Plant Requirements
A plant builds its entire body, wood, leaves, fruit, the lot, almost out of thin air. The carbon in a giant gum tree came mostly from carbon dioxide gas, not the soil. But "out of thin air" is not the whole story: plants also need water, light, mineral nutrients and oxygen, and they need them for two different jobs running at once. This lesson sorts out exactly what a plant takes in, and why.
Orient yourself
Predict, scan the goals and key terms, and clear the misconception this lesson exists to fix.
Before You Read
warm-upA gardener says: "Plants only need sunlight and water, that's why they're so easy."
Before reading: list everything you think a plant actually takes in from its environment, and write which of those it needs for photosynthesis and which for respiration.
Learning Intentions
goalsKnow
- The gases plants need for photosynthesis and respiration
- Why plants need water and what they use it for
- The key mineral nutrients plants require and why
Understand
- Why a plant needs both CO₂ and O₂, not just one
- That photosynthesis and respiration run together in a plant
- Why mineral nutrients are needed even though they are not in the photosynthesis equation
Can Do
- Describe the nutrient, water and gas requirements of plants
- Match each requirement to photosynthesis or respiration
- Explain a deficiency symptom in terms of a missing nutrient
Scan these before reading
vocabMisconceptions To Fix
watch outTwo truths, one lie: Three of these statements about plant requirements are true. Which one is the lie?
Gas Requirements
Why a plant needs both carbon dioxide and oxygen
Core Content
Gas Requirements
+5 XPWhy a plant needs both carbon dioxide and oxygen
A plant runs two opposite gas-using processes, photosynthesis and respiration, so it requires both carbon dioxide and oxygen, just at different times and places.
For photosynthesis a plant requires carbon dioxide (CO₂), the raw material from which glucose is built. For respiration, which occurs in every cell, day and night, a plant requires oxygen (O₂) to release energy from glucose. Gases enter and leave mainly through the stomata of the leaf, and through the surfaces of the roots.
During daylight, photosynthesis is faster than respiration, so the leaf takes in CO₂ overall and releases O₂. In the dark, only respiration occurs, so the plant takes in O₂ and releases CO₂, exactly like an animal. The roots, which cannot photosynthesise, always need a supply of oxygen, which is why waterlogged soil (low in O₂) drowns roots.
Pause, copy the highlighted gas requirements into your book, noting which gas pairs with which process.
A plant requires the gas _____ for cellular respiration to release energy from glucose.
Water Requirements
More than a raw material for photosynthesis
Water Requirements
+5 XPMore than a raw material for photosynthesis
We just covered the gases. That raises a question: what does the third great input, water, actually do? This card answers it → it is a reactant, a transporter, a coolant and a support.
Water is needed for far more than photosynthesis: it carries dissolved substances, keeps cells firm, and is itself a reactant.
A plant requires water (absorbed by the roots) as a reactant in photosynthesis, as the medium that dissolves and transports mineral nutrients and sugars, to keep cells turgid (firm) for support, and for transpirational cooling.
Text description
- Leaves: light energy and CO₂ in; O₂ out; water vapour out (transpiration).
- Roots, in the soil: water and mineral nutrients in; O₂ in, because root cells respire.
Pause, copy the highlighted four uses of water into your book.
Water is a reactant in photosynthesis as well as the medium that transports dissolved minerals.
Plant roots do not need oxygen because the leaves produce it during photosynthesis.
Carbon dioxide is the raw material from which glucose is built in photosynthesis.
Sort the Requirements by Process
Do the activity in your book, then check what you have understood.
Activity 1
Sort the Requirements by Process
Pattern A, Structured comparison
In your book, draw a two-column table headed Used in photosynthesis and Used in respiration. Sort these inputs into the correct column(s): carbon dioxide, oxygen, water, light energy, glucose, mineral nutrients. Note that some belong in both. Then write one sentence explaining why water appears as a need even though it is not "consumed" the way CO₂ is.
