Biology 5A Midterm 1
Biology 5A Midterm 1 Biol 5A
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This 16 page Study Guide was uploaded by Akash Patel on Wednesday April 13, 2016. The Study Guide belongs to Biol 5A at University of California Riverside taught by Sean Cutler in Winter 2016. Since its upload, it has received 78 views. For similar materials see Intro: Cell and Molecular Biology in Biology at University of California Riverside.
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Date Created: 04/13/16
TOPIC ONE: CHEMICAL CONTEXT OF LIFE ● All matter consists of elements that can be broken down by chemical reactions ○ Elements combine together to form compounds that have different characteristics than their elements ○ 92 elements found in nature, 25 are essential for human life ■ Carbon, hydrogen, oxygen, and nitrogen make up 96% of the mass of the human body ● Atoms are composed of neutrons, protons, and electrons ○ Nucleus consists of neutrons and protons (similar in mass of 1.7x10^24g=1 Da) ○ Electrons have a mass of 9.1x10^28g (1/2000 that of neutrons and protons) found around the nucleus ○ the identity of a particular element is dictated by the number of protons (atomic number) ○ mass number specifies the total number of neutrons and protons in an atom and approximates the atomic mass ● Atoms of identical atomic numbers but different numbers of neutrons are isotopes ○ Radioactive decay: one of the neutrons decays into a proton and electron (+ antineutrino) ○ Stability of an isotope is given by its halflife ■ N(t)=No(½)^1/t(½) ■ N(t):amount remaining, No=initial amount, t:time elapsed, and t(½):halflife ○ Isotopes can be substituted for a particular atom without affecting that atom’s chemical properties. ○ Radioactive atoms can be used to follow biochemical reactions or to detect biomolecules because the energy and particles produced by radioactive decay can be detected and measured ○ Radioactively labeled glucose detects cancer cells ○ Highenergy radioactivity can penetrate cells: can produce Xray images, sterilize food, treat cancer (damages DNA molecules ● The number of electrons in an atom determines its chemical reactivity ○ Electrons in outer shells have a higher potential energy ○ Electrons can absorb energy and occupy a higher energy level, or release energy and move to a lower energy level ○ If valence shell is complete, the atom is unreactive ○ Covalent bond: 2 atoms share a pair of valence electrons ○ The number of bonds that an atom can form is that atom’s valence ○ Different atoms can attract electrons with different strengths, defining their electronegativity ■ 2 atoms with similar electronegativity that form a covalent bond are nonpolar covalent bonds ● Always true for bonds between similar atoms ■ 2 atoms with different electronegativity that form a covalent bond are polar covalent bonds ○ When an electron is transferred to another atom, an ionic bond is formed ○ Covalent bonds are the strongest bonds in nature but weaker bonds (especially when their effects are added together) occur between many biomolecules ○ A hydrogen bond is a weak, noncovalent interaction that occurs between a hydrogen that is covalently bonded to an electronegative atom (N,O, F) and another electronegative atom (N or O) ■ form between water molecules ■ hold together the base pairs of doublestranded nucleic acids ■ contribute to stabilizing interactions between proteins or between proteins and DNA ○ Van der Waal interactions are shortlived regions of partial charge ■ stabilize interactions that occur among large macromolecules, including DNA and proteins TOPIC TWO: WATER ● Water forms a network of hydrogen bonds ○ H and O in water are joined by a polar covalent bond: the electrons in a water molecule spend more time near the oxygen ○ Water is a polar molecule with a partial negative charge near the oxygen and partial positive charge near the hydrogens ○ In its liquid state, water molecules can form four hydrogen bonds with other water molecules ○ The polar nature of water gives it several important properties ● Cohesion: when hydrogen bonds hold water molecules together ○ cohesion allows water to be transported in plants against the force of gravity ○ Water evaporates out of leaves and cohesion helps pull water up from the roots ○ contributes to surface tension ● Adhesion: when water interacts with other molecules ○ In a tree, it will form hydrogen bonds with the cell walls, helps it travel against the force of gravity ● Water is a polar