IB Biology HLTransport in Animals & PlantsPaper 1 & 2~12 min read
Translocation in Plants
Xylem only ever goes one way: up. Phloem is the clever one. It carries sugar from wherever the plant is making it to wherever the plant is spending it, and because those places change with the seasons, the direction of flow changes too. Same tube, opposite journeys.
📚 What you need to know
Translocation is the transport of organic solutes in the phloem.
The liquid inside is phloem sap: mostly sucrose, plus water and other dissolved substances such as amino acids. Together these are called assimilates.
Solutes travel from a source (where they are made or released) to a sink (where they are used or stored).
The same organ can be a source at one time of year and a sink at another.
Phloem is made of sieve tube elements joined end to end through perforated sieve plates, each supported by a companion cell.
Sucrose is actively loaded at the source using ATP, water follows by osmosis, and the resulting high hydrostatic pressure pushes the sap towards the sink.
This is mass flow – the whole solution moves together, not just the solute.
Sources and sinks
A source is anywhere organic solutes enter the phloem. A sink is anywhere they leave it. Everything else about translocation follows from those two definitions.
Sources
Sinks
Mature green leaves and green stems, where photosynthesis makes glucose that is converted to sucrose
Meristems, where cells are actively dividing
Storage organs such as tubers and tap roots, unloading their stores at the start of a growth period
Growing roots, and roots actively taking up mineral ions
Food stores in germinating seeds
Young leaves still in bud
Anywhere solutes are being stored: developing seeds, fruits, storage organs
A source can become a sink, and back again. A potato tuber is a sink in summer, filling with starch made by the leaves. In spring it is a source, breaking that starch down to sucrose and exporting it to the new shoots. Nothing about the tuber has changed except which way the sugar is moving.
Xylem could never do this. Water only ever travels from root to leaf; phloem sap goes wherever it is needed.
How phloem is built
Phloem is a complex tissue, meaning it contains more than one type of cell. The two you need are the sieve tube elements that carry the sap and the companion cells that keep them working.
Sieve tube elements
Feature
Why it helps
Cells joined end to end into a continuous tube
Sap can flow from one cell straight into the next without crossing full end walls
Perforated sieve plates between cells
Holes let assimilates pass through while the plate still supports the tube
No nucleus, vacuole or ribosomes in mature cells
Maximises the space available for sap
Thin layer of cytoplasm, few organelles
Reduces friction so sap moves freely
Cellulose cell wall
Strong enough to withstand the hydrostatic pressures that drive the flow
Companion cells
Feature
Why it helps
Nucleus and full set of organelles
Provides metabolic support for the sieve tube element, which has lost its own
Many mitochondria
Produce the ATP needed for active loading and unloading
Transport proteins in the plasma membrane
Move assimilates into and out of the sieve tube
Plasmodesmata linking the two cells
Cytoplasmic bridges that let assimilates pass into the sieve tube
A sieve tube element has thrown away almost everything a cell normally needs, purely to leave room for sap. It only survives because the companion cell next door runs its metabolism for it. If a question asks why companion cells have so many mitochondria, the answer is active loading needs ATP – not “for respiration”.
Mass flow: the mechanism
The five steps in order
Active transport loads sucrose from the source into the phloem, using ATP from the companion cells.
The high solute concentration lowers the water potential of the sieve tube, so water enters by osmosis, mostly from the neighbouring xylem.
This raises the hydrostatic pressure at the source. Since pressure at the sink is lower, a pressure gradient exists and the whole contents of the phloem flow along it.
At the sink, sucrose is unloaded into the surrounding cells, lowering their water potential.
Water follows the sucrose out of the phloem by osmosis, which keeps the pressure at the sink low and so maintains the gradient.
Notice that step 5 is not an afterthought. If water did not leave at the sink, the pressure there would rise, the gradient would disappear and the flow would stop.
🧠
Push, don’t pull
Xylem works by pulling (tension from the leaves). Phloem works by pushing (pressure from the source). One is passive and one costs ATP – and that is the cleanest way to tell them apart in an exam.
