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Week 2 Lecture 4: Lipids and Membranes (Lipids (Fats and oils…
Week 2 Lecture 4: Lipids and Membranes
Lipids
Sterols/steroids
Chemical messengers
They do the most messaging in the body
Very different to phospholipids structurally
Types
Cholesterol
Building block for other sterols
Large part of our diet
NOT a hormone
Structural component in many membranes
Sex hormones (produced from chemically modified cholesterol)
Testosterone
Male sex hormone
Contains a carbonyl group at the end of the compound and a hydroxyl group
Estradiol
Female sex hormone
Contains a hydroxyl group made from the
addition of a H onto the carbonyl group
, as well as another hydroxyl group
The balance of testosterone and estradiol determines your
biological gender
4 ring
structure
Fats and oils (
triglycerides
)
Energy storage
Most efficient way
of storing lots of chemical energy
Can burn fat
Using energy
Can accumulate fat
Eating food
Insulation
e.g. animals in cold climates
Structure
3 fatty acids
Do not normally exist as
free agents
Rather there are esterified to glycerol
#
Glycerol
To which the fatty acids are
esterified
Types
Fats
Solid
at room temperature
Due to
saturated
carbon bonds in acyl tails
Allows for greater packing – increased dispersion forces
This increases the melting point of fat relative to oil
e.g.
butter
Linear arrangement of acyl tails
Oils
Liquid
at room temperature
Due to one or more
unsaturated
carbon bonds in acyl tails
Causes less packing – fewer dispersion forces
This decreases the melting point of oil relative to fat
e.g.
olive oil
Phospholipids
Structural components of (plasma) membranes
All membranes in any cell are composed of a
phospholipid bilayer
Contains other components
Proteins
Glycoproteins (sugar/carbohydrate and protein)
Sterols (the phospholipids)
Different membranes have different ancillary components and different types of phospholipids
Structure
2 fatty acids
Do not normally exist as
free agents
Rather there are esterified to glycerol
#
Glycerol
To which the fatty acids are
esterified
Phosphate group
Interacts with OH and other functional groups
Examples
Phosphatidic acid
2 fatty acids, glycerol and phosphate
Phosphatidylinositol
Inositol
attached to phosphate group
Phosphatidylcholine
Choline
attached to phosphate group
Polarity
Known as
amphiphilic
or
amphipathic
There are two different 'loves'
Water-loving (i.e. fat-hating)
Hydrophilic
head
POLAR
Water-hating (i.e. fat-loving)
Hydrophobic
tails
(i.e. lipophilic)
NON-POLAR
The polar and non-polar properties of the phospholipids
allows for compartments
or a boundary
For a
cell
, this means a barrier between the intracellular (cytosolic aqueous solution) and extracellular environments (water/interstitial fluid)
For the
organelles
within a eukaryotic cell, this means a compartment between the cytoplasm and inside the organelle
Waxes
Protective coatings
In ears for
humans
On leaves for
plants
Bees
Calories are turned into
wax
, not fat
Fatty acids
(in triglycerides and phospholipids)
NOT lipids :red_cross:
Structure
1 carboxyl group (COOH)
'Acid' component
1 hydrocarbon tail (acyl tail)
'Fatty' component
The number of carbons on the tail can vary
The number of double bonds on the tail can vary
Examples
Stearic acid
No double bonds
Oleic acid
Its trans double bond
isomer
is
elaidic acid
, which has the same molecular formula but different arrangement of atoms
The trans arrangement of the acyl tail means there is a
kink/bend
which affects the
3D shape
of the molecule
1 double bond
Linoleic acid
2 double bonds
Arachidonic acid
4 double bonds
Esterification
A type of
condensation
reaction
Two molecules combine and form
ester bonds
or linkages
Involves the
removal/elimination
of H2O
From the combination of a
hydroxyl
(OH) group and a
carboxyl
(COOH) group
Triglycerides
Three H2O removed
Phospholipids
Two H2O removed
Types of cell transport across membranes
The cell or plasma membrane, i.e. phospholipid bilayer is
differentially permeable
It only allows certain substances in and out of the cell/organelle
This
regulates
what comes in and out
Passive transport
Movement of molecules
down
their concentration gradient
From a region of
high
concentration to a region of
low
concentration
No
metabolic energy required :forbidden:
3
types
Simple diffusion
What molecules
can
pass through the membrane?
Lipid-soluble
molecules
"The more lipid-soluble the molecule is, the more rapidly it diffuses through the membrane bilayer"
This statement holds true over a wide range of molecular weights
O2
CO2
What molecules
cannot
pass through the membrane?
Electrically charged
molecules
Ions
Polar (hydrophilic) or
lipophobic (lipid-insoluble)
molecules or large molecules
Sugars (due to OH groups)
Amino acids (due to their large size)
These substances form
hydrogen bonds
with water and ions, in the cytoplasm or even the extracellular environment
They cannot
move
into the hydrophobic region between the phospholipid bilayer
Facilitated diffusion
2
types of mediated facilitated diffusion, using...
