Life at the Cellular Level Flashcards
Prokaryotic Cells
- Bacteria, archaea
- lack nuclear membrane
- no membrane bound organelles
Eukaryotic Cells
- Animals, plants, fungi, algae and protozoa
- nucleus with membrane
- Membrance bound organelles
Describe human (eukaryotic) cells
- surrounded by a cell membrane
- contain a number of different organelles that from the machinery of the cellular factory
Describe the cell/plasma membrane /plasmalemma
- Comprises a double layer of lipid with attached phosphate groups = phospholipid bilayer.
- Forms a selective barrier, being choosy about what it allows to cross in or out of the cell.
- Embedded in the membrane are proteins which act as receptors to detect chemical messengers and signalling molecules in the fluid surrounding cells (extracellular fluid).
Describe the cytoskeleton
- Supports and maintains cell shape.
- Holds organelles in position – internal cell order.
- Helps move organelles around the cell – intracellular transport.
- Drives and guides cellular migration – movement.
- Protein fibres of the cytoskeleton connect with protein fibres in the extracellular space – Assembly of cells into tissues
What does the cytoskeleton include?
- Microfilaments - actin
- Intermediate filaments - keratin, neurofilament protein
- Microtubles - tublin
What do some cells have (cytoskelton related)
Surface projections supported by the cytoskelton:
1. cilia short usually many present, move with stiff power stroke and flexible recovery stroke (e.g. lung cilia)
2. Flagella longer, usually one or two present, movemnt is snakelike (e.g. sperm)
Both made of microtubules
Describe the nucleus
- Contains DNA - the brain of the cell.
- Nucleoli are sites of ribosomal RNA synthesis and ribosomal assembly.
- The nucleus is enclosed in the nuclear membrane/envelope which, like all biological membranes, is a phospholipid bilayer.
- It is closely associated with the Rough Endoplasmic Reticulum
Describe the Endoplasmic Reticulum
- Membrane bound organelle
- Endoplasmic reticulum comes in rough (RER) and smooth (SER) varieties
- RER has ribosomes attached giving it the “rough” appearance
- Ribosomes synthesise proteins
- RER modifies proteins.
- SER has no ribosomes attached and is mainly associated with lipid and steroid hormone production and metabolism of toxins
Describe the golgi apparatus
- Membrane bound organelle
- Golgi Apparatus packages up protein in preparation for transport out of the cell
Mitochondria
- Organelles bound by a phospholipid bilayer.
- Outer membrane contains pores
- Inner membrane has cristae (folds which inc surface area)
- Matrix contains most of the enzymes required for metabolising food molecules (e.g. Krebs cycle).
Other Mitochondria facts
- They have their own circular DNA.
- They have their own ribosomes – similar to bacterial ribosomes.
- They synthesise most of their own proteins.
- They can self-replicate.
Lysosomes and Peroxisomes
Lysosomes and peroxisomes are membrane bound vesicles containing enzymes – they separate enzymes from the rest of the cell. Enzymes are one form of protein packaged by the Golgi apparatus that catalyse reactions in cells, but need to be stored until needed.
Lysosomes
Lysosomal enzymes are degradative and they are responsible for the digestion of biological materials (cells own materials; autophagy) or digestion of engulfed particles (e.g. bacteria).
peroxisomes
Peroxisomal enzymes degrade long-chain fatty acids and other foreign toxic molecules. These reactions generate hydrogen peroxide, which is toxic. Peroxisomes break the hydrogen peroxide (catalase enzyme) protecting the cell.
Ribosome
Site of protein synthesis in a cell.
Reads mature mRNA transcript and sysnthesises sequence of amino acids, which can then be folded to form proteins
What are Stem Cells?
Stem cells are cells that can differentiate into many (multipotent) or any (pluripotent) cell types of the body.
Clinically they have been used in the treatment of certain diseases including some cancers.
Explain cell differentiation
- Undifferentiated stem cells divide and give rise to (genetically identicle) daughter cells
- Differences in gene expression and the local cellular environment cause daughter cells to develop into different cell types
Stem cells in the lab
Stem cells canbe differentiated into almost any type in the lad and used as models for reserch
What is constantly happening to our tissues
Tissues are collections of cells sharing a similar function and are as such contantly being remodelled with cells dying…
Apoptosis
Controlled and programmed cell death at a predetermined time. It is a normal process and essential for normal function.
necrosis
Untimely death of cells in **response to injury or infection. **It is NOT a normal process
What 2 things are intimately coupled?
