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BOGDAN FEDELES: Hello, and
welcome to 5.07 Biochemistry

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00:00:24,080 --> 00:00:26,610
on MIT OpenCourseWare.

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00:00:26,610 --> 00:00:28,800
I'm Dr. Bogdan Fedeles.

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00:00:28,800 --> 00:00:30,930
Let's metabolize some problems.

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00:00:30,930 --> 00:00:33,480
This series of videos
is meant to supplement

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00:00:33,480 --> 00:00:35,700
some of the other
materials on the site,

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00:00:35,700 --> 00:00:39,720
and to give you a more in-depth
and more interactive take

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00:00:39,720 --> 00:00:44,150
on some of the topics
covered in 5.07 Biochemistry.

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00:00:44,150 --> 00:00:46,530
Specifically, we're going
to be working together

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00:00:46,530 --> 00:00:49,980
through one problem from each
problem set from this course.

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00:00:53,620 --> 00:00:57,640
Today we're discussing the very
first homework problem of 5.07,

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00:00:57,640 --> 00:01:00,520
which is problem one
of problem set one.

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00:01:00,520 --> 00:01:02,530
This problem is
meant to give you

21
00:01:02,530 --> 00:01:05,349
a better sense of the
scales and dimensions

22
00:01:05,349 --> 00:01:07,870
of the cellular environment.

23
00:01:07,870 --> 00:01:10,120
Specifically, we're going
to be asking questions

24
00:01:10,120 --> 00:01:12,590
like, how big is this cell?

25
00:01:12,590 --> 00:01:14,260
What is the volume of a cell?

26
00:01:14,260 --> 00:01:16,960
And how many molecules
of a given protein

27
00:01:16,960 --> 00:01:18,200
are inside a cell?

28
00:01:21,690 --> 00:01:24,300
One fundamental idea
introduced in this problem

29
00:01:24,300 --> 00:01:27,750
is that the cellular
environment is very crowded.

30
00:01:27,750 --> 00:01:31,130
Things inside the cell
are very tightly packed.

31
00:01:31,130 --> 00:01:35,720
Now take a look at this
picture in your book.

32
00:01:35,720 --> 00:01:39,780
As you see here, the
constituents of a cell,

33
00:01:39,780 --> 00:01:44,370
like proteins, enzymes, the
organelles, metabolites,

34
00:01:44,370 --> 00:01:48,250
are all in very close
proximity to each other.

35
00:01:48,250 --> 00:01:51,240
Now, this is also reflected
in the concentration

36
00:01:51,240 --> 00:01:53,940
that we're given for the
proteins inside the cell.

37
00:01:53,940 --> 00:01:57,180
The problem tells us
there are 350 milligrams

38
00:01:57,180 --> 00:01:59,370
per milliliter of protein.

39
00:01:59,370 --> 00:02:02,770
Or in other words,
350 grams per liter.

40
00:02:02,770 --> 00:02:06,690
Now this is a very high number
in the context of biochemistry.

41
00:02:06,690 --> 00:02:09,270
Basically, if you
think one liter

42
00:02:09,270 --> 00:02:14,080
is 1,000 grams then 350
grams of that is protein.

43
00:02:14,080 --> 00:02:18,840
So we have 350 grams,
35%, of a cell is protein.

44
00:02:18,840 --> 00:02:21,900
And only 60% to 65% is water.

45
00:02:21,900 --> 00:02:24,810
Therefore, as the
problem says, when

46
00:02:24,810 --> 00:02:28,740
we're doing in-vitro experiments
using dilute solutions

47
00:02:28,740 --> 00:02:31,050
we rarely recapitulate
what actually

48
00:02:31,050 --> 00:02:33,280
happens inside the cell.

49
00:02:33,280 --> 00:02:35,840
Now how big is a mammalian cell?

50
00:02:35,840 --> 00:02:39,180
As you will see, sizes
of cells in an organism

51
00:02:39,180 --> 00:02:40,600
vary considerably.

