Plasma membrane

• Cholesterol molecules are embedded within the lipid bilayer to maintain its fluidity at different temperatures.
• Proteins can be found on both sides of the cell membrane, including integral proteins that span across it and peripheral proteins attached to one side.
• Cholesterol helps maintain fluidity by preventing excessive packing of lipid molecules.
• The membrane is too thin to see with a light microscope, but can be resolved with a transmission electron microscope.
• Cell membrane, also called plasma membrane. A phospholipid bilayer that encloses the contents of a cell, and control, the passage of substances into and out of the cell. Cell wall is fully permeable.
• Main components: Phospholipids, proteins, carbohydrates, cholesterol.
• Carbohydrates: 1. Glycoproteins are carbohydrates attached to an integral protein. Generally antigens/self markers. 2. Glycolipids are carbohydrates attached to a lipid. Involved in tissue identification and cell recognition.
• Each phospholipids has a polar, hydrophilic, positively charged, phosphate containing head, and two polar hydrophobic, no charge, fatty acid tails. Double layer (bilayer) of phospholipids. 2 fatty acids joined by a phosphate containing (head).
• Integral proteins extend to the hydrophobic space in the membrane. Transmembrane proteins are integral proteins that extend from one side of the membrane to the other. Outside bilayer, peripheral proteins interact only with the hydrophilic heads - temporary proteins, e.g cytochrome C.
• Integral proteins: channelling or transporting molecules across the membrane. Peripheral proteins: support, communication, enzymes, and molecule transfer in the cell.
• Diffusion is the tendency of molecules to disperse until they are evenly spread out, reached equilibrium, which is a low energy state.
• Facilitated diffusion occurs when substances move through channels (open/close depending on needs) or carrier proteins (bind to substances being transported and undergo a shape change to push molecules through) embedded within the plasma membrane. Channels allow polar or large molecules to pass easily. Carrier proteins bind to larger polar molecules and change shape to let them pass through. High concentration to low concentration.
• Active transport requires ATP to pump against concentration gradient. Transported substance moves into higher concentration area. Active transporters have specificity for certain molecules.
• Simple diffusion: diffusion of solutes across a phospholipid bilayer. Molecules can enter a cell through the bilayer, if they are small and non-polar. High concentration to low concentration.
• Steeper gradient=faster movement.
• Examples of simple diffusion: Oxygen and Carbon dioxide, glucose. Ions are unable to cross membrane because they are charged.
• Osmosis is the passive net movement of free-water molecules through a partially permeable membrane from a region of low solute concentration to a region of high solute concentration.
• Solute is a molecule dissolved in solvent.
• Semipermeable membrane: membrane which water is able to cross. Also called a partially permeable membrane or a differently permeable membrane.
• Osmolarity: total concentration of solute, in a solution. Isotonic: same osmotic pressure as the cytoplasm. Hypertonic: higher osmolarity than the cytoplasm. Hypotonic: lower osmolarity than the cytoplasm.
• Gated channel proteins=only open in response to certain stimuli e.g electric potential
• Active transport involves energy in the form of ATP. Involves polar or large molecules.
• ATP=adenosine triphosphate
• Bulk transport: type of active transport that moves large molecules or groups of molecules in and out of the cell.
• Endocytosis: Fold - the PM folds inwards to form a cavity that fills with extracellular fluid and the target molecules. Trap - the plasma membrane continues folding back until 2 ends meet and fuse, trapping the target molecules inside the vesicle. Bud - the vesicle pinches off from the membrane.  Then transported to the appropriate cellular location or to a lysosome for digestion
• Active transport is the active movement of molecules against their concentration gradient, across a membrane via specific transmembrane integral proteins (by a carrier protein) an active process. Low conc to high conc. Generally use protein pumps.
• Exocytosis: process of bulk movement (the active movement of a large molecule into or out of a cell, by enclosing it in a membrane sac, by which large molecules e.g proteins are secreted from a cell by the fusion of a vesicle with the plasma membrane. Possible because of the fluids nature of the cell membrane.
• Endocytosis: reverse of exocytosis, bulk transport into a cell from the extracellular fluid. Cell's membrane furrows, or "invaginates", to surround a particle, and engulfs (often vesicles as they contain digestive enzymes) it within a vacuole.
• Two types of endocytosis: Phagocytosis is the endocytosis of solid material such as bacteria. Pinocytosis is the process by which the cell takes in the fluids along with dissolved small molecules.
• Pinocytosis: endocytosis of fluid material, such as lipids.
• Turgidity: absorption of high fluid content.
• Osmosis moves down a concentration gradient. Does not require energy and proteins. E.g H2O.
• Simple diffusion moves down a concentration gradient. Does not require energy and proteins. E.g small non-polar molecules O2.
• Facilitated diffusion moves down a concentration gradient. Energy are not required. However, channel or carrier proteins are required. E.g large molecules like glucose.
• Active transport moves up a concentration gradient, energy is required, protein pumping or carrier proteins are required. E.g large polar molecules.