Movement Across the Plasma Membrane

• The cell membrane is semi-permeable

• A few molecules move freely– Water, Carbon dioxide, Ammonia, Oxygen

• Carrier proteins transport other molecules– Proteins embedded in lipid bilayer

– Fluid mosaic model – describes fluid nature of a lipid bilayer with proteins

Three Transport types:(don’t write this down)

1. Passive Transport

2. Active Transport

3. Membrane-assisted Transport– Endocytosis

(phagocytosis & pinocytosis)

– Exocytosis

1. Passive Transport

• No energy required

• Move due to gradient– differences in concentration, pressure, charge

• Move to equalize gradient– High moves toward low

Types of Passive Transport

a) Diffusion

b) Osmosis

c) Facilitated diffusion

a) Diffusion

• Molecules move to equalize concentration

Rate of diffusion- how fast diffusion occurs

Factors that affect the rate of diffusion are

a) Molecule size - bigger are slower

b) Molecule polarity - more polar are slower

c) Ion charge- more charged are slower

d) Temperature- directly proportional

e) Pressure- directly proportional

b) Osmosis

• Diffusion of water

– Into cell– Out of cell

Solution Differences & Cells

solvent + solute = solution

Solutions can be:

i. Hypotonic solution– More solutes in cell than outside– Outside solvent will flow into cell

• Draw it!

ii. Hypertonic solution– More solutes outside than inside cell– Fluid will flow out of cell

• Draw it!

iii. Isotonic solution– Solutes equal inside & out of cell

• Draw it!

c) Facilitated Diffusion

• Membranes are differentially permeable

• Still passive transport: why???

i. Specific channel proteins help molecule or ions enter or leave the cell– Hollow tubes– Ex. aquaporins

ii. Specific carrier proteins transport molecules across the cell membrane– Slower than channels (lower rate of diffusion)– Ex. Cytochromes: involved in ATP production

Process of Facilitated Transport

1. Protein binds with molecule

2. Shape of protein changes

3. Molecule moves across membrane

2. Active Transport

a) Primary Active Transport• against concentration gradient!• Requires energy (ATP)• Ex. sodium-potassium pump

• If the primary active transport involves ions it creates an electrochemical gradient– Combination of a concentration gradient and

charge– Stores potential energy

b) Secondary Active Transport• Uses the electrochemical gradient as a

source of energy• Used in uptake of amino acids and sugars• Ex. Hydrogen-sucrose pump

• http://bcs.whfreeman.com/thelifewire8e/content/cat_040/0504002.html

3. Membrane-assisted transport

cis-face

trans-face

a) Endocytosis

• Movement of large material– Particles– Organisms – Large molecules

• Movement is into cell

• Types of endocytosis– bulk-phase (nonspecific)– receptor-mediated (specific)

Process of Endocytosis

1. Plasma membrane surrounds material

2. Edges of membrane meet

3. Membranes fuse to form vesicle

Forms of Endocytosis

i) Phagocytosis – cell eating

ii) Pinocytosis – cell drinking

b) Exocytosis

• Reverse of endocytosis

• Cell discharges material

Steps

1. Vesicle moves to cell surface

2. Membrane of vesicle fuses

3. Materials expelled

• http://bcs.whfreeman.com/thelifewire8e/content/cat_040/0504003.html

End Chapter 2