52
00:02:40,600 --> 00:02:44,970
At one extreme we have very tiny
cells like endothelial cells,

53
00:02:44,970 --> 00:02:46,540
red blood cells.

54
00:02:46,540 --> 00:02:47,910
These are very, very small.

55
00:02:47,910 --> 00:02:51,060
On the other end, we
have reproductive cells,

56
00:02:51,060 --> 00:02:54,650
like the egg, which is 100
to 200 microns in diameter.

57
00:02:54,650 --> 00:02:57,750
Or even cells that can stretch
for centimeter, like the muscle

58
00:02:57,750 --> 00:03:00,640
cells and certain nerve cells.

59
00:03:00,640 --> 00:03:03,360
In this problem, we're going to
be dealing with the red blood

60
00:03:03,360 --> 00:03:06,880
cells, one of the most
abundant cells in the body.

61
00:03:06,880 --> 00:03:11,410
As you can see in this
very colorful picture,

62
00:03:11,410 --> 00:03:16,260
a red blood cell can be
approximated by a cylinder 6

63
00:03:16,260 --> 00:03:19,230
to 8 microns in diameter.

64
00:03:19,230 --> 00:03:22,200
So knowing that, we can actually
calculate some dimensions

65
00:03:22,200 --> 00:03:25,320
of the red blood cells,
such as the total volume

66
00:03:25,320 --> 00:03:26,730
and their surface area.

67
00:03:29,500 --> 00:03:35,770
Now here is a cylinder by which
we approximate a red blood

68
00:03:35,770 --> 00:03:37,090
cell.

69
00:03:37,090 --> 00:03:43,890
And let's say the cylinder
has the height, h,

70
00:03:43,890 --> 00:03:46,480
and the radius, r.

71
00:03:46,480 --> 00:03:51,790
From our problem we know h
is about 2 microns and r,

72
00:03:51,790 --> 00:03:55,660
well we know the diameter
is 6 to 8 microns.

73
00:03:55,660 --> 00:04:01,180
So r is going to be, let's go
with the middle, 3.5 microns.

74
00:04:01,180 --> 00:04:05,570
Now as you remember
from geometry,

75
00:04:05,570 --> 00:04:07,990
then the volume
of the cylinder is

76
00:04:07,990 --> 00:04:14,050
going to be pi r
squared h, which

77
00:04:14,050 --> 00:04:17,829
when we substitute
our units, we're going

78
00:04:17,829 --> 00:04:23,550
to get about 77 cubic microns.

79
00:04:26,430 --> 00:04:26,930
All right.

80
00:04:26,930 --> 00:04:30,680
We'll come back to discussing
the units in a little bit.

81
00:04:30,680 --> 00:04:32,750
Now the surface
area is just going

82
00:04:32,750 --> 00:04:39,050
to be the area surrounding the
cylinder plus the two circles--

83
00:04:39,050 --> 00:04:40,590
the top and the bottom.

84
00:04:40,590 --> 00:04:47,210
So the two circles are 2 pi r
squared plus the surrounding

85
00:04:47,210 --> 00:04:51,410
area is going to be 2 pi r h.

86
00:04:51,410 --> 00:05:00,590
And that comes out to be
about 120.95 square microns.

87
00:05:00,590 --> 00:05:05,519
Now how much is really
one cubic micron?

88
00:05:05,519 --> 00:05:07,310
Let's try to relate it
to a unit that we're

89
00:05:07,310 --> 00:05:10,610
more familiar such as liter.

90
00:05:10,610 --> 00:05:17,990
Well, one milliliter is
actually one cubic centimeter.

91
00:05:17,990 --> 00:05:24,920
One microliter is
one cubic millimeter.

92
00:05:24,920 --> 00:05:29,930
Now, one cubic micrometer,
it's like a billion times

93
00:05:29,930 --> 00:05:31,780
smaller than 1 microliter.