Carrier proteins
Bind substances and speed up their diffusion through the phospholipid bilayer
Glucose binds with GLUT1, a carrier protein
Channel proteins
integral transmembrane proteins
that form channels across the membrane through which certain substances can pass
Gated channel
Ligand binding causes a conformational change in 3D shape of channel, opening it
Ion channel
Requires specific amino acid
Unique diameter
Only allows certain passage of substances
Can have gates
They open or close depending on the chemical or electrical signals provided
For
lipid-insoluble
molecules
i.e.
water-soluble
molecules – ions like Ka+ and Ca2+
Osmosis
Special case of diffusion
The movement of water through a differentially permeable membrane from a region of
high
water concentration to a region of
low
water concentration
Water moves
down
its concentration gradient
Water moves from a higher potential to lower (more negative) potential
The greater the solute concentration, the more
negative
the water potential
Alternatively, the movement of water through a differentially permeable membrane from a region of
low
solute concentration to a region of
high
solute concentration
Tonicity
Terms used to compare the solute concentrations of two solutions separated by a membrane
Hypotonic
The solute concentration outside the cell is
less
than the solute concentration inside the cell
1 more item...
e.g. cell placed in distilled water
Isotonic
The solute concentration outside the cell is the
same
as the the solute concentration inside the cell
1 more item...
Hypertonic
The solute concentration outside the cell is
greater
than the solute concentration inside the cell
1 more item...
e.g. cell placed in concentrated salt solution
Relative to a cell's environment
Osmotic pressure
The pressure required to prevent the movement of water into a solution
Provided
the solution is separated from the water by a
selectively permeable
membrane
Ciliates
undergo osmosis and maintain osmotic pressure within
1.
Water goes into the contractile vacuole
2.
The ciliate uses their
cytoskeleton
to shrink the vacuole and pump out H2O from the intracellular environment
Active transport
Movement of molecules
against
their concentration gradient
From a region of
low
concentration to a region of
high
concentration
Metabolic energy
required
:check: :
3
types of proteins that carry out active transport
Uniporter
Moves a
single
substance in one direction
Ca-binding protein found in the plasma membrane and the ER membranes of many cells
Actively transports Ca2+ to locations where it is more highly concentrated
To outside the cell or inside the lumen of the ER
Coupled transporters (
2
types) – moving two substances at once
Symporter
Moves
two
substances in the same direction
Symporters in the gut (intestinal) endothelial cells must bind Na+ in addition to an amino acid in order to absorb amino acids from the intestine.
Antiporter
Moves
two
substances in
opposite
directions
One into the cell (or organelle)
The other out of the cell (or organelle)
e.g. Sodium–potassium pump
Moves Na+ out of the cell and K+ into it.
1.
Three
sodium ions bind to the protein channel of the antiporter
The channel is
open
on the intracellular domain
2.
One ATP donates an inorganic phosphate which binds/phosphorylates onto the protein surface inside the cell
This provides the energy to change the conformation of the antiporter
1 more item...
3.
With the inorganic phosphate still attached,
two
extracelullar potassium ions bind to the protein channel of the antiporter
The K+ ions binding causes another conformational change of the antiporter
1 more item...
Both substances are going
against
their concentration gradient using
ATP
It is
directional
, moving substances into or out of the cell
Bulk transport
Achieved via
membrane vesicles
Molecules that are too large and charged/polar to diffuse or be actively transported through biological membranes and pores/channels
Proteins
Polysaccharides
Nucleic acids
1.
Endocytosis
Small molecules, macromolecules, large particles, and even small cells into the eukaryotic cell
3
types
Phagocytosis
Meaning
"Cellular eating"
Engulfs large particles, pathogens, prey or even entire cells
Examples
Some white blood cells (
macrophages
) use phagocytosis to defend the body by engulfing foreign cells and substances
Unicellular protists use phagocytosis for
feeding
Ciliates (protozoa) eat
algae
by phagocytosis
Process
2.
The phagosome merges with a
primary lysosome
, a vesicle containing digestive enzymes
The lysosome is part of the
endomembrane system
3.
A
secondary lysosome
results and digestion takes place
1.
Phagocyte engulfs substance, forming a
phagosome
Pinocytosis
“Cellular drinking”
Fluids and dissolved substances from the extracellular environment are brought into the cell
The vesicles formed are smaller that that in phagocytosis
The particles are not as big at those undergoing phagocytosis, but are nevertheless too big to pass through membrane channels
Occurs constantly in the endothelium, the single layer of cells that separates a tiny blood capillary from the surrounding tissue
Pinocytosis allows cells of the endothelium to rapidly acquire fluids and dissolved solutes from the blood
Receptor-mediated endocytosis
Process
2.
A coated pit forms and invaginates to forms a coated vesicle around the bound macromolecule
"Coated": Cytoplasmic surfaces covered by proteins (e.g.
clathrin
)
"Pit": Slight depressions in the plasma membrane
3.
The clathrin molecules strengthen and stabilise the vesicle, which carries the macromolecule away from the plasma membrane and into the cytoplasm
4.
The vesicle loses its clathrin coat and may fuse with a lysosome, where the engulfed material is digested (by the hydrolysis of polymers to monomers) and the products released into the cytoplasm
1.
A transmembrane integral
receptor protein
binds to its specific ligand (i.e. a specific
macromolecules
) on the extracellular domain
High specificity of the receptor proteins for particular macromolecules
An efficient method of taking up substances that may exist at
low concentrations
in the cell’s environment
The plasma membrane
invaginates
(folds inwards), forming a small pocket around the materials from the environment
The pocket deepens, forming a
vesicle
, separating from the plasma membrane and migrating into the intracellular environment
2.
Exocytosis
Materials packaged in vesicles are secreted from a cell via
fusion with the plasma membrane
1.
The vesicle membrane fuses with the plasma membrane, making an
opening
to the extracellular environment
2.
The contents of the vesicle are released into the environment, and the vesicle membrane is smoothly
incorporated
into the plasma membrane
Materials packaged in vesicles are secreted from a cell via
pore formation
The vesicle touches the cell membrane and a pore forms, releasing the vesicle’s contents.
No
membrane fusion in this process, termed “kiss and run”
The vesicle can be
reused
Metabolic energy
required
:check: