Membranes And Movement Across Membranes Flashcards
Plasma Membranes - Fluid Mosaic Model
- All cells and organelle membranes have the same structure
- Fluid-mosaic model - mixture and movement of phospholipids, proteins, glycoproteins and glycolipids
- All these molecules arranged into phospholipid bilayer - create partially permeable membrane (cell surface membrane)
What is phospholipid bilayer?
- Align as bilayer - hydrophilic head attracted to water and hydrophobic tail repelled by water
- Extra cellular and intracellular
Cholesterol presence in membranes?
- Present in some membranes
- Restricts lateral movement of other molecules in the membrane - membrane is less fluid at higher temperatures and prevents water and dissolved ions leaking out the cell
What are glycoproteins and glycolipids?
- Glycoproteins - protein with carb attached
- Glycolipids - lipid with carb attached
What are protein channels?
- Transmembrane proteins that form tubes that fill with water to enable water-soluble (polar) ions to diffuse
What are Carrier proteins?
- Membrane proteins that bind to larger molecules like glucose and amino acids and change shape to transport them to the other side of a membrane
Molecules that can pass through plasma membranes?
- Lipid soluble substances
- Very small molecules
- (CO2, O2, H2O)
Molecules that can’t pass through plasma membrane?
- Water soluble (polar) substances (Sodium Ions)
- Large molecules (glucose)
5 types of transport across membranes?
- Simple diffusion
- Facilitated diffusion
- Osmosis
- Active transport
- Co-transport
What is Simple Diffusion?
- Net movement of molecules from an area of higher concentration to an area of lower concentration, until an equilibrium is reached.
- Doesn’t require ATP - uses kinetic energy
- Lipid soluble and small to diffuse across membrane
What is Facilitated diffusion?
- Passive process - proteins used to transport molecules
- Ions and polar molecules which can’t simply diffuse are transported across membrane using protein channels and carrier proteins
What is Osmosis?
- Movement of water from an area of higher water potential to an area of lower water potential across a partially permeable membrane
What is Water potential?
- pressure created by molecules
- measured in kPa
What’s the water potential of pure water?
0
What’s an isotonic solution?
Water potential is the same in the solution and the cell in the solution
What’s a hypotonic solution?
Water potential of the solution is more positive (closer to 0) than the cell
What’s a hypertonic solution?
Water potential of the solution is more negative than the cell
What is active transport?
Movement of substances from a low concentration to a high concentration using metabolic energy and a carrier protein
Steps of active transport
- Transport through a carrier protein across the cell membrane
- Molecules binds to a receptor with a complementary shape to the protein
- ATP binds to the carrier protein from inside of the cell and hydrolysed into ADP+Pi
- Causes the carrier protein to change shape and release molecules to the other side
- Phosphate ion released and the protein returns to its original shape
What is Co-transport of glucose and sodium ions in the ileum?
- To absorb glucose from the lumen to the gut, there must be a higher concentration of glucose in the lumen than the epithelial cell (for facilitated diffusion)
BUT - Usually more glucose in epithelial cells so active transport and Co-transport are required
Steps of Co-transport of glucose and sodium ions in the ileum?
- Sodium ions actively transported out of the epithelial cell into the blood
- This reduces sodium ion concentration in the epithelial cell
- Sodium ions can diffuse from the lumen down the concentration gradient into the epithelial cell
- The sodium ions diffuse through a Co-transported protein - glucose or amino acids also attach and transported into epithelial cells against a concentration gradient
- Glucose moves by facilitated diffusion from epithelial cells to the blood
Adaptations for rapid transport across membranes
- Increased surface area
- increased number of protein channels and carrier protein molecules in their membranes
