Module 1: Principles of Cell Function

Question 1

what are cellular membranes?

Answer 1

> fluid mosaics of lipids and proteins
phospholipids - abundant
phospholipids are amphipathic molecules
‘Fluid mosaic model’ - membrane is fluid with a ‘mosaic’ of proteins embedded in it.

Question 2

what does amphipathic mean?

Answer 2

part hydrophilic and part hydrophobic (membrane bound proteins)

Question 3

what is cholesterols role in the phospholipid bilayer?

Answer 3

> stops membrane from being too thin and fluid at higher temperatures
at low temps, it stops the membrane from being too viscous and thick
has a buffering effect.

Question 4

what are the components of a cellular membrane?

Answer 4

>Phospholipid bilayer
> Glycoproteins on the surface 
> carbohydrate
> glycolipids
> integral proteins
> peripheral proteins
> microfilaments of cytoskeleton
> cholesterol
> fibres of extracellular matrix

Question 5

what are the six major functions of membrane proteins?

Answer 5

> transport (ion channels and active transport)
enzymatic activity
signal transduction
cell-cell recognition
intercellular joining (into tissues)
attachment to the cytoskeleton and extracellular matrix

Question 6

how does the membrane structure result in selective permeability?

Answer 6

> lipid bilayers are impermeable to most essential molecules and ions (because they are lipids)
some permeability to water molecules and a few other small, uncharged molecules like oxygen and carbon dioxide

Question 7

what are lipid bilayers not permeable to ?

Answer 7

> ions such as K+, Na+, Ca2+, Cl-, HCO3-
small hydrophilic molecules like glucose
macromolecules like proteins and RNA

Question 8

what is passive transport?

Answer 8

is diffusion of a substance across a membrane with no energy investment.

Question 9

what is osmosis?

Answer 9

> diffusion of water through a selectively permeable membrane into another aqueous compartment containing solute at a higher concentration
water wants to be at equilibrium, same number of molecules in each compartment.

Question 10

what are osmotica

Answer 10

> ions - charged
sugars - water soluble
proteins - large
nutrients

Question 11

what is tonicity?

Answer 11

the ability of a solution to cause a cell to gain or lose water - compared to outside the cell etc.

Question 12

what is an isotonic solution?

Answer 12

solute concentration is the same as that inside the cell; no net water movement across the plasma membrane

Question 13

what is a hypertonic solution?

Answer 13

solute concentration is greater than that inside the cell; cell loses water - crenates

Question 14

what is a hypotonic solution?

Answer 14

solute concentration is less than that inside the cell; cell gains water - haemolysis

Question 15

what is an electrogenic pump?

Answer 15

a transport protein that generates voltage across a membrane

Question 16

what is a sodium-potassium pump?

Answer 16

the major electrogenic pump of animal cell

Question 17

what is the function of transport proteins?

Answer 17

to allow the passage of hydrophilic substances across the membrane.
> they are specific for the substance they move
> transport proteins speed the passive movement of molecules across the plasma membrane

Question 18

what are channel proteins?

Answer 18

they have a hydrophilic channel that certain molecules or ions can use as a tunnel
> e.g., aquaporins
> ion channels - open or close in response to a stimulus (gated channels)
> provide corridors that allow a specific molecule or ion to cross the membrane
> form pores through which molecules can pass

Question 19

what are carrier proteins?

Answer 19

they bind molecules and change shape to shuttle them across the membrane

Question 20

what is facilitated diffusion?

Answer 20

> passive transport aided by proteins

Question 21

what are the two categories that pore forming proteins fall under?

Answer 21

1) channels - facilitated diffusion

2) transporters - facilitated diffusion or active transport

Question 22

what is the difference in molecule flow in passive and active transport?

Answer 22

passive - molecules flow down the concentration gradient

active - pumped against concentration gradient

Question 23

what is cotransport?

Answer 23

> occurs when active transport of a solute indirectly drives transport of another solute
plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell. (Diagram)

Question 24

how does bulk transport across a membrane work?

Answer 24

enters via endocytosis

exits via exocytosis

Question 25

what are the different types of endocytosis?

Answer 25

>phagocytosis - engulfing of particles (food vaculoes) > pinocytosis - cells drinking - engulfs solution > receptor-mediated - compounds bind to cell surface receptors, vesicle is formed containing both receptor and ligand compound and other fluid.

Question 26

what is a specific purpose for exocytosis?

Answer 26

> the mechanism of neurotransmitters release from neurons.

Question 27

what are the different types of cell to cell communication?

