Carrier-Mediated Transport

• Number of molecules transported – Uniport carriers – Symport carriers – Antiport carriers • Specificity • Competition • Saturation – Transport maximum

© 2016 Pearson Education, Inc. Figure 5.10b Membrane Transporters

© 2016 Pearson Education, Inc. Carrier-Mediated Transport

• Facilitated diffusion uses carrier proteins – No energy input, down concentration gradient – Conformational change • Active transport uses carrier proteins – Energy input, against concentration gradient • Primary (direct) transport uses ATP • Secondary (indirect) transport use potential energy of another molecule – Competition and saturation

© 2016 Pearson Education, Inc. Figure 5.12a Carrier proteins

Carrier proteins, like the canal illustrated, never form a continuous passageway between the extracellular and intracellular fluids.

Closed gate

Pacific Atlantic Passage Ocean Ocean open to one side

Transition Pacific Atlantic state with Ocean Ocean both gates closed

Passage Pacific Atlantic open to Ocean Ocean other side

© 2016 Pearson Education, Inc. Figure 5.12b Carrier proteins

The ligand binding sites change affinity when the protein conformation changes.

Extracellular fluid Intracellular fluid

Gate closed Molecule to be Carrier transported Membrane

Gate closed

© 2016 Pearson Education, Inc. Figure 5.13 Facilitated diffusion of glucose into cells

© 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Figure 5.14 The sodium-potassium pump, Na+-K+-ATPase

© 2016 Pearson Education, Inc. Figure 5.15 Mechanism of the Na+-K+-ATPase

ECF

[Na+] high ATP

ADP + energy

High-affinity binding sites for Na+ appear. 3 Na+ from ICF bind to high-affinity sites. [Na+] low ICF

K-binding sites P lose their affinity ATPase is + for K and release phosphorylated 2 K+ into ICF. with Pi from ATP.

Protein changes Protein changes P conformation. conformation. Pi released. 2 K+ from ECF bind Na-binding sites lose their affinity + + to high-affinity sites. for Na and release 3 Na into ECF.

[K+] low

High-affinity P P binding sites for K+ appear. [K+] high

© 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Figure 5.16 Sodium-glucose cotransport

+ Na binds to carrier. Intracellular fluid

Lumen of intestine or kidney

Na+ [Na+] high SGLT protein

Glu [glucose] low [Na+] low [glucose] high

+ Na binding creates Na+ a high-affinity site for glucose.

Glu

Lumen ICF

Glucose binding changes carrier conformation so Na+ that binding sites now face the ICF.

Lumen ICF

Na+ is released into cytosol, where [Na+] is low. Release changes glucose-binding site to low affinity. Glucose Na+ is released. [Na+] low

[glucose] high

Lumen ICF

© 2016 Pearson Education, Inc. Figure 5.17a Transporter saturation and competition

The GLUT transporter brings glucose across cell membranes.

Extracellular fluid

Glucose

GLUT transporter Intracellular fluid

© 2016 Pearson Education, Inc. Figure 5.17b Transporter saturation and competition Maltose is a competitive inhibitor that binds to the GLUT transporter but is not itself carried across the membrane.

Glucose

Maltose

© 2016 Pearson Education, Inc. Figure 5.17c Transporter saturation and competition

Saturation. This graph shows that transport can reach a maximum rate when all the carrier binding sites are filled with substrate.

Transport maximum

cell

e of e transport into Rat Extracellular substrate concentration

Transport rate is proportional to substrate concentration until the carriers are saturated.

© 2016 Pearson Education, Inc. Figure 5.17d Transporter saturation and competition

Competition. This graph shows glucose transport rate as a function of glucose concentration. In one experiment, only glucose was present. In the second experiment, a constant concentration of galactose was present.

Glucose only rate Glucose and galactose

(1 mM)

transport Glucose

5 10 15 Glucose concentration (mM)

© 2016 Pearson Education, Inc. Vesicular Transport

• Phagocytosis – Vesicles created by the cytoskeleton – Cell engulfs bacterium or other particle into phagosome • Endocytosis – Membrane surface indents and forms vesicles – Active process that can be nonselective (pinocytosis) or highly selective – Receptor-mediated endocytosis uses coated pits • Clathrin most common protein in coated pits – Membrane recycling

© 2016 Pearson Education, Inc. Vesicular Transport

• Caveolae – No clathrin • Exocytosis

© 2016 Pearson Education, Inc. Figure 5.18 Phagocytosis Slide 1

Bacterium

The phagosome containing Phagocyte the bacterium separates Lysosome from the cell membrane and moves into the cytoplasm.

