Transport in Plants

1. Why do plants need a transport system?

• Plants are multicellular and big (think trees) → substances must travel long distances; diffusion / osmosis alone would be far too slow (days to years for one drop soil → leaf).
• To carry water + mineral ions from the soil water up to all parts of the plant — against the pull of gravity (gravity only acts downwards, so something must pull the water up).
• To carry food (sugars) from SOURCE to SINK.

Source = where sugar is made or released from store (usually mature leaf / mesophyll). Sink = where sugar is used or stored (roots, growing stem, fruits). Source ≠ always leaf — in a germinating seed with no leaves yet, the seed (endosperm) is the source and the growing stem is the sink.

So two systems: xylem → water & minerals; phloem → sugars (= assimilates). "Vascular" = "vessels" (like cardiovascular in humans). Vascular tissue = vascular bundles = xylem + phloem, always found side by side.

Three tissue types in a plant

Tissue	What it is	Examples
Vascular	the "vessels" for transport	xylem, phloem
Dermal	outermost layer (like our skin)	epidermis, incl. root hair cells & guard cells
Ground	everything else	mesophyll, cortex, endodermis, pith, pericycle — parenchyma (living, packing/support), sclerenchyma (dead, lignified), collenchyma (living, thick wall)

2. Transverse section (TS) of a dicot leaf

Layers (top → bottom): cuticle → upper epidermis → palisade mesophyll → spongy mesophyll (with vascular bundle) → lower epidermis (with stomata flanked by guard cells).

• Cuticle = waxy substance, not cellular, cuts down water loss. In a plan diagram you only draw cell layers (not the cuticle unless asked).
• Guard cells = specialised epidermal cells that do have chloroplasts (normal epidermal cells don't); the gap between them = stoma (pl. stomata).

How to tell XYLEM from PHLOEM in a leaf

Palisade vs Spongy mesophyll

Palisade mesophyll	Spongy mesophyll
one or two layers of cells	numerous layers
cylindrical cells	irregular-shaped cells
regularly / closely packed (no air gaps)	loosely packed cells
large central vacuole	large air spaces between cells
high chloroplast density	low chloroplast density
—	layer is thicker than palisade layer

3. TS of a dicot stem and a dicot root

Dicot stem

• Layers outside → in: epidermis → cortex → ring of vascular bundles → pith.
• In each bundle: phloem outer, xylem inner, with cambium between them (separator; rarely asked).

Dicot root

• Layers: root hair (extension of an epidermal cell) → epidermis → cortex → endodermis → pericycle → xylem + phloem. Everything inside the endodermis = stele.
• Vascular tissue is concentrated in the middle; xylem forms an "X" shape → root = easiest place to identify xylem; phloem = small vessels around/between arms.
• Endodermis cell walls contain suberin (waxy, hydrophobic) → joined round the ring = Casparian strip. Pericycle = living ground tissue (parenchyma), not in syllabus.

Vascular bundle contains vascular tissues → Xylem and Phloem.

4. Xylem tissue

Made of cells called xylem vessel elements, stacked one on top of another (like a pile of books) to form one long tubular xylem vessel — a hollow "straw" so water meets no obstruction.

Adaptations of a xylem vessel element (each: feature → why)

Feature	Why / function
Lignified walls (woody)	lignin is a hard material → rigidity, strength, resistance: supports the vessel, stops it collapsing under the huge pull / hydrostatic pressure of the water column.
Lignin is hydrophobic ("water-hating")	water does not leak out of the xylem (like an insulator for electricity).
No protoplasm — dead cells (no cytoplasm, nucleus, organelles, membrane)	nothing in the lumen → no resistance to flow of water through the vessel.
End walls between elements removed	water moves as a continuous column with no resistance.
Wide lumen	allows a large volume of water to move up.
Pits in the walls	pits = unlignified regions → the only places water can leave sideways (lateral movement) into adjacent tissues.

5. Phloem tissue

Function: translocation of solutes from SOURCE to SINK. Two cell types: ① sieve tube elements (STE) and ② companion cells (CC).

Phloem sap contains: sugars (sucrose), amino acids, plant hormones (+ other assimilates). Phloem is always full of sap, like blood vessels are always full of blood.

