UNIT 1.4

Question 1

What 2 key qualities do Cellular Membranes possess ?

Answer 1

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

Outline the process of Passive Transport

Answer 2

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)

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 3

Outline the process of Active Transport

Answer 3

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)

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 4

Define Osmosis and outline why it occurs

Answer 4

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)

Water is considered the universal solvent – it will associate with, and dissolve, polar or charged molecules (solutes)
Because solutes cannot cross a cell membrane unaided, water will move to equalise the two solutions
At a higher solute concentration there are less free water molecules in solution as water is associated with the solute
Osmosis is essentially the diffusion of free water molecules and hence occurs from regions of low solute concentration

Question 5

What is osmolarity ?

Answer 5

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

Question 6

How would the osmolarity of a tissue be calculated ?

Answer 6

The tissue would be bathed in soloutions with known osmolarity. If the tissue is isotonic with the soloution (as shown by no change in weight after bathing) the tissue has the same osmolarity as the known soloution.

Question 7

Define Crenation

Answer 7

When a cell shrivels as water leaves the cell due to it being Hypertonic

Question 8

Define Lysis (In terms of osmosis)

Answer 8

When a cell swells and potentially bursts as water enters the cell due to it being hypotonic.

Question 9

How do plant cells moderate Osmosis?

Answer 9

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 10

What is Facilitated Diffusion ?

Answer 10

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 11

What are carrier proteins ?

Answer 11

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

Question 12

What are channel proteins ?

Answer 12

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 13

Explain how facilitated diffusion is used by axons

Answer 13

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 14

what are potassium channels ?

Answer 14

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 15

What is Active Transport ?

Answer 15

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)

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 16

Outline the Sodium - Potassium pump

Answer 16

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

The process of ion exchange 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
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 17

Outline Endocytosis

Answer 17

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

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

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 18

Outline Exocytosis

Answer 18

Exocytosis

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