Homeostasis, Osmosis, Transport Unit 6 – Chapter 5

Diffusion Through Cell Boundaries All living cells need a watery environment to survive! The cell membrane helps organisms maintain Homeostasis (Equilibrium) by controlling what substances enter or leave the cell To remain alive, cells must maintain biological balance. Cells maintain this balance (homeostasis) in response to their immediate environment

Types of Cellular Transport Passive Transport cell does not use energy Diffusion Osmosis Facilitated Diffusion Active Transport cell does use energy Protein Pumps Endocytosis Exocytosis high low Weeee!!! high low This is gonna be hard work!!

3 Types of Passive Transport Diffusion – constant motion of molecules that causes them to spread out from high to low concentrations Osmosis – diffusion of water Facilitated Diffusion – diffusion with the help of transport proteins in the cell membrane

Diffusion concentration (concentration gradient) Equilibrium occurs when the concentration of solute (particles) is the same throughout (the particles still move!) Because diffusion depends upon random particle movements (kinetic energy), substances diffusion across membranes without cellular energy

The Dye = Solute Water= Solvent (In cells, water is always the Solvent). Equilibrium!

Law of Diffusion Substances ALWAYS diffuse from HIGH to LOW concentrations. This fact is key to understanding much of this chapter. This is called moving DOWN the Concentration Gradient.

OSMOSIS Osmosis is the name for an important type of diffusion. It is the diffusion of water across the cell membrane. Since cells are usually bathed in a watery environment, they have to deal with water moving in/out of them. Too much water in or out of the cell can become a problem.

Osmosis – water moves from high to low concentration 4 100% pure water 90% water 10% salt level falls level rises ...until water concentrations become equal membrane More water passes from Pure water to salt solution...

Water passes easily across membranes Osmosis Water passes easily across membranes Osmosis is the diffusion of water across a selectively permeable membrane Osmosis exerts a pressure known as osmotic pressure on the hypertonic side of a selectively permeable membrane

Osmosis between cells 20 If the concentration of the cell sap is greater in one cell than in its neighbour, water will pass by osmosis from the less concentrated to the more concentrated. cell sap more concentrated cell sap less concentrated Osmosis between one cell and the next plays a part in the movement of water throughout the tissues of the plant.

Osmosis in animal cells so water diffuses into the cell by osmosis There is a greater concentration of free water molecules outside the cell than inside and the cell swells up

Plant cells cell wall vacuole cytoplasm and cell membrane The cell absorbs water by osmosis .... ....but the cell wall stops the cell expanding any more

Solutions The relative concentrations of solutions to one another inside/outside of the cell can lead to 3 different situations. These situations are known as: Isotonic Hypertonic Hypotonic ** The next few slides will illustrate how these situations affect the cell.

Isotonic Concentration of Solute - (material dissolved in the water) is equal on both sides of the cell membrane

Hypertonic Solute concentration is greater outside the cell, so water moves OUT of the cell Remember, hypertonic, the cell shrinks The shrinking of cells is called Plasmolysis

Hypotonic Solution concentration is greater inside the cell, so water moves INTO the cell Remember, hypotonic, the cell POPS!!! The bursting of cells is called Cytolysis

How Single Celled Critters Deal with Osmosis Unicellular organisms in hypotonic environments need to get rid of the excess water that diffuses into them Contractile vacuoles are organelles that collect water and pump it out of the cell (uses energy)

How Multi-celled Critters Deal with Osmosis Other cells (especially in multicellular organisms) respond to a hypotonic environment by pumping solutes out of the cytoplasm Water molecules are less likely to diffuse into the cell

Facilitated Diffusion Ion Channels **Refer to the next 2 slides. Types of Passive Transport (How cells transport materials in/out of themselves) – NO CELL ENERGY REQUIRED Osmosis Facilitated Diffusion Ion Channels **Refer to the next 2 slides.

Facilitated Diffusion Some molecules cannot diffuse through the cell membrane because they are: Not soluble in lipids Or are too large to pass through the pores in the membrane (I.E. Glucose) These molecules are helped across the membrane by carrier proteins The carrier proteins change shape after the molecule binds to them The molecule is passed through the membrane (either in or out) No cellular energy is used, so it is still passive transport

Diffusion Through Ion Channels Ions such as sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl-) are important for cell functions Since they are not soluble in lipids they will not pass through the cell membrane on their own

Diffusion Through Ion Channels… Ion channels provide small tunnels across the cell membrane Each type of ion channel is usually specific for one type of ion Some channels are always open, some are gated The gates respond to three stimuli: Stretching of the cell membrane Electrical signals Chemicals in the cytosol or external environment No energy is used, so it is still passive transport

Active Transport – (cells actively work to move some substances in/out) – CELL ENERGY IS REQUIRED Pumps in the cell membrane – proteins in the cell membrane use cell energy to change their shape to actively pump molecules in/out of cell. Ex.) Sodium/Potassium Pump. Endocytosis – moving very large molecules INTO the cell. Cell wraps its membrane around the large molecule. This requires the cell to spend energy. Exocytosis – moving large OUT OF the cell. Cell membrane changes its shape to push molecule out of cell. This requires cell energy. ***See pages 101 to 104 in book.