Apoptosis and cell proliferation
What happens when the balance betweel apoptosis and proliferation is lost?
Cancers can occur:
* Divide without any control
* Fail to coordinate with normal cells.
* Fail to differentiate into specialized cells
* Displace and replace the normal cells – if not stopped.
What could happen when a cell is damaged during division?
- Cell damaged fixed
- Cell death - apoptosis
- Cell damage not fixed leading to uncontrolled proliferation
What are all human cells surrounded by?
Double membrane made up of lipid and phosphate - phospholipid bilayer
All substances entering/leaving the cell must cross the cell membrane
Chemical composition of the cell
- 4 organic basic elements: H,C,N,O
- 7 required in reatively large amounts: Na,Mg,K,Ca,P,S,Cl
- 12 trace elements needed in small amounts: Fe, Mn,Co,Ni,Cu,Zn,Mo,Se,I (from critical interactions in proteins; fe in heamoglobin)
Get these from our diet
Vanancy of 4 core organic molecules
H - 1
O - 2
N - 3
C - 4
Which is the most versitile element
C as can form bonds (sometimes 2/3/4) with H,O,N and C. This bonding versitity is why C forms the basis of most biomolecules
Molecule
2 or more atoms sharing (loosing or gaining e-) in a covalent bond
explain functional groups
Linked carbon atoms can form a variety of structures (linear chains, branched chains and cyclic structures).
Groups of elements that attach to these carbon structures are called “functional groups” and confer specific chemical properties to the molecule. Often more than one functional group per molecule. Important functional groups include: amino NH2, carboxyl COOH, Hydroxyl OH, Phosphate H2PO4
Define configuration
Fixed arrangement of atoms in a molecule
Explain configuration, cis/trans in more detial
Many biomolecules contain a double bond between carbon atoms (C=C) which is rigid (no freedom of rotation). They can therefore only interconvert between the two by breaking and re-forming bonds (energetically expensive!)
Can therefore only have two distinct configurations: cis/trans
1. Cis - groups on same side of bond
2. trans - groups on opposite side of bond
How can configuration be important biologically?
Changes in the configuration of a biological molecule can alter its conformation to other molecules and thus, if this configuration was to change binding and cellular pathways/reactions may change/be initiated too.
ex: Rhodopsin is a protein embedded in the (rods of the) retina of the eye that is bound to retinal. On exposure to light the configuration of retinal alters (from cis-retinal to trans-retinal) and this alteration stimulates nerves cells in the optic nerve to send messages to the brain where we “see”
Carbon molecules - symmetry
- Symmetric - achiral
- asymmetric - chiral
Asymmetric carbon molecules can be either L (laevo) or D (Dextro) - Most of the monosaccharides naturally occur in D- form
- The amino acid residues in proteins are L-amino acids
Describe conformation
the spatial arrangement of groups that can have different positions in space due to the freedom of rotation of single bonds
5 chemical reactions of life
- Redox reactions
- Making and breaking C-C bonds
- Internal rearrangements
- Group transfers
- Condensation and hydrolysis reactions
1.redox reactions
OILRIG
oxidising agent is reduced and gains e-
reducing agent is oxidised and looses e-
- redox example
glucose —(2NAD+ > 2NADH)—> 2 pyruvate —(2NADH > 2NAH+)—> 2 lactate
Give what is being reduced and the reducing agent?
- NAD+ is being reduced to NADH (gained e- as lost +ve charge)
- NADH is reducing agent - donates e- to pyruvate meaning pyruvate is reduced to lactate - thus NADH becomes oxidised to NAD+
Key points to remember about NADH and NAD+
- NADH is reducing agent
- NAD+ is oxidising agent
2 Making/breaking C-C bonds examples
Breaking C-C bonds = gycolysis (Fructose 1,6 - biphosphate –> dihydroxyacetone phosphate + glyceraldehyde 3-phosphate)
Making C-C bonds = gluconeogenesis (bicarbonate + pyruvate –> oxaloacetate)
3 Internal rearrangements example
glycolysis:
glucose 6-phosphate –> fructose 6-phosphate
4 Group Transfers example
Phosphoryl group (PO32-) is transfered from ATP to Fructose1,6-biphosphate. Reaction is catalysed by enzymes with ATP providing the energy for cellular reactions
5 condensation and hydrolysis definitions
condensation: : two smaller molecules combine to form a larger molecule, releaseing water
Hydrolysis: breaking a large molecule into smaller units using water
5 condensation/hydrolysis examples
The subunits of proteins, polysaccharides and nucleic acids are all joined by condensation and broken by hydrolysis reactions.