94
00:05:31,780 --> 00:05:33,830
One microliter is already
a million times smaller

95
00:05:33,830 --> 00:05:39,590
than a liter, so this is really
10 to the minus 15 liters

96
00:05:39,590 --> 00:05:42,800
is 1 cubic micrometer.

97
00:05:42,800 --> 00:05:46,770
Which is 10 to the minus
15 is the femto units.

98
00:05:46,770 --> 00:05:49,530
It's like one femtoliter.

99
00:05:49,530 --> 00:05:54,820
Now 77 femtoliters is an
incredibly small volume.

100
00:05:54,820 --> 00:05:59,150
But as we'll see next
when we look at bacteria,

101
00:05:59,150 --> 00:06:01,280
bacteria are even smaller.

102
00:06:01,280 --> 00:06:04,950
Now, let's take a look
at a Staph aureus cell.

103
00:06:04,950 --> 00:06:09,220
Now Staphylococcus aureus
is a very common bacteria

104
00:06:09,220 --> 00:06:14,117
that we often find on our skin
or in our respiratory tract.

105
00:06:14,117 --> 00:06:15,700
Now this is the same
bacteria that you

106
00:06:15,700 --> 00:06:18,670
might have heard in
that acronym MRSA,

107
00:06:18,670 --> 00:06:22,150
or Methicillin-Resistant
Staph Aureus.

108
00:06:22,150 --> 00:06:25,270
Now, this MRSA is a
pathogenic bacteria

109
00:06:25,270 --> 00:06:28,510
that can cause a lot of
problems in the hospitals

110
00:06:28,510 --> 00:06:31,210
nowadays, because it
is resistant to most

111
00:06:31,210 --> 00:06:33,370
of the antibiotics that we have.

112
00:06:33,370 --> 00:06:37,890
Now, here's a picture
of Staph aureus.

113
00:06:37,890 --> 00:06:41,140
As you can see, it's
a spherical cell.

114
00:06:41,140 --> 00:06:44,500
And of course this pretty
purple color is added in.

115
00:06:44,500 --> 00:06:48,640
This is just a electron
micrograph picture

116
00:06:48,640 --> 00:06:51,040
of a colony of Staph
aureus bacteria.

117
00:06:51,040 --> 00:06:54,850
So for it too we can
calculate the volume

118
00:06:54,850 --> 00:06:58,410
of the cell and
the surface area.

119
00:06:58,410 --> 00:07:02,980
If we assume Staph aureus
to be a sphere, of radius r,

120
00:07:02,980 --> 00:07:07,100
we are told r is 0.6 microns.

121
00:07:07,100 --> 00:07:09,640
Then we can calculate
the volume of the cell.

122
00:07:09,640 --> 00:07:16,110
The volume is simply going
to be 4 pi r cubed over 3.

123
00:07:16,110 --> 00:07:18,460
And if we plug-in
the numbers, we're

124
00:07:18,460 --> 00:07:27,880
going to get 0.11
cubic micrometer.

125
00:07:27,880 --> 00:07:30,280
Similarly for the
surface area, it's

126
00:07:30,280 --> 00:07:32,550
a surface area of a sphere.

127
00:07:32,550 --> 00:07:35,420
It's 4 pi r squared.

128
00:07:35,420 --> 00:07:42,610
And the units come out
to 1.13 square microns.

129
00:07:42,610 --> 00:07:46,120
Now 0.11 cubic microns.

130
00:07:46,120 --> 00:07:52,570
And we said a cubic micron
is one femtoliters, like 10

131
00:07:52,570 --> 00:07:54,890
to minus 15 liters.

132
00:07:54,890 --> 00:08:03,640
So 0.11 cubic micrometers is
110 times 10 to minus 18 liters.

133
00:08:03,640 --> 00:08:06,850
Now the prefix for 10
to minus 18 that's atto.

134
00:08:06,850 --> 00:08:09,625
So it's 110 attoliters.

