Week 2 notes
Week 2 notes BSCI 105 - 5666
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This 5 page Class Notes was uploaded by Natania Lipp on Wednesday February 10, 2016. The Class Notes belongs to BSCI 105 - 5666 at University of Maryland - College Park taught by Dr. Michael Keller in Winter 2016. Since its upload, it has received 18 views. For similar materials see Principles of Biology I in Biology at University of Maryland - College Park.
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Date Created: 02/10/16
Notes Week 2: Lipids, Proteins; the Membrane and its functions. Random fact to know for test: You can look at the construct of a molecule and say whether it is hydrophobic or hydrophilic, but you can’t assume something is water soluble without testing it. - Fatty acids in the body need to attach to something that is water soluble. They are carried around by blood protein. For example, steroids mostly just ﬂoat around in the blood and don’t do much. There are 4 main types of biological macromolecules: 1. Carbohydrates 2. Lipids 3. Proteins 4. Nucleic acids • Lipids are different from the other three macromolecules. - They are hydrophobic - They are not as big as the other three because a ﬁnite size, while the other three can keep connecting polymers and growing. • Lipid = any hydrophobic biological molecules - All lipids are nonpolar - They can be anything that ﬂoats in water and does not combine • Heterogenous group includes: - Fats/oils - Phospholipids - Steroids • Glycerol + fatty acids —> Triglycerol (fat, triglyceride) • Tryglicerol can vary in properties. • Fats are saturated — they have single hydrogen bonds, the shape is straight, and they are solid at room temperature. • Oils are unsaturated — they have a double bond in a fatty acid chain that causes the backbone to bend. - They are liquid because the change in shape makes them less dense. • Phospholipids: lipids with a hydrophilic head and two hydrophobic tails. - The molecule is amphipathic — has two ends with different actions. - Amphipathic molecules form micelles in water and help solublize fats. - The tails attract each other, and the heads on the outside attract water, creating a shell of water around the molecules. - This construct is self-maintaining, controlled by hydrophobic/hydrophilic interactions. Once made, they are extremely stable. - These bilayers are how all membranes form. • Steroids - derive from cholesterol - Not all cholesterol is bad — humans and animals need cholesterol - They are in 4 ring structures. Protein = polymers of amino acid monomers • The “workhorse” of molecules of the cell • They break through the cell membrane • Functions include: - Enzymes - Structural elements - Storage - Transport - Signaling - Contractile and motor proteins - Defense and toxins • There are 20 naturally occurring amino acids - Amino acids all differ in R-group - They can be polar/non polar, neutral/charged, acidic/basic, or neutral. • Amino acids all have: - A central (alpha) carbon - An amino group - A carboxyl group - An R-group (side chain) • Polypeptides: peptic bonds formed by dehydration synthesis - They link carboxyl terminus to amino terminus • Proteins have 4 levels of structure: 1. Primary structure: a string of amino acids 2. Secondary structure: hydrogen bonds along the backbone of the same polypeptides - Includes the alpha helix and beta sheet 3. Tertiary structure: interactions between R-groups of the same polypeptide. 4. Quaternary structure: more than 1 peptide stuck to each other. - Can make big, complicated things. - Also includes interacting R-groups Membrane Structure and Function The plasma membrane has selective permeability: it allows some substances to cross it more easily than others. - The ability to be selective about what enters is crucial to life, and the membrane is responsible for this function. 7.1 Cellular membranes are ﬂuid mosaics of lipids and proteins. - The stable ingredients of membranes are lipids and proteins. - Phospholipids are the most important lipids. - Phospholipids are amphipathic: they have a hydrophilic region (attracts water) and hydrophobic region (repels water). - The amphipathic membrane is what creates the phospholipid bilayer: a two-layered membrane arranged with hydrophilic tails on the inside and hydrophobic heads on the outside. - Most membrane proteins are also amphipathic, and they can inhabit the phospholipid bilayer. - A ﬂuid mosaic model is a mosaic of protein molecules bobbing in a ﬂuid bilayer of phospholipids. - Membranes have ﬂuidity. - Lipids and some proteins can shift laterally, and can even ﬂop across the membrane. - Phospholipids are constantly switching positions. - Factors that affect ﬂuidity: - Temperature: the hotter the temperature, the faster molecules move so the more ﬂuid the membrane is. - Unsaturated carbon tails are more ﬂuid than saturated, because they cannot pack together as closely. - Steroid cholesterol: when the temperature is higher, the membrane is more ﬂuid. However, it also lowers the temperature and then makes it less ﬂuid. - Viscous: the opposite of ﬂuidity, when the lipid tails are packed tightly together. - The model is called a mosaic model because membranes within each cell have unique collections of proteins. - Phospholipids are the fabric of the membrane and the proteins determine most of the membrane’s functions. - There are 2 major populations of membrane proteins: - Integral proteins: penetrate the hydrophobic interior of the lipid bilayer. - Mostly made up of transmembrane proteins which span the entire membrane as opposed to only extending part way. - Peripheral proteins: not embedded in the lipid bilayer — loosely bound to the surface of the membrane - Some proteins are held in place by the cytoskeleton and some are attached to ﬁbers of the extracellular matrix - Protein