Membrane Transport 1 Passive Transport All move HIGH

Membrane Transport

1) Passive Transport • All move HIGH to LOW “DOWN” the concentration gradient NO energy required

2) Active Transport • Requires Energy

3)DIFFUSION PASSIVE – Requires NO ENERGY Automatic due to kinetic energy of molecules Moves DOWN CONCENTRATION GRADIENT from [HIGH] → [LOW] until reaches equilibrium Ex: Oxygen/CO 2 cross capillary cell membranes

4) OSMOSIS • Specific example of passive transport. • Involves net movement of WATER from an area of low solute concentration to an area of high solute concentration. • No transport proteins are required.

Water Balance of Cells Without Walls • Tonicity – Is the ability of a solution to cause a cell to gain or lose water – Has a great impact on cells without walls 8

Isotonic Solutions • If a solution is isotonic – The concentration of solutes is the same as it is inside the cell – There will be NO NET movement of WATER 9

Hypertonic Solution • If a solution is hypertonic – The concentration of solutes is greater than it is inside the cell – The cell will lose water (PLASMOLYSIS) 10

Hypotonic Solutions • If a solution is hypotonic – The concentration of solutes is les than it is inside the cell – The cell will gain water 11

Water Balance in Cells Without Walls Animal cell. An animal cell fares best in an isotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water. 12

Water Balance of Cells with Walls • Cell Walls – Help maintain water balance • Turgor pressure – Is the pressure of water inside a plant cell pushing outward against the cell membrane • If a plant cell is turgid – It is in a hypotonic environment – It is very firm, a healthy state in most plants • If a plant cell is flaccid – It is in an isotonic or hypertonic environment 13

Water Balance in Cells with Walls Plant cells are turgid (firm) (firm and generally healthiest in a hypotonic environment, environment where the uptake of water is eventually balanced by the elastic wall pushing back on the cell. 14

How Will Water Move Across Semi -Permeable Membrane? • Solution A has 100 molecules of glucose per ml • Solution B has 100 molecules of fructose per ml • How will the water molecules move? There will be no net movement of water since the concentration of solute in each solution is equal 15

How Will Water Move Across Semi-Permeable Membrane? • Solution A has 100 molecules of glucose per ml • Solution B has 75 molecules of fructose per ml • How will the water molecules move? There will be a net movement of water from Solution B to Solution A until both solutions have equal concentrations of solute 16

How Will Water Move Across Semi-Permeable Membrane? • Solution A has 100 molecules of glucose per ml • Solution B has 100 molecules of Na. Cl per ml • How will the water molecules move? Each molecule of Na. Cl will dissociate to form a Na+ ion and a Cl - ion, making the final concentration of solutes 200 molecules per mil. Therefore, there will be a net movement of water from Solution A to Solution B until both solutions have equal concentrations of solute 17

s or organs to be used in medical procedures must be bathed in a solution with me osmolarity as the cytoplasm to prevent osmosis. The importance of osmotic control preventing damage to cells and tissues Common medical procedures in which an isotonic saline solution is useful: • fluids introduction to a patient’s blood system via an intravenous drip, e. g for rehydration • used to rinse wounds, skin abrasions etc. • keep areas of damaged skin moist before applying skin grafts • eye drops/wash • frozen and used pack donor organs for transportation http: //www. defenseimagery. mil/image. Retrieve. action? guid=8 c 9 d 5 fade 029 a 4 f 5 a 68 fe 667 d 1 ae 802 ba 9 f 30 dd 5&t=2

5) FACILITATED DIFFUSION With carrier proteins: PASSIVE- Requires NO ENERGY Trans-membrane proteins assist in movement Molecule specific Grab molecule, change shape, flip to other side Moves from [HIGH] → [LOW] Ex: Glucose • With AQUAPORINS (OSMOSIS) proteins move POLAR WATER molecules past phobic tails [HIGH] → [Low]

Facilitated Diffusion & Proteins • Channel proteins – Provide corridors that allow a specific molecule or ion to cross the membrane 20

Facilitated Diffusion & Proteins • Carrier proteins – Undergo a subtle change in shape that translocates the solute-binding site across the membrane A carrier protein alternates between two conformations, conformations moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either direction, direction with the net movement being down the concentration gradient of the solute. 21

6) MOLECULAR ACTIVE TRANSPORT • EITHER DIRECTLY OR INDIRECTLY USES ATP to transport substances against their concentration gradient using a transport protein.

7) BULK TRANSPORT • Type of ACTIVE TRANSPORT that involves the movement of large quantities of substances into or out of the cell through the use of vesicles.

