B4. Transport Across Cell Membranes Flashcards
Why are cell membranes important?
Separate contents of cells from their external environments; Control exchange between cell and its environments; Enables special compartments
What’s a phospholipid bilayer?
Phospholipids arranged in 2 molecule thick layer; Hydrophobic tail inwards away from water; Hydrophilic head outwards facing water
Why is the membrane described as a fluid mosaic model?
Fluid: Phospholipids & proteins move around via diffusion;
Mosaic: Scattered pattern produced by proteins look like a mosaic from above
Describe the structure of the cell membrane.
Phospholipid bilayer makes up membrane, contains proteins; Intrinsic proteins, Extrinsic protein
What molecules does the cell membrane contain?
Lipids: Phospholipid, cholesterol, glycolipids
Proteins: Glycoproteins, other proteins eg. transport
How are the lipid molecules in the cell membrane positioned?
Phospholipid - forms bilayer;
Cholesterol - fits between phospholipid molecules, orients same way, absent in prokaryotes, has hydrophobic tail & hydrophilic head
Glycolipids - Lipids with carbohydrate chains attached; Found on outer phospholipid monolayer
How are the protein molecules in the cell membrane positioned?
Glycoproteins - Proteins w/ carbohydrate chains attached; Found on outer phospholipid monolayer
Intrinsic proteins - embedded in membrane, arrangement determined by hydrophilic & hydrophobic regions; Most commonly span entire membrane (transmembrane protein) eg. transport protein;
Extrinsic proteins - found on outer/inner surface of membrane
What is the purpose of phospholipids in a cell membrane?
Form basic bilayer structure; Barrier against most water soluble substances (non polar fatty acid tail prevents passage of polar molecules); Prevents leakage of water soluble molecules; Sugars, amino acids, proteins; Can be chemically modified to act as signalling molecule
What is the purpose of cholestrol in a cell membrane?
Regulates fluidity of membrane; Prevents phospholipids packing too close when temp low (stops membrane from freezing & fracturing); Stabilises membrane at higher temps w/ phospholipid tail; Increases permeability of membrane to ions; Increases mechanical strength and stability
What is the purpose of glycoproteins & glycolipids in a cell membrane?
Receptor molecules; Binds w/ certain substances at cell surface; Signalling receptors for hormones & neurotransmitters; Receptors involved in endocytosis; Receptors involved in cell adhesion & stabilisation
What is the purpose of proteins in a cell membrane?
Transport proteins, hydrophilic channels allow ions & polar molecules cross cell membrane, channel proteins & carrier proteins; Each transport protein is specific to particular ions or molecules; Transport proteins control which substances enter or leave
What is the definition of diffusion?
Net movement, as a result of random motion of molecules or ions, of a substance from a region of higher concentration to lower concentration
What are the factors that increase rate of diffusion?
Steeper concentration gradient; Higher temperature, more kinetic energy; Higher surface area; Smaller molecules; Non-polar molecules
How do large polar molecules diffuse through the phospholipid bilayer membrane?
Facilitated diffusion; Channel proteins or carrier proteins
Describe channel proteins.
Water-filled pores; Allow charged substances to diffuse through cell membrane; Gated channels;
Describe carrier proteins.
Switches between 2 shapes; Binding site changes depending on shape; Diffusion down concentration gradient
What is the definition of osmosis?
Ned movement of water molecules from a region of higher water potential to a region of low water potential through a partially permeable membrane
What is the water potential of pure water at atmospheric pressure?
0kPa
What is plasmolysis?
Contraction of protoplast of a plant cell due to loss of water
What is a protoplast?
Entire cell excluding the cell wall
What is cytolysis?
Cell bursts due to osmotic imbalance that causes extra water to diffuse into the cell
Describe the investigation of water potential.
Potato cylinders are cut, same length and blotted dry; Initial mass recorded; Leave in solution for set time; Remove and dry excess liquid; Final length and mass recorded
What is the definition of active transport?
Movement of molecules and ions through a cell membrane from a region of lower concentration to a region of higher concentration using ATP
What structure does active transport use across the phospholipid bilayer?
Carrier protein, ATP used to change the shape
What is co-transport?
Coupled movement of substances across a cell membrane via carrier protein; Combination of facilitated diffusion and active transport
Describe how the active transport of glucose occurs across the ileum.
Na+/K+ pump, pumps Na+ from epithelial cell to blood; Na+ glucose pump, pumps Na+ & glucose, against conc gradient, from lumen of ileum into epithelial cell; Energy is given indirectly from conc gradient of sodium ions; Glucose facilitated diffusion through carrier protein into blood; Glucose is also transported by facilitated diffusion through carrier proteins
What are the factors that increase rate of facilitated diffusion?
Steep concentration gradient across exchange surface; High count of channels, carrier proteins
