1.4 Membrane transport

Question 1

Define diffusion

Answer 1

Diffusion is the movement of particles from a region of high concentration to a region of low concentration, and is the result of the random motion of particles.

Question 2

What are the two types of diffusion?

Answer 2

Simple and facilitated

Question 3

Diagram showing passive vs. active transport

Answer 3

Question 4

Both simple and facilitated diffusion are affected by factors such as ___

Answer 4

1) Temperature
2) Surface area of membrane
3) Size of particles
4) Concentration gradient of diffusing particles

Question 5

Effect of temperature on diffusion

Answer 5

Molecules diffuse faster if the temperature is higher.

Question 6

Effect of surface area of membrane on diffusion

Answer 6

An increase in surface area allows more molecules to diffuse.

Question 7

Effect of size of particles on diffusion

Answer 7

Smaller molecules diffuse faster than larger ones.

Question 8

Effect of concentration gradient of diffusing particles on diffusion

Answer 8

The steeper the gradient, the faster the diffusion.

Question 9

Simple diffusion

Answer 9

Simple diffusion occurs in a gas or liquid medium and only requires a concentration gradient. It occurs in both living and non-living systems.

Question 10

Example of simple diffusion

Answer 10

Non-polar molecules such as oxygen:

-If there is ahigher concentration of oxygen outside the cell than inside, there will be a net movement of oxygen across the plasma membrane into the cell.

Question 11

What happes if a particle is too big in the case of simple diffusion?

Answer 11

• If a particle is too big, it cannot pass through the phospholipid bilayer of the membrane.
• Similarly, charged particles (ions) are repelled by hydrophobic tails in the membrane.
• In both cases, when simple diffusion fails, facilitated diffusion can transfer the particle across the membrane.

Question 12

Diagram showing different types of diffusion

Answer 12

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Question 13

Facilitated diffusion

Answer 13

Facilitated diffusion is similar to simple diffusion, except that it requires channel proteins or carrier proteins, which are specific to the molecules being transported across the plasma membrane.

Question 14

What is required for facilitated diffusion?

Answer 14

• The size and shape of protein carriers and channels determine what substance can cross the membrane.
• An example is chloride ion channels that only allow the facilitated transport of chloride ions.
• The two types of proteins involved in facilitated diffusion are channel proteins and carrier proteins.

Question 15

Diagram of the types of proteins used in facilitated diffusion

Answer 15

Question 16

Important application of facilitated diffusion: movement of K+ ions in neurons during the generation of an action potential

Answer 16

• One important application of facilitated diffusion is in the movement of K+ ions in neurons during the generation of an action potential (a key step in the propagation of nerve impulses along neurons).
• It helps in moving K+ ions out of the axons to cause repolarisation.
• The K+ channels involved only allow the movement of K+.
• They are also voltage gated, that is, they open and close with changes in electrical potential to control movement of K+ ions.
• This is important in the functioning of neurons.

Question 17

Diagram showing potassium ions channel in the axon of a neuron

Answer 17

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Question 18

Define osmosis

Answer 18

Osmosis is the passive movement of water molecules from a region of lower solute concentration to a region of higher solute concentration across a partially permeable membrane.

Question 19

Osmosis

Answer 19

• Osmosis is a special form of diffusion that involves water molecules only.
• Solutes are particles that are dissolved in water, so water would be the solvent.
• Although solutes do not move during osmosis, they play an important role in determining the direction in which water molecules will move.
• Water molecules move through a partially permeable membrane from the side where the concentration of solute is low (more water molecules), to the side where the concentration of solute is high (fewer water molecules).

Question 20

Diagram showing osmosis

Answer 20

Question 21

Tissues and organs used in medical procedures

Answer 21

• In medical procedures, such as transplant operations, tissues need to be kept in a saline solution for storage.
• It is essential that the osmolarity (a measure of the solute concentration) of the saline solution is the same as that in the cytoplasm of the cells of the tissue to prevent any osmosis (gain or loss of water) that would damage the cells.

Question 22

What terms are used to describe the difference in solute concentration between two solutions?

