Cellular Respiration
Introduced| English |
|---|
| proton gradient/ˈprəʊtɒn ˈɡreɪdɪənt/ |
| cellular respiration/ˈseljʊlə ˌrespɪˈreɪʃn/ |
| glycolysis/ɡlaɪˈkɒləsɪs/ |
| aerobic respiration/eəˈrəʊbɪk ˌrespɪˈreɪʃn/ |
| fermentation/fɜːmənˈteɪʃn/ |
| citric acid cycle/ˈsɪtrɪk ˈæsɪd ˈsaɪkl/ |
| electron transport chain/ɪˈlektrɒn ˈtrænspɔːt tʃeɪn/ |
| ATP synthase/ˌeɪ tiː ˈpiː ˈsɪnθeɪs/ |
| chemiosmosis/ˌkemɪəzˈməʊsɪs/ |
| anaerobic respiration/ˌæneəˈrəʊbɪk ˌrespɪˈreɪʃn/ |
Why yeast can raise dough without oxygen
- Yeast releases carbon dioxide while fermenting sugar, so dough expands even without an oxygen supply. Yet aerobic cells can obtain much more ATP from the same sugar.
- Follow the electrons and proton gradient 质子梯度 to explain this difference; a list of reaction products alone cannot explain ATP production.
The overall reaction
- Cellular respiration 细胞呼吸 breaks glucose down using oxygen.
- $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$.
- The energy released is captured as ATP.
- Carbon dioxide and water leave as waste.
What is the purpose of cellular respiration?
Cellular respiration breaks down glucose to release energy as ATP.
Stage 1: glycolysis 糖酵解
- Glycolysis splits glucose in the cytoplasm.
- It makes two smaller molecules (pyruvate) and a little ATP.
- This first step needs no oxygen.
- If oxygen is present, the pyruvate goes on to the mitochondria.
The stages of respiration
Step through respiration — glucose is split, then broken down fully in the mitochondrion to release lots of ATP.
The first stage, splitting glucose in the cytoplasm, is called ____.
Glycolysis splits glucose into two pyruvate molecules and makes a small amount of ATP.
With or without oxygen
- Aerobic respiration 有氧呼吸 uses oxygen in the mitochondria to finish breaking glucose down.
- It releases a large amount of ATP.
- Without oxygen, muscle cells can use fermentation 发酵 to regenerate NAD⁺.
- Glycolysis then supplies only two net ATP per glucose; other products include lactate.
Which yields more ATP per glucose: complete aerobic respiration or glycolysis followed by fermentation?
Aerobic electron transport supports oxidative ATP production. Fermentation regenerates NAD⁺ but adds no ATP beyond glycolysis.
Glycolysis followed by fermentation yields much less ATP per glucose than complete aerobic respiration.
Fermentation permits glycolysis to continue with two net ATP per glucose, without oxidative ATP production.
Select all true statements about respiration.
Building glucose from CO₂ is photosynthesis — the opposite of respiration. The other three are correct.
Load the electron carriers
- Glycolysis yields two pyruvate, a net two ATP and NADH per glucose. Pyruvate oxidation and the citric acid cycle 柠檬酸循环 in the mitochondrial matrix release carbon dioxide and load NADH and FADH₂ with electrons.
- The cycle also makes a small amount of ATP directly. NADH and FADH₂ carry electrons to the inner-membrane electron transport chain 电子传递链, rather than carrying ATP themselves.
Build a proton gradient
- As electrons move along the chain, released energy pumps protons from the matrix into the intermembrane space. This builds a proton gradient: more H⁺ outside the inner membrane, so that side has the lower pH.
- Oxygen is the final electron acceptor in aerobic respiration and contributes to water formation. The folded inner membrane provides a large surface for transport proteins; aerobic prokaryotes use their plasma membrane instead.
During active mitochondrial electron transport, which compartment has the lower pH?
Proton pumping raises H⁺ concentration in the intermembrane space.
Let protons drive ATP synthase ATP合酶
- Protons flow back into the matrix through ATP synthase. This flow, called chemiosmosis 化学渗透, powers the joining of ADP and inorganic phosphate to make ATP.
- Electron transport and ATP production are coupled by the gradient. If an uncoupler lets protons cross elsewhere, electron transport may continue but ATP yield falls and more energy becomes heat.
A proton leak through the inner membrane can lower ATP yield even while electrons move along the chain.
The leak bypasses ATP synthase and weakens the gradient.
Fermentation keeps glycolysis running
- Fermentation transfers electrons from NADH to an organic molecule, regenerating NAD⁺. Without NAD⁺, glycolysis stops even if plenty of glucose remains.
- Lactate fermentation or alcohol fermentation adds no extra ATP beyond glycolysis. Some prokaryotes instead use anaerobic electron transport with an acceptor other than oxygen: this is not the same process as fermentation.
How does fermentation let glycolysis continue?
Glycolysis needs oxidized NAD⁺ to accept electrons.
Worked prediction: remove oxygen
- In an aerobic mitochondrion, blocking oxygen supply prevents the chain from passing electrons to its final acceptor. NADH is no longer efficiently oxidized; proton pumping and oxidative ATP production fall.
- A cell able to ferment can recycle NAD⁺ and maintain glycolysis with two net ATP per glucose. Fermentation does not restart the mitochondrial electron transport chain.
Using the stated net glycolysis yield, how many ATP are produced from five glucose molecules by glycolysis followed by fermentation?
5 × 2 = 10 net ATP. Fermentation regenerates NAD⁺ rather than adding ATP.
Fermentation and anaerobic respiration 无氧呼吸 are different. Fermentation regenerates NAD⁺ without an electron transport chain. Anaerobic respiration uses a chain with a terminal acceptor other than oxygen. Do not treat every oxygen-free pathway as the same mechanism.
- A yeast cell can keep glycolysis running without oxygen by converting pyruvate into ethanol and carbon dioxide, regenerating NAD⁺.
- Its ATP comes from glycolysis. Compare this with aerobic respiration, where an electron transport chain and a proton gradient enable a much larger ATP yield.
Cellular respiration releases energy from glucose as ATP. It starts with glycolysis in the cytoplasm (no oxygen needed). With oxygen, aerobic respiration finishes the job in the mitochondria and releases a lot of ATP; fermentation maintains glycolysis by recycling NAD⁺. Photosynthesis and respiration have related overall reactants and products, but use different pathways.