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Chapter 7 plant nutrition

Photosynthesis is the process by which plants convert light energy, carbon dioxide, and water into glucose and oxygen. It occurs in two stages: the light-dependent stage where light energy is absorbed by chlorophyll to split water into hydrogen and oxygen, and the light-independent stage where carbon dioxide and water are converted into glucose using the chemical energy produced in the first stage. Photosynthesis takes place in the chloroplasts located in the palisade mesophyll cells of leaves. The glucose produced can then be used by the plant for energy, converted into other organic compounds, or transported to other parts of the plant for storage.

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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 + Water Sunlight Chlorophyll
Light-dependent or light stage Light-independent or dark stage light energy chemical energy H2O12 photolysis of water O2 6 + 24 H water oxygen gas hydrogen atoms CO2 6 H2O6 watercarbon dioxide enzyme-controlled reactions C6H12O6 glucose + Overall equation of photosynthesis CO2 6 + H2O12 light energy chlorophyll C6H12O6 + O2 6 H2O6+ Or as a word equation carbon dioxide + water glucose + oxygen + water light energy chlorophyll Equations for Photosynthesis August 14, 2013Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 6
Where does Photosynthesis take place? In green leaves In 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 Bundle Made 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 mesophyll Cells 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 epidermis A single layer of cells which are transparent to allow sunlight to penetrate to mesophyll. No stoma present, minimising water loss Spongy mesophyll Cells 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 cells No 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 cells No 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 August 14, 2013Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 23 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 chlorophyll Light-independent stage • carbon dioxide glucosechemical energy water occurs in two stages light energy Photosynthesis Factors 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 August 14, 2013Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 27