Answer 22

• Hypertonic, isotonic, and hypotonic
• Usually, but not always, this is comparing between the solution outside the cell and the cytoplasm inside the cell.

Question 23

Hypertonic solution

Answer 23

The solution with the higher concentration of solutes is called the hypertonic solution.

Question 24

Hypotonic solution

Answer 24

The solution with the lower concentration of solutes is called the hypotonic solution.

Question 25

Water always moves by osmosis from the ___ to the ___

Answer 25

From the hypotonic solution to the hypertonic solution.

Question 26

The terms hypertonic and hypotonic are always ___

Answer 26

• Relative terms, not absolute terms, so can only be used when comparing two solutions.
• If you see it written that “A cell is put in a hypertonic solution”, this implies that the solution outside the cell is hypertonic relative to the cytoplasm of the cell.
• Logically therefore, the cytoplasm is hypotonic relative to the solution outside the cell.

Question 27

Isotonic solution

Answer 27

If the concentration of solutes is equal between the two solutions, we say that the solutions are isotonic.

Question 28

Diagram showing cellular responses to various solute concentrations

Answer 28

Question 29

It is of utmost importance that all tissues and organs to be used in medical procedures are kept in solutions that maintain the cells in a state ___

Answer 29

• As close to normal structure and function as possible.
• Tissue preservation solutions are generally designed to do this by minimising cell and tissue swelling and maintaining ionic balance​.

Question 30

Description and diagram of voltage-gated potassium channels

Answer 30

• Integral proteins with a hydrophilic inner pore via which potassium ions may be transported
• The channel is comprised of four transmembrane subunits, while the inner pore contains a selectivity filter at its narrowest region that restricts passage of alternative ions
• Potassium channels are typically voltage-gated and cycle between an opened and closed conformation depending on the transmembrane voltage

Question 31

What is essential for simple diffusion?

Answer 31

A concentration gradient

Question 32

Diffusion vs. osmosis chart

Question 33

Osmosis definition in terms of

Question 34

Define active transport

Answer 34

Active transport is the movement of particles across membranes, requiring energy in the form of ATP (mnemonic: Active TransPort requires ATP).

Question 35

What is the energy required for active transport used for?

Answer 35

• The energy is used to move substances against a concentration gradient, from a region of low concentration to one of a higher concentration.
• This is useful, for example, in the roots of plants.
• Some minerals may not be in a high enough concentration to diffuse into the cells of roots.
• Active transport can overcome this problem by using energy to move those substances in.
• In the epithelial cells of the small intestine, glucose is actively transported into the epithelial cells lining the small intestine.
• This creates a high concentration of glucose inside the epithelial cells, which then allows glucose to diffuse into the blood.

Question 36

Diagram showing active transport

Answer 36

Question 37

Active transport: sodium-potassium (Na+/ K+) pumps

Answer 37

• A very special form of active transport can be seen in the sodium–potassium (Na+/ K+) pumps.
• These pumps are very important in the nervous system for the maintenance of resting potential in neurons.
• Basically, the concentration of sodium ions needs to be relatively higher outside the neuron than inside a neuron, while potassium ions are more concentrated inside than outside the neuron.

Question 38

Stages of active transport by sodium-potassium (Na+/ K+) pumps

Answer 38

1) When the pump is open to the inside of the axon, three sodium ions (Na+) enter the pump and attach to their binding sites.
2) ATP donates a phosphate group to the pump.
3) The previous stage causes the protein to change shape, expelling Na+ to the outside.
4) Two potassium ions (K+) from outside then enter and attach to their binding sites.
5) The binding of the K+ leads to the release of the phosphate which causes the pump to change shape again so that it is only open to the inside of the axon.
6) K+ is released inside.
7) Na+ can now enter and bind to the pump again.

Question 39

Diagram showing how how sodium-potassium (Na+/ K+) pumps work

Answer 39

Question 40

What evidence would suggest that a substance entered a cell via active rather than passive transport?

Answer 40

1) The substance moved from a low concentration to a high concentration.
2) The substance moved across the membrane via a pump.
3) ATP was required for transport.

