Root hair cell — structure and function
A root hair cell sits in the epidermis (outer layer) of a root, just behind the growing tip, and grows one long, thin extension — the root hair — out into the soil. Its function is to absorb water and mineral ions from the soil. In a diagram or micrograph the single narrow, finger-like projection is the identifying feature; an ordinary root cortex cell has none. Root hairs occur only on young roots — further back the epidermis has matured and the hairs are lost.
Surface area — why a hair speeds uptake
The long, thin shape greatly increases the surface area of membrane in contact with soil water, compared with a root cell of the same volume and no hair. More area means more water molecules and mineral ions cross into the cell per second, so the rate of uptake increases. This is the surface-area-to-volume (SA:V) principle applied to one structure: for the same volume of cytoplasm, a thin projecting shape has far more surface than a compact one.
The water pathway — four stops in order
Water follows one fixed route: root hair cells (absorb from the soil) root cortex cells (cell to cell across the root) xylem (carries water and mineral ions up root and stem into the leaf, and gives support) mesophyll cells (in the leaf, where some water is used in photosynthesis and most evaporates — the start of transpiration, taught in B8.3). The examinable core is the order, plus naming xylem not phloem.
Drawn from real examiner reports.
"Root cell" is not "root hair cell"
The full three-word term root hair cell is required. "Root cell" drops the defining feature (the hair); "hair cell" is the name of an unrelated sensory cell in animal biology and earns nothing. Before naming the cell, check the diagram for the single long, thin, finger-like projection running out into the soil — that projection is what makes it a root hair cell.
Flagged Jun 2022 P41 Q1ai/aii
Phloem does not carry water
The B8.2.3 pathway names xylem as the tissue between the root cortex and the leaf mesophyll. Xylem transports water and mineral ions and gives support; phloem transports sucrose and amino acids (translocation). Naming phloem as the water-carrying tissue — or saying phloem carries "water", "nutrients" or "glucose" — is marked wrong.
Flagged Jun 2022 P32 Q10d, P42 Q7d · Jun 2023 P43 Q4c
"Xylem and phloem" is not a safe hedge
When a question asks which tissue carries water, only xylem is correct. Listing both tissues to cover yourself contradicts the right answer and is not credited — the mark scheme wants one named tissue. The same trap runs the other way: for sucrose transport the answer is phloem alone, never "xylem and phloem".
"Large surface area" is only half
Statement B8.2.2 is two ideas, not one: root hairs give the cell a large surface area, AND this increases the uptake of water and mineral ions. Writing the structural feature alone — or restating it in different words — drops the explanation mark. A 2-mark "explain" needs the feature and its functional consequence, explicitly linked together.
Root hair cell vs root cortex cell
Only the root hair cell has the long, thin projection into the soil. The root cortex cells that follow it in the pathway are ordinary, hairless cells that simply pass water further across the root toward the xylem at the centre. Labelling a cortex cell as a root hair cell, or expecting cortex cells to absorb water straight from the soil, is wrong.
Bigger surface area, not a bigger cell
A root hair does not give the cell more cytoplasm; it redistributes the same volume into a long, thin shape. The comparison a mark scheme wants is a larger surface area for the same volume — a higher SA:V ratio. "The cell is bigger so it absorbs more" misses the point and scores nothing.
Root hairs are not along the whole root
Root hairs form only in a narrow zone just behind the growing root tip. Drawing or describing hairs along the entire length of an old, woody root is a standard diagram error — that epidermis has already matured and lost its hairs, and new hairs appear further along as the tip advances.
Recite the chain, do not rebuild it
Learn root hair cells root cortex cells xylem mesophyll cells as one memorised sequence, not four separate facts. Under pressure the usual failure is scrambling the order — most often placing mesophyll before xylem.
Match the points to the tariff
Count the marks before you write. A 1-mark "state" wants the term only; a 2-mark "explain" wants the structural feature plus its consequence. Repeating one idea in different words never earns the second mark.
Find the projection before you label
In a diagram question, locate the single long, thin extension first, then write the label. A cell with no projection is a cortex cell. Then check you have written all three words — "root hair cell" — because a two-word label loses the mark.
Convert first, then give the unit
The numeric items here punish two habits: dividing before converting (magnification needs mm turned into µm first, and the formula is image ÷ actual, not the other way round), and leaving the unit off a final answer such as a rate in cm³/hour.
Define a root hair cell and state its function.
A photomicrograph shows a cell from the outer layer of a young root with a single long, thin projection extending into the surrounding soil.
(a) Identify this cell. (1 mark) (b) State its function. (1 mark)
