Life Processes · Lesson 7 of 13
Respiration
“Cells turn glucose into spendable energy, one pathway at a time.”
• Distinguish breathing from cellular respiration. • Trace the breakdown of glucose through different pyruvate pathways. • Compare aerobic respiration, fermentation and lactic-acid formation. • Explain ATP formation and use. • Describe gas exchange in plants. • Compare oxygen uptake in aquatic and terrestrial organisms.
Food contains stored chemical energy, but cells cannot use a piece of bread or a glucose molecule as a direct battery for every task. Cellular respiration releases that energy through controlled reactions and transfers part of it to ATP, a molecule that can power many cellular activities.
Cellular respiration is the set of cellular reactions that break down food molecules and release energy in a usable form.
Breakdown Of Glucose
The first stage occurs in the cytoplasm, where one six-carbon glucose molecule is broken into three-carbon pyruvate molecules. What happens next depends on oxygen availability and the organism or tissue involved.
| Condition and site | Products from pyruvate | Relative energy release | Example |
|---|---|---|---|
| Without oxygen in yeast cytoplasm | Ethanol and carbon dioxide | Low | Fermentation |
| With limited oxygen in muscle cells | Lactic acid | Low | Sudden vigorous activity |
| With oxygen in mitochondria | Carbon dioxide and water | High | Aerobic respiration |
Aerobic respiration is the breakdown of food using oxygen, producing carbon dioxide and water and releasing a relatively large amount of energy.
Anaerobic respiration releases energy from food without using oxygen and yields less energy than aerobic respiration.
Lactic Acid And Muscle Cramps
During sudden intense activity, oxygen delivery may not meet the muscle’s demand. Pyruvate is then converted to lactic acid with a smaller energy yield. Its accumulation is associated with fatigue and cramps. Rest and restored oxygen supply allow the body to process the accumulated products.
ATP As The Energy Currency
Adenosine triphosphate is the immediate energy-carrying molecule used to power most cellular processes.
Energy released during respiration is used to add inorganic phosphate to ADP, forming ATP. When ATP’s terminal phosphate linkage is broken with water, usable energy is released. Cells spend this energy on muscle contraction, protein synthesis, active transport and nerve-impulse conduction.
Gas Exchange In Plants
Gases diffuse through stomata and through intercellular spaces. At night, photosynthesis stops but respiration continues, so carbon dioxide release is prominent. During daylight, carbon dioxide from respiration is often used in photosynthesis, while oxygen production can exceed oxygen use, producing a net release of oxygen.
Aquatic And Terrestrial Respiration
Water contains much less available oxygen than air. Fish pass water over gills, where dissolved oxygen enters blood. Because each volume of water supplies relatively little oxygen, aquatic organisms often breathe faster than terrestrial organisms. Respiratory surfaces are thin, moist and extensive to support rapid diffusion.
Problem
Yeast breaks down sugar in a sealed container. Predict the main products and energy yield.
- 1.Oxygen is unavailable in the sealed conditions.
- 2.Pyruvate cannot follow the aerobic mitochondrial pathway.
- 3.In yeast it is converted into ethanol and carbon dioxide.
- 4.Only a relatively small amount of energy is released.
- 5.The pathway is anaerobic fermentation.
Problem
A runner sprints and develops muscle cramps. Explain the likely cellular pathway.
- 1.Energy demand rises rapidly.
- 2.Oxygen delivery may temporarily be insufficient.
- 3.Pyruvate is converted into lactic acid in muscle cells.
- 4.The pathway releases less energy than aerobic respiration.
- 5.Lactic-acid accumulation contributes to discomfort and fatigue.
Problem
Two equal glucose samples are broken down, one aerobically and one by yeast fermentation. Compare products and usable energy.
- 1.Both begin with glucose breakdown to pyruvate in the cytoplasm.
- 2.Aerobic respiration uses oxygen and continues mainly in mitochondria.
- 3.It produces carbon dioxide and water with a high energy yield.
- 4.Fermentation produces ethanol and carbon dioxide without oxygen.
- 5.Because fermentation is incomplete breakdown, much energy remains in ethanol and less ATP becomes available.
Plants respire throughout day and night. Photosynthesis does not replace respiration; it can merely conceal the carbon dioxide produced during daylight by using it rapidly.
Quiz
Where is glucose first broken into pyruvate?
Which pathway yields the most energy?
What are the products of yeast fermentation?
What is ATP mainly used for?
Why do fish often breathe rapidly?
Practice Problems
- Differentiate breathing and cellular respiration.
- Draw a text flowchart showing all three pathways followed by pyruvate.
- Explain why fermentation yields less usable energy than aerobic respiration.
- Predict net gas exchange in a green leaf during a bright day and during darkness.
- A cell cannot convert ADP to ATP. Explain the immediate effect on three energy-requiring activities.
Key Takeaways
• Glucose first breaks into pyruvate in the cytoplasm. • Pyruvate follows different pathways depending on oxygen and cell type. • Aerobic respiration releases more energy than anaerobic pathways. • Yeast fermentation produces ethanol and carbon dioxide. • Limited oxygen in muscles can lead to lactic-acid formation. • ATP transfers energy from respiration to cellular work. • Plants respire continuously while photosynthesis changes net gas exchange. • Aquatic organisms often breathe faster because water contains less available oxygen.