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Lecture 14 plasma membrane transport Active transport pp73-77.

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1 Lecture 14 plasma membrane transport Active transport pp73-77

2 Active Processes Whenever a cell uses the bond energy of ATP to move solutes across the membrane, the process is referred to as active. Substances moved actively across the plasma membrane are usually unable to pass in the necessary direction by passive transport processes. The substance may be too large to pass through the channels, incapable of dissolving in the lipid bilayer, or unable to move down its concentration gradient.

3 Membrane Transport: Active Processes Two types of active processes: – Active transport – Vesicular transport Both use ATP to move solutes across a living plasma membrane

4 Active transport like carrier-mediated facilitated diffusion, requires carrier proteins that combine specifically and reversibly with the transported substances. However, facilitated diffusion always honors concentration gradients because its driving force is kinetic energy. In contrast, the active transporters or solute pumps move solutes, most importantly ions (such as Na +, K +, and Ca 2+ ), “uphill” against a concentration gradient. To do this work, cells must expend the energy ofATP.

5 Figure 3.7b Lipid-insoluble solutes (such as sugars or amino acids) (b) Carrier-mediated facilitated diffusion via a protein carrier specific for one chemical; binding of substrate causes shape change in transport protein

6 Active Transport Requires carrier proteins (solute pumps) Moves solutes against a concentration gradient Types of active transport: – Primary active transport – Secondary active transport

7 Primary Active Transport Energy from hydrolysis of ATP causes shape change in transport protein so that bound solutes (ions) are “pumped” across the membrane

8 Primary Active Transport Sodium-potassium pump (Na + -K + ATPase) – Located in all plasma membranes – Involved in primary and secondary active transport of nutrients and ions – Maintains electrochemical gradients essential for functions of muscle and nerve tissues

9 Figure 3.10 Extracellular fluid K + is released from the pump protein and Na + sites are ready to bind Na + again. The cycle repeats. Binding of Na+ promotes phosphorylation of the protein by ATP. Cytoplasmic Na + binds to pump protein. Na + Na + -K + pump K + released ATP-binding site Na + bound Cytoplasm ATP ADP P K+K+ K + binding triggers release of the phosphate. Pump protein returns to its original conformation. Phosphorylation causes the protein to change shape, expelling Na + to the outside. Extracellular K + binds to pump protein. Na + released K + bound P K+K+ P PiPi

10 Figure 3.10 step 1 Extracellular fluid Cytoplasmic Na + binds to pump protein. Na + Na + -K + pump ATP-binding site Cytoplasm K+K+ 1

11 Figure 3.10 step 2 Binding of Na + promotes phosphorylation of the protein by ATP. Na + bound ATP ADP P 2

12 Figure 3.10 step 3 Phosphorylation causes the protein to change shape, expelling Na + to the outside. Na + released P 3

13 Figure 3.10 step 4 Extracellular K + binds to pump protein. P K+K+ 4

14 Figure 3.10 step 5 K + binding triggers release of the phosphate. Pump protein returns to its original conformation. K + bound PiPi 5

15 Figure 3.10 step 6 K + is released from the pump protein and Na + sites are ready to bind Na + again. The cycle repeats. K + released 6

16 Figure 3.10 Extracellular fluid K + is released from the pump protein and Na + sites are ready to bind Na + again. The cycle repeats. Binding of Na+ promotes phosphorylation of the protein by ATP. Cytoplasmic Na + binds to pump protein. Na + Na + -K + pump K + released ATP-binding site Na + bound Cytoplasm ATP ADP P K+K+ K + binding triggers release of the phosphate. Pump protein returns to its original conformation. Phosphorylation causes the protein to change shape, expelling Na + to the outside. Extracellular K + binds to pump protein. Na + released K + bound P K+K+ P PiPi

17 Secondary Active Transport Depends on an ion gradient created by primary active transport Energy stored in ionic gradients is used indirectly to drive transport of other solutes

18 Secondary Active Transport Cotransport—always transports more than one substance at a time – Symport system: Two substances transported in same direction – Antiport system: Two substances transported in opposite directions

