Back to: O level Biology NOTES Uganda syllabus
Both respiration and photosynthesis take place in a green plant.
Photosynthesis equation:
In darkness, Green plants do not photosynthesize however they continue to respire.
Here oxygen is used up (through respiration) and carbon dioxide given off and there is an overall net consumption of sugars and starch during respiration.
At low light intensity, some photosynthesis occurs and some carbon dioxide produced in respiration by plants is used up in photosynthesis. However, there is a net loss of Carbon dioxide.
As the light intensity increases, the rate of photosynthesis also increases until a point is reached when all the Carbon dioxide produced during the process of respiration is reused in the process of photosynthesis. This point is called the compensation point.
The compensation point is that point of light intensity at which the rate of Carbon dioxide produced by respiration is equal to the amount of Carbon dioxide consumed during photosynthesis.
At the compensation point, the rate of photosynthesis is equal to the rate of respiration ie the rate at which food (starch) is manufactured is equal to the rate at
which it is used up in the process of respiration and this means that there is no net gain or loss in the mass of the plant.
IMPORTANCE OF PHOTOSYNTHESIS
Photosynthesis is the method by which food is made from simple inorganic materials.
(i) Photosynthesis helps to purify the environment by removing excess Carbon dioxide from the atmosphere which is a pollutant.
(ii) During the photosynthesis process, oxygen is released back into the atmosphere and it is very vital in the respiration process of most organisms.
(iii) It provides energy. This energy is mainly organic in nature in form of fuels like coal, petroleum, firewood, all of which are products of photosynthesis.
Factors that affect the rate of photosynthesis
The rate of photosynthesis can be determined by considering how much oxygen is evolved by the plant or the amount of oxygen given off by the plant or increase in the weight of the plant due to accumulation of starch. Some of the factors include the following:
1) Amount of chlorophyll
The more chlorophyll, the more the light energy absorbed leading to increased rate of photosynthesis. The less the chlorophyll, the less light energy absorbed leading to decreased rate of photosynthesis
2) Amount of CO2 in the atmosphere
It is required as a raw material for photosynthesis thus the rate of photosynthesis increases in CO2 concentration and it decreases with the lowering of CO2 concentration.
3) Light intensity
The rate of photosynthesis increases with increase in light intensity. And it lowers with decrease in light intensity.
4) Temperature
It is required for the activity of enzymes that control the rate of photosynthesis. Thus the rate of photosynthesis increases with increase in temperature till the optimum temperature for enzyme action. Beyond which the enzymes are denatured leading to decrease rate of photosynthesis.
5) Number of stomata
The more the stomata, the more the gaseous exchange. This avails more CO2 to the plant leading to high rate of photosynthesis.
6) Surface area for photosynthesis
The larger the area for photosynthesis (more leaves) the more light energy is absorbed which causes increased rate of photosynthesis.
7) Amount of oxygen
The rate photosynthesis decreases with increase in oxygen concentration and it increases with the lowering of oxygen concentration.
8) Availability of water
MINERAL NUTRITION IN PLANTS
Pants need mineral elements for proper growth. Mineral elements are divided into two categories depending on the relative amounts of element needed.
- Essential macro (elements)
- Trace micro (elements)
Essential elements:
These are elements needed in large quantities for proper plant growth, e.g. nitrogen, phosphorus, magnesium, potassium, calcium, sulphur, carbon, hydrogen, oxygen, etc.
Trace elements:
These are elements need in small quantities for proper plant growth they include manganese zinc boron silicon aluminum copper, molybdenum, and iron.
Plants obtain minerals from mineral salts present in the soil; Mineral salts are absorbed in form of soluble salts e.g. nitrogen as nitrate, phosphorus as phosphates, sulphur as sulphate.
When a particular element is missing in the in the surroundings, a plant shows deficiency signs.
