Page 10 - Science Focus (issue20)

Mitochondria :
So Much More Than
the Powerhouses of the Cell
線粒體： 遠不只是細胞的發電站
By Kit Kan 簡迎曦
Powerhouse and Suicide Bomb
Fo r those of you w ho have some ba s ic
understanding of biology, you may already know the
mitochondria as the powerhouses of the cell. These
double-membrane structures serve as sites for cellular
respiration, in which sugar and oxygen are used to
produce adenosine triphosphate (ATP), the energy
currency that our cells use. Sperms have plenty of
mitochondria for their long distance swim in search of
an egg. Muscle cells need mitochondria to contract
and bring about movement. Neurons use the energy
generated by mitochondria to send electrical and
chemical signals. However, powering the cell is not all
that the mitochondria do.
Our cells engage in a process called apoptosis,
which is essentially suicide for cells. During embryonic
development, some cells have to die to carve out
the body structures that we now have. For instance,
our hands and feet would be webbed like frog’s
if the cells between our fingers and toes did not
undergo apoptosis. As apoptosis is also a way for
our body to eliminate cells that are potentially
cancerous, defective apoptosis can result in cancer.
One pathway of apoptosis involves mitochondria.
Mitochondria contain a protein called cytochrome c,
which when released to the cytoplasm, binds to other
proteins to form apoptosomes and facilitate apoptosis.
In short, mitochondria are not just powerhouses, but
also suicide bombs.
Origin and Inheritance
Mitochondria are not only interesting in the
functions they serve, but also their origin. The
mitochondria came from a process named
endosymbiosis. More than 1.45 billion years ago [1],
a unicellular organism is believed to have engulfed
another unicellular organism, and they formed a
symbiotic relationship. The endosymbiont (the one
being engulfed) became the mitochondria. There
are two different theories as to whether the host
was a prokaryote or a more full-fledged eukaryote,
but the hypothesis that the host was a prokaryote
is more widely accepted because the ubiquity of
mitochondria in eukaryotes can then be explained,
providing that eukaryotes were evolved from the
mitochondrial endosymbiosis in a prokaryote [1].
Mitochondria are similar to bacteria in many ways.
One major feature and evidence of endosymbiosis
is that mitochondria contain their own DNA that is
organized in circular chromosomes.
方便的方法追踪祖先。由於線粒體 DNA 僅是從母親那裡
線粒體 DNA 不僅在臨床上很重要，而且還提供了一種
發電站和自殺式炸彈
在 1987 年發表的一項研究中，科學家從不同族群採集了
147 個樣本，並根據其線粒體 DNA 序列的相似度畫成了
如果你對生物學有基本了解，你可能已經知道線粒體是
一棵演化樹。他們找到了證據證明目前所有的線粒體 DNA
細胞的發電站（註一）。這些擁有雙層薄膜的結構是細胞進
均源自非洲的一名婦女，這發現正正支持受普遍認同的非
行呼吸作用的地方，在過程當中糖和氧被用於生產腺苷三 繼承，因此科學家可以使用此特點追踪我們的母系血統。​
Unlike nuclear DNA, 磷酸（ATP），亦即是我們細胞使用的「能量貨幣」。精子含 洲起源說​[3]。在人類遺傳學中，所有活著的人類的母系最
t h e i n he r i t a n c e o f
有大量線粒體產生能量來支持長距離游泳以尋找卵子，肌 近的共同祖先通常被稱為「線粒體夏娃」。然而，這並不是
m i t o c h o n d r i a l D N A i s 肉細胞需要線粒體生產的 ATP 來收縮並引起運動，神經元 說線粒體夏娃是當時唯一活著的女性；我們的細胞核內的
strictly maternal. During 利用線粒體產生的能量來發送電和化學信號，但其實線粒 DNA 表明我們確實還有其他女性祖先 [4]。換句話說，我
fertilization, mitochondria
from the sperm are actively
removed. Therefore, no 體的功能並不只是為細胞提供能量。 們都有一個共同的母系曾……曾祖母，但同時我們也有其他
one inherits mitochondrial 女性祖先，只是不在母線（direct​female​line）而已。
DNA from his or her father. 我們的細胞有時會進行稱為細胞凋亡（apoptosis）的
Mitochondrial DNA encodes for genes that are 過程，簡單來說就是細胞的自殺。在胚胎發育過程中，一些 在你使用線粒體產生的能量閱讀本文，並驚嘆於這些
essential to the oxidative phosphorylation process 細胞必須死亡才能形成我們現在擁有的身體結構，例如如 複雜的結構之後，也許你可以去感謝你的母親，祖母和曾祖
in the mitochondria. Since the mitochondria mainly 果我們的手指和腳趾之間的細胞不發生凋亡，那麼我們的 母，因為她們把健康的線粒體遺傳給你！
generate energy by oxidative phosphorylation, the 手和腳就會像青蛙的掌一樣有蹼。
mutation in mitochondrial DNA can lead to disorders
由於凋亡也是我們身體消除潛在癌
in tissues with high energy demands, such as muscles 細胞的一種方法，故不正常的凋亡
and brain [2].
