Unit 5

Question 1

Endosymbiont Theory and 3 evidences

Answer 1

Mitochondria and chloroplasts originated from other prokaryotic cells. How do we know?

1) sizes and morphology are similar
2) divide by fission
3) contain their own DNA and ribosomes

Question 2

Could present day mitochondria and chloroplasts isolated from the cell survive independently?

Answer 2

No - endosymbiosis reduced the size of the organelle genomes by gene loss and transfer of genes to the nucleus

Question 3

Chloroplast and mitochondrial genome

Answer 3

Chloroplast genomes only encode 120 proteins, mostly for photosynthesis

Mitochondrial genome only encodes 13 and functions for the modern mitochondria

Question 4

Why are mitochondria and chloroplasts semi-autonomous?

Answer 4

The majority of mitochondrial/chloroplast proteins are encoded by nuclear genes and these proteins are imported into the organelle after synthesis in the cytosol

Question 5

How do nuclear-encoded proteins get to the mitochondria/chloroplasts?

Answer 5

They contain mito/chloro targeting sequences called transit peptides on their N-terminus (approx. 18 AAs). These transit peptides are then recognized by specific import receptors on organellar membranes

Question 6

How are transit peptides (targeting sequences) recognized?

Answer 6

They are recognized after the protein has been released from the ribosome, but chaperones inhibit it from further folding so it sticks out, which is required for binding to the receptor

Question 7

Post-translational targeting of nuclear-encoded proteins to mitochondrial matrix

Answer 7

Transit peptide is recognized and bound by receptors on the TOM complex on the outer membrane and TOM connects to TIM complex on the inner membrane. Protein unfolds as it enters. Once inside the matrix, the import signal is cleaved and the protein is refolded by chaperones

Question 8

Post-translational targeting of nuclear-encoded proteins to the thylakoid lumen

Answer 8

Transit peptide is recognized by receptor protein in TOC complex on the outer membrane, which is connected to TIC complex on the inner membrane. Protein unfolds and enters stroma and import signal is cleaved. A second hydrophobic signal sequence targets the protein to the thylakoid and protein refolds (with the help of chaperone proteins?) inside the thylakoid

NOTE: the chloroplast signal sequence has to be cleaved off in order to unmask the thylakoid signal, which allows it to be recognized

Question 9

Contact site

Answer 9

Where the inner and outer membrane transporters are close together to allow the protein to diffuse into the organelle

Question 10

Insertion of transmembrane proteins in chloro/mito

Answer 10

Has appropriate “start” and “stop” transfer sequences on the protein, similar to insertion into the ER membrane

Question 11

Flow of biological energy in mitochondria

Answer 11

Gets energy from sugars/fats and generates NADH/FADH2 and CO2 as byproduct

NADH/FADH2 has high E electrons used to catalyze the production of ATP and H2O is produced as a byproduct

Question 12

Flow of biological energy in chloroplast

Answer 12

Harness light energy and uses H2O to generate ATP and NADPH, which produces O2 as byproduct. ATP and NADPH are used to catalyze the fixation of CO2 into carbohydrates

Question 13

Mitochondria shapes

Answer 13

They often fuse and form elongated networks throughout the cytoplasm and are dynamic organelles, which is important for the delivery of ATP to different parts of the cell

NOTE: plane of section limits shape

Question 14

Where is pH the highest in the mitochondria? Why?

Answer 14

In the matrix because ATP formation is driven by H+ gradient across the inner membrane, therefore, the intermembrane space has to have a high concentration of H+

Question 15

Main function of mitochondria

Answer 15

Energy conservation, such as producing ATP from the oxidation of food (oxidative phosphorylation), which produce CO2 and H2O as byproducts

Question 16

Oxidation of glucose (glycolysis in cytosol) produces…

Answer 16

ATP, NADH (electron carrier), pyruvate. Pyruvate and NADH must be imported into mitochondria for further reactions

Question 17

Oxidation of acetyl-CoA (Krebs Citric Acid Cycle, also called TCA cycle, in the mitochondrial matrix) produces…

Answer 17

ATP, NADH and FADH2 (electron carriers used to power ETC)

Question 18

Oxidative phosphorylation

Answer 18

NADH and FADH2 donate electrons to the ETC. As electrons move down the ETC, protons move from the matrix to the intermembrane space across the inner membrane and establishes a proton gradient across the inner membrane and drives ATP production

Question 19

2 ways ATP is generated

Answer 19

1) Substrate-level phosphorylation - a singe enzyme transfers a phosphate group from one molecule (a substrate) to ADP to make ATP
Ex. glycolysis

2) Chemiosmotic coupling - membrane-based mechanism that uses proton gradient to power ATP synthesis
Ex. oxidative phosphorylation (mitochondria)
photophosphorylation (chloroplast)

Question 20

Why is the proton gradient higher in the chloroplast than in the mitochondrion?

Answer 20

The outer membrane of the mitochondria is very porous and allows H+ to diffuse out and doesn’t allow for much accumulation. When comparing it to chloro, the proton accumulation is much greater in the thylakoid because it is stuck there and allows ATP synthesis to be more efficient

Question 21

Chloroplasts are members of the plastid family. How are plastids involved in the different colours of plants

Answer 21

Plastids can differentiate according to the role they perform

Question 22

Do all plant cells have plastids?

Answer 22

Yes - plastids are sites of synthesis of purines and pyrimidines, most amino acids, and all fatty acids

Question 23

How does a plant cell gets the energy it needs to perform cellular processes?

Answer 23

Sugars made by photosynthesis in chloroplasts are used by respiration in mitochondria to make ATP

Question 24

Chloroplasts: energy transduction reactions

Answer 24

Occurs in the thylakoid membrane and is light dependent, such as photophosphorylation which synthesizes ATP.

Light E is used to generate protons from water and high E electrons in photosystem II. As electrons move down the ETC in thylakoid membrane, it creates a proton gradient across, which drives an ATP synthase located in the same membrane to generate ATP. During the process, NADP+ is reduced to NADPH

Question 25

Chloroplasts: carbon fixation reactions

Answer 25

Occurs in the stroma and is light independent, such as the Calvin cycle.

Fixation of carbon (from CO2) into sugars in photosynthesis is carried out by the enzymes of the Calvin cycle. E from ATP and NADPH is used for the production of sugars

Question 26

Why does ATP made in the chloroplast stay there? What do chloroplasts transport?

Answer 26

They do not have ATP transporter. They primarily transport sugars. These sugars can be used by mitochondria to make ATP. The excess sugars are stored in the chloroplasts as starch