biological membranes

phospholipid intrinsic and extrinsic proteins cholesterol fluid mosaic model external and intracellular membranes beetroot practical standard deviation

structure of a phospholipid

1 hydrophilic phosphate grp (neg charge so water soluble)

1 glycerol

2 hydrophobic fatty acid tails (uncharged and non polar so water repelling )

how are phospholipids arranged in the plasma membrane ?

the hydrophilic phosphate heads face the aqueous extracellular space (tissue fluid ) and the aqueous cytoplasm while the hydrophobic fatty acid tails face inwards away frm the aqueous solutions forming a phospholipid bilayer

what happens to phospholipids when submerged in water ?

they form micelles 

what is diffusion ?

the net movement of particles from an area of high conc to a low conc down a conc gradient 

passive process 

why is diffusion a passive process ?

it never requires energy from the cell as the particles alr have kinetic energy

which types of particles can pass through the phospholipid bilayer via simple diffuison 

small non polar and  lipid soluble e.g. steroids molecules 

why can water diffuse directly through the bilayer despite being polar ?

its small so it doesnt come into direct contact w/ the hydrophobic tails 

why cant large polar molecules diffuse directly through the bilayer

theyre rejected by the hydrophobic fatty acid tails which also  form an impermeable barrier to large molecules 

how do charged ions pass through the bilayer

via facilitated diffusion - passive

they must pass through channel proteins 

bind to a specific bonding site down their conc gradient 

how do large polar molecules that are not ions pass through the bilayer ?

via facilitated diffusion - passive 

molecule binds to binding site on carrier protein 

carrier has an allosteric change in shape 

molecule is released on the other side of the membrane 

moves down the conc gradient 

channel protein structure

intrinsic protein

spans entire bilayer

hydrophobic R grps face outwards to interact w/ hydrophobic fatty acid tails allowing for stability 

hydrophilic R grps face inwards allowing molecules to diffuse through 

structure of the bilayer and what it contains 

7nm thick

extrinsic and intrinsic proteins 

glycoproteins e.g. glycocalyx for cell recognition so cells grp tg to form tissues 

extrinsic proteins structure and functions

spans one part of the bilayer

cell signalling

Receptor sites where drugs, hormones and antibodies bind triggering chem reactions within the cell

cell signalling in extrinsic proteins

cells communicate w/ each other to control processes inside the body and respond to changes in the environment

one cell releases a hormone which travels in the blood to another cell

hormone detected by another cell as it binds to specific complimentary receptor on cell membrane of extrinsic protein

creating an up cascade of chem reactions within the specific cell

glycoprotein

protein + sugar (branch of carb )

glycolipid

sugar + fatty acid tails 

glycolipid and glycoproteins functions 

receptor sites where drugs hormones and antibodies bind triggering chem reactions within the cell

cell signalling 

act as antigens 

have OH and H grps that can form H bonds w/ surrounding molecules 

cholesterol structure

sits between phospholipid tails in both layers w/o it cells not supported esp if its not apart of a tissue e.g. rbc 

cholesterol function

maintains stability and fluidity of the membrane as it binds to fatty acid tails packing them closer tg

at body temp - holds fatty acid tails tg to make cell less fluid 

in icy areas - prevents crystallisation by inc fluidity (as the cell would otherwise shrink and become impermeable ) allowing cell to survive and function

what can inc the fluidity of the cell surface membrane at low temps ?

C=C double bonds in the fatty acid chains bc they kink the chains

fatty acids having shorter chains 

external ( cell surface ) membranes

control what comes in and out

cell recognition (antigens )
cell communication ( cell signalling )

intracellular /membranes within cells

membranes around organelles

compartmentalisation (keep spec conditions needed e.g. in rER )

form vesicles to transport substances to diff areas of the cell 

sites of chem reactions ( e.g. inner membrane of mito contains enzymes for respiration )

barrier between organelle and cell controlling movement of substances in and out of the organelle 

describe the fluid mosaic model of the plasma membrane 

fluid bcs the proteins can move freely through the bilayer phospholipids can sway side to side 

mosaic pattern from above formed by the scattered proteins 

model bcs its a visual created based on experimental and chemical evidence 

the ease by which they do this depends on the num of phospholipids w/ unsaturated fatty acids in the phospholipids as more = more fluidity 

why is facilitated diffusion passive ?

particles alr have KE and are moving down the conc gradient

how do bacteria maintain their shape if they have no cholesterol ?

murein cell wall which helps to maintain their shape

4 factors that affect the rate of diffusion ?

SA

Diffusion distance

conc gradient 
temp

how does an inc in temp affect the rate of diffusion ?

incs KE of molecules and speeds up diffusion of particles 

how does an inc in SA affect the rate of diffusion ?

the num of particles that can diffuse at one time incs so rate of difusion incs 

how does an inc in the steepness of the conc gradient affect the rate of diffusion ?

steeper conc gradient = faster rate of diffusion

how does an inc in the diffusion distance effect the rate of diffusion ?

larger distance = slower rate of diffusion 

what does Ficks law state ?

