Chapter 7:Nutrition in Plants

Do Plants Eat?• Like humans, plants need food for energy.

• Where does this food come from?

• It is through the food- making process of Photosynthesis

What is Photosynthesis?

• Photosynthesis is the process in which light energy is absorbed by chlorophyll and converted into chemical energy. The chemical energy is then used to synthesis carbohydrates from carbon dioxide and water. Oxygen is released in the process.

• The raw materials needed: (1) Carbon Dioxide : enters leaves by diffusion, via the stomata

found on the underside of the leaves.(2) Water: enters the plant through the roots and transported to leaves via xylem vessel

Equation of Photosynthesis

Carbon dioxide + Water Glucose + Oxygen + WaterSunlight

Chlorophyll

Light-dependent or light stage

Light-independent or dark stage

light energy chemical energy

H2O12photolysis

of waterO2

6 + 24 H

water oxygen gas hydrogen atoms

CO26 H2O6

watercarbon dioxide

enzyme-controlled reactionsC6H12O6

glucose

+

Overall equation of photosynthesis

CO26 + H2O12

light energy

chlorophyllC6H12O6 + O2

6 H2O6+

Or as a word equation

carbon dioxide + water glucose + oxygen + waterlight energy

chlorophyll

Equations for Photosynthesis

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Where does Photosynthesis take place?

In green leavesIn the mesophyll layers of each leaf

In the cytoplasm of the mesophyll cells

In an organelle called the chloroplast, which contains the green pigment chlorophyll

The Structures involved in Photosynthesis

• CHLOROPHYLL- green pigment that traps solar energy for photosynthesis

• CHLOROPLAST- organelle containing chlorophyll. Chloroplasts are the site of photosynthesis in plants

Internal Structure of the Leaf

(a) Upper Epidermis

(b) Palisade Mesophyll

(c) Spongy Mesophyll

(d) Lower Epidermis

Stoma which is made up of two guard cells

(e)Vascular Bundle

A) Upper epidermis layer – contains no chloroplasts allows sunlight to

pass through easily to the palisade mesophyll layer

– covered with a layer of cuticle reduces evaporation of water

– protects inner layer of cells

B) Palisade mesophyll layer– Main site of photosynthesis contains most number of chloroplasts– Palisade mesophyll cells are long and cylindrical– Cells are packed very closely to one another– During photosynthesis, oxygen will be produced

C) Spongy mesophyll layer– also contains some chloroplasts but main function is not for photosynthesis – main site of gaseous exchange – cells are loosely packed with intercellular air spaces – water will evaporate from surface of spongy mesophyll cells into the air spaces

water vapour in the air spaces will move out of the stomata into surrounding air by diffusion

Gaseous Exchange of water

Diagram showing movement of water out of the leaf

TRANSPIRATION the process where water vapour move out of stomata of leaves

Diagram showing movement of carbon dioxide

Gaseous exchange of Carbon dioxide

During photosynthesis, carbon dioxide from surrounding air will enter the stomata by diffusion into the intercellular air spaces it will then enter the spongy mesophyll cells

Gaseous exchange of Oxygen

Diagram showing movement of oxygen

The opposite happens for oxygen during photosynthesis from spongy mesophyll cells out of the stomata into the surrounding air

D) Lower epidermis layer – same as the upper epidermis EXCEPT that it also contains stomata (small pores) – stomata are always surrounded by two cells called the guard cells – guard cells contain chloroplasts for photosynthesis to occur– guard cells control the stomata, which in turn controls the amount of gases entering and leaving the leaf

(E) Vascular BundleMade up of the:

1) Xylem vessels (always on the top)Function: transport water from the roots to the

palisade mesophyll cells for photosynthesis to take place

- water in the palisade mesophyll cells will ALSO move to the spongy mesophyll cells to escape into the surrounding air

2) Phloem ( always on the bottom) Function: transports food made during photosynthesis

from the palisade mesophyll cells to other parts of the plant

(E) Vascular Bundle

Structure of a Dicotyledonous Leaf

The leaf lamina is very thin. It is made up of only a few layers of cells

Palisade mesophyllCells densely packed together like a ‘fence’ to maximise exposure to sunlight passing through epidermis. Cells contain highest concentration of chloroplasts; main site of photosynthesis

