Factors Affecting Photosynthesis: Internal, External and Blackman’s Law
Photosynthesis is the engine of plant productivity, and its rate directly determines the yield of plants, including crop plants. But that engine never runs on a single input — it is constrained at once by conditions inside the plant and conditions in the environment around it.
What you will test
- Internal (plant) factors: number, size, age and orientation of leaves, mesophyll cells and chloroplasts, internal CO2 concentration, and the amount of chlorophyll — all dependent on genetic predisposition and growth.
- External factors: availability of sunlight, temperature, CO2 concentration and water.
- Blackman’s Law of Limiting Factors (1905): when more than one factor affects a process, the rate is set by the factor nearest to its minimal value.
This quiz follows Lesson 15 and uses medium-difficulty, exam-style reasoning. Read each question carefully — several options are deliberately close.
Q1. Why is a clear understanding of the factors affecting photosynthesis considered necessary in plant physiology?
The lesson states that an understanding of these factors is necessary because the rate of photosynthesis is very important in determining the yield of plants, including crop plants.
Q2. Which of the following is classified as an internal (plant) factor affecting photosynthesis?
The plant or internal factors include the number, size, age and orientation of leaves, mesophyll cells and chloroplasts, internal ##\ce{CO2}## concentration and the amount of chlorophyll. Sunlight, temperature and atmospheric ##\ce{CO2}## are external factors.
Q3. The internal or plant factors affecting photosynthesis are said to be dependent on which of the following?
The lesson states that the plant or internal factors are dependent on the genetic predisposition and the growth of the plant.
Q4. Which set correctly lists only external factors affecting photosynthesis?
The external factors include the availability of sunlight, temperature, ##\ce{CO2}## concentration and water. The other options list internal (plant) factors.
Q5. Which of the following is NOT listed as an internal plant factor in the lesson?
Availability of sunlight is an external factor. The internal factors include leaf number, size, age and orientation, mesophyll cells and chloroplasts, internal ##\ce{CO2}## concentration and the amount of chlorophyll.
Q6. As a plant photosynthesises, how do the various internal and external factors act on its rate?
The lesson states that as a plant photosynthesises, all these factors will simultaneously affect its rate. Several factors interact and simultaneously affect photosynthesis or ##\ce{CO2}## fixation.
Q7. Although several factors interact and simultaneously affect photosynthesis, what is usually observed?
The lesson notes that though several factors interact and simultaneously affect photosynthesis or ##\ce{CO2}## fixation, usually one factor is the major cause or is the one that limits the rate.
Q8. At any given point, the rate of photosynthesis will be determined by which factor?
The lesson states that at any point the rate will be determined by the factor available at sub-optimal levels.
Q9. Blackman's Law of Limiting Factors is dated to which year?
The lesson refers to Blackman's (1905) Law of Limiting Factors.
Q10. According to Blackman's Law of Limiting Factors, if a chemical process is affected by more than one factor, its rate is determined by which factor?
Blackman's Law states that if a chemical process is affected by more than one factor, its rate will be determined by the factor which is nearest to its minimal value.
Q11. Blackman's Law describes the limiting factor as the one that directly affects the process under which condition?
The law states that the limiting factor is the factor which directly affects the process if its quantity is changed.
Q12. A green leaf is supplied with optimal light and optimal ##\ce{CO2}##, yet it shows no photosynthesis because the temperature is very low. What does this example best illustrate?
This is the lesson's example of Blackman's Law: despite a green leaf and optimal light and ##\ce{CO2}##, the plant may not photosynthesise if temperature is very low, because the factor nearest its minimal value limits the rate.
Q13. In the example of a green leaf with optimal light and ##\ce{CO2}## but very low temperature, what happens if the leaf is given the optimal temperature?
The lesson states that this leaf, if given the optimal temperature, will start photosynthesising.
Q14. Which of the following best states the relationship between the rate of photosynthesis and crop yield?
The lesson emphasises that the rate of photosynthesis is very important in determining the yield of plants, including crop plants.
Q15. Which statement about internal and external factors is consistent with the lesson?
Internal (plant) factors reside within the plant body and are dependent on genetic predisposition and growth, whereas external factors are environmental properties such as sunlight, temperature, ##\ce{CO2}## concentration and water.