Mitochondria: the cell’s power stations and the two stages
Science Prep 2 Term 1, Unit 3: pp. 75–81
GoalDescribe a mitochondrion and the two stages of cellular respiration.
New words
mitochondrion · an organelle where most energy is released from foodcristae · the folds of the inner membrane of a mitochondrionpyruvic acid · the molecule made when glucose is split in the first stage
Explanation
The mitochondrion is a bean-shaped organelle with an outer membrane and an inner membrane folded into cristae. The folds give more surface for releasing energy. Inside is a fluid, the matrix, with enzymes. Cells that need a lot of energy, such as muscle and liver cells, have many mitochondria. Most cells have mitochondria, but not all: mature red blood cells have none. Cellular respiration happens in two stages. Stage one takes place in the cytoplasm, without oxygen: one glucose molecule is split into two pyruvic acid molecules, and a little energy is released. Stage two takes place in the mitochondria, with oxygen: a series of reactions releases much more energy and makes carbon dioxide and water. In a mitochondrion we do not “make” energy: we release it from food. Think of a battery or a wallet: cells keep energy ready and spend it when needed. We do not use the Krebs cycle, ATP or other names here.
stage 1: cytoplasm, no oxygen: glucose → 2 pyruvic acid + a little energy→stage 2: mitochondria, with oxygen: much more energy, CO₂ and H₂O
Worked examples
Why does a muscle cell have more mitochondria than a skin cell? Muscle cells do a lot of work and need to release a lot of energy, so more mitochondria help them.
Which stage needs oxygen, stage one or stage two? Stage two, in the mitochondria. Stage one is in the cytoplasm and does not need oxygen.
Yeast in a bottle of warm sugar water, with a balloon over the neck, slowly inflates the balloon. The yeast releases carbon dioxide as it uses the sugar. This shows that yeast cells release energy from sugar, but it does not prove the two stages.
Class activity
Yeast balloon, with a grown-up. Put a spoon of dry yeast and a spoon of sugar in a clean plastic bottle, add warm (not hot) tap water, shake, and stretch a balloon over the neck. Leave it for 30 to 60 minutes and describe what the balloon does. Do not taste anything and keep the bottle open to a balloon only. Explain in two sentences what the yeast cells did with the sugar.
Practice
1
Describe the structure of a mitochondrion.
A bean-shaped organelle with an outer membrane, a folded inner membrane (cristae) and a matrix with enzymes.
2
Where does the first stage of respiration happen, and does it need oxygen?
In the cytoplasm; no.
3
What is made when one glucose is split in stage one?