solvent and will react with other polar molecules, called hydrophilic ● Hydrocarbon chains (regions involving C and H) are highly nonpolar and hydrophobic ● Water has a high specific heat capacity ○ Because of the many hydrogen bonds water molecules can form with each other in liquid, water requires a large amount of energy to change its temperature by even small amounts. ○ The specific heat of a substance is the amount of heat that must be added to increase 1g of the substance by 1 degree C ○ Specific heat of water: 4.18 J/g(degree C) ○ High specific heat allows it to absorb a large amount of energy without changing temperature much ● Water has a high heat of vaporization ○ hard to moderate the climate on earth ○ cooling effect of evaporating water keeps plant and animal life cool at higher temperatures ● Water is less dense when it is frozen ○ in liquid water, molecules are closer hydrogen bonds are breaking and reforming. ○ in ice, a regular array of water molecules forms, with stable hydrogen bonds ○ This property is important for life in earth’s bodies of water because if ice was denser than water, lakes and ponds would freeze from the bottom ● In liquid, sometimes a hydrogen nucleus transfers to another water molecule to produce a hydronium ion (H3O+) and a hydroxide ion (OH) ● Hydrogen has no neutron in its atomic nucleus ○ a hydrogen ion (H+) is the same thing as a proton ● The concentrations of H+ and OH are both 10^7 moles per L ○ When acids or bases are added, the relative concentration of the ions can be changed ○ Acids increase the concentration of H+ ions ○ Bases decrease the amount of H+ ions by accepting a proton or by producing OH ions ○ pH = log10[H+] ○ [H+][OH] = 10^14 ○ pH + pOH = 14 ○ in neutral water [H+]=10^7 so pH= log(10^7)= (7) = 7 ○ Acids have a pH < 7 ○ Bases have a pH > 7 ● A buffer is a substance that can minimize changes in H+ and OH in a solution by combining with excess H+ ions or donating them ○ As the pH rises, the equilibrium shifts to the right ○ As the pH falls, it shifts to the left ○ buffers allow biological systems to resist changes in pH so that biomolecules can stay in their optimum pH range TOPIC 3: CARBON ● Life on earth is carbonbased because of the properties of the carbon atom ○ Carbon has 4 valence electrons, allowing it to form covalent bonds with up to four different atoms ○ able to give rise to many complex compounds and polymers ○ Carbon Skeleton: interconnected carbons where various functional groups are attached ■ can be linear, branched, or in rings ■ usually involving single or double bonds ● The simplest carbon skeletons are hydrocarbons ○ molecules consisting only of carbon and hydrogen ○ Simple hydrocarbons containing no rings and only single bonds are alkanes ■ only have CH and CC bonds ○ named for the number of carbons in the longest chain ○ When carbon in a molecule has only single bonds, and all are available bonds are with hydrogen, it is saturated ○ Hydrocarbons with at least one double bond with 2 carbons, are called alkenes ○ Double bonds result in unsaturated molecules ■ unsaturated molecules are less flexible ○ Hydrocarbons involve only nonpolar bonds, hydrocarbons are hydrophobic ○ store a large amount of chemical energy ○ when completely oxidized, they generate CO2 and H2O and energy ○ Pure hydrocarbons are not generally found in cells but molecules with hydrocarbon portions ● Organic chemists draw carbon skeletons using a simplified notation ○ straight line is a single bond ○ double line is a double bond ○ end of a line or the place where two lines meet, is a carbon atom ○ atoms other than carbon are indicated by their chemical symbol ○ implied that all remaining bonds possible with a carbon are with hydrogens ● Two compounds are isomers if they share the same chemical formula but have different structures ○ creates different chemical properties ○ Structural isomers: differ in the arrangement of bonds ○ cistrans isomers: isomers that differ in arrangement of atoms around a C=C double bond ■ important in the properties of dietary fats and fatty acids in membranes ○ Enantiomers: isomers involving at least one asymmetric carbon ■ asymmetric carbon: one carbon joined to four different atoms or groups of atoms ■ 2 enantiomers are mirror images of each other and are named either L or D ■ In living systems, the L enantiomer is the most common form of amino acids ● There are 7 major functional groups ○ ○ Hydroxl, carbonyl, carboxyl, and amino contain oxygen or nitrogen and can