CELL MEMBRANE & CELL TRANSPORT

CELL MEMBRANE & CELL TRANSPORT

Homeostasis: Maintaining a Balance Organisms must adjust to changes in their environment. If not DEATH! A formal definition is maintaining a stable internal condition despite what is going on externally.

What Maintains Homeostasis? The PLASMA or CELL MEMBRANE maintains the proper concentrations of materials by controlling the passage of molecules in and out of the cell. Therefore, the cell membrane s function is to maintain HOMEOSTASIS through passive transport, active transport and cell communication!

Cell Membrane X 53, 260 This electron micrograph of the cell membrane shows the appearance of the phospholipid bilayer using several staining processes. The magnification on this structure is 53,260 X

Characteristics of the Cell Membrane

The cell membrane is Selectively Permeable (or) Semi-permeable It allows some things in/out and not others Oxygen, nitrogen,carbon dioxide, and other small, nonpolar molecules can diffuse directly Water was once thought to move directly through, but it is now understood to travel through aquaporins (a type of transport protein). Ions, sugars, and larger molecules move through transport proteins or in vesicles.

Plasma/Cell Membrane-Structure Phospholipid Bilayer with proteins embedded/ floating in it Phospholipid Structure: Polar Head (hydrophilic water loving ) Nonpolar Tails (hydrophobic water fearing )

Phospholipid Polar Head Nonpolar Tails Bilayer Arrangement Outside of cell Inside cell

Proteins are embedded/floating in the lipid bilayer.

Protein in membrane

Cell Membrane Structure-Proteins These proteins are needed for the membrane to function properly. 3 Types of Membrane Proteins 1. Transport - regulate what enters or leaves cell 2. Marker - identify the cell 3. Receptor - allow cells to communicate

Transport Proteins Channel Proteins Function as gates/passageways Allow polar sugars, amino acids, and ions to cross the membrane. Special channel proteins: Gated ion channels gates that open/close Carrier proteins change shape to allow specific molecule to pass Aquaporins allow water to diffuse through (called osmosis)

Marker Proteins Cell s Name Tag Protein sticks out of phospholipid layer Often has carbohydrates attached to outside end Functions in cell identification; identifies the cell to other cells and molecules Important in immunity so various white blood cells in your body do not mistake your cells for foreign cells - blood typing so you can t receive just anytype of blood

Receptor Proteins Function as messenger/receiver Receive information from the environment (extracellular fluid, blood, interstitial fluid) and transmit that info to the inside of the cell Protein has specific shape/charge to only allow certain molecules (like hormones) to bond Triggers a response in cell Ex) epinephrine can bind to a receptor protein and send a message inside that says break down glycogen

Solutions Solution: a mixture of a solute and a solvent water and sugar (koolaid :-) Solute: the substance in a solution that is dissolved by the solvent sugar Solvent: the substance in a solution that is dissolving the solute water

Isotonic Solution Concentration of solute and solvent molecules on one side of the membrane is equal to/the same as the solution on the other side (often the cytoplasm in cell) Water moves in and out at equal rates, no net movement into or out of cell since Cell size would stay the same

Red blood cells in isotonic solution X 1000 Note that all the cells appear normal.

Hypertonic Solution More solute molecules on one side of the membrane than the other When a cell is in a hypertonic solution, water moves out of the cell shrinks (dehydrates) Plants cells shrink (plasmolysis) because cell membrane pulls away from cell wall, so plant wilts because water has left the central vacuole. Animal cell shrink (crenate). In both cases, the cell may die.

Crenated red blood cells in hypertonic salt solution X 1000 Notice that the cells have shrunk.

Hypotonic Solution Fewer/less molecules on one side of the membrane than the other When a cell is in a hypotonic solution, water moves into the cell Cell will swell, and could burst (lyse) Plant cells have vacuoles to collect extra water

Red blood cells in hypotonic solution X 1000 Note that the pinkish cells have swollen (the little dip in the middle of a normal rbc is not visible and one side bows out).

CELL TRANSPORT Concentration gradient - a difference in the concentration of a particular substance across a space. Equilibrium is reached when the molecules are distributed evenly throughout a space.

Types of Transport Passive Transport no energy required, molecules move from high to low concentration (down or with concentration gradient) Diffusion Osmosis Facilitated Diffusion Active Transport energy required, molecules move from low to high concentration (up or against the concentration gradient) Pumps Vesicles

Passive Transport Diffusion, Osmosis, Facilitated Diffusion

Diffusion is the process by which molecules spread from areas of high concentration, to areas of low concentration Molecules are said to go down or with the concentration gradient. Passive Transportrequires no energy

Osmosis is the diffusion of water molecules through a semi-permeable membrane requires no energy Ex. Water will move toward the higher concentration of solute (and hence a lower concentration of water).

Facilitated Diffusion (uses Transport Proteins, passive) Moves substances (from high to low concentration) down the concentration gradient without using cell s energy using channel/carrier proteins located in membrane

Active Transport Requires energy from the cell because materials are being moved against the concentration gradient

Why do cells need active transport? Cells must maintain a certain concentration of molecules (amino acids, sugars, etc.) inside their cytoplasm from the surrounding fluid Some of these must be moved against the concentration gradient (there are already more molecules inside the cell)

How does active transport work? Some types involve carrier proteins that function as pumps Other types use vesicles Energy is provided by ATP (more on ATP later)

Active Transport Using Pumps Sodium/Potassium Pump Na + pumped out of a cell K + pumped into a cell Important because it prevents cells from bursting by lowering the sodium inside causing less water to enter through osmosis. Used by many cells, including nerve cells, to send a chemo-electric message.

Sodium-Potassium Pump Sodium/Potassium pump: 3 Na+ out of cell (yellow diamonds) 2 K + into cell (red/purple squares) Here the energy of a phosphate from ATP (shown in pink) is used to exchange sodium atoms for potassium atoms.

Proton (H+) Pump Forces protons out of a membrane enclosed space to create a proton gradient down which protons flow back in The cell uses one ATP to pump a proton out; that proton can be used to bring in large molecules with minimal energy output

Active Transport with Vesicles Exocytosis & Endocytosis

Endocytosis Substances are moved into a cell by a vesicle that pinches off from the cell membrane Requires energy (ATP)

Types of Endocytosis Pinocytosis - when the nutrient particles are dissolved in a liquid; cellular drinking Phagocytosis - when the nutrient particles are solids; cellular eating

Exocytosis Exocytosis- substances inside a vesicle are released from a cell as the vesicle fuses with the cell membrane Involves the cell expelling waste or the cell secreting cell products (ex. Hormones, insulin) Requires Energy (ATP)

Cell Transport Animations http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation phagocytosis.html http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120068/bio03.swf::sodiu m-potassium%20exchange%20pump http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120068/bio02.swf::endoc ytosis%20and%20exocytosis