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UNIT 1
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Active Transport:

• involves the movement of materials against a concentration gradient (low to high concentration)

• because materials are moving against a gradient, it requires ATP

• 2 types of active transport:

◦ primary (direct): involves direct use of metabolic energy to mediate transport

◦ secondary (indirect): involves coupling the molecule with another moving along an electrochemical gradient





Simple Diffusion
• diffusion: net movement of molecules from a region of high concentration to a region of low concentration

◦ directional movement along a gradient is passive and will continue until molecules become evenly dispersed
(equilibrium)

◦ small and non-polar molecules will be able to freely diffuse across cell membranes

‣ e
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O2, CO2, glycerol

• rate of diffusion can be influenced by a number of factors, include:

◦ temperature

‣ affect kinetic energy of particles in solution

◦ molecular size

‣ larger particles are subjected to greater resistance within a fluid medium

◦ steppes of gradient

‣ rate of diffusion will be greater with a higher concentration gradient



Osmosis
• net movement of water molecules across a semi-permeable membrane from a region of low solute concentration
to a region of high solute concentration

◦ water is considered the universal solve

‣ it will associate with and solve, polar or charged molecules/solutes

◦ because solutes cannot cross a cell mane unaided water all move to equalize the two solutions

◦ higher solute concentration there are less free water molecules in solution as water is associated with the
solute

◦ osmosis is essentially the diffusion of free water molecules and hence occurs from regions of low solute
concentration



Osmolarity
• measure of solute concentration, as defined by number of osmoses of a solute per liter solution

• solutions may be lonely categorized as hypertonic, hypotonic and isotonic according to relative osmolarity

‣ higher osmolarity: hypertonic

• gains water

‣ lower osmolarity: hypotonic

• loses water


‣ same osmolarity: isotonic

• no net water flow



Estimating Osmolarity

• of a tissue can be interpolated by bathing the sample in solutions with known omsolarities

◦ tissue will lose water when placed in hypertonic solutions and gain it in hypotonic solutions

◦ water loss or gain determined by weight sample before and after

◦ tissue osmolarity may be inferred by identifying the concentration of solution at which there is no weight gain

‣ e
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isotonic



• tissues or organs to be used in medical procedures must be kept in solution to prevent cellular desiccation

• solution must share the same osmolarity as the tissue/organ in order to prevent osmosis from occurring

• uncontrolled osmosis will have negative effects to cell viability

◦ hypertonic: water will leave cell causing it to shrivel (crenation)

◦ hypotonic: water will enter cell causing it to swell and potentially burst (lysis)

• in plant tissues, effects of uncontrolled osmosis are moderated by the presence of a cell wall

◦ hypertonic solutions: cytoplasm will shrink but cell will maintain shape

◦ hypotonic: cytoplasm will expand but be unable to rupture within the constants of cell wall



Facilitated Diffusion
• passive movement of molecules across the cell membrane via the aid of a membrane protein

◦ utilized by molecules that are unable to freely cross the phospholipid bilayer

◦ e
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large polar molecules and ions

◦ process mediated by 2 transport proteins: channel and carrier proteins





Carrier Proteins:

• integral glycoproteins which bind a solute and undergo a conformational change to translocate the solute across
the membrane

• will only bind a specific molecule via an attachment similar to an enzyme-substrate interaction

• may move molecules against concentration gradients in the presence of ATP

• much slower rate of transport than channel proteins



Channel Proteins

• integral lipoproteins which contain a pore via ions may cross from one side of the membrane to the other

• ion-elective and may be gated to regulate the passage of ions in response to certain stimuli

• only move molecules along a concentration gradient

◦ are not used in active transport

• much faster rate of transport than carrier proteins



• axons of nerve cells transmit electrical impulses by translocating ions to create a voltage difference across the
membrane

◦ at rest, sodium potassium pump expels sodium ions from the nerve cells

◦ when euro fires, ions swap locations via facilitate diffusion via sodium and potassium channels