give 4 biological macromolecules
- Carbohydrate
- Lipid
- Protein
- Nucleic Acid
What are carboydrates
give some facts
Glucose polymers:
Start with condensation reaction between 2 glucose monomers.
Glucose can exist in an open-chain or ring form
* When polymers are formed, one glucose monomer is linked to another which locks the additional glucose in a cyclic form
* When lots of monomers link together, all monomers of the chain are locked in the cyclic form except the end monomer, which can remain linear. This end monomer then forms a ‘reducing end’.
Glucose is termed a “reducing sugar”.
What are proteins
Proteins are long chains of amino acids (polypeptides are chains of amino acids as well but shorter than proteins). These chains are formed by condensation reactions. The amino group of one amino acid reacts with the carboxyl group of another to form a peptide bond (CONH) with a molecule of water being lost (condensation) as each bond is form.
Polysaccharide
an abundant carbohydrate found in food
Nucleic acids?
- Form core structure of DNA and RNA
- Polymers of nucleotide monomers linked by 3’,5’-phosphodieter bonds
How do DNA and RNA differ?
- DNA is double stranded nucleic acid with A-T and G-C base pairing
- RNA is a single stranded nucleic acid with A-U and G-C base pairing
Give classification of bases
Purines: adenine, guanine
Pyrimidines: thymine, cytosine, uracil
What are lipids?
Mainly repeating units of fatty acids which are long chains of C and H and can be saturated (C-C) opr unsaturated (C=C, mono or poly depending on number of double bonds).
Explain unsaturated FA in more detail
The more double bonds in a fatty acid chain (unsaturated it is), the more “bendy” the fatty acid molecule and the more fluid it is.This has important implications for cell membranes, which are largely composed of fatty acids.
Lipids: Triacylglycerides
- AKA triglycerides or fats
- Consist of Glycerol and 3 FA molecules.
- The fatty acid chains can be the same (simple triacylglycerides) or mixed (2 or more different fatty acids).
- Insoluble in water.
- Storage lipids (in adipocytes).
Lipids: Phosphates
- Glycerol + 2 fatty acid chains + phosphate group
- Phospholipids form biological membranes and due to their properties they are used for drug delivery
Lipids: Sterols
- Sterols are structural lipids present in cell membranes.
- They have an steroid nucleus – 4 fused rings.
- Most important one in humans: cholesterol
- Amphipathic: polar head group (OH) and a non-polar hydrocarbon body.
- Essential component of cell membranes (role in membrane fluidity and reduces the permeability of the cell membrane).
- Precursor to steroid hormones and fat soluble vitamins (A, D, E).
Summarise Functional Groups
Functional groups (groups of elements attached to carbon skeletons/orghanic molecules) confer specific properties to molecules. Typical of these are alcohols (having one or more hydroxyl groups OH); amines (amino groups NH2); aldehydes and ketones (carbonyl groups C=0); and carboxylic acids (carboxyl groups C=00H)
Summarise the main biological polymers
Proteins, nucleic acids, polysaccharides and lipids are polymers formed by amino acids, nucleotides, monosaccharides and fatty acids, respectively, joined together.
Why is water so essentail?
- it bathes our cells
- Dissolves and transports compounds
- allows compounds to move within and between cells
- perticipates in chemical reactions
- dissipates heat
Define electronegativity (give most electronegative elements)?
Atom’s ability to attract a pair of electrons to itself in a chemical (covalent) bond.
O,F,Cl all highly electronegative
Describe polarity and why it is imortant?
The distribution of electrons in a molecule.
it is key as it determines functionality: wheather a bond is non-polar (C-C/C-H) or highly polar (C-O/C-N/C-fuctional group) will alter the C bond reactivity/
Define a non-polar covalent bond
2 atoms equally share a pair of electrons with each other
Define a polar covalent bond
Pair of electrons is unequally shared between two atomes. More electronegative atom pulls the e- towards itself becoming partially negative.