Match each requirement to what the plant uses it for:
- Nitrogen (as nitrate)
- Magnesium
- Oxygen
- Potassium
- Building the chlorophyll molecule
- Helping regulate the opening of stomata
- Building amino acids, proteins and DNA
- Releasing energy from glucose in respiration
Mineral Nutrient Requirements
The ions that don't appear in the equations but are still essential
Mineral Nutrient Requirements
+5 XPThe ions that don't appear in the equations but are still essential
We just covered gases and water. That raises a question: if a plant makes glucose from CO₂ and water, why does it still need anything from the soil? This card answers it → to build everything glucose alone cannot.
Glucose supplies carbon, hydrogen and oxygen, but a plant must build proteins, DNA and chlorophyll too, and these need extra elements that come from soil minerals.
A plant requires mineral nutrients (inorganic ions) absorbed from the soil. Nitrogen (as nitrate) is needed to make amino acids, proteins and DNA; magnesium is needed to make chlorophyll; phosphorus is needed for DNA and ATP; and potassium helps regulate the opening of stomata.
These minerals do not appear in the photosynthesis or respiration equations, but without them the plant cannot build the machinery that runs those processes. A magnesium-deficient plant, for example, cannot make enough chlorophyll and its leaves turn yellow (chlorosis), reducing photosynthesis.
Add the highlighted mineral nutrients and their uses to your notes before the check below.
Which mineral nutrient is required by a plant to make chlorophyll?
Why fertiliser bags carry three numbers
Where this biology is doing real work outside the classroom.
Why fertiliser bags carry three numbers
Every bag of garden fertiliser is stamped with an "N-P-K" ratio, the proportions of nitrogen, phosphorus and potassium it supplies. Those three numbers exist because farmers and gardeners are topping up exactly the mineral nutrients a plant cannot make for itself. Hydroponic growers take this further, feeding plants a precisely balanced mineral solution with no soil at all, proof that a plant's requirements really do reduce to gases, water, light and a defined set of dissolved ions. Understanding plant requirements is the foundation of all crop science and food security.
Describe the nutrient, water and gas requirements of plants for photosynthesis and respiration. For each requirement, identify the process or processes it serves.
Describe asks for the features and characteristics of each requirement. The second sentence adds a condition — identify the process it serves — so an unlabelled list, however complete, cannot reach full marks.
6 marks across three categories. Work through gases, water and mineral nutrients in turn, and tag each one to photosynthesis, respiration, or both, as you go.
A plant requires carbon dioxide for photosynthesis, where it is the raw material from which the carbon in glucose is built. It also requires oxygen for cellular respiration, which occurs in every living plant cell day and night to release energy from glucose. Gases enter and leave mainly through the stomata of the leaf, and also across the surfaces of the roots.
The two requirements coexist because the two processes run together. In daylight photosynthesis is faster than respiration, so a leaf takes in carbon dioxide and releases oxygen overall. In darkness only respiration occurs, so the plant takes in oxygen and releases carbon dioxide, just as an animal does. Roots cannot photosynthesise at all and so always need an oxygen supply, which is why waterlogged, oxygen-poor soil drowns them.
A plant requires water, absorbed from the soil by the roots, for four distinct purposes: it is a reactant in photosynthesis; it is the medium in which mineral nutrients and sugars dissolve and are transported; it keeps cells turgid and so provides support to soft tissues; and its evaporation from the leaves provides transpirational cooling.
A plant requires mineral nutrients as inorganic ions absorbed from the soil. Nitrogen, taken up as nitrate, is needed to build amino acids, proteins and DNA; magnesium is needed to build chlorophyll; phosphorus is needed for DNA and ATP; and potassium helps regulate the opening and closing of the stomata.
These ions appear in neither the photosynthesis nor the respiration equation, yet both processes depend on them, because glucose supplies only carbon, hydrogen and oxygen while the plant must also build the proteins, enzymes, nucleic acids and pigments that run those reactions. A magnesium-deficient plant cannot make enough chlorophyll, so its leaves yellow and its rate of photosynthesis falls, even though magnesium is not a reactant.
