Cell structure and Organelles

Question 1

Why aren’t viruses organisms?

Answer 1

They can’t reproduce independently

Question 2

What is the function of the plasma membrane?

Answer 2

Maintains composition of external and internal compartments
Used to communicate between cells
Import and export of molecules

Question 3

What are defining features of prokaryotes?

Answer 3

They have no nucleus and are unicellular

Question 4

What are defining features of eukaryotes?

Answer 4

They contain a nucleus and membrane bound organelles with a cytoskeleton. Usually multicellular, by not always (yeast for example is a unicellular eukaryote)

Question 5

What are archaea?

Answer 5

Extremophile prokaryotes. Environmental conditions are extreme yet these organisms are specifically adapted to live in them

Question 6

What are the nucleus’ features?

Answer 6

Double membrane which forms nuclear envelope. There are nuclear pores in the envelope for milecules to enter and exit nucleus.
Nucleus contains nucleolus which is the site of ribosome synthesis.
DNA is packages with histones to form chromosomes in the nucleus

Question 7

What is the function of rER and sER?

Answer 7

rER function is to package proteins produced by the ribsomes attached to the rER. Proteins produced in rER are secreted by the cell.

sER are the site of lipid synthesis

Question 8

What is the function of the Golgi appartus?

Answer 8

Package proteins into secretory vesicles and modify them by adding sugars to them. The secretory vesicles are transported to where they need to go by the Golgi.

Question 9

What do lysosome do?

Answer 9

Contains enzymes used to degrade unwanted proteins

Question 10

What are peroxisomes?

Answer 10

Contains catalase to oxidise H2O2.

Question 11

What do the mitochondria do?

Answer 11

Site of aerobic respiration.

Question 12

What is the cytosol?

Answer 12

The medium within the cell for molecules to travel.

Question 13

What is a specialised function of the lysosome?

Answer 13

The pH of the lysosome is 5. If lysosome bursts open, enzymes would be outside the optimal pH range so would not function so cell components wouldn’t damaged.

Question 14

What 3 things the cytoskeleton made of?

Answer 14

Intermediate filament, microfilament (actin filament) and microtubules

Question 15

What is the diameter of microfilaments?

Answer 15

7nm

Question 16

What is the diameter of intermediate filaments?

Answer 16

10nm

Question 17

What is the diameter of microtubules?

Answer 17

25nm

Question 18

What is the structure of microfilaments?

Answer 18

Identical globular actin form filaments. Two of these filaments wind into a chain forming the microfilament.

The globular actin all point in the same direction giving the strands polarity.

Question 19

What are the two ends of the actin filaments?

Answer 19

-ve is the pointed end and the +ve end is the barbed end.

Question 20

How do microfilaments produce tread-milling?

Answer 20

Actin is bound to ATP or ADP

Growth is faster on the barbed end as ATP-actin binds more favourably at the barbed end.
There is a net loss at the pointed end and a net gain at the barbed end leading to tread-milling allowing movement.

Question 21

How does nucleation of actin filament occur?

Answer 21

For nucleation to occur, the G-actin trimer must be formed first.
It is energetically unfavourable for the trimer to be formed because of rapid dissociation of actin monomers. Dissociation rate reduces as more monomers are added.
Once the trimer is formed, it becomes favourable for the trimer to bind to the actin filament.

Question 22

What controls actin filament stability and disassembly?

Answer 22

When the trimer binds to the barbed end, ATP on the actin is hydrolysed to ADP. Hydrolysis decreases strength of binding between monomers so decreases the stability of the filament.

Phosphate dissociates slowly so only some of the polymerised subunits are converted to ADP bound actin. In this way ATP hydrolysis acts as a timer for filament stability and disassembly. ATP hydrolysis helps the cell to disassemble filaments after they are formed

Question 23

What is actin polymerisation regulated by?

Answer 23

Actin monomer binding proteins
Actin nucleators
Actin filament elongation factors
Actin filament capping proteins
Actin filament severing proteins
Actin filament cross-linking proteins

Question 24

How do actin filaments produce movement of cells?

Answer 24

Barbed ends of the actin filaments face the plasma membrane.
Actin filaments are anchored to the surface on which the cell is crawling.
Elongation of these anchored actin filaments pushes the membrane forward.
The retraction at the rear of the cell is mediated by myosin motors pulling on antiparallel bundles of actin filaments.

Question 25

How do actin filaments produce movement of muscles?

Answer 25

There are myosin filaments as well as actin filaments in muscle cells. The myosin head binds to actin and move along the actin filament to produce movement.

Question 26

What is the structure of microbtubules?

Answer 26

Tube-like structure composed of dimers of GTP/GDP bound α- and β-tubulin which are all arranged in the same direction as 13 parallel protofilaments. The microtubules towards the centromere is the -ve end, and away from the centromere is the +ve end

Question 27

Where are microtubules located in the cell?

Answer 27

There are found all throughout the cell, all microtubules emitting from the centrosomes.

Question 28

Where are microfilaments located in the cell?

Answer 28

They line the edge of the cell.

Question 29

Where are intermediate filaments located in the cell?

Answer 29

They are found interspersed throughout the cell, they also go beyond the cell, going through desmosomes to fill the space of adjacent cells and hemidesomosomes to link to connect to extracellular matrix.

Question 30

How do microtubules grow?

Answer 30

Microtubules emit from the centrosomes

Question 31

What is the function of the microtubules?

Answer 31

Due to dynamic instability, the +ve end of the microtubules can explore the cytoplasm for chromosomes during mitosis or delivery of vesicles to the plasma membrane

Question 32

What is the function of intermediate filaments?

Answer 32

IT strengthen cells against mechanical stress

Question 33

What is dynamic instability?

Answer 33

Microtubules grow, then rapidly shorten followed by regrowth. Hydrolysis of GTP bound β-tubulin drives dynamic instability because microtubules with GDP bound β-tubulin are unstable.

Question 34

What are motor proteins?

Answer 34

Kinesins and Dyneins move 'cargo' to and from the microtubules utilising ATP hydrolysis. Kinesins move to the +ve end Dyneins move to the -ve end

Question 35

What is the cis and trans region of the Golgi apparatus?

Answer 35

Cis region is the region that faces the rER and the trans region is the region that faces the plasma membrane.