Cell Biology: Topic 1.4 Membrane Transport

Question 1

What are the two key qualities of cell membranes?

Answer 1

“Cellular membranes possess two key qualities:

They are semi-permeable (only certain materials may freely cross – large and charged substances are typically blocked)
They are selective (membrane proteins may regulate the passage of material that cannot freely cross)”

Question 2

Why are cell membranes semi-permeable?

Answer 2

Phospholipid bilayers forms the cell membrane, and it is semi permeable because of the presence of polar phospholipid heads which are attracted to only charged/polar substances or molecules, whereas the non-polar fatty acid tails at the centre of the membrane repel charged molecules (unless very small such as ethanol and urea), hence preventing passage of ions through the membrane.

Question 3

What is Passive transport?

Answer 3

“Passive transport involves the movement of material along a concentration gradient (high concentration ⇒ low concentration)

Because materials are moving down a concentration gradient, it does not require the expenditure of energy (ATP hydrolysis)”

Question 4

List the types of Passive transport (and the materials they are known to transport)

Answer 4

“There are three main types of passive transport:

Simple diffusion – movement of small or lipophilic molecules (e.g. O2, CO2, etc.)
Osmosis – movement of water molecules (dependent on solute concentrations)
Facilitated diffusion – movement of large or charged molecules via membrane proteins (e.g. ions, sucrose, etc.)”

Question 5

What is Active transport?

Answer 5

“Active transport involves the movement of materials against a concentration gradient (low concentration ⇒ high concentration)

Because materials are moving against the gradient, it requires the expenditure of energy (e.g. ATP hydrolysis)
“

Question 6

List the types of Active transport

Answer 6

“There are two main types of active transport:

Primary (direct) active transport – Involves the direct use of metabolic energy (e.g. ATP hydrolysis) to mediate transport
Secondary (indirect) active transport – Involves coupling the molecule with another moving along an electrochemical gradient”

Question 7

Define Diffusion and outline the kind of transport and materials involved in this mechanism of transport

Answer 7

”
Diffusion is the net movement of molecules from a region of high concentration to a region of low concentration

This directional movement along a gradient is passive and will continue until molecules become evenly dispersed (equilibrium)
Small and non-polar (lipophilic) molecules will be able to freely diffuse across cell membranes (e.g. O2, CO2, glycerol)”

Question 8

The rate of diffusion can be influenced by a number of factors, including:

Answer 8

Temperature (affects kinetic energy of particles in solution)
Molecular size (larger particles are subjected to greater resistance within a fluid medium)
Steepness of gradient (rate of diffusion will be greater with a higher concentration gradient)

Question 9

Define Osmosis

Answer 9

Osmosis is the net movement of water molecules across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration (until equilibrium is reached)

Question 10

How does osmosis occur as a function or mechanism of transport of materials

Answer 10

Osmosis is due to differences in the concentration of substances dissolved in water ( solutes) . Substances dissolve by forming intermolecular bonds with water molecules. These bonds restrict the movement of the water molecules. Regions with a higher solute concentration, therefore have less free water molecules in solution as water is associated with the solute. Because solutes cannot cross a cell membrane unaided, water will move to equalise the two solutions. As Osmosis is essentially the diffusion of free water molecules, hence occurs from regions of low solute concentration which have more free water molecules that can move.

Question 11

What are aquaporins? (give examples)

Answer 11

Some cells have water channels called aquaporins, which greatly increase membrane permeability to water. Examples are kidney cells that reabsorb water and root hair cells that absorb water from the soil.

Question 12

Define osmolarity

Answer 12

Osmolarity is a measure of solute concentration, as defined by the number of osmoles of a solute per litre of solution (osmol/L)

Question 13

How are solutions categorised as per their relative osmolarity?

Answer 13

“Solutions may be loosely categorised as hypertonic, hypotonic or isotonic according to their relative osmolarity

Solutions with a relatively higher osmolarity are categorised as hypertonic (high solute concentration ⇒ gains water)
Solutions with a relatively lower osmolarity are categorised as hypotonic (low solute concentration ⇒ loses water)
Solutions that have the same osmolarity are categorised as isotonic (same solute concentration ⇒ no net water flow)”

Question 14

How is osmolarity of a tissue estimated?

