Table 1

Cell Size

N

Dry Weight* × 10-3 fg

Wet Weight × 10-3 fg

Volume × 10-6 µm3

Diameter (nm)

 

 

 

 

Naked

+ Memb.

+ Wall

100

1,007

3,022

2,747

174

186

206

200

1,678

5,035

4,577

206

218

238

300

2,349

7,048

6,407

230

242

262

450

3,356

10,067

9,152

260

272

292

950

6,711

20,132

18,302

327

339

359

* (DNA + RNA + Protein)

Table 2

Cell Composition

N

% Dry Weight (DNA + RNA + Protein)

 

 

Diameter from Table 1 (nm)

 

Genome

Ribosomes

Other RNAs

Other Proteins

 

Assuming each protein species present in 10 copies

 

 

 

100

6.6

74.5

18.4

0.5

206

200

6.5

74.5

18.4

0.6

238

300

6.4

74.5

18.5

0.6

262

450

6.3

74.5

18.5

0.7

292

950

6.3

74.5

18.5

0.7

359

Assuming each protein species present in 1,000 copies

 

 

100

4.4

49.9

12.4

33.3

231

200

4.1

46.8

11.6

37.5

272

300

3.9

45.7

11.3

39.1

303

450

3.8

44.8

11.1

40.3

340

950

3.7

43.8

10.9

41.6

422

As Dr. de Duve commented in opening the workshop, the results given in Tables 1 and 2 above must be regarded as unrealistically low. Referring to the values for cell composition listed In Table 2, he pointed out that In a cell with only 10 copies of each protein species, the ribosomes and the other RNA components of the protein-synthesizing machinery represent more than 90% of the dry weight. Even when 1,000 copies are present, a cell's protein-synthesizing machinery still accounts for more than 50% of its dry weight. Barring the unlikely event that the same ribosome actually serves in the synthesis of several distinct protein species, sizes significantly below the calculated values are possible only if a less bulky machinery makes proteins. Even a single ribosome surrounded by a membrane and a wall would occupy a sphere of 57 nm In diameter.



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