Power Point Lecture Slides prepared by Barbara Heard

Membrane Transport: Active Processes require ATP to move solutes across a living plasma membrane because – Solute too large for channels – Solute not lipid soluble – Solute not able to move down concentration gradient © 2013 Pearson Education, Inc.

Active Transport • Requires carrier proteins (solute pumps) – Bind specifically and reversibly with substance • Moves solutes against concentration gradient ( low to HIGH) – Requires energy © 2013 Pearson Education, Inc.

Figure 3. 10 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. Extracellular fluid Na+–K+ pump K+ ATP-binding site Na+ bound Cytoplasm 1 Three cytoplasmic Na + bind to pump protein. P K+ released 2 Na+ binding promotes hydrolysis of ATP. The energy released during this reaction phosphorylates the pump. 6 Pump protein binds ATP; releases K + to the inside, and Na + sites are ready to bind Na+ again. The cycle repeats. Na+ released K+ bound P Pi K+ 5 K+ binding triggers release of the phosphate. The dephosphorylated pump resumes its original conformation. 3 Phosphorylation causes the pump to change shape, expelling Na+ to the outside. P 4 Two extracellular K + bind to pump. © 2013 Pearson Education, Inc. Slide 1

Figure 3. 11 Secondary active transport is driven by the concentration gradient created by primary active transport. Extracellular fluid Slide 1 Glucose Na+-K+ pump Na+-glucose symport transporter loads glucose from extracellular fluid Na+-glucose symport transporter releases glucose into the cytoplasm Cytoplasm 1 Primary active transport The ATP-driven Na+-K+ pump stores energy by creating a steep concentration gradient for Na+ entry into the cell. © 2013 Pearson Education, Inc. 2 Secondary active transport As Na+ diffuses back across the membrane through a membrane cotransporter protein, it drives glucose against its concentration gradient into the cell.

Figure 3. 11 Secondary active transport is driven by the concentration gradient created by primary active transport. Extracellular fluid Na+-K+ pump Cytoplasm 1 Primary active transport The ATP-driven Na+-K+ pump stores energy by creating a steep concentration gradient for Na+ entry into the cell. © 2013 Pearson Education, Inc. Slide 2

Figure 3. 11 Secondary active transport is driven by the concentration gradient created by primary active transport. Extracellular fluid Slide 3 Glucose Na+-K+ pump Na+-glucose symport transporter loads glucose from extracellular fluid Na+-glucose symport transporter releases glucose into the cytoplasm Cytoplasm 1 Primary active transport The ATP-driven Na+-K+ pump stores energy by creating a steep concentration gradient for Na+ entry into the cell. © 2013 Pearson Education, Inc. 2 Secondary active transport As Na+ diffuses back across the membrane through a membrane cotransporter protein, it drives glucose against its concentration gradient into the cell.

Vesicular Transport • Transport of large particles, macromolecules, and fluids across membrane in membranous

Vesicular Transport • Functions: – Exocytosis—transport out of cell – Endocytosis—transport into cell •

Figure 3. 12 Events of endocytosis mediated by protein-coated pits. 1 Coated pit ingests substance. Protein coat (typically clathrin) 2 Protein-coated vesicle detaches. Extracellular fluid Plasma membrane Cytoplasm 3 Coat proteins are recycled to plasma membrane. Transport vesicle Uncoated endocytic vesicle Endosome 4 Uncoated vesicle fuses with a sorting vesicle called an endosome. Lysosome 5 Transport vesicle containing membrane compone -nts moves to the plasma membrane for recycling. 6 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). © 2013 Pearson Education, Inc. Slide 1

Figure 3. 12 Events of endocytosis mediated by protein-coated pits. 1 Coated pit ingests substance. Protein coat (typically clathrin) © 2013 Pearson Education, Inc. Extracellular fluid Plasma membrane Cytoplasm Slide 2

Figure 3. 12 Events of endocytosis mediated by protein-coated pits. 1 Coated pit ingests substance. Protein coat (typically clathrin) 2 Protein-coated vesicle detaches. © 2013 Pearson Education, Inc. Extracellular fluid Plasma membrane Cytoplasm Slide 3

Figure 3. 12 Events of endocytosis mediated by protein-coated pits. 1 Coated pit ingests substance. Protein coat (typically clathrin) 2 Protein-coated vesicle detaches. © 2013 Pearson Education, Inc. Extracellular fluid Plasma membrane Cytoplasm 3 Coat proteins are recycled to plasma membrane. Slide 4

Figure 3. 12 Events of endocytosis mediated by protein-coated pits. 1 Coated pit ingests substance. Protein coat (typically clathrin) 2 Protein-coated vesicle detaches. Uncoated endocytic vesicle 4 Uncoated vesicle fuses with a sorting vesicle called an endosome. © 2013 Pearson Education, Inc. Extracellular fluid Plasma membrane Cytoplasm 3 Coat proteins are recycled to plasma membrane. Endosome Slide 5

Figure 3. 12 Events of endocytosis mediated by protein-coated pits. 1 Coated pit ingests substance. Protein coat (typically clathrin) 2 Protein-coated vesicle detaches. Extracellular fluid Plasma membrane Cytoplasm 3 Coat proteins are recycled to plasma membrane. Transport vesicle Uncoated endocytic vesicle 4 Uncoated vesicle fuses with a sorting vesicle called an endosome. © 2013 Pearson Education, Inc. Endosome 5 Transport vesicle containing membrane compone -nts moves to the plasma membrane for recycling. Slide 6

Figure 3. 12 Events of endocytosis mediated by protein-coated pits. 1 Coated pit ingests substance. Protein coat (typically clathrin) 2 Protein-coated vesicle detaches. Extracellular fluid Plasma membrane Cytoplasm 3 Coat proteins are recycled to plasma membrane. Transport vesicle Uncoated endocytic vesicle Endosome 4 Uncoated vesicle fuses with a sorting vesicle called an endosome. Lysosome 5 Transport vesicle containing membrane compone -nts moves to the plasma membrane for recycling. 6 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). © 2013 Pearson Education, Inc. Slide 7

Endocytosis • Phagocytosis – Pseudopods engulf solids and bring them into cell's interior – Form vesicle called phagosome • Used by macrophages and some white blood cells – Move by amoeboid motion • Cytoplasm flows into temporary extensions • Allows creeping © 2013 Pearson Education, Inc.

