Respiration · System
Why every cell respires
Weeks without food. Days without water. Minutes without oxygen. The gap between those three numbers is the whole subject of this lesson.
Start here
Three weeks, three days, four minutes.
A person can survive around three weeks without food and three days without water. Without oxygen the number collapses to about four minutes before permanent damage begins. Nothing else the body needs has a deadline anything like that short.
Why is oxygen on such a different timescale?
Because there is no store of it and every cell needs it every second. Fat is a store of fuel that lasts weeks. There is no oxygen tank anywhere in the body — only what the blood is carrying at that moment, a few minutes' worth. Stop the supply and every cell in the body loses its main source of energy at once, which is why the deadline is minutes rather than weeks.
At the bench · five cells, one reaction
What is the energy actually for?
no cells cut off yet
Five very different cells, including one from a plant.
The cell
Heart muscle cell
Animal
Contracts about once a second, every second, for a lifetime, and never gets a rest day.
Where its energy goes
Contracting
70%
Pumping ions across the membrane
20%
Repair and rebuilding
10%
MitochondriaEnormous numbers — by some counts a third of the cell’s volume. The hardest-working cell in the body has the most machinery for the job.
Contraction stops within seconds. A heart deprived of oxygen for a few minutes suffers permanent damage, and it is heart muscle’s inability to be replaced afterwards that makes a heart attack so serious.
Nerve cell
Animal
Carries signals along a fibre that may be a metre long, hundreds of times a second.
Where its energy goes
Pumping ions back across the membrane after each signal
65%
Making transmitter chemicals
20%
Maintenance
15%
MitochondriaMany, and concentrated at the ends where signals are passed on. The brain is about a fiftieth of your body mass and uses roughly a fifth of your resting energy.
Fastest failure of any cell here. Brain cells begin to die within about four minutes, which is where the number at the top of this page comes from.
Root hair cell
Plant
Absorbs water and mineral ions from the soil, and pulls the minerals in against the concentration gradient.
Where its energy goes
Active transport of mineral ions
75%
Growth of the hair itself
15%
Maintenance
10%
MitochondriaPlenty — more than most plant cells, and the reason is on the line above. Active transport is expensive and this cell does almost nothing else.
Mineral uptake stops immediately, because moving ions from a low concentration to a high one cannot happen without an energy supply. Water still moves in on its own, which needs no energy, so the plant goes short of minerals long before it goes short of water.
Sperm cell
Animal
Swims, and does nothing else at all. Everything not needed for the journey has been stripped out.
Where its energy goes
Beating the tail
90%
Everything else
10%
MitochondriaA tight spiral of them wrapped around the base of the tail — the engine placed directly next to the propeller. You met this cell in Gametes and fertilisation.
It stops swimming and cannot start again. There is no store to fall back on, which is one reason so few of the hundreds of millions that set out arrive.
White blood cell
Animal
Crawls out of blood vessels, chases bacteria and engulfs them.
Where its energy goes
Crawling and engulfing
55%
Making antibodies and enzymes
30%
Dividing rapidly during an infection
15%
MitochondriaNumerous, and they multiply along with the cell when an infection begins — a cell that is about to work harder builds more of the machinery first.
It stops moving and stops engulfing. Everything on its list — crawling, swallowing a bacterium, building an antibody molecule — is work, and work has to be paid for.
Four things the energy pays for
Only one of them is moving.
Job one
Movement
Muscle contraction in animals, but also a white blood cell crawling and a sperm cell swimming. The only job on this list that is visible from outside.
Job two
Building molecules
Joining small molecules into large ones: amino acids into proteins, glucose into starch or cellulose. Growth and repair are this job, running for years.
Job three
Active transport
Moving a substance from where there is less of it to where there is more — the opposite of diffusion, and impossible without an energy supply. Root hairs and the gut lining both live on it.
Job four
Keeping warm
Mammals and birds hold their body temperature above their surroundings, and the energy comes from respiration. A reptile does not pay this bill, and eats far less as a result.
Key fact
Every living cell in every living organism respires continuously, because every other chemical process in a cell — movement, growth, repair, active transport and keeping warm — has to be paid for out of the energy respiration releases.
Think again
“Plants photosynthesise, animals respire.”
A plant is made of living cells, and a living cell that does not respire is a dead one. Every root cell, every stem cell, every leaf cell respires continuously, day and night, using oxygen and producing carbon dioxide exactly as yours do. Photosynthesis is not an alternative to respiration; it is an extra process that a plant can also do, in the cells that have chloroplasts, when there is light. Put the two together and the arithmetic works out like this: in bright light a leaf photosynthesises faster than it respires, so the net movement of gases looks like the opposite of an animal's, which is where the wrong idea comes from. Look at a root instead — buried, dark, no chloroplasts, and entirely dependent on sugar sent down from the leaves — and the plant looks exactly like you. The day-and-night version of this is the subject of Stomata and gas exchange in plants.
“You respire when you need energy — when you exercise.”
You respire fastest when you exercise. You respire all the time, and most of what you use in a day is spent while you are doing nothing you would describe as activity. Sitting still, your heart is contracting, your kidneys are filtering, your gut is transporting, your liver is running thousands of reactions, your brain is signalling, and every cell you own is pumping ions across its membrane to stay alive. That last one alone is a large and permanent bill. The clearest evidence is the four-minute figure at the top of this page: if respiration only mattered during effort, someone sitting quietly could hold their breath for an hour, and they cannot. Rest is not the state of not respiring — it is the state of respiring at your lowest rate, and that rate is still enough to keep you at 37 °C in a cold room.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · What needs energy
Which of these processes could not happen at all without respiration?
Rung 2 · The one that catches people
A potted plant is left in a completely dark cupboard. What is happening in its cells?
Rung 3 · Explain the root hair
A root hair cell absorbs mineral ions from soil where those minerals are far more dilute than they are inside the cell. Explain why this needs respiration, and why a root hair cell is packed with mitochondria.
Rung 4 · Take it somewhere new
A houseplant that is watered too often dies, and the leaves go yellow and droop as if it were short of water. Gardeners say the roots have "drowned". Explain what has actually happened, using respiration.
Key note
Respiration happens in every living cell of every living organism, plants included, continuously. The energy it releases pays for movement, for building large molecules from small ones during growth and repair, for active transport, and in mammals and birds for keeping the body warm. There is no store of oxygen, which is why the supply cannot be interrupted for more than a few minutes.
Going further
A dormouse in October has a problem an arithmetic teacher would recognise. Keeping a small warm body at 37 °C through a British winter costs more energy than a winter's food supply can provide, because a small animal loses heat quickly relative to its mass. Hibernation is the accounting solution: it abandons the temperature. Body temperature falls to a few degrees above the surroundings, the heart slows to a handful of beats a minute, and respiration drops to a tiny fraction of its normal rate — slow enough that a store of fat laid down in autumn can last until spring. The animal is not asleep in any ordinary sense and cannot simply wake up; getting back to operating temperature takes hours and costs a serious fraction of the reserve, which is why a hibernating animal disturbed too often in winter can starve before spring.
Before this lesson
Connects to
At GCSE this becomes
- Metabolism, ATP, active transport across membranes, and calculating metabolic rate from oxygen consumption.
Where to next
Ask Mr Badmus AI
Want to work out what a cell spends its energy on?
The energy shares on the bench are illustrative proportions chosen to show what each cell type is mainly doing, not measured values — real figures vary with the organism, its age and what it is doing at the time. Mitochondria counts are given as broad comparisons rather than exact numbers.
Lesson content © MrBadmusAI.