Photosynthesis Notes - A2 Biology OCR

Overview of photosynthesis

Plants use light energy to build organic molecules from inorganic molecules

o   6CO₂ + 6H₂O à C₆H₁₂O₆ + 6O₂

Organisms that feed this way – self sufficient, not needing organic molecules – are called autotrophs.

Not just plants, but phytoplankton and other bacteria.

Complex metabolic pathway – a series of reactions linked by steps, catalysed by enzymes. Split into two stages taking place in the chloroplast:

o   Light-dependent stage in the thylakoid membrane

o   Light-independent stage in the stroma of the chloroplast

Chloroplast structure

Found especially in the palisade mesophyll and spongy mesophyll tissue of the leaf. (Each plant cell contains 10x chloroplasts). Around 2-10μm long, consisting of double membranes forming an envelope around the organelle. Outside membrane is permeable to small ions. The inside membrane is less permeable but has transport proteins embedded in it, this membrane is folded into:

o   Membranes which form thin plates are called lamellae,

o   Fluid filled sacs called thylakoids

o   Thylakoids are packed into stacks called grana – site of light-dependent stage, and of light adsorption and ATP synthesis.

This is surrounded by ‘background material’ called stroma, which contains prokaryote type ribosomes, starch grains, their own small circular strand of DNA and lipid droplets. (Starch grains are a form of carbohydrate storage in plants). The stroma is also called a ‘fluid-filled matrix’ and the light-independent stage takes place here.

How the Chloroplast adapted to its role

• Inner membrane – controls the movement of substances with transport proteins between the cytoplasm (in cells)ßàThe stroma (in chloroplasts) 

o   Grana – has a large surface area for the light-dependent stage as it consists of hundreds of thylakoids. This means lots of photosynthetic pigments, electron carriers and ATP synthase. The grana is surrounded by the stroma therefore PRODUCTS from the light dependent stage can pass to the light independent stage when needed easily.

o   Stroma – catalyses reactions in the light-independent stage with enzymes.

o   Photosynthetic pigments – arranged as photosystems as it allows maximum adsorption of light energy.

o   Protein synthesis – The chloroplast can make some proteins used in photosynthesis with its chloroplast DNA and chloroplast ribosomes.

Photosynthetic pigments

Pigment – substances whose molecules absorb certain wavelengths (colours) of light. The try to capture as much light as possible.

The other lengths of light are either reflected or transmitted through the substance (the actual colour that hits our eyes/that we can see). Embedded in the membranes are several different kinds of synthetic pigments – coloured substances that absorb energy at certain wavelengths of light. These photosynthetic pigments are found in thylakoid membranes arranged in a funnel shape determined by proteins.

Chlorophyll

Pigments include the most abundant form chlorophyll; its two forms are chlorophyll a and chlorophyll b. They both absorb similar wavelengths of light.

 Chlorophyll a  slightly longer lengths…