Xylem and phloem side by side
Xylem
Phloem
Carries
Water and mineral ions
Sucrose, amino acids and other assimilates
Direction
Roots to leaves only
Source to sink, up or down
Cells
Dead, hollow, no contents
Living sieve tubes with companion cells
Walls
Lignified
Cellulose, no lignin
Driven by
Transpiration pull – tension
Hydrostatic pressure gradient – positive pressure
Energy
Passive; no ATP used
Active loading and unloading need ATP
Worked examples
WE 1
Explaining mass flow
Explain how sucrose loaded at a source causes phloem sap to move towards a sink. (4 marks)
Step 1: loading
Sucrose is actively transported into the sieve tube, using ATP produced by the companion cells.
Step 2: osmosis
The solute concentration rises so the water potential of the sieve tube falls, and water enters from the xylem by osmosis.
Step 3: pressure
The extra water raises the hydrostatic pressure at the source above that at the sink.
Step 4: flow
Sap moves by mass flow down this pressure gradient; unloading at the sink, followed by water leaving, keeps the gradient in place.
Load sucrose → water in → high pressure → mass flow to the sinkthe marks are in the sequence – write it as a chain and each link picks one up
WE 2
Calculating the rate of translocation
A radioactive tracer fed to a leaf was detected 84 cm further down the stem after 20 minutes. Calculate the rate of translocation in cm min−1 and in m h−1, and comment on the result. (4 marks)
Step 1: rate per minute84 cm ÷ 20 min = 4.2 cm min−1Step 2: convert to metres per hour
4.2 × 60 = 252 cm h−1 = 2.52 m h−1Step 3: comment
This is far faster than diffusion could achieve over such a distance, which supports the idea that the sap is being moved in bulk by a pressure gradient rather than diffusing.
4.2 cm min−1, which is 2.52 m h−1watch the units the question asks for, and keep the same number of significant figures as the data
WE 3
Interpreting a ringing experiment
A complete ring of bark, including the phloem, was removed from the trunk of a young tree. Several weeks later the trunk had swollen just above the ring, and the leaves were still healthy. Explain these observations. (3 marks)
Point 1: what was cut
Removing the bark removes the phloem but not the xylem, which lies deeper in the trunk.
Point 2: the swelling
Sucrose travelling down from the leaves cannot pass the gap, so assimilates accumulate above the ring and the tissue there swells.
Point 3: the healthy leaves
Water and mineral ions still reach the leaves through the intact xylem, so transpiration and photosynthesis continue in the short term.
Phloem cut, xylem intact – sugar piles up above the ringa strong answer adds that the roots will eventually starve, since they are a sink cut off from their source
💡 Exam tips
Say sucrose, not “sugar” or “glucose”. Glucose is converted to sucrose before transport.
Always name the direction as source to sink, never “up” or “down” – it can be either.
Use the phrase hydrostatic pressure gradient; “pressure difference” often loses the mark.
Whenever ATP appears in your answer, say what it is for: active loading and unloading.
Structure and function questions want a pair every time: the feature, then the reason.
If a question gives you a distance and a time, it wants a rate – show the division and the units.
⚠ Common mistakes
Saying translocation moves water. Water moves too, but translocation is defined by the organic solutes.
Saying phloem transport is diffusion. It is mass flow, driven by pressure, and the loading step is active.
Saying phloem only carries sugar downwards. Young leaves and fruits are sinks above the source.
Confusing sieve tube elements with companion cells. The sieve tube carries the sap; the companion cell supplies the ATP.
Writing that sieve tubes are dead. They are living, unlike mature xylem vessels.
Forgetting the sink end. Unloading and water leaving are what keep the gradient going.
That completes Transport in Animals & Plants. Look back and the whole topic is one question asked twice: how do you move something over a distance too great for diffusion? An animal builds a pump and a closed loop of vessels. A plant has no pump at all, so it uses evaporation to pull water up and a sugar gradient to push sap along – two very different answers to exactly the same problem.
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