135
00:08:12,584 --> 00:08:16,660
So this is an
incredibly small volume.

136
00:08:16,660 --> 00:08:18,790
So by calculating the
volume and the surface

137
00:08:18,790 --> 00:08:21,790
area of these cells,
we've essentially

138
00:08:21,790 --> 00:08:25,630
answered part 1 and
part 2 of this problem.

139
00:08:25,630 --> 00:08:28,120
Now next we're going to
explore the relationship

140
00:08:28,120 --> 00:08:30,690
between the volume and the
surface area of the cell.

141
00:08:34,450 --> 00:08:36,970
As you know from geometry,
the volume typically

142
00:08:36,970 --> 00:08:39,850
varies with the
cube of the radius,

143
00:08:39,850 --> 00:08:42,429
whereas the surface
area varies only

144
00:08:42,429 --> 00:08:44,800
with the square of the radius.

145
00:08:44,800 --> 00:08:46,900
Therefore, as the
radius of an object

146
00:08:46,900 --> 00:08:51,010
increases the ratio of
surface area over volume

147
00:08:51,010 --> 00:08:54,520
will decrease because the
volume increases quicker.

148
00:08:57,130 --> 00:09:00,950
Now, this is exactly what
we observe with cells.

149
00:09:00,950 --> 00:09:08,190
Therefore, for a red blood
cell surface area over volume

150
00:09:08,190 --> 00:09:20,000
it's going to be 12,095 square
microns over 77 cubic microns.

151
00:09:20,000 --> 00:09:25,065
That comes out to about
1.6 inverse microns.

152
00:09:35,250 --> 00:09:39,750
Now for a Staph aureus cell,
surface area over volume

153
00:09:39,750 --> 00:09:47,300
is going to be 1.13
square microns over 0.11

154
00:09:47,300 --> 00:09:49,500
cubic microns.

155
00:09:49,500 --> 00:09:56,720
That's approximately 10.

156
00:09:56,720 --> 00:09:58,090
Why is this important?

157
00:09:58,090 --> 00:10:00,590
It's because the
surface area of a cell

158
00:10:00,590 --> 00:10:05,300
controls how fast molecules
can go in and out of the cell.

159
00:10:05,300 --> 00:10:09,170
It essentially controls the flux
of molecules across the cell

160
00:10:09,170 --> 00:10:10,710
membrane.

161
00:10:10,710 --> 00:10:15,120
Now, if the surface area of
a volume is a large number,

162
00:10:15,120 --> 00:10:17,360
it means the molecules
can access that volume

163
00:10:17,360 --> 00:10:20,640
fairly quickly and efficiently.

164
00:10:20,640 --> 00:10:23,300
But as you can see,
the bigger the cell,

165
00:10:23,300 --> 00:10:26,840
the smaller the surface area
of a volume number becomes.

166
00:10:26,840 --> 00:10:30,840
And therefore, for big cells
molecules will have a hard time

167
00:10:30,840 --> 00:10:33,440
and will take a long time
to get inside the cell

168
00:10:33,440 --> 00:10:35,040
or out of the cell.

169
00:10:35,040 --> 00:10:38,960
That's why nature has designed
ways to transport molecules,

170
00:10:38,960 --> 00:10:42,640
to make them achieve the right
concentration efficiently.

171
00:10:42,640 --> 00:10:44,540
Now this is why, in
the case of bigger

172
00:10:44,540 --> 00:10:48,800
cells, such as the eukaryotic
cells or mammalian cells,

173
00:10:48,800 --> 00:10:51,860
in our case the red
blood cells, nature

174
00:10:51,860 --> 00:10:55,520
has evolved transport
mechanisms by which

175
00:10:55,520 --> 00:10:59,600
it can deliver small molecules
throughout the entire volume

176
00:10:59,600 --> 00:11:00,890
of the cell.

