5.6.2 Chloroplast Structure & Function

Question 1

Chloroplast structure

Answer 1

• 2-10μm diameter (larger than mitochondria)

Double membrane envelope
- Each containing phospholipid bilayer
- Outer membrane freely permeable to range of ions and small molecules
- Inner membrane selectively permeable because it contains transport proteins

Stroma
- Fluid
- CO2, enzymes, sugars and other molecules dissolved here
- Loop of DNA (codes for some chloroplast proteins)
- Contains 70S ribosomes (proteins coded by DNA are synthesised here)
- Starch grains (sugars formed during photosynthesis stored here)

Thylakoids
- In stroma
- Flattened, fluid filled sacs
- Pigments, enzymes, electron carriers
- Stack to form grana
- Grana connected by lamellae (ensures stacks are connected but distanced)
- Large SA
- So lots of light can be absorbed by large SA of pigment molecules
- Pigments arranged into photosystems
- Photosystems are funnel like structures in thylakoid membrane (each pigment molecule passes energy down to the next in the cluster til it reaches primary reaction centre)

Question 2

Where are stages of photosynthesis located?

Answer 2

Light dependent stage - on thylakoid membranes
Light independent stage - Stroma

Question 3

How are chloroplasts adapted to photosynthesis?

Answer 3

Stroma
- Contains enzymes catalysing reactions in light independent stage
- Stroma surrounds grana and membranes so transport of products from light dependent to light independent stage is quick

Grana
- Stacks to form large SA for presence of lots of photosystems so more light can be absorbed
- More membrane space for electron carriers and ATP synthase

DNA
- Contains genes coding for some proteins and enzymes used in photosynthesis

Ribosomes
- Translates proteins coded by the cpDNA

Inner membrane of chloroplast envelope
- Contains transport proteins to control flow of molecules between stroma and cytoplasm of plant cell