Cell Membranes Osmosis and Diffusion

Functions of Membranes 1. Protect cell 2. Control incoming and outgoing substances 3. Maintain ion concentrations of various substances 4. Selectively permeable - allows some molecules in, others are kept out

Phospholipid Bilayer

Fluid Mosaic Model

Blood-Brain Barrier Allows some substances into the brain, but screens out toxins and bacteria Substances allowed to cross include: water, CO 2, Glucose, O 2, Amino Acids, Alcohol, and antihistamines. HIV and bacterial meningitis can cross the barrier.

Solutions Solutions are made of solute and a solvent Solvent - the liquid into which the solute is poured and dissolved. We will use water as our solvent today. Solute - substance that is dissolved or put into the solvent. Salt and sucrose are solutes. Concentration Gradient- conentration diference across the membrane. Equilibrium- –concentration of molecules is equal throughout the space –Molecule still move even in equalibrium

Methods of Transport Across Membranes 1. Diffusion 2. Osmosis 3. Facilitated Diffusion 4. Active Transport

Methods of Transport Across Membranes 1. Diffusion -passive transport - no energy expended 2. Osmosis - Passive transport of water across membrane 3. Facilitated Diffusion - Use of proteins to carry polar molecules or ions across 4. Active Transport- requires energy to transport molecules against a concentration gradient – energy is in the form of ATP

Diffusion Movement of molecules from an area of high concentration to an area of low concentration. Movement from one side of a membrane to another, un-facilitated

Diffusion Solute molecules moving from an area of high concentration to an area of low concentration –Random motion drives diffusion –Movement is based on kinetic energy (speed), charge, and mass of molecules –Equilibrium is reached when there is an even distribution of solute molecules (water)

Diffusion

Factors that affect diffusion Concentration gradient Size of molecules Type of molecules Chemical nature of the membrane

Osmosis WATER molecules difule across a membrane- high to low conc. Of WATER

Osmosis

Tonicity is a relative term Hypotonic SolutionHypotonic Solution - One solution has a lower concentration of solute than another.- water moves in Hypertonic SolutionHypertonic Solution - one solution has a higher concentration of solute than another. - Water moves out Isotonic SolutionIsotonic Solution - both solutions have same concentrations of solute.- water moves back and forth in equalibrium

Plant and Animal Cells put into various solutions

Single Cell organisms Contractile Vacuoles collect excess water in the cell and excrete it Animation – cell/image/contractile_vacuole.gifhttp:// cell/image/contractile_vacuole.gif

Osmosis in Red Blood Cells Isotonic- equilibrium movement of water Hypotonic- higher conc outside- water moves in Hypertonic- higher conc inside- water moves out plasmolysis- cells shrink Cytolysis- cells swell and burst Normal cells

Osmosis in Plant Cells Hypertonic Hypotonic Plasmolysis- cells are “wilted” Turgor pressure- cells are rigid

Types of Transport

Passive Transport When a cell uses no energy to move particles across a membrane passive transport occurs Particles go DOWN their concentration gradient. Diffusion & osmosis are passive transport. Concentration gradient Plasma membrane

Passive Transport by proteins Passive transport of materials across the membrane using transport (carrier) proteins specific for each moleule is called facilitated diffusion. Beneficial because it can move both directions Channel proteins Concentration gradient Plasma membrane

Gated Ion Channels

Diffusion through Ion Channels Type of passive transport that involves membrane proteins known as ion channels: sodium (Na + ), potassium (K + ), calcium (Ca 2+ ), chloride (Cl - ) Ion channels provide small passageways through which ions can diffuse. Each types of channel is specific for one type of ion. Some are always open, others have gates which open and close in response to stimuli (stretching of cell membrane, electrical signals, chemicals in cytosol or external environment)

Active Transport active transport :Movement of materials through a membrane against a concentration gradient and requires energy from the cell. Concentration gradient Plasma membrane Carrier proteins Cellular energy

Animation: hill.com/sites/ /student_view0/chapter2/animation__how_the_sodium_ potassium_pump_works.html hill.com/sites/ /student_view0/chapter2/animation__how_the_sodium_ potassium_pump_works.html Sodium Potassium Pump animationanimation

Transport of Large Particles Endocytosis is a process by which a cell surrounds and takes in material from its environment. – The material is engulfed and enclosed by a portion of the cell’s plasma membrane. Phagocytosis- “cell eating” Pinocytosis- “cell drinking” –resulting vacuole with its contents moves to the inside of the cell Exocytosis is the expulsion or secretion of materials from a cell. Exocytose Endocytosis

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