177
00:11:00,890 --> 00:11:03,170
One example of such
transfer molecules

178
00:11:03,170 --> 00:11:05,780
is hemoglobin, which
is used to deliver

179
00:11:05,780 --> 00:11:09,230
oxygen. This is what we're
going to take a look at next.

180
00:11:13,130 --> 00:11:15,620
We are given that
hemoglobin constitutes

181
00:11:15,620 --> 00:11:19,680
95% of the proteins in
the red blood cells.

182
00:11:19,680 --> 00:11:22,100
So let's calculate the
concentration of hemoglobin

183
00:11:22,100 --> 00:11:24,110
in a red blood cell.

184
00:11:24,110 --> 00:11:27,590
Now let's start with the
average protein concentration

185
00:11:27,590 --> 00:11:30,560
in the cell, which we
mentioned earlier, which was

186
00:11:30,560 --> 00:11:35,250
350 milligrams per milliliter.

187
00:11:35,250 --> 00:11:38,900
Now if 95% of this
is hemoglobin,

188
00:11:38,900 --> 00:11:43,160
then the concentration
of hemoglobin

189
00:11:43,160 --> 00:11:50,440
is going to be about 322
milligrams per milliliter.

190
00:11:50,440 --> 00:11:56,440
Now I'm going to abbreviate
hemoglobin as Hb.

191
00:11:56,440 --> 00:12:03,200
We're told that hemoglobin has
a molecular weight of 67,000

192
00:12:03,200 --> 00:12:10,930
Daltons, which is another way
of saying 67,000 grams per mole.

193
00:12:10,930 --> 00:12:13,850
So then the concentration
of hemoglobin

194
00:12:13,850 --> 00:12:23,770
is going to be 322
milligrams per 67,000.

195
00:12:23,770 --> 00:12:26,200
Grams per mole is the
same as milligrams

196
00:12:26,200 --> 00:12:31,650
per millimole and
per milliliter.

197
00:12:31,650 --> 00:12:37,510
The grams go away
and we get 0.0048.

198
00:12:37,510 --> 00:12:39,790
It's going to be
millimole per milliliter

199
00:12:39,790 --> 00:12:41,680
and that's the same
as mole per liter.

200
00:12:41,680 --> 00:12:44,170
That's the molar concentration.

201
00:12:44,170 --> 00:12:50,230
Or we can write
it 4.8 millimolar.

202
00:12:50,230 --> 00:12:53,650
Now this is a pretty
important range

203
00:12:53,650 --> 00:12:56,130
to keep in mind, because
the most abundant proteins

204
00:12:56,130 --> 00:13:00,430
such as hemoglobin, are going to
be in the low millimolar range.

205
00:13:00,430 --> 00:13:02,080
Most of the other
proteins are going

206
00:13:02,080 --> 00:13:06,870
to be in the micromolar
range in a cell.

207
00:13:06,870 --> 00:13:11,390
Now let's calculate how
many molecules of hemoglobin

208
00:13:11,390 --> 00:13:12,800
we have in the red blood cell.

209
00:13:15,500 --> 00:13:21,370
So we calculated before that
the volume of a red blood cell

210
00:13:21,370 --> 00:13:26,590
is actually 77 femtoliters.

211
00:13:26,590 --> 00:13:31,460
That's once again 77 times
10 to minus 15 liters.

212
00:13:34,380 --> 00:13:38,370
Now we know in this volume the
concentration of hemoglobin

213
00:13:38,370 --> 00:13:40,920
is 4.8 millimolar.

214
00:13:40,920 --> 00:13:43,410
So we can calculate
the number of moles.

215
00:13:43,410 --> 00:13:46,020
So the new number
of moles is going

216
00:13:46,020 --> 00:13:50,860
to be the concentration
times the volume.

217
00:13:50,860 --> 00:13:58,500
So we have 4.8 millimolar
times 77 times 10 to minus 15

218
00:13:58,500 --> 00:14:02,840
liters equals--

219
00:14:02,840 --> 00:14:06,000
now of course, millimolar--

220
00:14:06,000 --> 00:14:07,890
we have to transform
this back into molar.

