细胞生物学资料：02课堂思考(8.30—9.13)

2002级细胞生物学

Testing & Thinking of Cell Biology

Testing and Thinking(8-30-2004)

1. Assume you were given a mixture consisting of one molecule each of all possible sequences of a smallish protein of molecular weight 4800. How big a container would you need to hold this sample? Assume that the average molecular weight of an amino acid is 120.

2 An adult human is composed of about 101

3 cells, all of which are derived

by cell division from a single fertilized egg.

A. Assuming that all cells continue to divide (like bacteria in rich

media), how many generations of cell divisions would be required to produce 1013 cells?

B. Human cells in culture divide about once per day. Assuming all cells

continue to divide at this rate during development, how long would

it take to generate an adult organism?

C. Why is it, do you think, that adult humans take longer to develop

than these calculations might suggest?

Answer

1. A protein with a molecular weight of 4800 is made of about 40 amino acids; thus there are 1.1 × 1052(= 2040) different ways to make such a protein. Each individual protein molecule weighs 8 × 10-21 g (= 4800/6 × 1023); thus a mixture of one molecule each weighs 9 × 1031 g (= 8 × 10-21 g ×

1.1× 1052), which is 15,000 times the total weight of the planet earth,

1.1weighing 6 × 1024kg. You would need a quite large container, indeed.

Answer:2

A. The number (n) of generations of cell divisions required to produce 1013

cells is 2n = 1013

It is useful to remember that 2110? 103 (2n produces the series: 2,

4, 8, 16, 32, 64, 128, 256, 512, 1024; thus, 2110= 1024 ? 103). If

103 cells result from ten generations of dividing, 1012 cells will

result from 4 x 10 = 40 generations. Thus, you can estimate quickly

that it will take a little over 40 generations to reach 1013 cells.

You can get a more accurate answer, 43.2, by plugging different

values of n into your calculator. Alternatively, you can solve the

equation for n, which tests your familiarity with logarithms.

Remember that

2 = 10log2 and 2n = 10nlog2

Substituting,

10nlog2 = 1013

Taking the log of both sides,

Nlog2 = 13

n = 13/log2 = 13/0.301

n = 43.2

B. If cells divided once per day and all cells continued to divide, it would

take 43.2 days to generate the number of cells in an adult human.

C. Obviously we don't become adults in 43 days. The simple answer is that

all cells don't continue to divide once per day and some cells are

programmed to die. As cells differentiate, they generally slow their

rate of division, ultimately in the adult dividing just often enough

to replace cells that are lost or die. Of course, the real answer

is much more complex, involving time for cell movements, for local

environments to be established, for extracellular matrices to be

laid down, for cells to differentiate, for global patterns to develop,

and so on.

Testing and Thinking(9-1-2004)

1． Friedrich Miescher (1844-1895), Gregor Mendel (1822-1884), Louis Pasteur (1822-1895), and Rudolf Virchow (1821-1902) were European scientists (from Switzerland, Austria, France, and Germany, respectively) whose most important discoveries were made in the 20-year period from 1855 to 1875. Assume that the four men met at a scientific conference in 1875 to discuss their respective contributions to biology.

(a) What might the four scientists have found they had in common?

(b) What do you think Pasteur might have found most intriguing or relevant about Virchow's work? Explain your answer.

(c) What do you think Virchow might have found most intriguing or relevant about Mendel's work? Explain your answer.

(d) In whose work do you think Miescher might have been the most interested?

Explain your answer.

Answers for (9-5-2003)

1．(a) The four scientists might have found that they had common interests in the origin of cells (from preexisting cells, not by “spontaneous

generation") and the nature of the genetic information that

organisms pass on to their progeny. (Bear in mind, however, that we

are answering the question from the perspective of almost 125 years

of further discoveries and insights. In 1875, the four scientists

might with equal likelihood have concluded that they had few, if any,

interests in common.)