Explain water and its properties as a solvent
Polar molecule; uneven charde distribution so H has slight +ve charge (dipole) and slight -ve charge (dipole) at O end.
This polarity means other polar substances will dissolve readily in water
How can biomolecules be classified based on their interactions with water?
- hydrophilic (likes water)
- Hydrophobic (hates water)
- Amphipathic (both parts)
Explain hydrophilic molecules
Those that dissolve in water due to being polar (like water)
give examples of hydrophilic molecules
Sugars
Alcohols
Aldehydes
Ketones
Compounds with N-H groups
Charged particles such as ions
How do charged hydrophilic molecules “dissolve” in water
Water forms a screen around the charged particles
When sodium chloride (common salt) is dissolved in water, the water forms ‘screens’ around each ion keeping the Na+ and Cl- ions in solution once dissolved. This screening is also tru for more complex biomolecules.
How do proteins/enzymes interact with water?
An ordered layer of water surrounds all solutes (“screens” both the enzyme and the substrate)
When a substrate interacts with an enzyme, ordered water molecules are displaced (move out of the gap allowing substrate to bind with Enzyme, stablised through H-bonding/ionic/hydrophobic interactions) and this increase in disorder favours (energetically speaking) the formation of enzyme-substrate complex.
Explain hydrophobic molecules
Mostly non-polar molecules which do not dissolve in water, but instead lipid (non-polar).
How do hydrophobic molecules arrange themselves in water?
So as to minimise contact with surrounding water molecules (know as the hydrophobic effect)
Give examples of hydrophobic molecules
- Fat soluble vitamins (A, D, E, K)
- Lipids
- Steroid hormones
- Oxygen
What are triacylglycerides in relation to solubility?
Insoluble in water due to their non-polar, hydrophobic interactions (long non-polar FA chains)
Explain Amphipathic molecules
- Contain both hydrophobic and hydrophilic parts
- Many proteins are amphipathic
- hydrophobic region of protein chain on inside and hydrophilic region on outside, thus allowing them to be water soluble
What does the polarity of protein R-roups determine?
Weather a potein is polar or non-polar
Give an example of an amphiphatic molecule
Cholesterol: polar head group (OH) and Non-polar hydrocarbon body
Give phospholipids as a more detailed ex of amphipathic molecules
Phosphate head = hydrophilic
FA tail = hydrophobic
In water phospholipids form micelle or bilayers such that the hydrophobic tail is directed away from contact with water
What happens if you put lots of phospholipids with water?
A sphere with a lipid bilayer outer shell (and maybe hollow core is produced - liposomes/micelles (both used in drug delivery)
Define Liposomes
Lipid bilayer outer shell with a hollow core
- allows for the delivery of hydrophilic drugs as phophate heads face drug in hollow core
Define Micelles
Single layer of phospholipd with no core
- used to deliver hydrophobic drugs as will dissolve in non-polar FA of phospholipid tails
How are lipids transported in the body
In a chylomicron
What is a chylomicron?
- Like a liposome with protein embeded shell, and lipid stored in its core
- The phospholipid heads and outer edges of the proteins form a hydrophilic outer shell
- The hydrophilic shell is essential to allow the chylomicron to be transported in the aqueous plasma of the blood.
Explain the ionization of water
Water molecules diccociate (ionize) into* hydrogen ions H+* and hydroxide ions OH- - giving the basis of the pH scale
Explain the basis of pH
- Way of designating H+ conc in aqueous solution
- Water has neutral pH as [H+] and [OH-] are equal
- Acid solutions have greater [H+] than [OH-]
- Basic (alkaline) solutions have greater [OH-] than [H+]
- is a log scale - 1 pH change is a 10fold inc/dec in [H+]
What should pH levels in the blood be?
7.35-7.45
Below is acidosis and above is alkalosis
What are strong acids/bases
those which dissociate fully in solution (e.g. HCL or NaOH)
Why are weak acids/bases more important in boilogical systems?
They only partially dissociate, giving them the ability to act as buffers
What are buffers?
Solutions that resist changes to pH
Why must we maintain pH?
