3.5 Cellular Respiration · Educerie AP Biology

Educerie · AP · Biology

AP Biology 3.5 Cellular Respiration

In one breath

Cellular respiration turns the energy in organic molecules into ATP, and all organisms respire or ferment.
Glycolysis in the cytosol splits glucose into two pyruvate, making a little ATP and NADH.
Pyruvate is oxidised in the mitochondrion and the Krebs cycle in the matrix releases CO₂, makes some ATP, and loads electrons onto NAD⁺ and FAD.
NADH and FADH₂ deliver those electrons to the electron transport chain in the inner membrane, where redox reactions pass them to oxygen, the terminal acceptor, and pump protons into the intermembrane space, so the matrix has the higher pH.
Protons flow back through ATP synthase, which makes most of the ATP by oxidative phosphorylation; folded cristae give room for more.
Let protons leak back another way and the energy becomes heat.
Without oxygen, fermentation regenerates NAD⁺ by making lactic acid or ethanol, so glycolysis can go on.

What you must be able to do

Describe what glycolysis, pyruvate oxidation and the Krebs cycle take in and give out, and where each happensMCQ: which stage releases CO₂; where the Krebs cycle takes place
Explain how electrons carried by NADH and FADH₂ pass along the electron transport chain to oxygenPredict what happens to NADH when oxygen runs out
Explain how the proton gradient across the inner mitochondrial membrane drives ATP synthaseA membrane model: which side has the higher pH; effect of a chemical that makes the membrane leaky
Explain how the folded inner membrane increases ATP productionMCQ comparing cells with more or fewer cristae
Explain how uncoupling electron transport from ATP synthesis generates heatQ4: explain how brown fat keeps a newborn mammal warm
Explain how fermentation lets glycolysis continue without oxygenMCQ: why yeast without oxygen still makes ATP; what lactic acid fermentation regenerates
Construct a graph of respiration data with appropriate type, labels, scale and error barsQ2: plot mean CO₂ production with error bars

Once you have read the note, tick off each one you can do in Check your understanding.

In the exam. Section I multiple choice on a model of the inner mitochondrial membrane (which side has more protons or the higher pH), on the inputs and outputs of each stage, on fermentation, and on data such as oxygen used by germinating seeds or CO₂ made by yeast. In Section II it is a common setting for Interpreting and evaluating experimental results with graphing (Q2), where you plot means with error bars; Analyze model or visual representation (Q5) with an electron transport chain diagram; and Conceptual analysis (Q4), such as predicting the effect of a poison or explaining how some animals make heat.

1The whole process at a glance

Aerobic respiration, summed up:

\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \to 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{energy (captured as ATP)}

It runs in stages, each in its own place.

Stage	Where (eukaryote)	Takes in	Gives out
Glycolysis	cytosol	glucose, NAD⁺, ADP + Pᵢ	pyruvate, NADH, a net gain of ATP
Pyruvate oxidation	mitochondrial matrix	pyruvate, NAD⁺	CO₂, NADH, a molecule that enters the Krebs cycle
Krebs cycle	mitochondrial matrix	that molecule, NAD⁺, FAD, ADP + Pᵢ	CO₂, NADH, FADH₂, ATP
Electron transport and oxidative phosphorylation	inner mitochondrial membrane	NADH, FADH₂, O₂, ADP + Pᵢ	NAD⁺, FAD, H₂O, most of the ATP

You do not need the steps of glycolysis or the Krebs cycle, the structures of the molecules, or the names of the enzymes, apart from ATP synthase. You need this table: what goes in, what comes out, and where.

Respiration and fermentation are found in all forms of life, plants included.

2Glycolysis, then into the mitochondrion

Glycolysis happens in the cytosol and does not need oxygen. One glucose molecule, with six carbons, is split into two molecules of pyruvate, with three carbons each. Along the way the cell makes a small net gain of ATP from ADP and inorganic phosphate, and reduces NAD⁺ to NADH. If oxygen is available, pyruvate is transported into the mitochondrion. There it is oxidised: it loses electrons and a carbon, which leaves as CO₂. The electrons go to NAD⁺, making NADH.

What is left enters the Krebs cycle, also called the citric acid cycle, in the matrix. Over the cycle:

the remaining carbon atoms are released as CO₂;
electrons are transferred to the coenzymes NAD⁺ and FAD, reducing them to NADH and FADH₂;
a small amount of ATP is made from ADP and inorganic phosphate.

By the end, every carbon of the glucose has left as CO₂, and most of the energy is held by electrons in NADH and FADH₂.

3The electron transport chain and ATP synthase

The inner mitochondrial membrane is folded into cristae. The folding gives a much larger area of membrane in the same space, so it can hold more electron transport chains and more ATP synthase, and the cell can make more ATP. Cells that use a lot of energy, such as muscle cells, have mitochondria with many cristae.