Answer 27

>paracrine signalling - neighbouring cells > autocrine signalling - neurotransmitter released from, and acts on, the same cell. > synaptic signalling - pre and post synaptic neurons > hormonal signalling - long distance

Question 28

what are the three stages of cell signalling?

Answer 28

1) reception 2) transduction 3) response

Question 29

what are the four receptor types?

Answer 29

> G protein coupled receptor > tyrosine kinase receptor > ion channel receptor > steroid receptor

Question 30

what are ion channel receptors?

Answer 30

``` > plasma membrane receptor >ligand binds to channel, > Na+ channels are opened > fastest acting receptor > e.g., Nictoninc receptors ```

Question 31

what are G protein coupled receptors?

Answer 31

> plasma membrane receptor > 7 transmembrane spanning domains > largest family of receptors > 1000 in human genome > 50% of current drugs target GPCRs > activated by various stimuli - light, ions (Ca2+), odourants, gustative molecules, neurotransmitters, hormones, peptides and proteins > interact with heterotrimeric ( 3 different peptides) G proteins to control the activity of enzymes, ion channels, and intracellular signal transduction pathways.

Question 32

what is the process involved in g protein coupled receptors

Answer 32

1) GDP is bound to the G - protein - inactive state 2) hormone ligand binds to the receptor, causing a conformational change, resulting in GDP being exchanged with GTP - active state 3) the activated G-protein dissociates from the receptor and diffuses across the membrane to bind to the enzyme Adenylyl cyclase, activating it. 4) adenylyl cyclase catalyses the conversion of ATP to cAMP - second messenger 4) cAMP activating another protein (Protein Kinase A - PKA) leads to a cellular response.

Question 33

what are the different types of enzymes that GPCRs can act on, to produce what second messengers?

Answer 33

> enzymes: Adenylyl cyclases, phospholipases, phosphodiesterases > second messengers: cAMP, cGMP, IP3, DAG

Question 34

what are tyrosine kinase receptors?

Answer 34

> plasma membrane receptor > 3rd fastest response > e.g., insulin receptors > used in metabolism, cell growth, cell reproduction

Question 35

what is the process involved in tyrosine kinase receptors?

Answer 35

1) before the signalling molecule binds, the receptors exist as individual monomers 2) the binding of signalling molecule causes two monomers to associate closely with each other to form a dimer in a process called dimerisation. 3) dimerisation activates the tyrosine kinase region of each monomer, each tyrosine kinase adds a phosphate from an ATP molecule to a tyrosine on the tail of the other monomer 4) now the receptor is fully activated, it is recognised by specific relay proteins. They too become phosphorylated and go on to activate various cellular responses

Question 36

what are the roles of protein phosphorylation?

Answer 36

``` > protein is phosphorylated > conformational change occurs > protein-protein interactions > change in cellular location > these changes lead to activation or inactivation of the target proteins ```

Question 37

What are steroid receptors?

Answer 37

> intracellular receptor | > slowest response

Question 38

describe the process of steroid receptors

Answer 38

1) the ligand passes through the plasma membrane 2) the ligand binds to the receptor in the cytosol, activating it 3) the 'ligand/hormone receptor complex' enters the nucleus and binds to specific genes where it eventually influences the transcription of proteins. * ligand must be small, non-polar, not charged and be hydrophobic.

Question 39

how can communication between adjacent cells occur?

Answer 39

through gap junctions with ions such as Na+, K+ and Ca+ (local regulators)

Question 40

how does communication across the extracellular fluid within tissue occur?

Answer 40

> growth factors (local regulators) and hormones (long distance regulators)

Question 41

how does communication between tissues occur?

Answer 41

secretion of chemical messengers into the circulatory system ( long distance regulators (hormones))

Question 42

how does cotransport work?

Answer 42

> a carrier protein, such as sucrose-H+ cotransporter in a plant cell, is able to use the diffusion of H+ down its electrochemical gradient into the cell to drive the uptake of sucrose > the H+ gradient is maintained by an ATP-driven proton pump that concentrates H+ outside the cell, thus storing potential energy that can be used for active transport, in this case sucrose. > Thus, ATP indirectly provides the energy necessary for cotransport.

Question 43

what are cell junctions?

Answer 43

> communication by direct cell contact > both animals and plants have cell junctions that allow molecules to pass readily between adjacent cells without crossing plasma membranes

Question 44

what is cell-cell recognition?

Answer 44

> communication by direct cell contact > two cells in an animal may communicate by interaction between molecules protruding from their surfaces > important in embryonic development and the immune response

Question 45

What is paracrine signalling?

Answer 45

a secreting cell acts on nearby target cells by discharging molecules of a local regulator (growth factor) into the extracellular fluid