3 Cells: The Living Units: Part B

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.

Requires carrier proteins (solute pumps) 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.

4 Two extracellular K+ bind to pump. 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. Slide 1 Extracellular fluid Na+ Na+–K+ pump K+ Na+ bound ATP-binding site Cytoplasm 1 Three cytoplasmic Na+ bind to pump protein. P K+ released 6 Pump protein binds ATP; releases K+ to the inside, and Na+ sites are ready to bind Na+ again. The cycle repeats. 2 Na+ binding promotes hydrolysis of ATP. The energy released during this reaction phosphorylates the pump. 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.

Primary active transport Figure 3.11 Secondary active transport is driven by the concentration gradient created by primary active transport. Slide 1 Extracellular fluid Glucose Na+-glucose symport transporter releases glucose into the cytoplasm Na+-glucose symport transporter loads glucose from extracellular fluid Na+-K+ pump Cytoplasm Primary active transport The ATP-driven Na+-K+ pump stores energy by creating a steep concentration gradient for Na+ entry into the cell. 1 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. 2 © 2013 Pearson Education, Inc.

Primary active transport Figure 3.11 Secondary active transport is driven by the concentration gradient created by primary active transport. Slide 2 Extracellular fluid Na+-K+ pump Cytoplasm Primary active transport The ATP-driven Na+-K+ pump stores energy by creating a steep concentration gradient for Na+ entry into the cell. 1 © 2013 Pearson Education, Inc.

Primary active transport Figure 3.11 Secondary active transport is driven by the concentration gradient created by primary active transport. Slide 3 Extracellular fluid Glucose Na+-glucose symport transporter releases glucose into the cytoplasm Na+-glucose symport transporter loads glucose from extracellular fluid Na+-K+ pump Cytoplasm Primary active transport The ATP-driven Na+-K+ pump stores energy by creating a steep concentration gradient for Na+ entry into the cell. 1 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. 2 © 2013 Pearson Education, Inc.

Vesicular Transport Transport of large particles, macromolecules, and fluids across membrane in membranous sacs called vesicles © 2013 Pearson Education, Inc.

Vesicular Transport Functions: Exocytosis—transport out of cell Endocytosis—transport into cell Phagocytosis, pinocytosis, receptor-mediated endocytosis © 2013 Pearson Education, Inc.

Figure 3.12 Events of endocytosis mediated by protein-coated pits. Slide 1 Coated pit ingests substance. 1 Extracellular fluid Plasma membrane Protein coat (typically clathrin) Cytoplasm Protein-coated vesicle deta- ches. 2 3 Coat proteins are recycled to plasma membrane. Transport vesicle Uncoated endocytic vesicle Endosome Uncoated vesicle fuses with a sorting vesicle called an endosome. 4 Transport vesicle containing 5 membrane compone -nts moves to the plasma membrane for recycling. Lysosome 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). 6 © 2013 Pearson Education, Inc.

Figure 3.12 Events of endocytosis mediated by protein-coated pits. Slide 2 Coated pit ingests substance. 1 Extracellular fluid Plasma membrane Protein coat (typically clathrin) Cytoplasm © 2013 Pearson Education, Inc.

Figure 3.12 Events of endocytosis mediated by protein-coated pits. Slide 3 Coated pit ingests substance. 1 Extracellular fluid Plasma membrane Protein coat (typically clathrin) Cytoplasm Protein-coated vesicle deta- ches. 2 © 2013 Pearson Education, Inc.

Figure 3.12 Events of endocytosis mediated by protein-coated pits. Slide 4 Coated pit ingests substance. 1 Extracellular fluid Plasma membrane Protein coat (typically clathrin) Cytoplasm Protein-coated vesicle deta- ches. 2 Coat proteins are recycled to plasma membrane. 3 © 2013 Pearson Education, Inc.

Figure 3.12 Events of endocytosis mediated by protein-coated pits. Slide 5 Coated pit ingests substance. 1 Extracellular fluid Plasma membrane Protein coat (typically clathrin) Cytoplasm Protein-coated vesicle deta- ches. 2 Coat proteins are recycled to plasma membrane. 3 Uncoated endocytic vesicle Endosome Uncoated vesicle fuses with a sorting vesicle called an endosome. 4 © 2013 Pearson Education, Inc.

Figure 3.12 Events of endocytosis mediated by protein-coated pits. Slide 6 Coated pit ingests substance. 1 Extracellular fluid Plasma membrane Protein coat (typically clathrin) Cytoplasm Protein-coated vesicle deta- ches. 2 Coat proteins are recycled to plasma membrane. 3 Transport vesicle Uncoated endocytic vesicle Endosome Uncoated vesicle fuses with a sorting vesicle called an endosome. 4 Transport vesicle containing 5 membrane compone -nts moves to the plasma membrane for recycling. © 2013 Pearson Education, Inc.

