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IGCSE Biology 0610 · Chapter 5 notes · ~11 min read

Plant nutrition

How plants make their own food: the photosynthesis equation, what glucose is used for, leaf structure, investigating light, chlorophyll and CO₂, limiting factors — and the minerals plants need.

PhotosynthesisLimiting factorsLeaf structureMineral ions
ASby Aiza Ma'am
mentorriyaz.in
No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end

In 30 seconds

  • Photosynthesis: plants make carbohydrates (glucose) from carbon dioxide and water, using light energy absorbed by chlorophyll in chloroplasts. Oxygen is released.
  • Word equation: carbon dioxide + water → glucose + oxygen. EXT Balanced: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
  • The leaf is adapted: broad and thin, palisade cells full of chloroplasts, stomata for gas exchange, air spaces, and veins.
  • Rate is limited by light, carbon dioxide or temperature. Plants need nitrate (for protein) and magnesium (for chlorophyll).

By the end you can

  • Define photosynthesis and write the word equation.
  • Write the balanced symbol equation. EXT
  • Describe the roles of chlorophyll, and the uses of the carbohydrate made.
  • Identify leaf tissues and explain how the leaf is adapted.
  • Describe the tests and controls for light, chlorophyll and carbon dioxide.
  • Explain the effect of light, CO₂ and temperature on the rate, including limiting factors. EXT
  • State the roles of nitrate and magnesium ions.

1Photosynthesis

What is photosynthesis?

PHOTOSYNTHESIScarbon dioxide + waterglucose + oxygenlight energy(needs chlorophyll)6CO₂ + 6H₂OC₆H₁₂O₆ + 6O₂light, chlorophyllraw materialsproductsCO₂ from the air(through stomata)water from the soil(through xylem)glucose: stored as starch,used for respiration,made into cellulose…oxygen: leaves through stomataLight energy is stored as chemical energy in glucose
Photosynthesis is the reaction that makes glucose from carbon dioxide and water, using light energy.

Photosynthesis — the process by which plants manufacture carbohydrates from raw materials using energy from light.

carbon dioxide + water → glucose + oxygen
word equation; light energy and chlorophyll are needed
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
EXT balanced symbol equation (check: 6 C, 12 H and 18 O on each side)

Chlorophyll — the green pigment in chloroplasts. It absorbs light energy and transfers it to chemical energy used to make carbohydrates.

  • The raw materials are carbon dioxide (from the air, through the stomata) and water (from the soil, through the roots and xylem).
  • The products are glucose and oxygen. Light energy is stored as chemical energy in the glucose.
  • Photosynthesis happens in chloroplasts. Most are in the palisade mesophyll cells at the top of the leaf.
sunlightCO₂ in(via stomata)O₂ outwater (+ ions)up the xylemfrom the rootsglucose → sucrosetravels down thephloemLEAF: chlorophyll inchloroplasts absorbs lightPhotosynthesis happens mainly in the leaves
Where each substance comes from and goes to.
Common mistake"Plants photosynthesise in the day and respire at night." Plants respire all the time and photosynthesise only when there is light.
Common mistake"Plants take in CO₂ and give out O₂, animals do the opposite, so plants do not respire." Plants respire too (see Gas exchange, respiration & excretion).

What does the plant do with glucose?

RESPIRATIONrelease energySTARCHinsoluble storeCELLULOSEcell wallsPROTEINSwith nitrate ionsFATS AND OILSenergy store (seeds)SUCROSEphloem transport; nectarGLUCOSEfrom photo-synthesisPlants cannot move food, so they convert glucose into whatever they need
Glucose is converted into other substances as the plant needs them.
  • Respiration — to release energy for all the cell's processes.
  • Starch — an insoluble energy store (in leaves, roots, seeds, tubers). Being insoluble, it does not affect the cell's water potential.
  • Cellulose — to build cell walls.
  • Sucrose — soluble, so it can be transported in the phloem. It is also found in nectar, which attracts insects.
  • Fats and oils — energy stores (especially in seeds). Proteins — made using nitrate ions (glucose + nitrate → amino acids).
Exam tipRemember: starch is stored, sucrose is transported. Starch is insoluble, sucrose is soluble.