Answer 14

“The osmolarity of a tissue may be interpolated by bathing the sample in solutions with known osmolarities

The tissue will lose water when placed in hypertonic solutions and gain water when placed in hypotonic solutions
Water loss or gain may be determined by weighing the sample before and after bathing in solution
Tissue osmolarity may be inferred by identifying the concentration of solution at which there is no weight change (i.e. isotonic)”

Question 15

Why is osmolarity important in relevance to tissues and organs used in medical procedures

Answer 15

“Tissues or organs to be used in medical procedures must be kept in solution to prevent cellular dessication

This solution must share the same osmolarity as the tissue / organ (i.e. isotonic) in order to prevent osmosis from occurring

Uncontrolled osmosis will have negative effects with regards to cell viability:

In hypertonic solutions, water will leave the cell causing it to shrivel (crenation)
In hypotonic solutions, water will enter the cell causing it to swell and potentially burst (lysis)”

Question 16

What are the effects of uncontrolled osmosis in plant tissues?

Answer 16

“In plant tissues, the effects of uncontrolled osmosis are moderated by the presence of an inflexible cell wall

In hypertonic solutions, the cytoplasm will shrink (plasmolysis) but the cell wall will maintain a structured shape
In hypotonic solutions, the cytoplasm will expand but be unable to rupture within the constraints of the cell wall (turgor)”

Question 17

Define Facilitated Diffusion and state what substances uses this transport mechanism and what additional structures are involved in this mechanism?

Answer 17

Facilitated diffusion is the passive movement of molecules across the cell membrane via the aid of a membrane protein

It is utilised by molecules that are unable to freely cross the phospholipid bilayer (e.g. large, polar molecules and ions)
This process is mediated by two distinct types of transport proteins – channel proteins and carrier proteins

Question 18

Outline the working of carrier proteins

Answer 18

1. Integral glycoproteins which bind a solute and undergo a conformational change to translocate the solute across the membrane
2. Carrier proteins will only bind a specific molecule via an attachment similar to an enzyme-substrate interaction
3. Carrier proteins may move molecules against concentration gradients in the presence of ATP (i.e. are used in active transport)
4. Carrier proteins have a much slower rate of transport than channel proteins (by an order of ~1,000 molecules per second)

Question 19

Outline the working of channel proteins

Answer 19

Integral lipoproteins which contain a pore via which ions may cross from one side of the membrane to the other
Channel proteins are ion-selective and may be gated to regulate the passage of ions in response to certain stimuli
Channel proteins only move molecules along a concentration gradient (i.e. are not used in active transport)
Channel proteins have a much faster rate of transport than carrier proteins

Question 20

Outline how nerve cells transmit electrical impulses using pumps (active transport) and facilitated diffusion

Answer 20

The axons of nerve cells transmit electrical impulses by translocating ions to create a voltage difference across the membrane

At rest, the sodium-potassium pump expels sodium ions from the nerve cell, while potassium ions are accumulated within
When the neuron fires, these ions swap locations via facilitated diffusion via sodium and potassium channels

Question 21

State the structure of potassium channels

Answer 21

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 22

Explain how potassium ions (dissolved in water) passes through the potassium channel?

Answer 22

The pore is 0.3 nm wide at its narrowest. Potassium ions are slightly smaller than 0. 3 nm, but when they dissolve they become bonded to a shell of water molecules that makes them too large to pass through the pore. To pass
through, the bonds between the potassium ion and the surrounding water molecules are broken and bonds form temporarily between the ion and a series of amino acids in the narrowest part of the pore. After the potassium
ion has passed through this part of the pore, it can again become associated with a shell of water molecules .

Question 23

Outline the specificity of the potassium channel

Answer 23

Other positively charged ions that we might expect to pass through the pore are either too large to pass through or are too small to form bonds with the amino acids in the narrowest part of the pore, so they cannot shed their shell of water molecules. This explains the specificity of the channel

Question 24

How is energy generated for active transport?

Answer 24

Active transport uses energy to move molecules against a concentration gradient

This energy may either be generated by:

The direct hydrolysis of ATP (primary active transport)
Indirectly coupling transport with another molecule that is moving along its gradient (secondary active transport)

Question 25

Outline how does active transport of materials occur?

Answer 25

Active transport involves the use of carrier proteins (called protein pumps due to their use of energy)

A specific solute will bind to the protein pump on one side of the membrane
The hydrolysis of ATP (to ADP + Pi) causes a conformational change in the protein pump
The solute molecule is consequently translocated across the membrane (against the gradient) and released

Question 26

What is cotransport and uniport?

Answer 26

The coupled transport of two distinct molecules is called co-transport
the movement of a single molecule is called uniport

Question 27

What are the types of co-transport?

Answer 27

If the two molecules are transported in the same direction it is called symport
If the two molecules are transported in opposite directions it is called antiport

Question 28

What kind of co-transport and active transport is the sodium potassium pump?

