B2.1: Membranes and Membrane Transport Flashcards
State that phospholipids naturally form continuous sheet-like bilayers in water.
A bilayer of phospholipids and other amphipathic molecules forms a continuous sheet that controls the passage of substances despite being 10nm or less across
What term refers to a property of fatty acid chains of a phospholipid
- Hydrophobic
List locations of lipid bilayers in cells.
- Nucleus
- Mitochondria
- Chloroplasts
State the primary function of the cell membrane and list 2 key qualtiies of the cell membrane that function to promote homeostatic regulation.
- The cell membrane acts as a barrier controlling the exchange of materials between the internal and external environment
- Composed of lipids, proteins and small amounts of carbs.
2 key qualities:
- Semi-permeability: Only certain materials are able to freely cross the cell membrane
- Selectivity: The cell can control the passage of any material that can’t freely cross the membrane
What could selective permeability change
Selective Permeability may involve a change in permeability
Outline the location of aqueous solutions in relation to the lipid bilayer.
- Hydrophilic phosphate heads of the inner layer of the phospholipid bilayer face the internal aqueous environment of the cell whereas heads of the outer layer face the external aqueous environment
- In an aqueous environment, phospholipid molecules spontaneously organise themselves in a way that their hydrophobic hydrocarbon tails are shielded from water.
Explain why the hydrophobic core of a lipid bilayer has low permeability to large molecules and hydrophobic particles.
- The 2 hydrocarbon “tails” of the phospholipid bilayer are hydrophobic and interact with each other to form the core of biological membranes hence, the biological membranes have a low permeability to all hydrophilic particles (including ions) and polar charged molecules like glucose which allows for charged particles to be kept in or out of a cell.
(The larger the molecule, the lower the permeability)
What proteins / transporters are involved in active transport
- Sodium Potassium pumps
- Sodium dependent glucose cotransporters
What proteins / transporters are involved in passive transport
- Channel proteins
- Pump proteins
Describe simple diffusion.
The movement of molecules of a subsatnce down a concentration gradient i.e. a region of where its concentration is higher to a region where its concentration is lower.
- It is a passive process meaning it doesn’t involve expenditure of energy by cells
What is simple diffusion controlled by
Controlled by the solutes involved and not the membrane
Outline the impact of concentration gradient, particle size and polarity or charge of molecules on the rate of diffusion across a lipid membrane.
- Concentration Gradient: A steeper concentration gradient results in a faster rate of diffusion. Higher concentration differences lead to more frequent collisions between molecules, increasing the rate of diffusion.
- Particle Size: Larger molecules (molecules with smaller SA:V ratio) experience greater resistance when moving through the lipid bilayer and have slower diffusion rate
- Polarity of molecules: Polar and charged molecules have a harder time diffusing across a lipid membrane compared to nonpolar molecules.
vice versa
Explain two examples of simple diffusion of molecules into and out of cells.
Into cells:
- O2 diffuses from O2-rich air into alveoli down the concentration gradient and to the blood in capillaries that surround the alveoli and this O2 is carried to tissues.
Out Cells:
- CO2 diffuses from the cells, where its concentration is higher, to blood, where its concentration is lower, where its then carried to Lungs. In Lungs, CO2 diffuses from blood to alveoli down the concentration gradient
Compare the location of integral and peripheral proteins in the membrane.
Integral Proteins:
- They are embedded in the lipid bilayer and are therefore difficult to isolate as extraction techniques disrupt the bilayer
- They are amphipathic molecules - the hydrophobic regions of integral proteins interact with the hydrophobic interior of the lipid bilayer hence, causing them to be embedded into the bilayer.
