5 - Lecture notes 5.6and5.7 PDF

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Biology 95 - 3002
Mon/Wed Class 4:30pm – 5:50pm
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5.6 – Movement of Ions and Molecules Pag e# 84

Vocabulary Word Concentration

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Concentration gradient

Difference in concentration between adjoining regions of fluid.

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Diffusion

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Hypertonic

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Hypotonic

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Isotonic

Net movement of molecules or ions from a region where they are more concentrated to a region where they are less so. Describes a fluid that has a high overall solute concentration relative to another fluid. Describes a fluid that has a low overall solute concentration relative to another fluid. Describes two fluids with identical solute concentration.

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Osmosis

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Osmotic pressure

Definition Number of molecules or ions per unit volume.

The diffusion of water across a selectively permeable membrane in response to a differing overall solute concentration. Amount of turgor that prevents osmosis into cytoplasm or other hypertonic fluid.

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Turgor

Pressure that a fluid exerts against a wall, membrane, or other structure that contains it.

5.7 – Membrane – Crossing Mechanisms 87

Active Transport

Energy requiring mechanism by which a transport protein pumps a solute across a cell membrane against its concentration gradient.

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Passive Transport

Mechanism by which a concentration gradient drives the movement of a solute across a cell membrane through a transport protein. Requires no energy input.

5.6 – Movement of Ions and Molecules  Ions and molecules tend to move from one region to another in response to concentration gradients.  Water diffuses across cell membranes by osmosis.  Links to Homeostasis 1.3, Ions and molecules 2.3, Solutes 2.5, Cell membrane structure 4.2, Plant cell walls 4.11 1) Understanding how metabolism works in cells begins with the behavior of solutions. a) How much of a solute is dissolved in a given amount of fluid is the solute’s concentration. b) A difference in solute concentration between adjacent regions of solution is a concentration gradient. c) Why do solute molecules or ions tend to move “down” their concentration gradient, from a region of higher concentration to one of lower concentration? i) Molecules & ions always in motion. ii) Collide at random & bounce off one another millions of times ea. second. iii) More crowded = more often they collide. iv) Rebounds from the collisions propel molecules away from one another. v) More molecules get bumped out of a region of higher concentration than get bumped into it. d) Diffusion: the net movement of molecules or ions in response to a concentration gradient. i) The rate of diffusion depends on (5) things (1) Size (2) Temperature (3) Steepness (4) Charge (5) Pressure 2) Diffusion across membranes. a) Tonicity refers to the total concentration of solutes in fluids separated by a selectively permeable membrane (such membranes allow some substances, but not others, to cross). b) Hypotonic: Describes a fluid that has a high overall solute concentration relative to another fluid. c) Hypertonic: Describes a fluid that has a low overall solute concentration relative to another fluid. d) Typical selectively permeable membrane – allows water to cross. i) When this type separates two fluids that are not isotonic, water will diffuse across the membrane, and move from the hypotonic fluid into the hypertonic one. ii) Osmosis: The diffusion of water across a selectively permeable membrane in response to a differing overall solute concentration. Figure 5.18 Animated - Selective permeability of lipid bilayers. A Hydrophobic molecules, gases, and water molecules can cross a lipid bilayer on their own. B Ions in particular and most polar molecules such as glucose cannot; they cross cell membranes only with the help of transport proteins in the bilayer.

3) Turgor, a) Cell walls of plants and many protists, fungi, and bacteria can resist an increase in the volume of cytoplasm even in hypotonic environments. b) Plant cells: cytoplasm usually contains more solutes than soil water does. c) Pressure that a fluid exerts against a structure that contains it is called turgor. d) When enough pressure builds up inside a plant cell, water stops diffusing into its cytoplasm. e) The amount of turgor that stops osmosis is called osmotic pressure. i) Osmotive pressure: keeps walled cells plump. ii) When soil dries out, it loses water but not soluets, so the conentration of solutes incresase in whatever water remains in it. iii) If soil water becomes hypertonic w/ respect to cytoplasm, water difuses out of the plant’s cells, so their cytoplasm shrinks. iv) As turgor inside the cells decreases, the plant wilts.

Take-Home Message 

What influences the movement of ions and molecules? Answer Molecules or ions tend to diffuse into an adjoining region of fluid in which they are not as concentrated. The steepness of a concentration gradient as well as temperature, molecular size, charge, and pressure affect the rate of diffusion. Osmosis is a net diffusion of water between two fluids that differ in water concentration and are separated by a selectively permeable membrane. Fluid pressure that a solution exerts against a membrane or wall influences the osmotic movement of water.

5.7 – Membrane – Crossing Mechanisms  Many types of molecules and ions diffuse across a lipid bilayer only with the help of transport proteins.  Link to Transport proteins 4.4

1) Passive Transport a) The movement of a solute (and the direction of the movement) through a transport protein is driven entirely by the solute’s concentration gradient. b) Also called facilitated diffusion. c) The solute simply binds to the passive transport protein, and the protein releases it in the other side of the membrane.

d) Passive transport. This model shows a glucose transporter. (1) A glucose molecule (here, in extracellular fluid) binds to a glucose transporter in the plasma membrane. (2) Binding causes the transport protein to change shape. (3) The glucose molecule detaches from the transport protein on the other side of the membrane (here, in cytoplasm), and the protein resumes its original shape. e) Which fluid is hypotonic: extracellular fluid or the cytoplasm? (Cytoplasm)

2) Active Transport a) A transport protein uses energy to pump a solute against its gradient across a cell membrane. b) After a solute binds to an active transporter, an energy input (often in the form of a phosphategroup transfer from ATP) changes the shape of the protein. c) The changes cause the transporter to release the solute to the other side of the membrane. Figure 5.21 Animated Active transport. This model shows a calcium pump embedded in a muscle cell membrane. A -Two calcium ions bind to the transport protein. B- Energy in the form of a phosphate group is transferred from ATP to the protein. The transfer causes the protein to change shape so that it ejects the calcium ions to the opposite side of the membrane. C- After it loses the calcium ions, the transport protein resumes its original shape.

Figure 5.22 Cotransport. This model shows how a sodium–potassium pump transports sodium ions (Na+, red) from cytoplasm to extracellular fluid, and potassium ions (K+, purple) in the other direction across the plasma membrane. A phosphate-group transfer from ATP provides energy for the transport.

Bear in mind, the membranes of all cells, not just those of animals, have active transporters. For example, active transporters in plant leaf cells pump sugars into tubes that distribute them throughout the plant body.

Take-Home Message How do molecules or ions that cannot diffuse through a lipid bilayer cross a cell membrane? Answer Transport proteins help specific molecules or ions to cross cell membranes. In passive transport, a solute binds to a protein that releases it on the opposite side of the membrane. The movement is driven by a concentration gradient. In active transport, a protein pumps a solute across a membrane, against its concentration gradient. The transporter requires an energy input, as from ATP....