Chapter 4: cell membranes and transport Flashcards
describe the ‘fluid mosaic model’
- both phospholipids and proteins can move about by diffusion
- the phospholipids move sideways, mainly in their own layers
- some protein molecules also move about within the phospholipid bilayer; others remain fixed to structures inside or outside the cell
describe the features of the fluid mosaic model
- double layer membrane of phospholipid molecules
- phospholipid tails point inwards
the more unsaturated the phospholipids, the more…the membrane
fluid. This is because the unsaturated fatty acid tails are bent, and therefore fit together more loosely
fluidity is also affected by…
tail length. The longer the tail, the less fluid the membrane. As temperature decreases, membranes become less fluid
proteins that are found within the membrane are called…
intrinsic proteins
proteins that span the whole membrane are called…
transmembrane proteins
‘extrinsic protein’ definition
proteins that are found on the inner or outer surface of the membrane
role of phospholipids
- form the bilayer
- their tails are non-polar so polar molecules can’t pass through
- they act as a barrier to most water-soluble substances
role of cholesterol
- they have hydrophilic heads and hydrophobic tails
- at low temperatures, cholesterol increases the fluidity of the membrane, preventing it from becoming to rigid
- the interaction of the phospholipid tails with the cholesterol molecules also helps to stabilise cells at higher temperatures when the membrane could become too fluid
the carbohydrate chains form a sugary coating to the cell, known as the…
glycocalyx
the carbohydrate chains perform what function?
they help the glycoproteins and glycolipids to act as receptor molecules, which bind with particular substances at the cell surgace
examples of receptors
- ‘signalling receptors’- part of a signalling system that coordinates the activities of cells
- receptors involved in endocytosis. They bind to molecules that are parts of the structures to be engulfed by the cell surface membrane
- receptors involved in binding cells to other cells in tissues and organs of animals
name the two types of transport proteins
- channel proteins
- carrier proteins
name the three basic ways in which a receptor can change the activity of a cell
- opeining an ion channel
- acting as a membrane-bound enxyme
- acting as an intracellular receptor when the initial signal passes straight through the cell surface membrane
define diffusion
the net movement, as a result of random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration, down a concentration gradient
name the three factors that affect the rate of diffusion:
- the ‘steepness’ of the concentration gradient
- the temperature
- the surface area across which diffusion is taking place
- the nature of the molecules or ions
define ‘facilitated diffusion’
the diffusion of a substance through transport proteins in a cell membrane; the proteins provide hydrophilic areas that allow the molecules or ions to pass through the membrane which would otherwise be less permeable to them
what is a partially permeable membrane?
a membrane that allows only certain molecules through
what is water potential
the tendency of water to move out of a solution
what is the solute potential?
the extent to which the solute molecules decrease the water potential of the solution
what is the protoplast of a cell?
the living part of the cell inside the cell wall
what is the role of the Na+-K+ pump?
to pump three sodium ions out of the cell at the same time as allowing two potassium ions into the cell for each ATP molecule used
active transport definition
the movement of molecules or ions through transport proteins across a cell membrane, against their concentration gradient, using energy from ATP
endocytosis definiton
the bulk movement of liquids or solids into a cell, by the infolding of the cell surface membrane to form vesicles containing the substance; endocytosis is an active process requiring ATP
exocytosis definition
the bulk movement of liquids or solids out of a cell, by the fusion of vesicles containing the substance with the cell surface membrane; exocytosis is an active process requiring ATP
