Module 2: Biological Membranes Flashcards
The role of membranes
1) Separating cell contents from outside environment.
2) Separating cell components from cytoplasm.
3) Cell recognition and cell signalling
4) Regulating transport of materials in and out of the cell.
5) Some stages of photosynthesis and respiration take place on the membranes of the chloroplasts or mitochondria.
Basic membrane structure
All membranes are made up of a phospholipid bilayer.
Phospholipid bilayers are the basic structure of ALL membranes in cells (prokaryotic and eukaryotic and membranes round organelles).
The are usually about 7‐10nm thick.
All membranes are permeable to water ‐ water diffuses through the bilayer. Some contain aquaporins (protein channels which allow water through) which make them much more permeable to water.
Cell surface membranes are partially permeable
membranes ‐ permeable to water and some solutes
Phospholipids
- Phosphate group head.
- Fatty acid tail.
The head is hydrophilic- water loving
The tail is hydrophobic- water hating
The phospholipids aren’t bonded to one another but because the hydrophilic head can’t pass through the hydrophobic region easily, the bilayer is quite stable.
The membrane is very fluid because of the lack of bonds ‐ the phospholipids can slide around one another.
What happens to phospholipids when put in water?
The head is hydrophilic because the charges on the head are unevenly distributed ‐ this lets it interact with the water molecules easily.
The tails are hydrophobic because the charges on them are evenly distributed ‐ this means they won’t mix with water and will repel the molecules.
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If surrounded completely by water, phospholipids can form a bilayer. The hydrophilic heads point towards the water and hydrophilic tails point towards each other ‐ away from water
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The fluid mosaic model of membrane structure
In 1972 Singer and Nicholson put forward their ‘fluid mosaic model’ of membrane structure ‐ it is still the model we accept today.
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Glycoproteins and Glycolipids:
Protein + carbohydrate = Glycoprotein
Phospholipid + carbohydrate = Glycolipid
- Glycolipids and glycoproteins are involved in cell signalling that they are ‘self’ to allow recognition by the immune system.
- They are receptors for hormones/drugs
- On the surface of pathogens they are antigens ‐ immune system can recognise them as ‘foreign.’
- Glycoproteins can bind cells together in tissue.
Proteins
1) Proteins which span whole bilayer = intrinsic proteins
2) Proteins which are partially embedded inside or outside = extrinsic proteins
Intrinsic proteins for transport ‐ these have to be hydrophobic molecules in order to span the whole bilayer (due to the hydrophobic region):
Channel proteins ‐ pores for small ions or small water soluble molecules to diffuse through membrane.
Carrier proteins ‐ can help larger molecules to diffuse through membrane, or can be used for active transport across membrane.
Cholesterol
‐ gives the membranes of eukaryotes more stability.
‐ fits between fatty acid tails and ‘plugs gaps’ ‐ makes the
membrane less permeable (through bilayer) to water
molecules and to ions.
‐ restricts too much movement within phospholipid layer.
Actin microfilaments
Actin microfilaments can be closely associated with the membrane and its proteins in order to help anchor them and stop them moving around too much.
Membranes and Temperature
Higher temperature = more kinetic energy for molecules = they move faster.
Faster movement of phospholipids and other parts of the membrane makes the membranes more fluid and ‘leaky’. This means they become more permeable and substances that couldn’t normally enter/leave the cell now can.
At very high temperatures, lipids can melt and proteins can denature ‐ this can completely cause the membrane to rupture and release everything that is inside
of it.
Organisms living in very hot or very cold environments need differently adapted molecules in their membranes e.g. cholesterol content, so that their membranes
can function correctly and remain intact at these temperatures.
Practical investigation into factors affecting diffusion rates in model cells.
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What is cell signalling?
Cell signalling is when cells communicate with one another by signals to help them work together and coordinate their actions.
This is very complex. Many molecules act as signals ‐ some signal during processes taking place inside cells; other signal from one cell to others.
Cytokines and hormones are examples of cell signals secreted from cells.
To detect signals, cells must have receptors on their surface. These are usually proteins or modified proteins e.g. glycoproteins.
Hormone receptors
Cells can communicate with hormones (endocrine signals) which are protein chemical messengers produced in glands and then released into the blood.
Cells with specific receptors for the hormones are called
target cells.
Hormone molecules bind to receptors on their target cell surface membrane. The hormone and receptor have complementary shapes. This binding brings about a response in the target cell.
