细胞生物.10 | Mitochondrion

1. Structure of Mitochondria

Mitochondria have a double membrane structure with four main compartments:

Outer Membrane:
- Smooth, permeable to small molecules (due to porins).
- Contains the TOM complex for importing proteins.
- 中文：外膜上有“通道蛋白”，可以让小分子自由通过，也负责最初的蛋白导入。
Intermembrane Space:
- The space between the outer and inner membranes.
- Plays a role in the proton gradient during ATP synthesis.
- 中文：质子会在这里堆积，为ATP合成提供能量。
Inner Membrane:
- Folded into cristae to increase surface area.
- Impermeable to most ions.
- Hosts electron transport chain (ETC) and ATP synthase.
- 中文：内膜是最关键的功能区，折叠形成嵴，上面布满了电子传递链和ATP合酶。
Matrix:
- Contains mitochondrial DNA, ribosomes, tRNAs, and enzymes for the Krebs cycle.
- 中文：基质里有自己的DNA、核糖体、tRNA、mRNA，还有柠檬酸循环的酶。

这是最核心的部分。分三步：

Located in the inner membrane.
Electrons from NADH and FADH2 are passed through complex I → III → IV, releasing energy.
This energy pumps H+ protons into the intermembrane space.
→ Creates a proton gradient (high H+ outside, low inside).
→ 中文：电子传递释放能量，把质子从基质泵到膜间隙，形成浓度差。

Protons can’t pass back freely due to inner membrane impermeability.
They can only flow back through ATP synthase, like water flowing through a turbine.
→ 中文：质子回流必须通过ATP合酶，回流过程中释放能量。

✔️ This is called oxidative phosphorylation.

Mitochondria have their own circular DNA (mtDNA), similar to bacteria.
It encodes 13 proteins, all part of the ETC or ATP synthase.
Also encodes 22 tRNAs and 2 rRNAs needed for protein synthesis.
But over 1,000 mitochondrial proteins are nuclear-encoded, made in the cytosol and imported.

→ 中文：线粒体有自己的DNA，但大部分蛋白还是细胞核制造后再运过来的。

这些是“蛋白转运通道”，很重要。

TOM (Translocase of the Outer Membrane):
- Recognizes signal sequences and imports proteins through the outer membrane.
- 中文：所有蛋白首先都得经过TOM。
TIM23 and TIM22 (Translocases of the Inner Membrane):
- TIM23: imports proteins into the matrix or inserts them into the inner membrane.
- TIM22: handles insertion of multi-pass membrane proteins.
SAM (Sorting and Assembly Machinery):
- Helps insert β-barrel proteins into the outer membrane.
- 中文：SAM专门负责外膜上的特殊结构蛋白。
OXA:
- Inserts proteins made inside mitochondria into the inner membrane.
- 中文：OXA是例外，它不负责外来蛋白，而是把线粒体自己制造的蛋白插入内膜。

Molecule Type	Count	Encodes	Location Used
DNA	1 circular genome	13 proteins, 22 tRNAs, 2 rRNAs	Matrix
rRNA	2 (12S and 16S)	Ribosome components	Matrix
tRNA	22	Translation of mtDNA mRNA	Matrix
mRNA	~13	ETC and ATP synthase proteins	Translated in matrix

RNA Type	Full Name	Transcribed From	Final Destination	Function
mRNA	Messenger RNA	DNA (in nucleus or mitochondria)	Cytoplasm (after exiting nucleus)	Carries genetic code from DNA to ribosome for protein synthesis
tRNA	Transfer RNA	DNA	Cytoplasm	Brings amino acids to ribosome during translation
rRNA	Ribosomal RNA	DNA (non-coding regions)	Cytoplasm (part of ribosome)	Forms the core structure of ribosome and catalyzes peptide bond formation

📌 补充说明（辅助中文）：

RNA Type	Full Name	Transcribed From	Final Destination	Function
mRNA	Mitochondrial Messenger RNA	Mitochondrial DNA (mtDNA)	Mitochondrial matrix	Carries coding information for ETC and ATP synthase subunits; translated by mitochondrial ribosomes
tRNA	Mitochondrial Transfer RNA	Mitochondrial DNA (mtDNA)	Mitochondrial matrix	Brings amino acids to mitochondrial ribosomes during translation
rRNA	Mitochondrial Ribosomal RNA	Mitochondrial DNA (mtDNA)	Mitochondrial matrix (ribosome)	Forms mitochondrial ribosomes for translating mitochondrial mRNA

📍补充说明（中英文）：