2.9 Photosynthesis Flashcards
What is photosynthesis?
Photosynthesis is the process by which cells synthesise organic compounds (e.g. glucose) from inorganic molecules (CO2 and H2O) in the presence of sunlight.
- Anabolic synthesis of organic compounds
Where does photosynthesis take place?
In the chloroplasts.
What does photosynthesis require
a pigment…
Photosynthesis requires a photosynthetic pigment (chlorophyll) and can only occur in certain organisms (plants, certain bacteria)
Absorption spectrum
The wavelengths of light absorbed by each pigment (chlorophyll a, chlorophyll b, cartenoids)
Light spectrum
Visible light has a range of wavelengths with violet the shortest wavelength and red the longest.
- Green is kind of in the centre, closer to the bliue
Chlorophyll and the visible light spectrum
Chlorophyll absorbs blue and red light most effectively and reflects green light more than other colours (hence the green colour of leaves).
Limiting factors of the rate of photosynthesis
- Temperature - inverted parabola (photosynthesis is also en enzyme-controlled reaction, therefore there will be an optimum temperature or peak)
- Carbon dioxide concentration - plateau
- Light intensity - plateau
Action spectrum
The overall rate of photosynthesis at each wavelength of light
What is the process of photosynthesis?
Photosynthesis is a two step process:
1. The light dependent reactions convert light energy from the Sun into chemical energy (ATP)
2. The light independent reactions use the chemical energy to synthesise organic compounds (e.g. carbohydrates)
What is the first part of the process of photosynthesis?
Light dependant reactions
Light is absorbed by chlorophyll and water molecules
- Light absorbed by chlorophyll results in the production of ATP (chemical energy)
- The water molecule gets split from the energy from sunlight (photolysis) to produce oxygen and hydrogen
- The hydrogen (carried by NADPH) and ATP are used in the light independent reactions and the oxygen is released from stomata as a waste product
What is the second part of the process of photosynthesis?
Light independant reactions
- ATP and hydrogen (carried by NADPH) are transferred to the site of the light independent reactions
- The hydrogen is combined with carbon dioxide to form complex organic compounds (e.g. carbohydrates, amino acids, etc.)
- The ATP provides the required energy to power these anabolic reactions and fix the carbon molecules together
Chromatographs
Photosynthetic organisms do not rely on a single pigment to absorb light, but instead benefit from the combined action of many. These pigments include chlorophylls, xanthophyll and carotenes.
Chromatography is an experimental technique by which mixtures can be separated
- A mixture is dissolved in a fluid (called the mobile phase) and passed through a static material (called the stationary phase)
- The different components of the mixture travel at different speeds, causing them to separate
- A retardation factor can then be calculated (Rf value = distance component travels ÷ distance solvent travels)
Two of the most common techniques for separating photosynthetic pigments are:
- Paper chromatography – uses paper (cellulose) as the stationary bed
- Thin layer chromatography – uses a thin layer of adsorbent (e.g. silica gel) which runs faster and has better separation
Design of experiments to investigate the effects of limiting factors on photosynthesis
How to measure the dependent variable
Skill
Measuring CO2 Uptake
1. By measuring a change in pH of water
- Carbon dioxide uptake can be measured by placing leaf tissue in an enclosed space with water.
- Water free of dissolved carbon dioxide can initially be produced by boiling and cooling water.
- Carbon dioxide interacts with the water molecules, producing bicarbonate and hydrogen ions, which changes the pH (↑ acidity)
- Increased uptake of CO2 by the plant will lower the concentration in solution and increase the alkalinity (measure with probe)
2. Alternatively, carbon dioxide levels may be monitored via a data logger
Measuring O2 Production
1. Oxygen production can be measured by submerging a plant in an enclosed water-filled space attached to a sealed gas syringe. Any oxygen gas produced will bubble out of solution and can be measured by a change in meniscus level on the syringe
2. Oxygen levels can also be measured with a data logger if the appropriate probe is available
Measuring Biomass (Indirect)
1. Glucose production can be indirectly measured by a change in the plant’s biomass (weight). This requires the plant tissue to be completely dehydrated prior to weighing to ensure the change in biomass represents organic matter and not water content.
2. An alternative method for measuring glucose production is to determine the change in starch levels (glucose is stored as starch). Starch can be identified via iodine staining (turns starch solution purple) and quantitated using a colorimeter
Oxygenation of earth
Billions of years ago photosynthesis did not exist on out planet.
Approximately 2.3 billion years ago, photosynthetic organisms began to saturate the environment with oxygen
This led to changes in the Earth’s atmosphere, oceans, rock deposition and biological life.
Oceans
- Earth’s oceans initially had high levels of dissolved iron (released from the crust by underwater volcanic vents)
- When iron reacts with oxygen gas it undergoes a chemical reaction to form an insoluble precipitate (iron oxide)
- When the iron in the ocean was completely consumed, oxygen gas started accumulating in the atmosphere
Atmosphere
- For the first 2 billion years after the Earth was formed, its atmosphere was anoxic (oxygen-free)
- The current concentration of oxygen gas within the atmosphere is approximately 20%
Rock deposition
- The reaction between dissolved iron and oxygen gas created oceanic deposits called banded iron formations (BIFs)
- These deposits are not commonly found in oceanic sedimentary rock younger than 1.8 billion years old
- This likely reflects the time when oxygen levels caused the near complete consumption of dissolved iron levels
- As BIF deposition slowed in oceans, iron rich layers started to form on land due to the rise in atmospheric O2 levels
