WJEC A Level Biology: ATP and Respiration Notes (Units 3.1 & 3.3)

ATP Structure and Function

ATP (adenosine triphosphate) is literally the fuel that powers every single thing your cells do. It's made of adenine (a nitrogen base), ribose (a sugar), and three phosphate groups - that's where the magic happens.

When your cells need energy, ATP breaks down into ADP (adenosine diphosphate) plus a phosphate group, releasing exactly 30.6 kJ/mol of energy. This hydrolysis reaction is catalysed by ATPase enzymes. What makes ATP brilliant is that it releases energy in perfectly sized chunks - not too much, not too little.

Your cells use this energy for everything: active transport, nerve impulses, muscle contraction, and building complex molecules. The best part? ATP is water-soluble, so it zips around your cells effortlessly, delivering energy exactly where and when it's needed.

Quick Tip: Remember that making ATP from ADP requires energy input (endergonic), whilst breaking it down releases energy (exergonic) - it's like charging and using a battery!

Chloroplasts vs Mitochondria

Both chloroplasts and mitochondria are cellular powerhouses with surprising similarities. They've got double membranes, their own circular DNA, 70S ribosomes, and both contain electron transport chains that pump out ATP like energy factories.

Here's where they differ: chloroplasts (in plants) do photosynthesis using NADP, whilst mitochondria (in animals and plants) handle cellular respiration using NAD and FAD. Think of chloroplasts as solar panels and mitochondria as power stations.

Aerobic respiration can theoretically make 38 ATPs per glucose molecule, but you never quite hit that maximum due to leaky membranes and energy costs. Anaerobic respiration only manages 2 ATPs per glucose - much less efficient but crucial when oxygen runs low.

When oxygen's scarce, your muscle cells switch to making lactate (hello, muscle burn!), whilst plants and bacteria produce ethanol instead. The key difference? Your lactate can be recycled when oxygen returns, but ethanol production is permanent and can become toxic.

Remember: Aerobic = lots of ATP but needs oxygen; Anaerobic = quick energy but much less efficient!

Glycolysis and Early Respiration Stages

Glycolysis kicks off in your cytoplasm and doesn't need oxygen - making it your cells' emergency energy system. Glucose gets phosphorylated by 2 ATP molecules, splits into two triose phosphates, then gets converted into 2 pyruvate molecules through substrate-level phosphorylation.

The clever bit? You invest 2 ATPs upfront but get 4 back directly, plus 2 reduced NADs that'll make 6 more ATPs later - net gain of 8 ATPs total. Not bad for the opening act!

Next comes the link reaction in the mitochondrial matrix. Each pyruvate gets dehydrogenated and decarboxylated, losing CO₂ and forming acetyl coenzyme A. Since you've got 2 pyruvates, this happens twice, producing 2 reduced NADs (worth 6 ATPs).

The Krebs cycle then takes over, spinning acetyl coenzyme A through a series of reactions that produce 2 ATPs directly, 6 reduced NADs, and 2 reduced FADs. That's 24 ATPs worth of energy carriers heading to the final stage!

Memory Trick: Glycolysis = glucose splitting; Link reaction = pyruvate prepping; Krebs cycle = the big energy harvest!

Electron Transport Chain and Alternative Fuels

The electron transport chain on the inner mitochondrial membrane is where the real ATP magic happens. Reduced NAD and FAD get oxidised, releasing electrons that power pumps, forcing H⁺ ions into the intermembrane space like inflating a balloon.

When these H⁺ ions flow back through ATPase (the stalked particles), they drive ATP synthesis - it's called oxidative phosphorylation. Each reduced NAD makes 3 ATPs, whilst reduced FAD makes 2 ATPs. Oxygen acts as the final electron acceptor, combining with electrons and H⁺ to form water.

Your body's remarkably flexible with fuel sources. Lipids get chopped into 2-carbon fragments that enter as acetyl coenzyme A - they're actually more energy-dense than glucose. Proteins can also be used, but only during starvation after deamination removes the nitrogen groups.

The grand total? Up to 38 ATPs per glucose molecule through aerobic respiration - that's why oxygen is so vital for complex life. Without it, you're stuck with just 2 ATPs from glycolysis alone.

Key Point: The electron transport chain produces about 89% of your ATP - it's definitely the star of cellular respiration!