Cellular Respiration
Senior 3 · Biology
Cellular Respiration
Lesson 1 of 4
25 min
When you are fast asleep you are not running, lifting or thinking hard, yet your body still uses energy every second. Your heart keeps beating, your breathing muscles keep working, your kidney cells keep pumping salts, new proteins are built in every cell, and your body stays at about 37 °C on a cold Musanze night. All of this energy comes from one process that never stops in any living cell, day or night: cellular respiration.
Definition
Cellular respiration: the chemical reactions in living cells that break down food molecules, mainly glucose, to release energy. It is controlled by enzymes.
ATP (adenosine triphosphate): the molecule that carries energy from respiration to the processes in a cell that need it.
Aerobic respiration uses oxygen; anaerobic respiration takes place without oxygen. (You will compare them in Lessons 2 and 3.)
In Senior 1 and Senior 2 you studied breathing and gas exchange: moving air in and out of the lungs and passing oxygen into the blood. Those are physical processes, carried out by organs. Respiration is a chemical process carried out inside every living cell, in plants as well as animals, by day and by night. Breathing supplies the oxygen that aerobic respiration uses and removes the carbon dioxide it makes. Respiration happens partly in the cytoplasm and mostly, when oxygen is present, in the mitochondria, which you met in Senior 1.
The energy in glucose is released in many small steps, and much of it is used to make ATP. An ATP molecule is built from a part called adenosine and a chain of three phosphate groups. When a cell needs energy, an enzyme splits the end phosphate off, leaving ADP (adenosine diphosphate), and energy is released exactly where it is needed. Respiration then uses energy from glucose to join a phosphate back on to ADP. The same molecules are used again and again.
Figure 1: The ATP cycle. ATP is made using energy from respiration, and broken down to release energy for work.
Cells use ATP for:
| Process | Example |
|---|---|
| Muscle contraction | Running, the heartbeat, breathing, peristalsis |
| Active transport | Root hair cells taking up mineral ions; the ileum absorbing glucose (Senior 2 and Unit 6) |
| Building large molecules | Making proteins from amino acids, for growth and repair |
| Cell division | Mitosis and meiosis (Unit 5) |
| Nerve impulses | Carrying messages along nerve cells |
| Keeping warm | Heat released by respiration keeps mammals and birds at a steady body temperature |
Tip
An analogy: savings and pocket money. Glucose is like money saved in a bank or on mobile money: a large store, but not directly spendable in every shop. ATP is like the coins in your pocket: a small amount, spent immediately, and topped up all the time from the savings. A cell holds only a little ATP at any moment, but remakes it constantly.
Worked example 1: does it need ATP?
Which of these processes need energy from ATP? (a) oxygen diffusing into a red blood cell; (b) a root hair cell taking up nitrate ions from soil that has fewer of them than the cell; (c) water entering a root hair cell by osmosis; (d) a ribosome joining amino acids into a protein.
Rule: movement down a gradient is free; movement against a gradient, and building things, costs ATP.
Worked example 2: energy in breakfast
A bowl of sorghum porridge contains 30 g of carbohydrate. Carbohydrate releases about 17 kJ per gram when it is respired. A pupil uses about 6 kJ per minute walking to school. (a) How much energy can the porridge release? (b) For how many minutes of walking is that enough?
Warning
Common mistakes. (1) Writing "respiration is breathing in and out": breathing moves air; respiration is a chemical process in cells. (2) Saying respiration "makes energy" or "produces energy": energy cannot be made; respiration releases energy stored in food. (3) Saying plants respire only at night: plants respire all the time; at night respiration is simply not hidden by photosynthesis.
What is ATP?, Amoeba Sisters. Watch how ATP becomes ADP when energy is released, and the examples of cell work that use it.
Practice: respiration and ATP
Key points
Sources and credits
Written by Ganzaa: the text, Figure 1, the table, the worked examples (illustrative figures) and the practice questions.
