2.5 biological membranes Flashcards
describe the fluid mosaic model of membranes
fluid - phospholipid bilayer in which individual phospholipids can move - flexible membrane
mosaic - extrinsic and intrinsic proteins of different shapes and sizes are embedded
what are the roles of membranes at the surface of cells?
- separates the cell’s components from the external environment
- allowing the cell to change shape
- regulates transport of materials into and out of the cell
- may contain enzymes involved in specific metabolic pathways
- has antigens for cell recognition
- involved in cell signaling
- may be the site of chemical reactions
what are the roles of membranes within cells?
- selective permeability to regulate what enters/leaves organelles
- compartmentalization to separate different organelle environments from the cytoplasm
- form vesicles to transport substances
- create concentration gradients
what is the structure of the plasma membrane?
phospholipid bilayer
- they form a bilayer, with the hydrophobic tails facing inwards and the hydrophilic heads outwards and interacting with the aqueous medium
what is the structure and role of phospholipids?
- structure of a phospholipid: glycerol backbone, the phosphate group head is polar and hydrophilic, while the 2 fatty acid chains of the tail are non-polar and hydrophobic
- can form either a micelle or a bilayer
- acts as a barrier to water-soluble (dissolved) substances
what is the role of cholesterol in the plasma membrane?
- important in controlling membrane fluidity - the more cholesterol, the more stable and rigid the bilayer
what is the role of glycolipids in the plasma membrane?
cell signalling and cell recognition
what are the roles of extrinsic proteins in the plasma membrane?
- binding sites/receptors (e.g. for hormones and drugs)
- antigens (glycoproteins)
- bind cells together
- involved in cell signalling
what is the difference between extrinsic and intrinsic proteins?
- intrinsic proteins span the entire width of the cell membrane
- extrinsic membranes are confined to the outer or inner part of the membrane
what are the roles of intrinsic proteins in the plasma membrane?
- receptors
- channel proteins (facilitated diffusion)
- carrier proteins (facilitated diffusion/active transport)
what are the roles of glycoproteins in plasma membranes?
- cell adhesion
- cell signalling
why is it important to maintain a fluid membrane?
- for the diffusion of substances across the membrane
- membranes to fuse with the cell
- cells to be able to move and change shape (e.g.phagocytosis)
what is the effect of heat on the bilayer?
- as temp. increases, so does the kinetic energy of the molecule, creating gaps in the bilayer and increasing permeability
what is the effect of solvents on the bilayer?
can dissolve the lipid bilayer and disrupt the structure of the membrane, increasing permeability
what is the effect of pH on the bilayer?
changes the tertiary structure of the membrane proteins - can increase or decrease permeability
what is simple diffusion?
the net movement of small, lipid-soluble molecules through the membrane from an area of high concentration to an area of lower concentration, down a concentration gradient
what are some of the features of simple diffusion?
- passive process - requires no energy from ATP hydrolysis
- liquids and gases (have kinetic energy)
- small, non-polar molecules
what is facilitated diffusion?
when substances move through a transport protein in the phospholipid bilayer, down a concentration gradient
what are some of the features of facilitated diffusion?
- still a passive process
- travel through carrier/channel proteins
- large and/or polar molecules
how do channel proteins work?
- hydrophilic channels bind to specific ions
- one side of the protein closes and the other opens
- usually used for ions/polar molecules
how do carrier proteins work?
- binds to complementary molecule
- conformational change releases molecule on the other side of the membrane
what is active transport?
- ATP hydrolysis releases phosphate group that binds to carrier protein, causing it to change shape
- carrier proteins transport molecules from an area of low concentration to an area of high concentration, against a concentration gradient
what is endocytosis?
the process of capturing a substance or particle from outside the cell by engulfing it with the cell membrane (invaginating)
(active process)
what is exocytosis?
the process by which materials are removed from, or transported out of, cells (the reverse of endocytosis) (active process)
usually involves vesicles fusing with the cell membrane
what are the factors that affect diffusion?
- temperature
- diffusion distance
- surface area
- size of molecule
- concentration gradient
what are the extra factors that affect active transport?
- number of carrier proteins
- number of mitochondria
- amount of oxygen for aerobic respiration
what is osmosis?
the movement of water molecules from an area of high water potential to an area of lower water potential through a partially permeable membrane (until a dynamic equilibrium is established)
what is water potential?
- pressure created by water molecules, measured in kPa
- more solute - water potential more negative
what happens when a plant cell is placed in a hypotonic vs hypertonic solution?
- hypotonic solution - net movement into cell, cell swells and becomes turgid, protoplast pushes against cell wall
- hypertonic solution - net movement out of cell, cell is plasmolyzed, protoplast pulls away from the cell wall
