AB3: Compartmentation

Question 1

What is the structure of the cytosol?

Answer 1

Semi-fluid substance between plasma membrane and nucleus and does not include organelles (cytoplasm= cytosol + organelle)- contains many soluble proteins

Question 2

What is the function of the cytosol?

Answer 2

An environment for metabolic reactions within the cell

Question 3

What is the structure of the mitochondria?

Answer 3

Has a double lipid bilayer constructed from the outer and inner membrane with many folds which are called cristae- increases surface area
These create 2 compartments- the intermembrane space and mitochondrial matrix
Also has mitochondrial DNA and free ribosomes.
Thought to have evolved from endocytosis of prokaryotes- endosymbiosis hypothesis

Question 4

What is the function of the mitochondria?

Answer 4

Mitochondrial matrix is the environment for Krebs cycle and the cristae provides a large surface area for membrane-bound enzymes involved in electron transport and oxidative phosphorylation

Question 5

What is the structure of the endoplasmic reticulum? what are the 3 types of endoplasmic reticulum?

Answer 5

Is part of the endomembrane system- creates a space between the ER membranes which is separate from the cytosol
The double membrane of the endoplasmic reticulum joins up with the double membrane of the nuclear envelope.
Comprises of rough, smooth and transitional ER- region that is adjacent to the Golgi

Question 6

What is the function of the rough endoplasmic reticulum?

Answer 6

Ribosomes dock here to synthesise secretory proteins (which enter the lumen of the ER and some proteins, such as integral membrane proteins, remain in the cell.
Newly synthesised proteins are ‘fed’ into the lumen of the ER if the protein is going to be packaged into vesicles and be released from the cell

Question 7

What is the function of the smooth endoplasmic reticulum?

Answer 7

Synthesises lipids and other hydrophobic metabolites e.g. cholesterol and steroids
Detoxifies poisons such as alcohol via Cytochrome P450 enzyme in the liver
Stores calcium in the lumen- important for cell signalling.
Enzymes are embedded in the membrane of smooth ER

Question 8

What is the function of the transition endoplasmic reticulum

Answer 8

Where the membrane vesicles bud off before departing to the Golgi- half way between the other endoplasmic reticulum’s and the Golgi

Question 9

What is the structure of the Golgi?

Answer 9

Consists of flattened membranous sacs/bags called cisternae- these are not connected to each other unlike in the ER
Their structure and enzyme content change from cis face (towards the ER) to trans faces (towards the plasma membrane)

Question 10

What is the function of the Golgi?

Answer 10

Modifies the products of the rough ER- e.g. glycosylation- modification of carbohydrate side chains on glycoproteins- modifying newly synthesised proteins from rough ER
Manufactures some macromolecules such as pectins (in plants) and polysaccharides
Transit time for a newly synthesised protein through the Golgi is around 10 mins

Question 11

What are the 2 hypothesis about how substances move through the Golgi?

Answer 11

1) Vesicular transport
The cisternae (containing enzymes which catalyse reactions) do not move. However, vesicles of membrane pass from the ER through the golgi network from cis to trans- carrying inside them proteins that have been synthesised in the rough ER
2) Cisternal maturation
The cisternae are moving in a cis to trans direction within the cell and proteins within the cisternae are ferried backwards in a trans to cis direction in membrane vesicles

Question 12

What is exocytosis and endocytosis?

Answer 12

Exocytosis of intracellular material- some of the vesicles made by the Golgi can move outwards towards the plasma membrane and fuse with the membrane- release the contents, e.g. insulin, into the extracellular space.
Endocytosis of extracellular material- extracellular proteins bind to receptors on the surface of the cell- part of membrane folds inwards and forms a vesicle on the inside of the cell

Question 13

Why is intracellular compartmentation important in Eukaryotes?

Answer 13

Cells perform different biochemical reactions in different compartments because some enzymes may be incompatible, requiring different conditions of pH, Ca2+ concentration or oxidising/reducing conditions

Question 14

How does pH vary across a mammalian cell?

Answer 14

• The extracellular fluid, e.g. the blood, has a pH of 7.4
• The cytosol has a pH of 7.1
• The mitochondrial matrix has a pH of 7.6- due to pH gradient
• The endosomal compartments, e.g. ER, Golgi, vesicles and lysosomes had a pH of 5-6

Question 15

How does Ca2+ concentration vary in a mammalian cell?

Answer 15

• In the extracellular fluid, it is 2mM per litre
• In the cytosol, it is 10-100nM per litre
• In the mitochondrial matrix, it is 1mM per litre
• In the endoplasmic reticulum, it is 1mM

Question 16

How does the oxidation/reduction conditions vary across a mammalian cell?

Answer 16

• In the cytosol, the conditions are relatively reducing as a result of glycolysis
• In the mitochondrial matrix, the conditions are relatively oxidising to drive oxidative phosphorylation
• The SER is relatively oxidative as a result of the detoxification reaction
• The peroxisome- strongly oxidative to drive beta-oxidation of fatty acids

Question 17

Why is intracellular compartmentation important in plants?

Answer 17

CN- substitutes for oxygen and blocks electron transport and ATP synthesis- Plants produce CN- compounds to protect themselves from herbivores- plants aren’t poisoned by their own cyanide molecules as they have a vacuole that holds the cyanide and keeps it separate from mitochondria

Question 18

Why is intracellular compartmentation important in prokaryotes

Answer 18

Some structures resemble organelles such as endospores- look like large organelle, but this apparent compartmentation is a specialised form of cell division- not strictly organelle formation
• pores are highly resistant to destruction e.g. by heat- pathological in humans