Membrane Proteins

Question 1

Fluid mosaic model

Answer 1

The plasma membrane
consists mainly of a fluid
mosaic bilayer of
phospholipids and
protein.
* Phospholipids will
naturally form bilayers in
aqueous environments
* Hydrophilic head groups
align to the extracellular
space or cytoplasm, and
the hydrophobic tails
point inwards.

Question 2

How are integral proteins held within the membrane?

Answer 2

Regions of hydrophobic R groups allow strong hydrophobic interactions that hold integral
membrane proteins within the phospholipid bilayer

Question 3

The phospholipid bilayer is a barrier to…

Answer 3

ions and most uncharged polar molecules. Some small molecules, such as oxygen and carbon dioxide, pass through the bilayer by
simple diffusion

Question 4

Some integral proteins are also

Answer 4

transmembrane proteins

Question 5

Peripheral proteins

Answer 5

Hydrophilic proteins bound to the surface of membranes mainly by ionic and hydrogen interactions.

Question 6

Facilitated diffusion

Answer 6

the passive transport of substances across the membrane through specific transmembrane proteins

Question 7

Ligand-gated channels

Answer 7

controlled by the binding of signal molecules

Question 8

voltage-gated channels

Answer 8

controlled by changes in ion concentration

Question 9

Transporter proteins

Answer 9

bind to the specific substance to be transported and undergo a
conformational change to transfer the solute across the membrane

Question 10

requirement for active transport

Answer 10

A source of metabolic energy

Question 11

How is energy provided for conformational changes?

Answer 11

Some active transport proteins hydrolyse ATP directly to provide the energy for the
conformational change required to move substances across the membrane

Question 12

formation of the electrochemical gradient

Answer 12

For a solute carrying a net charge, the concentration gradient and the electrical potential
difference combine. This determines the transport of the solute

Question 13

Ion pumps use energy from

Answer 13

the hydrolysis of ATP to
establish and maintain ion gradients

Question 14

The sodium-potassium pump process

Answer 14

The pump has high affinity for sodium ions inside the cell; binding occurs; phosphorylation
by ATP; conformation changes; affinity for sodium ions decreases; sodium ions released
outside of the cell; potassium ions bind outside the cell; dephosphorylation; conformation
changes; potassium ions taken into cell; affinity returns to star

Question 15

The sodium potassium pump and the small intestine

Answer 15

in the small intestine, the sodium gradient created by the sodium-potassium pump drives
the active transport of glucose
The glucose transporter responsible for this glucose symport transports sodium ions and
glucose at the same time and in the same direction

Question 16

Binding changes the conformation of the receptor, which

Answer 16

initiates a response within the cell

Question 17

Hydrophobic signalling molecules

Answer 17

can diffuse directly through the phospholipid bilayers of
membranes, and so bind to intracellular receptors

Question 18

transcription factors

Answer 18

The receptors for hydrophobic signalling molecules

Question 19

examples of hydrophobic signalling
molecules

Answer 19

steroid hormones oestrogen and testosterone

Question 20

Steroid hormones bind to specific receptors in..

Answer 20

the cytosol or the nucleus

Question 21

The hormone-receptor complex moves to the nucleus where…

Answer 21

it binds to specific sites on
DNA and affects gene expression

Question 22

Hydrophilic signalling molecules

Answer 22

bind to transmembrane receptors and do not enter the
cytosol

Question 23

Transmembrane receptors change conformation when

Answer 23

the ligand binds to the extracellular
face; the signal molecule does not enter the cell, but the signal is transduced across the
plasma membrane

Question 24

Transmembrane receptors act as signal transducers by

Answer 24

converting the extracellular ligand binding event into intracellular signals, which alters the behaviour of the cell

Question 25

Phosphorylation cascades

Answer 25

allow more than one intracellular signalling pathway to be activated

Question 26

Binding of the peptide hormone insulin to its receptor results in

Answer 26

an intracellular signalling cascade that triggers recruitment of GLUT4 glucose transporter proteins to the cell membrane of fat and muscle cells

Question 27

Diabetes mellitus can be caused by

Answer 27

failure to produce insulin (type 1) or loss of receptor function (type 2)

Question 28

What can also recruit GLUT-4 ?

Answer 28

Exercise also triggers recruitment of GLUT4, so can improve uptake of glucose to fat and muscle cells in subjects with type 2

Question 29

Resting membrane potential

Answer 29

a state where there is no net flow of ions across the membrane

Question 30

The transmission of a nerve impulse requires

Answer 30

changes in the membrane potential of the neuron’s plasma membrane

Question 31

action potential

Answer 31

a wave of electrical excitation along a neuron’s plasma membrane

Question 32

Depolarisation of the plasma membrane

Answer 32

as a result of the entry of positive ions triggers the opening of voltage-gated sodium channels, and further depolarisation occurs

Question 33

Restoration of the resting membrane potential

Answer 33

Inactivation of the sodium channels and the opening of potassium channels restores the resting membrane potential

Question 34

Depolarisation of a patch of membrane causes

Answer 34

neighbouring regions of membrane to depolarise and go through the same cycle, as adjacent voltage-gated sodium channels are opened

Question 35

When the action potential reaches the end of the neuron...

Answer 35

it causes vesicles containing neurotransmitter to fuse with the membrane — this releases neurotransmitter, which stimulates a response in a connecting cell

Question 36

Restoration of the resting membrane potential allows

Answer 36

the inactive voltage-gated sodium channels to return to a conformation that allows them to open again in response to depolarisation of the membrane

Question 37

Ion concentration gradients are

Answer 37

re-established by the sodium-potassium pump, which actively transports excess ions in and out of the cell

Question 38

The cytoskeleton

Answer 38

gives mechanical support and shape to cells

Question 39

The cytoskeleton consists of...

