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Biological Membranes
Structure of cell membranes
Fluid mosaic model = theory of cell membrane structure with proteins embedded in a sea of phospholipids
Glycolipid = lipid/phospholipid with a chain of carbohydrate molecules attached
Glycoprotein = protein with a chain of carbohydrate molecules attached
Plasma membrane = cell surface membrane
Partially permeable barriers
- Cell membranes need to allow some molecules through as they form a barrier between the cell and the
exterior environment
- Some organelles within the cell also have membranes that act as barriers too
- Membranes are partially permeable because they don’t let all types of molecule pass through
- The properties of the component molecules of the cell membrane determine its permeability
- Permeability = the ability to let substances pass through
1. Some very small molecules diffuse through between the cell membrane’s structural molecules
2. Some substances dissolve in the lipid layer and pass through
3. Some substances pass through protein channels or carrier proteins
Roles of membranes
Surface of cells
The plasma membrane has many roles:
1. Separates cell’s components from the exterior environment
2. Regulates transport of materials into and out of the cell
3. May contain enzymes involved in specific metabolic pathways
4. Has antigens to the organism’s immune system can recognise the cell and not attack it
5. Releases chemicals that signal to other cells
6. May contain receptors for these chemical signals = is a site for cell communication, hormones and
drugs bind to membrane-bound receptors
7. Site of chemical reactions
Within cells
Membranes around many organelles in eukaryotic cells separate the organelle contents from the cell
cytoplasm -> each organelles is a discrete entity and can perform its function
1. Mitochondria: Inner folded membranes called cristae give a large surface area for some reactions of
aerobic respiration and localise some of the enzymes needed for respiration to occur
2. Chloroplasts: Inner membranes called thylakoid membranes house chlorophyll, some reactions of
photosynthesis occur on the membranes
3. Epithelial cells: Digestive enzymes on the plasma membranes line the small intestine, enzymes
catalyse some stages of breaking down sugar
Fluid mosaic model of cell membrane structure
- Phospholipid bilayer (hydrophilic heads in contact with watery exterior/interior and hydrophobic tails in
centre of membrane), proteins floating in it -> mosaic pattern
- Lipid molecules can change places and proteins may move -> fluidity
- Cholesterol helps to regulate fluidity of the membrane, maintain mechanical stability and resist the effects
of temperature changes on the membrane structure (only eukaryotic)
- Total thickness of a cell membrane is 5-10nm
- Outside the membrane is the glycocalyx – formed from the carbohydrate chains attached to either lipids
(glycolipids) or proteins (glycoproteins) in the membrane
- Various proteins span the membrane
1. Some have pores -> act as a channel to allow ions (charged + surrounded by water molecules) to pass
2. Some are carriers -> by changing their shape carry specific molecules across the membrane
3. Some act as enzymes/antigens/receptor sites for complementary-shaped signalling chemicals e.g.
hormones -> attached to the carrier proteins
, Varying compositions of cell membranes
- Neurones/nerve cells: protein channels and carriers in the membrane covering the long axon allow
entry/exit of ions about the conduction of electrical impulses along the length
- Neurones have a myelin sheath formed by flattened cells wrapped around them several times – giving
several layers of cell membrane
Membrane forming myelin sheath is 20% protein, 76% lipid
- White blood cells: plasma membranes contain special protein receptors that enable them to recognise
antigens on foreign cells from invading pathogens or tissue/organ transplants
- Root hair cells: many carrier proteins actively transport nitrate ions from soil into cells
- Mitochondria: inner membranes 76% proteins and 24% lipids because they contain many electron carriers
made of protein, and hydrogen ion channels associated with ATP synthase enzymes
Simple diffusion
- Diffusion = movement of molecules from an area of high concentration to an area of low concentration,
may be across a membrane, does not involve metabolic energy (ATP)
- Diffusion is a passive process because they only use the kinetic energy of the molecules and don’t use ATP
1. All molecules have kinetic energy to move freely and randomly within gas or liquid on their own
2. If there is a high concentration in one area, the molecules will bump into each other as they naturally
and randomly move, and eventually spread apart from each other
3. More move into an area of low concentration until they are evenly dispersed
- When the molecules move down the concentration gradient they are still moving randomly but remain
evenly dispersed -> no net diffusion
- At this point have reached equilibrium
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