Figure 1 shows a stretch of inner membrane, with the intermembrane space above and the matrix below.

Figure 1 · Oxidative phosphorylation: electrons pass from NADH to oxygen, and the proton gradient they build drives ATP synthase

NADH delivers its electrons. NADH from glycolysis, pyruvate oxidation and the Krebs cycle gives up its electrons to the first carrier of the chain and is oxidised back to NAD⁺, which returns to pick up more. FADH₂ does the same a little further along the chain.

Electrons pass along the chain, and protons are pumped. The electrons move from carrier to carrier in a series of oxidation–reduction reactions, losing a little energy at each step. Some carriers use that energy to pump protons from the matrix into the intermembrane space. The intermembrane space now has a high proton concentration and the matrix a low one. Fewer protons means a higher pH, so the matrix has a higher pH than the intermembrane space.

Protons flow back through ATP synthase, which makes ATP. Protons diffuse down their electrochemical gradient, back into the matrix, through the channel in ATP synthase. The flow drives the enzyme to join ADP and inorganic phosphate into ATP. This is chemiosmosis, and in respiration the whole process is called oxidative phosphorylation. It makes most of the ATP from each glucose.

Oxygen accepts the electrons at the end. Oxygen is the terminal electron acceptor. It takes the electrons from the last carrier and combines with protons to form water. Without oxygen to clear them, electrons back up along the chain, the carriers stay reduced, and pumping stops.

You do not need the carriers' names. Compare the thylakoid in 3.4: there protons are pumped into the thylakoid and ATP is made in the stroma; here they are pumped out of the matrix and ATP is made in the matrix.

Prokaryotes have no mitochondria. Their electron transport chains sit in the plasma membrane, and protons are pumped across it to the outside of the cell. Aerobic prokaryotes use oxygen as the terminal electron acceptor; anaerobic prokaryotes use other molecules, such as sulfate or nitrate.

Electrons flow to oxygen, protons flow back through ATP synthase: the gradient is the link between them.

4Uncoupling: heat instead of ATP

Normally electron transport and ATP synthesis are coupled: protons can only get back to the matrix through ATP synthase, so pumping and ATP-making go together. If the inner membrane is given another route for protons, they flow back without passing through ATP synthase. Electrons still flow and oxygen is still used, but the energy of the gradient is released as heat instead of being captured in ATP. This is decoupling oxidative phosphorylation from electron transport.

Some endotherms, animals that warm their bodies with their own heat, do this deliberately. Mammals have brown adipose tissue (brown fat), whose mitochondria carry a protein that lets protons leak back across the inner membrane; newborn and hibernating mammals rely on it to stay warm.

5Fermentation: when there is no oxygen

Without oxygen, the electron transport chain stops. NADH can no longer hand over its electrons, so NAD⁺ is not regenerated. Glycolysis needs NAD⁺, so it would stop too, and the cell would make no ATP at all.

Fermentation solves this. The cell passes the electrons from NADH to pyruvate, or to a molecule made from it, regenerating NAD⁺ so glycolysis can continue. The products are organic molecules:

Lactic acid fermentation, in many bacteria and in human muscle cells short of oxygen: pyruvate becomes lactic acid (lactate).
Alcohol fermentation, in yeast and some bacteria: pyruvate becomes ethanol and carbon dioxide.

Fermentation itself makes no ATP. The ATP comes only from glycolysis, so the yield per glucose is far lower than in aerobic respiration (about 2 ATP against roughly 30). Its job is to keep glycolysis running.

6Graphing respiration data

Worked example. Invented data. Yeast is mixed with glucose solutions of three concentrations, three trials each, and the volume of CO₂ produced in 20 minutes is measured.

Glucose (%)	Trial 1 (mL)	Trial 2 (mL)	Trial 3 (mL)
0	0.2	0.0	0.1
5	3.8	4.4	4.1
10	6.9	7.6	7.1

The mean for 10% glucose, and its standard error from the formula sheet:

\bar{x} = \frac{6.9 + 7.6 + 7.1}{3} = 7.2\ \text{mL}

s = 0.36\ \text{mL} \qquad \textcolor{#838c91}{\text{sample standard deviation}}

SE_{\bar{x}} = \frac{s}{\sqrt{n}} = \frac{0.36}{\sqrt{3}} = 0.21\ \text{mL}

The other means are 0.1 mL (SE 0.06) and 4.1 mL (SE 0.17). Plot the three means against glucose concentration, with error bars of ±2 SE. The bars for 5% and 10% do not overlap, so the difference between them is likely to be real. CO₂ is made both by aerobic respiration and by alcohol fermentation in yeast, so this experiment measures the yeast's total sugar breakdown.

Practice questions

11 questions on this note, from easy to hard. Each one you get right lights a star of this topic's constellation.