lecture 2- Membranes and their ability to transport materials Flashcards
why is cholesterol important in digestion?
Cholesterol is also present in lipoproteins.
Lipoproteins are how lipids are transported around the body.
They contain:
*Hydrophobic core (contains triacylglycerols and cholesterol esters)
*Amphipathic coat of:
➢Apolipoproteins
➢Phospholipids
➢Cholesterol
ampiphillic
-means you are lipophillic and hydrophillic
what is a A chylomicron?
A chylomicron has the lowest density and contains dietary lipid & cholesterol esters. It transports absorbed fats to the liver for processing
what are lipoproteins?
Several classes of lipoproteins, named by density. As central triacylglycerols are degraded the density increases.
VLDL transports lipids generated in the liver to the tissues
IDL and HDL transport remaining lipids from the tissues back to the liver.
LDL recycles back to the liver and to tissues
Apolipoproteins act as signals for
◦Cellular uptake
◦Metabolism
summaries the importance of cholesterol
▪Cholesterol is a versatile molecule with a rigid sterol ring structure, a tail and a hydroxyl group.
▪Uptake of all lipids from the diet requires emulsification (requires bile acids) and that aids in the formation of chylomicrons for transport to the liver, for processing.
▪Bile acids not bound to lipid droplets can be toxic to the liver, as a result their binding to nuclear hormone receptors enables them to increase expression of enzymes that will degrade them through a transcriptional mechanism.
▪The liver generated lipoproteins enable transport of lipids around the body to tissues where it can either be stored or used, and then back to the liver. These use the amphipathic nature of some lipids to envelop those without this property.
▪The contents of the outside of lipoproteins dictate where the particles are destined fo
how does cholesterol fit into the cell membrane?
Cholesterol interacts with the top 3rdof the phospholipid
Thereforeit can have 2 effects;
*Interactions between phospholipid tails and the steroid ring give “stiffening” effect by reducing the movement of saturated fatty-acyl chains of the phospholipids.
*In bilayers where long chain, unsaturated fatty acid tails are prevalent the presence of cholesterol will decrease van der waalsinteractions, accompanied by an increase in fluidity.
what are amphipathic molecules and what is the shape of saturated fatty acids?
Amphipathic molecules enable a hydrophobic core and a hydrophilic outer surface.
Saturated fatty acids are more of a cone shape because their fatty acid tails are more linear.
how do hydrophobic molecules form a sphere and why?
Hydrophobic “middle” of the bilayer forces a shape change because it cannot interact with the aqueous environment.
This is because it is not energetically favourable as a sheet so creates a sphere
inner leaflet is usually slightely negatively charged
what are the different freedoms of movement for phospholipid bilayers?
-lateral diffusion
-flexion
-rotation
-flip-flop
Main ways for phospholipids to be dynamic within the membrane. the lateral diffusion, flexion and rotation are the most frequent to occur without enzymatic activity.
Flip-flop happens rarely unless it is enabled by an enzyme/channel
fluid mosaic suggests the bilayer is able to move and is made up of lots of different components that come together to make a whole
lipid density
Phospholipid tail saturation can affect movement and the thickness of the membrane.
Saturated tails extend further down if same number of carbons as the 30okink has the “diagonal length” effect.
what properties are provided by lipid density?
Correlation between tail saturation and fluidity;
Dense packing = greater rigidity.
Greater packing means less opportunity for movement.
regarding one Labelled green and one red if we are to fuse the two cells?Quickly write a hypothesis about what you think will happen next, based on what we have done so far.
Integration of Phospholipid Bilayers
The phospholipid bilayers of the two cells will merge to form a single continuous bilayer. This happens because phospholipids are amphipathic molecules with hydrophilic heads that face outward toward the aqueous environment and hydrophobic tails that face inward, away from water.
The merging of the two bilayers results in a larger, unified bilayer that encompasses both sets of cellular components.
what is Lipid raft formation?
Cholesterol can concentrate into small aggregates called lipid rafts on an artificial membrane with only phospholipids and cholesterol (red)
Lipid Raft: Dynamic assemblies of proteins and lipids that float freely within bilayer of plasma membrane but can also cluster to form larger, ordered platforms.
Cholesterol gathers inside the lipid rafts to stabilise the structure in that location
Properties:
*Acts as an anchor for proteins
*Prevent movement of membrane components including proteins around the “fluid mosaic” membrane, therefore reducing movement and diffusion.
Used when:
➢Require location specific functions e.g. receptors need to be in the synapse
➢Often contain specific proteins subunits e.g. associate in the membrane to be activated…or inactivated
For Lipid rafts Proximity is everything!
Support structures are close enough to assist when required.Can also separate structures to ensure that signals are prevented.
what is Lipinski’s rule of 5?
Five key physiochemical parameters for being able to move passively through a lipid membrane:
1)Molecular weight (MW) is less than 500 Da.
2)The calculated log P value is above five.