participate in hydrogen bonding ● Hydroxl group ○ Turns hydrocarbon into an alcohol ○ found in carbohydrates ○ can interact with water ● Carbonyl group: ○ double bonded oxygen ○ if it is present in the middle of a carbon skeleton, the compound is called a ketone ○ if it is at the end, it is called an aldehyde ● Carboxyl group: ○ carbonyl group attached to a carbon that is also attached to a hydroxyl ○ highly polar, in solution ionizes to release a proton ○ also called carboxylic acid (COO) ● Amino group: ○ forms amine compounds ○ in solution, the lone pair of electrons on the nitrogen can interact with a proton and will function as a base ■ NH3+ ● Sulfhydryl group: ○ polar ○ consists of sulfur and hydrogen ○ amino acid cysteine contains this ● Phosphate group: ○ phosphate atom bonded to four oxygen atoms ○ if at the end of the molecule, it has a net charge of 2 ○ if in the middle of the molecule, it has a net charge of 1 ○ important in the structure of nucleotides ○ addition or removal of phosphates is also important for regulating the functions of proteins in cells ● Methyl group: ○ small hydrocarbon group ○ hydrophobic, unlike the other groups ○ important in DNA structure and gene regulation ○ in thymine but not uracil ● The major molecules in life systems are carbohydrates, lipids, proteins, and nucleic acids ○ Carbohydrates, lipids, and proteins can become large because their monomers form long polymers ● Long polymers are built up by adding monomers together ○ the OH of one monomer joins with the H of another monomer to form a net covalent bond and releases water ○ dehydration: a water molecule is removed from the two monomers, joining the monomers together ○ Nucleic acid polymers are used to store or convey genetic information ○ Proteins are products of genes ○ For lipids and carbohydrates, building of long polymers is a way to store energy or build cell structures ● To break down polymers, the covalent bond holding two monomers together is broken ○ To each part, a H or OH is added in a reversal of dehydration ○ Hydrolysis: breakdown of polymers into smaller pieces using water ○ For proteins and nucleic acids, hydrolysis is used primarily to recycle monomers ○ For lipids and carbohydrates, hydrolysis will start the process of collecting stored energy TOPIC 4: MACROMOLECULES ● Carbohydrates: ○ used to make structures in cells (cellulose), found in plants, chitin (found in insects) ○ Known as sugars or saccharides ■ compounds consisting of carbon, oxygen, and hydrogen ■ Chemical formula of Cn(H2O)n; n is 3 or more ■ sugars contain a carbon skeleton with hydroxyl and carbonyl functional groups ■ polysaccharides are polymers of sugars joined by covalent bonds ○ Monosaccharides, the building blocks of larger carbohydrates, are classified according to the number of carbons, the location of the carbonyl group, and the handedness of asymmetric carbons ○ Sugars can be trioses, tetroses, pentoses, hexoses, and heptoses (37 carbons) ■ if carbonyl is at the end of the molecule, the sugar is call an aldose. ■ if it is in the middle, it is called a ketose. ■ Hexoses, glucose, and galactose are enantiomers ○ Trioses form during glucose breakdown or synthesis ■ Ribose is a sugar that is in RNA ■ Ribulose is an intermediate during photosynthesis ■ Glucose, galactose, and fructose are energy sources ○ Linear pentoses and hexoses exist in equilibrium with ring forms ■ requires forming covalent bond between the carbonyl carbon and oxygen in a terminal hydroxyl ■ carbonyl becomes a hydroxyl, carbon becomes an asymmetric carbon ■ position of the new hydroxyl can either be below the plane of the ring opposite the CH2OH2, making aglucose or above the ring making Bglucose ○ Two monosaccharides can be covalently joined together to generate a disaccharide ■ produced by a dehydration reaction to create a glycosidic bond ● in dehydration, 2 hydroxyls are involved ● only one of the two oxygens is used to make H2O, leaving an oxygen between the two sugar rings ○ Polysaccharides are very large sugar polymers produced by multiple dehydration reactions to produce glycosidic bonds ■ formed by specific enzymes ■ broken down by specific enzymes ■ most are polymers of aglucose or bglucose ○ Storage polysaccharides are polymers of aglucose that are used to store energy ■ plants store energy in starch inside cell organelles ■ animals use glycogen stored in the liver and muscles ● glycogen is more highly branched and compact than starch which is unbranched or has little branches ● glycosidic bonds are between carbons 1 and 4 but branches connect carbons 1 and 6 ○ Structural polysaccharides are used to build structures found in cells or tissues ■ cellulose is formed from many polymers of bglucose ● because of the alternating nature of the glucose monomers in cellulose, parallel cellulose molecules can form multiple hydrogen bonds between them ● the multiple hydrogen bonds make cellulose extremely strong and rigid ● bglucose polymers in cellulose are hard to digest unlike aglucose ■ Chitin is a polymer of a modified glucose monomer called Nacetylglucosamine ● component of the exoskeleton of insects ● used in medical applications as surgical thread ● derives its strength from extensive hydrogen bonding between parallel molecules ● Lipids ○ Large group of molecules that are mostly hydrophobic ○ consists of primarily CH and CC bonds, which are strong nonpolar covalent bonds ○ not true polymers because they can’t be made randomly large like carbohydrates, proteins, and nucleic acids can ○ Fats consist of fatty acids joined to glycerol ■ fatty acids are long hydrocarbon chains (1618 carbons long) and have a carboxylic acid at one end ■ fat is saturated when there are no double bonds between any carbons ● saturated fat is solid at room temperature ■ fat is unsaturated when it contains some double bonds ● when there are double bonds, they are most likely cis double bonds so the fat molecules have a bend or kink ● this causes them to pack together less tightly ● unsaturated fat forms liquids at room temperature called oils ■ hydrogenation can make plant oils solid ● In hydrogenation, some fatty acids with trans double bonds are made (trans fats) ○ Phospholipids make up cell membranes ■ similar to a glycerol with only 2 hydrophobic fatty acids attached but with additional functional groups that make a hydrophilic region ■ selfassemble in a water environment into lipid bilayers (basis for cell membranes) ● outer part of the lipid bilayer interacts with water but the inside is extremely hydrophobic ○ Steroids are structurally very different from fats ■ similar to cholesterol which has a flat planar structure that is almost all C or H ● Proteins ○ Proteins are the most structurally diverse of all the macromolecules. ○ Different proteins contribute to all the processes in the cell ○ Some proteins function as enzymes (catalysts for the biochemical reactions. Others provide structure to cells ○ Polypeptide: polymer of amino acids made during the process of translation ■ every polypeptide in a cell corresponds to a gene ○ represents one or more polypeptides that carry out a specific function that is controlled by their structure and shape ○ Amino acids have an amino group on one side and a carboxyl on the other ■ carbon in the middle called acarbon is attached to the 2 groups plus a hydrogen and a side chain (R group) ■ R group gives each amino acid its name ■ In water, amino group attracts a proton so its positively charged ■ Carboxyl group gives up a proton so its negatively charged ■ 20 amino acids found in proteins in nature ■ Side chains are carbon skeletons with various functional groups, so the acarbon is asymmetric ● grouped as to whether or not they interact with water ● nonpolar side chains are hydrophobic ● polar and electrically charged side chains are hydrophilic ■ Most amino acids are the L isomer ■ the sequence of various amino acids in a polypeptide gives a protein its characteristics ○ proteins are the most varied and diverse of the macromolecules in living systems because amino acids can combine multiple ways ○ Amino acids are joined together by peptide bonds in a dehydration reaction to form a polypeptide ■ the OH of a carboxyl group will bond with the H of an amino group to form a water molecule (H2O) and create the peptide bond ■ the carboxylic acid group becomes a carbonyl group ○ a dipeptide with two amino acids is created ■ one end of a polypeptide has an amino group and the other has a carboxyl group ● called the Nterminus and Cterminus ○ polypeptide backbone: the long chain of NCCNCC ○ Polypeptides don’t remain linear ■ fold into a particular shape because of the interactions between functional groups in the side chains and polypeptide backbone ○ Lysozyme: enzyme that digest peptidoglycans found in bacterial cell walls ■ found in human tears and saliva ○ There are four levels of protein structures. ■ first 3 levels are found in all proteins, the fourth involves interactions between multiple polypeptides together ■ Primary (1*) structure: ● the sequence of amino acids ● dictates the size of the protein and the identity of the side chains along its length ● controlled by the sequences of bases in a messenger RNA (mRNA) copied from a gene (DNA) ■ Secondary (2*) structure: ● describes hydrogen bonds that occur between the oxygen in C=O and hydrogen in HN between different regions of the polypeptide backbone ● generate 2 types of secondary structure ahelix and bpleated sheet ○ both stabilize a larger type of shape that is part of the folding of a protein ○ do not involve direct interactions of the amino acid side chains ○ the identity of the side chains allow formation of an ahelix or a bpleated sheet ■ Tertiary (3*) structure: ● describes interactions that occur between amino acid side chains ● these interactions in an aqueous environment ● four types of side chain interactions ○ hydrogen bonds: occur between functional groups of particular side chains ○ Regions containing hydrophobic side chains tend to exclude water so they interact with themselves (hence “hydrophobic interactions” ○ van der Waals interactions also stabilize the side chains ○ disulfide bridge: covalent bond that forms between 2 sulfhydryl functional groups on different cysteine amino acids ○ interactions can occur between electrically charged regions of opposite polarity (called electrostatic interaction but usually called an ionic bond) ■ Quaternary (4*) structure: ● when multiple polypeptides interact to form a protein ● same interactions that are in tertiary structures also stabilize quaternary structures. ● Nucleic Acids: ○ 2 types of nucleic acids: ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) ○ DNA carries genetic information that gets passed on to different generations ○ RNA is a shortlived intermediate that is used during gene expression ○ central dogma: flow of informations from transcription to translation ○ Building block is a nucleotide ■ has three parts: a ribose sugar, a nitrogenous base, and a phosphate group ■ sugar and nitrogenous base together is called a nucleoside ■ sugar is in a pentose ring structure and carbons are numbered 1’, 2’, etc. ■ 5’ carbon outside the ring is where the phosphate is joined ■ In RNA, a hydroxyl is on the 2’ carbon but in DNA it is only hydrogen ● this is why DNA is 2’deoxyRNA because there is no oxygen on the 2’ carbon of the ribose ○ 5 nitrogenous bases: Adenine (A), cytosine (C) , guanine (G), thymine (T), and uracil (U) ■ A and G are purine and have 2 rings ■ C, T, and U are pyrimidines and have 1 ring ■ T is found in DNA, U is found in RNA ○ In a nucleotide polymer (polynucleotide), nucleotides are joined by phosphodiester bonds ○ backbone consists of alternating sugars and phosphates with nitrogenous bases attached to the sugars ○ Polynucleotide strands pair together by forming hydrogen bonds between functional groups on the nitrogenous bases ■ A pairs with T (or U in RNA) and G pairs with C ■ forms a double helix ■ 5’ to 3’ direction on 1 strand and the complementary strand is 3’ to 5’ ○ Polynucleotide strands are synthesized by enzymes called polymerase ■ polymerase uses nucleoside triphosphates as building blocks and a preexisting DNA molecule as a template ○ DNA and RNA store genetic information as the order of the bases along the length of the molecule TOPIC 5: MEMBRANES ● Membranes are built on phospholipid bilayers but also contain carbohydrate and protein ○ have a hydrophilic head consisting of polar groups and a hydrophobic tail consisting of long hydrocarbon tails ○ The middle of the membrane strongly repels water, creating a barrier ● Fluidmosaic: model of cell membrane ○ says membranes are fluid structures where some proteins can be found with hydrophobic parts in the middle and their hydrophilic parts outside ○ later on shown that the middle of the membrane doesn’t have proteins ● Peripheral membrane proteins are associated with the outside of membrane ● Integral membrane proteins: proteins with portions that go through membranes ● Transmembrane proteins: proteins that pass completely through membranes ○ Portions of membrane proteins that are found in the lipid bilayer usually have ahelices that have hydrophobic amino acid side chains ■ In bacteria, can also be large “barrel” structures consisting of bpleated sheets ○ Example of transmembrane protein: CFTR or cystic fibrosis transmembrane conductance regulator ■ protein of 1480 amino acids ■ has 6 regions that have alpha helices that span the membrane ■ also a glycoprotein: has been covalently modified with carbohydrates ● lipids can also have covalent carbohydrate modifications, making them glycolipids ■ found in membranes in secretory epithelial cells (lungs, pancreas, and intestines) ■ All humans have the gene but some individuals carry a mutation in both copies that affect the CFTR function resulting in cystic fibrosis phenotype ■ the mutation disrupts the ATPdependent transport of ions ● Membranes are held together mainly by hydrophobic interactions between the fatty acid tails which are weak ○ movement of phospholipids occurs easily but the “flipping” of phospholipids from one side to another is rare ○ Frye and Edidin experiment: used cultured mouse and human cells and labeled the membrane proteins differently ■ used infection by Sendai viruse to fuse the two cell types together making a larger one ■ after an hour the membrane proteins were spread out inside the fused cell ■ shows that some proteins that are integral to membranes can travel ■ Recent studies show that they don’t move freely but have restricted regions when they can moving depending on its function ● At lower temperatures, membranes lose fluidity because phospholipids become more closely asociated ○ Similar to lipids, if the fatty acid tails have multiple cis double bonds, the membrane will keep its fluidity at low temperatures because the kinks keep the phospholipids apart ○ The presence of cholesterol is another important factor of membrane fluidity ■ cholesterol is mostly hydrophobic but has a small polar functional group (OH) ■ will interact with membranes by being mainly inside the bilayer with the polar group sticking into the hydrophilic portion ■ at high temperatures cholesterol restricts movement of phospholipids, making the membrane less fluid ■ at lower temperatures, cholesterol prevents tight association of phospholipids ■ this is why cholesterol can buffer effects of temperature on membrane fluidity ○ Membrane proteins have diverse functions ■ transports materials across the membrane ■ transduction of signals from outside the cell to the inside ■ cell to cell contact ■ attach cells to an extracellular environment ● Membranes are selectively permeable ○ the hydrophobic inside of membranes only allows nonpolar molecules (hydrocarbons, CO2, O2) pass directly through cell membranes ○ Solutes: substances that move across membranes (because membranes are usually in a water environment) ○ 2 types of movement of solutes across a membrane ■ Passive and active transport ● Passive transport is the process of diffusion across a membrane ○ does not require energy besides the energy in the system that causes random collisions ○ movement of solutes will be from where they are more concentrated to where they are less concentrated ■ behavior of solute particles is because of random collisions ■ at equilibrium, the solute continues to move across the membrane in both directions ■ overall concentration on both sides of the membrane stay the same ● Membrane can be permeable to a solute with transport proteins ○ channel protein: protein that creates a hydrophilic tunnel through the membrane ○ carrier protein: binds to particular substances and by changing shape, allows the substance to enter the cell ■ diffusion that occurs this way is called facilitated diffusion ○ Ion channels: channel proteins that allow ions to move across membranes ■ some are “gated”, only allowing ions to flow under certain conditions ● gated ion channels are important for function of neurons ● Osmosis is the movement of water across a semipermeable membrane that is permeable only to water ○ water will diffuse from the region of lower solute concentration to the region of higher solute concentration ○ water moves from where there is more free water to where there is less free water (more solute) ● Osmotic pressure: the amount of pressure that would have to be applied on the righthand side to force the water levels to be equalized ○ measure the difference in osmotic pressure by measure the differences in heights between the 2 sides ○ the greater the difference in solute concentration between the 2 sides, the greater the osmotic pressure ○ one “osmole” equals one mole of osmotically active particles ■ Example: 1M glucose = 1Osmolar; 1M of NaCl= 2 Osmolar because Na+ and Cl ions dissociate in solution ● Osmotic pressure is important because cell membranes are semipermeable ○ Isotonic environment: ■ concentration of solutes is approximately equal ■ no net osmotic pressure in cell’s