Rate of diffusion is:

proportional to the SA

proportional to the conc gradient

inversely proportional to the diffusion distance

explain the graph 

the conc gradient at the beginning must be steep in order for diffusion to occur

As time incs w/ more substances moving down the conc gradient the steepness is reduced so the rate of diffusion decreases till equilibrium is reached so there is no longer a conc gradient.

explain the graph

Linear graph line

As the conc gradient incs the steepness of the conc gradient incs

Diffusion incs in a linear form.

According to fick’ s law the rate of diffusion is proportional to conc gradient therefore the rate incs

describe the graph that relates to facilitated diffusion (conc of substance)?

as the conc incs initially the rate of diffusion also incs (as conc must be a LF)

When the conc of the substance continues to inc the steepness of the conc gradient incs

The graph will plateau and the rate will reach saturation (v max) as the number of protein / carrier channels are saturated. This has become a limiting factor

active transport process ?

particles move from a low conc to a high conc against the conc gradient

active process so needs ATP and carrier proteins

facilitated diffusion vs active transport

both :

carrier proteins 

involve the movement of particles using an intrinsic protein 

FD : 

channel proteins 

no ATP - passive 

down the conc gradient 

AT:

no channel proteins 

ATP - active 

against conc gradient 

in a beetroot why cant betalain diffuse out of the membrane 

too large to fit in between the tails of the fatty acids 

polar so rejected by fatty acid tails 

beetroot practical method

use a cork borer to cut out pieces of the beetroot from the same source variety and age  

use a ruler and a sharp scalpel to cut these into 5mm discs all with the same SA 

place all discs into small beaker and wash under running water for 5 mins to remove excess pigment 

label 7 test tubes with temps 20-80

measure 5cm³ of distilled water w/ a measuring cylinder and add into boiling tubes 

make a water bath using large beaker and water frm a kettle 

when the water bath reaches 30 using a thermometer place beetroot peices in boiling tube then put boiling tube into water bath 

after 30 mins stir same num of times go no more than 60 mins  

what are the control variables in the beetroot practical?

sa of discs 

source for same conc of pigment 

same variety and age for same conc/vol of pigment and steepness of conc gradient 

temp - water bath at 25 as too low to denature proteins

precautions in the beetroot practical

rinsing the discs to remove excess stain bcs it produces an over estimation in absorption and shows membrane has incd fluidity / more permeability than it rlly is

shake the curvette before taking absoprtion reading - distribution of pigment precise measurement

wash piece of appratus and get new one each time - prevents contamination

how could the data be made more precise in the beetroot practical instead of judging absorbance by observing the colour ?

pour the solution into a cuvette and put into colorimeter

choose green wavelength 

add the distilled water in a cuvette to make sure absorbance is 0 - this is a control 

repeat process 3 times for each temp to allow anomalous data to be identified 

controls / precautions when making data more precise in beetroot practical 

water in colorimeter ensures all values r measured to the same standard and comparable incs validity 

repeats produce a mean value or statistic test e.g. SD to inc reliability 

limitations of beetroot practical and solutions 

colorimeter can only detect small range of absorbance - theres a definite max absorbance that can be detected so darker solutions js give same reading 

solutions:

dilute each sample before placing in cuvette 

use smaller num of discs per boiling tube 

what factors damage a membrane

temp

pH

ethanol

detergent 

how does temp damage the membrane ?

phospholipids move round more no longer held in a tightly gelled state

membrane becomes more fluid and permeable

pigment begins to diffuse out

steepest gradient indicates higher temp so phospholipids gain even more KE and begin to melt

membrane becomes even more fluid and permeable

h bonds vibrate due to incd KE , they break , proteins denature

membrane breaks down -most permeable atp so most pigment -which has greatest KE- diffuses out w/ fastest rate of diffusion

how does pH damage the membrane

when the pH is away from the optimum:

too acidic (H+ions )
too alkali (OH- ions )

these ions break ionic and H bonds in the tertiary structure so proteins denature

cell membrane breaks down bcs of incd permeability and all pigment diffuses out

how do ethanol and detergent damage the membrane and what does the plateau on the graph showing this mean ?

ethanol -non polar and fat solvent so more ethanol = more phospholipids dissolve membrane becomes more permeable so pigment diffuses out 

plateau means : 

equilibriums been reached or if membranes fully dissolved atp then all pigments js leaked out 

membrane more permeable and fluid  

 

in the beetroot practical when using ethanol or pH state independent and dependent variables

independent - conc of ethanol ( 5 diff concs so 0-2mol dm^-3 ) serial dilutions to get diff concs

or

pH in which case pH buffer and probe are used 

dependent - absorption of the solution surrounding the beetroot (A.U)

what is standard deviation ?

a measure of the spread around the mean

what does it mean when theres no overlap between standard deviations and theyre little?

low sd shows results r consistent w/ little variation

no overlap between sd bars means there is a likelihood of a sig diff between results so data is repeatable and precise - greater certainty

what does it mean when theres overlap between standard deviations as theyre large?

large sd shows variable results 

overlap between sd bars shows theres a likelihood of no sig difference between data so less certainty as data is unrepeatable unprecise 

why is sd better than range for dispersion of data 

sd is less affected by anomalies than range and also takes into acc all values 

water potential 

measure of the ability of water to leave a solution - pressure so in kPa

water potential of pure water and why ?

highest possible water potential of 0kPa bcs its the easiest possible solution for water to leave 

osmosis

the net movement and diffusion of water molecules from a high water potential to a low water potential down the water potential gradient through a partially permeable membrane - passive process 

what is a hypertonic solution ?

contains a higher concentration of solutes - has a lower water potential compared to another solution.

what wld happen to rbcs if they were put into a hypertonic solution ?

