AS & A Level Biology · 9700 · Topic 7 · 2025–2027 Exam

Transport in Plants

Two parallel vascular pipelines, one design problem: distribute water and assimilates throughout a body too large for diffusion alone. The cellular architecture of xylem and phloem; the cohesion-tension theory of water ascent; xerophytic adaptations; and active phloem loading driven by proton-coupled cotransport.

Sub-sections 7.1–7.2 AS Level Papers 1–3 Xylem · Phloem · Transpiration · Translocation

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Topic 7.1 · AS

Structure of transport tissues

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Flowering plants do not have compact bodies like animals: leaves and roots spread out widely to capture light, water, mineral ions, and CO₂. Distances are too great for diffusion alone, so plants have two specialised vascular tissues arranged as continuous pipelines:

Xylem — transports water and dissolved mineral ions upwards from roots to leaves, plus mechanical support
Phloem — transports assimilates (sucrose, amino acids) from sources to sinks in any direction

Note: plants do not have a system for transporting respiratory gases. O₂ and CO₂ diffuse directly through air spaces within stems, roots, and leaves — covered in Topic 9.

Dicot stem — transverse section

In a herbaceous dicotyledonous stem (e.g. sunflower), vascular tissue is arranged in discrete vascular bundles in a ring near the perimeter. Each bundle has the same orientation:

Outer to inner

Stem TS layers

Epidermis: single outer layer; cuticle prevents water loss
Cortex: ring of parenchyma cells inside the epidermis — storage
Vascular bundles: arranged in a ring around the stem (a key dicot feature)
Pith: central parenchyma tissue

Within each bundle

Phloem outside, xylem inside

Phloem on the outer side of the bundle
Cambium — thin layer of dividing cells between phloem and xylem (gives rise to secondary thickening)
Xylem on the inner side of the bundle

Dicot root — transverse section

The arrangement is fundamentally different from the stem:

Outer to inner

Root TS layers

Epidermis (piliferous layer): with root hairs — long extensions giving huge surface area for water and ion uptake
Cortex: large band of parenchyma cells — storage and route for water
Endodermis: a single ring of cells with the Casparian strip — a band of waterproof suberin/lignin around each cell
Pericycle: thin layer just inside the endodermis — gives rise to lateral roots
Vascular tissue: in the centre, in a star-shaped arrangement

Star-shaped centre

Xylem in the middle, phloem in the gaps

Xylem forms a central star or X-shape with arms reaching out
Phloem sits in the gaps between the xylem arms
This central placement gives roots tensile strength to resist pulling forces from above

Dicot leaf — transverse section

The vascular tissue runs through the leaf inside the veins:

Outer surfaces

Leaf TS layers

Upper epidermis: single layer covered with a waxy cuticle reducing water loss; usually transparent
Palisade mesophyll: column-shaped cells packed with chloroplasts — main site of photosynthesis
Spongy mesophyll: rounded cells with large air spaces, allowing gas exchange
Lower epidermis: contains most of the stomata (gaps flanked by guard cells) — gas exchange and the exit point for transpiration

Within each vein

Xylem upper, phloem lower

Xylem on the upper side of the vein
Phloem on the lower side of the vein
This is the opposite orientation to the stem — common confusion in exams

Common mistake

Position of xylem and phloem differs by organ: stem — xylem inner, phloem outer (in a ring of bundles); root — central xylem star with phloem in the gaps; leaf vein — xylem upper, phloem lower. Memorise all three or you will lose marks on TS identification questions.

Xylem vessel elements

The main water-conducting cells of xylem are xylem vessel elements. Their structure is a textbook example of form fitting function:

Adaptation 1

Dead and hollow

At maturity, xylem vessel elements have no cytoplasm, nucleus, or organelles — the cells die during development. The hollow lumen offers minimum resistance to water flow. End walls have largely broken down so individual elements join end-to-end into long continuous tubes.

Adaptation 2

Lignified walls

The cell wall is thickened and impregnated with lignin — a tough waterproof polymer. Lignin is laid down in patterns (rings, spirals, or solid sheets with gaps for pits). The rigid wall withstands tension from transpiration pull, preventing the vessel from collapsing inwards under negative pressure.

Adaptation 3

Pits

Pits are unlignified gaps in the side walls. They allow lateral movement of water from one vessel to another (or out to surrounding cells), and provide a bypass route if a vessel becomes blocked by air bubbles (cavitation).

