Passive movements •Requires no energy from cell •Diffusion •Facilitated Diffusion •Osmosis •Filtration
Active Movements •Requires energy from cell •Ion pumps •Endocytosis •Exocytosis
Diffusion • Tendency 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 • Substances move down their concentration gradient (from high to lower concentration) • Concentration gradient = difference in concentrations
Diffusion of sugar In water
Requirements for diffusion across cell membrane 1. Cell membrane must be permeable to substance. •
Oxygen, Carbon Dioxide, and Steroid Hormones easily diffuse across the cell membrane.
2. A concentration gradient must exist across the cell membrane
Oxygen enters cells & Carbon Dioxide leaves cells by diffusion
Facilitated Diffusion •Diffusion that uses membrane proteins, called carrier proteins. •Carrier proteins include ion channels and other proteins that “carry” substances across the cell membrane. •Transports ions, glucose, and some hormones across the cell membrane. •Similar to diffusion in that substances only move down their concentration gradient.
The number of carrier proteins limits the rate of facilitated diffusion.
Example of facilitated diffusion transporting substances into a cell.
Osmosis • Water diffuses across the cell membrane •Movement of water from areas of higher water concentrations to areas of lower water concentrations. • Rule of thumb: Water follows salt.
Osmosis
• Aquaporins (channel proteins) allow water to freely diffuse across the cell membrane. • Solutes such as salts, proteins, sugars are too large to cross membrane. • H2O moves into areas containing higher solute concentrations.
Membrane is impermeable to proteins
A
B
60% H2O 40% protein
80% H2O 20% protein
Higher solute concentration
Higher H2O concentration
Osmotic Pressure = Pressure generated by osmosis.
Osmotic Pressure
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. • Extracellular solution of 0.9%NaCl = Isotonic • Extracellular Solution above 0.9%NaCl = Hypertonic • Extracellular Solution below 0.9%NaCl = Hypotonic
Isotonic Solution
• Extracellular [NaCl] is equal to Intracellular [NaCl]. • Results in no osmotic pressure
Red Blood Cell in isotonic solution No osmotic pressure = normal shape & size
Hypertonic Solution
• Extracelluar [NaCl] is greater than Intracellular [NaCl] • Water moves out of cell & cell may shrink
Red Blood Cell in hypertonic solution. Water leaves the cell causing it to shrink.
Hypotonic Solution • Extracellular [NaCl] is less than Intracellular [NaCl]. • Water moves into cell & the cell swells • Cell may lyse (burst)
Red Blood Cell in hypotonic solution. Water enters the cell causing it to swell.
Filtration • Molecules are forced through membranes • Force created by hydrostatic pressure, such as blood pressure. • Commonly separates solids from liquids.
In filtration of water and solids, gravity forces water through filter paper.
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
• Movement against a concentration gradient. • From lower concentration to higher concentration. • Cell must use energy from cellular metabolism (usually in the form of ATP).
• Up to 40% of a cell’s energy supply is used for active transport.
Active Transport • Active transport requires carrier proteins.
Sodium/Potassium Pump: Active Transport •Pumps Na+ out of the cell •Creates a high extracellular [Na+] •Pumps K+ into the cell •Creates a high intracellular [K+]
ATP = currency of energy for cell
Endocytosis •Cell engulfs larger substances. •Cell membrane invaginates and surrounds a substance. •Forms a vesicle from a portion of the cell membrane.
Endocytosis Examples • Pinocytosis = endocytosis of a fluid
Endocytosis Examples • Phagocytosis = endocytosis of a solid
Endocytosis Examples • Receptor-Mediated Endocytosis
Moves specific types of molecules into the cell. Specific target molecules are called ligands.
Receptor-Mediated Endocytosis • Think of the receptor as a lock, and only a ligand “key” with the correct shape can bind to it. Ligand = Key Receptor = Lock
• The ligand fits onto a receptor and forms a receptor-ligand complex.
• The receptor-ligand complex is engulfed by cell
Receptor-Mediated Endocytosis • Provides specificity • Removes substances even in very low concentrations
Exocytosis
•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
Trancytosis of HIV
End of Section 2, Chapter 3