221
00:14:07,890 --> 00:14:09,480
We have mole per liters.

222
00:14:09,480 --> 00:14:10,970
The liters are
going to cancel out

223
00:14:10,970 --> 00:14:12,570
so we're going to get moles.

224
00:14:12,570 --> 00:14:20,740
And it's 369, or so, times
10 to minus 18 moles.

225
00:14:20,740 --> 00:14:23,820
Remember 10 to minus
18 that's attomoles.

226
00:14:23,820 --> 00:14:31,920
So 369 attomoles of hemoglobin
we have in a red blood cell.

227
00:14:31,920 --> 00:14:36,450
Now we know one mole
contains the Avogadro

228
00:14:36,450 --> 00:14:38,560
numbers of molecules.

229
00:14:38,560 --> 00:14:41,100
So if you multiply this
with the Avogadro number,

230
00:14:41,100 --> 00:14:44,420
we should get the actual
number of molecules.

231
00:14:44,420 --> 00:14:51,770
So number of molecules is just
Avogadro number times number

232
00:14:51,770 --> 00:15:00,440
of moles, and Avogadro number
is approximately 6.022 times 10

233
00:15:00,440 --> 00:15:04,430
to the 23rd power,
so a gigantic number,

234
00:15:04,430 --> 00:15:11,090
times 369 times 10
to minus 18 moles.

235
00:15:11,090 --> 00:15:16,760
We're going to get about 2.2
times 10 to the 8 molecules.

236
00:15:21,410 --> 00:15:28,700
Or in other words, this
is 220 million molecules.

237
00:15:28,700 --> 00:15:34,640
So the problem was telling us
about a Google search in which

238
00:15:34,640 --> 00:15:38,000
we came up for different numbers
and one of them was 2,000,

239
00:15:38,000 --> 00:15:43,070
one of them was 200 million,
so obviously the answer we got,

240
00:15:43,070 --> 00:15:47,600
220 million, is closer to
the 200-300 million molecules

241
00:15:47,600 --> 00:15:50,862
that our Google search returned.

242
00:15:50,862 --> 00:15:52,820
That's the answer for
that part of the problem.

243
00:15:56,610 --> 00:16:00,320
Finally, let's see how the
size of a hemoglobin molecule

244
00:16:00,320 --> 00:16:02,480
compares to the size of a cell.

245
00:16:02,480 --> 00:16:11,070
We're told hemoglobin is
roughly spherical in shape,

246
00:16:11,070 --> 00:16:16,350
with a diameter of
about 55 Angstrom.

247
00:16:16,350 --> 00:16:19,290
Now as you recall from
Intro to Chemistry,

248
00:16:19,290 --> 00:16:24,510
one Angstrom is 10 to
the minus 10 meters.

249
00:16:24,510 --> 00:16:27,540
That's 0.1 nanometer.

250
00:16:27,540 --> 00:16:33,530
So our radius here is going
to be half the diameter,

251
00:16:33,530 --> 00:16:43,250
so it's 27.5 Angstrom
is 2.75 nanometers.

252
00:16:43,250 --> 00:16:46,340
So the volume of a
hemoglobin molecule

253
00:16:46,340 --> 00:16:51,080
is going to be 4
pi r cubed over 3.

254
00:16:51,080 --> 00:16:56,040
And with plugging
in 2.75 nanometers.

255
00:16:56,040 --> 00:17:00,500
So it's going to come up to
be 8.7 times 10 to the minus

256
00:17:00,500 --> 00:17:03,005
eighth cubic microns.

257
00:17:06,130 --> 00:17:09,550
Now, you remember the
volume of a red blood cell

258
00:17:09,550 --> 00:17:16,780
was 77 cubic microns,
so if you look

259
00:17:16,780 --> 00:17:20,920
at the relationship
between the two,

260
00:17:20,920 --> 00:17:26,109
how many volumes of a hemoglobin
can we fit in a red blood cell?

