Roots: Structure, Water Uptake and Mineral Absorption

The hidden half

If you pull up a dandelion, the long root often snaps off and the plant grows right back. If you pull up a clump of grass, you get a tangled mat of thin roots holding a ball of soil. Roots are the hidden half of a plant, and in many plants there is as much living material underground as above it.

Roots have four main jobs: they anchor the plant, absorb water and minerals, conduct them to the stem, and often store food (carrots, beets and sugar beets are storage roots). In this lesson you'll look inside a root and see how its structure fits these jobs.

Two kinds of root systems

• A taproot system has one thick main root that grows straight down, with smaller lateral roots branching off it. Most dicots have taproots (carrots, dandelions, oaks). Taproots reach deep water and are good at storing food.
• A fibrous root system has many thin roots of similar size spreading out near the surface. Most monocots have fibrous roots (grasses, corn, onions, wheat). Fibrous roots hold soil very well, which is why grasses are used for erosion control.
• Adventitious roots grow from stems or leaves instead of from another root. Corn plants grow prop roots from the stem near the ground, and a stem cutting makes adventitious roots when you put it in water.

Inside a root: the tissues from outside to inside

Cut a young dicot root straight across and look under a microscope. From the outside in:

1. Epidermis: one layer of cells. In the maturation zone, many of these cells grow root hairs, tube-like extensions only one cell thick. Root hairs give a huge surface area for absorption. There is no waxy cuticle here: a root needs water to get in.
2. Cortex: many layers of loosely packed parenchyma cells with air spaces. The cortex stores starch and moves water inward.
3. Endodermis: a single ring of tightly fitted cells, the innermost layer of the cortex. Each cell has a waterproof band, the Casparian strip, in its walls.
4. Pericycle: a layer just inside the endodermis. Its cells can divide to start lateral roots, which push out through the cortex.
5. Vascular cylinder (stele): in a dicot root, xylem forms an X- or star-shaped core, with phloem in the spaces between the arms. (In a monocot root, xylem and phloem alternate in a ring around a central pith.)

How water gets in: osmosis

Root cells contain sugars, salts and other dissolved substances, so their cytoplasm usually has a higher solute concentration than the water in moist soil. Water moves by osmosis from the lower solute concentration (the soil) to the higher solute concentration (the root cell), across the cell membrane.

This only works if the soil water is less concentrated than the root cells. If the soil becomes very salty (for example, beside a road salted all winter), water can even move out of the root, and the plant wilts although the soil is wet.

How minerals get in: active transport

Minerals such as nitrate (NO₃⁻), phosphate and potassium (K⁺) are often more concentrated inside root cells than in the soil. To keep collecting them, root cells use carrier proteins and energy from ATP to pump ions in, against the concentration gradient. This is active transport.

ATP comes from cellular respiration, which needs oxygen. That's why roots need air spaces in the soil. In waterlogged soil, the air spaces fill with water, the roots run short of oxygen, mineral uptake stops, and roots can die. Overwatering is one of the most common ways houseplants are killed, for exactly this reason.

There's a bonus: pumping ions in raises the solute concentration inside the root, which pulls in even more water by osmosis.

Two routes across the root, and one checkpoint

Water can cross the cortex in two ways:

• Apoplast route: through the porous cell walls and spaces between cells, without entering any cytoplasm. It is fast, like walking through hallways.
• Symplast route: into a cell's cytoplasm and then from cell to cell through tiny channels called plasmodesmata, like walking through connected rooms.

At the endodermis, the Casparian strip seals the walls, so the apoplast route is blocked. All water and minerals must cross a cell membrane to get into the endodermis cells. Membranes are selectively permeable, so the plant can control which ions enter the xylem and can keep out many toxic substances. The strip also stops minerals that were pumped into the xylem from leaking back out.

Root partners: fungi and bacteria

• Mycorrhizae: most land plants have fungi living on or in their roots. The fungal threads (hyphae) spread far through the soil and are much thinner than roots, so they absorb water and minerals, especially phosphate, from a larger volume of soil. The plant pays the fungus with sugar. Both partners benefit (a mutualism).
• Root nodules: legumes (peas, beans, clover, alfalfa, soybeans) have swellings on their roots filled with nitrogen-fixing bacteria (Rhizobium). The bacteria turn nitrogen gas from the air into ammonium the plant can use. You'll see why farmers value this in Lesson 16.