Notes for the Second Week
Notes for the Second Week BSC1010
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This 20 page Class Notes was uploaded by Amanda Howard on Thursday May 26, 2016. The Class Notes belongs to BSC1010 at University of North Florida taught by Dr. Michael R. Lentz in Summer 2016. Since its upload, it has received 16 views. For similar materials see General Biology I in Biology at University of North Florida.
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Date Created: 05/26/16
Chapter 3: Protein Structure and Function (con’t) Proteins (I’m reinserting the entire section from last week and adding to it) Proteins are crucial to most tasks required for cells to exist o Catalysis: Enzymes speed up chemical reactions o Defense: Antibodies and complement proteins attack pathogens o Movement: Motor and contractile proteins move the cell or molecules within the cell o Signaling: Proteins convey signals between cells o Structure: Structural proteins define cell shape and comprise body structures o Transport: Transport proteins carry materials o Membrane proteins control molecular movement into and out of the cell o All senses are based on protein signals o Proteins break down food into amino acids What do proteins look like? o The unparalleled diversity of proteins in size, shape, and other aspects of structure is important because: Function follows from structure o Proteins can serve diverse functions in cells because they are diverse in size and shape and diverse in the chemical properties of their amino acids o Since amino acids can be combined in limitless combinations and shapes the number of possible proteins is limitless, however, that doesn’t mean all the proteins created would be functional o All proteins have just four basic levels of structure: Primary: Protein primary structure: its unique sequence of amino acids The number of possible primary structures is practically limitless 20 types of amino acids available Lengths range from two amino acid residues to tens of thousands Determines everything else If the sequence is correct then the secondary and tertiary structure should follow spontaneously The amino acid R-groups affect a polypeptide’s properties and function A single amino acid change can radically alter protein function Secondary Protein’s secondary structure is formed by hydrogen bonds o Hydrogen bonds occur between the carbonyl group of one amino acid and the amino group of another Hydrogen bonding between sections of the same backbone o Is possible only when a polypeptide bends in a way that puts C O and N–H groups close together, forming: -helices -pleated sheets Or a combination of both o Each group is used for a hydrogen bond within the structure o Sidechains would stick out of the secondary coil or sheet Secondary structure depends on the primary structure: o Some amino acids are more likely to be involved in -helices o Others are likely to be involved in -pleated sheets Tertiary The tertiary structure of a polypeptide results from interactions between R-groups or between R-groups and the peptide backbone o These contacts cause the backbone to bend and fold o Bending and folding contribute to the distinctive three-dimensional shape of the polypeptide The chains are folded into a stable 3D shape Infinite possibilities, only one is correct Structure with the lowest energy is the correct shape R-group interactions include: o Hydrogen bonds Form between hydrogen atoms and the carbonyl group in the peptide-bonded backbone Or they form between hydrogen and negatively charged atoms in side chains o Hydrophobic interactions: these interactions within a protein increase stability of surrounding water molecules by increasing hydrogen bonding o Van der Waals interactions: Weak electrical interactions between hydrophobic side chains o Covalent disulfide bonds: bonds between sulfur- containing R-groups o Ionic bonds form between groups that have full and opposing charges Important part of the part of the folding process is in the tertiary level: o Most side chains are hydrophobic o This process happens in water o All the hydrophobic side chains are moved to the middle of the protein to minimize their interaction with water Quaternary (new information) Many proteins contain several distinct polypeptide subunits that interact to form a single structure The bonding of two or more distinct polypeptide subunits produces quaternary structure o Very hierarchical o Many proteins actually only have three structures o Quick rundown: Protein structure is hierarchical Quaternary structure is based on tertiary structure, which is based in part on secondary structure All three of the higher-level structures are based on primary structure Combined effects of primary, secondary, tertiary, and sometimes quaternary structure allow for amazing diversity in protein form and function Folding and Function o Protein folding is often spontaneous because of the hydrogen bonds and van der Waals interactions that make the folded molecule more energetically stable than the unfolded molecule o A denatured (unfolded) protein is unable to function normally o Proteins called molecular chaperones help proteins fold correctly in cells o Protein folding is often regulated since the function of a protein is dependent on its shape Controls when or where it is folded Regulates the protein’s activity