outer membrane ○ Hypotonic environment ■ the solution outside the cell has a lower solute concentration ■ more water will move into the cell, causing them to burst (lyse) ■ makes plant cells turgid (normal condition) ○ Hypertonic environment ■ the solution outside the cell has a higher solute concentration ■ more water leaves the cell, cell becomes shriveled ■ In plants, the cells shrink away from cell walls, condition called plasmolysis ● occurs when plants wilt ● Active transport requires input of energy, usually from ATP ○ cells can move solutes against the concentration gradient, from low concentration to high concentration by using energy ○ active transport uses only carrier proteins ○ required for cells to keep certain solutes out and other in ○ Example of active transport: sodium potassium pump ■ cells keep a higher concentrations of Na+ outside and higher concentration of K+ inside ■ 3 Na+ ions bind the pump, which becomes phosphorylated by a phosphate transferred from ATP ● changes the shaped of the carrier protein so the Na+ ions face the outside of the cell ■ Na+ ions are released and 2 K+ ions can bind ■ Phosphate is removed and K+ ions are released inside the cell ○ membrane potential: difference in charge resulting from a higher positive charge outside the cell than inside ○ Plants, bacteria, and fungi pump hydrogen ions across membranes using a proton pump ■ creates higher concentration of protons outside the cell ○ Electrochemical gradient: with active transport of ions, made by combinded difference in ions and charge ■ can be harnessed to do work ● Example: cotransported: carrier protein that harnesses the energy of ions moving down their concentration gradient to actively transport another solute ■ proton pump creates a proton gradien t ● used by cotransporter to import H+ ions ● Vesicles move large molecules in and out of cells ○ Small molecules can either diffuse directly or use transport proteins to travel in and out of cells but larger molecules move using vesicles ○ Vesicles: small sacs of membranes that completely enclose their contents ■ can either bud off existing membranes or fuse with them ■ transports contents between organelles and digesting molecules ○ Exocytosis: vesicles fuse with outer cell membrane, causing contents inside the cells to be outside the cell ■ how cells add new membrane for growth ■ used by secretory cells: releases insulin into bloodstream ■ neurons release neurotransmitters using exocytosis ○ Endocytosis: vesicles bud off the outer membrane to enclose substances outside the cell and bring them inside. Three types of endocytosis ■ Phagocytosis: “cell eating”, internalization of solids, like bacteria ● used for intake of nutrients by some cells ● used by immune system to destroy pathogens ● cells extends pseudopods to surround the particle and the vesicle will form around it, bring it into the cell ● internal contents will fuse with a lysosome (vesicle found inside cells that has digestive enzymes) ■ Pinocytosis: “cell drinking” forms small vesicles ● used to internalize liquids and small particles ● nonspecific about substances engulfed ■ Receptormediated endocytosis: cells bring specific substances determined by interaction of the substances with specific receptors on the surface of the membrane ● ligands: substances that bind a receptor ● when ligands bind the receptors, pit will form, coated on the cytoplasmic side with protein ● vesicle will form and bring contents into cell ■ LDL receptor: cholesterol in the bloodstream is carried by LDLs (lowdensitylipoproteins), complex protein and lipid ● bind LDL receptors then the LDL+cholesterol is taken into the cell in a coated vesicle ● humans with defects in the gene encoding the LDL receptor have familial hypercholesterolemia (disorder causing a buildup of cholesterol in the blood) TOPIC 6: CELL STRUCTURE ● All cells consist of a lipid bilayer, the plasma membrane, inside had cytosol or cytoplasm ○ plants have a cell wall outside the plasma membrane ○ cells contain genetic material (DNA) and ribosomes (where translation takes place) ● Prokaryotes: ○ include bacteria ○ do not contain significant internal membranes ○ DNA is concentrated in a region called the nucleoid ○ 15 micrometers across ● Eukaryotes: ○ includes plants, animals, protists, and fungi ○ DNA can be found in the nucleus ○ has other membranebound organelles that divide cell functions ○ 10100 micrometers in diameter but can be larger
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