Lecture 1.1: Overview of Biological Membranes Flashcards
What are the Functions of Biological Membranes? (5)
- Form continuous and highly selective permeability
barrier - Control enclosed chemical environment
- Facilitate communication between organelle/cell and
environment - Allow recognition through the presence of signalling
molecules, adhesion proteins and allow immune
surveillance - Generate signals in response to stimuli (electrical,
chemical)
Composition of Biological Membranes and Percentages (Dry Weight)
- 60% protein
- 40% lipid
- 1-10 % carbohydrate
What percentage of total membrane weight is water?
20 %
Phospholipid Structure
- Hydrophilic Group (choline, amines or sugars,
phosphate) - Glycerol (NOTE: sphingomyelin contains sphingosine)
- Hydrophobic Group (two fatty acids of varied length,
C16 and C18 are most common, and degree of
saturation- cis double bond introduces a kink)
What allows phospholipids to be less packed in the lipid bilayer?
- Unsaturated fatty acids introduce a kink in
the hydrophilic part of the phospholipid - This allows them to be less packed/rigid and
more flexible
Cholesterol Structure
Contains hydroxyl group and rigid steroid ring
and hydrocarbon side chain
How does cholesterol fit in-between phospholipids?
It fits into spaces created by the kinks of
unsaturated fatty acids in phospholipids
What are the 2 types of carbohydrate found in Glycolipds?
- Cerebrosides: head group with monosaccharide
- Gangliosides: head group with oligosaccharide
What are the 3 Types of Membrane Proteins?
- (Integral) Transmembrane Proteins
- (Integral) Lipid Anchored Proteins
- Peripheral Proteins
Where are Transmembrane Proteins found?
Deeply embedded in the lipid bilayer of membrane
Examples of Transmembrane Proteins (4)
- Adhesion Proteins
- Ion Channels
- Transporters
- Receptors
Where are Lipid Anchored Proteins found?
- Attached covalently to lipids without entering
core portion of lipid bilayer
Examples of Lipid Anchored Proteins (1)
G-coupled proteins
Where are Peripheral Proteins found?
- Associated with the surface of membrane
- Electrostatic and hydrogen bond interactions
What denatures Peripheral Proteins?
- pH change
- Temperature change
Examples of Peripheral Proteins (1)
Cytoskeletal proteins (e.g. actin)
Functions of Membrane Proteins by Function?
- Transporters (control movement across membrane)
- Anchors (act as attachment points to membrane)
- Recognition (markers for cell-cell interactions)
- Glue (junctions connecting cells)
- Enzymes (localising metabolic pathways)
- Transduction (receptors carry signals into the cell)
What is the Fluid Mosaic Model of Membranes?
- Biological membranes are fluid structures as
lipids and proteins - Are not placed rigidly in particular location
- Despite their fluidity they are stable
- As in mosaic, proteins are scattered throughout
membranes
Mobility of Phospholipid Bilayer (4 Types of Movement)
- Flexion
- Rotation
- Flip-flop
- Lateral Diffusion
Examples of Hydrophobic Molecules (4)
- O2
- CO2
- N2
- Benzene
Examples of Small Uncharged
Polar Molecules (3)
- H2O
- Urea
- Glycerol
Examples of Large Uncharged
Polar Molecules (2)
- Glucose
- Sucrose
Why do we want to transport things across the membrane? (6)
- Maintenance of ionic composition
- Maintenance of intracellular pH
- Regulation of cell volume
- Control concentration of metabolic fuels and building
blocks - Extrusion of waste products of metabolism and toxic
substances - Generation of ion gradients necessary for the
electrical excitability of nerve and muscle
What is Passive Transport?
Diffusion of hydrophobic molecules across the membrane with their concentration gradient (from higher concentration to lower concentration)
What is Passive Facilitated Transport?
Diffusion of hydrophilic substances through specific membrane proteins:
* Carrier proteins (ping-pong)
* Protein pores (channels)
* Protein flip-flop
What are the Types of Channels (6)
- Gap Junction
- Uniporter
- Ligand Gated
- Antiporter
- Voltage Gated
- Symporter
Types of Channels: Gap Junction
Pore between cells for electric and chemical continuity
Types of Channels: Uniporter
Transports 1 molecule across a membrane
Types of Channels: Ligand Gated
Opens when chemical binds to a receptor
Types of Channels: Antiporter
Transports 2 molecules in opposite directions
Types of Channels: Voltage Gated
Opens when potential difference across membrane change
Types of Channels: Symporter
Transports 2 different molecules across membranes in the same direction
Primary ATP-Powered Pumps: P-class
What does it transport? (4) Across what membrane?
- H+
- Na+
- K+
- Ca2+
- Across several different membranes
Primary ATP-Powered Pumps: F-class
What does it transport? Across what membrane?
