section 2 of chapter 3
movements into & out of cells
The cell membrane creates a barrier through which molecules must cross into and out of the
cell.
Passive movementsRequires no energy from cell
DiffusionFacilitated DiffusionOsmosisFiltration
Active MovementsRequires energy from cell
Ion pumpsEndocytosisExocytosis
DiffusionTendency of atoms or molecules in water or air to move from areas of higher concentration to lower concentration
Moving substances eventually become diffuse, or evenly distributed
Diffusion occurs because all substances are in constant motion
Diffusion of sugar In water
DiffusionSubstances move down their concentration gradient (from high to lower concentration)
Concentration gradient = difference in concentrations
Requirements for diffusion across cell membrane
2. A concentration gradient must exist across the cell membrane
1. Cell membrane must be permeable to substance.• Oxygen, Carbon Dioxide, and Steroid Hormones easily diffuse across
the cell membrane.
Oxygen enters cells & Carbon Dioxide leaves cells by diffusion
Illustrating diffusion across cell
Substances move down their concentration gradient.
Diffusion through carrier proteins within the cell membrane
Carrier proteins include ion channels and other proteins that “carry” substances across the cell membrane.
Facilitated diffusion transports ions, glucose, and some hormones across the cell membrane.
facilitated diffusion
Diffusion of water across a semipermeable membrane
Osmosis
Water freely crosses the membrane, but solutes (sugars, salts, and proteins) cannot cross the membrane.
Remember: Water follows salts!
water moves down its concentration gradient, i.e. from area of higher [water] into area of lower [water]
60% H2O40% protein
80% H2O20% protein
Higher H2O concentration
A B
Higher soluteconcentration
Membrane is impermeable to proteins
Osmotic Pressure = Pressure generated by osmosis.
H2O moved down its concentration gradient
Osmotic Pressure exerted on Cells
Intracellular fluid (fluid inside cells) = 0.9% NaCl
Extracellular Solutions (fluid outside cells) may exert osmotic pressure onto cells.
Isotonic = extracellular solution of 0.9%NaCl
Hypertonic = extracellular solution above 0.9%NaCl
Hypotonic = extracellular Solution below 0.9%NaCl
Red Blood Cell in isotonic solutionNo osmotic pressure = normal shape & size
Isotonic Solution
Extracellular [NaCl] is equal to Intracellular [NaCl].
Results in no osmotic pressure
H2O
H2O
Water moves into and out of the cell
Red Blood Cell in hypertonic solution.Water leaves the cell causing it to shrink.
Hypertonic Solution
Extracelluar [NaCl] is greater than Intracellular [NaCl]
Water moves out of cell & cell may shrink
H2O
net water movement out of the cell. Cell shrinks
Red Blood Cell in hypotonic solution.Water enters the cell causing it to swell.
Hypotonic Solution
Extracellular [NaCl] is less than Intracellular [NaCl].
Water moves into cell & the cell swells
Cell may lyse (burst)
H2O
net water movement into the cell. Cell swells and my lyse (burst)
In filtration of water and solids, gravity forces water through filter paper.
FiltrationMolecules are forced through membranesForce created by hydrostatic pressure, such as blood pressure.Commonly separates solids from liquids.
Filtration in body
Blood pressure forces water and smaller solutes through tiny openings in capillary wall.
Larger molecules, proteins, and cells remain inside the capillaries.
Active Transport
Up to 40% of a cell’s energy supply is used for active transport.
i.e. From lower concentration to higher concentration.
Movement against a concentration gradient.
Requires cellular energy (usually in the form of ATP).
Active transport uses energy to “pump” particles through a carrier protein
ATP = currency of energy for cell
Sodium/Potassium Pump: example of active transport
3 Na+ are pumped out of the cell, while 2 K+ are pumped into the cell.
The Na+/K+ pump creates a high extracellular [Na+] and a high intracellular [K+]
endocytosisCells take in larger substances by engulfing them.
The cell membrane surrounds the substance and forms a vesicle from a portion of the cell membrane.
Pinocytosis –cell takes in fluids
Phagocytosis –cell takes in solid particles
Receptor-Mediated Endocytosis
Cell takes in specific molecules
Removes substances even in very low concentrations
Reverse of Endocytosis
Cell organelles produce chemicals and proteins, and then package them in vesicles.
The vesicles fuse with the cell membrane releasing the chemicals from the cell.
Exocytosis
Example of Exocytosis: Neurotransmitters are secreted from neurons by exocytosis
Transcytosis
Combines endocytosis & exocytosis
Quickly transports substances across cell
Trancytosis transports HIV across linings of the mouth, anus, and female reproductive tract
End of Section 2, Chapter 3Transcytosis of HIV