Potassium Channels:

• integral proteins with hydrophilic inner pore via which potassium ions may be transported

• channel is compromised of 4 transmembrane subunits, while inner pore contains a selectivity filter that restrict
passage of alternate ions

• voltage-gated and cycle between an open and closed conformation depend one the transmembrane voltage



Active Transport
• uses energy to move molecules against concentration gradient

• energy may be generated by:

◦ direct hydrolysis of ATP (primary)

◦ indirectly coupling transport with another molecule that is moving along its gradient (secondary)



• active transport involves the use of carrier proteins (called protein pumps)

◦ specific solute will ind to the protein pump on onside of membrane


◦ hydrolysis of ATP causes a conformational change in the protein pump

◦ solute molecule is consequently translocated across the membrane (against the gradient) and released



• axons of nerve cells transmit electrical impulses by translocating icons to create a voltage difference across the
membrane

◦ at rest, the sodium-potassium pump expels sodium ions from the nerve cell, while potassium ions (moves
into cell)

◦ when the neuron fires, these ions swap locations via facilitated diffusion via sodium and potassium channels



Sodium Potassium Pump

• an integral protein that exchanges 3 sodium ions (moves out of cell) with two potassium ions (move into cell)

• process of ion exchange against the gradient is energy-dependent and involves a number of key steps:

◦ 3 sodium ions bind to intracellular sites on the sodium-potassium pump

◦ phosphate group is transferred to the pup via the hydrolysis of ATP

◦ pump undergoes a conformational change, translocating sodium across the membrane

◦ conformational change exposes 2 potassium binding sites on the extracellular surface of the pump

◦ phosphate group is released which causes the pump to return to its original conformation

◦ translocates the potassium across the membrane, completing the ion exchange



Vesicular Transport
• materials destined for secretion are transported around the cell in membranous containers called vesicles



Endoplasmic Reticulum

• ER is a membranous network that is responsible for synthesizing secretory materials

◦ rough ER embedded with ribosomes and synthesizes proteins destined for extracellular use

◦ smooth ER is involved in lipid synthesis and also plays a role in carbohydrate metabolism

• materials transported from ER when membrane bulges and then buds to create a vesicle surrounding the
material



Golgi Apparatus

• vesicle is then transported to the Golgi apparatus and fuses to the internal face of complex

◦ materials move via vesicles from the internal cis face of the Golgi to the externally oriented trans face

◦ while within the golgi apparatus, materials may be structurally modified



Plasma Membrane

• vesicles containing materials destined for extracellular use will be transported to the plasma membrane

• vesicle will fuse with the cell membrane and its materials will be expelled into the the extracellular fluid

• materials sorted by golgi apparatus may be either:

◦ released immediately into the extracellular fluid

◦ stored within an intracellular vesicles for a delayed release in response to a cellular signal



Bulk Transport
• membrane is principally held together by weak hydrophobic associations between the fatty acid tails of
phospholipids

• weak association allows for membrane fluidity and flexibility, as the phospholipids can move around to some
extent

• allows for spontaneous breaking and reforming of the bilayer, allowing larger materials to enter or leave the cell
without having to cross membrane (active transport and requires ATP hydrolysis)



Endocytosis

• process by which large substances (or bulk amounts of smaller substance) enter the cell without crossing the
membrane

◦ the invagination of the membrane forms a flask-like depression which envelopes the extracellular material

◦ invagination is then sealed off to form a intracellular vesicle containing the material

• two main types of endocytosis:

◦ phagocytosis

‣ process by which solid substances are ingested

• transported to lysosome

◦ pinocytosis


‣ process by which liquids/dissolved substances are ingested

• allows for faster intra rather than via protein channels



Exocytosis

• process by which large substances exit the cell without crossing the membrane

◦ vesicles fuse with the plasma membrane, expelling their contents into the extracellular environment

◦ process of exocytosis adds vesicular phospholipids to the cell membrane, replacing those lost when
vesicles are formed via endocytosis










































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