19 Symport carries 2 or more solutes thru the membrane simultaneously in the SAME direction Cotransport = process

20 Antiport Carriers 2 or more solutes in the OPPOSITE directions Counter transport = process

21 Figure 3.11 The ATP-driven Na + -K + pump stores energy by creating a steep concentration gradient for Na + entry into the cell. As Na + diffuses back across the membrane through a membrane cotransporter protein, it drives glucose against its concentration gradient into the cell. (ECF = extracellular fluid) Na + -glucose symport transporter loading glucose from ECF Na + -glucose symport transporter releasing glucose into the cytoplasm Glucose Na + -K + pump Cytoplasm Extracellular fluid 12

22 Figure 3.11 step 1 The ATP-driven Na + -K + pump stores energy by creating a steep concentration gradient for Na + entry into the cell. Na + -K + pump Cytoplasm Extracellular fluid 1

23 Figure 3.11 step 2 The ATP-driven Na + -K + pump stores energy by creating a steep concentration gradient for Na + entry into the cell. As Na + diffuses back across the membrane through a membrane cotransporter protein, it drives glucose against its concentration gradient into the cell. (ECF = extracellular fluid) Na + -glucose symport transporter loading glucose from ECF Na + -glucose symport transporter releasing glucose into the cytoplasm Glucose Na + -K + pump Cytoplasm Extracellular fluid 12

24 Vesicular Transport Transport of large particles, macromolecules, and fluids across plasma membranes Requires cellular energy (e.g., ATP)

25 Vesicular Transport Functions: – Exocytosis—transport out of cell – Endocytosis—transport into cell – Transcytosis—transport into, across, and then out of cell – Substance (vesicular) trafficking—transport from one area or organelle in cell to another

26 Endocytosis and Transcytosis Involve formation of protein-coated vesicles Often receptor mediated, therefore very selective

27 Types Endocytosis – vesicular process that brings matter INTO the cell Exocytosis – vesicular process that releases matter OUTSIDE the cell

28 Exocytosis Figure 3.12a

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30 Vesicular Transport Transcytosis – moving substances into, across, and then out of a cell Vesicular trafficking – moving substances from one area in the cell to another Phagocytosis – pseudopods engulf solids and bring them into the cell’s interior

31 Vesicular Transport Fluid-phase endocytosis – the plasma membrane infolds, bringing extracellular fluid and solutes into the interior of the cell Receptor-mediated endocytosis – clathrin- coated pits provide the main route for endocytosis and transcytosis Non-clathrin-coated vesicles – caveolae that are platforms for a variety of signaling molecules

32 Figure 3.12 Coated pit ingests substance. Protein- coated vesicle detaches. Coat proteins detach and are recycled to plasma membrane. Uncoated vesicle fuses with a sorting vesicle called an endosome. Transport vesicle containing membrane components moves to the plasma membrane for recycling. Fused vesicle may (a) fuse with lysosome for digestion of its contents, or (b) deliver its contents to the plasma membrane on the opposite side of the cell (transcytosis). Protein coat (typically clathrin) Extracellular fluid Plasma membrane Endosome Lysosome Transport vesicle (b) (a) Uncoated endocytic vesicle Cytoplasm

33 Figure 3.12 step 1 Coated pit ingests substance. Protein coat (typically clathrin) Extracellular fluid Plasma membrane Cytoplasm 1

34 Figure 3.12 step 2 Coated pit ingests substance. Protein- coated vesicle detaches. Protein coat (typically clathrin) Extracellular fluid Plasma membrane Cytoplasm 1 2

35 Figure 3.12 step 3 Coated pit ingests substance. Protein- coated vesicle detaches. Coat proteins detach and are recycled to plasma membrane. Protein coat (typically clathrin) Extracellular fluid Plasma membrane Cytoplasm 1 2 3

36 Figure 3.12 step 4 Coated pit ingests substance. Protein- coated vesicle detaches. Coat proteins detach and are recycled to plasma membrane. Uncoated vesicle fuses with a sorting vesicle called an endosome. Protein coat (typically clathrin) Extracellular fluid Plasma membrane Endosome Uncoated endocytic vesicle Cytoplasm

37 Figure 3.12 step 5 Coated pit ingests substance. Protein- coated vesicle detaches. Coat proteins detach and are recycled to plasma membrane. Uncoated vesicle fuses with a sorting vesicle called an endosome. Protein coat (typically clathrin) Extracellular fluid Plasma membrane Endosome Transport vesicle Uncoated endocytic vesicle Cytoplasm Transport vesicle containing membrane components moves to the plasma membrane for recycling.