o For example: The inactive form of a protein has a disordered shape When active protein is needed It folds into an ordered, active conformation Prions and Protein folding o Misfolding can be “infectious” o Prions are improperly folded forms of normal proteins They are present in healthy individuals Amino acid sequence does not differ from a normal protein but its shape is radically different Prions can induce normal protein molecules to change their shape to the altered form What do proteins do? o Diversity of function is reflected in a diversity of shape o Proteins are crucial to most tasks required for cells to exist o Catalysis: Enzymes speed up chemical reactions Proteins are only useful if they interact with something else A catalyst is something that increases the rate of reaction Enzymes organize molecules to create a more efficient reaction Catalysis may be the most fundamental of protein functions Reactions take place when reactants collide in precise orientation Reactants have enough kinetic energy to overcome repulsion between the electrons that come in contact during bond formation Enzymes perform two functions: Bring substrates together in precise orientation so that the electrons involved in the reaction can interact Decrease the amount of kinetic energy that reactants must have for the reaction to proceed o Defense: Antibodies and complement proteins attack pathogens o Movement: Motor and contractile proteins move the cell or molecules within the cell o Signaling: Proteins convey signals between cells o Structure: Structural proteins define cell shape and comprise body structures o Transport: Transport proteins carry materials Membrane proteins control molecular movement into and out of the cell Chapter 4: Nucleic Acids and the RNA World Key Concepts: Nucleotides consist of a sugar, a phosphate group, and a nitrogen-containing base. Ribonucleotides polymerize to form RNA. Deoxyribonucleotides polymerize to form DNA DNA’s primary structure consists of a sequence of nitrogen containing bases. Its secondary structure consists of two DNA strands , running in opposite directions, which are held together by complimentary base pairing and twisted into a double helix. DNA’s structure allows organisms to store and replicate the information needed to grow and reproduce RNA’s primary structure consists of a sequence of nitrogen containing bases. Its secondary structure includes short regions of double helices and structures called hairpins Because RNA molecules can carry information as well as catalyze chemical reactions, it is likely that RNA was the first self-replicating molecule and a forerunner to the first life forms What is a Nucleic Acid? A nucleotide is similar to an amino acid in a protein A nucleic acid is a polymer of nucleotide monomers Three components of a nucleotide: o A phosphate group o A five-carbon sugar Ribonucleotides The sugar is ribose Deoxyribonucleotides The sugar is deoxyribose (deoxy means lacking oxygen) These two sugars differ by a single oxygen atom: Ribose has an –OH group bonded to the 2′ carbon Deoxyribose has an H instead at the same location In both of these sugars an –OH group is bonded to the 3′ carbon o A nitrogenous (nitrogen-containing) base There are two groups of nitrogenous bases: Purines: these have a single ring Adenine Guanine Pyrimidines: these have a double ring Cytosine Uracil Thymine The base uracil (U) is found only in ribonucleotides The base thymine (T) is found only in deoxyribonucleotides The phosphate is bonded to the sugar molecule and in turn, the sugar molecule is bonded to the nitrogenous base Nucleotides polymerize to form nucleic acids Nucleic acids form when nucleotides polymerize Phosphodiester linkage (bond) occurs between the phosphate group on the 5′ carbon of one nucleotide and the –OH group on the 3′ carbon of another o Forms through condensation reaction o Two types of nucleotides are involved Ribonucleotides: contain the sugar ribose and form RNA Deoxyribonucleotides: contain the sugar deoxyribose and form DNA The sugar phosphate backbone is directional o The sugar-phosphate backbone of a nucleic acid is directional (has polarity) One end has an unlinked 5′ carbon The other end has an unlinked 3′ carbon o The nucleotide sequence is written in the 5′ 3′ direction Reflects the order that nucleotides are added to a growing molecule The nucleic acid’s primary structure is the nucleotide sequence o Nucleotides are always added to the 3’ end o In a single strand of RNA or DNA one end has an unlinked 5′ phosphate and the other end has an unlinked 3′ hydroxyl o There is an identical backbone for all nucleotides: Phosphate and Sugar o Bases come off of these and provide functionality The polymerization of nucleic acids is endergonic o Polymerization of nucleic acids is an endergonic process which is catalyzed by enzymes o Energy for polymerization comes from the phosphorylation of the nucleotides Phosphorylation: Is the transfer of a phosphate group(s) to a substrate molecule Raises the potential energy of the substrate Enables endergonic reactions Endergonic process: it requires energy o Nucleoside triphosphate: two phosphates are transferred they are created during nucleic acid polymerization o Figure 4.4a shows an example of an activated nucleotide o This molecule is called adenosine triphosphate, or ATP Lots of energy stored