# Mitochondria, Plastids and the Plant Cell

Biology I · Cells and Tissues · https://tryals.app/learn/biology-i/mitochondria-plastids-and-the-plant-cell

## Compartments That Were Once Cells

**Mitochondria** and **plastids** share an origin and a design: a double membrane, their own circular DNA, 70S ribosomes, and division by fission independent of the host.

A mitochondrion has an outer membrane and a heavily folded inner one. The folds, **cristae**, carry the electron transport chain, and folding is the point: it multiplies the area available for ATP synthesis within a fixed volume. Cells with high energy demand have both more mitochondria and more densely folded cristae, and heart muscle is the extreme case.

**Plastids** are a family. **Chloroplasts** photosynthesise, with an internal thylakoid membrane system stacked into grana, suspended in the stroma. **Chromoplasts** store the pigments that colour fruits and flowers. **Amyloplasts** store starch and, in roots, also sense gravity. All three interconvert, a tomato ripening from green to red is chloroplasts becoming chromoplasts.

**Peroxisomes** handle oxidative reactions producing hydrogen peroxide and immediately destroy it with catalase, one of the fastest enzymes known. In plant seeds, specialised glyoxysomes convert stored fat into sugar so a seedling can grow before it can photosynthesise.

Two structures make a plant cell recognisable. The **central vacuole** occupies up to 90 % of the cell volume, and it does far more than store: it maintains **turgor pressure** against the wall, which is what holds a non-woody plant upright. A wilting plant is one whose vacuoles have lost water.

The **cell wall** is outside the membrane, built of cellulose microfibrils in a matrix. The **primary wall** is thin and extensible, allowing growth; the **secondary wall**, deposited inside it once growth stops, is thicker and often lignified. Walls are perforated by **plasmodesmata**, channels connecting adjacent cytoplasms so that a plant tissue is partly one continuous compartment.

> **Common pitfall:** describing the cell wall as simply "for protection". It also determines cell shape, limits how far the cell can expand, and bears the turgor pressure that supports the whole plant. A cell with a wall grows by *loosening* it, growth is a controlled failure of the wall, not an inflation against it.

## Practice questions

7 of this lesson's 11 practice questions, with answers. The full set is in the app.

### 1. Why does a plant wilt when it loses water?

A. The vacuoles lose turgor pressure, so the cells no longer press against their walls
B. The chloroplasts cease photosynthesising, depriving the cell wall of structural ATP
C. The cellulose microfibrils in the primary wall break down rapidly without moisture
D. The plasmodesmata constrict and seal completely, preventing fluid flow between cells

**Answer:** A. The vacuoles lose turgor pressure, so the cells no longer press against their walls

**Why:** A non-woody plant is held up by turgor: full vacuoles press outward against rigid walls, and the tissue becomes stiff. Lose the water and the pressure falls, so the tissue goes limp, support is hydraulic, not skeletal.

Page: https://tryals.app/practice/biology-i/mitochondria-plastids-and-the-plant-cell/why-does-a-plant-wilt-when-it-loses-water

### 2. The central vacuole can occupy up to 90 % of a mature plant cell’s volume. Set the percentage of the volume left for everything else, cytoplasm, nucleus and all the organelles.

**Answer:** 10 (within ±3)

**Why:** Just **10 %** is left for the cytoplasm, nucleus and every organelle. The cytoplasm is pressed into a thin layer against the wall, which is also why chloroplasts in a plant cell are found close to the surface, where the light is.

Page: https://tryals.app/practice/biology-i/mitochondria-plastids-and-the-plant-cell/the-central-vacuole-can-occupy-up-to-90-of-a-mature-plant-cells

### 3. Chloroplasts, chromoplasts and amyloplasts can convert from one type into another.

**Answer:** True

**Why:** True, plastids are a single interconvertible family. A ripening tomato is chloroplasts converting to chromoplasts, and a potato tuber exposed to light shows amyloplasts becoming chloroplasts as it greens.

Page: https://tryals.app/practice/biology-i/mitochondria-plastids-and-the-plant-cell/chloroplasts-chromoplasts-and-amyloplasts-can-convert-from-one-type

### 4. Sort each organelle by the kind of cell it is found in.

**Answer:**

- Plant cells only: Chloroplast, Large central vacuole, Cellulose cell wall
- Both plant and animal cells: Mitochondrion, Peroxisome, Golgi apparatus

**Why:** Chloroplast, central vacuole and cellulose wall identify a plant cell. Note that plants have mitochondria too, photosynthesis makes sugar, and respiring that sugar still requires a mitochondrion.

Page: https://tryals.app/practice/biology-i/mitochondria-plastids-and-the-plant-cell/sort-each-organelle-by-the-kind-of-cell-it-is-found-in

### 5. Match each plastid or organelle to its role.

**Answer:**

- Chloroplast → Captures light and fixes carbon
- Chromoplast → Stores the pigments colouring fruit and flowers
- Amyloplast → Stores starch and senses gravity in roots
- Peroxisome → Runs oxidative reactions and destroys peroxide

**Why:** The three plastids share an origin and interconvert. The amyloplast is doubly useful: dense starch grains settle under gravity, and the root uses that settling to tell which way is down.

Page: https://tryals.app/practice/biology-i/mitochondria-plastids-and-the-plant-cell/match-each-plastid-or-organelle-to-its-role

### 6. Mitochondria and chloroplasts divide independently by fission rather than being assembled de novo by the host cell. What does this autonomy imply about how a daughter cell inherits these organelles during cell division?

A. They must be partitioned from the pre-existing organelles of the parent cell
B. They are reassembled by 70S ribosomes translating nuclear-encoded signals
C. They replicate synchronously with the nuclear genome before mitosis begins
D. They differentiate spontaneously from simpler storage plastids after division

**Answer:** A. They must be partitioned from the pre-existing organelles of the parent cell

**Why:** Because these endosymbionts arise only from binary fission of existing organelles, anucleate segregation is vital; losing them entirely is lethal since the host cannot synthesise them from scratch. Synchrony with the nucleus is not required, nor can they spontaneously generate from non-plastid precursors.

Page: https://tryals.app/practice/biology-i/mitochondria-plastids-and-the-plant-cell/mitochondria-and-chloroplasts-divide-independently-by-fission-rather

### 7. Arrange these events in the order they occur as a tomato ripens.

**Answer:**

1. The fruit is green, its cells full of chloroplasts
2. Chlorophyll is broken down and photosynthesis ceases
3. Carotenoid pigments accumulate in the plastids
4. The plastids are now chromoplasts and the fruit is red

**Why:** The same plastids are converted rather than replaced: chlorophyll is broken down and carotenoids accumulate in place. Ripening is a plastid conversion visible from across the room.

Page: https://tryals.app/practice/biology-i/mitochondria-plastids-and-the-plant-cell/arrange-these-events-in-the-order-they-occur-as-a-tomato-ripens