U1 KA3 - MEMBRANE PROTEINS -1)movement Of Molecules Across Membranes

Question 1

How is the structure of the plasma membrane described ?

Answer 1

The structure of the plasma membrane is usually described using the fluid mosaic model

Question 2

What are membranes composed of

Answer 2

• the membrane of cells are comprised of a phospholipid bi-layer with globular proteins penetrating the bi layer or attached to the surface
• this is called the fluid mosaic model of the membrane structure : the phospholipid bi layer can move so it has fluid character ,and the separate proteins are like the small pieces of a mosaic

Question 3

Phospholipid molecule composition

• hydrophobic / hydrophilic
• bilayer

Answer 3

A phospholipid molecule has a hydrophilic head and hydrophobic tails.

• the head region of phospholipid molecule is charged and therefore hydrophilic (attracted to water)
• the tail is uncharged and non polar , therefore hydrophobic (repelled by water
• the phospholipid forms a bi layer , with the hydrophilic heads towards the aqueous cytoplasm and the aqueous external fluid and the hydrophobic tails to the inside of the bi layer

Question 4

The two types of membrane proteins

Answer 4

• intregal

- peripheral

Question 5

Integral proteins

• placement
• how are they held in the bi layer
• size (transmembrane / other) and examples

Answer 5

• integral membrane proteins are held firmly in place within the membrane. They penetrate the hydrophobic interior of the phospholipid bi layer.
• they are folded so they have regions of hydrophobic R groups. The R groups on their amino acids allow integral proteins to be held permanently within the bi layer because of strong hydrophobic interactions with it. (Integral membrane proteins interact exclusively with the hydrophobic region of membrane phospholipids)
• some integral membrane proteins are transmembrane- this means they span the entire width of the membrane. Examples of transmembrane proteins include channels , transporters and many receptors

Look at diagrams on page30 brightred and 70scholar

Question 6

Peripheral proteins

• placement
• how are they held
• other interactions

Answer 6

• peripheral membrane proteins are only loosely associated with the plasma membrane and are not embedded in the phospholipid bi layer.
• they have hydrophilic R groups on their surface and are bound to the surface of membranes, mainly by ionic and hydrogen bond interactions.
• many peripheral proteins interact with the surfaces of integral membrane proteins
• some peripheral proteins on the inside of the membrane are attached to the cytoskeleton , which helps give mechanical support and shape to the cells.

Question 7

How can small molecules such as oxygen and carbon dioxide pass through the hydrophobic centre of the phospholipid bi layer?

Answer 7

• small molecules such as oxygen and carbon dioxide can pass through the bi layer centre directly as they are SMALL NON POLAR molecules, and pass through by simple diffusion

Question 8

What do most (other molecules that are not oxygen or carbon diode ) molecules require to be able to enter / exit the cell.
- why is this ?

Answer 8

• most molecules require membrane proteins to enter or exit the cell.
• this is because the phospholipid bi layer is a barrier to ions (eg H+ , Na+, amino acids) and most uncharged polar molecules (water or glucose)

Question 9

Explain how most molecules pass through the membrane specifically (the two ways how they can pass through)

Answer 9

• any ion or polar molecule that can pass across the membrane can only do so through a specific channel protein or transporter protein.
• for example there are specific transmembrane proteins that act as sodium channels , glucose transporters or proton pumps

Question 10

To perform specialised functions different cell types have different _______ and ___________ proteins

Answer 10

To perform specialised functions different cells have different channel and transporter proteins

Question 11

What is facilitated diffusion

Answer 11

Facilitated diffusion is the passive transport of substances across the membrane through specific transmembrane proteins

Question 12

Channel proteins

• what are they / definition
• how do they allow facilitated diffusion
• selective yes or no

Answer 12

• channels are multi- subunit proteins with the subunits arranged to form water filled pores that extend across the membrane ( look at brightred page 30 or scholar71
• the passage of ions or molecules through channel proteins is an example of facilitated diffusion because the protein makes it easier for the ion or molecule to move passively across the membrane
• most channel proteins in animal and plant cells are highly selective

Question 13

What is a water channel called, how does it allow water to pass through

Answer 13

• Aquaporin
• most water passes through the membrane via aquaporin which can allow up to 3 billion water molecules to move across the membrane per second. The direction of water movement is dependent upon the osmotic gradient

Question 14

What are gated channels

Answer 14

• some channel proteins can be opened to allow diffusion of their ion , or closed to prevent diffusion of their ion - these are described as gated channels and they change conformation to allow or prevent diffusion
• channels respond to a stimulus which causes them to open or close

Question 15

The two types of gated channels /stimulus

Answer 15

• ligand gated channels (chemical)

- voltage gated channels (electrical )

Question 16

Ligand gated channels

• how are they controlled
• example

Answer 16

• ligand gated channels are controlled by the binding of signal molecules : signal molecules bind to the ligand gated channel protein and change its conformation
• eg to pass a signal at a synapse , a neurotransmitter ligand binds to a ligand gated channel protein to trigger it OPEN and let Na+ ions through
• in rods and cones in the retina , a ligand keeps the sodium channel closed until light activates a pathway to release the ligand and the channel lets ions through to generate a signal.

Look at diagrams brightred pg 31 and scholarpg72

Question 17

Voltage gated channels

• how are they controlled
• example

Answer 17

• if there is a large enough change in the ion concentration across the membrane, this can change the conformation of a voltage gated channel protein.
• eg in the transmission of a nerve impulse along a nerve, the movement of ions across the membrane causes the voltage across the membrane to reach a critical level , so sodium channels open. The voltage change continues as more ions move across the membrane, which causes the channel to close again.

Look at brightred pg 31 and scholar pg 73 for diagrams

Question 18

Transporter proteins

• how do they work ?
• why do they alternate between two conformations

Answer 18

• transporter proteins bind TO the ions/molecule to be transported , causing the protein to undergo conformational change- the proteins actually pass the ions or molecules across the membrane , rather than just providing a route through.
• transporters alternate between two conformations so that the binding site for a solute is sequentially exposed on one side of the bi layer than the other, therefore allowing the solute to move into or out of the cell

Question 19

Active transport

Answer 19

Active transport uses energy to transfer substances across the membrane against their concentration gradient (low to high)

Question 20

What brings about active transport (in terms of proteins)

Answer 20

• active transport uses pumps that are transporter proteins , that transfer substances across the membrane against their concentration gradient
• pumps that mediate active transport are transporter protein coupled with an energy source (a source of metabolic energy is required for active transport)

Question 21

Active transport proteins

• ATPases
• how does it work

Answer 21

• some active transport proteins are ATPases, which hydrolyse ATP directly to provide the energy for the conformational change required to moved substances across the membrane