Lecture 5

Lecture 5: Movement of material across cell

membranesCovers 5.2

• Now that we have learned about cell theory and the structure of cells, lets talk about how material moves across the membrane.

• Material moves in response to gradients and concentrations

• If there is more of molecule A on the inside of the cell than on the outside, molecule A will want to move to an area of lesser concentration (molecule A will want to move OUTSIDE)

• If there is more of molecule A on the outside of the cell, molecule A will want to move INSIDE

• Molecules always want to move from an area of high concentration to low concentration

Two types of transport*

• Passive transport: movement from area of high concentration to lower concentration– 3 types of passive transport:

• Simple diffusion• Facilitated diffusion• Osmosis

• Energy-Requiring Transport: sometimes it is necessary for molecules to move from an area of low concentration to an area of high concentration. Because this is against the gradient, energy is required to make this happen– 3 types:

• Active transport• Endocytosis• Exocytosis

Passive transport*

• Simple Diffusion: substances move DOWN their concentration gradient (high to low concentration) THROUGH THE PHOSPHOLIPID BILAYER (water, O2, CO2, and lipid soluble material like ETOH and Vitamins A, D, E)

• Facilitated Diffusion: substances move DOWN their concentration gradient THROUGH PROTEIN CHANNELS (ions, sugars, amino acids)

• Osmosis: the movement of WATER ONLY THROUGH PROTEIN CHANNELS

Fig. 5-7

water glucose

carrierprotein

aquaporinchannelprotein

phospho-lipidbilayer

(cytoplasm)

(extracellularfluid)

Cl–

O2

(a) Simple diffusion throughthe phospholipid bilayer

(b) Facilitated diffusionthrough channel proteins

(c) Osmosis throughaquaporins or thephospholipid bilayer

(d) Facilitated diffusion through carrier proteins

Types of Diffusion Through the Plasma Membrane

Osmosis affects solute concentration

• Solutes are the solid particles inside a cell• Water moving in and out of cell through osmosis

can make solutes more concentrated (if water moves out) or less concentrated (if water moves in)

• Movement can also depend on solution the cell is bathed in (isotonic/hypotonic/hypertonic)

• Ex: Red Blood Cell

Fig. 5-8

No net flowof water

Water flows out;the balloon shrinks

Water flows in;the balloon expands

(a) A balloon in anisotonic solution

(b) A balloon in ahypertonic solution

(c) A balloon in ahypotonic solution

Fig. 5-9

Energy-Requiring Transport*: Active Transport

• substances move AGAINST concentration gradient (low concentration to high concentration) THROUGH PROTEIN CHANNELS.

• ATP IS REQUIRED. (more about ATP later, but recall it is a molecule with 3 phosphates attached to it. There is energy in the phosphate bonds that can be used for work in the body.)

Active Transport

Fig. 5-11

The transportprotein binds bothATP and Ca2

Energy from ATPchanges the shape of thetransport protein and movesthe ion across the membrane

The protein releases the ion andthe remnants of ATP(ADP and P) and closes

ATPbindingsite

recognitionsite

ATP

P

ADP

Ca2

(extracellular fluid)

(cytoplasm)

ATP

1 2 3

Endocytosis• Cells can engulf and bring particles or fluids into the cell through

endocytosis.

• Plasma membrane will “dimple” inward and create a vesicle (small space encircled by plasma membrane) containing the material to be brought into cell.

• Then vesicle will break apart once inside cell and “drop off” the contents.

• Three types:

– Pinocytosis: movement of FLUIDS into cell

– Receptor-Mediated Endocytosis: receptor proteins on cell surface recognize particles that need to be brought into cell and then that material is brought into cell by creation of a vesicle

– Phagocytosis: movement of LARGE PARTICLES into cells (in humans, WBC’s can do this)

Pinocytosis

Fig. 5-12

(extracellular fluid)

(cytoplasm)

vesicle containingextracellularfluid

(a) Pinocytosis

(b) TEM of pinocytosis

A dimple forms in the plasma membrane, which deepens and surrounds the extracellular fluid. The membrane encloses the extracellular fluid, forming a vesicle.

extracellular fluid

cytoplasm

1

2

3

1 23

1

23

proteincoating

coated pit

coated vesicleextracellular particlesbound to receptors

plasma membrane

(extracellular fluid)

(cytoplasm)

0.1 micrometer

(cytoplasm)

nutrient molecule (extracellular fluid)

receptor

coated vesicle

coated pit

(a) Receptor-mediated endocytosis

Receptor proteins for specific moleculesor complexes of molecules are localized atcoated pit sites.

A vesicle ("coated vesicle") containingthe bound molecules is released into thecytoplasm.

The coated pit region of the membraneencloses the receptor-bound molecules.

The receptors bind the molecules andthe membrane dimples inward.

(b) TEM of receptor-mediated endocytosis

1

2

3

4

1

2

34

1

2 34

Fig. 5-13

Phagocytosis

Fig. 5-14

food particlepseudopods

food vacuole

(a) Phagocytosis

(b) An Amoeba engulfs aParamecium

(c) A white blood cell ingestsbacteria

(extracellular fluid)

(cytoplasm)

The plasma membrane extends pseudopods toward an extracellular particle (for example, food). The ends of thepseudopods fuse, encircling the particle. A vesicle calleda food vacuole is formed containing the engulfed particle.

123

1 2

3

123

Exocytosis

• Cells can dispose of waste AND send molecules out of cell via exocytosis

• A membrane enclosed vesicle carrying the material to be expelled moves toward the plasma membrane.

• Vesicle binds to plasma membrane and releases material to the outside of the cell.

Exocytosis

Fig. 5-15