A plant requires carbon dioxide for photosynthesis, as the raw material from which the carbon in glucose is built, and oxygen for cellular respiration, which occurs in every living cell day and night to release energy from glucose; both gases enter and leave mainly through the stomata of the leaf and across the root surfaces. Both are required because the two processes run together: in daylight photosynthesis exceeds respiration so the leaf takes in carbon dioxide and releases oxygen overall, while in darkness only respiration occurs so the plant takes in oxygen and releases carbon dioxide, and the roots, which cannot photosynthesise, always need oxygen — which is why waterlogged soil drowns them. A plant requires water, absorbed by the roots, for four purposes: as a reactant in photosynthesis, as the medium in which mineral nutrients and sugars dissolve and are transported, to keep cells turgid and so support soft tissues, and for transpirational cooling. It also requires mineral nutrients as inorganic ions from the soil: nitrogen as nitrate to build amino acids, proteins and DNA; magnesium to build chlorophyll; phosphorus for DNA and ATP; and potassium to help regulate the opening of the stomata. These ions appear in neither equation, but both processes depend on them, because glucose supplies only carbon, hydrogen and oxygen while the plant must also build the proteins, enzymes, nucleic acids and pigments that run the reactions — a magnesium-deficient plant cannot make enough chlorophyll, so its leaves yellow and its rate of photosynthesis falls even though magnesium is never a reactant.
A hydroponic grower supplies a crop with light, warmth, water and dissolved nitrate, phosphate and potassium, but omits magnesium. After three weeks the older leaves are pale yellow and growth has slowed. Account for this result.
Account for means give the reasons for an observed result — a causal chain running from the change made to the outcome seen. It is not enough to name magnesium; you must follow the consequence through to both symptoms.
5 marks and two symptoms to explain: the yellowing, and the slowed growth. Build one chain that reaches both.
Magnesium is a mineral nutrient required to build chlorophyll, the green pigment in the chloroplasts that absorbs light energy for photosynthesis. Because the grower supplies every other requirement, magnesium is the only input in short supply.
Without a magnesium supply the plant cannot synthesise enough chlorophyll. Since chlorophyll is what makes leaves green, leaf tissue lacking it appears pale yellow, a deficiency symptom called chlorosis. It shows first in the older leaves because the plant remobilises the magnesium it already has, moving it out of mature leaves to supply the new growth.
Chlorophyll is also the site at which light energy is absorbed, so less chlorophyll means less light energy captured and therefore a lower rate of photosynthesis. Less photosynthesis means less glucose produced.
Glucose is both the substrate respired to release the energy the plant needs and the raw material from which new cellulose, proteins and other structures are built, so a reduced glucose supply slows respiration and growth together. Growth has therefore slowed for the same reason the leaves are yellow: an ion that appears in neither the photosynthesis nor the respiration equation is nonetheless required to build the machinery both processes run on.
Magnesium is the mineral nutrient required to build chlorophyll, the pigment in the chloroplasts that absorbs light energy for photosynthesis, and since the grower supplies every other requirement it is the only input in short supply. Without it the crop cannot synthesise enough chlorophyll, so the leaf tissue lacking the green pigment appears pale yellow — the deficiency symptom known as chlorosis. It appears first in the older leaves because the plant remobilises the magnesium it already holds, withdrawing it from mature leaves to supply new growth. Because chlorophyll is also the site at which light energy is absorbed, less chlorophyll means less light energy captured and therefore a lower rate of photosynthesis, so less glucose is made. Glucose is both the substrate respired to release energy and the raw material from which new cellulose, proteins and other structures are built, so a reduced supply slows respiration and growth together. The slowed growth and the yellowing are therefore two consequences of one shortage, and the case shows that an ion appearing in neither the photosynthesis nor the respiration equation can still be an absolute requirement, because it builds the machinery on which both processes depend.
Practise independently
Answer in your own words first. The model answers are for checking, not for copying.
Multiple Choice
+5 XPA fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
Short Answer, 3 questions · 14 marks
+5 XPUnderstandBand 5(5 marks) 1. Describe the nutrient, water and gas requirements of plants for photosynthesis and respiration. For each requirement, identify the process it serves.
