2.1.5 Biological membranes
(a) the roles of membranes within cells and at the surface of cells
Permeability: the ability to let substances pass through
Some very small molecules, such as oxygen and carbon dioxide, diffuse through the cell
membrane between its structural molecules.
Some substances dissolve in the lipid layer and pass through
Some substances pass through channel proteins or are carried across by carrier
proteins.
Plasma membranes do not let all types of molecule pass through them so are called partially
permeable membranes.
Roles of membranes
AT THE SURFACE OF CELLS:
Separates cells components from its external environment
Regulates transport of materials in/out of the cell
Site of chemical reactions
Has antigens to prevent the immune system attacking the cell
Site of cell communication & signalling: may release chemicals that signal to other cells,
contains receptors for chemical signals (hormones may bind to membrane’s receptors).
WITHIN CELLS:
Separate organelle contents from the cytoplasm
Separates the sites of different reactions, allowing each to have their specific conditions
such as chemical gradients.
Provides surfaces for metabolic processes to take place:
o In mitochondria, the folded inner membrane (cristae) increases surface area for
aerobic respiration
o In chloroplasts, the inner membrane (thylakoid) houses chlorophyll and is
where some photosynthesis reactions occur
(b) the fluid mosaic model of membrane structure and the roles of its
components
Fluid mosaic model: theory of cell membrane structure with proteins embedded in a sea of
phospholipids that can move around each other.
(a) the roles of membranes within cells and at the surface of cells
Permeability: the ability to let substances pass through
Some very small molecules, such as oxygen and carbon dioxide, diffuse through the cell
membrane between its structural molecules.
Some substances dissolve in the lipid layer and pass through
Some substances pass through channel proteins or are carried across by carrier
proteins.
Plasma membranes do not let all types of molecule pass through them so are called partially
permeable membranes.
Roles of membranes
AT THE SURFACE OF CELLS:
Separates cells components from its external environment
Regulates transport of materials in/out of the cell
Site of chemical reactions
Has antigens to prevent the immune system attacking the cell
Site of cell communication & signalling: may release chemicals that signal to other cells,
contains receptors for chemical signals (hormones may bind to membrane’s receptors).
WITHIN CELLS:
Separate organelle contents from the cytoplasm
Separates the sites of different reactions, allowing each to have their specific conditions
such as chemical gradients.
Provides surfaces for metabolic processes to take place:
o In mitochondria, the folded inner membrane (cristae) increases surface area for
aerobic respiration
o In chloroplasts, the inner membrane (thylakoid) houses chlorophyll and is
where some photosynthesis reactions occur
(b) the fluid mosaic model of membrane structure and the roles of its
components
Fluid mosaic model: theory of cell membrane structure with proteins embedded in a sea of
phospholipids that can move around each other.
