IB Biology HLCoordinating Body SystemsPaper 1 & 2~9 min read
Regulating Plant Growth & Fruit Ripening
Leave one ripe banana in a bowl of green ones and within a couple of days the whole bowl has turned. That is not coincidence and it is not ripeness spreading by contact. It is a gas, and it is one of the very few examples of positive feedback you will meet in this course.
📚 What you need to know
Auxin causes cell elongation and is produced in the shoots. Cytokinin causes cell division and is produced in the roots.
Each must be transported to where it is not produced: cytokinin in the xylem (root to shoot), auxin in the phloem (shoot to root).
Their activities are complementary, and the interaction depends on concentration.
Ethylene (ethene under IUPAC naming) is a gas produced by fruit during the later stages of ripening.
It diffuses to adjacent fruit and triggers further ethylene release — a positive feedback loop.
In positive feedback the response makes the factor deviate even more, enhancing the original stimulus.
Integrating root and shoot growth
A plant has a problem of coordination. The shoots need to grow, and so do the roots, and the two have to stay in proportion — a huge shoot on a tiny root system would collapse or dry out. Two hormones, auxin and cytokinin, interact to ensure the integration of root and shoot growth.
The complication is that each is made at the wrong end of the plant for the other’s job:
Auxin is responsible for cell elongation and is produced in the shoots.
Cytokinin is responsible for cell division and is produced in the roots.
So both hormones have to be transported to the areas of the plant where they are not produced, and they use different tissues to get there:
Cytokinin travels in the xylem, because the direction of flow in the xylem is always root to shoot.
Auxin travels in the phloem sap, from shoot to root.
How the two interact
At certain concentrations the two hormones work together to ensure root and shoot growth is regulated. Their activities can be called complementary, because their signalling is integrated. The detail runs like this:
At low concentrations, auxin limits the action of cytokinin.
An increase in cytokinin counteracts that inhibitory effect, and leads to an inhibition of auxin signalling.
At higher concentrations of both hormones, these interactions between cytokinin and auxin are prevented.
Concentration is the variable. There is no single fixed relationship between auxin and cytokinin — which one dominates depends on how much of each is present. That is exactly what allows the balance between root and shoot growth to shift as the plant develops.
Positive feedback in fruit ripening
Almost every control mechanism you have met so far has been negative feedback: something drifts from normal, and the response pushes it back. Fruit ripening is the exception.
In positive feedback the original stimulus produces a response that causes the factor to deviate even more from the normal range. The response enhances the effect of the original stimulus rather than reversing it.
Negative feedback
Positive feedback
Effect of the response
Reverses the change, returning the factor to normal
Amplifies the change, pushing the factor further from normal
Result
A stable value is maintained
The process accelerates until something ends it
Example here
Ventilation rate returning to resting after exercise
Ethylene production during fruit ripening
Ethylene
Ethylene — named ethene by IUPAC — is a gas produced by fruit during the later stages of ripening. Because it is a gas, it does not need a transport tissue at all. It simply diffuses out of the fruit and into the air around it.
When that gas reaches an adjacent fruit, it triggers further release of ethylene from that fruit. That fruit now produces gas of its own, which reaches more fruit, which produce more gas. The loop feeds itself, and the effect is that all the fruit ripens at the same time.
🧠
Two names, one molecule
Biologists say ethylene; chemists and IUPAC say ethene. Same gas. If a question uses one name and you use the other, that is fine — but mentioning that they are the same shows the examiner you know the molecule rather than just the word.
Worked examples
WE 1
Explain the transport of two hormones
Explain why auxin and cytokinin must both be transported within a plant, and name the tissue each uses. (3 marks)
Step 1: where each is made
Auxin is produced in the shoots and cytokinin in the roots, but each is needed in regions where it is not produced.
Step 2: cytokinin
Cytokinin travels in the xylem, because xylem flow is always from root to shoot.
Step 3: auxin
Auxin travels in the phloem sap, from shoot to root.
Cytokinin up the xylem, auxin down the phloemthe xylem detail is worth stating as a reason, not just a fact — cytokinin can use it precisely because the flow only goes one way
WE 2
Explain why ripening is positive feedback
Explain why ethylene production in ripening fruit is an example of positive feedback, and contrast this with negative feedback. (4 marks)
Step 1: the original stimulus
A fruit in the later stages of ripening produces ethylene gas.
Step 2: the response
The gas diffuses to adjacent fruit, which begin to ripen and release ethylene themselves.
Step 3: why that makes it positive
The response increases the ethylene concentration further, so the factor deviates even more from its starting value. The response enhances the original stimulus.
Step 4: the contrast
In negative feedback the response reverses the change and returns the factor towards normal, as ventilation rate does after exercise. Here nothing is being returned to normal.
The response amplifies the change instead of reversing ita contrast question needs the other side stated explicitly. One sentence naming a negative feedback example secures the fourth mark
WE 3
Apply the mechanism to a practical problem
A supplier transports unripe bananas over several weeks. Suggest, with reasons, two things they could do to slow ripening during the journey. (3 marks)
Step 1: identify the mechanism to interrupt
Ripening accelerates because ethylene diffuses from fruit to fruit and triggers further release, so the loop has to be broken.
Step 2: first measureVentilate the storage space or remove the gas, so ethylene cannot accumulate around the fruit and the concentration reaching each one stays low.
Step 3: second measureSeparate the fruit, or remove any fruit that has already begun to ripen, so gas from one fruit does not reach its neighbours.
Prevent the gas accumulating, and prevent it reaching neighbouring fruit"suggest" means apply what you know to a new situation. Always link the suggestion back to the mechanism — a measure with no reason attached scores nothing
💡 Exam tips
Auxin: shoots, elongation, phloem. Cytokinin: roots, division, xylem. Learn the two sets as blocks.
Say complementary when describing the auxin and cytokinin relationship — it is the term used in the syllabus.
Define positive feedback as the response causing the factor to deviate even more. That exact idea is the mark.
Remember ethylene is a gas, so it moves by diffusion and needs no transport tissue.
Note that ethylene is produced in the later stages of ripening, not at the start.
⚠ Common mistakes
Swapping the transport tissues. Cytokinin uses xylem; auxin uses phloem.
Saying auxin causes cell division. That is cytokinin. Auxin causes elongation.
Describing positive feedback as "a good thing". Positive and negative describe the direction of the response, not whether it benefits the organism.
Saying ethylene is carried in the phloem. It is a gas and diffuses through the air.
Writing that ripening spreads by touch. The fruit do not need to be touching, only close enough for the gas to reach them.
Claiming auxin and cytokinin always oppose each other. The interaction depends on the concentration of both.
That completes Coordinating Body Systems. Look back at what actually connects the nine pages: a stimulus is detected by a receptor, information travels to a coordinator, and an effector produces a response. It was true for a hand pulling off a hot pan, for a heart speeding up during exercise, and for a shoot bending towards a window. Different organisms, different messengers, same logic — and if you can spot that pattern in an unfamiliar exam question, you can usually work out the answer from first principles.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