38 Figure 3.12 step 6 Coated pit ingests substance. Protein- coated vesicle detaches. Coat proteins detach and are recycled to plasma membrane. Uncoated vesicle fuses with a sorting vesicle called an endosome. Fused vesicle may (a) fuse with lysosome for digestion of its contents, or (b) deliver its contents to the plasma membrane on the opposite side of the cell (transcytosis). Protein coat (typically clathrin) Extracellular fluid Plasma membrane Endosome Lysosome Transport vesicle (b) (a) Uncoated endocytic vesicle Cytoplasm Transport vesicle containing membrane components moves to the plasma membrane for recycling.

39 Endocytosis Phagocytosis—pseudopods engulf solids and bring them into cell’s interior – Macrophages and some white blood cells

40 Figure 3.13a Phagosome (a) Phagocytosis The cell engulfs a large particle by forming pro- jecting pseudopods (“false feet”) around it and en- closing it within a membrane sac called a phagosome. The phagosome is combined with a lysosome. Undigested contents remain in the vesicle (now called a residual body) or are ejected by exocytosis. Vesicle may or may not be protein- coated but has receptors capable of binding to microorganisms or solid particles.

41 Endocytosis Fluid-phase endocytosis (pinocytosis)— plasma membrane infolds, bringing extracellular fluid and solutes into interior of the cell – Nutrient absorption in the small intestine

42 Figure 3.13b Vesicle (b) Pinocytosis The cell “gulps” drops of extracellular fluid containing solutes into tiny vesicles. No receptors are used, so the process is nonspecific. Most vesicles are protein-coated.

43 Endocytosis Receptor-mediated endocytosis—clathrin- coated pits provide main route for endocytosis and transcytosis – Uptake of enzymes low-density lipoproteins, iron, and insulin

44 Figure 3.13c Vesicle Receptor recycled to plasma membrane (c) Receptor-mediated endocytosis Extracellular substances bind to specific receptor proteins in regions of coated pits, enabling the cell to ingest and concentrate specific substances (ligands) in protein-coated vesicles. Ligands may simply be released inside the cell, or combined with a lysosome to digest contents. Receptors are recycled to the plasma membrane in vesicles.

45 Exocytosis Examples: – Hormone secretion – Neurotransmitter release – Mucus secretion – Ejection of wastes

46 Summary of Active Processes Also see Table 3.2 ProcessEnergy SourceExample Primary active transportATPPumping of ions across membranes Secondary active transport Ion gradientMovement of polar or charged solutes across membranes ExocytosisATPSecretion of hormones and neurotransmitters PhagocytosisATPWhite blood cell phagocytosis PinocytosisATPAbsorption by intestinal cells Receptor-mediated endocytosis ATPHormone and cholesterol uptake

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49 Thank you

50 Membrane Potential Separation of oppositely charged particles (ions) across a membrane creates a membrane potential (potential energy measured as voltage) Resting membrane potential (RMP): Voltage measured in resting state in all cells – Ranges from –50 to –100 mV in different cells – Results from diffusion and active transport of ions (mainly K + )

51 Generation and Maintenance of RMP 1.The Na + -K + pump continuously ejects Na + from cell and carries K + back in 2.Some K + continually diffuses down its concentration gradient out of cell through K + leakage channels 3.Membrane interior becomes negative (relative to exterior) because of large anions trapped inside cell

52 Generation and Maintenance of RMP 4.Electrochemical gradient begins to attract K + back into cell 5.RMP is established at the point where the electrical gradient balances the K + concentration gradient 6.A steady state is maintained because the rate of active transport is equal to and depends on the rate of Na + diffusion into cell

53 Figure K + diffuse down their steep concentration gradient (out of the cell) via leakage channels. Loss of K + results in a negative charge on the inner plasma membrane face. K + also move into the cell because they are attracted to the negative charge established on the inner plasma membrane face. A negative membrane potential (–90 mV) is established when the movement of K + out of the cell equals K + movement into the cell. At this point, the concentration gradient promoting K + exit exactly opposes the electrical gradient for K + entry. Potassium leakage channels Protein anion (unable to follow K + through the membrane) Cytoplasm Extracellular fluid

