignin covering the cellulose within the straw stops the cellulose from being broken down by such enzymes. The lignin must therefore be removed before this can occur.

Use the information provided above and your own knowledge to answer the following questions.

Compare and contrast the structure of a hemicellulose molecule and a cellulose molecule.

Molecules of cellulose are joined in chains, which then layer together to create cellulose fibres (lines 2-3). Describe the structure of a cellulose molecule.

The lignin covering the cellulose within the straw stops the cellulose from being broken down by cellulose-digesting enzymes (lines 14-15). Explain why these enzymes cannot simply break down the lignin.

Hemicellulose is mostly made up of pentose sugar monomers (lines 3-4).

Give one named example of a pentose sugar.

Pathogens enter the bodies of organisms, where they cause disease. Viruses are a type of pathogen. People infected with the human immunodeficiency virus, commonly referred to as HIV, develop something known as acquired immunodeficiency syndrome, commonly referred to as AIDS. This is a very serious condition as it stops the infected individual from producing an immune response in defence against other pathogens.

Researchers have been trying to create vaccines against HIV for a long time but it is proving very difficult, mainly due to three properties of HIV. Firstly, HIV has high antigenic variability. Secondly, the virus can enter human body cells within a very short time of an individual being infected. Thirdly, HIV destroys T-cells.

There are other methods used to help delay the onset of AIDS. Although not a long-term cure, a drug called azidothymidine (AZT) can be used to temporarily stop and slow down the development of AIDS. The drug does not destroy HIV in the body but inhibits the enzyme responsible for synthesising DNA from the RNA of the human immunodeficiency virus.

Use the information provided above and your own knowledge to answer the following questions.

Viruses are a type of pathogen (lines 1-2). Other than viruses, name one other type of pathogen.