Answer 28

The sodium-potassium pump is an example of an antiporter as sodium and potassium are pumped in opposite directions

This is primary active transport as both molecules are pumped against their gradient and require ATP hydrolysis

Question 29

What type of co-transport and active transport is the example of glucose uptake in kidneys?

Answer 29

Glucose uptake in the kidneys is an example of symport as its movement is coupled to the parallel transport of sodium

This is secondary active transport as the sodium is moving passively down an electrochemical gradient

Question 30

Explain the structure and functioning of the sodium potassium pump

Answer 30

An integral protein that exchanges 3 sodium ions (moves out of cell) with two potassium ions (moves into cell)

The process of ion exchange (generating concentration gradients) against the gradient is energy-dependent and involves a number of key steps:

Three sodium ions bind to intracellular sites on the sodium-potassium pump
A phosphate group is transferred to the pump via the hydrolysis of ATP
The pump undergoes a conformational change, translocating sodium across the membrane
The conformational change exposes two potassium binding sites on the extracellular surface of the pump to which potassium ions bind.
The phosphate group is released which causes the pump to return to its original conformation
This translocates the potassium across the membrane, completing the ion exchange

Question 31

What kinds of materials are transported via vesicles?

Answer 31

Materials destined for secretion are transported around the cell in membranous containers called vesicles

Question 32

Describe stage 1 of the entire process of vesicular transport

Answer 32

The endoplasmic reticulum is a membranous network that is responsible for synthesising secretory materials

Rough ER is embedded with ribosomes and synthesises proteins destined for extracellular use
Smooth ER is involved in lipid synthesis and also plays a role in carbohydrate metabolism

Materials are transported from the ER when the membrane bulges and then buds off to create a vesicle surrounding the material

Question 33

Describe stage 2 of the entire process of vesicular transport

Answer 33

The vesicle is then transported to the Golgi apparatus and fuses to the internal (cis) face of the complex

Materials move via vesicles from the internal cis face of the Golgi to the externally oriented trans face
While within the Golgi apparatus, materials may be structurally modified (e.g. truncated, glycosylated, etc.)

Material sorted within the Golgi apparatus will either be secreted externally or may be transported to the lysosome

Question 34

Describe stage 3 of the entire process of vesicular transport

Answer 34

Vesicles containing materials destined for extracellular use will be transported to the plasma membrane

The vesicle will fuse with the cell membrane and its materials will be expelled into the extracellular fluid

Question 35

How are materials sorted in the Golgi apparatus according to two types of secretion?

Answer 35

Materials sorted by the Golgi apparatus may be either:

Released immediately into the extracellular fluid (constitutive secretion)
Stored within an intracellular vesicle for a delayed release in response to a cellular signal (regulatory secretion)

Question 36

What functional and structural property is responsible for the process of material transport by endocytosis and exocytosis to take place?

Answer 36

The membrane is principally held together by weak hydrophobic associations between the fatty acid tails of phospholipids

This weak association allows for membrane fluidity and flexibility, as the phospholipids can move around to some extent

This allows for the spontaneous breaking and reforming of the bilayer, allowing larger materials to enter or leave the cell without having to cross the membrane (this is an active process and requires ATP hydrolysis)

Question 37

What is endocytosis and how does it occur?

Answer 37

The process by which large substances (or bulk amounts of smaller substances) enter the cell without crossing the membrane

An invagination of a small region of the membrane forms a flask-like depression which envelopes the extracellular material
The invagination is then pinched off the membrane to form an intracellular vesicle containing the material

Question 38

Two types of endocytosis?

Answer 38

There are two main types of endocytosis:

Phagocytosis – The process by which solid substances are ingested (usually to be transported to the lysosome)
Pinocytosis – The process by which liquids / dissolved substances are ingested (allows faster entry than via protein channels)

Question 39

What is exocytosis and its two main contributions?

Answer 39

The process by which large substances (or bulk amounts of small substances) exit the cell without crossing the membrane
Vesicles (typically derived from the Golgi) fuse with the plasma membrane, expelling their contents into the extracellular environment
The process of exocytosis adds vesicular phospholipids to the cell membrane, replacing those lost when vesicles are formed via endocytosis

Question 40

What are the two broad purposes of exocytosis?

Answer 40

1. Secretion of useful substances for use outside the cell (Digestive enzymes are released from gland cells by exocytosis. The polypeptides in the enzymes are synthesized by the rER, processed in the Golgi apparatus and then carried to the membrane in vesicles for exocytosis)
2. Exocytosis can also be used to expel waste products or unwanted materials. An example is the removal of excess water from the cells of unicellular organisms. The water is loaded into a vesicle, sometimes called a contractile vacuole, which is then moved to the plasma membrane for expulsion by exocytosis.