Upper epidermisA single layer of cells which are transparent to allow sunlight to penetrate to mesophyll. No stoma present, minimising water loss

Spongy mesophyllCells more loosely packed, air spaces present to allow penetration of air from stomata to upper layer for gaseous exchange

Lower epidermis‘Pores’ for gaseous exchange present. The pores are called stomata, the opening of each stoma controlled by a pair of guard cellsNo layer of cuticle

The leaf is modified for its main function, which is photosynthesis

Structure of a Dicotyledonous Leaf

Lower epidermis‘Pores’ for gaseous exchange present. The pores are called stomata, the opening of each stoma controlled by a pair of guard cellsNo layer of cuticle

One stoma

Guard cells

Air movement

Surface view

The lower epidermis- Stomata for Gaseous Exchange

What Happens to the Manufactured ‘Food’?

Glucose produced by photosynthesis in leaf is converted to sugars (mainly sucrose) and translocated to different parts of the plant

To growing regions to be used as energy for growth

To storage organ (fruit)to be stored mainly as sugars

To storage organ (tubers in roots)To be stored mainly as starch

Transport in the phloem occurs in both directions up and down the plant (bidirectional movement)

If the plant requires more energy than can be produced, food stores are mobilised, converted back to sugars, and transported to wherever it is needed

The movement of sugars and amino acids via the phloem is called translocation

Glucose

In photosynthesis, carbon dioxide and water form

Used immediately by plant cells• for cellular respiration• to form cellulose cell walls.

1 Converted into sucrose or into starch in leaves• Excess glucose is converted into sucrose and transported to storage organs as starch or in other forms. • In daylight, excess glucose is converted into starch for temporary storage in the leaf. It may be converted into glucose at night or into sucrose and transported away for storage.

2

Used to form amino acids and proteins• Glucose reacts with nitrates to form amino acids in the leaf. • The amino acids are combined to form proteins, which make up new protoplasm in the leaf.• Excess amino acids are transported away for synthesis of new protoplasm or for storage as proteins.

3

Used to form fats• for storage• used in cellular respiration• for synthesis of new protoplasm

4

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Fates of Glucose Formed During Photosynthesis

Factors limiting Photosynthesis• Carbon dioxide concentration

– Carbon dioxide is one of the raw materials needed for photosynthesis. Therefore if there is absence or lack of carbon dioxide, a plant will not be able to photosynthesize. The normal atmospheric concentration is 0.03%.

• Light intensity– Without enough light a plant cannot photosynthesize very fast,

even if there is abundance of water and carbon dioxide. Increasing the light intensity will increase the rate of photosynthesis.

• Temperature – The reactions in photosynthesis are enzyme-dependent. At too

high a temperature they are denatured; at too low a temperature the reactions progress very slowly.

Graphs showing the effect of these three factors on the rate of photosynthesis

Importance of photosynthesis

1) Photosynthesis makes chemical energy available to animals

- Light energy is converted into chemical energy and stored in food.- Carbohydrates produced during photosynthesis is a source of food for all living

organisms

2) Photosynthesis removes carbon dioxide and provides oxygen

- Oxygen is produced for respiration and removes carbon dioxide from the air. - Acts as a purification process.

Light-dependent stage

• light energy chemical energy

• water hydrogen and oxygen

chlorophyllLight-independent stage

• carbon dioxide glucosechemical energy

water

occurs in two stages

light energy

PhotosynthesisFactors affecting photosynthesis• carbon dioxide• sunlight• temperature• water• chlorophyll

Importance of photosynthesis• provides food for animals• stores energy from the sun as chemical energy• maintains the balance oxygen and carbon dioxide in the atmosphere

Fates of glucose

Broken down to release energy for vital activities

Used in synthesis of cell wall

Converted into amino acids and proteins

Converted into fats

Excess stored temporarily as starch in leaves

Glucose

produces

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