Chapter 5 Homeostasis and Transport

Homeostasis & Transport BIG IDEAS: Cell Membranes help organisms maintain homeostasis by controlling what substances enter and leave the cell. Some substances can cross the cell membrane without any energy input from the cell. Some substances require a cell to expend energy to enter or leave the cell.

How Materials Enter and Leave Cells How does the cell move materials into and out of a cell? One way molecules of a substance get into and out of a cell is by a process known as passive transport Passive transport is the diffusion or movement of molecules across a cell membrane. It does NOT REQUIRE any ENERGY input from the cell. For cells to remain alive, a certain combination of chemicals must be able to be present inside its cell membrane. The cell membrane is one of the structures that helps maintain this special environment inside and outside the cell.

The Cell Membrane The cell membrane is Selectively Permeable. Meaning, it allows the passage of some substances and blocks other or not required substances. Gym Closet filled with balls. Open door – fall out. Diffusion happens automatically with a cell membrane if it permeable to the molecules and if there is a different concentration of molecules on either side of the cell membrane. This movement is called passive transport because no energy is required by the cell to get the work done.

The Cell Membrane This selectively permeable membrane only allows certain things in/out of the cell. Made of protein and lipids BILAYER-two layers; the head of the molecules face outward while the tails face inwards. PHOSPHOLIPID-molecule that makes up the cell membrane. Has two parts. 1. The phosphate head (hydrophilic – water loving) 2. Two lipid tails (hydrophobic – water hating)

Passive transport Molecules diffuse or move from an area of higher concentration to an area of lower concentration. THEY MOVE DOWN A CONCENTRATION GRADIENT! Substances crossing the cell membrane without any input of energy by the cell – move down their concentration gradient Types: Simple Diffusion Osmosis Facilitated Diffusion Ion Channels http://www.youtube.com/watch?v=JShwXBWGMyY

?? Concentration Gradient = a gradient results from an unequal distribution of dissolved molecules across the cell membrane. When this happens, solutes downconcentration gradient. This kind of movement is called diffusion. DOWN A CONCENTRATION GRADIENT. .

Diffusion Diffusion Stops when…Molecules reach EQUILIBRIUM The concentration of the molecules is the same throughout a space. No more diffusion! But molecules keep moving. EXAMPLE: food coloring, cookie smell in your house, etc. The simplest type of passive transport is diffusion. Diffusion is the natural movement of molecules from an area of high concentration to an area of low concentration. Molecules naturally intermingle as a result of their kinetic energy of random motion. .

Diffusion Add a sugar cube to a beaker of water It sinks to the bottom making the concentration of sugar higher there As the cube dissolves sugar molecules break away and move from the bottom to the top of the beaker Because of their kinetic energy, the molecules of sugar are in constant motion They keep moving until they hit something and then they rebound 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

Diffusion The molecules are still moving, but they are just as likely to move in one direction as the other – they balance each other

Diffusion Across a Cell Membrane REMENBER - The cell membrane is Selectively Permeable. There are tiny pores or holes in it that allow some molecules to diffuse into the cell. For example, oxygen molecules diffuse into your blood stream through the membranes of cells in your lungs. http://videos.howstuffworks.com/hsw/23424-cell-diffusion-across-the-cell-membrane-video.htm The molecules are still moving, but they are just as likely to move in one direction as the other – they balance each other

Osmosis takes place through passive transport. A cell is made up mostly of water and dissolved substances. Water can move into or out of a cell through the cell membrane. The diffusion of water through a cell membrane is called osmosis. When plants are put in a salt or sugar solution, the abundant water inside the plant cell automatically moves out of the cell where there are more salt (solute) or sugar molecules and relatively few water molecules.

Osmosis Osmosis maintains the balance of water inside and outside the cell. Question?? If there is more salt outside of the cell, will water move into the cell or out of the cell?? The direction that water moves depends on the amount of dissolved substances inside and outside of a cell. If there are more dissolved substances inside the cell, water will move into the cell and vise-versa. Water will move to where there is LESS WATER. When plants are put in a salt or sugar solution, the abundant water inside the plant cell automatically moves out of the cell where there are more salt (solute) or sugar molecules and relatively few water molecules.

Water goes where it aint !! 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

Example of Osmosis http://www.stolaf.edu/people/giannini/flashanimat/transport/osmosis.swf

3 Types of Environments The net movement of water, or osmosis, depends on the type of environment the cell is in. Three types of environments: Isotonic Hypertonic Hypotonic Not an absolute – it is a relationship between two entities.

Isotonic: Concentration of water is equal inside and outside of the cell. Water will diffuse into and out of the cell at an equal rate No net movement of water. Cell stays the same.

