Questions on cell specialisation and biological organisation

Question 1

Discuss the arguments for and against using embryonic stem cells rather than induced pluripotent stem cells in medical treatment.

Accepted Answer

B1 both are pluripotent and so can in principle be used to produce almost any tissue type, offering the same range of possible treatments B1 obtaining embryonic stem cells destroys the early embryo it came from, which raises an ethical objection that some people hold to be a serious moral cost B1 induced pluripotent cells are made by reprogramming adult cells, commonly from skin, so they avoid the need to use an embryo at all B1 induced pluripotent cells can be taken from the patient themselves, giving a much lower risk of immune rejection than tissue derived from a donor embryo B1 reprogramming and time spent in culture can alter induced pluripotent cells, so they carry their own, different, safety uncertainties that have to be weighed against the ethical cost of using embryonic cells

Question 2

A stem cell in the bone marrow and a ciliated epithelial cell from the trachea carry exactly the same DNA. Explain how the two cells come to have such different structures.

Accepted Answer

B1 both cells hold a complete copy of the same genome, so the genes present in the two cells are the same B1 only a fraction of those genes is transcribed in any one cell, so different proteins are made and different structures develop B1 which genes are expressed is set by the signals a cell receives, such as hormones, growth factors, its neighbours and its position in the embryo B1 once established the pattern of expression is maintained through subsequent divisions, so a cell's daughters stay the same type

Question 3

A phloem sieve tube element has lost its nucleus and almost all of its organelles, yet it is still alive. Explain how this suits it to its function, and explain why it cannot work without a companion cell.

Accepted Answer

B1 losing the nucleus and most organelles clears the centre of the tube, so sap flows through with little obstruction B1 the end walls survive as sieve plates perforated with pores, through which the contents pass from one element to the next B1 with no nucleus and no ribosomes the element cannot make its own proteins, so the companion cell makes them and passes them through plasmodesmata B1 the companion cell has dense cytoplasm and many mitochondria, and supplies the ATP for loading sucrose actively into the sieve tube

Question 4

A xylem vessel and a phloem sieve tube in the same vascular bundle have almost nothing in common when they are mature. Explain how both can be produced by a single population of dividing cells.

Accepted Answer

B1 the vascular cambium, or the procambium in a young stem, is a meristem of unspecialised cells that go on dividing B1 one daughter of a division stays as cambium, so the dividing population is not used up, and the other differentiates B1 a daughter left on the inner side of the cambium differentiates into xylem and one left on the outer side into phloem, so position decides the fate B1 both daughters carry the same genome, and the signals each receives in its position switch on a different set of genes

Question 5

Describe the structure of cartilage, and describe two examples of where it is found in the body.

Accepted Answer

B1 chondrocytes, the cells that secrete the tissue, lie isolated in small spaces called lacunae B1 they are surrounded by a firm, elastic extracellular matrix of collagen fibres and glycoproteins that they secreted B1 cartilage contains no blood vessels, so its chondrocytes are supplied only by diffusion through the matrix B1 any two examples of where it is found: capping the ends of bones at a joint, shaping the ear or nose, or the cartilage rings of the trachea

Question 6

Compare the structure of a xylem vessel with the structure of a phloem sieve tube element, referring to whether each is living, its wall and its end walls.

Accepted Answer

B1 a xylem vessel is dead with no cell contents, whereas a sieve tube element is alive, though it has lost its nucleus B1 a xylem vessel wall is thickened with lignin, whereas a sieve tube element wall remains unthickened cellulose B1 a xylem vessel's end walls have broken down completely, whereas a sieve tube element's end walls survive as perforated sieve plates B1 xylem transports water and mineral ions upward only, whereas phloem transports sucrose and amino acids in either direction

Question 7

A student says that a striated muscle fibre, which contains many nuclei, and a virus, which contains none, both show that cell theory is wrong. Evaluate this claim.

Accepted Answer

B1 a muscle fibre forms when many cells fuse, so it is still made from cells and by cells B1 it is the unit of structure and function of muscle tissue, so it is a modified cell and fits the theory B1 a virus is not made of cells and cannot carry out any function outside a host cell, so it is classed as non-living and cell theory, which concerns living organisms, does not apply to it B1 judgement: neither case contradicts cell theory; one is a modified cell and the other is outside the theory's scope

Question 8

Compare a totipotent cell, a pluripotent cell and a multipotent cell, giving one place each type is found.

