chloroplasts

Question 1

chloroplast functions

Answer 1

• site of photosynthesis
• involved in other metabolic processes (e.g fatty acid and AA biosynthesis, nitrogen and sulfur assimilation
• only in plants

Question 2

chloroplast structure

Answer 2

• double membrane-bound organelle
• outer membrane
• IMS
• inner membrane
• stroma
• thylakoid membrane
• thylakoid lumen

Question 3

mitochondria envelope

Answer 3

consists of inner and outer membranes
outer membrane: contains porins - not as permeable to ions as OM of mitochondria
inner membrane: highly impermeable, contains transporters

Question 4

Thylakoids

Answer 4

3rd (internal) membrane system
- grana thylakoids are flattened membranous disks arranged in stacks
- stromal thylakoids are between stacks
thylakoid membrane: site of ATP synthase, maintain H+ gradient in thylakoid lumen
thylakoid lumen: aqueous interior: high [H+]

Question 5

stroma

Answer 5

aqueous interior of chloroplast
- contains enzymes involved in carbohydrate synthesis
- contain plastid genome
- contains ribosomes

Question 6

what does the plastid genome encode

Answer 6

• ribosomal proteins
• tRNAs and rRNAs
• some RNA polymerase subunits

Question 7

stromules

Answer 7

connect chloroplasts
- stroma filled membrane tubules
- branched, rapidly extend and contract
- allows for efficient metabolite transfer, communication between chloroplasts and/or other organelles

Question 8

thylakoid protein targeting in chloroplasts

Answer 8

2 pathways that both have the same first step
- all chloroplast proteins have a stromal import sequence that moves the protein through the TOC complex and then the TIC complex
- a protease in the storma cleaves the stromal import sequence which reveals the thylakoid-targeting sequence

Question 9

thylakoid protein targeting - SRP-dependent pathway

Answer 9

• the thylakoid targeting signal is similar to a signal sequence in ER-targeting
• the stroma of the chloroplast contains an SRP that binds the thylakoid targeting signal
• the stromal SRP binds proteins AFTER translation has been completed
• stromal SRP is recognized by the thylakoid SRP receptor which brings the protein into contact with the thylakoid translocon (SecY)
• the protein passes through the translocon and into the lumen of the thylakoid, the thylakoid targeting signal is cleaved and the protein is folded

Question 10

thylakoid protein targeting - pH-dependent pathway

Answer 10

• once the protein is in the stroma, a protease cleaves the stroma-import sequence which reveals a thylakoid targeting sequence (which is 2 argenines - RR)
• the pH gradient ([H+] in the thylakoid lumen > [H+] in the stroma) allows the protein to move through the thylakoid membrane
• once in the thylakoid, the 2 argenines are chopped off, preventing it from moving back out to the stroma