Adaptation 4

Narrow lumen

Vessels are narrow (typically 20–200 μm diameter). This may help maintain the continuity of the water column under tension — narrow tubes are less likely to break than wide ones — and contributes to capillary action.

Phloem — sieve tube elements and companion cells

Phloem has two cell types working as a unit: sieve tube elements conduct the phloem sap, while companion cells sustain them metabolically.

Conducting cell

Sieve tube element

Living cell, but lacks a nucleus and most organelles at maturity (very few mitochondria, no ribosomes)
End walls are perforated by large pores, forming sieve plates at the boundary between adjacent elements
Sieve plates allow phloem sap to flow from one element to the next under pressure
Reduced organelle content keeps the lumen clear — minimum resistance to mass flow
Joined end-to-end forming continuous sieve tubes

Support cell

Companion cell

Smaller cell positioned alongside each sieve tube element
Contains a nucleus, dense cytoplasm, and many mitochondria — supplying ATP
Connected to the sieve tube element via numerous plasmodesmata (cytoplasmic bridges through the wall)
Carries out metabolic functions for the sieve tube element — including active loading of sucrose into the phloem

Why the structures differ — xylem dead, phloem alive

Xylem transport (water + mineral ions) is passive — driven by tension from transpiration with no metabolic cost in the conducting cell. So the cells can be dead, hollow, and rigid — reducing resistance and providing structural support.

Phloem transport (sucrose, amino acids) is driven by active loading at the source, then mass flow under pressure. Active loading needs ATP, which the sieve tube element cannot make for itself (no mitochondria). The companion cell provides this ATP via plasmodesmata. Both must be alive.

MCQ · Topic 7.1 · Paper 1 style

Which feature of xylem vessel elements specifically helps withstand the tension created by transpiration pull?

A. The presence of plasmodesmata between adjacent cells
B. Lignified cell walls reinforcing the tube
C. The presence of a large central vacuole
D. The presence of many mitochondria providing ATP

Answer: B — Water in the xylem is under negative pressure (tension) due to transpiration pull. Without rigid walls, the tube would collapse inwards. Lignification provides the mechanical strength needed to withstand tension. (A) describes phloem; (C) is wrong — xylem vessel elements are dead and have no vacuole; (D) is wrong — xylem cells have no mitochondria because xylem transport is passive.

Structured · Topic 7.1 · Paper 2 style · 7 marks

Phloem sieve tube elements and companion cells work together to transport assimilates.

(a) Compare the structure of xylem vessel elements and phloem sieve tube elements. Give THREE differences. [3]
(b) Explain how the structure of companion cells supports the function of sieve tube elements. [4]

(a) Three differences [3 marks]

Acceptable difference points (any three)

Xylem vessel elements are dead at maturity; sieve tube elements are alive
Xylem vessel elements have lignified walls; sieve tube elements have non-lignified cellulose walls
Xylem vessel elements have pits; sieve tube elements have sieve plates with large pores
Xylem vessel elements have no cytoplasm or organelles; sieve tube elements retain cytoplasm (though they lack a nucleus and most organelles)
Xylem vessel elements have a wider lumen than sieve tube elements (acceptable)

(b) How companion cells support sieve tube elements [4 marks]

Acceptable points

Companion cells contain a nucleus and many mitochondria, which the sieve tube element lacks [1]
Mitochondria carry out aerobic respiration, providing ATP [1]
The ATP is used to actively load sucrose into the sieve tube element (via proton pumps and cotransporter proteins) [1]
The companion cell connects to the sieve tube element via plasmodesmata, allowing materials and ATP to pass between them [1]

Mark scheme guidance: Each comparison in (a) must explicitly contrast both cell types — bare statements about one alone do not score. The companion cell description in (b) must link structure to function (mitochondria → ATP → active loading), not just list components.

Topic 7.2 · AS

Transport mechanisms

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Plants use two transport pathways with different mechanisms. Water and dissolved minerals move through the xylem from roots to leaves, driven mainly by transpiration pull (passive). Assimilates move through the phloem from sources to sinks, driven by active loading followed by mass flow down a hydrostatic pressure gradient (active overall).

