サルでもわかる細胞の呼吸

呼吸は細胞質基質ミトコンドリアで行われる

細胞の呼吸、つまり細胞がグルコースなどの栄養をどのようにからだが利用できるエネルギーに変えているかについて、わかりやすく簡単に説明します!

呼吸は大きく3つのパートにわかれる

呼吸は大きく「解糖系⇒クエン酸回路⇒電子伝達系」の三段階にわけられます。

解糖系

解糖系は、名前がついているように「糖を分解する」パートです。グルコースを分解してピルビン酸2分子が作られます。解糖系は細胞質基質(細胞の内側の細胞膜で囲まれた部分)で行われます。また解糖系そのものでは酸素を必要としません。

グルコース×1 + リン×2 ⇒ ピルビン酸×2 + ATP×2 + H⁺×2 

クエン酸回路

クエン酸回路は、解糖系で作られたピルビン酸を用います。反応がぐるぐる回り続けているので、回路と呼ばれていて、ピルビン酸が変化したアセチルCoAが最初に入る回路がクエン酸なので、クエン酸回路と呼ばれています。

クエン酸回路は細胞内にたくさんある ミトコンドリア のマトリックスで行われます。マトリックスとはミトコンドリアの内膜に囲まれた部分のことを指します。

ピルビン酸×1 + H₂O×3 ⇒ ATP×2 + CO₂×3 + 水素原子×10

実際に起こっているのは以下の反応です。

2C₃H₄O₃ + 6H₂O + 8NAD⁺ + 2FAD → 6CO₂ + 8(NADH+H⁺) + 2FADH₂ + 2ATP

実際には、解糖系ではピルビン酸は2個作られたので、トータルでは上記の倍になります。ここまでの反応での使われた物質と生成物は以下のようにまとめられます。

つかわれたもの
・ グルコース1分子
・ 水 6分子

作られたもの
・ ATP 4分子
・ 二酸化炭素 6分子
・ 水素原子 24個

電子伝達系

次に使うのは、クエン酸回路でできた水素原子です。水力発電のダムのような仕組みでATPが生成されるのが電子伝達系です。ここは言葉だけではかなり理解しにくいと思いますので、生物学を学んでいない人はさらっと読み飛ばしてもいいかもしれません。

電子伝達系はミトコンドリアの内膜で行われます。ミトコンドリア内膜はクリステというひだのような構造になっており、反応が効果的に行われます。ミトコンドリア内膜にはシトクロムというタンパク質が散らばっており、またATP合成酵素という酵素もあります。

水素原子はまず、水素イオン(H⁺)と電子(e⁻)に分かれます。

電子は、内膜に散らばっているシトクロムの中を次々と受け渡されていき、そのたびに電子のエネルギーは放出されます。そのエネルギーを用いて、水素イオンがクリステのひだの内側に入りこむことで、水素イオンの濃度勾配ができ、その濃度勾配がさらにほかの水素イオンを内膜へと流れ込ませます。この濃度勾配がATP合成酵素を動かしてATPが作られます。

水素イオンと電子は最後には、酸素(O₂)に結合し、H₂Oを作ります。最後にこの反応がおきるまで、クリステでのバケツリレーのような電子の伝達が行われるのです。なので、酸素がなかったら、反応が起こらないのです。

では、電子伝達系で生成された物質をまとめてみましょう。

10(NADH+H⁺) + 2FADH₂ + 6O₂ → 10NAD⁺ + 2FAD + 12H₂O + 34ATP(最大)

よくわからない…

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呼吸の反応のまとめ

全反応をまとめてみると以下のようになります。

反応が起こる場所酸素の有無ATP合成量
解糖系細胞質基質不要2分子
クエン酸回路ミトコンドリアのマトリックス消費はしないがないと反応が停止2分子
電子伝達系ミトコンドリアの内膜必要34(最大)
全体細胞内全体としては必要38(最大)

酸素がなければ電子伝達系が働かず、よって脱水素酵素が働かなくなるためクエン酸回路も止まってしまい、ピルビン酸が変化しません。なので、解糖系の脱水素酵素も働けなくなるため、解糖系も止まってしまうと考えられます。

下の化学反応式を見てみると、解糖系だけ、クエン酸回路だけでそれぞれ反応することができないということがわかります。

化学反応式
解糖系C₆H₁₂O₆ + 2NAD⁺ → 2C₃H₄O₃ + 2(NADH+H⁺) + 2ATP
クエン酸回路2C₃H₄O₃ + 6H₂O + 8NAD⁺ + 2FAD
→ 6CO₂ + 8(NADH+H⁺) + 2FADH₂ + 2ATP
電子伝達系10(NADH+H⁺) + 2FADH₂ + 6O₂
→ 10NAD⁺ + 2FAD + 12H₂O + 34ATP(最大)
全体C₆H₁₂O₆+10NAD⁺+6H₂O+2FAD+6O₂
→ 10NAD⁺+2FAD+12H₂O+38ATP(最大)

全体の反応を見てみると、反応に必要な総量のNAD⁺分子十個が、全体の反応式のNAD⁺の生成量と一致しています。また、反応に必要なFAD分子の個数も一致しています。こうして、呼吸はぐるぐると物質が細胞の中を巡り巡っているのです。

ただし、酸素が本当にない場合は、解糖系の生成物であるピルビン酸と水素を結合させて、乳酸に変更することができます。

ピルビン酸(C₃H₄O₃)+2H⁺ → 乳酸(C₃H₆O₃)

なので、筋トレをしたときは、「乳酸が出ている」などという表現をするんですよね。すべての反応経路をまわる時間がないため、とりあえず、解糖系が働きまくっているという状態です。

このような酸素を必要とせずにATPを合成できる嫌気呼吸は利点もありますが、好気呼吸では最大38ATPをつくることができ、真核生物は生命活動の維持に必要なエネルギーの大部分を好気呼吸によって得ています。

しかし、酵母菌や乳酸菌などの微生物となると、嫌気呼吸を行って得られるエネルギーを使って生きているのです。

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