Biological Molecules and Cellular Components

Question	Answer
Which sterol modulates membrane fluidity and how does it do this?	Cholesterol provides a short rigid structure to fill the gaps produced by kinks of unsaturated hydrocarbons of membrane phospholipids. This stiffens the bilayer and makes it more rigid and less permeable.
What is the function of a flippase?	Flippases are enzymes which transfer phospholipids and embedded membrane proteins from the cytosolic bilayer leaflet to the outer leaflet after their insertion into the cytosolic leaflet in the endoplasmic reticulum. This maintains the heterogeneous concentrations of phospholipids in the halves of the bilayer.
Which structures do simple amphipathic sodium or potassium salts of the long chain fatty acids, and amphipathic lipids form in solution?	The amphipathic fatty acid salts form micelles, which are present in detergents to burrow hydrophobic tails into great and pull it apart with the hydrophilic head's interaction with water, and amphipathic lipids form bilayers which are stable in solution.
What is a wax?	An ester of a long-chain alcohol (may be saturated or unsaturated) with a long-chain fatty acid (usually saturated). Waxes tend to be insoluble as they have no polar regions.
Where and how are glycolipids inserted into the outer leaflet of the plasma membrane?	The sugar molecules of glycolipids are added to the lipids in the Golgi apparatus after their formation in the endoplasmic reticulum. The glycolipids are only present on the exterior of the cell and so this section of membrane forms a vesicle with the plasma membrane on the inside so that when the vesicle fuses with the lipid bilayer of the cell the orientation of the bilayer will be correct.
What is a terpene?	An unsaturated hydrocarbon formed by two or more molecules of isoprene, which are usually important in olfactory reactions.
What is a steroid?	A terpene-based lipid with a basic structural unit of four rings - cholesterol is the most common steroid and the precursor for all other animal steroids.
How are fats and oils distinguished?	Fats are solid at 25 degrees celsius, while oils are liquid.
What are three major types of membrane proteins?	Transporters, anchors, receptors and enzymes.
How do transmembrane proteins associate with the lipid bilayer?	They extend through the bilayer and have both hydrophobic and hydrophilic regions.
How do monolayer-associated proteins associate with the lipid bilayer?	They associate with only one leaflet of the bilayer and often an amphipathic alpha-helix anchors the cytosolic proteins to the bilayer.
How are lipid-linked proteins associated with the bilayer?	They lie entirely outside of the bilayer (on either side) and are attached to it by a single covalently bonded lipid.
What happens to a phopholipid bilayer if torn?	It is not energetically favourable for 'free edges' with non-polar regions to be exposed to polar environments so to restore the energetic balance the bilayer will tend to fold in on itself into a spherical shape.
How are protein-attached proteins associated with the phospholipid bilayer?	They are bound indirectly to the membrane via a different transmembrane protein. Those on the outer leaflet are also called peripheral membrane proteins, and can be released relatively easily.
The higher the proportion of unsaturated hydrocarbon tails...	...the more fluid the phospholipid membrane.
How and why do alpha-helices and pores of transmembrane proteins form?	Alpha-helices form because, although transmembrane proteins have hydrophobic regions which can interact with the lipid bilayer, they still have a polar backbone and so this limits the interaction of the hydrophilic backbone and the membrane. The pores form similarly but are composed of multiple alpha-helices, and tend to be for the transport of water-soluble molecules across the membrane rather than receptors which just tend to have one alpha-helix.
Describe the fluidity of the phospholipid bilayer.	Biological members have a more liquid consistency than gel-like because of the temperature maintained in higher organisms, and the composition of phospholipids with unsaturated hydrocarbon tails. It is energetically unfavourable for the phospholipids to flip from one plane to another, but there is continuous swapping places of molecules within a monolayer - as a result of thermal motions.
Name a type of membrane pore, other than an alpha-helix pore.	A beta-sheet barrel, a water pore which is quite wide because there is a limit to how tightly the B-sheet can be curved to form the barrel.
What dictates which direction a protein will face in a membrane?	A protein will always have its amino terminus and its carboxyl terminus on a specific side of the membrane and it is energetically unfavourable for a molecule of its size with its hydrophobic region to switch directions - this dictates how the protein will be produced.
What are fatty acids?	Carboxylic acids with a long hydrophobic hydrocarbon tail (R-COOH) which compose nearly all lipid structures. The hydrocarbon tail may be saturated or monounsaturated/polyunsaturated.