- H+ ions
- Across inner mitochondrial membrane
Primary ATP-Powered Pumps: V-class
What ion does it transport? Across what membrane?
- H+ ions
- Across lysosomal membrane
Primary ATP-Powered Pumps: ATP-Binding Cassette (ABC)-class
What does it transport? Across what membrane?
- Ions
- Drugs
- Xenobiotics
- Across various cell membranes
What is Primary Active Transport?
- ATP-powered pumps that have ATPase
activity - They use energy from hydrolysis of ATP
What is Secondary Active Transport?
- Transport proteins do not have ATPase activity
- Their function depends indirectly on ATP hydrolysis by
ATPases (sodium pump) - They use the ion gradient generated by ATPases
- This is co-transport
What is co-transport?
The movement of one substrate depends on the movement of another substrate
When is pH controlled by the activity of
cell membrane transporters?
When cellular buffering capacity is exceeded
How is pH controlled by cell membrane transporters ?
Most of them use H+ gradient generated by Na+-K+-ATPase
What cell membrane transporters cause Alkalinisation? (3)
- Na+/H+ antiporter
- Na+/HCO3- symporter
- Na+/Cl-/HCO3-/H+ antiporter
What cell membrane transporter causes Acidification?
Cl-/HCO3- antiporter
How is Cellular Volume regulated?
- Regulated by transport of osmotically active ions
- Na+, K+, Cl- or organic osmolytes (amino acids)
- Water follows them affecting cell volume
- Most of cell membrane transporters use H+
gradient generated by Na+-K+-ATPase
Mechanisms to Resist Cell Swelling (3)
- Conductive Systems
- Efflux of osmotically active ions in response to
cell swelling - Co-transport Systems
Mechanisms to Resist Cell Shrinking (4)
- Conductive Systems
- Influx of osmotically active ions in response to
cell shrinking - Co-transport Systems
- Organic Osmolytes
Types of Transport across the Placenta (4)
- Simple Diffusion
- Facilitated Diffusion
- Active Transport
- Pinocytosis
What is transported via simple diffusion across the placenta? (5)
- Water
- Sodium
- Chloride
- Gases
- Drugs (midazolam & paracetamol)
What is transported via facilitated diffusion across the placenta? (3)
- Glucose
- Fatty Acids
- Drugs (cephalosporins, glucocorticoids)
What is transported via active transport across the placenta? (5)
- Amino Acids
- Iron
- Calcium
- Vitamins
- Drugs (norepinephrine, dopamine)
What is Neonatal Abstinence Syndrome (NAS)?
- Foetus is exposed to drugs (e.g. opioids) via placental
transfer - Mother and foetus/baby with dependency to a drug
- Withdrawal symptoms after birth
Complications of NAS (4)
- Preterm Birth
- Intrauterine Growth Restriction
- Withdrawal symptoms after Birth
- Death
What is Coeliac Disease?
- It is a lifelong autoimmune disease caused by an
abnormal immune system reaction to gluten - The damage (atrophy of the villi), and inflammation to
the lining of small intestine reduces body’s ability to
digest and absorb nutrients from food
What are the Symptoms of Coeliac Disease? (8)
- Bloating
- Diarrhoea
- Nausea
- Wind
- Constipation
- Tiredness
- Weight Loss
- Anaemia
What is the Placenta?
It is a disc-shaped organ which forms the physical barrier between mother and foetus
What are the Functions of the Placenta? (4)
- Gas Exchange
- Metabolic Transfer
- Hormone Secretion
- Foetal Protection
What is is the primary barrier limiting nutrient transfer
across the placenta?
Syncytiotrophoblast
What is the Structure of the Syncytiotrophoblast?
- Maternal-facing microvillous plasma membrane
(MVM) - Basal plasma membrane (BM) oriented towards foetal
circulation
How does nutrient and drug transfer across the placenta occur? (4)
- Simple Diffusion
- Facilitated Diffusion
- Active Transport
- Pinocytosis (type of endocytosis)
Villus of Small Intestine Important Structures
- Lacteals (dilated lymphatic vessels) are
important in the absorption of fat - Smooth muscle produces local rhythmic
movements to improve the efficiency of
absorption - Simple columnar epithelium:
- Enterocytes (absorptive cells)
- Goblet cells (mucus- secreting cells)
- Enteroendocrine cells (gastric inhibitory
peptide hormone- producing cells) - Paneth cells (enzymes and anti-microbial
compounds- secreting cells)
Small Intestine: protein networks between cells and microvilli. What are they? Why are they important/What is their role?
- Specific connections formed by protein-protein
complexes link adjacent cells in a controlled way - This allows for paracellular transport of substances
What useful substances are transported across the Placenta?
- Nutrients
- Hormones
- Gases
What harmful substances are transported across the Placenta?
- Drugs
- Alcohol
- CO
