plant nutrition
book: cambridge o level, biology, hodder education
chapter: 6
pages: 78-97
Photosynthesis:
• the process by which plants make carbohydrates from raw materials using light energy.
• when complicated food molecules combine to make simpler molecules, the process is called synthesis.
all living organism's require food to build new cells and tissues as they grow and as a source of energy, as food is a kind of fuel for vital living processes.
plants too require food, however their source of raw materials is most likely the soil, but the weight gained by the plants is more than the weight lost by the soil, which means they have other sources of raw materials.
a theory is that plant uses the air, water and soil minerals to make its food. we know that the product of photosynthesis is glucose and oxygen.
glucose = C⁶ H¹² O⁶. which means it has elements like carbon, hydrogen and oxygen. the carbon and the oxygen could be supplied by the air, while the hydrogen is supplied by the water in the soil. nitrogen and sulfur, which are needed to make proteins may be supplied by the soil as well in the form of nitrates and sulfates.
word equation of photosynthesis:
carbon dioxide + water (light energy/chlorophyll) glucose + oxygen
chemical equation of photosynthesis:
6CO² + 6H²O (light energy/chlorophyll) C⁶ H¹² O⁶ + 6O²
Process Of Photosynthesis:
while this theory is not the same for all plants, it is widely accepted.
photosynthesis takes place mainly in the leaves.
the water is absorbed by the roots of the plants, carried in the xylem vessels, up to the stem and then the leaf, the stomata absorbs the carbon dioxide from the air. the green pigment in the leaves absorbs sunlight and with that energy the chemical reaction of combining the water and carbon dioxide takes place in which the water molecules are split into oxygen and hydrogen molecules, the oxygen escapes and the hydrogen combines with the carbon dioxide molecules to form glucose.
Uses Of The Products:
• Glucose: used for respiration, quickly changed into sucrose which is transported around the plant.
• Starch: unused glucose is turned to starch and stored, the starch molecules are added to the starch granules in the chloroplast, this is because if the glucose concentration increased, it could affect the osmotic balance and as starch is insoluble it does not alter the concentration. some plants store starch grains in the cells of their stems or roots, and some plants such as the potato, have special storage organs called tubers to hold starch reserves.
• Sucrose: the starch is broken down to soluble sucrose, which is transported to the phloem (food carrying cells), the sucrose is transported to all the parts of the plant that do not photosynthesis, e.g: the growing buds, fruits etc.
• Cellulose: are a long chain of glucose, the cell wall of plants is made of cellulose,
• Respiration: glucose is used to provide energy in the process of respiration.
Gaseous Exchange In Plants:
air contains nitrogen, oxygen, carbon dioxide and water vapour. the taking in or releasing of these gases is called gas exchange.
in the daylight, the plants take in carbon dioxide and give out oxygen (opposite of respiration), however plants respire as well. so in the daytime, they are actually respiring as well as photosynthesizing, so the carbon dioxide produced by respiration is being used for photosynthesis and the oxygen needed for respiration is supplied by photosynthesis, however when the rate of photosynthesis is higher than the rate of respiration, the excess oxygen will be given out while the carbon dioxide is taken in.
Effects Of External Factors On The Rate Of Photosynthesis:
• Limiting Factor: something present in the environment in such short supply that it limits the life process.
• Light Intensity: increase in light intensity, results in the increase of photosynthesis, however to an extent. after that extent, the increasing light intensity will not affect photosynthesis. one reason for this could be that the chlorophyll are fully engaged in light absorption or because any other reactant has become a limiting factor.
• Temperature: increase in temperature, will result in an increase in the rate of photosynthesis as the enzymes will speed up but this will also be an extent, as the enzymes will soon denature or any other factor has become a limiting factor, and the increased temperature will not affect photosynthesis anymore.
• Carbon Dioxide: increase in carbon dioxide results in increased photosynthesis, which will again be to an extent as at some point, another factor may become the limiting factor.
Leaf Structure: refer to a labeled picture of a leaf.
functions:
veins: bring water and minerals ions to the leaf cells and carry away the food made by them, they also support the softer tissues.
leaf stalk: supports the leaf
lamina: keeps the leaf in a position where it can absorb maximum sunlight.
midrib: distributes water and food to the veins.
adaptations:
• the thin layer of cells makes diffusion easier.
refer to a picture of a leaf structure under a microscope
functions:
cuticle (waxy layer): prevents the cells/water from drying out.
upper epidermis: protects the cells
mesophyll cells: cells that do most of the photosynthesis.
spongy layer: lots of air spaces.
xylem vessels: transport water and minerals.
phloem vessels: transports sugars and ammino acids away (translocation).
lower epidermis: protects the cells
stomata: a tiny whole that lets oxygen and carbon dioxide get out of and into the leaf.
guard cells: control the opening and closing of the stomata.
Mineral Nutrition: plants require nitrate ions to make ammino acids, which is used to make proteins, which is used to make the enzymes and cytoplasm of the cell. (absorbed by the soil, through the roots.)
plants also require magnesium to make chlorophyll. (absorbed by the soil, through the roots.)
- without these minerals the plant would have a stunted growth and yellowish leaves.
test yourself:
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