2The leaf

Structure and function

lightwaxy cuticleupper epidermispalisade mesophyll(many chloroplasts)spongy mesophyll(air spaces)xylem + phloem(vein)lower epidermisguard cellstomaair spaces let CO₂ diffuse to the cells; stomata control gas exchange
A section through a leaf. The arrangement of the tissues is the key to how a leaf works.
PartFunction
waxy cuticlewaterproof layer: reduces water loss
upper epidermistransparent single layer: lets light through
palisade mesophylltall cells packed with chloroplasts: most photosynthesis happens here
spongy mesophyllloosely packed cells with air spaces so CO₂ and O₂ can diffuse
lower epidermis with stomata and guard cellsstomata let CO₂ in and O₂ and water vapour out; guard cells open and close them
xylem (in the veins)brings water and mineral ions to the leaf
phloem (in the veins)carries sucrose (and amino acids) away

How the leaf is adapted

  • Broad and flat — a large surface area to absorb light.
  • Thin — a short distance for gases to diffuse and light to reach all cells.
  • Palisade cells at the top — nearest to the light, packed with chloroplasts.
  • Air spaces in the spongy layer — carbon dioxide diffuses to the cells quickly.
  • Stomata — let gases in and out. Guard cells open them in the light and close them to save water.
  • Veins — supply water and carry sugars away. They also support the leaf.
Exam tipExam answers need feature + advantage: "thin leaf, so a short diffusion distance for CO₂."

3Investigating photosynthesis

Testing a leaf for starch

1 BOIL2 ETHANOL3 RINSE4 IODINEkills the leaf andstops reactions(leaf: green)hot ethanol removeschlorophyll(leaf turns white)softens the brittleleaf(leaf: white)blue-black = starchbrown = no starch(drops of iodine)Safety: never heat ethanol with a flame — use a hot water bath
Four steps. Starch is evidence that photosynthesis has been happening.
  • 1 Boil the leaf in water for about a minute: this kills it and stops reactions.
  • 2 Put the leaf in hot ethanol in a water bath (flame off) to remove the chlorophyll.
  • 3 Rinse in warm water to soften the leaf.
  • 4 Add iodine solution: blue-black where there is starch; orange-brown where there is none.

Light, chlorophyll and carbon dioxide

LIGHTleaf with foil stripCHLOROPHYLLvariegated leafCARBON DIOXIDEleaf with NaOH / soda limeNaOHnormal aircovered strip: brown(no starch)uncovered: blue-blackwhite (no chlorophyll):brown, no starchgreen: blue-blackno CO₂: no starch(brown)with CO₂: blue-blacklight isNEEDEDchlorophyll isNEEDEDCO₂ isNEEDED
Three experiments. In each, a control shows that only one factor is different.
  • Destarch the plant first: keep it in the dark for 24–48 hours so that any starch present at the end was made during the experiment.
  • Light: cover part of a leaf with foil. Only the uncovered part makes starch.
  • Chlorophyll: use a variegated leaf (green and white). Only the green parts make starch.
  • Carbon dioxide: put one plant with soda lime or sodium hydroxide (absorbs CO₂) and a control with normal air. Only the control makes starch.

Measuring the rate of photosynthesis

distance dgas collects here(oxygen bubbles)pondweed (Elodea)under a funnelwater + NaHCO₃(source of CO₂)move the lamp to change the light intensitycount bubbles per minute at each distance · keep the temperature constant
Count the oxygen bubbles released by pondweed at different distances from a lamp.
  • The number of bubbles per minute is the rate of photosynthesis (oxygen is released).
  • Independent variable: distance of the lamp (changes the light intensity). Other variables to keep the same: temperature, pondweed, and CO₂ (add sodium hydrogencarbonate).
  • A water bath or heat shield stops the lamp from warming the water.
Worked example

Bubbles counted in three one-minute periods at one distance: 38, 41 and 39. Calculate the mean rate.

  1. total = 38 + 41 + 39 = 118
  2. mean = 118 ÷ 3 = 39.3
  3. about 39 bubbles per minute
Hydrogencarbonate indicatorhigh CO₂(more respiration)air level(no change)low CO₂(photosynthesis)Alightleaf in lightBfoilleaf in dark (foil)Ccontrolno leaf (control)Photosynthesis uses CO₂ (→ purple) · respiration releases CO₂ (→ yellow)
Hydrogencarbonate indicator changes colour with the carbon dioxide concentration.
  • Yellow = more CO₂ (more respiration). Red/orange = air level. Purple = less CO₂ (photosynthesis using it up).
  • A leaf in light takes in CO₂ (purple). A leaf in the dark only respires, giving out CO₂ (yellow). The tube without a leaf is the control.
Worked example

A leaf in a tube of hydrogencarbonate indicator is kept in the dark. The indicator turns yellow. Explain why.

