photosynthesis OCR A2

What is photosynthesis?

Process whereby light energy is converted from the sun is transformed into chemical energy and used to synthesise large organic molecules from inorganic substances. The chemical potential energy is then available for consumers and decomposers

6CO2  +  6H2O  (+light energy)     à        C6H12O6   +  6O2

Autotrops- organisms that use light energy to chemical energy and inorganic molecules such as CO2 and H2O, to synthesise large organic molecules

Heterotrophs- organisms that ingest and digest large organic molecules, releasing the chemical potential energy stored in them

Animals, fungi and some bacteria are heterotrophs

Plants, some bacteria and some protoctists are photoautotrophs

Structure of chloroplast

·      The outer membrane is permeable to many small ions

·      The inner membrane is less permeable and has transporter proteins embedded in it. It is folded to lamellae, which are stacked up. Each stack is called a granum

·      Between the grana are intergranal lamellae.

The light-dependent reactions use light energy to split water and make some ATP and energetic hydrogen atoms. This stage takes place within the thylakoid membranes of chloroplasts, and is very much like the respiratory chain, only in reverse.

The light-independent reactions don’t need light, but do need the products of the light-dependent stage (ATP and H), so they stop in the absence of light. This stage takes place in the stroma of the chloroplasts and involve the fixation of carbon dioxide and the synthesis of glucose.

We shall see that there are many similarities between photosynthesis and respiration, and even the same enzymes are used in some steps.

Chloroplasts

Photosynthesis takes place entirely within chloroplasts. Like mitochondria, chloroplasts have a double membrane, but in addition chloroplasts have a third membrane called the thylakoid membrane. This is folded into thin vesicles (the thylakoids), enclosing small spaces called the thylakoid lumen. The thylakoid vesicles are often layered in stacks called grana. The thylakoid membrane contains the same ATP synthase particles found in mitochondria. Chloroplasts also contain DNA, tRNA and ribososomes, and they often store the products of photosynthesis as starch grains and lipid droplets.

Chlorophyll is a fairly small molecule (not a protein) with a structure similar to haem, but with a magnesium atom instead of iron. Chlorophyll and the other pigments are arranged in complexes with proteins, called photosystems. The photosystems are located in the thylakoid membranes and they hold the light-absorbing pigments in the best position to maximise the absorbance of photons of light. The chloroplasts of green plants have two kinds of photosystem called photosystem I (P700) and photosystem II (P680). These absorb light at different wavelengths and have slightly different jobs in the light dependent reactions of photosynthesis.

The Light-Dependent Reactions

The light-dependent reactions take place on the thylakoid membranes using four membrane-bound protein complexes called photosystem I (PSI) mainly in intergranal lamellae, photosystem II (PSII) exclusive to granal lamellae,

1. PSII contains an enzyme that in the presence of light, the splitting (or photolysis…