Cell: The Building Block of Life · Lesson 6 of 9
Mitochondria — The Powerhouse of The Cell
“Mitochondria power the place while plastids and vacuoles handle food, colour and storage.”
• Describe how mitochondrial structure supports energy release. • Explain the role of ATP in cellular activities. • Describe the structure and function of chloroplasts. • Distinguish chloroplasts, chromoplasts and leucoplasts. • Compare mitochondria and chloroplasts. • Explain how vacuoles provide storage and support. • Relate vacuolar water content to plant firmness and wilting.
A cell needs usable energy, a plant needs a place to make food, and both plant and animal cells need storage. These jobs are handled by different organelles. Their structures are not decorative details: folds, pigments, membranes and fluid-filled spaces directly support what each organelle does.
A mitochondrion is surrounded by two membranes. The outer membrane is smooth and porous. The inner membrane is folded into finger-like projections called cristae. These folds increase the available surface area for reactions involved in energy release.
The process in which glucose and other molecules are broken down to release energy inside cells.
Adenosine Triphosphate, a molecule that stores released energy in a form that can be used for cellular activities.
| Mitochondrial feature | Structural detail | Functional importance |
|---|---|---|
| Outer membrane | Smooth and porous | Forms the outer boundary |
| Inner membrane | Folded | Supports energy-releasing reactions |
| Cristae | Finger-like inner folds | Increase surface area |
| Own DNA and ribosomes | Present within the organelle | Allow production of some of its own proteins |
Problem
Two mitochondria have the same outer size, but one has more cristae. Explain the likely advantage.
- 1.Cristae are folds of the inner mitochondrial membrane.
- 2.More folds provide a greater membrane surface area within the same outer volume.
- 3.A greater surface can accommodate more of the reactions involved in energy release.
- 4.The mitochondrion with more cristae can therefore support greater energy-producing activity under suitable conditions.
Plastids — Centre for Food Synthesis in The Plant Cells and Beyond
Plastids are specialised organelles in plant cells. Some capture light energy, some provide colour, and others store food. Their differences arise from the pigments or stored material present within them.
A chloroplast is enclosed by two membranes. Its interior contains a semi-fluid stroma and disc-shaped membrane structures containing chlorophyll. Chlorophyll absorbs light energy for photosynthesis. Sugars produced during photosynthesis and starch granules may be present in the stroma.
| Feature | Mitochondrion | Chloroplast |
|---|---|---|
| Main process | Cellular respiration | Photosynthesis |
| Energy relationship | Releases energy from food and stores it in ATP | Absorbs light energy to synthesise sugars |
| Internal membrane feature | Cristae | Chlorophyll-containing disc-shaped membranes |
| Semi-fluid internal region named in the chapter | Not specifically named here | Stroma |
| Own DNA and ribosomes | Present | Present |
| Occurrence | Plant and animal cells | Photosynthetic plant cells |
Mitochondria and plastids have their own DNA and ribosomes and can make some of their own proteins. These similarities to certain bacteria suggest a shared evolutionary history with single-celled organisms, as described in the chapter.
Problem
Describe how a green leaf cell uses both chloroplasts and mitochondria.
- 1.Chlorophyll in chloroplasts absorbs light energy.
- 2.The chloroplast uses that energy during photosynthesis to synthesise sugars.
- 3.The cell can break down sugar through cellular respiration in mitochondria.
- 4.Released energy is stored in ATP.
- 5.ATP supplies usable energy for the leaf cell’s activities.
Chromoplasts contain yellow, orange or red pigments rather than chlorophyll as their main visible pigments. They contribute to the bright colours of flowers, fruits and vegetables. These colours can attract pollinators and fruit-eating animals that assist pollination or seed dispersal.
Leucoplasts lack visible pigments and are colourless. They store food such as starch, oils or proteins. Leucoplasts in potato and Colocasia cells can store starch, showing why an underground plant part can contain plastids even when it does not photosynthesise.
| Plastid | Pigment | Main role | Example context |
|---|---|---|---|
| Chloroplast | Chlorophyll | Photosynthesis | Green leaf cells |
| Chromoplast | Yellow, orange or red pigments | Colour | Flower petals and ripe fruits |
| Leucoplast | No visible pigment | Food storage | Starch storage in potato cells |
Problem
Match a green leaf cell, an orange flower petal and a potato storage cell with the most relevant plastid.
- 1.A green leaf cell performs photosynthesis and contains chlorophyll, so chloroplast is the relevant plastid.
- 2.An orange petal contains a bright non-green pigment, so chromoplast is relevant.
- 3.A potato cell stores starch and need not contain a visible pigment, so leucoplast is relevant.
- 4.All three are plastids, but their contents and roles differ.
Vacuoles — The Organelles for Storage and Support
A mature plant cell usually contains one large central vacuole surrounded by a selectively permeable membrane. It contains cell sap and stores water, minerals, sugars and wastes. When filled with water, the vacuole presses the living contents outward against the cell wall, helping the cell remain firm.
When a plant does not receive enough water, its vacuoles lose water. The cells become less firm and the plant wilts. Animal cells may contain smaller vacuoles for temporary storage, but they do not normally have the single large central vacuole shown in a mature plant cell.
Problem
Explain step by step why a plant wilts when its roots cannot obtain enough water.
- 1.Less water enters the plant through its roots.
- 2.Water available to the cells decreases.
- 3.Central vacuoles lose water and become smaller.
- 4.The outward pressure of cell contents against cell walls decreases.
- 5.Cells and tissues become less firm, so leaves and stems wilt.
Pause and Ponder
A white flower may lack strongly coloured pigments in its petals. This does not mean that its cells contain no plastids. Colourless plastids can occur, and the visible colour of a tissue depends on the pigments and structures present.
Threads of Curiosity
Scientists synthesised a copy of the DNA of a simple bacterium and inserted it into a related bacterial cell whose own DNA had been removed. The recipient cell then grew and divided according to the inserted information. Only the DNA was synthetic; the cytoplasm, membrane and other working parts came from an existing cell. The investigation demonstrated the controlling role of DNA without creating a complete cell from non-living materials.
• Chloroplasts make sugars using light energy; mitochondria release usable energy from food molecules. • Roots can contain plastids even when they do not contain chloroplasts, because leucoplasts can store food.
Quiz
Which description best matches Cellular respiration?
Which description best matches ATP?
Which term matches this description: The process in which glucose and other molecules are broken down to release energy inside cells.
Which term matches this description: Adenosine Triphosphate, a molecule that stores released energy in a form that can be used for cellular activities.
Which statement is a key takeaway from this lesson?
Practice Problems
- Explain how cristae support the work of a mitochondrion.
- Compare mitochondria and chloroplasts using membranes, DNA, ribosomes and function.
- Which plastid would you expect in a coloured fruit, and how can its colour benefit the plant?
- Why can potato cells contain plastids even though potatoes grow underground?
- Trace the change from vacuolar water loss to visible wilting.
Key Takeaways
• Mitochondria release energy during cellular respiration and store it in ATP. • Chloroplasts contain chlorophyll and carry out photosynthesis. • Chromoplasts provide non-green colours, while leucoplasts store food. • Mitochondria and plastids contain their own DNA and ribosomes. • A large central vacuole stores cell sap and helps keep a plant cell firm.