in the bonds, the breaking of these bonds is what drives processes in the body ATP is constantly being synthesized and broken down in cells What is the nature of DNA’s secondary structure DNA and RNA are double stranded Chemists: o Had worked out the structure of nucleotides o Knew that DNA polymerized through the formation of phosphodiester linkages o Watson and Crick knew that the molecule had a sugar-phosphate backbone o Erwin Chargaff established two empirical rules for DNA: The total number of purines and pyrimidines is the same The numbers of A’s and T’s are equal and the numbers of C’s and G’s are equal o Rosalind Franklin and Maurice Wilkins calculated the distances between groups of atoms in the DNA molecule by bombarding DNA with X-rays and analyzing how it scattered the radiation this is called X-ray crystallography The scattering patterns showed that three distances were repeated many times Inferred that DNA molecules had a regular and repeating structure The pattern of X-ray scattering suggested that the molecule was helical, or spiral, in nature o James Watson and Francis Crick determined DNA strands run in an antiparallel configuration: Antiparallel means that they run in opposite directions DNA strands form a double helix The hydrophilic sugar-phosphate backbone faces the exterior Nitrogenous base pairs face the interior o Purines always pair with pyrimidines Strands form complementary base pairs A-T and G-C Across the DNA Molecule is even spacing created by proper base pairs These pairs help to hold the two DNA strands together These base pairs are highly specialized to only hydrogen bond with the pair A-T have two hydrogen bonds C-G have three hydrogen bonds o DNA has two different-sized grooves: The major groove The minor groove Summary of DNA’s secondary structure o DNA’s secondary structure consists of two antiparallel strands twisted into a double helix o The molecule is stabilized by hydrophobic interactions in its interior by hydrogen bonding between the complementary base pairs A-T and G-C DNA Contains Biological Information DNA can store and transmit biological information DNA carries the information required for the organism’s growth and reproduction The language of nucleic acids is contained in the sequence of the bases DNA carries the information required for the growth and reproduction of all cells DNA is basically instructions to make a human The phosphate and deoxyribose makes for a very stable strand Information is stored in the bases How does DNA replicate “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism.” —Watson and Crick Here’s the key insight about DNA o Primary structure serves as a mold or template for the synthesis of a complementary strand o Contains the information required for a copy of itself to be made Complementary base pairing provides a simple mechanism for DNA replication and each strand can serve as a template for the formation of a new complementary strand The fact that their model readily suggested a copying mechanism was very important DNA replication requires two steps: o Separation of the double helix o Hydrogen bonding of deoxyribonucleotides with complementary bases On the original template strand Followed by phosphodiester bond formation between the deoxynucleotides to form the complementary strand The hydrogen bonds that hold the two strands together are much weaker than the carbon bonds holding the nucleotides together This allows the two strands to be pulled apart from each other without destroying the DNA Is DNA a catalytic molecule? DNA’s stability o Makes it a reliable store for genetic information o It is less reactive than RNA o It is more resistant to chemical degradation Stable molecules such as DNA make poor catalysts Biologists think that the first life-form was made of RNA not DNA DNA does not appear to catalyze any chemical reaction Remember the difference between RNA and DNA comes from the addition of a single O in into the sugar of RNA creating Ribose which makes an OH group making RNA highly reactive RNA is single stranded Orderliness and stability make DNA a dependable information repository but extraordinarily inept at catalysis RNA Structure and Function RNA (like DNA) has a primary structure consisting of a sugar-phosphate backbone formed by phosphodiester linkages and extending from that backbone, a sequence of four types of nitrogenous bases The primary structure of RNA differs from DNA o RNA contains uracil instead of thymine o RNA contains ribose instead of deoxyribose The presence of the –OH group on ribose makes RNA o Much more reactive o Less stable than DNA o Single strand o Can fold over on itself making either a hairpin or stem loop structure o Still has stable base pairs, however its one strand folding on itself instead of two strands forming a double helix RNA’s secondary structure o RNA’s secondary structure results from complementary base pairing o The bases of RNA typically form hydrogen bonds with complementary bases on the same strand o The RNA strand folds over, forming a hairpin structure The bases are on one side of the fold The bases align with an antiparallel RNA segment on the other side of the fold Example of an RNA in the form a ribosome o This is the basis of a ribosome o If not folded properly then the ribosome doesn’t work RNA molecules can also have tertiary structure o Forms