(b) Given Pasteur’s demonstration that living, preexisting yeast cells

are needed to carry out the fermentation of sugar into alcohol, he

would almost certainly have been interested in Virchow’s conclusion

that all cells come from preexisting cells. (In fact, he would most

likely have found Virchow’s addition to the cell theory of Schleiden

and Schwann particularly encouraging in light of the earlier belief,

debunked by Pasteur’s work, that yeast cells arose by “spontaneous

generation" in the presence of such substrates as fermentable grape

juice.)

(c) In light of Virchow’s conclusion that cells (and therefore organisms

as well, presumably) arise only from preexisting cells, he would

likely have been very interested in Mendel’s evidence that traits

such as those he observed in pea plants were heritable and could be

passed from one generation to the next by transmission of “hereditary

factors," because the genetic continuity between generations

implied by Mendel’s findings would have fit well with the physical

continuity of generations inherent in Virchow’s contribution to cell

theory.

(d) If Miescher were sufficiently foresighted, he might have been most

interested in the work of Mendel because Miescher’s discovery of what

we now call DNA was eventually to provide the chemical basis for the

heritability of traits described by Mendel.

Testing and Thinking(9-6-2004)

1. What does the ‘p’ in pH stand for?

2. The proteins in a mammalian cell account for 18% of its net weight. If

the density of a typical mammalian cell is about 1.1 g/mL and the

volume of the cell is 4 x 10-9mL, what is the concentration of protein

in mg/mL?

3. Each of the following statements is an answer; indicate in each case what

the question is.

(b) This property of water makes it possible for land animals to cool

themselves by surface evaporation with minimum loss of body fluid.

Answers for (9-6-2004)

1. Although the symbol ‘p’in common usage denotes the ‘negative logarithm

of’what it stands for is unclear. In the original 1909 paper in which

the concept of pH was developed, the author--Danish chemist Soren

EL. Sorensen--was not explicit. In textbooks where it is commented

on at all, it is most commonly reputed to stand for the French or

German words for power or potential. Close examination of the

original paper reveals that the ‘p’ in pH is likely a consequence

of the author's arbitrary choice to call two solutions by the letters

‘p’and ‘q’. The q solution had the known H+ concentration of 1, the

p solution had the unknown H+ concentration. If the solutions had

been switched, do you think qH would ever have caught on?

2. The concentration of protein is about 200 mg/mL (0.18 x 1.1 g/mL = 198

mg/ mL). Given the density of the cell, you don't need to know its

volume to calculate the concentration of protein.

3. (b) What is the temperature-stabilizing capacity of water? (Or,

alternatively, what is the specific heat of water?)

Testing and Thinking(9-13-2004)

7-3. Gorter and Grendel Revisited. Gorter and Grendel's classic conclusion that the plasma membrane of the human erythrocyte consists of a lipid bilayer was based on the following observations:

(i) the lipids that they extracted with acetone from 4.74 ×109 erythrocytes formed a

monolayer 0.89 m2 in area when spread out on a water surface; and

(ii) the surface area of one erythrocyte was about 100μm2, according to their measurements.

(a) Show from these data how they came to the conclusion that the erythrocyte

membrane is a bilayer.

(b) We now know that the surface area of a human erythrocyte is about 145μm2.

Explain how Gorter and Grendel could have come to the right conclusion when one of their measurements was only about two-thirds of the correct value.

Answer: 7-3.

(a) If the lipids from 4.74 x 109 erythrocytes have an area of

0.89 m2 (= 0.89 x 1012 um2). then each cell has a monolayer

area of 0.89 x 1012/4.74 x 109 = 188 um2.This is almost twice

the surface area of an erythrocyte as Gorter and Grendel

estimated it at the time, leading to the conclusion that

the surface of each cell was covered with two layers (i.e.,

a bilayer) of lipid.

(b) As it turned out, the lipid extraction technique that Gorter

and Grendel used was not quantitative, so they

underestimated the amount of lipid per cell. In fact, they

extracted only about two-thirds of the total lipid from the

erythrocytes, so both of their values were off by about the

same extent—a classic case of the right conclusion but from

flawed data.