 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 membrane a) Molecule size: faster for smaller molecules b) Molecule polarity: faster for non-polar or small polar molecules c) 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