Lecture 7- Membrane transport Flashcards
What is selective permeability?
Membranes allow some substances but not others to pass through them
What two types of processes do substances cross biological membranes by?
Active transport
Passive transport
What are the two types of passive transport?
Simple diffusion
Facilitated diffusion
What is diffusion?
The process of random movement towards a state of equilibrium
What is a solution said to be at when there is no net change in distribution?
Equilibrium
Define diffusion
The net movement from regions of greater concentration to regions of lesser concentration
What 4 factors does the rate of diffusion depend on?
- Diameter of molecules/ions
- Temperature
- Electric charge
- Concentration gradient
How long does it take solutes to diffuse across an organelle?
1 milisecond
Over what distances is diffusion less useful?
1cm- one hour to diffuse
-Not adequate over the length of the human body
What is a membrane called when it allows solutes across easily?
Permeable
Explain an experiment that tests the hypothesis that diffusion leads to a uniform distribution of solutes.
Add equal amounts of three dyes to still water in a shallow container
Sample different regions at different times
How do solutes distribute within a solution?
By diffusion, uniformly and independently of each other.
What is simple diffusion?
Small molecules pass through the phospholipid bilayer membrane
What type of molecule moves by simple diffusion?
Hydrophobic, lipid soluble molecules
Also water
What types of molecules do not pass readily though a membrane by simple diffusion?
Proteins, amino acids, ions (electrically charged, polar molecules)
Why do polar, charged molecules not pass through a membrane by simple diffusion?
Polar substances form polar bonds with water- being surrounded prevents their escape
The interior of the membrane is hydrophobic, hydrophilic substances are excluded
What is osmosis defined as?
Water diffuses from regions of higher concentrations to a region of its lower concentration
What does osmosis depend on?
Number of solute particles present (not kinds)
What is an isotonic solution?
Solutions with equal solute concentrations
What is a hypertonic solution?
Solution has higher solute concentration than the other solution with which it is being compared
What is a hypotonic solution?
Solution has lower solute concentration than the other solution with which is it being compared
What happens when a red blood cell is placed in a hypotonic solution?
It takes up water, swells and then bursts
What happens when a red blood cell is placed in a hypertonic solution?
Cells lose water and shrivel
What two processes are facilitated diffusion?
Channel proteins
Carrier proteins
What is the structure of a channel protein?
A central pore lined with polar amino acids and water, non-polar amino acids outside to keep it embedded in the bilayer
What is the movement of ions into and out of cells important in?
Electrical activity of the nervous system
Opening of pores in leaves
What is a channel protein that can be opened or closed called?
A gated channel
What are some types of gated channel proteins?
Ligand-gated channel
Voltage-gated channel
What two factors does the speed of ions through channel proteins depend on?
- Concentration gradient
- Electrochemical gradient
What is a charge imbalance across a plasma membrane called?
A membrane potential
What is the Nernst equation?
Ek=58log[Ko/Ki]
Where Ek is the membrane potential in milivolts
Ko is concentration of ion outside cell
Ki is concentration of ions inside cell
What is the membrane potential of animal cells?
-70mV (inside negative with respect to outside)
How are ion channels specific?
K+ is larger but lets go of its water shell and is attracted to oxygen atoms of the channel protein
Smaller Na+ ions are more distant from the oxygen so are surrounded by more water
How does water move through membranes faster than expected?
Hydrating ions as they pass through channels
Aquaporins
What are aquaporins?
Channel proteins that allow water to pass through them
Where are aquaporins found?
Plant vacuoles (to maintain turgor pressure) Mammalian kidney (retain water)
What do carrier proteins transport?
Polar substances such as amino acids and sugars
What carrier protein transports glucose?
The glucose transporter
What does the glucose transporter do?
Facilitate glucose uptake into the cell
How is simple diffusion similar to carrier protein transport?
The rate of both depends on concentration gradient across the membrane
How is simple diffusion and carrier protein transport different?
Carrier-mediated transport, a point is reached at which increasing concentration does not increase rate of diffusion- saturation
Why is there a saturation point for carrier-mediated transport?
The limited number of carrier proteins are fully loaded with solute molecules
What is active transport?
The movement of a substance across a biological membrane against a concentration gradient, requiring expenditure of chemical energy
What are the three types of membrane protein involved in active transport?
Uniports
Symports
Antiports
What do uniports do?
Move a single substance in one direction
Give an example of a uniport?
Calcium-binding protein in plasma membrane and endoplasmic reticulum actively transport Ca2+ outside the cell or inside the ER
What do symports do?
Move two substances in the same direction
Give an example of a symport.
Uptake of amino acids from the intestine by cells that line it requires simultaneous binding of Na+ and amino acid
What do antiports do?
Move two substances in opposite directions
Give an example of an antiport.
Sodium-potassium pump moves 3 molecules of Na+ out and 2 molecules K+ in
What are symports and antiports known as?
Coupled transporters
Because they move two substances at once
What are the two types of active transport?
Primary active transport
Secondary active transport
What is primary active transport?
Requires direct participation of energy rich ATP
What is secondary active transport?
Does not use ATP directly
Energy is supplied by an ion concentration gradient established by primary active transport
What does secondary active transport aid in?
Uptake of amino acids and sugars
Why cant macromolecules such as proteins, polysaccharides and nucleic acids pass through membranes?
They are too large, too charged and/or polar
How do large macromolecules pass biological membranes?
Endocytosis
Exocytosis
What is endocytosis?
Process by which mall molecules, macromolecules, large particles and even small cells are brought into the eukaryotic cell
What are the three types of endocytosis?
Phagocytosis
Pinocytosis
Receptor-mediated endocytosis
What is phagocytosis?
Plasma membrane engulfs large particles or entire cells
Where is phagocytosis used?
Unicellular protists as cellular feeding
White blood cells to defend body
Food vacuole or phagosome fuses with lysosome for digestion
What is pinocytosis?
Small vesicles bring small, dissolved substances or fluids into the cell
Where does pinocytosis occur?
Endothelium- the single layer of cells separating tiny blood capillaries from surrounding tissue for cells to acquire fluids from the blood
What is receptor-mediated endocytosis?
Specific reactions at the cell surface trigger the uptake of specific materials
What does receptor-mediated endocytosis depend on?
Receptor-proteins
What is the first step in receptor-mediated endocytosis?
Receptor proteins on the extracellular surface bind with specific substances. These sites are called coated pits
What is the cytoplasmic surfaces behind a coated pit coated in?
Other proteins such as clathrin
What happens once a receptor binds to a specific ligand?
Coated pit invaginates and forms a coated vesicle
What does clathrin do?
Strengthen and stabilize the coated vesicle
What happens to a coated vesicle once inside the cell?
The clathrin is lost
May fuse with lysosome
Engulfed material is processed and released into the cytosome
How is cholesterol taken up by mammalian cells?
Receptor-mediated endocytosis
Where are LDL’s taken up?
The liver for recycling
What is exocytosis?
The process by which materials packaged in vesicles fuses with the plasma membranes and materials are moved out of the cell
Where is exocytosis important?
Secretion of indigestible materials to the environment
Secretion of substances, including digestive enzymes, neurotransmitters, materials for plant cell wall construction
