# Chloroplast and Photosynthetic Structures

> A chloroplast turns light into sugar. Light reactions in the thylakoid membranes capture energy and release oxygen, and the Calvin cycle in the stroma uses that energy to build sugar from carbon dioxide.

Web page: https://cellnaut.com/topics/chloroplast-photosynthesis

![Teaching illustration: Chloroplast and Photosynthetic Structures](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/chloroplast-photosynthesis-teaching.jpg)

## Labelled parts

1. **Light direction**: Light arrives from the sun and is absorbed by chlorophyll in the thylakoids.
2. **Outer chloroplast membrane**: The outer boundary of the double-membrane envelope; it lets small molecules pass.
3. **Double-membrane envelope**: Two membranes surround the organelle, a clue to its bacterial origin.
4. **Granum**: A stack of thylakoid discs, like a pile of coins, where the light reactions happen.
5. **Thylakoid stack**: Stacking packs a large area of membrane and chlorophyll into a small volume.
6. **Stroma lamellae**: Unstacked thylakoid membranes that connect one granum to the next.
7. **Stroma**: The fluid where the Calvin cycle builds sugar from carbon dioxide.
8. **Circular chloroplast DNA**: The organelle's own small circular genome, much like a bacterium's.
9. **Starch granule**: Temporary storage of sugar made during the day.
10. **Oxygen release**: Oxygen is a by-product, released when water is split in the light reactions.

## Key points

- Thylakoid membranes hold chlorophyll and carry out the light reactions; stacks of thylakoids are called grana.
- The stroma is the fluid around the thylakoids, where the Calvin cycle makes sugar.
- Oxygen is a by-product of splitting water, and excess sugar is stored as starch.

## Check yourself

**1. Where do the light reactions take place?**

- A. Thylakoid membranes
- B. Stroma
- C. Outer membrane
- D. Starch granule

**Answer:** A. Thylakoid membranes - Chlorophyll sits in the thylakoid membranes, so light is captured there.

**2. The oxygen released by photosynthesis comes from which molecule?**

- A. Carbon dioxide
- B. Water
- C. Glucose
- D. Chlorophyll

**Answer:** B. Water - The light reactions split water, releasing O₂.

## Related lessons

- [Mitochondria and Chloroplasts](https://cellnaut.com/learn/energy-organelles)

## See it in 3D

- [Plant Cell](https://cellnaut.com/studio/plant)
- [Chlamydomonas](https://cellnaut.com/studio/chlamydomonas)
- [Cyanobacterium](https://cellnaut.com/studio/cyanobacteria)