Question 41

When are endocytosis and exocytosis needed?

Answer 41

• Often cells have to secrete or absorb much larger quantities of a substance or even take up other cells.
• This takes the form of bulk transport and can be of two types: endocytosis and exocytosis.
• Both of these methods of transport utilise energy and are highly dependent on the flexibility of the plasma membrane.

Question 42

Diagram showing endo and exocytosis

Answer 42

Question 43

What is endocytosis?

Answer 43

A cellular process where cells take in molecules or substances from outside of the cell by engulfing them in the cell membrane.

Question 44

What is exocytosis?

Answer 44

The opposite of endocytosis and involves the ejection of waste products or useful substances (such as hormones) from the inside of the cell.

Question 45

Why are endo and exocytosis critical to the survival of cells?

Answer 45

Since many important molecules cannot normally pass through the cell’s hydrophobic plasma membrane.

Question 46

What two groups can endocytosis be divided into?

Answer 46

Phagocytosis and pinocytosis

Question 47

Pinocytosis

Answer 47

Pinocytosis is the taking in of liquid substances by cells (‘cell drinking’).

Question 48

Phagocytosis

Answer 48

Phagocytosis involves the absorption of solids (‘cell eating’).

Question 49

Example of endocytosis (phagocytosis)

Answer 49

When a white blood cell or phagocyte engulfs a pathogenic microbe.

Question 50

Example of endocytosis (phagocytosis)

Answer 50

When a white blood cell or phagocyte engulfs a pathogenic microbe.

Question 51

Diagram showing endocytosis

Answer 51

Question 52

Depending on the type of material ejected from the cell, exocytosis can be subdivided into ___

Answer 52

Excretion and secretion

Question 53

Excretion

Answer 53

Following phagocytosis, any undigested remains of the microbe that are not useful to the cell are excreted outside the cell.

Question 54

Secretion

Answer 54

Proteins synthesised by ribosomes on rough endoplasmic reticulum are first passed to the Golgi apparatus via vesicles, where they are processed and packaged (to give the enzymes or hormones the correct conformations) before being released in vesicles that in turn fuse with the plasma membrane for secretion outside the cell.

Question 55

Diagram showing exocytosis

Answer 55

Question 56

Role of vesicles in endo and exocytosis

Answer 56

• Vesicles play a very important role in both exocytosis and endocytosis as they allow the movement of materials within the cell.
• Many of the organelles in the cell are membrane-bound and that means that they can make their own vesicles.
• This is a common way for the cell to move molecules around inside cells.
• For example, enzymes might be made by bound ribosomes in the rough ER (rER).
• The rough ER will package the enzymes in a vesicle formed from the membranes of the rough ER.
• The vesicles carrying the enzymes will then move to the Golgi apparatus, and fuse with the Golgi apparatus membrane.
• The enzymes will then be modified further in the Golgi apparatus and then packed in a vesicle created using the Golgi apparatus membrane.
• The vesicle will then move toward the plasma membrane and undergo exocytosis and release the enzymes out of the cell.
• The opposite is also true, as Figure 2 shows: substances can be taken into the cell and moved around.

Question 57

Diagram showing the role of vesicles in moving materials within the cell

Answer 57

Question 58

What is the order of destinations of a protein undergoing exocytosis?

Answer 58

Ribosomes on rER, Golgi apparatus, Vesicles fusing with plasma membrane.

Question 59

Exocytosis is a process by which cells ___

Answer 59

Release substances from the cell via vesicles.

Question 60

Estimating the osmolarity in tissues (1.4.4)

Question 61

Define osmolarity

Answer 61

The concentration of a solution in terms of moles of solutes per litre of solution.

Question 62

When the concentration of water is the same on both sides of a membrane there is no net movement. This is shown on a graph where the trendline crosses the x-axis. What is this point called?

Answer 62

Isotonic point

Question 63

Table showing what the phospholipid bilayer is and isn’t permeable to

Answer 63

Question 64

Are endo and exocytosis active or passive processes?

Answer 64

As these transport processes require energy, they are known as active transport processes.