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2.5Structures and Functions in Living Organisms

Nutrition — Edexcel Linear - OL Revision Notes & Topical Questions

20 min read 28 parts Lesson 20 of 25 Unit quiz

📚 Edexcel Linear - OL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

Photosynthesis and the Factors that Limit its Rate — EDEXCEL LINEAR - OL notes: Write the equation for photosynthesis, describe the uses of glucose and interpret how temperature, light and carbon dioxide limit the rate.Photosynthesis is the process in which energy from sunlight i…

  • chlorophyll
  • carbon dioxide and water
  • limiting factor
  • Worked example 1
  • Worked example 2
  • Worked example 3

Part 1 of 6The process of photosynthesis

Photosynthesis is the process in which energy from sunlight is transferred to the chloroplasts of green plants.

1Chlorophyll
The light is absorbed by chlorophyll, the green pigment in chloroplasts.
2Carbon dioxide and water
Green plants use this energy to make the carbohydrate glucose from the raw materials carbon dioxide and water,
  • and oxygen is released as a waste product.
3
Photosynthesis can be defined as the process by which plants manufacture carbohydrates from raw materials using energy from light.
4
Plants are producers, the first organisms in all food chains.
5
If asked for the raw materials, the answer is carbon dioxide and water; light is energy, not a substance.

Word equation

  • carbon dioxide + water → glucose + oxygen (in light, with chlorophyll).
  • Symbol equation: 6CO2 + 6H2O → C6H12O6 + 6O2.
  • This is the exact reverse of aerobic respiration, so learning one gives you the other.

Part 2 of 6Uses of the glucose made

Plants use glucose as a source of energy in respiration.

Uses of the glucose made
They can also use it to produce starch for storage, to synthesise lipids as an energy source in seeds, to form cellulose for cell walls,
  • and to produce amino acids (for proteins) when combined with nitrogen and other mineral ions absorbed by the roots.
Once the carbon from carbon dioxide has been converted into glucose it can be used to build many biological molecules.

Part 3 of 6Limiting factors

A limiting factor is something present in the environment in such short supply that it restricts a life process; the rate of photosynthesis is limited by whichever factor is lowest.

Limiting factors
The three main factors are temperature, light intensity and carbon dioxide concentration.
Water is not considered a limiting factor because the amount needed is small compared with the amount lost by transpiration.
The number of chloroplasts or the amount of chlorophyll also affects the rate.
FactorEffect on the rate
TemperatureRaises kinetic energy and collisions between reactants and enzymes, so the rate increases up to the optimum; above it the enzymes are denatured and the rate falls
Light intensityMore light gives a faster rate until another factor becomes limiting
Carbon dioxideA raw material; more gives a faster rate until another factor becomes limiting
Chlorophyll / chloroplastsMore means faster; reduced by disease (tobacco mosaic virus), lack of magnesium, loss of leaves and variegated leaves

Part 4 of 6Reading limiting factor graphs

Where the line is rising, the limiting factor is the one on the x-axis.

Reading limiting factor graphs
Where the line is horizontal, the limiting factor is something else: temperature, light intensity or carbon dioxide concentration (not the one on the axis).
For example, on a light intensity graph, the plateau means light is no longer limiting and carbon dioxide or temperature is.
Describe the graph then explain it.

Part 5 of 6Photosynthesis and respiration compared

FeaturePhotosynthesisAerobic respiration
WhereChloroplastsMitochondria (and cytoplasm)
EnergyAbsorbs light energyReleases energy
ReactantsCarbon dioxide and waterGlucose and oxygen
ProductsGlucose and oxygenCarbon dioxide and water
WhenIn lightAll the time

Photosynthesis therefore takes carbon dioxide out of the atmosphere and adds oxygen, whereas respiration does the reverse, and the two processes keep the atmosphere balanced.

Part 6 of 6Worked examples

  1. State the word equation for photosynthesis. [2]2 marks

  2. Explain why the rate of photosynthesis levels off as light intensity increases. [3]3 marks

  3. Why does the rate fall above about 45 °C? [2]2 marks

  4. Name two uses of glucose in a plant. [2]2 marks

  5. Explain why water is rarely a limiting factor. [1]1 mark

Key terms

TermMeaning
PhotosynthesisProcess by which plants make glucose from CO2 and water using light energy.
ChlorophyllGreen pigment in chloroplasts that absorbs light.
Limiting factorA factor in short supply that restricts a process.
ProducerAn organism that makes its own food, forming the start of a food chain.
Raw materialA substance used up in a reaction.

Avoid these mistakes

1
Listing light as a raw material.
2
Saying plants photosynthesise only in the day and respire only at night.
3
Saying temperature always increases the rate — enzymes are denatured at high temperatures.
4
Reading the wrong factor as limiting on the flat part of a graph.

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