Levels of Organisation in Multicellular Organisms
Senior 1 · Biology
Levels of Organisation in Multicellular Organisms
Lesson 3 of 5
25 min
The eucalyptus trees along Rwanda's roads can grow taller than a ten-storey building. Every drop of water in their highest leaves entered through microscopic hairs on roots in the soil, rose through dead, hollow pipes with no heart to pump it, and was used by leaf cells arranged in layers like rows of solar panels. Four kinds of specialised plant cells make this possible.
Definition
Mesophyll: the tissue in the middle of a leaf, where most photosynthesis happens. It has two layers: the palisade mesophyll (tall, tightly packed cells near the upper surface) and the spongy mesophyll (rounded cells with air spaces between them).
Root hair cell: a cell in the outer layer of a young root, part of which grows out into a long, thin hair.
Xylem vessel: a long, hollow tube made of dead cells joined end to end, which carries water and mineral salts up the plant. Its walls are strengthened with lignin, a hard, waterproof substance.
Figure 1: A leaf cut across, showing the palisade and spongy mesophyll.
Figure 2: A root hair cell growing out between soil particles.
Figure 3: A xylem vessel, lengthways and cut across.
A xylem vessel starts as a column of living cells. As it matures, lignin is laid down in the side walls, the end walls between the cells break down, and the cells die, leaving an empty tube that can run from the root to the leaves.
| Cell | Job | Key adaptations |
|---|---|---|
| Palisade mesophyll cell | photosynthesis | tall and tightly packed near the upper surface; many chloroplasts |
| Spongy mesophyll cell | photosynthesis and gas exchange | rounded, with air spaces between cells; thin walls |
| Root hair cell | absorbs water and mineral salts | long, thin hair with a large surface area; thin wall; no chloroplasts |
| Xylem vessel | carries water and mineral salts upwards; support | dead and hollow; no end walls; walls thickened with lignin |
Warning
Xylem vessels are dead. A common mistake is to describe a working xylem vessel as a living cell with a nucleus. It does its job only after its contents have died and disappeared: a living cell full of cytoplasm would block the pipe.
Worked example 1: how long does water take to reach the top?
On a sunny day, water rises through the xylem of a eucalyptus tree at about 3 m per hour. The tree is 30 m tall. How long does water take to travel from the roots to the top leaves?
Answer: about 10 hours: water taken up at dawn reaches the top leaves by late afternoon. At night, when the stomata close, the flow slows almost to a stop.
Worked example 2: name the cell
Identify each cell and give the evidence.
| Cell | Description | Identity and evidence |
|---|---|---|
| P | tall, column-shaped, packed with chloroplasts, just under the upper surface of a leaf | palisade mesophyll: shape, position and chloroplasts |
| Q | long outgrowth into the soil, no chloroplasts | root hair cell: large surface for absorption, in the dark |
| R | empty tube with rings of lignin and no end walls | xylem vessel: dead, hollow and strengthened |
| S | rounded, loosely packed, air spaces around it, few chloroplasts | spongy mesophyll: air spaces for gas exchange |
Root hairs can only take in mineral salts that are dissolved in the soil water. Fertiliser spread on dry soil in the long dry season does little until the rains dissolve it. This is also one reason why coffee and banana farmers in Rwanda cover the soil around their plants with mulch of dry grass or banana leaves: the soil stays moist, and the fine, thin-walled root hairs near the surface keep working.
Specialisation in Plant Cells: Photosynthetic, Xylem, Phloem and Root Hair Cells, I'm Stuck. It also describes phloem, which you will study later; focus on the palisade, root hair and xylem cells.
Tip
Memory hook: follow the water: root hair (in) → xylem (up) → spongy (spread out) → palisade (used with light to make food).
Practice: specialised plant cells
Key points
Sources and credits
Figures 1 to 3, the tables, the worked examples and the practice questions are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
