The Cell Membrane and Homeostasis
Homeostasis Maintaining a Balance A cell must always maintain an internal balance (homeostasis) while carrying out essential life processes. These include
Obtaining food Converting energy Eliminating wastes Reproducing Growth and repair Transporting substances
The Cell Membrane The cell membrane helps the cell achieve homeostasis by regulating the entrance and exit of molecules into and out of the cell. Selectively permeable
Structure of the Cell Membrane: 2 Parts 1) Phospholipid bilayer (2 layers of phospholipids) with a fluid consistency. 2) Embedded with proteins and strengthened with cholesterol molecules.
What s a Phospholipid? A phospholipid contains a phosphate head and 2 lipid tails Hydrophilic(water-loving) polar heads face the outside and inside of the cell and the hydrophoebic(water-fearing) nonpolar tails hang inside and face each other.
What s a Phospholipid?
Membrane Proteins Proteins may be either peripheral or integral. Integral proteins span from inside the bilayer to outside the cell. Integral = Peripheral proteins are found either on the inside or outside surface of the bilayer. Peripheral =
Functions of Membrane Proteins Peripheral proteins help stabilize the shape of the cell membrane.
Integral proteins can be of five types: Channel proteins allow substances to move freely across the cell membrane. Carrier Proteins combine with a substance to help it move across the cell membrane Receptor Proteins shaped so that a specific molecule can bind to it and bring about a cellular response Enzymatic proteins catalyze a specific reaction Recognition proteins acts as a fingerprint or barcode. Become important for blood and organ donation.
Diagram Time! Grab 7 different colours! In the worksheet in your notes, draw the model from the board, including all labels. Colour in as you work.
Selective Permeability of the Cell Membrane: Crossing the cell membrane can either be passive or active. We will discuss Passive Transport first. Passive transport does not use chemical energy and involves diffusion, facilitated transport, or osmosis
Passive Transport 1. Diffusion movement of particles from an area of high concentration to an area of low concentration. -Continues until an equilibrium is reached (no gradient). -Dynamic equilibrium particles move freely and are evenly distributed. 2. Facilitated diffusion embedded proteins act as tunnels allowing particles to fall through.
3. Osmosis Diffusion of water across a selectively permeable membrane. Occurs until water is balanced on both sides of the membrane. Osmosis Video - http://www.youtube.com/watch?v=hqkl Lm2MjkI
Cell Concentrations Isotonic solutions A solution with equal amounts of solute and water as inside the cell. If a cell is placed in isotonic solution there is no net gain or loss of water. Hypertonic solutions A solution with more solute and less water than inside the cell. If a cell is placed in hypertonic solution, water will leave the cell and the cell will shrink or shrivel Hypotonic solutions A solution with less solute and more water than inside the cell. If a cell is placed in hypotonic solution water enters the cell and the cell swells and can even burst.
Grab 2 colours! Using the blank chart at the end of your notes fill in the chart as Mrs.Connor creates it on the board.
Overcoming Osmosis When cells are placed in high water environments, they must overcome the passive movement of water. This is especially true for organisms or cells that live in aquatic environments. There are 3 ways this can occur: plasmolysis, turgor pressure and using contractile vacuoles.
Contractile vacuoles expel excess water from bacterial cells that live in water. Plasmolysis water moves out of the cell because it was placed in a hypertonic solution. This causes a shrinking of the cytoplasm in a plant cell. Results in wilting. Turgor pressure water moves into a cell because it was placed in a hypotonic solution. The increased amount of water causes an outward pressure on the cell wall in a plant cell.
Contractile Vacuole Video http://www.youtube.com/watch?v=ig6d d3coug4
Active Transport Active transport requires both a carrier protein and chemical energy (ATP). This can involve endocytosis and exocytosis. An example is facilitated diffusion.
A Closer Look at Facilitated Diffusion: Facilitated diffusion can also be carried out by carrier proteins. Molecule to be transported enters carrier protein, molecule combines with carrier protein and carrier protein undergoes a change in shape that releases the molecule on the other side of the membrane.
Active Transport 3 Types of Facilitated Diffusion Carrier proteins embedded proteins change shape to open and close passages across the membrane. Endocytosis taking something into the cell. Exocytosis expelling something from the cell.
Active Transport: Ions or molecules move through the plasma membrane against their concentration gradient this requires energy (up to 40% of a cell s energy supply) ATP (energy) is required for the carrier protein to change its shape and deposit the substance on the other side of the membrane. Proteins involved in active transport are often called pumps because they use energy to move substances against their concentration gradients.
Example of Carrier Protein: Sodium-Potassium Pump Active in all animal cells, especially nerve and muscle cells Moves sodium ions to the outside of the cell and potassium ions to the inside. The carrier protein involved is called a sodium-potassium pump
For every 3 ions of sodium taken out of the cell, 2 ions of potassium are moved into the cell. http://www.youtube.com/watch?v=gth Wig1vOnY
Endocytosis: Vesicles form to take substances into the cell. Two types: Phagocytosis when vesicles form around large material, such as a food particle or another cell. Pinocytosis - When vesicles form around a liquid or very small particles to bring them into the cell
An amoeba taking in food through phagocytosis
Exocytosis: During exocytosis, vesicles fuse with the plasma membrane as secretion occurs In humans, molecules transported out of the cell by exocytosis include digestive enzymes produced by the pancreatic cells and hormones such as growth hormone produced by pituitary cells.
Tear fluid leaving a tear duct through exocytosis