Introduction to Respiration
In this chapter, we are looking at how animals (including humans!) get the energy they need to stay alive. Whether you are running a marathon, thinking about a difficult math problem, or even just sleeping, your body is constantly hard at work. This work requires energy, and that energy comes from a process called cellular respiration.
Important Note: Many people confuse "respiration" with "breathing." In biology, breathing is just the physical act of moving air in and out of your lungs. Respiration is a chemical reaction that happens inside every single living cell to release energy from food.
1. Respiration as an Exothermic Reaction
Respiration is described as a continuous exothermic reaction. Let’s break that down:
1. Continuous: It happens all the time. If your cells stopped respiring, they would run out of energy and die.
2. Exothermic: This is a chemistry term that means the reaction releases energy to the surroundings. Some of this energy is used for movement and growth, while some is released as heat (which helps mammals maintain their body temperature).
2. Aerobic Respiration
Aerobic respiration is the "standard" way cells get energy. It uses oxygen from the air to break down glucose (sugar) completely.
Where does it happen?
Aerobic respiration takes place in the mitochondria of your cells. You might remember from Topic 1 that mitochondria are the "powerhouses of the cell"—this is exactly why!
The Aerobic Equation
You need to know this word equation and the balanced symbol equation:
Word Equation:
Glucose + Oxygen \(\rightarrow\) Carbon Dioxide + Water (+ Energy released)
Symbol Equation:
\(C_{6}H_{12}O_{6} + 6O_{2} \rightarrow 6CO_{2} + 6H_{2}O\)
Why is it the "best" way?
Aerobic respiration is very efficient. Because the glucose is fully broken down, it releases a large amount of energy per molecule of glucose.
3. Anaerobic Respiration
Sometimes, your body cannot supply enough oxygen to your muscles fast enough—like when you are sprinting or lifting very heavy weights. In these "emergency" situations, your cells switch to anaerobic respiration.
Anaerobic literally means "without oxygen."
The Anaerobic Equation (in Animals)
In animal cells, glucose is only partially broken down. Because no oxygen is used, the products are different:
Word Equation:
Glucose \(\rightarrow\) Lactic Acid (+ Energy released)
The Downsides of Anaerobic Respiration
1. Less Energy: Because the glucose isn't fully broken down, it releases much less energy than aerobic respiration.
2. Lactic Acid: This is a waste product that builds up in your muscles. It causes that "burning" sensation during exercise and can lead to muscle fatigue and cramps.
Quick Summary Table: Aerobic vs. Anaerobic
- Oxygen used? Aerobic: Yes | Anaerobic: No
- Glucose breakdown: Aerobic: Complete | Anaerobic: Incomplete
- Products: Aerobic: \(CO_{2}\) and \(H_{2}O\) | Anaerobic: Lactic acid
- Energy release: Aerobic: High | Anaerobic: Low
4. Core Practical 8.11: Investigating Respiration Rate
You may be asked about how to measure the rate of respiration in living organisms (like small insects or germinating seeds) using a piece of equipment called a respirometer.
How it works:
1. The organisms are placed in a sealed container.
2. A chemical called soda lime (or potassium hydroxide) is placed in the container to absorb any carbon dioxide the organisms breathe out.
3. As the organisms respire, they take in oxygen from the air inside the container.
4. Since the \(CO_{2}\) is being absorbed and the oxygen is being used up, the air pressure inside the tube drops.
5. This drop in pressure sucks a drop of colored liquid (ink) through a glass tube.
The Calculation:
You can measure the distance the drop moves in a certain amount of time to calculate the rate.
\(Rate = \frac{Distance}{Time}\)
Common Exam Trap: If the question asks why we use soda lime, the answer is always: To absorb carbon dioxide so that the change in gas volume is only due to oxygen being taken in.
5. Real-World Connection: The "Stitch"
Have you ever felt a sharp pain in your side while running? That’s often caused by your body switching to anaerobic respiration. Your muscles are working so hard they've run out of oxygen, and the build-up of lactic acid is irritating the tissues. Once you stop and catch your breath, your body uses oxygen to break down that lactic acid, and the pain goes away!
Key Takeaways for Revision
- Respiration is an exothermic reaction that happens in all living cells.
- Aerobic respiration uses oxygen, happens in the mitochondria, and releases lots of energy.
- Anaerobic respiration happens without oxygen, produces lactic acid, and releases very little energy.
- Respirometers measure respiration rate by tracking how much oxygen is consumed.
Note: For more on how oxygen and glucose get to the cells, see the next chapters on "Blood vessels" and "The Heart".