when secondary structures fold into more complex shapes RNA (like DNA) can function as an information-containing molecule capable of self-replication Structurally/chemically, RNA is intermediate between o The complexity of proteins o The simplicity of DNA RNA self-replication o Step 1 A complimentary copy of the RNA is made Free ribonucleotides form hydrogen bonds with complementary bases on the original template strand of RNA o Step 2 Their sugar-phosphate groups form phosphodiester linkages to produce a double-stranded RNA molecule o Step 3 The hydrogen bonds between the double-stranded product must be broken by heating or by a catalyzed reaction RNA versatility o The newly made complementary RNA molecule now exists independently of the original template strand o If steps 1–3 were repeated with the new strand serving as a template (steps 4–6) then the resulting molecule would be a copy of the original o RNA can function as a catalytic molecule Ribozymes are enzyme-like RNAs o RNA has a degree of structural and chemical complexity which makes it capable of catalyzing a number of chemical reactions The First Life-Form: RNA (not super important) The theory of chemical evolution: o Life began as a naked self-replicator o A molecule that existed by itself in solution o Without being enclosed in a membrane To make a copy of itself, the first living molecule had to: o Provide a template that could be copied o Catalyze polymerization reactions that would link monomers into a copy of that template RNA is capable of both processes Most origin-of-life researchers propose that the first life-form was made of RNA RNA is not very stable but it might have survived long enough in the prebiotic soup to replicate itself and so it may have been the first life-form RNA replicase is a ribozyme that o Can catalyze the addition of ribonucleotides to a complementary RNA strand o Can replicate RNA Chapter 6: Lipids, Membranes, and the First Cells Key Concepts: Phospholipids are amphipathic molecules- they have a hydrophilic region and a hydrophobic region. In solutions, phospholipids spontaneously form bilayers that are selective and permeable- meaning that only certain substances cross them readily Ions and molecules diffuse spontaneously from regions of high concentration to regions of low concentration. Water moves across lipid bilayers from regions of high water concentration to regions of low water concentration via osmosis- a special kind of diffusion In cells, membrane proteins are responsible for the passage of ions, polar molecules, and large molecules that can’t cross the membrane on their own because they are not soluble in lipids. Some membrane proteins form channels, some facilitate diffusion by binding to substrates, and some use energy from ATP to actively pump ions or molecules The Importance of Membranes The plasma membrane, or cell membrane separates life from nonlife The plasma membrane separates the cell’s interior from the external environment Membranes function to: o Keep damaging materials out of the cell o Allow entry of materials needed by the cell o Facilitate the chemical reactions necessary for life Lipids Lipids are carbon-containing compounds found in organisms and are largely nonpolar and hydrophobic Hydrocarbons are nonpolar molecules that contain only carbon and hydrogen Lipids do not dissolve in water, because of a major hydrocarbon component called a fatty acid o Fatty acids are hydrocarbon chains bonded to a carboxyl (–COOH) functional group o They can be saturated or unsaturated o Saturated hydrocarbon chains consist of only single bonds between the carbons and therefore has the maximum possible number of hydrogen atoms o Unsaturated hydrocarbon chains one or more double bonds exist in the hydrocarbon chains o Bond saturation also profoundly affects the physical state of lipids Highly saturated fats Such as butter Are solid at room temperature More packed in Saturated lipids that have extremely long hydrocarbon tails Such as waxes Form particularly stiff solids at room temperature Highly unsaturated fats are liquid at room temperature (ex: olive oil)- the kinks cause it to not hold together as well Fatty acids and isoprene are the key building blocks of lipids Lipid structure is characterized by a physical property: their insolubility in water, instead of a shared chemical structure This insolubility is based on: o The high proportion of nonpolar C–C and C–H bonds o Relative to polar functional groups The three most important types of lipids found in cells: o Fats (triacylglycerols or triglycerides) Fats are composed of three fatty acids linked to glycerol these are also called triacylglycerols or triglycerides Fats are found in blood When the fatty acids are polyunsaturated they form liquid triacylglycerols called oils The primary role of fats is energy storage Structure Fats form when o A dehydration reaction occurs between o A hydroxyl group of glycerol + the carboxyl group of a fatty acid The glycerol and fatty acid molecules become joined by an ester linkage o Steroids Steroids are a family of lipids distinguished by a bulky, four-ring structure Steroids differ from one another by the functional groups or side groups attached to different carbons in those hydrophobic rings Structure A steroid example is cholesterol: a hydrophilic hydroxyl group attached to the top ring and an