advanced higher biology KA 1.3 + 1.4

describe structure of cell membrane- fluid mosaic model

all cells are surrounded by a partially permeable membrane that controls what substances can enter and exit the cell. A cell needs to be able to import the substances it needs to survive and to export waste materials and substances that are needed outside the cell

different types of membrane proteins and describe how they interact with the membrane

cell membranes have regions of hydrophobic R groups that allow strong hydrophobic interactions to hold integral membrane proteins within the phospholipid bilayer. Integral proteins extend into or are completely embedded within phospholipid bilayer. Integral proteins extend into or are completely embedded within bilayer the integral proteins that completely span width of the membrane are called transmembrane integral proteins

peripheral membrane proteins have hydrophilic R groups on their surface and are bound to the surface of membrane mainly by ionic or hydrogen bond interactions many peripheral membrane proteins interact with the surfaces of integral membrane proteins

relate structure of channel proteins membrane to its function

phospholipid bilayer acts as barrier to ions and most uncharged polar molecules due to its hydrophobic nature but some small non-polar molecules such as oxygen and carbon dioxide pass through bilayer by simple diffusion facilitated diffusion is passive transport of substances across membrane through specific transmembrane proteins ( channels or transporters )

to perform specialised functions different cell types have different channel and transporter channels most channel proteins in animal and plant cells are highly selective. Channels are multi-subunit proteins with the sub-units arranged to form water filled pores that extend across the membrane. Some channel proteins are gated and change conformation to allow or prevent diffusion ( sodium or potassium channels )

relate structure of gated ligand channels and transporter proteins membrane to its function

gated channels respond to a stimulus which causes them to open or close. Ligand gated channels are controlled by binding of signal molecules and voltage gated channels are controlled by changes in ion concentration

transporter proteins bind to the specific substance to be transported and undergo conformational change to actually transfer the solute across the membrane rather than just provide a route through

Transporters alternate between two conformations so that the binding site for a solute is sequentially exposed on one side of the bilayer then the other

facilitated and active transport

active transport uses protein pumps that transfer substances across the membrane against their concentration gradient.

this process requires a source of metabolic energy. Some active transport proteins hydrolyse ATP directly ( use ATPases ) to provide the energy for the conformational change required to move substances across the membrane

pumps that mediate active transport are transporter proteins coupled to an energy source

explain purpose of sodium potassium pump

movements of solutes against a concentration gradient by active transport is an essential part of the cells metabolism

for a solute carrying a net charge the concentration gradient and the electrical potential different combine to form an electrochemical gradient that determines the transport of the solute

a membrane potential ( an electrical potential difference ) is created when there is a difference in electrical energy on the 2 sides of the membrane

how sodium potassium pump works

actively transports sodium ions out of the cells and potassium ions in

the pump transport ion against a steep concentration gradient using energy directly from ATP hydrolysis

for each ATP hydrolysed three sodium ions are transported out of the cell and two potassium ions are transported into the cell this establishes both concentration gradients and an electrical gradient

sequence of events which takes place in the operation of the sodium potassium pump

1. initially pump is open to cytosol 3 sodium ions can bind with high affinity to the 3 Na + binding sites and the ATPase function phosphorylates the protein

2. this causes conformational change which makes the protein open to the extracellular environment

3. the Na+ binding sites have now decreased affinity for sodium and the 3 Na+ ions are released outside of the cell. The 2 K+ binding sites ( high affinity for K+ ) can then be occupied

4. again conformational change occurs following release of the phosphate ( dephospho-rylation ) which switches the protein back to the open to inside structure ( the original conformation ) The K+ sites now have lowered affinity for potassium and the 2 K+ ions are released into cell

For each ATP hydrolysed 2 sodium ions are transported out of the cell and 2 potassium ions are transported into the cell this establishes both concentration gradients and an electrical gradient

the sodium potassium pump is found is most animal cells accounting for a high proportion of the basal metabolic rate in many organisms

importance of sodium pump

in small intestine the sodium gradient created by sodium potassium pump drives active transport of glucose

in intestinal epithelial cells the sodium potassium pump generates a sodium ion gradient across the plasma membrane= high Na outside/ low Na inside

Transporter proteins are involved in coupled transport - this is when movement of one molecule down its concentration gradient is used to transport another molecule across membrane against its concentration gradient

like glucose symport found in cell lining of vili- Na move into cell down concentration gradient and simultaneous transport of glucose molecules against their concentration gradient into cells

how do multicellular organisms signal between cells

extracellular signalling molecules

role of receptor proteins

Receptor molecules of target cells are proteins with a binding site for a specific signal molecule

what happens when ligand binds to receptor protein and tissue specific response

Binding changes the conformation of the receptor, which initiates a response within the cell

Different cell types produce specific signals that can only be detected and responded to by cells with the specific receptor

In a multicellular organism, different cell types may show a tissue-specific response to the same signa

describe the action of the hydrophobic signalling molecules in the control of transcription

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

The receptors for hydrophobic signalling molecules are transcription factors

The steroid hormones oestrogen and testosterone are examples of hydrophobic signalling molecules

Steroid hormones bind to specific receptors in the cytosol or the nucleus

The hormone-receptor complex moves to the nucleus where it binds to specific sites on DNA and affects gene expression

explain hydrophilic signalling molecules and control of transcription

Hydrophilic signalling molecules bind to transmembrane receptors and do not enter the cytosol like peptide hormone and neurotransmitter