Uptake of water and ions by roots

Water and dissolved mineral ions enter the plant through the root hairs — long thin extensions of root epidermal cells. Adaptations:

Enormous surface area in contact with soil (millions of root hairs per plant)
Thin cell wall — short diffusion distance for water and small solutes
Many mitochondria — supply ATP for active uptake of mineral ions
The cytoplasm contains dissolved solutes, lowering its water potential below that of soil water — water enters by osmosis (Topic 4.2)

Mineral ions (e.g. nitrate, phosphate) are taken up actively across the root hair membrane via carrier proteins, against their concentration gradient (Topic 4.2). Once inside the cytoplasm, they are dissolved in the cell water and travel onwards into the rest of the root.

Water across the root: apoplast and symplast pathways

From the root hair, water has two parallel pathways across the cortex to the central xylem:

Pathway 1

Apoplast pathway

Water moves through the cell walls and intercellular spaces — never crossing a plasma membrane. The cellulose cell wall is freely permeable to water, which simply diffuses through.

Faster route — least resistance
Carries dissolved mineral ions along with water
Stops at the endodermis — the Casparian strip blocks the apoplast

Pathway 2

Symplast pathway

Water moves through the cytoplasm of cells, passing from cell to cell via plasmodesmata (cytoplasmic bridges through the wall).

Slower than apoplast (must cross plasma membranes initially)
Allows the plant to filter and control what enters the xylem
Continues across the endodermis — not blocked by the Casparian strip

The Casparian strip — gateway to the xylem

The endodermis is a single layer of cells separating the cortex from the central vascular tissue. Each endodermal cell has a band of suberin and lignin in its radial walls, called the Casparian strip. Suberin is waterproof, so the strip blocks water and solute movement through the cell wall.

The consequence: water moving by the apoplast pathway must, at the endodermis, cross a plasma membrane and enter the cytoplasm (i.e. switch to the symplast pathway). This forces all water entering the xylem to pass through at least one selectively permeable membrane — allowing the plant to control which mineral ions pass into the xylem and exclude harmful substances.

The cohesion-tension theory of water ascent

The cohesion-tension theory explains how water moves up the xylem from roots to leaves — sometimes against gravity over distances of more than 100 m in tall trees — with no metabolic energy directly driving the column.

Mechanism — transpiration pull

Water evaporates from the surfaces of mesophyll cells inside the leaf and diffuses out as water vapour through stomata in the lower epidermis — this is transpiration
Loss of water from mesophyll cell walls lowers the water potential in those walls; water moves out of the xylem at the leaf to replace it (down the water potential gradient)
Withdrawal of water from the top of the xylem column creates tension (negative pressure) in the column
The tension pulls the entire column of water upwards from the roots
Two properties of water make this possible: cohesion (water molecules attract each other through hydrogen bonds, holding the column together) and adhesion (water molecules stick to the cellulose cell walls of xylem vessels)
At the root, water enters the xylem from cortex/endodermis to replace the rising column — an unbroken stream from soil to atmosphere

Why hydrogen bonding matters specifically

Water's hydrogen bonds (Topic 2.4) provide both cohesion (between water molecules) and adhesion (to the polar cellulose walls of xylem vessels). Without cohesion, the column would break under tension. Adhesion contributes to capillary movement up narrow tubes. The lignified xylem walls themselves stop the negative pressure from collapsing the vessel inward.

Energy balance: the energy does have a source — ultimately the sun, evaporating water at the leaf. No direct ATP cost is needed to lift water up the xylem.

Factors affecting transpiration rate

Factor	Effect on rate	Reason
Light intensity	Higher light → higher rate	Stomata open in light to allow CO₂ in for photosynthesis — water vapour escapes the same way
Temperature	Higher T → higher rate	More kinetic energy → water evaporates faster from mesophyll cell surfaces; air can hold more water vapour
Humidity	Higher humidity → lower rate	Smaller water vapour gradient between leaf interior and external air — less driving force for diffusion out
Air movement (wind)	More wind → higher rate	Removes humid air immediately around stomata, maintaining a steep water vapour gradient
Soil water availability	Less soil water → lower rate (eventually)	Stomata close when the plant cannot replace lost water — reduces further loss

Investigating transpiration with a potometer

A potometer measures the rate of water uptake by a cut shoot — assumed to be approximately equal to the rate of transpiration (most water taken up is lost as vapour, with a small amount used in photosynthesis and held in the plant).