Mitochondrial DNA is not just clinically important, 可導致癌症。細胞凋亡的其中一種
but also provides a convenient way for ancestry 途徑涉及線粒體。線粒體含有一種
tracing. Since mitochondrial DNA is only inherited from 叫做細胞色素 c（cytochrome​c）
the mother, scientists can trace our maternal ancestry 的蛋白質，當被釋放到細胞質時，它
using this feature. In a study published in 1987, taking 將與其他蛋白質結合形成凋亡小體
147 samples from different populations, scientists
mapped the samples onto an evolutionary tree based （apoptosome）並促進細胞凋亡。
on the similarity of their mitochondrial DNA sequence. 簡而言之，線粒體不僅是發電廠，而
且亦是自殺式炸彈。
They found proof that all current mitochondrial DNA
originated from one woman in Africa, supporting
the popular “out-of-Africa” model [3]. The matrilineal 起源與遺傳
most recent common ancestor of all living humans is
often called the Mitochondrial Eve in human genetics. 線粒 體 的 有趣 之 處 不僅在 於
However, it is not to say that Mitochondrial Eve was
the only woman alive at the time; our nuclear DNA 其 功 能，它 們 的 來 歷 更 是 稀 奇。
suggests that we do have other female ancestors 線粒 體 來 自 稱 為 內共 生 的 過 程
[4]. In other words, we all share a common maternal （endosymbiosis）。科 學 家 相 信
great-great…grandmother, but we also have female 在超過 14.5 億年前​[1]，一種單細胞生物吞噬了另一種單
ancestors, just not in the direct female line. 細胞生物，然後它們形成了共生關係，當中內共生體（被吞
After you use the energy generated by your 1​ 編按：在英文中，線粒體通常被比喻為「the​powerhouse​of​the​cell（細胞的
mitochondria to read this article and marvel at the 噬的單細胞生物）變成了線粒體。關於宿主是原核生物還 發電站）」，外國網民甚至創作了不同迷因來揶揄這個一成不變的比喻，有人覺
beauty of these complex structures, maybe you 是發展得較成熟的真核生物，學術界存在兩種不同的理論， 得它代表了學校裡會教授但在現實生活上不太實用的知識。但無論如何，教育
can go thank your mother, grandmother and great 但是宿主是原核生物的假說被更廣泛地接受，因為在真核 局似乎沒有把這個比喻帶進香港中學的中文課程裡。
grandmother for the functional mitochondria that 生物是從原核生物的線粒體內共生進化而來的前提下，這
they passed on to you!
種說法可以解釋為何線粒體普遍存在於真核生物中​[1]。線 References 參考資料：
粒體在許多方面與細菌相似。內共生的其中一個主要特徵
和證據是線粒體含有自己的環狀 DNA 染色體。 [1] Martin WF, Mentel M. The Origin of Mitochondria. Nat
Educ. 2010;3(9):58.
與細胞核內的 DNA 不同，線粒體 DNA 是母系遺傳的。 [2] Chial H, Craig J. mtDNA and Mitochondrial Diseases. Nat
在受精過程中，來自精子的線粒體會被移除，因此沒有人能 Educ. 2008;1(1):217.
遺傳父親的線粒體 DNA。線粒體 DNA 包含了線粒體中氧 [3] Cann RL, Stoneking M, Wilson AC. Mitochondrial DNA and
化磷酸化過程必不可少的基因。由於線粒體主要通過氧化 human evolution. Nature. 1987;325:31-36.
[4] Takahata N. Allelic Genealogy and Human Evolution. Mol
磷酸化產生能量，因此線粒體 DNA 的突變可以導致一些 Biol Evol. 1993;10(1):2-22.
與肌肉和大腦等能量需求較高的組織相關的病症​[2]。
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