3)There are no more than five hydrogen bond donors (e.g. –NH–, –OH)
4)No more than 10 hydrogen bond acceptors (e.g. –O–)
5)Low overall charge
Under what circumstances might something need to cross the cell membrane?
- Nutrient Uptake
Essential Nutrients: Cells need to take in nutrients (like glucose, amino acids, fatty acids, vitamins, and minerals) for energy, growth, and repair.
Transport Mechanisms: This can occur via simple diffusion, facilitated diffusion (through transport proteins), or active transport (requiring energy). - Waste Removal
Metabolic Byproducts: Cells generate waste products (like carbon dioxide, urea, and ammonia) that must be expelled to maintain cellular health and prevent toxicity.
Exocytosis: Waste can be eliminated via exocytosis, where vesicles fuse with the membrane to release substances outside the cell. - Signal Transduction
Hormones and Signaling Molecules: Extracellular signals (like hormones) need to bind to receptors on the cell membrane to initiate a response inside the cell.
Second Messengers: Some signaling pathways involve the production of second messengers that diffuse across the membrane to activate intracellular signaling cascades. - Cell Communication
Cell-Cell Interaction: Molecules involved in communication, such as neurotransmitters, cytokines, and cell adhesion molecules, must cross the membrane to transmit signals between cells.
Gap Junctions: In some cases, small molecules can pass directly between neighboring cells through gap junctions. - Immune Response
Antigen Presentation: Immune cells may need to present antigens on their surfaces, requiring the movement of protein complexes across the membrane.
Pathogen Invasion: Pathogens like viruses may enter cells by hijacking the cell’s machinery to cross the membrane. - Cell Growth and Division
Cell Cycle Regulation: During cell division, cellular components (like organelles and DNA) must be moved and divided, requiring the transport of various substances across the membrane.
Membrane Expansion: As a cell grows, it must also incorporate new lipids and proteins into the membrane, necessitating membrane transport. - Homeostasis Maintenance
Ion Transport: Cells regulate their internal environment by controlling the movement of ions (like sodium, potassium, calcium, and chloride) across the membrane to maintain membrane potential and overall cell function.
pH Regulation: Maintaining pH levels may require the transport of hydrogen ions and bicarbonate ions across the membrane. - Energy Production
ATP Production: In mitochondria, the transport of protons across the inner membrane is essential for ATP synthesis through oxidative phosphorylation.
Metabolite Exchange: The exchange of metabolites (like pyruvate and NADH) between the cytosol and mitochondria requires membrane crossing.
Explain what principles govern whether a molecule can traverse the membrane alone or whether it needs a channel or carrier protein to do so
- Molecule Size
Small Molecules: Small nonpolar molecules (like oxygen, carbon dioxide, and some hydrocarbons) can diffuse through the lipid bilayer freely due to their size and nonpolar nature.
Large Molecules: Larger molecules (such as glucose, amino acids, or proteins) cannot easily cross the membrane and typically require transport proteins. - Polarity and Charge
Nonpolar Molecules: Nonpolar (hydrophobic) molecules can easily pass through the lipid bilayer because they do not interact favorably with the polar heads of phospholipids.
Polar Molecules: Polar molecules (like water) can only cross the membrane at a very slow rate and may need specific channels (e.g., aquaporins for water) to facilitate their passage.
Ions: Charged particles (ions such as Na⁺, K⁺, Ca²⁺) cannot pass through the lipid bilayer directly due to their charge and typically require ion channels or pumps to move across the membrane. - Concentration Gradient
Diffusion: Molecules will move from an area of higher concentration to an area of lower concentration (down their concentration gradient) until equilibrium is reached. This process can occur without the need for transport proteins if the molecule is small and nonpolar.
Facilitated Diffusion: For polar molecules and ions, facilitated diffusion through channels or carriers allows for movement down the concentration gradient without energy expenditure. - Transport Mechanisms
Passive Transport: Molecules that can traverse the membrane by simple diffusion (like oxygen and carbon dioxide) do so passively, without the need for energy or transport proteins.
Facilitated Transport: Molecules that cannot cross the membrane directly rely on facilitated transport through specific channel or carrier proteins. This mechanism can be passive (no energy required) or active (energy required).
Active Transport: Some molecules need to be moved against their concentration gradient (from low to high concentration), which requires energy input (usually in the form of ATP) and transport proteins. - Specificity of Transport Proteins
Channel Proteins: These proteins create a hydrophilic pathway through the membrane that allows specific ions or small polar molecules to pass through. They can be gated (opening/closing in response to stimuli) or always open.
Carrier Proteins: Carrier proteins undergo a conformational change to transport larger molecules or specific substrates across the membrane. They typically bind to the molecule they transport, facilitating its movement. - Membrane Fluidity and Composition
Fluidity: The fluid nature of the lipid bilayer can influence how easily some molecules can cross. Higher fluidity can facilitate the movement of certain molecules, while lower fluidity (due to saturation of fatty acids or cholesterol content) can hinder it.
Lipid Composition: The specific types of phospholipids and their arrangements can affect the permeability of the membrane to different substances.
