今日は細胞についてお話ししたいと思います！
細胞ってとっても小さなイメージがあるなぁ。顕微鏡を使わないと見えないような…
でも、実際はそうでもないんですよ。例えば、毎朝、みなさんが食べている卵の黄身はひとつの細胞です！
へー。あれも細胞なんだ。目に見えるものもあるんだなぁ。
細胞にもいろいろな種類があります。大きさや形が違うし、働きも違います。１メートル以上ある細胞もあるんです！
ええっ！ 1メートル！？
生命の最小単位が細胞
細胞というと漠然と「小さいもの」というイメージがありますが、生物学では「生命の最小単位」のことを細胞といいます。
ミドリムシやアメーバなど細胞１個でひとつの生物である「単細胞生物」と、ヒトやイヌやバラやミジンコのように２つ以上の細胞が集まって生きている「多細胞生物」の２種類があります。ミジンコは小さいですが、いろいろな細胞が集まっているので多細胞生物です。
細胞の中にもいろいろな器官があり、それぞれが別々の働きをしています。たとえば、植物が緑である理由である葉緑体は、太陽の光を利用して水と二酸化炭素からデンプンをつくる光合成というはたらきを行っています。その葉緑体は下の図にあるように、約１㎛( マイクロメートル )です。
髪の毛や爪もたくさんの細胞が集まってできています。しかし、自分の細胞ひとつをとって「これが生物だ」とは言わないことが多いと思います。ヒトなどの多細胞生物は、様々な種類の細胞がたくさん合わさって「ひとつの生物」となっています。
細胞には小さなものから大きなものまであり、例えば、ヒトの 神経細胞 には1メートル以上の長さがあります。キリンの首にある神経細胞には３メートルもの長さがあるものもあります。
しかし、大きな動物の細胞が大きいということはありません。ヒトの細胞もネズミの細胞もクジラの細胞もほとんど同じ大きさです。ただクジラはヒトよりも多くの細胞が集まってできた生物なのです。
ヒトの細胞は約200種類、細胞の数は約60兆個と言われています。
細胞の中はどうなっているのか
へーっ！ ヒトには200種類もの細胞が集まっているんだ！
そうなんです。そして、人間のからだに心臓や肺、肝臓というような、それぞれ異なるはたらきをする器官があるのと同じように、細胞にも「器官」があり、それぞれが自分の仕事をしています。
じゃあ、200種類の細胞があるなら、それぞれ違う「器官」を持っているっていうこと？
そうです！ 細胞の「器官」のことを細胞小器官といいます。細胞独自の細胞小器官もありますし、どの動物にも哺乳類なら心臓や肺があるように、ほとんどの細胞が共通して持っている細胞小器官もあります。
主な細胞小器官には以下のようなものがあります。
・ 核
・ ミトコンドリア
・ 小胞体
・ リボソーム
・ ゴルジ体
・ 細胞壁
核にはDNAが入っています。DNAは核にある物質で、自分のコピーを作る能力を持っています。それもものすごい正確さで、ほぼ完璧に自分の分身を作り出すことができるのです。命を持っているものは「DNAを持っているもの」と言いかえることもできます。生きていることは自分の複製を作り出せることが条件のひとつに挙げられます。
バクテリアのようなに、核がない生物もいます。しかし、DNAがないのではありません。DNAが核に入っていない生物を原核生物と言います。
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