Cell Membrane

Structure Cell membranes consist of mostly a phospholipid bilayer (fat). The membrane is fluid, meaning it can change shape and parts can move around the cell membrane. The ends of the lipids are charged (Hydrophilic) and pointed toward the edge of the membrane (both in and out) The central region is long fatty acid chains which are non-polar (Hydrophobic) Cholesterol is used to support and fill gaps within the membrane

Integrated into the lipid complex are integral proteins used for stability and transport (also called Carrier Proteins) Two Types Integrated proteins (transmembrane) go all the way through the membrane (connect the intercellular matrix to the extra-cellular matrix) Peripheral membrane proteins attach to the outside on the membrane and only partially penetrate the membrane Carbohydrates are used as receptor sites and recognition sites on the membrane

What can pass? Polar molecules can pass through the polar heads of the phospholipids, but cannot pass the non-polar region. Non-charged molecules can pass through the fatty acid, non-polar region. Molecules have to be small to pass the polar region. The main route in or out of the cell is through carrier proteins which form channels (can be opened or closed)

Passive transport A cell can allow molecules to move freely (without using energy) through channels and pores by the osmotic gradient. Molecules move from high concentration to low concentration (diffusion). The goal is to reach equilibrium. This happens naturally, but can go both ways: molecules can move in, in the case of low interior concentration, or can move out of the cell when in high concentrations

Diffusion

Osmotic Gradients The diffusion of water across the membrane is known as Osmosis. Osmosis is dependent on the concentration of solutes 3 types of gradients: Hypotonic – solute concentration is higher inside of the cell so water flows into the cell (turgor pressure) Isotonic – both inside and outside concentrations are equal, allowing an even flow in and out Hypertonic – solute concentration is higher outside the cell causing water to flow out of the cell

Kidney Function Salt is filtered out of the ascending loop into the kidney cells. This creates a concentration gradient between the fluid and kidney cells, causing water to flow out of the collecting duct and descending loop.

Reverse Osmosis Using salt water and fresh water A tank of fresh water is placed across a semi-permeable membrane from a fresh water tank. What should happen? To create reverse osmosis you simply need to push enough force to overcome the osmotic force

Facilitated Diffusion Facilitated diffusion is the diffusion of molecules using a protein channel rather than going through the lipid bi-layer. Molecules still flow from a level of high concentration to low concentration and do not require energy to move. Combined transport: Symport: movement of two particles in the same direction (requires both particles to activate) Antiport: movement of two particles in opposite directions (requires both particles to activate)

Active Transport Active transport requires energy to move a particle across the cell membrane. Can move particles against the concentration gradient (low to high concentration) Can move large molecules such as carbohydrates, lipids and proteins

Movement across the Membrane The particle attaches to the outer membrane at active sites (recognition sites) 2 possibilities: Using ATP causes the integral protein to change shape, allowing the particle to pass through a now open channel Or the cell can activate the process of endocytosis (also called phagocytosis)

Pumps Pumps are membrane proteins that require energy in order to push materials across the membrane. Can move particles against the concentration gradient. Can increase the rate at which particles move by simple diffusion.

Endocytosis Also called phagocytosis When a molecule it attached at a recognition site, the cell membrane will pinch in slightly to form a groove around the molecule The cell membrane then folds over top of the molecule to encase it in a membrane (like a scab covering a cut) The interior portion of the membrane is removed, and the molecule is now in the cytoplasm (think PAC-MAN)

Example = Nerve Cells How does a neuron fire? In order to send a signal to your brain, your nerves must fire an electrical signal that is carried to the Central Nervous System. A nerve cell uses what is called a Sodium-Potassium Pump (active antiport). Sodium (Na+) is actively pumped out of the cell to create a concentration gradient (high outside and low inside) Potassium (K+) is actively pumped the opposite direction, high concentrations inside, low outside

When a nerve needs to fire a signal, the pumps stop which allows Na+ ions to flow into the cell and K+ ions to flow out. The result of the ion flow is an electrical signal

Nerve Cell In order to create a concentration gradient what mechanism needs to be working in a nerve cell? Would it be a pump or endocytosis?

Breathing What part of your lung facilitates the exchange of oxygen between the air and your blood? Does air ever directly contact your blood? Why or why not? What is the concentration gradient between the air and your blood? Is your blood high or low in O2? CO2? Is the air high or low in O2? CO2?

Exocytosis Removal of particles from inside the cell Two functions Removes large wastes, diseases, etc Can pass cellular products like proteins, carbohydrates (Plants) for usage in other areas or for storage