Cell membrane

• The plasma membrane, also known as the cell surface membrane, is a semi-permeable barrier that controls the movement of substances into and out of the cell.
• The plasma membrane is composed of phospholipids, cholesterol, proteins, glycolipids, and glycoproteins.
• One fatty acid chain in a triglyceride is replaced by a phosphate group.
• The phospholipid bilayer is fluid due to the movement of phospholipids and proteins within its monolayer.
• The fluidity of the plasma membrane is affected by temperature, the ratio of unsaturated to saturated fatty acids, the length of phospholipid tails, and the presence of cholesterol.
• Cholesterol regulates the membrane's fluidity by increasing fluidity at lower temperatures and decreasing fluidity at higher temperatures.
• Cholesterol helps maintain fluidity by preventing excessive packing of lipid molecules.
• Membrane proteins can be extrinsic or peripheral, which are located on the inner or outer surface of the membrane, or intrinsic or integral, which extend into the hydrophobic core and maybe mobile or fixed.
• Phospholipids are amphiphilic molecules with hydrophobic tails and polar heads that form the bilayer structure of the cell membrane.
• Some membrane proteins are transmembrane proteins, which span across the membrane.
• Steepness of the concentration gradient, temperature, number of transport proteins available, surface area of the membrane, and osmotic pressure influence the rate of uptake of glucose into animal cells across the plasma membrane.
• Osmosis involves the diffusion of water from a region of high water potential to low water potential down the water potential gradient across a partially permeable membrane until equilibrium.
• In an experiment, maltose diffuses out, causing less water to diffuse in by osmosis.
• If uptake of glucose into cells was passive diffusion, the rate would continue to rise.
• Water potential, Ψ, depends on the concentration of the solution and the pressure applied to it.
• When the external solution is hypertonic to the plant cell, water leaves the plant cells by osmosis.
• Water potential, Ψ, in plant cells is determined by the concentration of the solution and the presence of a cell wall.
• If uptake of glucose into cells was active transport, the rate would be independent of concentration except at low concentration.
• Glucose enters cells by facilitated diffusion because the rate of uptake increases with increasing glucose concentration, up to a plateau.
• Membrane proteins can have various roles such as cell signaling, cell recognition, cell-to-cell adhesion, site for enzymes to catalyse reactions, anchoring for the cytoskeleton, and forming H bonds with water for stability.
• The cell membrane is composed of phospholipids, cholesterol, proteins, and carbohydrates.
• The process of cell signalling involves secreted ligands, transported ligands, ligands binding to cell surface receptors, receptor activation, and the production of secondary messengers by a G protein.
• Intrinsic proteins have both hydrophobic and hydrophilic regions, with the hydrophobic regions interacting with the hydrophobic core of phospholipids and the hydrophilic regions extending into the aqueous external environment inside or outside the cell.
• Glycolipids interact with water to stabilize membrane structure, form H bonds with water molecules, and play roles in cell-to-cell adhesion and cell recognition.
• Membrane proteins play roles such as transport proteins, enzymes, receptors for cell signalling molecules, anchoring cytoskeleton, and cell-to-cell adhesion.
• Cell signalling involves how cells detect and respond to stimuli, and how cells communicate, involving specific chemicals known as ligands that lead to specific responses.
• Glycolipids are composed of glycerol and fatty acids, with carbohydrate chains attached to phospholipids or proteins, and form a sugary coat on the cell known as the glycocalyx.
• Glycoproteins are composed of carbohydrate chains attached to proteins, with carbohydrate chains facing outside of the cell and acting as cell surface antigens.
• Glycoproteins interact with water to stabilize membrane structure, form H bonds with water molecules, and play roles in cell-to-cell adhesion and cell recognition.
• Exocytosis is the process where substances packaged into secretory vesicles fuse with the cell surface membrane and release their contents.
• When the external solution is hypotonic to the plant cell, water diffuses into cells by osmosis, increasing water potential in cells.
• Hydrogen pumps in cells are involved in the translocation of sucrose into phloem.
• When the external solution is isotonic to the plant cell, there is no net movement of water and the protoplasm begins to shrink away from the cell wall.
• Active transport is the movement of molecules or ions through carrier proteins from a region of low concentration to high concentration against the concentration gradient, using energy in the form of ATP.
• Endocytosis is the process where substances are packaged into secretory vesicles and fuse with the cell surface membrane, releasing their contents.
• Protoplast pushes against cell wall, causing the cell to become turgid.
• Turgor pressure in cells builds up, increasing water potential of the cell further.
• Water potential in cell, Ψ = Ψs + Ψp.
• Sodium-potassium pump is a carrier protein that pumps 2 K+ in, 3 Na+ out, resulting in the inside of the cell becoming less positively charged than outside, and uses 1 ATP.
• Plasmolysis occurs when there is no pressure on the cell wall, resulting in Ψp = 0 and Ψ = Ψs only.