Photosynthesis
- A limiting factor is a factor that restricts the rate of photosynthesis.
- We can speed up the rate of photosynthesis by:
1) Giving the plant extra light
2) Giving the plant extra CO₂
3) Increasing the temperature
- A shortage of chlorophyll could also limit the rate of photosynthesis and water shortage.
- If we increase the light intensity and measure the rate of photosynthesis,
- A plateau occurs when another limiting factor is involved.
- The rate of photosynthesis increases from:
A→B due to an increase in light intensity
If we increase temperature,
Increasing CO₂ concentration,
- The limiting factor is the low light intensity.
- Between B and C, light intensity is higher and is no longer the limiting factor. As you increase light intensity, the rate of photosynthesis doesn’t change, as there is another limiting factor. Which could it be CO₂ concentration or temperature.
- Shade-tolerant plants have a higher ratio of leaves to roots, thinner leaves, more chlorophyll, and a larger surface area than other plants, as they need to capture as much sunlight as possible.
Testing for Starch
B1.4d
- We test leaves for starch because they store glucose in the form of starch. This demonstrates that the presence of starch indicates the occurrence of photosynthesis, which results in the production of glucose.
- First, conduct the test in water to soften it.
- Then, keep the leaf in ethyl until it turns white. This removes chlorophyll, the green pigment, so that you can see the iodine results.
- Afterwards, return the leaf to the boiling water, as the presence of ethanol can cause it to become brittle.
- Then dry the leaf and add iodine to it. Iodine is the test for starch.
- The leaf will turn blue/black if starch is present.
- The leaf will remain orange if starch isn’t present.
- Variegated leaves: The process of photosynthesis exclusively takes place in the centre of the leaf. Some areas (usually outside) are white, and the middle is green.
Storage:
- A plant in a sealed glass bottle will not die.
- The clear glass lets in sunlight, condensation produces oxygen, and respiration produces carbon dioxide.
Products
Uses of Glucose:
- For respiration, glucose is a reactant.
- Seeds can convert glucose into lipids (fats and oil) for storage.
- Glucose can be built into cellulose, which strengthens cell walls.
- Nitrate ions and other minerals can combine with glucose to form amino acids.
- Then, amino acids combine to form proteins.
- Starch can form long chains from glucose. Starch is the main energy store and is also insoluble.
Oxygen:
- Oxygen is a waste product of photosynthesis.
- Plant cells use some of this oxygen during respiration, and the plant releases the rest into the air through the stomata.
- Plants respire all the time, which means they constantly use oxygen and release carbon dioxide.
- During hours of daylight, they also photosynthesise.
- Plants photosynthesise much faster than they respire.
- Plants release more oxygen than they use and also use much more carbon dioxide than they release.
- Over a 24-hour period, plants produce much more glucose through photosynthesis than they use for respiration.
Photosynthesis
B1.4b
- To make food plants have to take in:
1) Carbon dioxide - This diffuses from the air into the plant through the stomata to reach the palisade layer, the site of photosynthesis.
2) Water - This enters the roots, the soil solution, through the root hair cells by osmosis, and then transported to leaves through the xylem.
B1.4c
Equations:
Word: Carbon Dioxide + Water → (light) → Glucose + Oxygen
Symbol: 6CO₂ + 6H₂O → (light) → C₆H₁₂O₆ + 6O₂
- Photosynthesis is a series of reactions that use sunlight, it's endothermic.
- Photosynthesis energy is transferred from the environment to chloroplasts, by light.
- Photosynthesis takes place inside the plants chloroplasts and mainly occurs in the leaf.
- Leaves and stems are green due to the chlorophyll inside the chloroplasts.
- Light transfers energy from the sun to chlorophyll where carbon dioxide and water react to make glucose. Glucose stores energy within its chemical bonds.
- Photosynthesis is an example of an endothermic reaction. This means that energy must be transferred from the surroundings to keep it going.
Stages:
1) Light dependent: Energy transferred from light splits water molecules into oxygen and hydrogen ions.
2) Light independent: Carbon dioxide gas combines with the hydrogen ions to make glucose. Technically indirectly light dependent, stage 2 can’t occur without stage 1.
B1.4a: Photosynthetic organisms are the main producers of food, and therefore biomass of life on Earth.
- Respiration in plants constantly happens at a lower rate - lower than photosynthesis when daylight.
→ Plant will always make more O₂ than needed for respiration.