Proteins and cellular transport

Question 1

Folding of protein

Answer 1

Done after translation. Physical properties of proteins dictate how it folds

Question 2

Effect of protein folding on function

Answer 2

Tertiary and quaternary structure most important for function. E.g. if a receptor is the wrong shape a hormone is not able to bind. If the structure of the protein is not correct it will not function properly leading to diseas

Question 3

Clustering

Answer 3

Occurs when there are polar and non-polar side chains. Polar side chains will be on the outside, as they are hydrophilic and the non-polar side chains will cluster in the centre because they are hydrophobic

Question 4

Primary structure

Answer 4

linear sequence of amino acids in the polypeptide, amino acids will have different properties

Question 5

Secondary structure

Answer 5

Local structures within the peptide chain, caused by hydrogen bonds. Classically they are alpha-helix (spiral structures) and beta pleated sheet (ribbon structures).

Question 6

Tertiary structure

Answer 6

The folding of the secondary structure to form a 3d shape. Include hydrogen bonds, electrostatic interactions, van der waals attraction, hydrophobic interaction. Individually weak but collectively strong

Question 7

Quaternary structure

Answer 7

Formation of a protein complex with two or more polypeptide chain. Can be a globular protein.

Question 8

Quality control mechanism of the secretory pathway

Answer 8

1) The signal recognition particle (SRP) will bind to the ribosome and pause translation. The ribosome will have a specific N-terminal signal sequence.
2) The SRP bound ribosome binds to the SRP receptor on the surface of the RER.
3) The SRP drops off, the protein is now being fed into the lumen of the RER via the protein translocator, translation continues.
4) The protein is now in the secretory pathway.
5) In the RER the polypeptide chain will fold to form its tertiary structure and where post translational modifications will occur. Disulphide bonds will form.

Question 9

Where do proteins synthesised from free ribosomes end up?

Answer 9

Cytosol, mitochondria, nucleus and peroxisome. Because proteins can not move back from the ER to these organelles.

Question 10

Protein destination

Answer 10

Protein destination is dependent on the location of the ribosome during translation. Proteins are normally translated in the ribosomes in the cytosol unless signalled to go to the RER (if they have an N-terminus)

Question 11

How do proteins travel from the RER to the organelle they are needed in

Answer 11

Proteins are transported in vesicles from the RER to the golgi apparatus. =They are then packed in vesicles, the protein signals where it needs to go. Part of the membrane of the organelle bud off into the vesicles containing our protein. The vesicles then fuse and release their cargo proteins into the destination organelle.

Question 12

Exocytosis

Answer 12

Vesicles fuse with the plasma membrane and release their cargo outside the cell

Question 13

Endocytosis

Answer 13

vesicles form at the plasma membrane to capture proteins and move them into the cell

Question 14

What is intracellular fluid?

Answer 14

Fluid within cells. Accounts for two thirds of fluid

Question 15

What is extracellular fluid?

Answer 15

Fluid outside cells. Accounts for a third of our body fluid, divided between interstitial and plasma fluid

Question 16

What is plasma fluid?

Answer 16

The extracellular components of the blood present in the heart and blood vessels (intravascular compartments)

Question 17

What is interstitial fluid?

Answer 17

Bathes the non-blood cells of the the body

Question 18

What is transcellular fluid?

Answer 18

The fluid between epithelial lined spaces i.e. joint fluid

Question 19

Movement between fluid compartments

Answer 19

Water can move freely between the different compartments but solutes can not because of cell membranes

Question 20

Contents of intracellular fluid

Answer 20

High concentrations of K+ and Mg+ driven by Na+/K+ ATPase pump

Question 21

Contents of extracellular fluid

Answer 21

Na+ is the major cation, Cl- and HCO2- are the major anions. Interstitial is similar to plasma but less proteins

Question 22

Osmolarity in fluid compartments

Answer 22

Electrolytes (ionised substances) contribute the most to solute concentration (osmolality). Normally osmolality is the same in each compartment and they are said to be in equilibrium. If there is a change in one compartment, water will move between the compartments and restore this equilibrium. Fluid composition is driven by cell membranes and transport proteins

Question 23

Channel proteins

Answer 23

Integral membrane proteins which allow direct access to the cell and facilitate uncoupled transport of a solute down a concentration gradient i.e. aquaporins.

Question 24

Gated channels

Answer 24

Integral membrane proteins that open in response to a stimuli and facilitate uncoupled transport of a solute down a concentration gradient. The stimuli which allow them to open can be a voltage, mechanical (if the cell is stretching) or ligand binding. If the gate is closed a solute cannot move even if there is a concentration gradient.

Question 25

Carrier mediated (uniporters)

Answer 25

Solute moves due to an electrochemical gradient i.e. glucose transporters. A carrier opens into the extracellular space, the solute then binds to the open transporter which will cause a change in shape. The outer gate will then close and the inner gate will open, allowing the solute to move through the membrane

Question 26

Pros and cons of carrier mediated (uniporters)

Answer 26

pro- greater flux then passive diffusion
con- reaches saturation at lower concentration because there is a definitive number of carriers. System can be poisoned by a similar looking solute

Question 27

Secondary active transporters

Answer 27

They move one solute down its concentration gradient which provides the energy to move another solute against theirs. Both solutes are bound to the transporter. Two types symporter and antiporters

Question 28

Symporters

Answer 28

Secondary active transporters which move two ions in the same direction e.g. sodium and glucose transporter

Question 29

Antiporters

Answer 29

Secondary active transporters that move two ions in opposite directions. I.e. sodium/ proton acceptor

Question 30

Primary active transporters

Answer 30

Uses the energy from the hydrolyses of ATP to move solutes against their concentration gradient. The transporter protein has a domain which can bind to ATP and catalyse its hydrolysis to ADP and inorganic phosphate. The energy released is used to change the shape of the transporter

Question 31

Types of primary active transporters

Answer 31

P-type ATPase, F-ATPase, V-ATPase and ABC transporters. The first three are ATPases and the final one is an ATP binding cassette transporter.

Question 32

Passive diffusion

Answer 32

Permeable substances move from an area of high concentration to an area of low concentration down an electrochemical gradient through a plasma membrane. As substances get bigger and more heavily charged they become less permeable.

Question 33

How permeability effects transport

Answer 33

Solutes with high permeability can move straight through the plasma membrane, those with low permeability will need a transport protein.