261
00:17:26,109 --> 00:17:32,920
Well, we just divide the
volume of the red blood cell

262
00:17:32,920 --> 00:17:34,690
to the volume of the hemoglobin.

263
00:17:34,690 --> 00:17:39,010
77 over 8.7 times 10
to the minus eighth.

264
00:17:39,010 --> 00:17:43,550
Both are cubic microns.

265
00:17:43,550 --> 00:17:49,750
And that gives us 8.8 times
10 to the eight molecules.

266
00:17:52,510 --> 00:17:58,900
So this is 880 million
molecules of hemoglobin

267
00:17:58,900 --> 00:18:02,950
would fit in the volume
of a red blood cell.

268
00:18:02,950 --> 00:18:05,770
If only hemoglobin
would be in there.

269
00:18:05,770 --> 00:18:07,930
Obviously this number
is an overestimation,

270
00:18:07,930 --> 00:18:12,040
because when you're
packing spherical objects,

271
00:18:12,040 --> 00:18:14,750
they're not going to pack
very tightly with each other.

272
00:18:14,750 --> 00:18:18,220
And as we said, the shape is
only approximately spherical,

273
00:18:18,220 --> 00:18:21,250
but nevertheless it's on
the same order of magnitude

274
00:18:21,250 --> 00:18:25,360
as the 200- 300 million
molecules of hemoglobin

275
00:18:25,360 --> 00:18:28,240
that we calculated earlier
based on the concentration.

276
00:18:28,240 --> 00:18:30,940
So from both the volume
standpoint and concentration

277
00:18:30,940 --> 00:18:34,600
standpoint we now
have calculated

278
00:18:34,600 --> 00:18:38,960
how many molecules of hemoglobin
can fit in a red blood cell.

279
00:18:38,960 --> 00:18:42,070
This result we just got actually
highlights a very important

280
00:18:42,070 --> 00:18:44,870
take home message,
which is, if we

281
00:18:44,870 --> 00:18:48,160
look at the molecular
and atomic scale,

282
00:18:48,160 --> 00:18:50,920
it is as distant from
the cellular scale

283
00:18:50,920 --> 00:18:52,810
as the cellular
scale is different

284
00:18:52,810 --> 00:18:55,510
from the macroscopic scale.

285
00:18:55,510 --> 00:18:58,510
Now if we take one
milliliter of blood,

286
00:18:58,510 --> 00:19:03,580
we find a few billion red
blood cells inside it.

287
00:19:03,580 --> 00:19:05,800
Now within each
red blood cell we

288
00:19:05,800 --> 00:19:09,820
find hundreds of millions of
molecules such as hemoglobin.

289
00:19:09,820 --> 00:19:11,470
That's it for this problem.

290
00:19:11,470 --> 00:19:14,770
I hope you now have a
better sense of the sizes

291
00:19:14,770 --> 00:19:17,770
and scales relevant for
biochemistry and cell biology

292
00:19:17,770 --> 00:19:19,280
in general.

293
00:19:19,280 --> 00:19:21,700
Keep in mind our discussion
of the surface area

294
00:19:21,700 --> 00:19:27,010
to volume ratio and why as
the cell size gets bigger,

295
00:19:27,010 --> 00:19:29,320
we need transport
mechanisms to make sure

296
00:19:29,320 --> 00:19:32,560
the nutrients and
metabolites get to where

297
00:19:32,560 --> 00:19:34,810
they need to go efficiently.

298
00:19:34,810 --> 00:19:37,930
Also keep in mind some of
the concentration ranges

299
00:19:37,930 --> 00:19:40,180
that we discussed,
as these will become

300
00:19:40,180 --> 00:19:43,870
very important in understanding
the biological significance

301
00:19:43,870 --> 00:19:45,880
of some of the
constants that we're

302
00:19:45,880 --> 00:19:50,460
going to calculate for
enzymes later in the course.