Membrane Transport

Question 1

What is passive transport?

Answer 1

Passive transport does not require the cell to expend any energy and involves a substance diffusing down its concentration gradient across a membrane.

Question 2

What are the features of passive transport? (4)

Answer 2

• No energy input (diffusion)
• May require carrier protein (facilitated transport)
• Movement down the concentration gradient
• Molecules need to be small and hydrophobic

Question 3

What is diffusion?

Answer 3

Diffusion:
o Passive movement of molecules down concentration gradient until equilibrium is reached
>substance tends to move from an area of high concentration to an area of low concentration until its concentration becomes equal throughout a space

Question 4

What does it mean if we to the plasma membranes are amphipathic?

Answer 4

> The phospholipids of plasma membranes are amphipathic: they have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.

Question 5

What does the hydrophobic core of the plasma membrane help with?

Answer 5

> The hydrophobic core of the plasma membrane helps some materials move through the membrane, while it blocks the movement of others.

Question 6

Which type of molecules have trouble crossing the membrane?

Answer 6

> Polar and charged molecules have much more trouble crossing the membrane.
Polar molecules can easily interact with the outer face of the membrane, where the negatively charged head groups are found, but they have difficulty passing through its hydrophobic core.

Question 7

> Water molecules, for instance, cannot cross the membrane ____ (although thanks to their small size and lack of a full charge, they can cross at a slow rate).

Answer 7

rapidly

Question 8

> Additionally, while small ions are the right size to slip through the membrane, their charge prevents them from doing so. What are the implications of this?

Answer 8

> This means that ions like sodium, potassium, calcium, and chloride cannot cross membranes to any significant degree by simple diffusion, and must instead be transported by specialized proteins (which we’ll discuss later).

Question 9

> Larger charged and polar molecules, like sugars and amino acids, also need help from _____ to efficiently cross the membrane.

Answer 9

proteins

Question 10

o Polar molecules and ions impeded by the lipid bilayer, diffuse ____ with the help of transport proteins.

Answer 10

passively

Question 11

What are the two types of proteins?

Answer 11

Channel and Carrier

Question 12

Some molecules, such as carbon dioxide and oxygen, can diffuse across the plasma membrane
directly, but others need help to cross its hydrophobic core. How do molecules diffuse in facilitated diffusion?

Answer 12

In facilitated diffusion, molecules diffuse across the plasma membrane with assistance from membrane proteins, such as channels and carriers.

Question 13

Is the concentration gradient effective in allowing charged molecules to diffuse?

Answer 13

A concentration gradient exists for these molecules, so they have the potential to diffuse into (or out
of) the cell by moving down it. However, because they are charged or polar, they can’t cross the
phospholipid part of the membrane without help.

Question 14

How do charged molecules cross the membrane?

Answer 14

Facilitated transport proteins shield these molecules from the hydrophobic core of the membrane, providing a route by which they can cross. Two major classes of facilitated transport proteins are channels and carrier proteins.

Question 15

What is osmosis?

Answer 15

Osmosis is the spontaneous net movement or diffusion of solvent molecules through a selectively permeable membrane from a region of high water potential to a region of low water potential, in the direction that tends to equalize the solute concentrations on the two sides.

Question 16

How do carrier proteins change their shape? (2)

Answer 16

> can change their shape to move a target molecule from one side of the membrane to the other (carrier proteins are typically selective for one or a few substances).
Often, they will change shape in response to the binding of their target molecule, with the shape change moving the molecule to the opposite side of the membrane.

Question 17

What do the carrier proteins in facilitated diffusion provide hydrophilic molecules with?

Answer 17

> The carrier proteins involved in facilitated diffusion simply provide hydrophilic molecules with a way to move down an existing concentration gradient (rather than acting as pumps).

Question 18

Isotonic, hypotonic and hypertonic refers to the ____ a cell is placed in.

Answer 18

solutions

Question 19

> Organisms without cell walls on their cells must have other adaptations for the _____ of the organism

Answer 19

osmoregulation

Question 20

Cell wall containing cells water control: (3)

Answer 20

> turgor pressure: cell wall exerts pressure on the interior of the cell to resist more water uptake. (Turgid cells)*hypotonic exterior
Isotonic cells & environment= flaccid (no tendency,net tendency or water
to be absorbed )
too little water= (plasmolysis) (hypertonic environments)

Question 21

What do channel proteins do?

Answer 21

> span the membrane and make hydrophilic tunnels across it, allowing their target molecules to pass through by diffusion.

Question 22

> Channels are very ____ and will accept only one type of molecule (or a few closely related molecules) for transport.

Answer 22

selective

Question 23

What does a passage through a channel protein allow for?

Answer 23

> Passage through a channel protein allows polar and charged compounds to avoid the hydrophobic core of the plasma membrane, which would otherwise slow or block their entry into the cell.

Question 24

What are ionic channels?

Answer 24

> ionic channels: protein channels carrying ions -gated channels: they open/close to a stimulus
(includes ion channels)

Question 25

What are aquaporins?

Answer 25

Aquaporins are channel proteins that allow water to cross the membrane very quickly, and they play important roles in plant cells, red blood cells, and certain parts of the kidney (where they minimize the amount of water lost as urine).

Question 26

What is active transport?

Answer 26

Movement of molecules/ions against the concentration concentration gradient. Movement occurs from high to low concentration.

Question 27

Active transport require ______ proteins (only carrier).

Answer 27

transport

Question 28

What does active transport allow the cell to maintain?