  1. there is no light, so no photosynthesis
  2. the leaf still respires and releases CO₂
  3. the CO₂ makes the solution more acidic, so the indicator turns yellow

Limiting factors

light intensityrateABCO₂ concentrationrateABtemperaturerateACA: more light = faster(light is limiting)B: flat — CO₂ ortemperature limitsA: more CO₂ = faster(CO₂ is limiting)B: flat — light ortemperature limitsA: warmer = fasterC: too hot — enzymesdenature, so rate fallsRate of photosynthesis and its limiting factors
Rate against light intensity, CO₂ concentration and temperature.

Limiting factor — the factor in shortest supply that sets the maximum rate of the process.

  • Light intensity: rate increases with light until another factor limits it, then levels off.
  • Carbon dioxide concentration: same pattern. CO₂ is only about 0.04% of the air, so it is often limiting.
  • Temperature: rate increases up to about 30–40 °C. Above that the enzymes are denatured and the rate falls.
  • The flat part of a graph means some other factor is limiting, not the one on the x-axis.
light intensityrate of photosynthesishigh CO₂low CO₂rising part:LIGHT is thelimiting factorflat blue part: CO₂ (or temperature)is now limiting — more lightmakes no differenceAdd more CO₂ and the rate can rise again — until something else limits it
EXT With more CO₂ the rate rises again until another factor limits it.
  • At night: light is limiting. On a cold, bright day: temperature is limiting. On a bright, warm day: CO₂ may be limiting.
  • Growers in greenhouses add light, heat and CO₂ (for example paraffin heaters) to get more growth.
Worked example

A greenhouse is bright and warm, but the tomato plants grow slowly. Suggest a likely limiting factor and a solution.

  1. light and temperature are not limiting
  2. carbon dioxide is only about 0.04% of the air, so it may be limiting
  3. add carbon dioxide (for example a paraffin heater or a CO₂ generator) to raise the rate
Common mistake"A flat line means the factor on the x-axis has stopped working." The plant is limited by another factor that is in short supply.
Exam tipGraph question? Say which factor is limiting: rising part → the x-axis factor. Flat part → a different factor.

4Mineral ions

Nitrate and magnesium

NITRATE (NO₃⁻)used to makeAMINO ACIDS → PROTEINS(for growth)if lacking:stunted growthyellow older leavesMAGNESIUM (Mg²⁺)used to makeCHLOROPHYLL(to absorb light)if lacking:yellow leaves(poor photosynthesis)healthyyellow (deficient)healthyyellow (deficient)both are absorbed from the soil as ions, by active transport in root hairs
Two mineral ions, their jobs and what goes wrong without them.
  • Nitrate (NO₃⁻) — needed to make amino acids and then proteins. A lack gives stunted growth and yellow older leaves.
  • Magnesium (Mg²⁺) — needed to make chlorophyll. A lack gives yellow leaves (chlorosis), so less photosynthesis.
  • Roots take up mineral ions from the soil by active transport. Farmers add fertilisers to replace them.
Common mistake"Plants get their food from the soil." Plants make food by photosynthesis. Soil provides water and mineral ions.

5Test yourself

Test yourself — cover the answers first

Quick check

  1. Name the reactants in photosynthesis.
  2. Name the green pigment and where it is found.
  3. Which leaf cells contain most chloroplasts?
  4. Why is it important that a plant is destarched before a starch test experiment?
  5. What do guard cells do?
  6. Name two uses of the glucose made in photosynthesis.
  7. What does a plant need nitrate ions for?
  8. EXT Write the balanced symbol equation for photosynthesis.

Exam-style

  1. Explain how a leaf is adapted for photosynthesis. [4]
  2. A leaf is kept in the dark for 48 hours, then half of it is covered with foil and the plant is put in light for a day. Describe and explain the result of the starch test. [3]
  3. EXT At low light intensity, increasing the CO₂ concentration does not increase the rate of photosynthesis. Explain why. [2]
  4. Describe the symptoms of magnesium deficiency and explain them. [2]
Show answers
  1. carbon dioxide and water
  2. chlorophyll in chloroplasts
  3. palisade mesophyll cells
  4. so that any starch found must have been made during the experiment
  5. open and close the stomata
  6. respiration, starch store, cellulose, sucrose (any two)
  7. to make amino acids/proteins
  8. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
  9. thin and broad: large surface area and short diffusion path
    palisade cells packed with chloroplasts at the top
    stomata and air spaces let CO₂ in
    veins bring water
  10. the covered half is orange-brown, the uncovered half is blue-black
    light is needed for photosynthesis
    no light, so no starch is made in the covered part
  11. light is the limiting factor
    so CO₂ is not in short supply
  12. yellow leaves
    magnesium is needed to make chlorophyll
Practise this chapter
PhotosynthesisLeaf structureMineral requirements
Next chapterHuman nutrition →
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