Photosynthesis

Photosynthesis All life on Earth is powered by the sun. This light travelling 160 000 000 km from the sun is captured by chloroplasts in green plants. Here it is turned into the energy-rich foods and oxygen that all other organisms depend on.

Chlorophyll This is found in the chloroplasts in any part of the plant which is green. Most photosynthesis takes place in the leaves. Chlorophyll is found in the thylakoids. These are stacked one on top each other in piles called Grana.

Chlorophyll The grana stacks are joined to each other by membranous extensions of the thylakoids called lamella. Photosynthetic pigments, enzymes and carrier molecules are embedded in the thylakoid membranes. The clear fluid in the chloroplast is called stroma.

Chlorophyll Chlorophyll is similar to haemoglobin, the red pigment in our blood except it is based around magnesium. There are several different kinds of chlorophyll. The most important one is chlorophyll A. Other chlorophylls help to pick up extra wavelengths of light.

Chlorophyll This energy is then passed to chlorophyll A. Certain carotenoids known as carotene and xanthophyll also do this. Chlorophyll absorbs light in the red and violet end of the visible spectrum, and reflects green.

Photosynthesis Equation The overall equation for photosynthesis is: Sunlight 6CO2 + 6H2O C6H12O6 + 6O2 Chlorophyll

NADP - the Hydrogen Carrier Nicotinamide Adenine Dinucleotide Phosphate. This acts as the hydrogen carrier in photosynthesis. H2 NADP NADPH2 H2

Photosynthesis Phtosynthesis takes place in 2 steps: The light phase The light independent phase This is known as the Calvin Cycle. Used to be called the Dark phase but causes confusion as it still takes place in the light. It is independent of light.

The Light Phase This is the photo part. It takes place in the thylakoids Chlorophyll absorbs light and uses the energy to: Split water into hydrogen and oxygen, the hydrogen is picked up by NADP, the oxygen is a waste product. Charge up ADP to ATP.

The Calvin Cycle This cyclic process occurs in the stroma. Carbon dioxide is captured from the air and fixed into a molecule in the stroma (carbon dioxide fixation) An important enzyme called Rubisco (ribulose carboxylase) catalyses the first step of fixation by adding CO2 to ribulose biphosphate.

The Calvin Cycle Rubisco is the most abundant enzyme in chloroplasts. The product of this fixation is a 6C substance, which is so unstable it immediately splits into 2 molecules of 3- phosphoglycerate. This, along with electrons from NADPH2 and the energy from ATP, is turned into glyceraldehyde-3- phosphate (G3P).

The Calvin Cycle Some of this G3P is returned to the cycle The rest is used to make organic compounds such as glucose, lipids and proteins

The Relationship between Photosynthesis and Respiration Respiration takes place ALL the time, so oxygen is taken in and carbon dioxide is produced continuously. Photosynthesis takes place in the daylight hours, so carbon dioxide is taken in and oxygen is produced.

The Relationship between Photosynthesis and Respiration During the day there is usually more photosynthesis than respiration. The net result is that oxygen is produced and carbon dioxide is used during the day. During the night, oxygen is used and carbon dioxide is released.

The Relationship between Photosynthesis and Respiration Compensation point – this is the point where the amount of photosynthesis is the same as the amount of respiration. There is no net gain or loss of gases. Ref pg 137 Pattern’s of Life

Absorption of Carbon Dioxide If you keep adding CO2 to a greenhouse, photosynthesis will increase with increased light and temperature. Eventually a limit is reached and the rate levels off.

Absorption of Carbon Dioxide

Factors that Affect Photosynthesis Enzymes Photosynthesis is run by enzymes. So any factor which affects enzymes affects the rate of photosynthesis. Temperature At cold temperatures photosynthesis is reduced. It speeds up as temperature rises until an optimum is reached. Eventually the enzymes become denatured and photosynthesis stops.

Factors that Affect Photosynthesis Amount of Carbon dioxide Increase in CO2 will increase photosynthesis up to a point. Light Intensity and Wavelength Chlorophyll uses red and blue wavelengths of light. Carotene and xanthophyll pick up different wavelengths. Plants are adapted to different intensities of light.

Factors that Affect Photosynthesis Chlorophyll and minerals In general, the more chlorophyll, the more photosynthesis. If plants lack magnesium, their leaves go yellow and do not photosynthesise. Also need iron and other trace elements from the soil.

Factors that Affect Photosynthesis Absorption of Carbon Dioxide Linked to the opening and closing of the stomata in the leaves, which is in turn linked to water loss. If the leaf is losing too much water it closes its stomata. Sufficient Water Water is needed to keep the cells in the leaf wet. Gases are absorbed over a wet surface. Water also needed for photosynthesis.