Method

Cut a leafy shoot under water (avoids drawing air into xylem) and connect it to the potometer through a rubber bung — airtight seal
Fill the apparatus with water; introduce a single air bubble into the capillary tube
Measure the time taken for the air bubble to move a fixed distance along the tube (e.g. 50 mm)
Calculate volume of water taken up = distance × cross-sectional area of capillary tube
Rate = volume per unit time
Reset the bubble to the start using the reservoir, then repeat
Vary one factor (light, temperature, humidity, wind) at a time to test its effect

Limitations: measures water uptake not directly transpiration; cut shoots may behave differently from intact plants; small amounts of water are used in photosynthesis or retained in the plant.

Xerophyte adaptations

Xerophytes are plants adapted to dry environments — deserts, sand dunes, exposed mountains. Their adaptations all serve one purpose: reduce water loss while still allowing photosynthesis.

Adaptation 1

Thick waxy cuticle

The cuticle on the upper (and sometimes lower) epidermis is greatly thickened. The waxy layer is impermeable to water, so virtually all water loss must occur through stomata. Increases the diffusion path length for water vapour escaping any non-stomatal route.

Adaptation 2

Sunken stomata in pits

Stomata are recessed into pits or grooves in the leaf surface. The pits trap moist air immediately above the stomata, raising the local humidity and reducing the water vapour gradient between the leaf interior and the atmosphere — transpiration rate falls.

Adaptation 3

Hairs (trichomes) on the surface

Dense hairs on the leaf epidermis trap a layer of moist air, similar effect to sunken stomata. Examples: edelweiss, lamb's ear, many desert plants.

Adaptation 4

Rolled leaves

Some grasses (e.g. marram grass on sand dunes) roll their leaves under dry conditions, with stomata on the inner surface. The rolled-up tube traps moist air, raises local humidity, and dramatically reduces transpiration.

Adaptation 5

Reduced leaf area / spines

Leaves may be small, needle-shaped (conifers in cold dry mountains), or reduced to spines (cacti) — with the stem taking over photosynthesis. Smaller surface area = less area for transpiration.

Adaptation 6

Deep or extensive root systems

Long taproots (tap deep groundwater — e.g. mesquite) or extensive shallow networks (catch any rain — e.g. cacti) maximise water absorption from the limited supply.

Adaptation 7

Water storage tissues (succulence)

Cacti and succulents have specialised parenchyma tissue that stores water during wet periods, allowing the plant to survive long dry periods. Often associated with thick stems.

Adaptation 8

Stomata closed by day

Some xerophytes (CAM plants) keep stomata closed during the hot day and open them at night to take in CO₂, fixing it temporarily until daytime light is available for photosynthesis. (CAM mechanism is beyond syllabus, but the strategy is exam-relevant.)

Translocation: assimilates from sources to sinks

Translocation is the movement of assimilates (sucrose, amino acids) through the phloem from sources to sinks. Sucrose is the main transported sugar in flowering plants because it is highly soluble, chemically stable, and metabolically inert.

Term	Definition	Examples
Source	Site where assimilates are made or released from storage — net producer of assimilates	Photosynthesising leaves (during day); storage organs releasing sugars (potato tubers in spring)
Sink	Site where assimilates are used or stored — net consumer of assimilates	Growing tips (root and shoot); developing fruits and seeds; storage organs accumulating starch

Sources and sinks are flexible

The same organ can be a source at one time and a sink at another. A potato tuber stores starch (sink) during summer and autumn, then releases sucrose (source) in spring to feed the growing shoot. A young leaf is a sink while still importing sugars; once mature it becomes a source. The direction of phloem flow can therefore reverse as the plant develops.

Phloem loading at the source

Sucrose must be loaded into sieve tube elements at the source against its concentration gradient. The mechanism is active loading via companion cells, using a clever indirect coupling of ATP to sucrose transport:

Mechanism — proton pump + cotransporter

The companion cell membrane contains proton (H⁺) pumps — carrier proteins that use ATP to actively pump H⁺ ions out of the companion cell into the surrounding apoplast (cell wall space)
This creates a steep H⁺ concentration gradient — H⁺ is high outside, low inside
The companion cell membrane also contains sucrose-H⁺ cotransporter proteins that allow H⁺ to flow back in down its gradient, but only by binding sucrose at the same time and bringing it in too
This loads sucrose into the companion cell against its concentration gradient — powered indirectly by the H⁺ gradient set up by the proton pump (this is secondary active transport)
From the companion cell, sucrose moves into the sieve tube element through plasmodesmata (passive, down its gradient)

The point of secondary active transport

The proton pump is the only step that directly hydrolyses ATP. The sucrose loading itself uses the energy stored in the H⁺ gradient. This is the same general principle behind glucose absorption in the small intestine (Topic 4.2 mention) — gradients of one solute provide the energy to move another against its gradient.

