7 Transport across cell membranes Flashcards
What is the function of the cell membrane?
partially permeable, controlling which substances enter or leave the cell
How are the phospholipids arranged in a membrane?
in a bilayer, with the hydrophobic fatty acids tails in the centre and hydrophilic phosphate heads pointing towards the outside
What are the functions of the phospholipids in a membrane?
partially permeable barrier; allow passage of lipid-soluble substances and prevent passage of water-soluble ones; make membrane flexible
Where are intrinsic proteins positioned in a phospholipid bilayer?
span the entire bilayer
What are the roles of intrinsic proteins?
span the bilayer, are enzymes, carrier proteins, and channel proteins
Where are extrinsic proteins found in a phospholipid bilayer?
found on surface or embedded in one layer
What is the role of extrinsic proteins?
provide mechanical support; together with glycolipids act as cell receptors for hormones and other molecules
What are the roles of glycoproteins?
are receptors for chemical signals (eg neurotransmitters); receptor sites; adhere cells together in tissues
What is the role of glycolipids?
cell recognition / act as antigens
What is the role of cholesterol in a membrane?
may be present; restricts movement other membrane components, making membranes less fluid, providing mechanical stability
What do carrier proteins do?
bind to ions/ molecules, then change shape in order to move molecules across membrane
Why is the term ‘fluid mosaic’ used to describe a phospholipid bilayer membrane?
fluid- phospholipids are able to move relative to one another
mosaic- describes pattern of proteins scattered amongst phospholipids
so its role is to provide mechanical stability and regulate fluidity
Definition for simple diffusion
the net movement of molecules/ ions in a liquid/ gas from an area of high to low concentration; passive process
Can electrically charged (polar) molecules pass through the membrane?
no (non-polar, hydrophobic tails in phospholipid bilayer)
What are 2 adaptations of cells to maximise membrane transport?
large surface area; large number of specific protein channels and carrier molecules embedded in membrane (e.g. liver cells takes up large amounts glucose as have many glucose channels)
List factors that can affect the rate of diffusion
temperature; diffusion pathway distance; size of molecule; concentration gradient; surface area; movement of molecules/ ions (stirring)
Definition for facilitated diffusion
the passive movement of molecules across a membrane from area of high to low concentration, using carrier proteins/ protein channels
What is the difference between carrier and protein channels?
protein channels: form pores in membrane, specific to an ion/ molecule
carrier proteins: change shape once ion/ molecule is attached to allow molecule through membrane
Definition for osmosis
net movement of water molecules from region of high to low water potential across a partially permeable membrane
What is water potential?
measure of the ability of water molecules to diffuse; pure water has highest water potential of 0 kPa
What 3 states can animal and plant cells experience in solutions of different concentrations of water?
hypotonic (high conc.)
isotonic (equal)
hypertonic (low conc.)
Definition for active transport
the movement of molecules/ ions across a membrane against the concentration gradient, using ATP
What do carrier proteins do in active transport?
require energy from hydrolysis of ATP to ADP and inorganic phosphate; transport molecules/ ions faster than by diffusion
What is co-transport?
special type of facilitated diffusion; carried out by co-transporter proteins which enables diffusion of large molecules with an ion
