Root hair cells — the site of uptake
Root hair cells are epidermal cells just behind the root tip, each drawn out into a long thin hair that pushes between the soil particles. They are the site of uptake: water enters by osmosis, mineral ions by active transport using energy from respiration. The many hairs together give a very large total surface area, so more particles cross per second and the rate of uptake rises — the same design principle as alveoli and villi.
The pathway — soil to xylem in three steps
The route is soil solution → root hair cell → cortex cells → xylem, and every cell-to-cell step is osmosis. Each cell further in has a slightly lower water potential than the one outside it, so water moves down a continuous water-potential gradient. The xylem is made of dead, hollow tubes that offer little resistance; from there the transpiration stream draws the water up to the leaves.
(Extended) Eosin traces the water pathway
Eosin — a red, water-soluble dye — traces the pathway. Stand a root or young stem in dilute eosin, leave it, then cut thin sections and view them under a microscope. The dye is carried passively with the water, so it colours the xylem vessels and the cells the water passed through, never the phloem. That result confirms the soil → root hair cell → cortex → xylem route described in the Core content.
Drawn from real examiner reports.
Water enters the xylem, not the phloem
"Water enters the root hair cell and then moves into the phloem" scores nothing. Phloem carries dissolved sugars; xylem carries water and mineral ions from root to shoot. In a root cross-section the xylem is the central star and the phloem sits between its arms, so a mislabelled diagram costs the mark too.
Water pathway errors in root: candidates said water enters phloem before reaching xylem. Consistent across all sittings (s22 Papers 13/23 Q16/Q18; w22 Paper 41 Q3; s23 Papers 11/13 Q17).
Do not skip the cortex
The route is root hair cell → cortex → xylem. Jumping straight from the root hair to the xylem drops the middle mark, because "describe the pathway" questions award one mark per correct step in sequence. The cortex is the wide band of cells between the epidermis and the central vascular tissue.
Same digest entry as the phloem error: root water-pathway steps omitted or reordered (s22 Papers 13/23 Q16/Q18; w22 Paper 41 Q3; s23 Papers 11/13 Q17).
Root hairs absorb; xylem transports
Asked what root hair cells do, many candidates write "transport water to the leaves". That is the xylem's job. Root hair cells absorb — they take water and mineral ions up out of the soil. The mark scheme wants "absorption" or "uptake"; "transport" as a root hair cell function is not credited.
Root hair cell functions confused with xylem transport: describing transport of water as a function of root hair cells rather than absorption (s23 Paper 32 Q2c).
Water potential, not concentration
"There is a higher concentration of water in the soil" is not credited. Cambridge osmosis mark schemes require the term water potential: water moves from the higher water potential of the soil solution to the lower water potential inside the root hair cell, which its dissolved salts have lowered.
Water potential terminology required but missing: many wrote "concentration gradient" for water potential questions; the mark scheme demands "water potential" explicitly (s23 Paper 41 Q2a ii; w22 Paper 41 Q3c; s22 Paper 31 Q1).
Osmosis answers must name the membrane
A full osmosis explanation has three parts: the direction given in water-potential terms, the process named as osmosis, and the membrane named as partially permeable. Candidates routinely give the first two and omit the third, losing an easy mark. "Water diffuses in" also fails — diffusion is not specific to water.
Same water-potential digest entry (s23 Paper 41 Q2a ii; w22 Paper 41 Q3c; s22 Paper 31 Q1).
Osmosis ≠ active transport
Water and mineral ions do not enter the same way. Water enters passively by osmosis, down the water-potential gradient, using no energy. Mineral ions enter by active transport, against their concentration gradient, through carrier proteins and using energy from respiration. Swapping the two loses both marks.
Number the steps of the pathway
One mark per correct step, in order. For each step give the structure (soil, root hair cell, cortex, xylem), the process (osmosis, or active transport for ions) and the driving force (higher to lower water potential). An out-of-order list earns nothing.
Use the same three-part osmosis sentence
Write it the same way every time: "from the higher water potential of the soil to the lower water potential in the cell, by osmosis, across the partially permeable membrane." All three elements in one sentence secures the marks.
Run the four-error check
Before moving on, check you have not written phloem for xylem, "concentration gradient" for water potential, "transport" instead of absorption for root hair cells, or left out the partially permeable membrane.
Label the root cross-section carefully
Xylem is the central star; phloem lies between its arms — putting phloem in the centre is the usual error. Label the root hair as an epidermal extension, and never leave the cortex off a pathway diagram.
Root hair cells: epidermal cells at the surface of roots that have a long, thin extension (the root hair) projecting into the soil. They are the site of water and mineral ion uptake into the plant.
Osmosis (definition — two elements required): the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.
Active transport: the movement of mineral ions from the soil (low concentration) into root hair cells (high concentration) — i.e. against the concentration gradient — using energy from respiration. This requires carrier proteins in the cell membrane.
Water potential: a measure of the tendency of water to move. Pure water has the highest water potential; adding dissolved solutes lowers it. Water always moves by osmosis from higher to lower water potential.
Xylem: a tissue made of dead, hollow, lignified tubes that conducts water and dissolved mineral ions from the roots up through the stem to the leaves.
State the function of root hair cells.
(a) Name the type of cell that is the main site of water uptake in plant roots.
(b) State two substances absorbed by these cells from the soil.
(c) State one way in which the structure of these cells is adapted to increase the rate of uptake.
