A Closer Look at Cell Membranes

Chapter 5 Part 2

5.5 Membrane Trafficking

By processes of endocytosis and exocytosis, vesicles help cells take in and expel particles that are too big for transport proteins, as well as substances in bulk

Membrane trafficking• Formation and movement of vesicles formed from

membranes, involving motor proteins and ATP

Exocytosis and Endocytosis

Exocytosis• The fusion of a vesicle with the cell membrane,

releasing its contents to the surroundings

Endocytosis• The formation of a vesicle from cell membrane,

enclosing materials near the cell surface and bringing them into the cell

Endocytosis and Exocytosis

Fig. 5-12, p. 86

Endocytosis ExocytosisA Molecules get concentrated inside coated pits at the plasma membrane.

coated pit

B The pits sink inward and become endocytic vesicles.

D Many of the sorted molecules cycle to the plasma membrane .

E Some vesicles are routed to the nuclear envelope or ER membrane. Others fuse with Golgi bodies.

C Vesicle contents are sorted.

F Some vesicles and their contents are delivered to lysosomes.

lysosome Golgi

Three Pathways of Endocytosis

Bulk-phase endocytosis• Extracellular fluid is captured in a vesicle and

brought into the cell; the reverse of exocytosis

Receptor-mediated endocytosis• Specific molecules bind to surface receptors,

which are then enclosed in an endocytic vesicle

Phagocytosis• Pseudopods engulf target particle and merge as

a vesicle, which fuses with a lysosome in the cell

Receptor-Mediated Endocytosis

Fig. 5-13, p. 86

plasma membrane aggregated lipoproteins

Animation: Phagocytosis

Phagocytosis

Fig. 5-14a, p. 87

A Pseudopods surround a pathogen ( brown ).

Fig. 5-14b, p. 87

B Endocytic vesicle forms.

C Lysosome fuses with vesicle; enzymes digest pathogen.

D Cell uses the digested material or expels it.

Membrane Cycling

Exocytosis and endocytosis continually replace and withdraw patches of the plasma membrane

New membrane proteins and lipids are made in the ER, modified in Golgi bodies, and form vesicles that fuse with plasma membrane

Exocytic Vesicle

Fig. 5-12, p. 86

F Some vesicles and their contents are delivered to lysosomes.

lysosome

B The pits sink inward and become endocytic vesicles.

C Vesicle contents are sorted.

Exocytosis

D Many of the sorted molecules cycle to the plasma membrane .

E Some vesicles are routed to the nuclear envelope or ER membrane. Others fuse with Golgi bodies.

Golgi

Endocytosis

A Molecules get concentrated inside coated pits at the plasma membrane.

coated pit

Stepped Art

Animation: Membrane cycling

5.5 Key Concepts: Membrane Trafficking

Large packets of substances and engulfed cells move across the plasma membrane by processes of endocytosis and exocytosis

Membrane lipids and proteins move to and from the plasma membrane during these processes

5.6 Which Way Will Water Move?

Water diffuses across cell membranes by osmosis

Osmosis is driven by tonicity, and is countered by turgor

Osmosis

Osmosis• The movement of water down its concentration

gradient – through a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration

Tonicity• The relative concentrations of solutes in two fluids

separated by a selectively permeable membrane

Tonicity

For two fluids separated by a semipermeable membrane, the one with lower solute concentration is hypotonic, and the one with higher solute concentration is hypertonic• Water diffuses from hypotonic to hypertonic

Isotonic fluids have the same solute concentration

Osmosis

Fig. 5-16, p. 88

hypotonic solution

hypertonic solution solutions become isotonic

selectively permeable membrane

A Initially, the volume of fluid is the same in the two compartments, but the solute concentration differs.

B The fluid volume in the two compartments changes as water follows its gradient and diffuses across the membrane.

Animation: Tonicity and water movement

Experiment: Tonicity

Fig. 5-17a, p. 89

Fig. 5-17a, p. 89

2% sucrose

2% sucrose 10% sucrose water

A What happens to a semipermeable membrane bag when it is immersed in an isotonic, a hypertonic, or a hypotonic solution?

2% sucrose

A What happens to a semipermeable membrane bag when it is immersed in an isotonic, a hypertonic, or a hypotonic solution?

Fig. 5-17, p. 89

2% sucrose 10% sucrose water

B Red blood cells in an isotonic solution do not change in volume.

C Red blood cells in a hypertonic solution shrivel because water diffuses out of them.

D Red blood cells in a hypotonic solution swell because water diffuses into them. Stepped Art

Fig. 5-17 (b-d), p. 89

Fig. 5-17 (b-d), p. 89

B Red blood cells in an isotonic solution do not change in volume.

C Red blood cells in a hypertonic solution shrivel because water diffuses out of them.

D Red blood cells in a hypotonic solution swell because water diffuses into them.

Animation: Osmosis experiment

Effects of Fluid Pressure

Hydrostatic pressure (turgor)• The pressure exerted by a volume of fluid against

a surrounding structure (membrane, tube, or cell wall) which resists volume change

Osmotic pressure• The amount of hydrostatic pressure that can stop

water from diffusing into cytoplasmic fluid or other hypertonic solutions

Hydrostatic Pressure in Plants

Fig. 5-18a, p. 89

5.6 Key Concepts: Osmosis

Water tends to diffuse across selectively permeable membranes, to regions where its concentration is lower

Animation: Active transport

Animation: Endocytosis and exocytosis

Animation: Fluid mosaic model

Animation: Passive transport II

Animation: Solute concentration and osmosis

Video: One bad transporter and cystic fibrosis

Video: Diffusion of dye in water

Video: Contractile vacuole