Homeostasis and Transport Chapter 5 Homeostasis and Transport

Homeostasis The property of a system that regulates its internal environment and tends to maintain a stable, constant condition Cell membranes help organisms maintain homeostasis by controlling what substances enter and leave cells

Passive Transport Substances crossing the cell membrane without any input of energy by the cell – move down their concentration gradient Types: Diffusion Osmosis Facilitated Diffusion Ion Channels

Concentration Gradient Concentration - how many of a substance’s molecules are sitting in a specific volume of a solution Gradient - a measurement of how much something changes as you move from one region to another Concentration Gradient - the difference in the concentration of molecules across a space

Concentration Gradient

Diffusion Movement of molecules from an area of high concentration to an area of low concentration The process by which molecules intermingle as a result of their kinetic energy of random motion.

Diffusion

Diffusion Add a sugar cube to a beaker of water It sinks to the bottom making the concentration of sugar higher there

Diffusion As the cube dissolves sugar molecules break away and move from the bottom to the top of the beaker

Diffusion Because of their kinetic energy, the molecules of sugar are in constant motion They keep moving until they hit something and then they rebound

Diffusion If no object blocks their movement, molecules continue on their path They move down their concentration gradient from areas of high concentration to low concentration until They reach equilibrium

Do Now What is homeostasis? Do forms of Passive Transport require cell energy? What is diffusion?

Equilibrium Diffusion will eventually cause the concentration of molecules to be the same throughout Equilibrium – when the concentration of the molecules of a substance is the same throughout a space The molecules are still moving, but they are just as likely to move in one direction as the other – they balance each other

Equilibrium

Diffusion Through Cell Boundaries Cell membrane regulates movement of dissolved molecules from the liquid on one side of the membrane to the other Cell membrane - selectively permeable - some substances can pass across it and some cannot Nonpolar molecules can diffuse through the lipid bilayer as well as small molecules

Diffusion Through Cell Boundaries If a molecule can pass through a cell membrane, it will diffuse from an area of high concentration on one side to an area of low concentration on the other The cell is not required to use energy for diffusion.

Diffusion in a Cell

Osmosis Solution = solute + solvent

Osmosis Both solute and solvent molecules can diffuse In cells: the solutes are organic and inorganic compounds the solvent is water

Osmosis Osmosis - diffusion of water through a selectively permeable membrane (cell membrane) Water moves down its concentration gradient It moves from areas of high concentration of water to low concentration of water Osmosis does not require energy

Osmosis When water is more concentrated on one side of the membrane, water will move to an area of lower concentration in order to re-establish equilibrium. Question?? If there is more salt outside of the cell, will water move into the cell or out of the cell??

Osmosis The net direction of osmosis depends on the concentration of solutes on the two sides of the semi-permeable membrane In a cell, this can have important consequences

Types of Solutions Solutions can be: Isotonic Hypotonic Hypertonic

Isotonic Solution The concentration of solutes is the same inside and outside cell Water will diffuse into and out of the cell at equal rate No net movement of water

Hypotonic Solution The solution has a lower solute concentration than the cell Water diffuses into the cell until equilibrium is established Net movement of water – into the cell

Hypertonic Solution The solution has a higher solute concentration than the cell Water diffuses out of the cell until equilibrium is established Net movement of water is out of the cell

Osmotic Pressure Osmosis exerts a pressure (osmotic pressure) on the hypertonic side of a selectively permeable membrane. (This could cause water to rush into cells and cells could bust) This does not happen in animal cells because they are usually in isotonic fluids example: blood

Animal Blood Cells in Different Solutions Isotonic Hypertonic Hypotonic

The Effects of Osmosis on Animal Cells

Osmotic Pressure Plant and Bacteria cells are usually in hypotonic environments (water wants to diffuse into the cell: exposed to tremendous osmotic pressure Rigid cell wall keeps plant and bacteria cells from bursting.

Plant Cells in Different Solutions Hypotonic Isotonic Hypertonic

Facilitated Diffusion Some molecules easily pass through the cell membrane because they dissolve in lipids (alcohols) - others can not (glucose) Specific carrier proteins allow these other molecules to pass through the cell membrane easily This does not require energy (type of diffusion) - only occurs when concentration is higher on one side of the membrane than the other.

Facilitated Diffusion Carrier protein binds to molecule and changes shape Carrier protein shields molecule from hydrophobic lipid bilayer Carrier protein releases molecule inside cell Carrier protein returns to its original shape

Facilitated Diffusion

Ion Channels Some ions are important for cell functions (Na+, K+, Ca2+ and Cl-) Since they are charged and hydrophilic, they can’t get across the lipid bilayer Ion channels – specific membrane proteins that help ions get across cell membrane

Active Transport Requires cell’s energy Cells must move substances up their concentration gradient Substances go from areas of lower concentration to areas of higher concentration

Types of Active Transport Cell Membrane Pumps Endocytosis Exocytosis

Cell Membrane Pumps Carrier proteins also help out in active transport and work against the concentration gradient: Low  High Similar to facilitated diffusion, but cell’s energy is required

Sodium (Na+) - Potassium (K+) Pump Many types of animal cells need to have: a high concentration of Na+ outside the cell a high concentration of K+ inside the cell The sodium-potassium pump uses cell energy to maintain this concentration difference

Sodium (Na+) - Potassium (K+) Pump Three Na in the cytoplasm bind to the protein pump At the same time, the protein splits a phosphate from an ATP molecule (energy) Protein carries the three Na across the lipid bilayer and releases them out of cell

Sodium (Na+) - Potassium (K+) Pump Next the protein binds two K from outside of cell As K bind, the phosphate is released and the protein changes shape again Finally the protein releases the K into the cytoplasm of the cell This creates an electrical gradient across cell membrane (nerve and muscle cells)

Endocytosis and Exocytosis Some substances (macromolecules and food particles) are too large to enter cell through membrane proteins Endocytosis and exocytosis can move these substances across cell membrane Requires cell energy – active transport

Endocytosis Taking material into the cell by means of infoldings, or pouches in the cell membrane Pouches pinch off and form vesicles (organelles) in cytoplasm.

Two Types of Endocytosis Phagocytosis: cell membrane engulfs food or whole cells (bacteria and viruses) Pinocytosis: cell membrane engulfs liquids

Exocytosis Membrane of vesicle surrounding material fuses with cell membrane, forcing contents out of the cell Reverse of endocytosis Release of proteins