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In chemical terms this can be expressed as follows:
6CO 2 + 6H 2 O +light -> 1C 6 H 12 O 6 + 6O 2
(when in the presence of chlorophyll)
CO 2 = carbon dioxide from the air
H 2 O = water absorbed by osmosis through the roots
H = hydrogen
1C 6 H 12 O 6 = the chemical formula for glucose which is a carbohydrate (made from carbon and water)
O 2 = oxygen molecule
Two stages – one of which requires light and the other does not.
The photosynthetic process –light dependent phase
A molecule of chlorophyll is excited by a photon of light and an electron ‘escapes’ into the thylakoid membrane of the chloroplast. This energy is used to split molecules of water (H 2 O) into their component hydrogen and oxygen. The oxygen is released as a by-product into the atmosphere.
The energy released by breaking the chemical bonds in the water is incorporated, with the hydrogen, into energy carrying molecules called ATP and NADPH2
Photosynthesis – light independent phase
This phase is also called the Calvin Cycle after Melvin Calvin who discovered it.
In this phase the energy stored in the ATP and NADPH2 produced in the light dependent cycle is used in the main body of the chloroplast to convert carbon dioxide from the atmosphere into glucose. You do not need to know the complex chemical and enzyme reactions which are involved in this for this course.
This phase takes place both in the day and at night – it does not need light to operate.
Limiting factors for photosynthesis
Light – only part of the wavelength of light from the sun is used (PAR – photosynthetically active radiation). If there is not enough PAR then photosynthesis slows or may cease.
Carbon dioxide – more than enough of this in the open air, but under glass may become depleted. If there is a lack of carbon dioxide photosynthesis slows or may cease.
Water – required but the amount used in photosynthesis itself makes up only a tiny proportion of the plant’s total needs. However, if the plant has insufficient water for transport of nutrients and to maintain its structure, then the stomata will close and carbon dioxide will not enter the leaf.
Temperature – the chemical reactions in photosynthesis are temperature sensitive. Ideal range 25 °C - 36°C.
The law of limiting factors
If a process is influenced by more than one factor, the rate of the process will be limited by the factor that is in lowest supply
Therefore if temperature is low (10 °C) increasing light intensity or carbon dioxide concentration will not increase the rate of photosynthesis.
Why does this matter ? The slower the rate of photosynthesis the slower the rate of growth.
Glucose + oxygen ->carbon dioxide + water+ energy
(in the mitochondria of the cell)
1C 6 H 12 O 6 + 6O 2 -> 6CO 2 + 6H 2 O + energy
Looks familiar? It’s the chemical equation for photosynthesis in reverse!
Respiration takes place in the mitochondria of the cells of the plant. Some of the energy released is used to combine simple carbohydrates and minerals into more complex compounds like cellulose. Some is used to power cell division for growth at the meristems.
Limiting factors of respiration
Temperature – the enzymes (chemical compounds that enable processes in cells) are temperature sensitive. The ideal range is 36 °C (ideal) down to 0°C.
Oxygen and carbon dioxide – respiration requires oxygen in sufficient concentration; reduced oxygen leads to slower respiration. This is why cold storage for fruits is often in a low oxygen environment. If carbon dioxide builds up then respiration slows.
Water loss – closure of stomata due to water loss prevents oxygen from entering the plant and slows respiration.
Why do the limiting factors of respiration matter?
Respiration continues even if photosynthesis stops! So in cut flowers, picked fruit and cuttings water levels drop, stopping photosynthesis but respiration continues to use up carbohydrates. So respiration rates can be lowered by manipulating the limiting factors.
For example – cold storage of picked fruits keeps them in better condition for sale by slowing respiration. That way the sugars in the apple that we like to taste are not used up by respiration in the fruit.
Transportation of water and nutrients
Xylem – transports water from the roots to the shoots; enters by osmosis and is drawn up by transpirational pull.
Xylem cells are long, wide and open ended; dead once mature. Linked end to end and also laterally through pores called pits
Transportation of water and minerals
Phloem – transports carbohydrates and plant growth regulators around the plant from the leaves.
Composed of cells called sieve cells joined end to end to form sieve tubes . Living cells but lack nuclei and most cytoplasm, they are connected by plasmodesmata. Connected to companion cells laterally.
Companion cells have a nucleus and organelles such as mitochondria. They provide the energy needed to transport the nutrients etc through the phloem. This active transport means that the carbohydrates can be moved to parts of the plant (such as tap root cortex) that are already rich in these substances.
Osmosis and diffusion
Osmosis – the movement of water molecules from an area of low salt concentration (high water potential) to an area of high salt concentration (low water potential) through a semi- permeable membrane.
Diffusion - the movement of molecules of a gas or liquid from an area of high concentration to an area of relatively low concentration. For example in transpiration water vapour moves from within the leaf where there is a high concentration of water vapour to the external atmosphere where the concentration of water vapour is lower.
Effects of humidity and temperature on transpiration
High humidity reduces the rate of transpiration because the rate of diffusion will fall. This is why greenhouse growers damp down the floor of the greenhouse in hot weather.
The higher the temperature outside the leaf the more quickly water in the spaces in the spongy mesophyll will evaporate. The more water vapour within the leaf the faster the rate of transpiration through diffusion.
1. Photosynthesis and respiration
1.1 State the basic equation for photosynthesis in words.
1.2 Describe how the efficiency of photosynthesis is determined by temperature, light, carbon dioxide and water.
1.3 State the basic equation for respiration in words.
1.4 Describe how the efficiency of respiration is determined by oxygen, water and temperature.
2. Transportation within the plant
9.1 Define the term: ‘transpiration’ and state how water and minerals move through the plant. (review)
9.2 Name the tissue that is involved in this process .(review)
9.3 Explain the role of phloem and describe its location within stems and roots. (review and additional detail)
9.4 Describe osmosis and diffusion and their roles within the plant. (review)
9.5 Describe how stomata control the release of water from the leaf. (review)
9.6 State what is meant by evaporation and how the plant may limit water loss.
9.7 State the effects of humidity and temperature on transpiration and water loss.