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BIOLOGY · Stage 9 · Chapter 9: Plants and Photosynthesis

Biology 9.1 What do we know about plants? — Plant Organs and Biomass

5 min read 6 parts Lesson 1 of 9

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Lesson summary

9.1 What do we know about plants?

  • Other uses
  • Plant science
  • Common features
  • Palisade cells
  • Root hair cells
  • Guard cells

Section 1 of 6Why plants matter

  • Food
    • plants make carbohydrates using energy from the Sun and carbon dioxide from the atmosphere.
    • They are the producers in food chains and webs all over the world.
    • Crops are the source of most human food, both directly and as animal feed.
  • Other uses
    • plants are part of the water cycle and give materials for medicines, clothing, building and biofuels.
  • Plant science
    • biologists study plant structure, breeding and genetic material, nutrients, pests and diseases to improve crops.

Section 2 of 6Organs of a flowering plant

  • A flowering plant has a root system and a shoot system, and each organ is made of tissues, which are groups of similar cells.
  • Roots
    • they anchor the plant and absorb water and mineral ions through root hairs, which give a large surface area.
  • Stem
    • it supports the leaves, flowers and fruit and holds them in the light.
    • It contains xylem and phloem that carry water and sugars.
  • Leaves
    • they are the main organs of photosynthesis, using chlorophyll to capture energy from the Sun.
    • They are flat and thin with a large surface area, and stomata let gases in and out.
  • Flowers
    • they are the organs of sexual reproduction, producing pollen and ovules and later forming fruits and seeds.
    • They appear only at certain times in the life cycle.
OrganMain function
RootAnchorage; absorption of water and minerals
StemSupport; transport
LeafPhotosynthesis; gas exchange
FlowerReproduction

Section 3 of 6Plant cells and their features

  • Common features
    • plant cells have a cell wall of cellulose, a large permanent vacuole and chloroplasts in green parts.
  • Palisade cells
    • they have many chloroplasts and are packed at the top of the leaf to absorb light.
  • Root hair cells
    • they have no chloroplasts but a long extension that increases surface area for absorption.
  • Guard cells
    • they open and close the stomata.

Section 4 of 6Biomass

1Biomass
= the mass of living material, such as a plant, an animal or an ecosystem.
2Gaining biomass3 points
  • plants build carbon from the air into sugars by photosynthesis, then convert them to starch, cellulose, proteins and fats.
  • Most of the dry mass of a plant comes from carbon dioxide in the air, not from the soil.
  • Water from the soil supplies hydrogen and oxygen atoms.
3Measuring biomass2 points
  • use dry mass, found by heating the plant gently in an oven until the mass no longer changes.
  • Fresh mass varies with water content, so it is not reliable.

Section 5 of 6Investigating plant growth

  • Methods
    • measure height or leaf area every few days, or dry the plants and find their mass at the end.
  • Van Helmont's willow experiment (1600s)
    • a 2.3 kg willow grew in 90 kg of soil for five years and reached 76 kg while the soil mass hardly changed.
    • He concluded that the plant came from water.
    • We now know the main source of the extra mass is carbon dioxide.

Section 6 of 6Crop yields

  • Trend
    • each crop produces more now than in 1961, and the yield of some crops, such as maize and wheat, has more than doubled.
  • Causes
    • the extra food results from scientists learning more about plants, along with the crop plant, farming methods, fertilisers and climate.

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