Cholesterol has a relatively high insulating capacity. In what cells is it found in high proportions?	In the myelin sheath of Schwann cells that line myelinated axons.
Give an example of an anchored protein function.	Supporting antigens on the plasma membrane of erythrocytes.
Why is the most prevalent isomer of unsaturated fatty acids the cis isomer?	Because cis isomers have a bend/kink in the hydrocarbon tail which prevents close packing and allows for some fluidity in biological membranes, which is important for cofactors, protein signalling systems, and endocytosis and exocytosis.
What is a lipid raft?	An area of less fluidity than the rest of the lipid bilayer, as a result of a high cholesterol concentration and some specific proteins, which adds to the structural support of the membrane.
What is the lipid bilayer permeable and impermeable to?	Permeable to: gases (e.g. O2, CO2), and small uncharged polar molecules (e.g. ethanol, H2O). Impermeable to: large uncharged polar molecules (e.g. glucose), ions (e.g. K+, Mg+) and charged polar molecules (e.g. amino acids, ATP).
What is the significance of the distribution of positive and negative ions being maintained as equal within the cell and the surrounding fluid?	To prevent the cell from being torn apart by electrical forces, and also to prevent osmotic effects.
Selectively permeable membrane transport proteins traverse biological membranes. What two categories can these be divided into?	Channels, which select solutes based on size and/or charge, and transporters, which discriminate based on whether a solute fits its binding site and causes its own conformation to permit transport.
What are triacylglycerols?	TAGs are structures composed of glycerol in which each -OH group has undergone a condensation reaction to form an ester link with a fatty acid. The three fatty acids may be different or identical. TAGs are the major lipid energy reserve found in adipose tissue which is also useful for insulation.
Cells or organelles have varying proportions of different transporters. Give an example.	Mitochondria have a large proportion of ATP transporters because ATP is generated by the electron transport chain which occurs in the mitochrondrial inner membrane.
What is facilitated diffusion?	Facilitated diffusion is passive transport of a solute based on its relative concentrations. Membrane transport proteins are involved for the solute's transport across, but the transport protein does not expend any energy in its movement.
What is active transport?	Active transport is the movement of a solute from one compartment to another, usually against its concentration gradient, involving the expenditure of energy in the form of ATP (the transport is coupled with this process, or another).
What is the difference between simple diffusion and facilitated diffusion?	1) Proteins are involved in the latter. 2) Simple diffusion is only limited by the concentration of solute, whereas facilitated diffusion is limited by the number of proteins - these are saturable.
The rate of transport is dependent on which gradients?	The concentration and electrochemical gradients. Transport is fast if these operate in the same direction but slow if they are operating in opposing directions.
What charge does the cytoplasmic side of the plasma membrane have and what effect does this have?	It is generally negatively charged which draws positively charged ions in along the electrical gradient and repels negatively charged ions out of the cell.
List the 3 main ways of cells carrying out active transport.	Coupled transport, ATP-driven pumps, and light-driven pumps.
What is a glycerophospholipid? Give an example of one.	A glycerophospholipid consists of a molecule of glycerol with 2 of the -OH groups esterified with fatty acids and the third carbon attached to a phosphate group. A variety of polar structures are usually attached to this phosphate group, such as choline or ethanol amine. 1-stearoyl-2-oleoyl-phosphatidyl choline is a common constituent in natural membranes.
The ATP-driven Na+ pump has a central role in membrane transport in animals. How does it work?	The ATP-driven NA+ pump hydrolyses ATP to ADP to provide the energy to transport Na+ out of the cell against its electrochemical gradient (which makes it an enzyme as well as a transporter). When Na+ re-enters the cell, it does so through a coupled transporter and thus provides an energy source for the active movement of many other substances against their concentration gradients, including K+ ions. This works by NA+ and the other solute changing the conformation of the transporter by binding.
What is osmosis?	The passive movement (diffusion) if water across a selectively permeable membrane down a concentration gradient.
Which plasma membrane channels facilitate osmosis?	Aquaporins.
Which membrane protein predominantly works to maintain the osmotic balance/osmotic pressure across a membrane?	The Na+-K+ ATPase pump which controls osmotic effects of water to prevent influx and swelling or efflux and structural damage.