Mass flow from source to sink

Once sucrose is loaded into the sieve tube at the source, it drives mass flow of phloem sap to the sink:

Mass flow mechanism

Active loading raises sucrose concentration in the sieve tube at the source → lowers water potential in the sieve tube
Water enters the sieve tube from surrounding xylem and apoplast by osmosis → raises hydrostatic pressure at the source end of the sieve tube
At the sink, sucrose is unloaded into surrounding cells (used for respiration, growth, or stored as starch). This raises water potential in the sieve tube at the sink end
Water leaves the sieve tube at the sink by osmosis → lowers hydrostatic pressure at the sink end
The pressure difference (high at source, low at sink) drives mass flow of phloem sap (sucrose, amino acids, water) down the hydrostatic pressure gradient from source to sink
Flow can occur in either direction in the same plant simultaneously — different sieve tubes carrying sap to different sinks

Xylem vs phloem transport — comparison

Feature	Xylem (transpiration stream)	Phloem (translocation)
Substances	Water; dissolved mineral ions	Sucrose; amino acids; water
Direction	Roots → leaves (mostly upwards)	Sources → sinks (any direction; can reverse over time)
Driving force	Tension from transpiration pull (negative pressure at top)	Hydrostatic pressure gradient set up by active loading at source
Pressure	Negative (tension)	Positive (high pressure at source, lower at sink)
ATP required?	No (passive overall)	Yes — for active loading at source (proton pumps)
Conducting cells	Dead xylem vessel elements	Living sieve tube elements + companion cells

MCQ · Topic 7.2 · Paper 1 style

Which combination of properties of water is most directly responsible for the cohesion-tension mechanism of water transport in xylem?

A. Solvent properties and high specific heat capacity
B. Cohesion (between water molecules) and adhesion (to xylem walls), both due to hydrogen bonding
C. Low density of ice and high latent heat of vaporisation
D. Polarity of water as a solvent for sucrose

Answer: B — The cohesion-tension theory depends specifically on cohesion (water molecules holding the column together) and adhesion (water sticking to xylem walls). Both arise from hydrogen bonding (Topic 2.4). Solvent properties and thermal properties matter for water's role in living systems generally, but not directly for the ascent up the xylem.

Structured · Topic 7.2 · Paper 2 style · 9 marks

A student investigates the rate of water uptake by a leafy shoot connected to a potometer in different conditions.

(a) Describe how the student should set up the potometer to give reliable readings. [3]
(b) The shoot is then placed in front of a fan. Predict and explain the effect on the rate of water uptake. [3]
(c) Explain how loading sucrose into a phloem sieve tube at a leaf source eventually causes movement of phloem sap towards a root sink. [3]

(a) Reliable potometer setup [3 marks; any three]

Acceptable points

Cut the shoot under water to avoid drawing air into the xylem [1]
Insert the shoot through a rubber bung to make an airtight seal between the shoot and apparatus [1]
Introduce a single air bubble into the capillary tube as the marker for water uptake [1]
Allow time for the shoot to equilibrate before taking readings [1]
Take repeat measurements and calculate a mean rate [1]
Keep all factors except the one being investigated constant [1]

(b) Effect of fan [3 marks]

Acceptable points

The rate of water uptake increases [1]
The fan removes humid air from around the leaves, maintaining a steep water vapour potential gradient between the leaf interior and the surrounding air [1]
This increases the rate of transpiration / diffusion of water vapour out through the stomata, which increases tension in the xylem and pulls water up faster — so uptake at the cut end increases [1]

(c) Loading sucrose drives mass flow [3 marks]

Acceptable points

Loading sucrose into the sieve tube at the leaf source raises sucrose concentration there, which lowers the water potential inside the sieve tube [1]
Water moves into the sieve tube from surrounding tissues by osmosis, raising the hydrostatic pressure at the source [1]
At the root sink, sucrose is unloaded; water leaves the sieve tube by osmosis, reducing pressure. Phloem sap flows down the hydrostatic pressure gradient from source (high pressure) to sink (low pressure) — mass flow [1]

Exam Prep

Topic 7 Practice — Comprehensive

Mixed practice covering both sub-sections in 9700 P1/P2 style. Try each before revealing the answer.