isoprenoid “tail” attached at the bottom Cholesterol is an important component of plasma membranes in many organisms o Phospholipids Phospholipids are amphipathic The head region contains highly polar covalent bonds consisting of a glycerol, a phosphate, and a charged group making it hydrophilic The tail region is comprised of two nonpolar fatty acid or isoprene chains which is hydrophobic When placed in solution, these lipids form membranes: The phospholipid heads interact with water The tails do not Phospholipids in water Phospholipids do not dissolve in water Water molecules cannot form hydrogen bonds with the hydrocarbon tail Water molecules interact with the hydrophilic heads not with the hydrophobic tails- This drives the hydrophobic tails together Membranes Composed of the phospholipids discussed earlier An important part of this is how phospholipids interact in water: o Phospholipids do not dissolve in water o Water molecules cannot form hydrogen bonds with the hydrocarbon tail o Water molecules interact with the hydrophilic heads not with the hydrophobic tails- This drives the hydrophobic tails together o Water always wants to form hydrogen bonds and it is always energetically unfavorable to interfere with water’s hydrogen bonds in the case of membranes it is best to let all the tales interact with themselves pointed to the center while the heads point out allowing them to interact with water no energy is required to make this happen since this is the configuration since the thermodynamics push towards this configuration Upon contact with water, phospholipids form either o Micelles: Heads face the water and tails face each other o Phospholipid bilayers (lipid bilayers) Phospholipid bilayers form when Two sheets of phospholipid molecules align Hydrophilic heads in each layer face a surrounding solution The hydrophobic tails face one another inside the bilayer Phospholipid bilayers form spontaneously with no outside input of energy is required Artificial membranes as an Experimental System o The hydrophilic heads on both sides of the bilayer remain in contact with the aqueous solution and water is present both inside and outside the vesicle o Artificial membrane-bound vesicles like these are called liposomes o Planar bilayers: lipid bilayers constructed across a hole in a glass or plastic wall separating two aqueous solutions Permeability o The permeability of a structure is its tendency to allow a given substance to pass across it o Lipid bilayers are highly selective o Phospholipid bilayers have selective permeability Small or nonpolar molecules move across phospholipid bilayers quickly Charged or large polar substances cross slowly, if at all Ions can rarely cross o Many factors influence the behavior of the membrane Number of double bonds between the carbons in the phospholipid’s hydrophobic tail Length of the tail Number of cholesterol molecules in the membrane Temperature o Permeability is essential because it is important that cells be able to receive supplies and remove waste Bond saturation and membrane permeability o The make-up of the membrane can be changed in order to effect permeability in response to the environment and needs o Double bonds in a hydrocarbon chain can cause a “kink” in the hydrocarbon chain Prevents the close packing of hydrocarbon tails Reduces hydrophobic interactions o Unsaturated hydrocarbon chains have at least one double bond and therefore the membranes are much more permeable o Saturated hydrocarbon chains are without double bonds and have more chemical energy than unsaturated fats do which makes them much less permeable Other factors that affect permeability o Fluidity and permeability are connected Membrane fluidity decreases With temperature When molecules in the bilayer are moving more slowly Decreased membrane fluidity causes decreased permeability If the membrane loses fluidity than it is unable to allow transport Individual phospholipids can move laterally throughout the lipid bilayer however they rarely flip between layers How quickly molecules move within and across membranes is a function of Temperature The structure of hydrocarbon tails The number of cholesterol molecules in the bilayer o Hydrophobic interactions become stronger as saturated hydrocarbon tails increase in length Membranes containing phospholipids with longer tails have reduced permeability o Adding cholesterol to membranes increases the density of the hydrophobic section o Cholesterol decreases membrane permeability Solute movement across lipid bilayers Materials move across the cell membrane in different ways Passive transport does not require an input of energy Active transport requires energy to move substances across the membrane o This is how polar molecules can be transported across a membrane Small molecules and ions in a solution o Are called solutes o Have thermal energy o Are in constant, random motion This random movement is called diffusion Diffusion is a form of passive transport Diffusion along a concentration gradient o A concentration gradient is created by a difference in solute concentrations o Molecules and ions move randomly when A concentration gradient exists There is a net movement from high-concentration regions to low-concentration regions o Diffusion along a concentration gradient increases entropy and is spontaneous o Equilibrium occurs when the molecules