Many biomolecules (proteins, DNA) are affected by pH as their shape is dictated by the pH of their environment (~7.4), so for optimal activity they must be at an optimal pH - or else they could be denatured/irreparable cell damage, disastrous metabolic effects and death
- Optimal pH is maintained by using weak acids and their bases as buffer systems within cells and organisms.
Phosphate buffer system
- In cytoplasm of all cells
- buffers ICF
- resists pH change of 5.9 to 7.9
- (weak acid) H2PO4- <=> H+ +HPO42- (base)
Bicarbonate buffer system
- In plasma
- More complex because conc of H2CO3 depends on CO2 conc
- CO2 + H2O <=> H2CO3 <=> H+ + HCO3-
What is the bicarbonate buffer system affected by?
- Lung inc or dec in ventilation will change CO2 levels
-
Kidney changes in bicarbonate* reabsorption or sectetion* change overall plasma bicarbonate
Diseases of lungs/kidneys can cause acid-base disorders
What can the Henderson-Hasselbalch be used to calc?
How the pH of a physiological solution will respond to changes in either the conjugate acid or base
Give the Henderson-Hasselbalch equation
pH = pKa + log [A-]/[HA]
where:
* pKa - indicates how weak acid is, lower value=stronger acid
* A- = conjugate base
* HA = weak acid
Simplified equation: pH = [A-]/[HA]
Summarise the Henderson-Hasselbalch principle
pH is proportional to the ratio of conjugate base to weak acid, or, for example, proportional to the ration of HCO3- (conjugate base ) to H2CO3 (weak acid)
Explain the bicarbonate buffer system in more detail…
Without the bicarbonate buffer system, the blood pH would fluctuate wildly as, for example, cellular products of acids (e.g. lactic acid) would cause marked drops in blood pH.
Instead, as these acidic products are released into the blood, the bicarbonate ions act to buffer the H+ to prevent a rapid fall in blood pH. As free [H+] rises so does H2CO3 as the equation is pushed to the left. The bicarbonate “mops up” free H+ and limits (but does not completely prevent) the fall in pH.
CO2 + H2O < = > H2CO3 < = > H+ + HCO3-
By measuring blood pH, [HCO3-] and [CO2] the Henderson-Hasselbalch equation can be used to check how much buffering capacity a patient has.
Give 2 acid-base disorders
- Higher [H+] = acidosis (low PH)
- Lower [H+] = alkalosis (high pH)
Only 2 chemicals which can change to cause these disorders are HCO3- and CO2, which will both then affect [H+]
CO2 + H2O < = > H2CO3 < = > H+ + HCO3-
What would happen if CO2 conc inc?
[H+] inc as eq shifted to right, (product conc dec and reactant conc inc). This (inc in CO2) would lead to respiratoy acidosis and (dec in HCO3-) metabolic acidosis
Pluripotent
Differentiate into any cell type (embryonic)
Multipotent
Differentiate into many but not all cell types (adult stem cell)
Give 1st law of Energy change
Energy can be converted between forms but total energy of the universe must remain constant
Give 2nd law of energy change
All energy changes lead to disorder in the universe (inc entropy)
- as usuable energy dec unusable energy inc
What do cells try to create/maintain?
Order; to do so cells perform lots of chemical reactions which require energy
Do cells follow the 2nd law?
Cells use enrgy to grow/form complex molecules/systems, and since they don’t live in isolation they take energy from the sun/food molecules to generate this order required for life. The chemical reactions which generate the cell’s order produce heat which is discharged into the environment, inc total entropy - so no
How can we describe the useful or free energy in a system?
Gibb’s free enrgy: G = H - TS
H - enthalpy (heat released in bonds breaking/forming)
S - Entropy (randomness/disorder)
T - Absolure temperature, K
What can free energy change be used to define?
The spontaneity of a reaction, which will occur if a system:
- gives up energy
- becomes more random and inc entropy
What must happen in a spontaneous process
Enthalpy (H) decrease and/or Entropy (S) increase
What value must deltaG take for a spontaneous process?
Negative (system releases energy)
#G = #H -T#S
What do most biological processes require in terms of energy?
Require more order so reactions generating proteins/SNA/cells/organs… require energy (+ve #G)
How can thermodynamically unfavourable biochemical reactions occur?
Reaction coupling where a catabolic reaction is paired to an anabolic one, providing the energy it requires - free energy floes between catabolic and anabolic processes allowing them to occur
How does free enrgy work with reaction equilibrium?
