Diffusion and Osmosis

Cells have needs too… All cells need to: Take in food or nutrients Take in oxygen and release carbon dioxide Remove wastes made inside the cell Control their water balance All of these activities must happen across the cell membrane of every single cell

Methods of Transport Passive Transport Active Transport

1. Passive Transport Passive transport- the movement of substances across the membrane without the use of chemical energy. Diffusion and Osmosis Passive transport relies on the movement of substances down a concentration gradient.

Diffusion Diffusion- the movement of molecules from an area of higher concentration to an area of lower concentration. A difference in the concentrations of molecules across a space is called a concentration gradient. Diffusion is driven by the movement of molecules The molecules like to “spread out”

Diffusion 2nd Law of Thermodynamics governs biological systems universe tends towards disorder (entropy) Movement from high concentration of that substance to low concentration of that substance. Diffusion movement from high  low concentration

Animation

The process of diffusion is used to deliver oxygen from your lungs into your blood.

Equilibrium Equilibrium occurs when the concentration of molecules is the same throughout a space. Molecular movement still occurs, but in all directions  the balance is maintained.

Membranes Some molecules can diffuse across a membrane (cell membrane). If a molecule can move through a membrane, that membrane is permeable to that molecule. Ex. Water, oxygen, some ions Not all molecules can diffuse across all membranes. Molecular movement depends on the size and type of the molecule as well as the membrane structure. Ex. Glucose, large proteins

Membranes again… Animation

Osmosis… Osmosis- special case of diffusion, in which water molecules diffuse across a cell membrane from an area of high concentration of water to areas of low concentration of water. The direction of water movement depends on the concentrations of water and solute dissolved in the solution. Remember… Solution- homogeneous mixture in which one or more substances are uniformly dissolved in another substance. Solute- substance dissolved in a solution.

Which way will it go? In many situations, the solute molecules can’t actually diffuse across the membrane, which creates an imbalance. Very Simply, the concentrations of water and solutes “want” to be equal on each side of the membrane, so the water must move.

Osmosis is the movement of water from areas of high concentration of water to areas of low concentration of water Animation

Water flows “downhill” If the concentration of water molecules outside the cell is higher than the concentration in the cell, the solution is hypotonic to the cell. Water moves into the cell until equilibrium is reached. Hypo means “lower”

Hypotonic- the cell swells Solute 90% Water Cell Membrane Draw arrows representing the movement of water. 80% Water

Water flows “downhill” continued… If the concentration of water molecules outside the cell is lower than the concentration inside the cell, the solution is hypertonic to the cell. Water moves out of the cell until equilibrium is reached. Hyper mean “higher”

Hypertonic- the cell shrivels Solute 70% Water Cell Membrane Draw arrows representing the movement of water. 90% Water

Water flows “downhill” continued… When concentration of water molecules outside the cell is equal to the concentration inside the cell, the solution is isotonic to the cell. Water moves into and out of the cell at an equal rate. “iso” means equal

Isotonic- at equilibrium Solute 80% Water Cell Membrane Draw arrows representing the movement of water. 80% Water

Pressure If water is moving into the cell, turgor pressure increases. If the cell takes too much water in it can burst. This is termed cytolysis. If water is moving out of the cell, turgor pressure drops and the cell wilts. This is termed plasmolysis.

Managing water balance Cell survival depends on balancing water uptake & loss freshwater balanced saltwater

Passive Movement Passive transport- the movement of substances across the membrane without the use of chemical energy (No ATP needed) Diffusion and Osmosis (see previous) Facilitated Diffusion- carrier molecules speed up the diffusion of molecules that move slowly across the membrane. No energy is required. Larger molecules Ex.: Glucose into and out of cells.

Membrane Components Carrier molecules- transport molecules across the cell membrane from high concentration to low concentration.

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2. Active Transport Active Transport- The movement of materials across a membrane that requires the use of energy. (Requires ATP) Usually moves molecules Uphill, against the concentration gradient

It takes energy to go uphill Cell Membrane pumps- carrier proteins that move molecules against a concentration gradient. From areas of low concentration to areas of higher concentration. This movement requires the use of chemical energy. Animation

Cells eat too… Endocytosis- the process by which cells engulf substances that are too large to pass through the cell membrane. The cell forms a vesicle around the material. “Endo”- means “in” or “internal”

Types of Endocytosis Pinocytosis- the transport of fluids or solutes into the cell. (the cell “drinks”) Phagocytosis- movement of large molecules or entire cells/microorganisms into the cell. (the cell “eats”)

And get rid of wastes Exocytosis- the passage of large molecules out of the cell. Vacuoles merge with the cell membrane, expelling their contents. Animation

Managing water balance Isotonic animal cell immersed in mild salt solution Concentration of water outside=concentration inside example: blood cells in blood plasma problem: none no net movement of water flows across membrane equally, in both directions volume of cell is stable balanced

Managing water balance Hypotonic a cell in fresh water Concentration of water outside>concentration inside example: Paramecium problem: gains water, swells & can burst water continually enters Paramecium cell solution: contractile vacuole pumps water out of cell ATP plant cells turgid ATP freshwater

conformational change Active Transport Cells may need to move molecules against concentration gradient shape change transports solute from one side of membrane to other protein “pump” “costs” energy = ATP conformational change low high Some transport proteins do not provide channels but appear to actually translocate the solute-binding site and solute across the membrane as the protein changes shape. These shape changes could be triggered by the binding and release of the transported molecule. This is model for active transport. ATP “The Doorman”