The phagocytic white blood cell encounters a bacterium The phagosome fuses with that binds to the cell lysosomes containing membrane. digestive enzymes.

The phagocyte uses its The bacterium is killed cytoskeleton to push its and digested within the cell membrane around the vesicle. bacterium, creating a large vesicle, the phagosome.

© 2016 Pearson Education, Inc. Figure 5.18 Phagocytosis Slide 2

Bacterium

Phagocyte Lysosome

The phagocytic white blood cell encounters a bacterium that binds to the cell membrane.

© 2016 Pearson Education, Inc. Figure 5.18 Phagocytosis Slide 3

Bacterium

Phagocyte Lysosome

The phagocytic white blood cell encounters a bacterium that binds to the cell membrane.

The phagocyte uses its cytoskeleton to push its cell membrane around the bacterium, creating a large vesicle, the phagosome.

© 2016 Pearson Education, Inc. Figure 5.18 Phagocytosis Slide 4

Bacterium

The phagosome containing Phagocyte the bacterium separates Lysosome from the cell membrane and moves into the cytoplasm.

The phagocytic white blood cell encounters a bacterium that binds to the cell membrane.

The phagocyte uses its cytoskeleton to push its cell membrane around the bacterium, creating a large vesicle, the phagosome.

© 2016 Pearson Education, Inc. Figure 5.18 Phagocytosis Slide 5

Bacterium

The phagosome containing Phagocyte the bacterium separates Lysosome from the cell membrane and moves into the cytoplasm.

The phagocytic white blood cell encounters a bacterium The phagosome fuses with that binds to the cell lysosomes containing membrane. digestive enzymes.

The phagocyte uses its cytoskeleton to push its cell membrane around the bacterium, creating a large vesicle, the phagosome.

© 2016 Pearson Education, Inc. Figure 5.18 Phagocytosis Slide 6

Bacterium

The phagosome containing Phagocyte the bacterium separates Lysosome from the cell membrane and moves into the cytoplasm.

The phagocytic white blood cell encounters a bacterium The phagosome fuses with that binds to the cell lysosomes containing membrane. digestive enzymes.

The phagocyte uses its The bacterium is killed cytoskeleton to push its and digested within the cell membrane around the vesicle. bacterium, creating a large vesicle, the phagosome.

© 2016 Pearson Education, Inc. Figure 5.19 Endocytosis, Exocytosis, and Membrane Recycling

Extracellular fluid Ligand binds to membrane receptor.

Exocytosis Receptor-ligand migrates to clathrin-coated pit.

Transport vesicle and cell membrane Clathrin-coated Endocytosis fuse (membrane pit recycling).

Receptor

Clathrin

Vesicle loses Transport vesicle clathrin coat. with receptors moves to the cell membrane.

Receptors and ligands To lysosome or separate. Golgi complex

Endosome Ligands go to lysosomes or Golgi for processing. Intracellular fluid

© 2016 Pearson Education, Inc. Figure 5.19 Endocytosis, Exocytosis, and Membrane Recycling

Extracellular fluid Ligand binds to membrane receptor.

Receptor

Intracellular fluid

© 2016 Pearson Education, Inc. Figure 5.19 Endocytosis, Exocytosis, and Membrane Recycling

Extracellular fluid Ligand binds to membrane receptor.

Receptor-ligand migrates to clathrin-coated pit.

Clathrin-coated pit

Receptor

Clathrin

Intracellular fluid

© 2016 Pearson Education, Inc. Figure 5.19 Endocytosis, Exocytosis, and Membrane Recycling

Extracellular fluid Ligand binds to membrane receptor.

Receptor-ligand migrates to clathrin-coated pit.

Clathrin-coated Endocytosis pit

Receptor

Clathrin

Intracellular fluid

© 2016 Pearson Education, Inc. Figure 5.19 Endocytosis, Exocytosis, and Membrane Recycling

Extracellular fluid Ligand binds to membrane receptor.

Receptor-ligand migrates to clathrin-coated pit.

Clathrin-coated Endocytosis pit

Receptor

Clathrin

Vesicle loses clathrin coat.

Intracellular fluid

© 2016 Pearson Education, Inc. Figure 5.19 Endocytosis, Exocytosis, and Membrane Recycling

Extracellular fluid Ligand binds to membrane receptor.