54 Cell-Environment Interactions Involves glycoproteins and proteins of glycocalyx – Cell adhesion molecules (CAMs) – Membrane receptors

55 Roles of Cell Adhesion Molecules Anchor cells to extracellular matrix or to each other Assist in movement of cells past one another CAMs of blood vessel lining attract white blood cells to injured or infected areas Stimulate synthesis or degradation of adhesive membrane junctions Transmit intracellular signals to direct cell migration, proliferation, and specialization

56 Roles of Membrane Receptors Contact signaling—touching and recognition of cells; e.g., in normal development and immunity Chemical signaling—interaction between receptors and ligands (neurotransmitters, hormones and paracrines) to alter activity of cell proteins (e.g., enzymes or chemically gated ion channels) G protein–linked receptors—ligand binding activates a G protein, affecting an ion channel or enzyme or causing the release of an internal second messenger, such as cyclic AMP

57 Figure Ligand (1st messenger) binds to the receptor. The activated receptor binds to a G protein and activates it. Activated G protein activates (or inactivates) effector protein (e.g., an enzyme) by causing its shape to change. Extracellular fluid Intracellular fluid GDP Active 2nd messenger Activated kinase enzymes Effector protein (e.g., an enzyme) Receptor G protein Ligand Cascade of cellular responses (metabolic and structural changes) Activated effector enzymes catalyze reactions that produce 2nd messengers in the cell Second messengers activate other enzymes or ion channels Kinase enzymes transfer phosphate groups from ATP to specific proteins and activate a series of other enzymes that trigger various cell responses. Inactive 2nd messenger

58 Figure 3.16 step 1 1 Ligand (1st messenger) binds to the receptor. Extracellular fluid Intracellular fluid Receptor Ligand

59 Figure 3.16 step 2 1 Ligand (1st messenger) binds to the receptor. The activated receptor binds to a G protein and activates it. Extracellular fluid Intracellular fluid GDP Receptor G protein Ligand 2

60 Figure 3.16 step 3 1 Ligand (1st messenger) binds to the receptor. The activated receptor binds to a G protein and activates it. Activated G protein activates (or inactivates) effector protein (e.g., an enzyme) by causing its shape to change. Extracellular fluid Intracellular fluid GDP Effector protein (e.g., an enzyme) Receptor G protein Ligand 2 3

61 Figure 3.16 step 4 1 Ligand (1st messenger) binds to the receptor. The activated receptor binds to a G protein and activates it. Activated G protein activates (or inactivates) effector protein (e.g., an enzyme) by causing its shape to change. Extracellular fluid Intracellular fluid GDP Active 2nd messenger Effector protein (e.g., an enzyme) Receptor G protein Ligand Activated effector enzymes catalyze reactions that produce 2nd messengers in the cell Inactive 2nd messenger 2 3 4

62 Figure 3.16 step 5 1 Ligand (1st messenger) binds to the receptor. The activated receptor binds to a G protein and activates it. Activated G protein activates (or inactivates) effector protein (e.g., an enzyme) by causing its shape to change. Extracellular fluid Intracellular fluid GDP Active 2nd messenger Activated kinase enzymes Effector protein (e.g., an enzyme) Receptor G protein Ligand Activated effector enzymes catalyze reactions that produce 2nd messengers in the cell Second messengers activate other enzymes or ion channels Inactive 2nd messenger

63 Figure 3.16 step 6 1 Ligand (1st messenger) binds to the receptor. The activated receptor binds to a G protein and activates it. Activated G protein activates (or inactivates) effector protein (e.g., an enzyme) by causing its shape to change. Extracellular fluid Intracellular fluid GDP Active 2nd messenger Activated kinase enzymes Effector protein (e.g., an enzyme) Receptor G protein Ligand Cascade of cellular responses (metabolic and structural changes) Activated effector enzymes catalyze reactions that produce 2nd messengers in the cell Second messengers activate other enzymes or ion channels Kinase enzymes transfer phosphate groups from ATP to specific proteins and activate a series of other enzymes that trigger various cell responses. Inactive 2nd messenger


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