MCQ · TS anatomy · Paper 1

In a transverse section of a herbaceous dicot root, where is the xylem typically found?

A. In a ring of vascular bundles around the perimeter, with phloem on the outside
B. In a central star-shaped arrangement, with phloem in the gaps between the arms
C. On the upper side of veins, with phloem below
D. Scattered throughout the cortex

Answer: B — Roots have a central star-shaped xylem arrangement with phloem in the gaps between the arms. (A) describes a stem; (C) describes a leaf vein; (D) is a monocot stem feature, not a dicot.

MCQ · Casparian strip · Paper 1

What is the main function of the Casparian strip in the endodermis?

A. To increase the surface area of the root for water uptake
B. To force water into the symplast pathway, allowing the plant to control which solutes enter the xylem
C. To support the root mechanically against gravity
D. To actively pump mineral ions into the xylem

Answer: B — The Casparian strip is a band of waterproof suberin/lignin in the radial walls of endodermal cells. It blocks the apoplast pathway, forcing all water (and dissolved solutes) to cross at least one selectively permeable membrane to enter the xylem — giving the plant control over which solutes pass through.

MCQ · Xerophytes · Paper 1

Marram grass leaves are rolled into tubes with stomata on the inner surface. This adaptation reduces water loss because:

A. The rolled leaf prevents light from reaching the stomata, so they remain closed.
B. The rolled leaf traps moist air inside, raising local humidity and reducing the water vapour gradient.
C. The rolled leaf has a smaller total surface area for photosynthesis.
D. The rolled leaf prevents wind from moving air around the stomata.

Answer: B — The trapped moist air increases the humidity immediately around the stomata, which reduces the water vapour potential gradient between leaf interior and external air — less driving force for transpiration. (D) is partially true (air movement is also reduced) but the primary mechanism is the humidity effect.

Structured · Synoptic · Topic 2 + Topic 7 · Paper 2 · 8 marks

Water has unusual properties that are essential for its role in plant transport.

(a) Explain how cohesion and adhesion of water enable the ascent of sap up the xylem in tall trees. [4]
(b) Explain why xylem vessel elements have lignified cell walls. [2]
(c) Suggest why a tall tree continues to lose water by transpiration even when soil water is plentiful. [2]

(a) Cohesion and adhesion in xylem ascent [4 marks]

Acceptable points

Water evaporates from mesophyll cells / leaves → transpiration creates tension in the xylem column [1]
Cohesion: water molecules attract each other through hydrogen bonds, holding the water column together as a continuous stream / preventing it from breaking under tension [1]
Adhesion: water molecules form hydrogen bonds with cellulose in the xylem walls, helping water cling to the walls / contributing to capillary movement [1]
Tension is transmitted from the leaves down through the entire column to the roots, pulling water upwards over many metres [1]

(b) Why xylem walls are lignified [2 marks]

Acceptable points

Water in the xylem is under tension (negative pressure) due to transpiration pull [1]
The lignified walls are rigid and strong, preventing the xylem vessel from collapsing inwards under this tension — also provide structural support to the plant [1]

(c) Why transpiration continues even with abundant soil water [2 marks]

Acceptable points

Stomata must be open during the day to allow CO₂ in for photosynthesis — water loss is an inevitable consequence [1]
Transpiration also drives the movement of water and dissolved mineral ions up the xylem; the plant needs continuous water flow to deliver minerals to the leaves and to keep the leaves cool by evaporative heat loss [1]

Synoptic note: This question links Topic 2.4 (water properties — hydrogen bonding) with Topic 7 (transport). The same hydrogen bonding behind cohesion-tension also underlies water's high specific heat capacity and high latent heat of vaporisation, which protect the plant from temperature extremes.

