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THE TRANSPORT OF SUBSTANCES ACROSS A CELL MEMBRANE
PASSIVE TRANSPORT
Processes that enable substances to move into and out of cells without an input of energy from the cell
Powered by a concentration gradient
Three types:
a) Diffusion
b) Osmosis
c) Facilitated Diffusion
PASSIVE TRANSPORT: DIFFUSION
Random movement of substances from regions of higher concentration to lower concentration
Continues until equilibrium (afterward no net change)
PASSIVE TRANSPORT: DIFFUSION
Factors affecting diffusion
- Relative concentrations
- How readily a molecule or ion crosses the membranea) Molecule size: faster for smaller moleculesb) Molecule polarity: faster for non-polar or small polar
moleculesc) Molecule/Ion charge: faster for uncharged molecules or
ions
- Temperature: faster at higher temperatures
PASSIVE TRANSPORT: OSMOSIS
The movement of WATER from higher concentration to lower concentration across a SEMI-PERMEABLE MEMBRANE
- Membrane is impermeable to the solute
- Water moves until concentrations are equal (afterward, no net change)
PASSIVE TRANSPORT: OSMOSIS
HYPERTONIC: Solution with higher concentration
HYPOTONIC: Solution with lower concentration
ISOTONIC: Solutions with equal concentrations
PASSIVE TRANSPORT: OSMOSIS
PASSIVE TRANSPORT: FACILITATED DIFFUSION
Diffusion across a membrane faciliated by a membrane protein
Two types of proteins exist
- Channel Proteins
- Carrier Proteins
PASSIVE TRANSPORT: FACILITATED DIFFUSION
CHANNEL PROTEINS
- Tubular shape composed of one or more helixes
- Can remain open or have gates that respond to a variety of signals (Ex: hormones, electric charge, presssure, light, etc)
- Generally permit ions or small polar molecules to pass
PASSIVE TRANSPORT: FACILITATED DIFFUSION
CHANNEL PROTEINS
- Exterior: amino acids with non-polar side chains
- Internal: highly specific for the molecules that can enter (Ex: sodium channels allow Na+ to enter)
PASSIVE TRANSPORT: FACILITATED DIFFUSION
CHANNEL PROTEINS
Cystic Fibrosis – defective chloride ion channel iwhich interrupts movement of water into and out of cell
- Results in thick mucus in the breathing passages and pancreas
PASSIVE TRANSPORT: FACILITATED DIFFUSION
CARRIER PROTEINS
- Bind a specific molecule and change shape to transport them across cell membrane
- Generally transport larger molecules (Ex: glucose, amino acids)
- Slower rate of diffusion than channel proteins
PASSIVE TRANSPORT: FACILITATED DIFFUSION
CARRIER PROTEINS
- Exterior: non-polar amino acids
- Interior: lined with amino acids that are specific to the molecule being carried
PASSIVE TRANSPORT: FACILITATED DIFFUSION
CARRIER PROTEINS
Cystinurea – inability for carrier proteins to remove cystine and other amino acids from urine
- Cystine crystallizes into painful stones that block urine flow
ACTIVE TRANSPORT
Active transport of substances across a cell membrane against their concentration gradient
Requires energy usually in the form of ATP
ACTIVE TRANSPORT
ATP – Adenosine triphosphate
- Derived from an adenosine nucleotide but with a triple phosphate group
- The hydrolysis of the end phosphate releases energy
ACTIVE TRANSPORT
ATP – Adenosine triphosphate
ADP – Adenosine diphosphate
AMP – Adenosine monophosphate
ACTIVE TRANSPORT
Two types of active transport:
a) Primary Active Transport
b) Secondary Active Transport
ACTIVE TRANSPORT- PRIMARY ACTIVE TRANSPORT
Cellular process using ATP directly to move molecules or ions across a cell membrane
ACTIVE TRANSPORT- PRIMARY ACTIVE TRANSPORT
SODIUM-POTASSIUM PUMP
- 3 Na+ transported out of the cell against its gradient
- 2 K+ transported into the cell against its gradient
ACTIVE TRANSPORT- PRIMARY ACTIVE TRANSPORT
SODIUM-POTASSIUM PUMP
- 3 Na+ bind the ion pump on the inside of the cell membrane
- ATP binds the ion pump and is hydrolysed to ADP + Pi
- ADP is released and Pi attaches to ion pump
- Ion pump changes shape
- Na+ released outside cell and 2 K+ bind
- Pi is released from the pump and the pump returns to its original shape releasing 2 K+ inside cell
ACTIVE TRANSPORT- PRIMARY ACTIVE TRANSPORT
SODIUM-POTASSIUM PUMP
ACTIVE TRANSPORT- SECONDARY ACTIVE TRANSPORT
Ion pumps result in:
- A difference in charge/electric potential
- A difference in concentration gradient
*** Electrochemical gradient ***
ACTIVE TRANSPORT- SECONDARY ACTIVE TRANSPORT
Use of an electrochemical gradient as a source of energy to transport molecules or ions across a cell membrane
ACTIVE TRANSPORT- SECONDARY ACTIVE TRANSPORT
HYDROGEN-SUCROSE PUMP
- H+ pumped out of the cell by a hydrogen ion pump using ATP
- Resulting electrochemical gradient powers movement of sucrose against its gradient from outside to inside of cell
ACTIVE TRANSPORT- SECONDARY ACTIVE TRANSPORT
HYDROGEN-SUCROSE PUMP
MEMBRANE-ASSISTED TRANSPORT
Used to transport large macromolecules
Requires energy from cell
Two types:
a) Endocytosis
b) Exocytosis
MEMBRANE-ASSISTED TRANSPORT - ENDOCYTOSIS
Cell engulfs material
Folds cell membrane around material and pinches off to form a vesicle inside the cell
Three types:
a) Phagocytosis
b) Pinocytosis
c) Receptor-Mediated Endocytosis
MEMBRANE-ASSISTED TRANSPORT - ENDOCYTOSIS
PHAGOCYTOSIS – cell eating
- Endocytosis with solid particles
MEMBRANE-ASSISTED TRANSPORT - ENDOCYTOSIS
PINOCYTOSIS – cell drinking
- Endocytosis with liquid particles
MEMBRANE-ASSISTED TRANSPORT - ENDOCYTOSIS
RECEPTOR-MEDIATED ENDOCYTOSIS
- Receptor proteins are found on a portion of the cell membrane (coated pit)
- Receptor proteins bind molecules and pit folds inwards to form a vesicle
- Contents are used by cell or digested by cell
- Receptor proteins are recycled into the cell membrane
MEMBRANE-ASSISTED TRANSPORT - ENDOCYTOSIS
RECEPTOR-MEDIATED ENDOCYTOSIS
MEMBRANE-ASSISTED TRANSPORT - EXOCYTOSIS
Vacuoles fuse with cell membrane to release contents outside the cell
Vesicle becomes part of cell membrane
MEMBRANE-ASSISTED TRANSPORT - EXOCYTOSIS
Ex: Plant cells – used to construct cell wall
Animal cells – releasing hormones, neurotransmitters, digestive enzymes, etc