BIOLOGY, SCIENCE, NUTRITION, PLANT

Notes on IGCSE Biology Part 2E- Plant Nutrition

2.18 understand the process of photosynthesis and its importance in the conversion of light energy to chemical energy

2.19 know the word equation and the balanced chemical symbol equation for photosynthesis

2.20 understand how varying carbon dioxide concentration, light intensity and temperature affect the rate of photosynthesis

2.21 describe the structure of the leaf and explain how it is adapted for photosynthesis

2.22 understand that plants require mineral ions for growth, and that magnesium ions are needed for chlorophyll and nitrate ions are needed for amino acids

2.23 practical: investigate photosynthesis, showing the evolution of oxygen from a water plant, the production of starch and the requirements of light, carbon dioxide and chlorophyll

 

Photosynthesis

  • Simple organic molecules, carbon dioxide and water, make glucose and oxygen in the presence of chlorophyll and light

  • Chemical energy in the product, glucose, comes from the light which is trapped by photosynthesis in the chlorophyll (converts light energy to chemical energy)

 

Equations

 

Starch production

  • A plant does not make starch directly.

  • Glucose is produced first from photosynthesis and joined together to form starch molecules

 

Photosynthesis and respiration

  • Concentration of carbon dioxide in the air around a plant changes throughout a day

  • There is the lower carbon dioxide levels when photosynthesis occurs in the day, and lowest when photosynthesis peaks in the afternoon

    • Because carbon dioxide absorbed and used for photosynthesis by the plants

  • At night carbon dioxide levels rise due to no photosynthesis

    • Less carbon dioxide absorbed by plants

    • Carbon dioxide added to air from respiration of all organisms

 

Effect of different conditions on photosynthesis

Light intensity

  • Increasing light intensity increases the photosynthesis rate

  • Until another factor is in short supply and becomes the limiting factor

  • Without sufficient light, the plant cannot photosynthesise regardless of how much carbon dioxide and water there is

 

Carbon dioxide concentration

  • Increasing CO2 concentration increases photosynthesis rate as it is one of the products

  • Atmosphere has a set CO2 concentration, but levels can be adjusted in glasshouses (discussed in our Food Production article)

 

Temperature

  • Enzymes control the chemical reactions in photosynthesis

  • As temperature increases, enzyme rate of reaction increases as particles move faster and collide with more energy (activation energy reached more easily)

  • However, once the optimum temperature of the enzyme is reached, they begin to denature

    • Rate of reaction drops

Enzymes are discussed in more detail in the article on Biological Molecules

 

The Leaf

There are layers of cells in the leaf which have different functions.

 

Upper and lower epidermis

  • Has few chloroplasts (not used for photosynthesis)

  • Covered by a thin layer of a waxy material called the waxy cuticle

    • Reduces water loss by evaporation

    • Acts as a barrier to the entry of pathogens (disease-causing microorganisms) such as bacteria and fungi

 

Stomata

  • The lower epidermis has many holes or pores called stomata. The upper epidermis has few or no stomata.

  • Allows carbon dioxide to diffuse into the leaf and reach the photosynthetic tissues (to be used in photosynthesis)

  • Allows oxygen (waste product) and water vapour (for transpiration stream) to diffuse out

  • Each stoma is formed as a gap between two specialised cells called guard cells

    • Guard cells change their shape to open or close stomata by absorbing water and becoming turgid, or decreasing water content and becoming flaccid

 

Palisade mesophyll

  • Tissue made of long, narrow cells

  • Cells contain hundreds of chloroplasts - main site of photosynthesis

  • Located close to the source of light under the relatively transparent upper epidermis layer so light can pass through to the chloroplasts

 

Spongy mesophyll

  • Has fewer chloroplasts than the palisade cells

  • Tissue is made oof round and loosely packed cells with many air spaces - main gas exchange surface in the leaf

  • Air spaces allow gases to diffuse in and out of the mesophyll cells

  • Carbon dioxide is absorbed, oxygen and water vapour are released

 

Xylem

  • Tissue that supplies the leaf through vessels with water and mineral ions

  • Part of a continuous transport system throughout the plant

  • Water absorbed by the roots passes up through the stem into the leaves by the transpiration stream.

    • Water leaves the xylem and supplies the cells in the leaf

 

Phloem

  • Tissue that carries away products of photosynthesis like sugars (glucose)

  • Supplies the products to other tissues and organs of the plant

 

Stomata, xylem and phloem are discussed in more detail in our article on transport in plants

 

Plant Minerals

Mineral Use
Potassium Allows stomata to function, enzyme activator
Phosphorus Component of DNA genetic code
Magnesium Chlorophyll
Nitrate Make amino acids (for proteins)
updated_at 17-07-2020