- Most integral proteins are transmembrane proteins (extend across the protein)
Peripheral Proteins:
- They are found on the surface of the membrane and interact only with the hydrophilic regions of the internal proteins hence, this means that these molecules are easier to remove from biological membranes
What is Glycocalyx
Eukaryotic cells have this
- This is protein and polysaccharide amtrix covering the surface of a cell
- Used for cell recognition and adhesion of cells together
e.g: Cells lining the intestine have a glycocalyx to protect them from physical stress of food flowing through them
List at least four functions (with example) of membrane bound proteins [JETRAT]
JETRAT
- Junctions - serve to connect and join 2 cells together
- Enzymes - fixing to membranes localises metabolic pathways
- Transport - responsible for facilitated diffusion and active transport
- Recognition - May function as markers for cellular identification
- Anchorage - Attachment points for cytoskeleton and extracellular matrix
- Transduction - Function as receptors for peptide hormones
Define osmosis.
Osmosis is the diffusion of water molecules across a partially permeable membrane from high water potential to low water potential
- Highest water potential is 0.
- Low water potentials are negative
Predict the direction of water movement based upon differences in solute concentration.
Water diffuses from an area of higher water potential to an area of lower water potential through the selectively permeable membrane.
This continues until the concentration becomes the same on both sides of the membrane. Once this happens, although the random movement of water molecules continues, there is no net movement of water.
Outline the structure and function of aquaporin proteins.
Structure:
- Aquaporins are integral proteins composed of 4 monomeric subunits (this protein is called a tetrameric protein) where each
subunit has a water channel hence, an aquaporin molecule has 4 identical water channels.
Function:
- They have water channels linked with specific hydrophilic chains (of which are amino acid residues) which allows the passage
of water molecules but not of ions.
- Aquaporins permit the rapid movement of water in and out of the cell by forming hydrophilic channels that span across
the membrane - The volume of water that needs to be transported across the cell membranes determines the # of aquaporins
Describe the structure and function of channel proteins.
Structure:
- Channel proteins are tubular shaped, acting as a passageway for the molecules to pass from one side of the membrane to the other. They are composed of multiple amino acid subunits that are identical to each other
Function:
- assemble to form channels for the passage of polar molecules
What are carrier proteins
carrier proteins are transmembrane transport proteins that play an important role in facilitated diffusion. They are highly specific meaning they can be involved in both facilitated diffusion or active transport
Most channels open or close in response to what specific stimuli?
- Changes in voltage across the membrane or voltage-gated channels
- Binding of small molecules to the channel proteins or ligand-gated channels
- Mechanical Forces like pressure
Define facilitated diffusion.
The movement of a molecule’s particles down its concentration gradient with the help of specialised transport proteins – carrier and channel proteins – across the cell membrane
What are channel proteins
Form pores or channels in the cell membrane, allowing specific molecules or ions to pass through - facilitated diffusion occurs
What are carrier proteins
Bind to large molecules and then change shape to move them across the membrane (they are also specific for a particular molecule)
Outline the specificity of channel proteins for ions.
Selectivity of the ion channels is due to:
- The binding sites of the hydrophilic amino acid side chains lining the channel being highly ion-specific
- The size of the pore acting as a size filter
List types of gates on channel proteins.
- Voltage-gated channels
- Ligand-gated channels
Describe one example of facilitated diffusion through a protein channel.
Ion channels, the tiny pores of which act as pathways for ions like Na+ and K+.
- When the gates open, the ions pass through the pore down the concentration gradient.
- When the gates close, the pore’s plugged which prevents the passage of the ion.
Define Active transport
- When molecules are transported against the concentration gradient i.e. from a region of their lower concentration to a region of their higher concentration
- Requires ATP of which is generated by mitochondria during cellular respiration
What is the main function of membrane pumps
To move substances against the concentration gradient
Describe the structure and function of pump proteins, including the role of specificity, conformational change and ATP.
- ATP: During the transport cycle, ATP is hydrolysed to provide the necessary energy for ion transport. The energy released from ATP is used to pump ions against their concentration gradients allowing it to actively transport ions or molecules
- Conformational Change: When specific ions or molecules bind to the pump’s active sites, it changes shape to allow the ions to move across the membrane.