The rbcs wld lose water through osmosis via the partially permeable membrane

water would diffuse from a high to low water potential.

They would shrink and crenate

what is a hypotonic solution ?

has a lower solute concentration so a higher water potential compared to another solution

what would happen to normal rbcs placed in a hypotonic solution ?

The rbcs wld gain water through osmosis via the partially permeable membrane

water would diffuse from a high to low water potential.

They would expand then eventually rupture and lyse 

what is an isotonic solution ?

2 solutions separated by a partially permeable membrane have the same water potential.

There is no net movement of water particles into either solution.

the movement of water into the cell is equal to the movement of water out

why would a rbc eventually burst in distilled water ?

A rbc does not have a cell wall

The influx of water produces a higher (hydrostatic) pressure

the cell membrane expands but cant resist the increasing pressure of the water.

why would a plant cell be less likely to burst when placed in distilled water ?

It has a cell well to resist the pressure exerted by the water and expanding cell membrane

why do plant cells become turgid ?

Plant placed in a solution that has a higher water potential compared to plant cells (hypotonic)

Water diffuses into plant cells via osmosis through a partially permeable membrane.

The water diffuses into the vacuole which expands exerting turgor pressure causing membrane to expand against the cell wall

The plant cell becomes turgid

describe what happens when plant cells are placed in a concentrated salt solution ?

water leaves the cell by osmosis down the water potential gradient through the partially permeable membrane bcs the water potential inside the cells is higher than the salt solutions outside 

this causes the causes the cells to become plasmolysed bcs they were placed in a hypertonic solution

plasmolysis

where a plant cell has lost so much water its cytoplasm vacuole and membrane have shrunken away frm the cell wall 

what’s the formula for the dilution of the stock concentration ?

vol of stock conc = stock conc required / og conc x vol

precautions when doing dilutions ?

use a new clean micropipette / measuring cylinder for each dilution

ensure adequate mixing at each step

what are the independent and dependent variables in the investigation titled : how does increasing the concentration of a solute (salt) in water effect the mass of potato cubes?

independent - conc of salt

dependent - change in mass

osmosis practical method

cut cylinders from the same part of the same veg using a cork borer- controls conc / water potential

place cylinders next a ruler and cut to the same length (10mm) using a ruler - controls SA

wipe w/ a paper towel to remove excess liquid and measure mass of each cylinder using weighing scales

make up range of diff salt concs and place the same vol of each into separate boiling tubes

add into 25C -optimum-water bath w/ temp controlled -same KE same rate

immerse up to 3 cylinders tissue into each for 30 mins so osmosis can occur

remove cylinders from each solution - blot off excess liquid gently to ensure no overestimation of mass

control variables of osmosis practical

cells from diff veggies will have diff water potentials

time

temp

SA

how are the control variables controlled in osmosis practical ?

use cells frm the same onion

leave onion pieces in solution for sufficient time for osmosis to occur - at least 5 mins

perform exp at same time in a water bath so inion cells r all at the same temp

use a cork borer to ensure diameter is the same and a ruler to cut to same length

what happens if control variables arent controlled in osmosis practical ?

diff water potential gradients e.g. steeper gradients = faster rate

longer time = more osmosis

KE incs = rate of osmosis incs

Greater SA = more particles diffuse at a time = faster rate

osmosis practical precautions

blot potatoes before weighing - no over estimation = no greater % diff

skin potato - skins water proof prevents osmosis

wash and use new piece of apparatus each time - prevents contamination

use thermometer to check temp of water bath -when switching on take time to reach desired temp

same variety potatoes - diff varieties = diff water potential

common improvements in practicals

at least 3 repeats - accurate mean , identify anomalies - carry on for concordant results - improves repeatability

more intermediate values - allows for clearer trend

endocytosis

process of taking material into the cell by means of infoldings or pockets of the cell membrane - usually putting them into a vesicle

how is ATP used in endocytosis

ATP isnt needed for the conformational change of a carrier protein

it is to form the vesicles and fuse w/ the membrane and then to move these vesicles using motor proteins along cytoskeleton threads into the cell interior

exocytosis

a process in which the membrane of the vesicle surrounding the material fuses w/ the cell membrane forcing the contents out the cell

how is ATP used in exocytosis

ATP is needed for the fusion and also to move the motor proteins along the cytoskeleton threads before the fusion to the outside of the cell