Sieve tube element adaptations (feature → why)

Feature	Why / function
Sieve plates perforated (sieve pores)	easy flow of phloem sap from one STE to the next. (Companion cells have no such holes → sap flows through STEs, not CCs.)
Sieve plates separate neighbouring STEs	may prevent the sieve tube bursting due to pressure.
Scanty cytoplasm (only at edges)	middle empty → easy flow of phloem sap.
Has a cell surface membrane	prevents leakage of sugars out of the STE (sugars move via membrane proteins; membrane controls entry/exit) — unlike dead xylem, STEs are living.
Thin walls	minimises diffusion distance, maximises rate of diffusion.
Joined to a companion cell via plasmodesmata	allows movement to and from the companion cell; CC supplies the life-support (energy, proteins) the STE lacks.

6. Differentiate: xylem vessel element vs phloem sieve tube element

Xylem vessel element	Phloem sieve tube element
Dead cells	Living cells
Lignin deposited in cell walls	Non-lignified cell walls
Thick cell walls	Thin cell walls
No cytoplasm	Scanty cytoplasm
No organelles	Few organelles, e.g. ER, mitochondria

Xylem vessel element	Phloem sieve tube element
End walls removed	End walls exist as sieve plates (pores)
Side walls have pits	Side walls do not have pits
No plasmodesmata in side walls	Plasmodesmata in side walls (to companion cell)

Also: xylem vessels have the wider lumen; thicker-walled = xylem when identifying in a section.

7. Movement of water through the plant

Two pathways between adjacent cells

Symplast pathway: water moves between adjacent cells via plasmodesmata (cytoplasm to cytoplasm); involves crossing the cell surface membrane (osmosis — driven by a water potential gradient, high → low WP).

Apoplast pathway: water moves between adjacent cells through cellulose cell walls (cell wall to cell wall); does NOT cross a membrane → not osmosis. Driving force = transpirational pull (water replaces what is lost, like water soaking into a tissue — no gradient needed).

The transport of water is predominantly a PASSIVE process (no ATP needed in xylem). Five stages — we study them (a) → (e), from leaf back to soil:

1. mesophyll cell walls → external atmosphere
2. xylem vessel (leaf) → mesophyll cell wall
3. xylem vessel (root) → xylem vessel (leaf)
4. root hair cells → xylem vessel (root)
5. soil water → root hair cell

(a) Mesophyll cell walls → atmosphere (transpiration)

• Mesophyll cell walls are covered with a thin film of moisture.
• The water of the film evaporates into the intercellular air spaces as water vapour. Energy for evaporation comes from sunlight (heat; faster if hot / windy).
• Vapour saturates the air spaces and diffuses down the concentration gradient (high in leaf → low outside) to the atmosphere via the stomata (open in daytime).
• This loss of water vapour = TRANSPIRATION ⇒ provides the "transpirational pull" needed to move water from roots to leaf.

(b) Xylem (leaf) → mesophyll cell wall

• Water lost from the mesophyll walls is replaced by water from the xylem in the leaf.
• Water leaves xylem through the pits (non-lignified regions).
• The mesophyll cell next to xylem gets water directly, then passes it on to adjacent cells — one mesophyll cell to the next… via the apoplast or symplast pathway.
• Xylem now has less water → its hydrostatic pressure (HSP) at the top drops (see (c)).

(c) Xylem (roots) → xylem (leaf): cohesion–tension / mass flow

1. Water moving out of leaf xylem towards mesophyll cells lowers the hydrostatic pressure at the top of the xylem.
2. HSP at the base (roots) is high (xylem has just received water from the soil). This creates a hydrostatic pressure gradient (high → low) → water moves in a continuous column from roots to leaf.
3. This bulk movement of water molecules in a continuous column = mass flow — upwards, against gravity.

Tension at the top of the xylem, due to transpiration, pulls the water up from the roots' xylem (transpirational pull). This mechanism = the cohesion–tension model.

• Cohesion = attraction between water molecules (hydrogen bonds) → column doesn't break. Adhesion = attraction of water to the xylem walls → keeps water from falling back.
• Like a house water pipe: if air gets in, the column breaks into sections and flow turns jerky. Cavitation = formation of air-filled spaces in the xylem (click sound).
• Pulling water up = a pull; sugar moving down in phloem = a push. Hydrostatic pressure gradients work either way, whatever the direction relative to gravity.
• Mass flow is always due to a PRESSURE (hydrostatic) gradient — not osmotic/water-potential differences. Osmosis is only for cell-to-cell steps (e.g. xylem→phloem).