Structured · Phloem loading · Paper 2 · 6 marks

Sucrose is loaded into phloem sieve tube elements at a source by an indirect mechanism that uses ATP. Explain how proton pumps and cotransporter proteins together achieve this active loading. [6]

Six creditable points (any six):

Acceptable points

The companion cell membrane contains proton pumps which use ATP to actively pump H⁺ ions out of the companion cell [1]
This creates a steep H⁺ concentration gradient across the membrane (high outside, low inside) [1]
The companion cell membrane also contains sucrose-H⁺ cotransporter proteins [1]
These cotransporters allow H⁺ to move back into the companion cell down its gradient, but only when sucrose binds and is brought in at the same time [1]
This loads sucrose into the companion cell against its concentration gradient — secondary active transport, powered indirectly by ATP via the H⁺ gradient [1]
From the companion cell, sucrose passes through plasmodesmata into the adjacent sieve tube element (passive, down its gradient) [1]
The increase in sucrose concentration in the sieve tube lowers its water potential, drawing water in by osmosis and raising hydrostatic pressure — driving mass flow [1]

Exam Prep

Topic 7 — Common Mistakes

🌿
Mixing up xylem/phloem position in stem vs root vs leafStem: ring of vascular bundles, phloem outer / xylem inner. Root: central xylem star with phloem in the gaps. Leaf vein: xylem upper / phloem lower. The orientation reverses in the leaf vs stem — very high-frequency exam point.
🧳
Saying water moves “up a water potential gradient”Water moves from higher to lower water potential, i.e. down the water potential gradient. The xylem column moves up through the plant body, but the water potential decreases from soil → xylem → leaf air — movement is always down the gradient. Don't conflate spatial direction with potential direction.
♻
Saying transpiration is positive pressure / pushing waterWrong. Transpiration creates negative pressure (tension) in the xylem. Water is pulled, not pushed. This is why the xylem walls must be lignified — to withstand tension without collapsing inwards.
🌿
Calling the Casparian strip a structure that “blocks water entry”The Casparian strip blocks the apoplast pathway specifically, forcing water to switch into the symplast pathway. Water still enters the xylem — it just must cross a selectively permeable membrane to do so. This is a control point, not a barrier.
✅
Saying xylem cells have many mitochondriaXylem vessel elements are dead at maturity — no mitochondria, no cytoplasm, no organelles. Many mitochondria belong to root hair cells (for active uptake of mineral ions) and to companion cells (for ATP to power proton pumps). Don't confuse these.
🧸
Treating xylem and phloem as “tubes” without distinguishing themXylem = dead, lignified, hollow, low-resistance, water under tension. Phloem = living sieve tube elements + companion cells, with sieve plates and pressure-driven mass flow. Be specific about the cell type and the driving force.
🤯
Saying ATP directly powers sucrose loadingLoading is secondary active transport. ATP directly powers the proton pump (H⁺ out). The H⁺ gradient then powers sucrose loading via the cotransporter. Saying “ATP loads sucrose” misses the mechanism — explain the two-step coupling.
🍃
Saying phloem flow is always upwards or always downwardsPhloem flows from source to sink, in any direction. A leaf source supplies upper sinks (developing fruit, shoot tips) by upward flow and lower sinks (roots, storage organs) by downward flow — both can happen simultaneously in different sieve tubes.
🍃
Treating “sources” and “sinks” as fixed identitiesThe same organ can be source or sink at different times. A potato tuber stores starch (sink) in summer; releases sucrose in spring (source) for the new shoot. A young leaf is a sink while expanding; a mature leaf is a source.
🏣
Saying xerophyte adaptations “eliminate” transpirationNo xerophyte stops transpiration completely — CO₂ uptake for photosynthesis requires open stomata at some point, and water loss accompanies this. Adaptations reduce transpiration. Phrasing matters in the mark scheme.
📉
Confusing potometer rate with transpiration rateThe potometer measures water uptake, not transpiration directly. They are approximately equal because most water taken up is lost by transpiration, but a small fraction is retained or used in photosynthesis. Mention this approximation in evaluation questions.

Topic 7 strategy

Topic 7 connects backwards to Topic 2 (water properties drive cohesion-tension), Topic 4 (osmosis, water potential, active transport), and Topic 6 (companion cells need ATP and proteins from gene expression). Forward links to Topic 9 (gas exchange — stomata) and Topic 13 (photosynthesis — the source of sucrose for translocation). Highest-yield items: TS anatomy of stem/root/leaf with xylem-phloem positions, structure-function of xylem vessels, sieve tubes, and companion cells, apoplast/symplast/Casparian strip pathway, the cohesion-tension theory with hydrogen bonding, the four standard transpiration-rate factors, xerophyte adaptations, and the proton-pump + cotransporter loading mechanism with mass flow. Practical 7 (potometer) is a high-frequency Paper 3 question.