Homeostasis and Transport Organisms respond to the environment (this is a characteristic of life!) Cells, tissues, organs, and whole organisms must maintain a biological balance with their immediate environment Each cell sustains this balance (homeostasis) by regulating what substances can enter and/or leave The organelle that regulates this is… the CELL MEMBRANE

The transport of substances across the membrane can occur in 2 different ways… PASSIVE TRANSPORT: NO energy is required ACTIVE TRANSPORT: Energy (in the form of ATP) is required

Passive Transport Diffusion: Passive Transport: the movement of some substances across the cell membrane without any energy input Diffusion: A natural movement of molecules from an area of high concentration to an area of low concentration In other words: the molecules move down their concentration gradient

Passive Transport (cont.) Continues until the system has reached a state in which the concentration of molecules is equal everywhere in the system → this state is called EQUILIBRIUM Molecules are still in motion in a system which has reached equilibrium, but there is no concentration change

Diffusion across the cell membrane Molecules diffusing across the cell membrane move DOWN their concentration gradient Small, non-polar molecules are able to simply diffuse through the lipid bilayer → examples: CO2 and O2 It is important to remember that NO ENERGY is being used here

Facilitated Diffusion ... is used to transport molecules needed by the cell that are very large and insoluble in lipids Examples? IONS, ORGANIC POLYMERS Their diffusion must be facilitated (helped) by a part of the membrane Ion Channels Some proteins embedded in the membrane create a pore through which specific ions (charged atoms) can travel Gated channels open/close in response to stimuli Carrier Transport Some proteins embedded in the membrane actually “carry” a specific needed molecule across the cell membrane A molecule is “received” on one side, slides through the carrier, and is “spit out” the other side

OSMOSIS Hypotonic: Describes an area of low solute concentration Osmosis: Diffusion of water molecules through a selectively permeable membrane Hypotonic: Describes an area of low solute concentration (Therefore, solvent concentration is high) Hypertonic: Describes an area of high solute concentration (Therefore, solvent concentration is low) Isotonic: Describes a situation in which there is no difference in solute concentration Rule of Thumb: Water moves towards high solute

Osmosis and Living Cells If a living cell found itself in a solution that was extremely hypotonic to its internal conditions, what problems could this cause? → Hint: you should first think about which way water would tend to move in this situation The Paramecium Unicellular organism that lives in fresh (hypotonic) water Has an organelle called a contractile vacuole which pumps out water that is constantly diffusing into the cell *** Since this pumping action is being done against the concentration gradient, it requires energy!

Why do plants wilt? QUESTIONS: In addition to the cell membrane… What “barrier” do plants possess? Where is it located? Plant cells, when watered regularly, find themselves living in hypotonic solution Water, therefore, moves into the plant cells by osmosis and the cell membrane swells, pressing it against the cell wall (this force is called TURGOR pressure) Plant cells, when NOT watered regularly, find themselves instead living in hypertonic solution Water, therefore, moves out of the plant cells by osmosis and the cell membrane shrinks away from the cell wall (this condition is called PLASMOLYSIS) → wilting plants! REMEMBER: Rule of Thumb: Water moves towards high solute

Evaluating Osmotic Scenarios Solution Water will move… Result Hypotonic Into the cell Animal cells undergo cytolysis (burst) Plant cells gain turgor pressure Hypertonic Out of the cell Animal cells shrivel/shrink Plant cells undergo plasmolysis Isotonic in/out of the cell (at equilibrium) Cells are happy and healthy

HYPOTONIC HYPERTONIC Cells in Pure water Cells in Salt-water

Active Transport ACTIVE TRANSPORT Requires the input of energy (ATP) Recall: Passive transport occurs down a concentration gradient, and therefore does not use energy! ACTIVE TRANSPORT Requires the input of energy (ATP) Transports molecules from an area of lower concentration to an area of higher concentration

Cellular “Pumps” Some membrane carrier proteins (like the ones involved in facilitated diffusion) also have a function in active transport These proteins use ATP and act like pumps, which force specific molecules against (up) their concentration gradient

The Sodium-Potassium (Na+ / K+) Pump Transports sodium and potassium ions UP their concentration gradients A common example of a cellular pump found in many animal cells Result: Three sodium ions are moved out of the cell Two potassium ions are moved into the cell Cost: 1 ATP

Endocytosis Allows extremely large particles (such as food) entry into the cell without passing through the membrane The cell membrane folds inward and pinches off, which results in the creation of a vesicle Pinocytosis – the transport of bulk fluids into the cell Phagocytosis – the transport of bulk solids into the cell

Exocytosis Allows extremely large particles (waste, for example) to exit the cell without passing through the membrane A vesicle inside the cell fuses with the cell membrane, releasing its contents outside Picture endocytosis occurring backwards