G is incompatible with life, so reactions usually dont reach eq as E passes from the environment to the organism then back to the environment. This works because organisms use pathways of reactions (e.g. food molecules to excretory products)
What is the dynamic steady state?
Give example
Series of reactions with maintain consistant internal composition, but different from eq conc
For ex:
food molecule —a—> intermediate molecule —b—> excretory molecule
(a and b are rates of process shown)
when a=b the system is in a synamic steady state because the intermediate molecule although being made and degraded does not change conc
Explain metabolic pathways
- Each step catalysed by an enzyme
- Spontaneous reactions move towards ea but don’t reach it
- spontaneous is not necessarily instant
- Enzymes function to sectively alter the rate of particular parts of metabolic pathways
What generally happens with catabolic and anabolic pathways?
Energy released from catabolic reaction (e.g.ADP to ATP) is then used in an anabolic pathway (ATP to ADP) - reaction coupling
what are intermediary metabolites?
Components of several pathways
Intermediary metabolism refers to the sum of all intracellular chemical processes by which nutritive material is converted into cellular components. It includes anabolism (synthesis of macromolecules) and catabolism (breakdown of macromolecules).
Give glucose as an ex. of metabolism
Glucose in state of high potential energy. Reacting with O2 realeases CO2, H20 (low potential enrgy) and heat. Thjis reaction occurs in cells buit in series of small chemical reactions facilitated by enzymes. Small reactions allow the Ep to be used or stored at particular points in the pathway (rather than realeasing all energy as heat)
Define anabolism
synthesis of complex molecules from simple ones, requiring energy
Define catabolism
breakdown of complex molecules to simple ones, releasing energy
Define metabolism
Sum of all chemical and physical changes that take place in the body and enable its continued growth and function (anabolism and catabolism combined)
What happens with ATP hydrolysis?
ATP-ADP releasing free energy (exergonic) which is used from this catabolic process to drive thermodynamically unfavourable reactions (+#G)
What does an exergonic and endergonic pathway do?
- Exergonic, catabolic saves free energy withing a sytem by forming ATP
- Endergonic anabolic pathway is supplied with free energy within a system through conversion of ATP to ADP
Show ATP/ADP interconversions
ATP + H20 —-hydrolysis—-> ADP + Pi
<—-condensation——
ATP higher Ep than ADP/Pi so hydrolysis occurs with dec in free enregy (exergonic) #G=-7.3kcal/mol
ADP to ATP condensation reaction requires free enrgy input (endergonic) #G=+7.3kcal/mol
What does ATP allow?
Anabolic, thermodynamically unfavourable reactions to proceed therough coupling of catabolic thermodynamically favourable reaction
Explain PEP as an intermediate in glucose metabolism
Glucose releases E when degraded. Intermediate step in process produces phosphoenolypyruvate (PEP) which acts as an intermediate for a reaction which goes on to produce ATP during PEP –> pyruvate conversion.
some potential energy released from PEP (originally from glucose) to pyruvate conversion is “stored” in the form of ATP with some being released as its converted toi pyruvate
Explain phosphoryl transfers
Phosphoryl group added to ADP, producing ATP in a process with stores energy. When phosphate release from ATP forming ADP + Pi, energy is relased. Thus, phosphpate group transfers allow energy to be stored and released
How does Ep arise in food:?
Because they ahve many H atoms: carbs (glucose), fatty acids (palmitate)
How can an EMF arise?
redox reactions transfer e- which causes an e- flow to arise, producing energy required by cells. essentially, EMF describes electrons accomplishing work as they pass through chemical intermediates
give EMF with the example of glucose
oxidation of glucose releases e- that spontaneously flow through series of intermediate steps to another chemical species, e.g. O2
What electron carrier molecules do cellular redox reactions use?
Activated carreirs of ennergy act as coenzymes in biochemical pathways to facilitate oxidation and biosynthesis reations:
NAD+ —> NADH
FAD+ —> FADH
What do both NADH and FADH contain
A reactive site where oxidation/reduction occurs through net gain/loss of electrons
where do the phosphate and bicarbonate buffers usually exist
Phosphate = ICF
Bicarbonate = plasma
What does an increase in C=C double bonds of a FA in the cell membrane do
Inc flexibility of membrane
Double bonds inc flexibility of FA chain