- Specificity: Pump proteins are highly specific for the ions or molecules they transport
Where is the sodium potassium pump found
- In the neurons
Whats the role of sodium potassium pumps
They use indirect active transport to have a high concentration of Na+ outside the cell and high concentration of K+ inside the cell
How is the sodium potassium pump functioned
It will pump out 3 Na+ ions and pump into the cell 2 K+ ions hence, creating an electronegative gradient (outside is more positive than the inside although the inside still stays positive)
- 3 Na+ ions bind to intracellular sites on the sodium-potassium pump
- A phosphate group is transferred to the pump via the hydrolysis of ATP
- The pump undergoes a conformational change, translocating sodium across the membrane
- The conformational change exposes 2 K+ binding sites on the extracellular surface of the pump
- The phosphate group is released which causes the pump to return to its original conformation
- This translocates the K+ ions across the membrane, completing the ion exchange
Compare active transport using a pump protein to facilitate diffusion using a channel protein.
In diffusion a substance moves down its concentration gradient - from an area of high concentration to a low concentration (no energy required)
In active transport, a substance moves against its concentration gradient - from an area of low concentration to a high concentration (energy required)
Explain one example of active transport of molecules into and out of cells through a protein pump.
- Active transport functions to maintain the right concentrations of ions in the cells; active transport helping RBC maintain their internal sodium levels
Define selective permeability.
The property of a biological membrane that allows certain substances to pass through while restricting the passage of others, based on factors such as particle size, polarity, and chemical properties.
Outline how channels and pumps in the membrane allow for selective permeability.
- Gated ion channels like Cl- channels only bind to Cl- ions
- Carrier proteins are highly specific. For example, GLUT (glucose transporter) recognises and binds only to glucose and a few other monosaccharide molecules.
How is simple diffusion not selective?
Simple diffusion depends only on the concentration gradient, particle size and polarity molecules.
Outline the structure of glycoproteins and glycolipids.
Glycoproteins:
- Covalent bonding of oligosaccharides (short carb chains) to the protein molecules reuslts in glycoproteins
- Carb groups often stick out into the extracellular environment
Glycolipids:
- Covalent bonding of carbs to lipids results in glycolipids
- Amphipathic molecules; Carb groups are polar whereas non-polar lipid component lies embedded in the bilayer
- contribute to stability of membrane as they form H-bonds with water molecules surrounding the cell
Describe the role of glycoproteins and glycolipids in cell adhesion and cell recognition.
Cell adhesion:
- They both help cells attach and bind to other cells to form tissues.
Cell recognition:
- They both act as markers on the cell surface and help cells of the body recognise one another. Also help cells of the immune system to recognise foreign cells
“fluid” meaning in Fluid-Mosaic model
- The phospholipid bilayer is viscous and individual phospholipids can move position
“mosaic” meaning in Fluid-Mosaic model
- The phospholipid bilayer is embedded with proteins, resulting in a mosaic of components
Draw and label a two-dimensional representation of the fluid mosaic model of membrane structure. Include:
* Phospholipid bilayer with hydrophobic tails facing inward and hydrophilic heads facing outwards
* Integral proteins shown embedded the membrane
* Peripheral proteins on membrane surface or anchored to an integral protein
* A transmembrane protein channel with a pore
* Glycoproteins with a carbohydrate side chain facing the outside of the cell
* Glycolipid with a carbohydrate side chain facing the outside of the cell
* Cholesterol embedded between phospholipids in the hydrophobic region
* An indication of thickness (8-10nm)
pk
Compare and contrast the structure, melting point and relative fluidity of saturated and unsaturated fatty acids in lipid bilayers.
Saturated fatty acids:
- Structure: Fatty acid chain lies parallel to each other
- Melting point: Higher due to no double bonds disrupting the layers
- Relative fluidity: Provide stability to membrane
Unsaturated fatty acids:
- Fatty acid chain is kinked hence preventing membrane lipids with unsaturated fatty acids from packing together closely therefore maintaining fluidity.
- Structure: - Melting point: Lower due to double bonds causing kinks
- Relative fluidity: Ensure fluidity of the membrane
What is the connection between saturated / unsaturated acids and melting points
Unsaturated fatty acids have lower melting points
