13.2 Chloroplasts and Other Plastids

Question 1

() are organelles responsible for photosynthesis, thus they are only present in plant cells and some protists

Answer 1

chloroplasts

Question 2

what are the similarities between chloroplasts with mitochondria?

Answer 2

1. both generate metabolic energy
2. both contain their own genetic systems
3. both evolved from endosymbiosis
4. both replicate by division

Question 3

what occurs in chloroplasts?

Answer 3

1. conversion of CO2 into carbohydrates
2. synthesis of amino acids, fatty acids, and lipids for chloroplast membrane
3. reduction of nitrite to ammonia

Question 4

chloroplasts are bounded by a double membrane called the (1) and contain a third internal membrane system called the (2)

Answer 4

1. chloroplast envelope
2. thylakoid membrane system

Question 5

the thylakoid membrane forms a network of flattened disks (1) that are arranged in stacks called (2) (unique to chloroplasts)

Answer 5

1. thylakoids
2. grana

Question 6

what are the 3 distinct internal compartments of a chloroplast?

Answer 6

1. intermembrane space. - found within the chloroplast envelope
2. stroma - inside the envelope but outside the thylakoid membrane
3. thylakoid lumen - interior of thylakoids

Question 7

the outer membrane of the chloroplast envelope is similar to that of the outer mitochondrial membrane in that it ()

Answer 7

has porins that allow for the diffusions of small molecules

Question 8

compare and contrast the inner membrane of the chloroplast envelope and the inner mitochondrial membrane

Answer 8

Compare: they both allow entrance of ions and metabolites through specific membrane transporters, thus serving as the main barrier between the cytosol and the organelle
Contrast: the inner envelope of the chloroplast envelope is not involved in electron transport and photosynthesis (ATP generation)

Question 9

the () is the structural equivalent of the mitochondrial matrix

Answer 9

stroma

Question 10

how is the stroma equivalent to the mitochondrial matrix?

Answer 10

it contains the chloroplast genetic system as well as a variety of metabolic enzymes (including those involved in CO2 conversion to carbs during photosynthesis)

Question 11

the () is equivalent to the inner mitochondrial membrane in terms of its role in electron transport and ATP generation

Answer 11

thylakoid membrane

Question 12

unlike mitochondria, protons are concentrated not in the stroma, but in the (), making it the equivalent of the intermembrane space

Answer 12

thylakoid lumen

Question 13

unlike mitochondria, chloroplast tRNAs are sufficient to ()

Answer 13

translate all the mRNA codons using the universal genetic code (they don’t need additional wobble)

Question 14

the critical enzyme that catalyzes the addition of CO2 to ribulose-1,5-biphosphate during the Calvin cycle; also the most abundant protein on Earth

Answer 14

rubisco (ribulose biphosphate carboxylase/oxygenase)

Question 15

like mitochondrial proteins encoded by nuclear genomes, proteins imported to chloroplasts are imported as ()

Answer 15

completed polypeptide chains

Question 16

most chloroplast proteins are targeted for import to chloroplasts by ()

Answer 16

transit peptides

Question 17

transit peptides are ()-terminal sequences (usually 30-100 AA long) that direct protein translocation across the two membranes of the chloroplast envelope

Answer 17

N-terminal sequences

Question 18

overview of protein import into stroma

Answer 18

1. transit peptides target the polypeptides to the Toc complex (translocase of the outer chloroplast membrane); energy required for this comes from ATP hydrolysis
2. some Toc proteins bind and hydrolyze GTP to GDP to generate energy because there is no electrochemical potential across the inner membrane to drive translocation
3. proteins are then transferred to the Tic complex (translocase of the inner chloroplast membrane) from which they are translocated into the stroma
4. Hsp93 chaperone uses energy from ATP hydrolysis and draws the protein through the Tic complex
5. stromal processing peptidase (SPP) in the stroma cleave the transit peptide
6. stromal Hsp70 chaperones then bind to the polypeptide to aid in folding and maturation

Question 19

proteins that must cross the thylakoid membranes have a second signal sequence () that is exposed after the transit peptide is cleaved

Answer 19

thylakoid signal sequence

Question 20

thylakoid signal sequences are cleaved by ()

Answer 20

thylakoid processing protease (TPP)

Question 21

what are the 2 pathways that proteins can take to enter the thylakoid lumen

Answer 21

1. Tat pathway (Twin Arginine translocation) - related to bacterial membrane translocation
2. Sec pathway - related to pathway for translocation of proteins across ER

Question 22

describe the Tat pathway for proteins to enter the thylakoid lumen

Answer 22

1. proteins are recognized through their tat signal sequence
2. proteins are then transported across the thylakoid membrane in a fully folded state
3. translocation is driven by potential energy stored in the proton gradient across the thylakoid membrane

Question 23

describe the Sec pathway for proteins to enter the thylakoid lumen

Answer 23

1. thylakoid signal sequence is recognized by SecA protein
2. proteins are translocated through Sec translocon in an unfolded state
3. translocation is dependent on ATP hydrolysis by SecA

Question 24

what are the pathways used by proteins to incorporate with the thylakoid membrane

Answer 24

1. Sec pathway
2. SRP pathway

Question 25

describe the Sec pathway for the incorporation of proteins into the thylakoid membrane

Answer 25

1. thylakoid signal sequence is recognized by SecA protein
2. proteins are translocated through Sec translocon in an unfolded state
3. proteins exit Sec translocon laterally once a transmembrane sequence is encountered

Question 26

describe the SRP pathway for the incorporation of proteins into the thylakoid membrane

Answer 26

• 1. during translation, chloroplast signal recognition particles (cSRPs) identify proteins
  2. cSRPs target proteins to Alb3 translocase
  3. proteins exit Alb3 laterally into the thylakoid membrane

Question 27

chloroplasts are part of a larger family of plant organelles called ()

Answer 27

plastids

Question 28

different types of plastids are usually classified according to the ()

Answer 28

pigments they contain

Question 29

pigment in chloroplasts

Answer 29

chlorophyll

Question 30

pigments in chromoplasts

Answer 30

carotenoids

Question 31

plastids with no pigments; involved in storage of energy sources in nonphotosynthetic tissues

Answer 31

leucoplasts

Question 32

2 kinds of leucoplasts

Answer 32

1. amyloplasts - store starch
2. elaioplasts - store lipids

Question 33

all plastids develop from (), which are small undifferentiated organelles presend in rapidly dividing cells of plant roots and shoots

Answer 33

proplastids

Question 34

aside from developing from proplastids, different kinds of plastids can ()

Answer 34

change from one type to another

Question 35

development of plastids is regulated by both (1) and (2)

Answer 35

1. environmental signals
2. intrinsic developmental signals