2. Hypertonic More water inside the cell, less outside. More solutes outside the cell than INSIDE the cell. Water moves out of the cell until equilibrium is reached. Net movement of water is out of the cell. Can cause the cell to shrink or shrivel. Crenation in animal cells Plasmolysis in plant cells

3. Hypotonic More water outside the cell, less inside the cell. More solutes inside, less solutes OUTSIDE the cell. Net movement of water into the cell. Can cause cell to burst. CYTOLYSIS-cells take in so much water that they burst.

The Effects of Osmosis on Animal Cells

Plant cells store water Osmosis in Plants Remember Plant cells store water in vacuoles. http://www.kscience.co.uk/animations/turgor.htm FYI - Plants whose cells do not have enough water appear dry and may droop. However, once the plant receives water, its cells may absorb the water through osmosis. Once water is absorbed into its cells, the plant swells. It will probably stand up straight and appear healthy. This is referred to as “Turgor” and is determined by osmosis. 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.

DRAW Osmosis in Plant Cells Plant cell membranes can be seen pulling away from their cell wall, in a process called plasmolysis, as their cells lose water. DRAW Osmosis in Plant Cells Plant Cell Cell Wall Vacuole Cytoplasm Cell Membrane The Cell absorbs water …But the cell wall stops by osmosis… the cell from expanding Limp celery in water… As water moves into a plant cell, the cell expands as the cell membrane pushes against the cell wall and the plant becomes firm or, turgid again (turgor pressure)

DRAW Cells in 3 Types of Solution! crenation cytolysis plasmolysis

Remember: SALT SUCKS!!! Wherever there is more solute (i.e. salt, sugar), that is where water will be pulled to  WATER WILL MOVE TO WHERE IT AINT!!

Do Now Respond to the following in your journal: Why do supermarkets have sprinkler systems set up in their produce section? Explain.

Facilitated Diffusion Like diffusion, molecules are still moving from an area of higher concentration to an area of lower concentration. Facilitated Diffusion: Facilitated diffusion is the form of passive transport in which carrier proteins embedded in the cell membrane help or facilitate the movement across it. It does not require the cell to use any energy. The carrier proteins act as gates. Glucose molecules move across the membrane by 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

Ion Channels Like diffusion, molecules are still moving from an area of higher concentration to an area of lower concentration. 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 c ell membrane – some are always open, some are gated and require a stimuli to open. EXAMPLE: impulse from nerve travels to muscle cell-causes some gated channels to open. It does not require the cell to use any energy.

Active Transport Active Transport: Most plants get water and minerals they need from their roots. Minerals enter the root by active transport. Active transport is also needed to move certain waste materials out of a cell. Active Transport: Movement through the cell membrane that is the opposite of diffusion. Molecules of certain substances move through the cell membrane from an area that is less crowded to an area that is more crowded is called ACTIVE TRANSPORT. Energy is needed to move these materials across the cell membrane - Molecules move AGAINST the concentration gradient!! http://www.youtube.com/watch?v=STzOiRqzzL4&feature=related Moving substances against their gradient requires energy, because they are being moved away from equilibrium. Cells use active transport, which requires ATP, to move substances in this way.

Types of Active Transport Cell Membrane Pumps Endocytosis Exocytosis

SODIUM (Na+)/Potassium (K+) Pump Cell Membrane Pumps SODIUM (Na+)/Potassium (K+) Pump Uses a special carrier molecule. K+ (potassium) goes into the cell. Na+ (sodium) leaves the cell Uses energy from the cell Carrier proteins also help out in active transport and work against the concentration gradient: Low High Similar to facilitated diffusion, but a cell’s energy is required. 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

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 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 cells)

Passive vs. Active Transport Low Concentration High Concentration Passive Transport High Concentration Active Transport Low Concentration

Endocytosis & Exocytosis Cells have two processes of moving large particles across the cell membrane: Endocytosis and Exocytosis Endocytosis is the process by which the cell Membrane wraps around the large particle, forms a vesicle, and pulls it inside the cell. Once inside, the vesicle releases the particle. 2 Ways: 1. Pinocytosis-movement of solute or fluid into the cell. 2. Phagocytosis-movement of large food particles or entire microorganisms into the cell. 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 & Exocytosis Exocytosis is exact opposite. When the cell wants to get rid of a large particle, it encloses the particle inside a vesicle. The vesicle fuses with the cell membrane and the cell can then release the particle. http://media.pearsoncmg.com/bc/bc_campbell_biology_7/media/interactivemedia/activities/load.html?7&G

Endocytosis & Exocytosis