Figure 3.12 Events of endocytosis mediated by protein-coated pits. Slide 7 Coated pit ingests substance. 1 Extracellular fluid Plasma membrane Protein coat (typically clathrin) Cytoplasm Protein-coated vesicle deta- ches. 2 Coat proteins are recycled to plasma membrane. 3 Transport vesicle Uncoated endocytic vesicle Endosome Uncoated vesicle fuses with a sorting vesicle called an endosome. 4 Transport vesicle containing 5 membrane compone -nts moves to the plasma membrane for recycling. Lysosome 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). 6 © 2013 Pearson Education, Inc.

Used by macrophages and some white blood cells 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 3.13a Comparison of three types of endocytosis. 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. Receptors Phagosome © 2013 Pearson Education, Inc.

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 3.13b 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 3.13c Comparison of three types of endocytosis. 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. Vesicle © 2013 Pearson Education, Inc.

Substance enclosed in secretory vesicle Functions Exocytosis Substance enclosed in secretory vesicle Functions Hormone secretion, neurotransmitter release, mucus secretion, ejection of wastes © 2013 Pearson Education, Inc.

Fusion pore formed Cytoplasm Figure 3.14 Exocytosis. Slide 1 The process of exocytosis Plasma membrane SNARE (t-SNARE) Extracellular fluid Fusion pore formed 3 The vesicle and plasma membrane fuse and a pore opens up. Secretory vesicle Vesicle SNARE (v-SNARE) 1 The membrane- bound vesicle migrates to the plasma membrane. Molecule to be secreted Cytoplasm 4 Vesicle contents are released to the cell exterior. 2 There, proteins at the vesicle surface (v-SNAREs) bind with t-SNAREs (plasma membrane proteins). Fused v- and t-SNAREs © 2013 Pearson Education, Inc.

Photomicrograph of a secretory vesicle releasing its contents Figure 3.14b Exocytosis. Photomicrograph of a secretory vesicle releasing its contents by exocytosis (100,000x) © 2013 Pearson Education, Inc.

Table 3.2 Active Membrane Transport Processes (1 of 2) © 2013 Pearson Education, Inc.

Table 3.2 Active Membrane Transport Processes (2 of 2) © 2013 Pearson Education, Inc. 26

Extracellular fluid + + + + + + + + Figure 3.15 The key role of K+ in generating the resting membrane potential. Slide 1 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 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 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 © 2013 Pearson Education, Inc.

Extracellular fluid + + + + + + + + Figure 3.15 The key role of K+ in generating the resting membrane potential. Slide 2 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 + + + + + + + + – – – – – – – Potassium leakage channels – Protein anion (unable to follow K+ through the membrane) Cytoplasm © 2013 Pearson Education, Inc.

Extracellular fluid + + + + + + + + Figure 3.15 The key role of K+ in generating the resting membrane potential. Slide 3 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 2 K+ also move into the cell because they are attracted to the negative charge established on the inner plasma membrane face. + + + + + + + + – – – – – – – Potassium leakage channels – Protein anion (unable to follow K+ through the membrane) Cytoplasm © 2013 Pearson Education, Inc.

Extracellular fluid + + + + + + + + Figure 3.15 The key role of K+ in generating the resting membrane potential. Slide 4 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 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 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 © 2013 Pearson Education, Inc.

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. Ligand (1st messenger) Receptor G protein Enzyme 2nd messenger Ligand* (1st messeng- er) binds to the receptor. The receptor changes shape and activates. The activated receptor binds to a G protein and acti- vates it. The G protein changes shape (turns “on”), causing it to release GDP and bind GTP (an energy source). Activated G protein activates (or inactivates) an effector protein by causing its shape to change. 1 2 3 Extracellular fluid Effector protein (e.g., an enzyme) Ligand Receptor Activated effector enzymes catalyze reactions that produce 2nd messengers in the cell. (Common 2nd messengers include cyclic AMP and Ca2+.) 4 Inactive 2nd messenger G protein GDP Active 2nd messenger Second messengers activate other enzymes or ion channels. Cyclic AMP typically activates protein kinase enzymes. 5 Activated kinase enzymes Kinase enzymes activate other enzymes. Kinase enzymes transfer phosphate groups from ATP to specific proteins and activate a series of other enzymes that trigger various metabolic and structural changes in the cell. 6 Cascade of cellular responses (The amplification effect is tremendous. Each enzyme catalyzes hundreds of reactions.) * Ligands include hormones and neurotransmitters. Intracellular fluid © 2013 Pearson Education, Inc.