(d) Root hair cells → xylem (root)

• Root hair cells have the highest water potential compared with adjacent cortical cells.
• Water moves from a region of ↑ WP to ↓ WP: root hair cells → cortical cells, one after the other (osmosis).
• Movement from one cortical cell to another through a selectively permeable membrane and plasmodesmata = the symplast pathway.
• Water also moves by the apoplast pathway between adjacent cortical cells. However the apoplast pathway is BLOCKED at the endodermis by a waxy, hydrophobic substance suberin that forms the Casparian strip.
• At the endodermis all water molecules must enter via the symplast pathway ⇒ the membrane filters soluble toxins and unwanted ions before water enters the xylem.

(e) Soil water → root hair cell

• Soil water has a higher water potential than the root hair cell.
• Water enters from soil to root hair cell by osmosis.
• (Teacher: minerals are first taken up by active transport, lowering the root hair cell's WP — that keeps the gradient steep.)
• The surface area for water absorption is increased by root hairs = cytoplasmic extensions of epidermal cells. Only some epidermal cells are root hair cells.

8. Translocation of sucrose in the phloem

• Active process (needs energy), involves movement of assimilates from source to sink.
• Travels as sucrose, not glucose: glucose is very osmotically active (would upset water potentials in xylem/phloem), so mesophyll cells convert glucose → sucrose straight after photosynthesis.

Active loading at the source (mesophyll → phloem)

After loading: water enters the sieve tube

• Sucrose entering lowers the water potential in the sieve tube (solute ↑).
• Water moves from neighbouring xylem into phloem by osmosis → region of HIGH hydrostatic pressure at the source end.

Active unloading at the sink

• Sucrose leaves the sieve tube → companion cell → sink cells by active unloading (energy needed).
• Water potential of the phloem increases → water leaves phloem, re-enters xylem (osmosis) → region of LOW hydrostatic pressure at the sink.
• The hydrostatic (pressure) gradient → mass flow of sucrose & other nutrients through the phloem, source → sink.

Munch's pressure-flow hypothesis (MCQ favourite!)

• Semi-permeable membrane mimics the cell surface membrane.
• The high-sugar bulb = SOURCE; the dilute one = SINK. Water enters the source bulb by osmosis → pressure builds → sugar pushed along the tube.
• Tube where solute particles move = phloem; tube where only water moves (back the other way) = xylem. Direction of sugar movement tells which end is source / sink.

9. Comparing xylem and phloem

Similarities

• Both involve diffusion and osmosis (at cell-to-cell level).
• Mass flow occurs in both, driven by a hydrostatic pressure gradient.
• Both transport across long distances.

Differences

Xylem	Phloem
unidirectional flow (soil → leaf only; water always from the ground)	bidirectional flow (depends on source/sink and growth stage: seed → stem, later leaves → roots)
transports water and minerals	transports assimilates (sucrose, amino acids…)
passive process (no ATP)	active process (active loading/unloading, H⁺ pump)
slower rate of flow	faster rate of flow
involves transpiration pull, mass flow and root pressure	involves mass flow and active transport

10. Xerophytes — plants of dry places

Xerophytes live where water is in short supply; leaf adaptations keep water loss to a minimum. Transpiration = diffusion of vapour → depends on the concentration (water potential) gradient; adaptations reduce it or lengthen the path.

Adaptation	How it reduces water loss
Very few stomata	fewer openings for vapour to escape.
Sunken stomata (in pits)	moist air is trapped in the pit → outside air near the stoma is humid → smaller gradient → ↓ transpiration.
Hairs (trichomes) around stomata	trap moist air, reduce the water potential gradient, decrease transpiration rate.
Needle-shaped / small leaves (cactus spines)	small surface area for evaporation; green stem photosynthesises and stores water.
Thickened waxy cuticle	thicker waterproof layer → less loss through the epidermis.
Rolled / curled leaf (hinge cells flaccid)	creates a humid space, less exposed surface.
Shallow + deep roots	shallow roots absorb any rainfall; deep roots reach the water table.
Thick epidermis (e.g. 3 layers in oleander)	↑ diffusion distance → less water lost.

11. Exam practice (from class)

Practical corner: potato osmosis (Paper 3)

• Dilutions (serial/simple) of salt or sucrose stock; cut potato cylinders of the stated size (cork borer → even cylinders); record initial mass/length; immerse for the given time (e.g. 15–20 min).
• Blot DRY before weighing (surface liquid adds mass). Record final mass/length.
• % change = final − initialinitial × 100
• Solute concentration = independent variable (x-axis); % change on y.
• Dilute (high WP) → water enters → potato gains mass; concentrated (low WP) → water leaves → loses; no change = isotonic with the potato.
• Errors/improvements: poor drying; uneven potato (hard to measure length) → repeat 2–3×, use mean; use a cork borer.

★ Chapter cheat-sheet