8) PRIMARY ACTIVE TRANSPORT • A protein pump is phosphorylated by ATP which activates the pum and allows it to pump substances against their concentration gradient. – EX. Sodium/Potassium Pump • Pump moves 3 Na+ ions across the membrane, changing the shape of the pump which allows 2 K+ ions on the other side of the membrane to bind and be transported in the opposite direction.

Active Transport • The sodium-potassium pump – Is one type of active transport system 1 Cytoplasmic Na+ binds to the sodium-potassium pump. Na+ binding stimulates phosphorylation by ATP. EXTRACELLULAR [Na+] high FLUID [K+] low 2 Na+ Na+ Na+ CYTOPLASM [Na+] low [K+] high Na+ ATP P ADP Na+ Na+ K+ is released and Na+ 3 sites are receptive again; the cycle repeats. K+ Phosphorylation causes the 4 protein to change its conformation, expelling Na+ to the outside. K+ K+ 5 Loss of the phosphate restores the protein’s original conformation. K+ P i 6 Extracellular K+ binds to the protein, triggering release of the Phosphate group. 25

9) SECONDARY ACTIVE TRANSPORT • Transport proteins are used, BUT they do not get phosphorylated by ATP, instead the energy used to power the proteins come from the concentration gradients created by protein pumps that do use ATP.

ANTIPORTERS • Protein transporters that move substances in opposite directions – Ex. Shown SODIUM/CALCIUM EXCHANGER PROTEIN SYMPORTERS Protein transporters that move substance in the same direction. -Ex. Shown SODIUM/GLUCOSE transporter

10) EXOCYTOSIS • Process that removes intracellular contents to the extracellular environment through the fusion of vesicles with the cell membrane. – Ex. Golgi transport

11) ENDOCYTOSIS • Moves extracellular contents into the intracellular environment through engulfing them with the cell membrane which then pinches off to form a vesicle.

• Three Types of Endocytosis In phagocytosis, phagocytosis a cell engulfs a particle by Wrapping pseudopodia around it and packaging it within a membraneenclosed sac large enough to be classified as a vacuole The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes. In pinocytosis, pinocytosis the cell “gulps” droplets of extracellular fluid into tiny vesicles. It is not the fluid itself that is needed by the cell, but the molecules dissolved in the droplet. Because any and all included solutes are taken into the cell, pinocytosis is nonspecific in the substances it transports. PHAGOCYTOSIS 30

Receptor-mediated endocytosis enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid. The receptor proteins are usually already clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coat proteins. Extracellular substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules. Notice that there are relatively more bound molecules (purple) inside the vesicle, other molecules (green) are also present. After this ingested material is liberated from the vesicle, the receptors are recycled to the plasma membrane by the same vesicle. RECEPTOR-MEDIATED ENDOCYTOSIS Coat protein Receptor Coated vesicle Ligand Coated pit A coated pit and a coated vesicle formed during receptormediated endocytosis (TEMs). Coat protein Plasma membrane 0. 25 µm 31

12) PHAGOCYTOSIS • Means “cell eating” • Form of endocytosis which involves engulfing large solid contents – Example: white blood cells that phagocytose bacteria

13) PINOCYTOSIS • Means “cell drinking” • Form of endocytosis which involves engulfing liquid contents

14) RECEPTOR MEDIATED ENDOCYTOSIS • Allows the cell to engulf target substances that bind to receptors on the cell membrane causing a pit to form. This pit will eventually become a coated vesicle.

More on Pumps • PUMPS Can move AGAINST concentration gradient [LOW] → [HIGH] • Used to create electrochemical gradients across cell membranes – 1. SODIUM-POTASSIUM (Na+-K+) PUMP – 2. PROTON PUMP

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Proton Pump • Main electrogenic pump in PLANTS • ATP provides energy to pump H+ ions across a membrane • Stored H+ = potential energy to do work EX: COTRANSPORT (see next slide) ATP PRODUCTION during cellular respiration/photosynthesis

Co-transport • Na+-K+/ Proton pumps use ATP to create concentration gradient Movement of substance is linked to return of Na+/H+ as it flows back down its concentration gradient • EX: sucrose is linked to H+ transport

Comparison of Passive & Active Transport Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane. Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP Diffusion. Hydrophobic Facilitated diffusion. Many hydrophilic molecules and (at a slow substances diffuse through membranes with the rate) very small uncharged assistance of transport proteins, polar molecules can diffuse through the lipid either channel or carrier proteins. bilayer. 39

Review Sites • David Knuffke Water Potential Simulation Transport tutorial PROTEIN TRANSPORT VESICLE BUDDING