Learn first: Cell structure
Metabolism is the sum of all the chemical reactions in an organism. Every living cell constantly takes in substances from its surroundings, transforms them and gets rid of what it does not need; when metabolism stops, life stops. Metabolism consists of two linked processes: building substances (energy is used) and breaking them down (energy is released). The cell’s universal energy “currency” is ATP. Exams very often ask about the stages of energy metabolism, where they take place and what they produce, and about calculating ATP, oxygen, CO₂ and energy from glucose breakdown.
They depend on each other: catabolism supplies ATP for synthesis, and anabolism makes the substances that are broken down and the enzymes that run the process. In a growing organism anabolism dominates; in old age and starvation catabolism dominates.
ATP ↔ ADP + P: splitting ATP releases energy (about 40 kJ/mol); making it absorbs energy.
By how they obtain organic matter, organisms are:
By type of respiration: aerobes (need oxygen) and anaerobes (live without oxygen: some bacteria, yeasts in oxygen-free conditions).
In aerobes energy metabolism has three stages. The first is the preparatory stage:
The second stage, glycolysis, takes place in the cytoplasm without oxygen. One molecule of glucose is broken down by many enzyme-controlled steps into two molecules of pyruvic acid (pyruvate):
C₆H₁₂O₆ → 2C₃H₄O₃ (pyruvate) + 2 ATP
About 200 kJ of energy is released: 2 × 40 = 80 kJ (about 40%) is stored in ATP and the remaining 120 kJ is lost as heat.
If oxygen is short, pyruvate does not go on to the next stage and fermentation occurs:
Fermentation is inefficient — only 2 ATP per glucose. During hard exercise lactic acid builds up in muscles, one cause of fatigue and muscle ache; during rest it is carried in the blood to the liver and processed.
The third stage takes place in mitochondria with oxygen. Pyruvate enters the mitochondrion and is oxidised to CO₂ in the Krebs cycle (in the matrix); the hydrogen atoms (electrons) removed are passed along the respiratory chain in the inner membrane (cristae) to oxygen, forming water, and a lot of ATP is made:
2C₃H₄O₃ + 6O₂ → 6CO₂ + 6H₂O + 36 ATP
This stage releases about 2600 kJ, of which 36 × 40 = 1440 kJ is stored in ATP.
The overall equation for the complete oxidation of glucose:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 38 ATP
In total about 2800 kJ is released and 38 × 40 = 1520 kJ (about 55%) is stored in ATP. Aerobic respiration is 38 / 2 = 19 times more efficient than fermentation — which is why aerobic organisms can lead more active lives.
Rules for calculations:
People use energy even at rest — this is basal metabolism (heartbeat, breathing, keeping body temperature, cell processes). Mental and especially physical work raise energy use. The energy value of food should match what is used: too much food leads to obesity, too little to wasting and greater risk of disease.
Metabolism is controlled by the nervous system and hormones: for example, insulin moves blood glucose into cells and stores it in the liver as glycogen; adrenaline and glucagon break glycogen down and release glucose into the blood; the thyroid hormone thyroxine raises the overall metabolic rate. In problems distinguish “increased by 30%” from “increased 3 times”: a 30% rise means × 1.3, a threefold rise means × 3.
In a muscle 5 mol of glucose were broken down completely and 3 mol incompletely (to lactic acid). How many moles of ATP were formed, and how many moles of O₂ were used and CO₂ released?
A cell made 3800 ATP by complete oxidation, and another 50 glucose molecules went only through glycolysis. How many glucose molecules were used in total, and how much ATP was made altogether?
At rest a person uses about 300 kJ per hour. Mental work raises this by 30%, and sport raises it 3 times. How much energy is needed for 2 hours of mental work and 1 hour of sport?