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:
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/co
ntent/cat_040/0504002.html
3. Membrane-assisted transport
trans-face
cis-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/co
ntent/cat_040/0504003.html
End Chapter 2