Cell membranes/transport

• Cell membranes are vital structures found in all cells, separating different areas and controlling the exchange of material across them
• Cellular membranes are partially permeable, allowing substances to cross by diffusion, osmosis, and active transport
• Cellular membranes are formed from a bilayer of phospholipids, roughly 7nm wide and visible under an electron microscope at high magnifications
• The fluid mosaic model of the membrane explains how biological molecules are arranged to form cell membranes, aiding in passive and active movement between cells and their surroundings, cell-to-cell interactions, and cell signaling
• Phospholipids structurally contain a polar head (hydrophilic) and two nonpolar tails (hydrophobic), forming a bilayer in cell membranes
• If phospholipids are mixed with water, they form spheres called micelles with the hydrophilic heads facing out towards the water and the hydrophobic tails facing in towards each other
• Phospholipids can also form two-layered structures called phospholipid bilayers, which is the basic structure of the cell membrane
• Phospholipid bilayers are composed of two layers of phospholipids, with hydrophobic tails facing inwards and hydrophilic heads outwards, forming compartments in cells
• Cell membranes also contain proteins, which can be intrinsic (integral) or extrinsic (peripheral), contributing to the fluid mosaic model of cell membranes
• Cholesterol in cell membranes regulates fluidity, while glycolipids and glycoproteins contain carbohydrate chains and are found on the outer phospholipid monolayer
• Transport proteins, a type of transmembrane protein, cross the whole membrane, aiding in the movement of substances across the cell membrane
• Cell surface membranes, formed by phospholipids, act as a barrier to most water-soluble substances, ensuring molecules like sugars and amino acids cannot leak out of the cell
• Cholesterol regulates the fluidity of the membrane by preventing phospholipids from packing too closely together at low temperatures, thus preventing membranes from freezing and fracturing
• Cholesterol stabilizes the cell membrane at higher temperatures by interacting with phospholipid tails, causing them to pack more closely together
• Glycolipids and glycoproteins contain carbohydrate chains that exist on the surface of the cell, enabling them to act as receptor molecules
• Glycolipids and glycoproteins can bind with certain substances at the cell’s surface, acting as cell markers or antigens for cell-to-cell recognition
• Proteins in the cell membrane, like transport proteins, create hydrophilic channels to allow ions and polar molecules to travel through the membrane
• Transport proteins, including channel (pore) proteins and carrier proteins, are specific to particular ions or molecules, allowing the cell to control which substances enter or leave
• Facilitated diffusion enables the movement of large polar molecules and ions across the phospholipid bilayer with the help of channel proteins and carrier proteins
• Channel proteins are water-filled pores that allow charged substances like ions to diffuse through the cell membrane
• Carrier proteins can switch between two shapes, opening the binding site to one side of the membrane first and then to the other side when the protein changes shape
• Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane
• Water potential describes the tendency of water to move out of a solution, with dilute solutions having high water potential and concentrated solutions having low water potential
• Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane
• In plant cells, if placed in pure water or a dilute solution, water enters the cell through its partially permeable cell surface membrane by osmosis, causing the cell to become turgid
• Turgidity in plant cells is important for providing support and strength for the plant, making it stand upright with its leaves held out to catch sunlight
• If a plant cell is placed in a solution with a lower water potential than the cell (e.g., a concentrated sucrose solution), water leaves the cell through osmosis, causing the cell to shrink and plasmolyze
• Animal cells, unlike plant cells, do not have a supporting cell wall, so the effects of water loss or gain through osmosis are more severe, leading to cell shrinkage or bursting depending on the environment's water potential
• In an isotonic environment, where the solution outside the cell has the same solute concentration as inside the cell, there is no net movement of water into or out of the cell
• In an osmosis experiment, a potato cylinder in the strongest sucrose concentration will have decreased in mass the most due to the greatest concentration gradient between the potato cells (higher water potential) and the sucrose solution (lower water potential)
• If a potato cylinder neither increases nor decreases in mass, it indicates no overall net movement of water into or out of the potato cells, meaning the solution had the same water potential as the cytoplasm of the potato cells
• The concentration of sucrose inside potato cylinders can be found by drawing a graph showing how the percentage change in mass varies with the concentration of sucrose solution
• Active transport is the movement of molecules and ions through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration
• Active transport requires carrier proteins specific to certain molecules or ions and energy from ATP produced during respiration
• Co-transport is the coupled movement of substances across a cell membrane via a carrier protein, involving a combination of facilitated diffusion and active transport
• Co-transport involves two types of molecules moved across the membrane simultaneously, with the movement of one dependent on the other
• Cells adapted for rapid transport of molecules have factors affecting diffusion rate, including temperature, surface area, concentration gradient, and thickness of the exchange surface
• Specialised cells for diffusion:

  • Root hair cells and epithelial cells of the small intestine are adapted for the rapid transport of molecules across their membranes
  • Root hair cells are adapted for the absorption of water and mineral ions from soil, with a specialized shape (the root ‘hair’) increasing surface area for greater water uptake by osmosis
  • Epithelial cells of the small intestine have microvilli to increase surface area for greater diffusion of digestion products
• Many cells adapted for diffusion have increased surface area, like root hair cells in plants and cells lining the ileum in animals
• Specialised cells for facilitated diffusion:

  • Neurones, muscle cells, and some kidney cells are adapted for rapid transport of molecules across their membranes via facilitated diffusion
  • Certain kidney cells have cell membranes with a high number of aquaporins, allowing facilitated diffusion of water
  • Neurones and muscle cells have cell membranes with channel proteins for sodium, potassium, and calcium ions, crucial for electrical impulse transmission