Receptor-ligand migrates to clathrin-coated pit.

Clathrin-coated Endocytosis pit

Receptor

Clathrin

Vesicle loses clathrin coat.

Receptors and ligands separate.

Endosome

Intracellular fluid

© 2016 Pearson Education, Inc. Figure 5.19 Endocytosis, Exocytosis, and Membrane Recycling

Extracellular fluid Ligand binds to membrane receptor.

Receptor-ligand migrates to clathrin-coated pit.

Clathrin-coated Endocytosis pit

Receptor

Clathrin

Vesicle loses clathrin coat.

Receptors and ligands To lysosome or separate. Golgi complex

Endosome Ligands go to lysosomes or Golgi for processing. Intracellular fluid

© 2016 Pearson Education, Inc. Figure 5.19 Endocytosis, Exocytosis, and Membrane Recycling

Extracellular fluid Ligand binds to membrane receptor.

Receptor-ligand migrates to clathrin-coated pit.

Clathrin-coated Endocytosis pit

Receptor

Clathrin

Vesicle loses Transport vesicle clathrin coat. with receptors moves to the cell membrane.

Receptors and ligands To lysosome or separate. Golgi complex

Endosome Ligands go to lysosomes or Golgi for processing. Intracellular fluid

© 2016 Pearson Education, Inc. Figure 5.19 Endocytosis, Exocytosis, and Membrane Recycling

Extracellular fluid Ligand binds to membrane receptor.

Receptor-ligand migrates to clathrin-coated pit.

Transport vesicle and cell membrane Clathrin-coated Endocytosis fuse (membrane pit recycling).

Receptor

Clathrin

Vesicle loses Transport vesicle clathrin coat. with receptors moves to the cell membrane.

Receptors and ligands To lysosome or separate. Golgi complex

Endosome Ligands go to lysosomes or Golgi for processing. Intracellular fluid

© 2016 Pearson Education, Inc. Figure 5.19 Endocytosis, Exocytosis, and Membrane Recycling

Extracellular fluid Ligand binds to membrane receptor.

Exocytosis Receptor-ligand migrates to clathrin-coated pit.

Transport vesicle and cell membrane Clathrin-coated Endocytosis fuse (membrane pit recycling).

Receptor

Clathrin

Vesicle loses Transport vesicle clathrin coat. with receptors moves to the cell membrane.

Receptors and ligands To lysosome or separate. Golgi complex

Endosome Ligands go to lysosomes or Golgi for processing. Intracellular fluid

© 2016 Pearson Education, Inc. Epithelial Transport

• Apical (mucosal) membrane • Basal membrane • Paracellular transport – Through junctions between adjacent cells • Transcellular transport – Through cells themselves – Transcytosis with vesicular transport

© 2016 Pearson Education, Inc. Epithelial Transport

• Absorption from lumen to extracellular fluid (ECF) • Secretion from ECF to lumen • Transcellular transport of glucose uses membrane proteins

© 2016 Pearson Education, Inc. Figure 5.20 Transporting epithelia are polarized

Lumen of intestine or kidney

Apical membrane with microvilli Secretion faces the lumen. Transporting epithelial cell limits Tight junction Absorption (transcellular) movement of substances between the cells.

Transport proteins Absorption (paracellular) Basolateral membrane faces the ECF.

Extracellular fluid

© 2016 Pearson Education, Inc. Figure 5.21 Transepithelial absorption of glucose

+ [Glucose]low Glu Na [Na+] Lumen of kidney high + or intestine Na -glucose symporter brings glucose into cell against its gradient using energy stored in the Na+ concentration gradient.

Apical membrane GLUT transporter transfers glucose to ECF by facilitated diffusion.

+ + [Glucose]high Glu Na [Na ]low

Na+-K+-ATPase pumps Epithelial Na+ out of the cell, keeping cell ICF Na+ concentration low.

Basolateral FIGURE QUESTIONS membrane Glu Na+ K+ 1.Match each transporter to its location. Choose either 1. GLUT (a) apical membrane 2. Na+-glucose symporter (b) basolateral + + ATP 3. Na -K -ATPase membrane 2. Is glucose movement across the Extracellular basolateral membrane active or fluid passive? Explain. 3. Why doesn't Na+ movement at the apical [Glucose] Glu + Na+ + low [Na ]high K membrane require ATP?

© 2016 Pearson Education, Inc.