Insulin
Made in: beta‐cells in islets of Langerhans in pancreas.
Made when: blood glucose is too high.
Target cells: insulin receptors on many cells inc. muscle and liver cells.
Effect: cells take up more glucose from blood = reduces blood glucose level.
Medicinal drugs
Can be made to be complementary to receptor molecules so they block them.
Beta‐blockers stop heart muscle from increasing heart rate in people at risk of heart attacks.
Some drugs act like natural neurotransmitters that some people can’t make.
e.g. Schizophenics can’t control the levels of dopamine in the brain ‐ the drugs act like dopamine in order to supress the symptoms of the disorder.
Poisons
Poisons can bind with receptors.
Toxin from bacterium Clostridium botulinum binds with receptors on muscle fibres and stops them from working ‐ this causes paralysis of the muscles.
The toxin is one of the most lethal neurotoxins known to man but in TINY doses it is used for BOTOX surgery. It paralyses small muscles in the face to reduce wrinkles.
Hijacking receptors
Viruses can get inside cells when they bind with their receptors (normally used for signalling).
Human Immunodeficiency Virus (HIV) [causes AIDS] is so dangerous because it can enter the immune system’s cells e.g. T‐lymphocytes [one type of white blood cell].
The HIV virus can lie dormant and the reproduce inside the cell and cause it to burst open and be destroyed. The main symptom of AIDS is a lowered immune system.
Functions of glycoproteins
- Cell signalling ‐ communication between cells to help them work together/coordinate actions.
- Act as antigens for…
- cell recognition ‐ recognition as self/non‐self.
- receptors found on target cells… for hormones/cytokines to trigger reactions/responses in cells.
- cell adhesion ‐ hold cells together in tissues.
- form bonds with water molecules to stabilise membrane.
- receptors on transport proteins.
Diffusion
Diffusion is the net movement of molecules from a region of high concentration to a region of lower concentration of that molecule, down a concentration gradient. This is a passive- doesn’t require energy.
No net movement is called equilibrium.
The steeper the concentration gradient, the quicker the rate of diffusion will be.
i.e the bigger the difference between the concentrations in the regions of high and lower concentration then the faster the rate of diffusion.
Cells maintain conc gradients to ensure constant net movement of the molecules they need into the cell/organelle.
Osmosis
Osmosis is the movement of water molecules from a region of high concentration to a region of lower concentration of water molecules down the concentration gradient, across a partially permeable membrane. This is a passive process and is effected by the concentration gradient.
Active transport
Active transport is the movement of molecules or ions across membranes from a region of low concentration to a region of higher concentration of that molecule against a concentration gradient. Active transport requires energy in the form of ATP.
Factors affecting the rate of diffusion
Temperature ‐ higher temp = more kinetic energy for molecules = higher rate of random movement = higher rate of diffusion
Stirring/moving ‐ stirring increases movement of molecules = higher rate of diffusion
Surface area ‐ diffusion across membranes is faster if there is a larger SA to diffuse across. e.g. alveoli
Distance/thickness ‐ thinner membrane = faster diffusion e.g. alveoli
Size of molecule = smaller molecules = faster diffusion
Diffusion across membranes
The non‐polar, hydrophobic tails of phospholipids in membranes are a barrier to most substances, but a few can diffuse directly across the bilayer :
- Lipid‐based molecules ‐ are fat soluble so dissolve and diffuse readily across the bilayer e.g. steroid hormones.
- Very small molecules ‐ oxygen and carbon dioxide are small enough to pass between phospholipid molecules. Water and urea molecules are very small but because they are polar (charged) they pass across the bilayer much slower.
Generally, the smaller and less polar a molecule the quicker it will diffuse across a membrane.
Charged particles (ions) can’t diffuse across the bilayer even if they are small; nor can larger molecules. When these substances diffuse across a membrane, they are helped by 2 types of intrinsic proteins ‐ this is known as facilitated diffusion.
Different membranes have different carrier and channel proteins ‐ this means different membranes will allow different substances across ‐ specific to their cell or organelle’s needs.
Channel proteins
‐ form pores in membranes.
‐ hydrophilic conditions inside pore.
‐ are specific to certain small‐water soluble molecules or ions e.g. sodium, calcium ions.
‐ allow diffusion in or out of cell/organelle.
‐ can be ‘gated’ which means they can be opened or closed by a signal or change in voltage across the membrane.