1 mark: CO₂ for photosynthesis · 1 mark: O₂ for respiration · 1 mark: water (reactant in photosynthesis + transport/support) · 1 mark: a named mineral nutrient with its use · 1 mark: links each input correctly to photosynthesis and/or respiration
UnderstandBand 4(4 marks) 2. A plant is kept in continuous darkness for 48 hours. Describe how the gases the plant requires, and the gases it exchanges with the air, differ between this period and a bright sunny day.
1 mark: in darkness only respiration occurs, so the plant takes in O₂ and releases CO₂ · 1 mark: in bright light photosynthesis also occurs and exceeds respiration · 1 mark: states the net daytime exchange is CO₂ in and O₂ out · 1 mark: makes clear respiration continues in the light, so both gases are required at all times
AnalyseBand 5–6(5 marks) 3. A hydroponic grower supplies a crop with light, warmth, water and dissolved nitrate, phosphate and potassium, but forgets magnesium. After three weeks the older leaves are pale yellow and growth has slowed. Analyse this result by relating the missing nutrient to the plant's requirements for photosynthesis and respiration.
1 mark: identifies magnesium as required to build chlorophyll · 1 mark: less chlorophyll means less light energy absorbed · 1 mark: therefore the rate of photosynthesis falls · 1 mark: less glucose is made, so less substrate is available for respiration and growth · 1 mark: explains the yellowing (chlorosis) as the visible symptom of missing chlorophyll
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Answer
Q1 (5 marks): For photosynthesis, a plant requires carbon dioxide (the raw material from which glucose is built) and water (a reactant, and the source of hydrogen), along with light energy. For respiration, which occurs in every cell day and night, a plant requires oxygen to release energy from glucose. Water is required for both processes and additionally to transport dissolved minerals and sugars and to keep cells turgid for support. A plant also requires mineral nutrients absorbed from the soil, such as nitrogen (as nitrate) to make amino acids and proteins, magnesium to make chlorophyll, and phosphorus for DNA and ATP. These minerals do not appear in the photosynthesis or respiration equations but are essential for building the molecules that carry out both processes.
Q2 (4 marks): In continuous darkness the plant carries out respiration only. It therefore requires oxygen, which it takes in from the air, and releases carbon dioxide, exactly as an animal does. In bright light the plant carries out both respiration and photosynthesis, and photosynthesis runs faster than respiration, so the net exchange reverses: carbon dioxide is taken in and oxygen is released. The important point is that respiration does not stop in the light, it continues in every cell day and night, so the plant requires both gases at all times. What changes between the two conditions is not the requirement but which process dominates, and therefore which way the net exchange runs. Roots, which never photosynthesise, require oxygen from the soil at all times.
Q3 (5 marks): Magnesium is required because it sits at the centre of the chlorophyll molecule. Without it the plant cannot build enough chlorophyll, and chlorophyll is the pigment that absorbs light energy, so less light energy is captured and the rate of photosynthesis falls. Less photosynthesis means less glucose, and glucose is both the plant's building material and the substrate for respiration, so the plant has less energy available and growth slows, which is exactly what was observed. The pale yellow colour, chlorosis, is the visible symptom: the green pigment is missing and the other leaf pigments show through. This example shows why mineral nutrients belong in a list of plant requirements even though they appear nowhere in the photosynthesis or respiration equations, they build the machinery those equations depend on. The symptom appears in the older leaves first because the plant moves its scarce magnesium to new growth.
Retrieve and reflect
Use the spaced-review session, then compare how your thinking has changed since your opening prediction.
Check what actually stuck
A short retrieval session mixing this lesson with earlier ones, so the ideas move into long-term memory instead of fading by the exam.
Lock it in
bossYou can now describe every raw material a plant takes in and match each to photosynthesis or respiration. Next lesson zooms in on photosynthesis itself: the conditions it needs to run.
↩ Back to focus-area hubHow did your thinking change?
At the start the gardener said plants only need sunlight and water. In fact a plant requires carbon dioxide (for photosynthesis), oxygen (for respiration), water (reactant, transport and support), light energy, and a set of mineral nutrients from the soil. The gardener missed CO₂, oxygen and minerals entirely.
If you assumed plants don't need oxygen, revisit the misconception box: every plant cell respires all the time and needs O₂, roots especially.