Plant Nutrition

  • Nutrition is utilization of food to grow, repair and maintain our bodies or getting the right amount of nutrients from healthy foods in the right combinations.
  • In other words we can say it is the way that living things obtain their food.
  • The chemical substances that provide nourishment to living organisms are called nutrients.
  • Depending on the mode of nutrition the organisms are classified as autotrophs and heterotrophs.

Autotrophs nutrition

The process by which organisms make their own food is called autotroph. Green plants obtain their food by utilizing carbondioxide as their sole source of carbon for the formation of organic food by the process of photosynthesis


Heterotrophs nutrition         

Organisms which are incapable of photosynthesizing, obtain certain organic compounds from other autotrophs and they are called heterotrophs and this type of nutrition is referred to as heterotrophic nutrition. Different types of heterotrophic nutrition are as follows:

  1. Parasitic Holozoic nutrition: The process by which organisms live in or on other living organisms and feed from them as well as causing harm to the living organisms. e.g Cuscuta (Dodder)
  2. Saprotrophic nutrition: Saprophytes get their nutrition from dead and decaying organic matter.e.g. fungi
  3. Holozoic nutrition: a method of nutrition that involves the ingestion of liquid or solid organic material, digestion, absorption and assimilation of it to utilize it.  e.g. Amoeba

External structure of a plant


Mid rib / veins: helps in transport of matter, ions and food

Leaf stalk: all leaves receive maximum amount of sunlight without over lapping or over shadowing each other.

Thin broad lamina: increase surface area of the leaf for maximum absorption of sunlight and CO2.

Stomata: this enables the leaf to have efficient gaseous exchange.

Internal structure of a leaf


Palisade mesophyll: Consists of closely packed, long cylindrical palisade cells arranged at right angle to the epidermis to allow maximum light absorption. These cells contain large no of chloroplast containing chlorophyll to absorb maximum sunlight. So these are main photosynthesis cells.

Spongy mesophyll: Contains loosely packed cell with fewer chloroplasts. There are many air spaces in this layer which provide an easy passage for gases to diffuse through the stoma.

Lower epidermis: From the lower epidermal boundary to the leaf. This cell do not contain chloroplast except for the guard cells

Upper epidermis: Secretes the waxy cuticle. The cells in the upper epidermis contain no chloroplasts allowing light to reach the mesophyll layers.

Guard cell: Controls the size of the stomata.

Stomata: The stomata are openings surrounded by the guard cells. Stomata allows gaseous exchange to take place, the size of the stoma is controlled by the turgidity of the guard cells.


Photosynthesis is the process by which plants make carbohydrates from raw materials, using energy from light. During photosynthesis:

  • light energy is absorbed by chlorophyll – a green substance found in chloroplasts in green plant cells and algae
  • absorbed light energy is used to convert carbon dioxide (from the air) and water (from the soil) into a sugar called glucose
  • oxygen is released as a by-product

The following equations summaries what happens in photosynthesis:


Some glucose is used for respiration, while some is converted into insoluble starch for storage. The stored starch can later be turned back into glucose and used in respiration.


Investigation on photosynthesis


Investigations to see if chlorophyll, light and CO2 are needed for photosynthesis.

  1. Chlorophyll is necessary for photosynthesis


  • Take a potted plant withvariegated (green and white) leaves.
  • Destarchthe plant by keeping it in complete darkness for about 48 hours.
  • Expose the plant to thesunlight for a few days.

Test one of the leaves for starch with iodine solution.



Areas with previously green patches test positive (turn blue black). And areas with previously pale yellow patches test negative (remain brown).



Photosynthesis takes place only in green patches because of the presence of chlorophyll. The pale yellow patches do not perform photosynthesis because of the absence of chlorophyll.


  1. Light is essential for photosynthesis


  • Take a potted plant.
  • Destarch the plant by keeping it in complete darkness for about 48 hours.
  • Test one of it leaves for starch, to check that is does not contain any.
  • Fix a leaf of this plant in between two strips of a thick paper on leaf.
  • Place the plant in light for a few days.
  • Remove the cover from the leaf and test it for starch.



Positive starch test will be obtained only in the portion of the leaf exposed to light and negative test in parts with paper strip.


Light is necessary for photosynthesis.


  1. Carbon dioxide is essential for photosynthesis


  • Take two destarched potted plants.
  • Cover both the plants with bell jars and label them as A and B.
  • Inside Set-up A, keep NaHCO3(sodium bicarbonate). It produces CO2.
  • Inside Set-up B, keepNaOH (Sodium hydroxide). It absorbs CO2.
  • Keep both the set-ups in the sunlight at least for 6 hours.
  • Perform the starch test on both of the plants.



Leaf from the plant in which NaHCO3 has been placed gives positive test. Leaf from the plant in which NaOH has been kept gives negative test.

Conclusion: Plant in Set up A gets COwhereas plant in Set-up B does not get CO2.It means COis must for photosynthesis.

Factors affecting photosynthesis

Rate of photosynthesis is how many molecules of product are formed in a given period of time. Three factors can limit the rate of photosynthesis: light intensity, carbondioxide concentration and temperature.

If any of these factors is in short supply, the rate of photosynthesis will be less than its possible rate.

Light intensity

Without enough light, a plant cannot photosynthesis very quickly – even if there is plenty of water and carbon dioxide. Increasing the light intensity will boost the rate of photosynthesis.

At the certain point future increase in light intensity can no longer increase the rate of photosynthesis. This point is the light saturation point. After reaching this point however much the light intensity increased, the rate of photosynthesis will not increase because now CO2 is in short supply and become the limiting factor.

Before reaching the light saturation point, light is the limiting factor. At very high light intensity the rate of photosynthesis slows down as excessive amount of ultraviolet rays can damage chlorophyll molecules.


Carbon dioxide concentration

Even if there is plenty of light, a plant cannot photosynthesize if there is insufficient carbon dioxide. The more CO2 plant is given the faster it can photosynthesis up to CO2 saturation point. Up to this point CO2 is the limiting factor. Beyond this point increasing the CO2 concentration dose not speed up the rate of photosynthesis because light is in short supply and become the limiting factor.



If it gets too cold, the rate of photosynthesis will decrease. Plants cannot photosynthesis if it gets too hot. The chemical reactions of photosynthesis are enzyme controlled reactions. Therefore temperature has an effect on the rate of photosynthesis. When temperature is increased the rate of photosynthesis also increase until it reaches the optimum temperature, beyond the optimum rate of photosynthesis slows down as the enzyme gradually become denatured.

Importance of mineral ions

Plants need minerals for healthy growth. They are absorbed through the roots by active transport as mineral ions dissolved in the soil water.

Magnesium and nitrate

Magnesium ions and nitrate ions are needed by plants. A plant will not grow well if it cannot get enough of these ions, and it will show symptoms of mineral deficiency.

Mineral ions


Needed for Effects of deficiency
Magnesium Making chlorophyll Leaves turn yellow
Nitrate Making amino acids Stunted growth


Nitrogen fertilizers

Fertilizers are used to replace minerals used by plants.

Nitrogen fertilizers – such as ammonium nitrate and ammonium phosphate – provide plants with water-soluble sources of nitrogen that they can absorb through their roots. They allow farmers to increase the yield and quality of their crops.

However, overuse of fertilizers can cause problems. Eutrophication happens when excess nitrate enters rivers or lakes from fields. This can lead to the death of fish and other aquatic animals.