Accepted Answer

B1 a totipotent cell can produce any cell type in the organism and the extra-embryonic tissues such as the placenta, and is found in the zygote and the first few divisions B1 a pluripotent cell can produce any cell type in the body but not the placenta, and is found in the embryonic stem cells of the blastocyst B1 a multipotent cell can produce only a limited range of cell types, and is found in adult tissues such as bone marrow

Question 9

Long-term smokers secrete more mucus than usual and the cilia on their airway epithelium are damaged. Suggest why such a smoker develops a persistent cough and gets more chest infections.

Accepted Answer

B1 cilia normally beat in a coordinated wave that sweeps mucus up towards the throat to be swallowed B1 with the cilia damaged the mucus is not moved, so it collects in the airways and coughing is the only way left to shift it B1 that mucus holds trapped dust and bacteria, which now stay in the airway and can multiply there, so infections are more likely

Question 10

Explain how the structure of squamous epithelium suits it to lining the alveoli, referring to diffusion.

Accepted Answer

B1 squamous epithelium is a single layer of extremely flat, thin cells B1 this gives a short diffusion path across the alveolar wall B1 the shorter the path, the faster oxygen and carbon dioxide diffuse across it, so gas exchange is rapid

Question 11

A student is given three structures to classify: ciliated epithelium from the trachea, the trachea itself, and the gas exchange system. Identify which is a tissue, which is an organ and which is an organ system, giving a reason for each identification.

Accepted Answer

B1 ciliated epithelium is a tissue, because it is one type of similar cells performing one function B1 the trachea is an organ, because it is built from several different tissues, such as epithelium, cartilage and smooth muscle, working together B1 the gas exchange system is an organ system, because it is a group of organs, including the trachea, bronchi, lungs and diaphragm, working together for an overall function

Question 12

Explain how the loss of its nucleus and mitochondria suits a mature red blood cell to its function of carrying oxygen.

Accepted Answer

B1 losing the nucleus and mitochondria leaves more space inside the cell, filled instead with roughly 270 million molecules of haemoglobin, increasing the oxygen-carrying capacity of the cell B1 with no mitochondria the cell respires anaerobically, so it does not consume the oxygen it is carrying on its way to deliver it to other tissues B1 the trade-off is that the cell cannot divide or repair itself, which is why each one lasts only about 120 days

Question 13

A surgeon proposes to repair a patient's damaged trachea using cartilage grown in the laboratory from a small sample of the patient's own cartilage. Suggest why cartilage is a realistic tissue to grow and transplant in this way, and suggest one reason repair might still be slow once it is in place.

Accepted Answer

B1 cartilage is mostly extracellular matrix secreted by a comparatively small number of chondrocytes, so a small number of cells can be grown to produce a useful mass of tissue B1 cartilage has no blood vessels of its own, so a piece grown outside the body does not need a blood supply to survive before or immediately after grafting, unlike a more vascular tissue B1 the chondrocytes in the graft are supplied only by diffusion through the matrix, which is slow, so nutrients reaching the repair site are limited and healing is correspondingly slow

Question 14

State the three parts of cell theory.

Accepted Answer

B1 all living organisms are made of one or more cells B1 the cell is the fundamental unit of structure, function and organisation in living organisms B1 new cells arise only from the division of existing cells

Question 15

State what is meant by a tissue, and state what is meant by an organ.

Accepted Answer

B1 a tissue is a group of similar cells working together to perform a particular function B1 an organ is several different tissues working together to perform a particular function

Question 16

State what is meant by a meristem, and state where the vascular cambium lies in a vascular bundle of a dicot stem.

Accepted Answer

B1 a tissue of unspecialised cells that continue to divide, and from which specialised cells differentiate B1 as a layer between the xylem on the inner side of the bundle and the phloem on the outer side

Question 17

State two ways, other than the loss of most of its cytoplasm, in which the structure of a sperm cell is adapted to its function.

Accepted Answer

B1 any one of: an acrosome, a cap of digestive enzymes used to digest through the outer layers of the egg; a mid-piece packed with many mitochondria, supplying ATP for the movement of the tail; or a long flagellum for swimming B1 any second, different adaptation from the same list

Question 18

Explain why cell theory is described as a unifying concept in biology.

Accepted Answer

B1 it applies to every living organism, from bacteria to plants and animals B1 processes such as respiration, protein synthesis, growth and repair are explained at the level of the cell, and tissues and organs are built from cells