Answer 28

Method allows for cell to maintain internal concentrations that differ from its environment

Question 29

What energy does active transport utilise?

Answer 29

Requires ATP (energy) to pump solute against the gradient
o ATP transfers P to carrier protein
o Results in carrier protein changing shape
• Sodium-potassium pump is one example that uses this method—>Na+ is pumped out and K+ in
o Carrier protein acts as a pump and transfers the ions to and from the extracellular fluid

Question 30

What is the sodium-potassium pump?

Answer 30

One of the most important pumps in animal cells is the sodium-potassium pump, which moves Na+ out of cells, and K+ into them. Because the transport process uses ATP as an energy source, it is considered an example of primary active transport.

Not only does the sodium-potassium pump maintain correct concentrations of Na+ and K+ in living cells, but it also plays a major role in generating the voltage across the cell membrane in animal cells. Pumps like this, which are involved in the establishment and maintenance of membrane voltages, are known as electrogenic pumps. The primary electrogenic pump in plants is one that pumps hydrogen ions (H*) rather than sodium and potassium^2,3.

Instead, the sodium-potassium pump acts primarily by building up a high concentration of potassium ions inside the cell, which makes potassium’s concentration gradient very steep. The gradient is steep enough that potassium ions will move out of the cell (via channels), despite a growing negative charge on the interior. This process continues until the voltage across the membrane is large enough to counterbalance potassium’s concentration gradient. At this balance point, the inside of the membrane is negative relative to the outside. This voltage will be maintained as long as K+ concentration in the cell stays high, but will disappear if K+ stops being imported^4,5.

Question 31

How does the sodium-potassium pump work? (6)

Answer 31

1. To begin, the pump is open to the inside of the cell. In this form, the pump really likes to bind (has a high affinity for) sodium ions, and will take up three of them.
2. When the sodium ions bind, they trigger the pump to hydrolyze (breakdown) ATP. One phosphate
   group from ATP is attached to the pump, which is then said to be phosphorylated. ADP is released as a by-product.
3. Phosphorylation makes the pump change shape, re-orienting itself so it opens towards the extracellular space. In this conformation, the pump no longer likes to bind to sodium ions (has a low affinity for them), so the three sodium ions are released outside the cell.
4. In its outward-facing form, the pump switches allegiances and now really likes to bind to (has a high affinity for) potassium ions. It will bind two of them, and this triggers removal of the phosphate group attached to the pump in step 2.
5. With the phosphate group gone, the pump will change back to its original form, opening towards the interior of the cell.
6. In its inward-facing shape, the pump loses its interest in (has a low affinity for) potassium ions, so the two potassium ions will be released into the cytoplasm. The pump is now back to where it was in step 1, and the cycle can begin again.

Question 32

What is the electromechnaical gradient?

Answer 32

> The electrochemical gradient is the sum of the forced acting on an ion as a result of a concentration gradient (chemical reasoning) and the electrical gradient ( difference in charge across a membrane)
Therefore, am ion passively moves down its electrochemical gradient, not just down a concentration gradient

Question 33

> Inner membrane(cytoplasm side)= _____.

>Outer membrane(ECM)= _____.

Answer 33

-ve [Na]

+ve [Na]

Question 34

The sodium-potassium pump is known as an electrogenic pump; creates/transports a voltage across the membrane. Elaborate on this. (2)

Answer 34

• every 2 K+ ions actively transported into a cell is met with 3Na+ ions actively transported out.
• external (ECM) is therefore more positive

Question 35

What can a co-transporter do? (2)

Answer 35

> a cotransporter (transport protein) can coupler a downhill & uphill transport.
(Both with and against a gradient) -2 different substances
passive diffusion & active transport take place simultaneously

Question 36

What is secondary transport?

Answer 36

The electrochemical gradients set up by primary active transport store energy,
which can be released as the ions move back down their gradients. Secondary
active transport uses the energy stored in these gradients to move other
substances against their own gradients.

Question 37

How does secondary transport work?

Answer 37

In secondary active transport, the two molecules being transported may move either in the same direction (i.e., both into the cell), or in opposite directions (i.e., one into and one out of the cell). When they move in the same direction, the protein that transports them is called a symporter, while if they move in opposite directions, the protein is called an antiporter.

Question 38

In secondary active transport, the movement of the sodium ions down their gradient is coupled to the uphill transport of other substances by a shared carrier protein (a cotransporter). For instance, in the figure below, a carrier protein lets _____ ions move down their gradient, but simultaneously brings a glucose molecule up its gradient and into the cell. The carrier protein uses the energy of the sodium _____ to drive the transport of _____ molecules.

Answer 38

solium
gradient
glucose

Question 39

What is exocytosis?

Answer 39

• Exocytosis: vesicles releasing substances from the cell to the external environment—> eg. the secretion of substances produced by the cell
o Vesicle attaches to plasma membrane, releases substances, and then forms part of the membrane

Question 40

What does endocytosis involve?

Answer 40

• Endocytosis: involves substances being brought into the cell from the external environment
> vesicles transport these substances

Question 41

In what ways does endocytosis occur? (3)

Answer 41

1. Phagocytosis
   - Membrane engulfs food particles—>forms vacuole (larger)
   - Broken down by lysosome
2. Pinocytosis
   - Membrane surrounds the particles —> forms vesicle
   - Vesicles form around liquid or small particles
   - Non-specific particles are ‘gulped’
3. Receptor-mediatedendocytosis
   - A form of pinocytosis BUT receptor proteins on the plasma membrane attract specific molecules
   - Molecules attach to receptors on membrane—> forms vesicle