Why can plants tolerate a large osmotic difference across their plasma membranes that animals can't?	Plant cells have tough cell walls which exert a countering pressure on the water inside it, resulting in turgor pressure which is actually beneficial for the cell as it keeps it distended with water and their cell walls tense, allowing maximum structure and surface area for light to hit for photosynthesis.
What is an ether glycerophospholipid?	A structure consisting of a glycerol molecule with a fatty acid esterified to the central carbon, an unsaturated alkyl group esterified to C1 and a phosphate group attached to C3.
What are is primary and secondary active transport?	Primary active transport involves using ATP or light energy to transport a solute against its electrochemical gradient - this is active and saturable. Secondary active transport involves using a co-transport exchange system in which one ion moving down its electrochemical gradient is coupled with another solute moving against its own - this is also active and saturable.
Membranes can be involved directly in transport between organelles or cells by breaking off around a substrate. List some situations in which this happens.	The budding of vesicles from the Golgi complex, exocytosis and endocytosis, the fusion of an endosome and a lysosome, viral entry to a cell in infection, fusion of sperm and ova, the fusion of small vacuoles in plants, and the separation of two plasma membranes during cytokinesis.
What are sphingolipids?	Instead of glycerol, sphingosine is the backbone to the structure. Typically, a fatty acid links to its amino group via an amide linkage, and, in sphingomyelins, another group such as phosphorylcholine attaches to C1. Sphingomyelins are important in the nervous tissue of higher animals.
What are glycosphingolipids?	A structure consisting of a ceramide (sphingosine + fatty acid) with one or more sugar residues linked via a beta-glycosidic linkage to the C1 -OH of sphingosine. These sugars may be charged or uncharged.
Like amino acids, monosaccharides exist as L-isomers and D-isomers, which are present in sugars?	D-isomers.
Name some of the 16 optical isomers of C6H12O6.	Glucose, galactose, mannose.
What are 5-membered rings and 6-membered rings called?	5-membered rings are called furanoses and 6-membered rings are called pyranoses.
Which bonds form between monosaccharides to produce polysaccharides, and which reactions produce these bonds?	1,4-glycosidic bonds (straight-chains) and 1,6-glycosidic bonds (branches).
What are some functions of sugars?	The production and storage of energy, in mechanical supports such as cellulose in plants cells walls or chitin of insect exoskeletons, and small oligosaccharides can be covalently linked to proteins to form glycoproteins or lipids to form glycolipids, both of which are cell membrane constituents and often antigens.
Lipids are characterised by..	...low solubility in water and high solubility in non-polar solvents, such as benzene.
Define amphipathic.	Possessing both polar and non-polar groups.
How are amphipathic properties balanced in pharmacology?	Pharmacological lipids must have hydrophilic properties in order to be dissolved in the gut or in saline for delivery to tissues, but it is equally important for them to have hydrophobic properties in order to cross hydrophobic membranes, such as the lipid bilayer.
What are the four major groups of small organic biological molecules in cells and what are they the components of?	Amino acids - proteins, oligosaccharides - polysaccharides, nucleotides - nucleic acids, fatty acids - lipids.
What is the basic structure of an amino acid?	An alpha carbon atom with an amino (NH2) group, a carboxyl (COOH) group and a hydrogen group, along with a highly variable R group which defines the amino acid's properties and makes the molecule chiral (except glycine, in which the R group is H).
What are the different types of amino acid side chains?	Neutral alkyl side chains - non-polar, hydrophobic on increased size; hydroxyl containing chains - small and polar; acidic and amide side chains - polar; basic side chains - large and polar; sulphur-containing side chains - not particularly polar; aromatic and heterocyclic side chains - varying polarity, more than one element makes up the ring.
Proline is an...	...imino acid, which means its side chain is non-polar (its amino group is incorporated into a ring, the rest of which is its R group) and it cannot rotate around itself like other amino acids.
Polypeptides are formed by ? bonds between amino acids, by ? reactions.	Peptide/amide bonds formed by condensation reactions.
Amino acids exist as optical isomers in D (deutero) and L (livo) forms. Which is found in proteins and where is the other isomer found?	L-isomers are found in proteins while D-isomers are found in bacterial cell walls and some antibiotics.
What happens when the functional groups of amino acids are ionised?	The charges can affect the shape and properties of a particular protein.
In its zwitterion form, what charged groups are present on an amino acid (~pH7)?	NH3+ and COO- groups.
What is the general formula of a sugar?	(CH2O)n

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