This topic asks you to locate photosynthesis at several scales: the plant organ, the tissue, the cell, and the cell structure. Move from the largest scale to the smallest and check whether the named part contains chloroplasts and receives light.
Use a location ladder
Leaves carry out most photosynthesis because they expose a broad area to light and contain many chloroplast-rich cells. Other green parts, including green stems, can also photosynthesise. Ordinary underground roots, woody bark, and non-green petals do not provide the usual combination of chlorophyll and light. Roots absorb water from soil and also take in dissolved mineral ions; they do not collect ready-made food.
Inside a green plant cell, photosynthesis occurs in chloroplasts. Chlorophyll is the pigment within those structures that absorbs light. Keep the names linked as container and contents: a chloroplast contains chlorophyll. Mitochondria have a different role because respiration occurs there.
| Leaf location | Chloroplast pattern and consequence |
|---|---|
| Palisade mesophyll near the upper surface | Its cells have the greatest number of chloroplasts and receive strong light |
| Spongy mesophyll | Its cells have some chloroplasts, but fewer than palisade cells |
| Upper epidermis | Its ordinary cells lack chloroplasts and are transparent, allowing light through |
| Guard cells | They are the exception among epidermal cells because they contain chloroplasts |
Colour gives useful evidence. Green tissue contains chlorophyll and can capture light. A white region of a variegated leaf lacks chlorophyll, so it cannot photosynthesise or make starch there. A green leaf looks green because chlorophyll reflects green light while absorbing red and blue light for photosynthesis.
Common location mix-ups
| Tempting route | How to correct it |
|---|---|
| Swap chloroplast and chlorophyll | Use the container-and-pigment check: the structure holds the pigment, not the reverse. |
| Choose roots as the main food-making organ | Roots take in water from soil and also absorb mineral ions. Photosynthesis needs chloroplasts and light, which are concentrated in green shoot tissues. |
| Choose spongy or epidermal cells as having the most chloroplasts | The palisade layer is near the incoming light and is densely supplied with chloroplasts. |
| Expect white tissue to make starch after receiving sugar from green tissue | The starch result shows where photosynthesis made a local surplus; tissue without chlorophyll cannot make that surplus. |
| Choose an upper or lower epidermal cell, or a palisade cell, as the epidermal exception | The exception is the pair of guard cells around a pore. Palisade cells are not part of the epidermis. |
| Assume the upper covering must photosynthesise because it faces the light | Its ordinary cells are clear so light reaches the chloroplast-rich layer underneath. |
Try it
Inside which part of a plant cell does photosynthesis take place?
- In the mitochondria
- In the cell wall
- In the nucleus
- In the chloroplasts
Why can the ordinary underground roots of a plant not carry out photosynthesis?
- Their cells contain no chloroplasts to capture light
- Their cells take in ready-made food from the soil instead
- Their cells are too busy absorbing water to make sugar
- Their cells only make sugar at night, when the soil is cool
Which statements about why a leaf looks green are true? Select all that apply.
- Green light is mostly reflected from the leaf
- Green light is the colour chlorophyll absorbs best
- Red and blue light are absorbed and used for photosynthesis
- Every colour of light is absorbed equally by chlorophyll