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How things get into cells:Principles of diffusion, osmosis, and the nature of biological membranes.

DiffusionMovement of substances from an area of high

concentration to an area of low concentrationDepends on 2nd law of thermodynamics

Osmosis: special case of diffusion of water Biological membranes

Lipids =barriers; proteins = channels.

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Movement of molecules depends on:

1. Kinetic energy

higher temperature = more kinetic energy

2. Concentration

gradient – more of something in one area than another

Second Law of Thermodynamics: all things tend toward entropy.

If there’s more of something in one area, it will spread out.

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Diffusion Passive process

Depends on concentration and kinetic energyDoes not require energyMoves substances from an area of high

concentration to an area of low concentration Down a concentration gradient

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Osmosis: a special instance of diffusion

The most concentrated form of water is

pure water.

To make water less concentrated, we dissolve substances in it.

Concentration of one solution relative to another

Isotonic – equal concentrations Hypertonic – more concentrated Hypotonic – less concentrated

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Tonicity – relative concentrations of solutions Isotonic – two solutions contain the same

amount of substance dissolved in them- equal concentrations

Hypertonic – a solution containing a greater amount of dissolved substance- more concentrated

Hypotonic – a solution containing a lesser amount of dissolved substance – less concentrated

Osmosis: The movement of water across a selectively permeable membrane, down a concentration gradient.

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Osmosis: special case of the diffusion of water.

Movement of water across a semi permeable membrane.

If the environment is:Isotonic: No NET flow.Hypertonic: Water flows OUT

of cell.Hypotonic: Water flows IN.

Water flows from where it (the water) is in high concentration to where it is in low concentration.

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Osmosis is an important phenomenon in biology Red blood cells must be collected carefully

In a hypotonic solution (distilled water), water enters the RBCs and explodes them.

The action of penicillin depends on itBacteria are protected by a cell wall. When

drugs damage the wall, water rushes in and explodes the bacterium, killing it.

Filtering action of the kidney depends on itWater is drawn back out of urine.

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The physical law that molecules travel down a concentration gradient (from a region of high to low concentration) drives most movement of molecules in and out of cells.

Other movement of molecules and particles e.g., movement “against” a concentration gradient, requires some form of metabolic energy.

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Entry of particles and molecules attached to the cell membrane

http://bio.winona.msus.edu/bates/genbio/images/endocytosis.gif

Requires:

•Direct access of cell membrane to outside

•Cytoskeleton

•Source of energy

Thus NOT done by cells with cell walls (plants, fungi, bacteria) or cells without a cytoskeleton (bacteria).

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The type of molecule affects how it gets through a membrane

Small molecules can pass through a membraneWater; Gases such as O2 and CO2

Lipid molecules canDissolve in lipid bilayer, pass through membraneMany drugs, vitamins, hormones are lipid soluble

Larger, hydrophilic molecules cannot Ions, sugars, amino acids cannot Transport proteins required

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Transport through membranes Simple diffusion

Molecules travel down concentration gradientMembrane is not a barrier to their passage

Facilitated diffusionMolecules travel down concentration gradientCannot pass through lipid bilayer; their passage is

facilitated by protein transporters Active transport

Molecules travel against concentration gradientRequires input of metabolic energy (ATP),

transporter

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How molecules get through the membrane

http://www.rpi.edu/dept/chem-eng/Biotech-Environ/Membranes/bauerp/diff.gif

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Active transport: movement of molecules against a concentration gradient. Requires Energy.

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Transport into cells: a step in the middle of a process All cells need raw materials

For maintaining cells, building new ones All cells need a source of energy

For continuing cell functions like movementFor building new molecules, new cells

Chemical reactions that use or release energy: Metabolism

17 http://www.nlm.nih.gov/medlineplus/ency/images/ency/fullsize/8710.jpg

Food preparation, consumption, digestion (break down into smaller molecules.)

Small molecules absorbed or transported into cells.

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Chemical reactions carried out by enzymes Once inside the cell, molecules are used

as raw materials or for their energy. The chemical reactions involved are

carried out by protein molecules called enzymes.

Enzymes are the tools of the cell and have several important properties.

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Enzymes are catalysts

Catalysts speed up the rate of a chemical reaction2H2 + O2 2H2O

Reactions must occur quickly in a cell. A catalyst is not used up

Enzymes are tools, so think “hammer”Used over and over to pound nails

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Enzymes overcome “activation energy”

Enzymes give an extra push to reactions that don’t require energy to finish.

Enzymes are facilitators: they get all the reactants together on the enzyme’s surface so they can react.

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Enzymes are specific

There are thousands of different reactions that take place in a cell, most of them going on at the same time.To speed up each one, there is a different

enzyme. Each type of enzyme can speed up only 1 type of chemical reaction.

Enzymes are proteins, and their 3D shape is what makes them specific. Think “wrenches”

Instructions for making enzymes thus found in the DNA

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The next lecture(s) will discuss how cells take molecules, break them down and release the energy in them, and put them back together in the unique way required by the cells.