The Leaf and Minerals

Explained

The leaf, and why photosynthesis alone is not enough

A leaf is a flat sheet held out in the light, and every one of its layers has a job that follows from that. Learn the layers in the order light meets them and the adaptations come with them.

The layers

The waxy cuticle is a waterproof covering on the upper surface. It is not a layer of cells; it is a secretion, and it reduces water loss from the surface most exposed to the sun.

The upper epidermis beneath it is a single layer of transparent cells with no chloroplasts. Transparent is the whole point. Light passes straight through to the cells below.

The palisade mesophyll is the layer that does most of the photosynthesis. Its cells are tall, packed closely together and crowded with chloroplasts, and they sit near the top surface where the light is strongest.

The spongy mesophyll below has fewer chloroplasts and large air spaces between the cells. Those spaces connect to the stomata and let carbon dioxide reach every cell by diffusion.

The lower epidermis contains the stomata, pores controlled by guard cells. Most stomata are on the lower surface, away from the direct sun, which reduces water loss while still allowing gas exchange.

Running through the middle are the veins, containing xylem carrying water up to the leaf and phloem carrying sugars away.

Adaptations, and the reason for each

Every adaptation answers a question that starts with why, so learn the pairs rather than the list.

Broad and flat gives a large surface area to catch light and to exchange gases. Thin gives a short diffusion distance, so carbon dioxide reaches the palisade cells quickly. Many chloroplasts in the palisade layer put the chlorophyll where the light is. Air spaces in the spongy layer allow gases to move freely inside the leaf. Stomata let carbon dioxide in and oxygen out. Veins deliver water and remove the glucose.

A question asking for two adaptations wants the feature and its purpose each time. Broad on its own is a description; broad to absorb more light is an adaptation.

Why a plant still needs minerals

Photosynthesis makes glucose from carbon dioxide and water, so it supplies carbon, hydrogen and oxygen. Those three elements are all it can supply.

A plant cannot build proteins from them, because proteins contain nitrogen. It cannot build DNA or ATP, because those contain phosphorus. It cannot even build chlorophyll, because chlorophyll contains magnesium.

So minerals are absorbed from the soil as ions dissolved in water, taken in through the root hair cells by active transport, and carried up in the xylem. Active transport, rather than diffusion, because the concentration of these ions is usually higher inside the root than in the soil around it.

What deficiency looks like

Without nitrate a plant cannot make proteins, so it grows poorly and its older leaves turn yellow as it moves what nitrogen it has to the new growth. Stunted growth and yellow older leaves together point to nitrate.

Without magnesium a plant cannot make chlorophyll, so the leaves turn yellow between the veins while the plant keeps growing at a fairly normal size. That yellowing is called chlorosis.

Both look like yellow leaves, and the difference is whether growth has also stopped. A question that mentions stunted growth alongside the colour has told you which mineral it means.