or ions are randomly distributed Molecules are still moving randomly but there is no more net movement Osmosis Water moves quickly across lipid bilayers The movement of water is a special case of diffusion called osmosis Water moves from regions of low solute concentration to regions of high solute concentration This movement dilutes the higher concentration It also equalizes the concentration on both sides of the bilayer Osmosis only occurs across a selectively permeable membrane- moving from areas of high to low concentration. This is especially true in the case of water when it can cross a membrane that the solute cant Osmosis and Relative Solute Concentration o The concentration of a solution outside a cell may differ from the concentration inside the cell o An outside solution with a higher concentration is hypertonic to the inside of a cell Water will move out of the cell by osmosis The cell will shrink o A solution with a lower concentration is hypotonic to the cell Water will move into the cell by osmosis The cell will swell o If solute concentrations are equal on the outside and inside of a cell solutions are isotonic to each other There will be no net water movement The cell size will remain the same Fluid-Mosaic Model of Membrane Structure Phospholipids provide the basic membrane structure Plasma membranes contain as much protein as phospholipid The fluid-mosaic model of membrane structure suggests: o Some proteins are inserted into the lipid bilayer o Thus making the membrane a fluid, dynamic mosaic of phospholipids and proteins Membrane Proteins o Integral proteins are Amphipathic Able to span a membrane With segments facing both its interior and exterior surfaces o Transmembrane proteins are Integral proteins that span the membrane Involved in the transport of selected ions and molecules across the plasma membrane Able to affect membrane permeability o Peripheral proteins are Found only on one side of the membrane Often attached to integral proteins o Membrane proteins affect ions and molecules Transport proteins are transmembrane proteins that transport molecules Three broad classes of transport proteins: Channels o Ion channels are specialized membrane proteins: They circumvent the plasma membrane’s impermeability to small, charged compounds o Electrochemical gradients occur when ions build up on one side of a plasma membrane they establish both a concentration gradient and a charge gradient o Ions diffuse through channels down their electrochemical gradients o Channel proteins are selective Each channel protein has a structure Permits only a particular type of ion or small molecule to pass through it o Gated channels Open or close in response to a signal Examples: The binding of a particular molecule, or a change in the electrical voltage across the membrane o The flow of ions and small molecules through membrane channels is carefully controlled o The movement of substances through channels is passive- it does not require an input of energy o Passive transport is powered by diffusion along an electrochemical gradient o Cells have many different types of channel proteins in their membranes Each type of protein features a structure that allows it to admit a particular type of ion or small molecule o These channels are responsible for facilitated diffusion The passive transport of substances that would not otherwise cross the membrane Carrier proteins or transporters o Facilitated diffusion can occur through channels or through carrier proteins, or transporters Change shape during the transport process Move only down a concentration gradient, reducing differences between solutions o Ex: Lipid bilayers are only moderately permeable to glucose Glucose is a building block for important macromolecules and a major energy source A glucose transporter named GLUT-1 increases membrane permeability to glucose Pumps o Cells can transport molecules or ions They move against an electrochemical gradient They require energy in the form of ATP The process is called active transport o Pumps are membrane proteins that provide active transport of molecules across the membrane + + o The sodium–potassium pump (Na /K -ATPase) uses ATP to transport Na and K + These ions move against their concentration gradients o Pumps move materials against their concentration gradients Pumps also set up electrochemical gradients o These gradients make it possible for cells to engage in secondary active transport, or cotransport o The gradient provides the potential energy required to power the movement of a different molecule against its particular gradient Summary of Three mechanisms of membrane transport: Diffusion Facilitated diffusion Active transport Passive transport Involves diffusion and facilitated diffusion Moves materials down their concentration gradient Does not require an input of energy Membrane Transport Active transport o Moves materials against their concentration gradient o Requires energy provided by ATP or an electrochemical gradient Each affects membrane permeability Plasma Membrane and the Intracellular Environment o Enables cells to create an internal environment that is much different from the external one o The selective permeability of the lipid bilayer o The specificity of the proteins involved in passive transport and active transport
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