functions: a) Transport b) Enzymatic activity c) Signal transduction d) Cell-cell recognition: a cell’s ability to distinguish one type of neighboring cell from another - Important for defense and sorting cells into tissues and organs - Most membrane carbohydrates are covalently bonded to proteins, making glycoprotiens - Diversity of location on the cell’s surface allow them to function as markers to distinguish one cell from another. - Glycolipids: the membrane carbs that are covalently bonded to lipids. e) Intercellular joining f) Attachment o the cytoskeleton and extracellular matrix (ECM) 7.2 Membrane structure results in selective permeability. - Nonpolar molecules are hydrophobic, so they can dissolve easily in the lipid bilayer and can pass through without the help of membrane proteins. - Polar molecules are harder to pass through the lipid bilayer and require proteins to help transport them. - Transport proteins help move some ions and polar molecules through the membrane. There are 2 main types: 1. Channel proteins are a kind of transport protein that have a hydrophilic channel through which certain molecules or atomic ions can use as a tunnel to get through the membrane. - Aquaporins: a kind of channel protein that allow 3 billion water molecules to pass into the cell every second. 2. Carrier proteins: hold onto their passengers and change shape in a way that shuttles them across the membrane. 7.3 Passive transport is diffusion of a substance across a membrane with no energy investment. - Diffusion: the movement of particles of any substance so that they spread out into the available space. - How it works: the membrane’s pores allow molecules to pass through them. Molecules will move randomly through pores, and (in the absence of other forces) they move from where it is more concentrated to where it is less concentrated in order to achieve equilibrium. - Concentration gradient: the region along which the density of a chemical substance increases or decreases. Every substance diffuses down its concentration gradient. - Passive transport: the diffusion of a substance across a biological membrane that requires no energy from the cell. - Osmosis: the diffusion of water molecules across a membrane through the pores of the membrane. - Tonicity: the ability of surrounding solution to cause a cell to gain or lose water. - Depends on the solute concentration and membrane permeability - If there is higher concentration water will leave the cell, if there is lower, water will enter the cell. - Isotonic: when there is no net movement of water in or out of the cell because water diffuses in and out of the membrane at the same rate - Hypertonic: a solution that does not maintain enough water — the water leaves the cell faster than it enters. - Could cause the cell to shrivel and die. - Hypotonic: solution that has too much water — the water enters the cell faster than it leaves - Could cause the cell to swell and maybe burst. - In both hypertonic and hypotonic situations, the cells lack rigid cell walls - These cells need other adaptation to achieve osmoregulation: control of solute concentrations and water balance. - Cell wall — helps regulate the cell’s osmosis to achieve osmoregulation. - When the cell swells, the cell wall pushes back pressure called turgor pressure and make the cell turgid (very ﬁrm), which is a healthy state. - When the cell is isotonic, the cell becomes ﬂaccid (limp). - When the cell is hypertonic the plant cell shrivels in and the cell undergoes plasmolysis which can be deathly. - Facilitated diffusion: when polar molecules and ions diffuse passively with the help of transport proteins on the membrane. - 2 types of transport proteins carry out facilitated diffusion: channel and carrier. - Ion channels: channel proteins that transport ions. - Many ion channels function as gated channels: open or close in response to stimulus (i.e. electrical stimulus). - Channel proteins slightly change their shape and move the solute across the membrane during their shape change. 7.4 Active transport uses energy to move solutes against their gradients. - Active transport: the pumping of a solute across a membrane that requires work. - Only carrier proteins do active transport. - Allows cells to maintain internal concentrations of small solutes that are different from the concentrations in the cell’s environment. - Examples: - Animal cells have higher concentrations of potassium ions and lower concentrations of sodium ions so the membrane helps by pumping sodium out of the cell and potassium into the cell. The sodium-potassium pump exchanges sodium for potassium across the membrane. - ATP supplies energy for active transport by transferring its terminal phosphate group to the transport protein which causes the protein to change shape and move the solution. - Voltage: electrical potential energy - All cells have voltage - Membrane potential: voltage across a membrane, which is negative on the cytoplasmic side and positive on the extracellular side because of an unequal distribution of anions and cations - Electrochemical gradient: the combination of two forces that drive the diffusion of ions across a membrane — a chemical force of the ions concentration gradient, and an electrical force, the effect of the membrane potential on the ion’s movement. - Electrogenic pump: the transport protein that generates voltage across a membrane. - Proton pump: the main electrogenic pump of plants, fungi, and bacteria - Helps store energy that can be tapped for cellular work. - Cotransport: when a transport protein couples the “downhill” diffusion of the solute to the “uphill” transport of a second substance against its own concentration gradient. - Helps drive the amino acids, sugars, and other nutrients into the cell.
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