Figure 3. 13 a Comparison of three types of endocytosis. Receptors Phagosome © 2013 Pearson Education, Inc. Phagocytosis The cell engulfs a large particle by forming projecting pseudopods ("false feet") around it and enclosing 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.

Endocytosis • Pinocytosis (fluid-phase endocytosis) – Plasma membrane infolds, bringing extracellular fluid and dissolved solutes inside cell • Fuses with endosome – Most cells utilize to "sample" environment – Nutrient absorption in the small intestine – Membrane components recycled back to membrane © 2013 Pearson Education, Inc.

Figure 3. 13 b Comparison of three types of endocytosis. Pinocytosis The cell "gulps" a drop of extracellular fluid containing solutes into tiny vesicles. No receptors are used, so the process is nonspecific. Most vesicles are protein-coated. Vesicle © 2013 Pearson Education, Inc.

Figure 3. 13 c Comparison of three types of endocytosis. Vesicle © 2013 Pearson Education, Inc. Receptor-mediated endocytosis Extracellular substances bind to specific receptor proteins, 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.

Exocytosis • Substance enclosed in secretory vesicle • Functions – Hormone secretion, neurotransmitter release,

Figure 3. 14 Exocytosis. Slide 1 The process of exocytosis Plasma membrane Extracellular SNARE (t-SNARE) fluid Secretory vesicle Vesicle SNARE (v-SNARE) Molecule to be secreted Cytoplasm Fused v- and t-SNAREs © 2013 Pearson Education, Inc. Fusion pore formed 1 The membranebound vesicle migrates to the plasma membrane. 2 There, proteins at the vesicle surface (v -SNAREs) bind with t -SNAREs (plasma membrane proteins). 3 The vesicle and plasma membrane fuse and a pore opens up. 4 Vesicle contents are released to the cell exterior.

Figure 3. 14 b Exocytosis. Photomicrograph of a secretory vesicle releasing its contents by

Table 3. 2 Active Membrane Transport Processes (1 of 2) © 2013 Pearson Education,

Table 3. 2 Active Membrane Transport Processes (2 of 2) © 2013 Pearson Education,

Figure 3. 15 The key role of K+ in generating the resting membrane potential. 1 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. Extracellular fluid + + – Cytoplasm © 2013 Pearson Education, Inc. – Slide 1 – + – Potassium leakage channels + – + – Protein anion (unable to follow K+ through the membrane) 2 K+ also move into the cell because they are attracted to the negative charge established on the inner plasma membrane face. 3 A negative membrane potential (– 90 m. V) 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.

Figure 3. 15 The key role of K+ in generating the resting membrane potential. 1 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. Extracellular fluid + + – Cytoplasm © 2013 Pearson Education, Inc. – Slide 2 – + – Potassium leakage channels + – + – Protein anion (unable to follow K+ through the membrane)

Figure 3. 15 The key role of K+ in generating the resting membrane potential. 1 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. Extracellular fluid + + – Cytoplasm © 2013 Pearson Education, Inc. – Slide 3 – + – Potassium leakage channels + – + – Protein anion (unable to follow K+ through the membrane) 2 K+ also move into the cell because they are attracted to the negative charge established on the inner plasma membrane face.

Figure 3. 15 The key role of K+ in generating the resting membrane potential. 1 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. Extracellular fluid + + – Cytoplasm © 2013 Pearson Education, Inc. – Slide 4 – + – Potassium leakage channels + – + – Protein anion (unable to follow K+ through the membrane) 2 K+ also move into the cell because they are attracted to the negative charge established on the inner plasma membrane face. 3 A negative membrane potential (– 90 m. V) 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.

Figure 3. 16 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell. Slide 1 The sequence described here is like a molecular relay race. Instead of a baton passed from runner to runner, the message (a shape change) is passed from molecule to molecule as it makes its way across the cell membrane from outside to inside the cell. 1 Ligand* (1 st messenger) binds to the receptor. The receptor changes shape and activates. Ligand (1 st Receptor G protein Enzyme messenger) 2 The activated receptor binds to a G protein and activates it. The G protein changes shape (turns “on”), causing it to release GDP and bind GTP (an energy source). 2 nd messenger 3 Activated G protein activates (or inactivates) an effector protein by causing its shape to change. Extracellular fluid Effector protein (e. g. , an enzyme) Ligand Receptor G protein GDP Inactive 2 nd messenger Activated kinase enzymes * Ligands include hormones and neurotransmitters. © 2013 Pearson Education, Inc. 4 Activated effector enzymes catalyze reactions that produce 2 nd messengers in the cell. (Common 2 nd messengers include cyclic AMP and Ca 2+. ) 5 Second messengers activate other enzymes or ion channels. Cyclic AMP typically activates protein kinase enzymes. 6 Kinase enzymes activate other enzymes. Kinase enzymes transfer phosphate groups from ATP to specific proteins and activate a Cascade of cellular responses series of other enzymes that trigger (The amplification effect is various metabolic and structural tremendous. Each enzyme changes in the cell. catalyzes hundreds of reactions. ) Intracellular fluid