Answer 39

different protein structures including microtubules, which are found in all eukaryotic cells

Question 40

Microtubules

Answer 40

control the movement of membrane-bound organelles and chromosomes

Question 41

Cell division requires

Answer 41

remodelling of the cytoskeleton

Question 42

Formation and breakdown of microtubules involves

Answer 42

polymerisation and depolymerisation of tubulin

Question 43

The cell cycle consists

Answer 43

interphase and mitotic (M) phase

Question 44

Mitotic phase involves

Answer 44

mitosis and cytokinesis

Question 45

Mitosis consists of

Answer 45

prophase, metaphase, anaphase and telophase

Question 46

Progression through the cell cycle is controlled by

Answer 46

checkpoints

Question 47

Cyclin proteins that accumulate during cell growth are

Answer 47

involved in regulating the cell cycle

Question 48

G1 checkpoint

Answer 48

retinoblastoma protein (Rb) acts as a tumour suppressor by inhibiting the transcription of genes that code for proteins needed for DNA replication

Question 49

Phosphorylation by G1 cyclin-CDK

Answer 49

inhibits the retinoblastoma protein (Rb)

Question 50

G2 checkpoint

Answer 50

the success of DNA replication and any damage to DNA is assessed

Question 51

DNA damage triggers

Answer 51

the activation of several proteins including p53 that can stimulate DNA repair, arrest the cell cycle or cause cell death

Question 52

metaphase checkpoint

Answer 52

controls progression from metaphase to anaphase

Question 53

An uncontrolled reduction in the rate of the cell cycle may result in

Answer 53

degenerative disease

Question 54

uncontrolled increase in the rate of the cell cycle may result in

Answer 54

tumour formation

Question 55

proto-oncogene

Answer 55

a normal gene, usually involved in the control of cell growth or division, which can mutate to form a tumour-promoting oncogene

Question 56

Apoptosis is triggered

Answer 56

by cell death signals that can be external or internal

Question 57

External death signal molecules bind to

Answer 57

a surface receptor protein and trigger a protein cascade within the cytoplasm

Question 58

An internal death signal resulting from DNA damage causes

Answer 58

activation of p53 tumour-suppressor protein

Question 59

Both types of death signal result in

Answer 59

the activation of caspases (types of protease enzyme) that cause the destruction of the cell

Question 60

Apoptosis is essential during

Answer 60

development of an organism to remove cells no longer required as development progresses or during metamorphosis

Question 61

absence of growth factors

Answer 61

Cells may initiate apoptosis

Question 62

Mitosis phases

Answer 62

Prophase Metapahse Anaphase Telaphase

Question 63

Prophase

Answer 63

◦ DNA condenses into chromosomes, each of which consists of two sister chromatids which are joined at the centromere. ◦ The nuclear membrane breaks down. ◦ Spindle microtubules extend from the MTOC (by polymerisation) and attach to chromosomes via their kinetochores (areas where spindle fibres bind) in the centromere region.

Question 64

Metaphase

Answer 64

Chromosomes align at the metaphase plate (the equator of the spindle).

Question 65

Anaphase

Answer 65

◦ Spindle fibres shorten by depolymerisation. ◦ This causes the sister chromatids to become separated and so the chromosomes are pulled to opposite poles. ◦ Each chromatid can now be called a chromosome. ◦ By the end of anaphase, the two poles of the cell each have an identical and complete set of chromosomes.

Question 66

Telophase

Answer 66

◦ The chromosomes decondense (uncoil) and nuclear membranes form around them. ◦ Cytokinesis also takes place during this period, which involves the separation of the cytoplasm into two daughter cells.

Question 67

Cyclins combine with and activate

Answer 67

cyclin-dependent kinases (CDKs).

Question 68

Active cyclin-CDK

Answer 68

Active cyclin-CDK complexes will phosphorylate proteins, specifically the proteins which regulate the progression through the cycle.

Question 69

the cell cycle is allowed to progress if...

Answer 69

there is sufficient phosphorylation

Question 70

G1

Answer 70

If an insufficient threshold is reached, the cell is held at a checkpoint. ◦ During this time there is low levels of CDK activity so Rb binds with a transcription factor preventing the transcription of genes required for entering the S phase. ◦ As CDK activity increases, Rb is repeatedly phosphorylated. Once sufficient phosphorylation has taken place Rb no longer binds with the transcription factor allowing them to promote the transcription of genes required for DNA replication in the S phase. ◦ The ‘go ahead’ signal is then given at the G1 checkpoint. (G1S)

Question 71

G2

Answer 71

During the S phase, DNA should be replicated successfully. ◦ The G2 checkpoint will determine how successfully the DNA has been replicated. ◦ The G2 checkpoint also allows for any damage to DNA to be assessed. ◦ As DNA damage can ultimately lead to cancer, if DNA damage is detected, the activation of several proteins, such as p53, is triggered. ◦ This can: ◦ stimulate DNA repair ◦ arrest the cycle ◦ cause cell death (apoptosis)

Question 72

Metaphase checkpoint

Answer 72

A metaphase checkpoint will control the progression from metaphase to anaphase. ◦ Progression will be halted until the chromosomes are aligned correctly on the metaphase plate and attached to the spindle microtubules. ◦ This ultimately ensures that each cell receives the correct number of chromosomes.

Question 73

Type 1 diabetes is caused by

Answer 73

insufficient production of insulin

Question 74

Explain how a single signalling molecule such as testosterone is able to influence the expression of multiple genes.

Answer 74

Each gene will possess hormone response elements/DNA sequences that bind the hormone-receptor complex/transcription factor.

Question 75

Name the glucose transporter that enables glucose transport in these cells.

Answer 75

GLUT-4