2.5 Biological Membranes
Cell membranes
- Partially permeable barriers that separate cell contents from exterior
environment, and isolate organelles from cytoplasm.
- Site for biological reactions.
- Allow cell to change shape.
- Selectively allow molecules to pass through, into or out of cell.
○ Some very small molecules diffuse through cell membrane, in between
structural molecules.
○ Some substances dissolve in lipid layer and pass through.
○ Other substances pass through special protein channels or carried by
carrier proteins.
- Permeability of cell membrane determined by properties of component
molecules.
- Partially permeable membrane allows some molecules to pass through, e.g.
carbon dioxide and water, but not others.
At surface of cells
Plasma membrane, or cell surface membrane:
- Separates cell's components from external environment
- Regulates transport of materials into and out of cell
- May contain enzymes involved in specific metabolic pathways
- Has antigens, so that organism's immune system can recognise cell as 'self'
and not attack it
- May release chemicals that signal to other cells
- Contains receptors for these chemical signals, so is site for cell
communication or signalling; hormones and drugs may bind to membrane-
bound receptors
- May be site of chemical reactions
Within cells
Membranes around organelles in eukaryotic cells separate organelle contents from
cell cytoplasm, so that each organelle separate and able to perform function.
In some organelles, metabolic processes occur on membranes.
- Mitochondria have cristae, giving large surface area for reactions of aerobic
respiration and holding enzymes needed for respiration to occur.
- Thylakoid membranes of chloroplasts house chlorophyll. Some reactions
of photosynthesis occur here.
- Plasma membranes of epithelial cells in small intestine house digestive
enzymes to catalyse some of final stages in breakdown of certain types of
sugars.
Fluid mosaic model suggests that fabric of cell membranes consists of
phospholipid bilayer with proteins floating in it, making up a mosaic pattern.
Lipid molecules can change places with each other and some proteins may move,
giving fluidity.
Lipid bilayer made up of two layers of phospholipid molecules, whose
hydrophilic heads in contact with watery exterior or interior, with hydrophobic
tail regions at centre of membrane, away from water.
Eukaryotic cell membranes contain cholesterol, which regulates fluidity of
membrane, maintains mechanical stability, and resists effects of temperature
changes.
Membrane between 5 and 10nm thick.
Cell membranes have different compositions to suit functions.
Module 2 - Foundations in biology Page 1
Cell membranes
- Partially permeable barriers that separate cell contents from exterior
environment, and isolate organelles from cytoplasm.
- Site for biological reactions.
- Allow cell to change shape.
- Selectively allow molecules to pass through, into or out of cell.
○ Some very small molecules diffuse through cell membrane, in between
structural molecules.
○ Some substances dissolve in lipid layer and pass through.
○ Other substances pass through special protein channels or carried by
carrier proteins.
- Permeability of cell membrane determined by properties of component
molecules.
- Partially permeable membrane allows some molecules to pass through, e.g.
carbon dioxide and water, but not others.
At surface of cells
Plasma membrane, or cell surface membrane:
- Separates cell's components from external environment
- Regulates transport of materials into and out of cell
- May contain enzymes involved in specific metabolic pathways
- Has antigens, so that organism's immune system can recognise cell as 'self'
and not attack it
- May release chemicals that signal to other cells
- Contains receptors for these chemical signals, so is site for cell
communication or signalling; hormones and drugs may bind to membrane-
bound receptors
- May be site of chemical reactions
Within cells
Membranes around organelles in eukaryotic cells separate organelle contents from
cell cytoplasm, so that each organelle separate and able to perform function.
In some organelles, metabolic processes occur on membranes.
- Mitochondria have cristae, giving large surface area for reactions of aerobic
respiration and holding enzymes needed for respiration to occur.
- Thylakoid membranes of chloroplasts house chlorophyll. Some reactions
of photosynthesis occur here.
- Plasma membranes of epithelial cells in small intestine house digestive
enzymes to catalyse some of final stages in breakdown of certain types of
sugars.
Fluid mosaic model suggests that fabric of cell membranes consists of
phospholipid bilayer with proteins floating in it, making up a mosaic pattern.
Lipid molecules can change places with each other and some proteins may move,
giving fluidity.
Lipid bilayer made up of two layers of phospholipid molecules, whose
hydrophilic heads in contact with watery exterior or interior, with hydrophobic
tail regions at centre of membrane, away from water.
Eukaryotic cell membranes contain cholesterol, which regulates fluidity of
membrane, maintains mechanical stability, and resists effects of temperature
changes.
Membrane between 5 and 10nm thick.
Cell membranes have different compositions to suit functions.
Module 2 - Foundations in biology Page 1