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

Transmembrane receptors act as signal transducers by converting the extracellular ligand binding event into intracellular signals, which alters the behaviour of the cell Transduced hydrophilic signals often involve G-proteins or cascades of phosphorylation by kinase enzymes

Phosphorylation cascades allow more than one intracellular signalling pathway to be activated

descrive events when insulin binds to receptor molecule on membrane of fat and muscle cell

when BGL rise above norm the pancreas releases extra insulin.

insulin is peptide hormone and is therefore hydrophilic

insulin binds to specific receptor protein on outer surface of fat and muscle ( target ) cells

insulin binding to receptors causes conformational change that triggers phosphorylation of the receptor ( receptor occupied by insulin ) acts as transducer and begins signal transduction pathway

causes phosphorylation cascade inside cell

leads to recruitment of GLUT-4 containing vesicles being transported to cell membrane

glucose enters cell and thus BGL concentration lowered to the norm

an enzyme at end of transduction pathway converts glucose to glycogen

action of testosterone

lipid soluble so diffuses into cells through plasma membrane

target cells have specific hormone receptor protein in the cytosol

testosterone binds the hormone receptor protein to form an activated complex

conformational change makes the hormone receptor complex into an active gene transcription factor ( gene regulatory protein )

the complex binds to hormone response elements ( specific DNA sites ) affecting gene expression

proteins are synthesised that support sperm production and male sexual characteristics

general action of signalling molecule

receptor molecules of target cells are proteins with a complementary binding site for a specific signal molecule

binding changes the conformation of the receptor which initiates a response in the cell. Different cell types produce specific signals which can only be detached and responded to by cells with a specific receptor

in a multicellular organism different cell types may show a tissue specific response to the same signalling molecule

signalling molecules may have different effects on different target cell types due to differences in intracellular signalling molecules and pathways that are involved

type 1 and 2 diabetes and excersise impact

Binding of the peptide hormone insulin to its receptor results in an intracellular signalling cascade that triggers recruitment of GLUT4 glucose transporter proteins to the cell membrane of fat and muscle cells

Diabetes mellitus can be caused by failure to produce insulin (type 1) or loss of receptor function (type 2)

Type 2 is generally associated with obesity

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

explain what the resting potential of a neurone is

the membrane potential of a neurone that isnt transmitting signals

Generation of a nerve impulse Resting membrane potential is a state where there is no net flow of ions across the membrane

The transmission of a nerve impulse requires changes in the membrane potential of the neuron’s plasma membrane

key stages in production of an action potential

An action potential is a wave of electrical excitation along a neuron’s plasma membrane

neurotransmitters diffuse across synaptic cleft

neurotransmitters bind to ligand gated Na+ channels ( receptors ) on next neurone

this causes conformational change in the receptor opening the channel and enabling the influx of Na+ ions

the initial influx of Na+ depolarises membrane

if this is sufficient and membrane is depolarised beyond threshold value this will trigger opening of the local voltage gated channels Na+ which further depolarises the membrane and Na+ ions enter the cell down their electrochemical gradient

such rapid depolarisations spreads to other neighbouring Na+ channels along the neurone ( dominoes ) creating action potential that spreads length of enitre axon

explain how resting potential is re-established

once wave of depolarisation has passed along neurone then the resting potential has to be re-established to enable the next nerve impulse to be passed along

therefore when the voltage reaches a critical high level voltage gated Na+ channels become inactivated or closed and voltage gated K+ channels open

k+ ion then diffuse out of neurone in opposite direction to Na+ to restore the resting membrane potential once restored K+ channels close and Na+ channels return to a conformation that allows them to open again in response to a conformation that allows them to open again in return to a conformation that allows them to open again in response to depolarisation of the membrane

the sodium potassium pump involved in generating and maintaining ion concentration gradient for resting potential in neurones through active transport of excess ions in and out of cell

following depolarisation the sodium potassium ion concentration and gradients are reduced

the sodium potassium restores the sodium and potassium ions back into resting potential levels

2 types of photoreceptors in retina of vertebrate eye

Rods function in dim light but do not allow colour perception. Cones are responsible for colour vision and only function in bright light.

rod and cone cells

in animals the light sensitive molecule retinal is combined with opsin a membrane protein to form photoreceptors of eye

sequence of events which takes place when retinal absorbs photon of light in rod cells

when stimulated by photon of light retinal absorbs photon and rhodopsin undergoes conformational change and becomes photo-excited rhodopsin

the resulting cascade of protein amplifies the signal - photo excited rhodopsin activates a G protein called transducin which in turns activates the enzyme phosphodiesterase ( PDE )

a single photo excited rhodopsin activates hundreds of molecules of G-protein

each activated G-protein activated one molecule of PDE

each active PDE molecule catalyses the hydrolysis of thousands of cyclic GMP molecules per second

the reduction of cGMP concentration results in closure of ion channels in membrane of rod cells which triggers nerve impulses in neurones of the retina

the very high degree of amplification results in rod cells being able to respond to low light intensities and dosent allow colour perception

mechanism of cone cells

different forms of opsin ( differ by few amino acids = different structure combine with retinal to give different photoreceptors proteins

these different photoreceptors proteins have a maximum sensitivity to specific wavelengths like blue red green plus UV in fish bird insects

the colour seen depends on the relative degree of stimulation of the 3 different types of cone cells they are less sensitive than rod cells as they have fewer photo receptors in membranes and so only function in bright loves