2.1.5 Membranes Flashcards
What are the 5 functions of the membrane?
- Formation of partially permeable barriers
- Compartmentalisation
- Sites of enzyme attachment and there fore of chemical reactions
- Maintenance of concentration gradients
- • Sites of cell signalling
what is a partially permeable barrier?
partially permeable means that only specific molecules/ions may pass through the membrane, others may not, giving control over what may pass into/out of a cell or organelle.
where to partially ermeable barriers come in handy?
between the cell and its environment, or between organelles and the cytoplasm, and within organelles
what does partially permebale mean?
a membrane which allows some substances through but not others
what is compartmentalisation?
membranes define organelles within eukaryotic cells by forming their boundaries. These membrane‐bound organelles can then function as specialised compartments, effective in carrying out specific roles.
give two examples of compartmentalisation
- transport vesicles carry proteins from RER to Golgi
- the membrane of a lysosome keeps the hydrolytic enzymes separate from other parts of the cell so that these are not accidently digested
why is compartmentalisation important?
Within each organelle, the relevant substrates and enzymes can be concentrated and pH can be optimised, giving higher reaction rates.
why is membranes being sites of enzyme attachment important? give it in an example form
membrane of mitochondria is folded into cristae so there is more surface area for attachment of ATP synthase proteins and hence a higher rate of ATP synthesis.
why is it important for the maintenance of concentration gradients?
the presence of a partially permeable membrane means that a concentration gradient can be set up and maintained across it, potentially giving faster rates of diffusion
give an example of a concentration gradient
At the alveoli (air sacs in the lungs), ventilation (breathing) plus the flow of blood in capillaries maintain steep concentration gradients for oxygen and carbon dioxide, resulting in rapid diffusion of these across the alveolar wall.
what is cell signalling?
A complex system of intercellular communication. This is when chemicals binf to complementary receptors casuing a direct or cascade of events in a cell.
How does cell signalling work
Plasma (i.e. cell surface) membranes contain receptors (typically made from glycoprotein), which have a binding site of complementary shape to a signalling molecule (e.g. a hormone or cytokine) which has been released by another cell. When the signalling molecule binds to its complementary receptor (found only in the plasma membrane of appropriate target cells), this triggers a change in the tertiary structure (3D shape) of the receptor. This in turn will result in a response (a change in activity) in the target cell, e.g. a change in metabolism due to enzyme activation, or changes to gene expression.
how do medicinal and r3ecreational drugs work?
Some receptors in plasma membranes act as sites where (medicinal or recreational) drugs can bind.
What is the model of membranes called?
the fluid mosaic model
why is the fluic mosaic model called a model?
The term model is used to describe a simplified representation of a complex structure or process
whats the aim of using a model?
The aim of a model is to enable clear communication of the key features of the structure or process, such that they are easier to understand.
what is the fluid mosaic model?
The fluid mosaic model is a representation of the key features known about biological membrane structure. The model helps us to visualise the different components found in a biological membrane and how they are organised
how thick is a membrane?
7nm
what is included in the fluid mosaic model?
A phospholipid bilayer from the basis of any biological membrane. Embedded in the bilayer are the other components: cholesterol, glycolipids, glycoproteins and proteins.
what does the term fluid refer to in the fluid mosaic model?
The term fluid refers to the idea that most of the membrane components, especially the phospholipids, are not in fixed positions but rather are free to move past one another; the membrane is essentially in liquid phase
what does the term mosaic mean in the fluid mosaic model?
The term mosaic refers to the overall appearance of the membrane from above. large proteins are embedded, with what appears to be random distribution, in the bilayer of phospholipids. The phospholipids appear to fill in all the spaces between the proteins
what is the role of a phospholipid in a membrane? 3
Forms a bilayer that functions as a barrier with the property of partial permeability;
Due to hydrophobic fatty acid tails, prevent passage of ions (e.g. Na+) and polar
molecules (e.g. glucose)
Allow passage of molecules that are lipid‐soluble (e.g. steroid hormones) and/or very small (e.g. water).
what is the role of cholesterol in the bilayer? 4
Slots between fatty acid tails of phospholipids in the bilayer;
Due to largely hydrophobic nature, increases effectiveness of the bilayer as a barrier to ions and polar molecules
Regulates the fluidity (degree of movement) of phospholipid molecules within the bilayer to keep this optimal despite temperature fluctuations
Increases mechanical strength of the membrane.
what is the role of glycolipids in the bilayer? 2
Carbohydrate component (always projecting to the outside of the cell) can act as a self‐antigen, with a role in recognition of self‐cells by the immune system
Carbohydrate component is sticky hence plays a role in cell‐to‐cell adhesion
what is the role of proteins in the bilayer? 3
May be cylindrical in shape, functioning as channel proteins providing a pore through the membrane that allows passage of specific ions or small polar molecules by facilitated diffusion
May function as carrier proteins, changing shape to transport specific ions or small polar molecules by active transport across the membrane
May functions as enzymes, e.g. ATP synthase in the inner mitochondrial membrane makes ATP from ADP and Pi.
what is the role of glycoprotein in the bilayer? 3
Act as receptors for cell signalling, with a complementary binding site for a specific signalling molecule (e.g. hormone, cytokine, neurotransmitter or drug)
Carbohydrate component (always projecting to the outside of the cell) can act as a self‐antigen, with a role in recognition of self‐cells by the immune system
Carbohydrate component is sticky hence plays a role in cell‐to‐cell adhesion.
what are the effects on membrane structure due to temperature increase? 2
Increased kinetic energy (KE) of phospholipids (PLs) means they move around more, so bilayer becomes more fluid and hence more/larger gaps appear between PLs, resulting into increased permeability
Membrane proteins denature, resulting in gaps opening up around them;
If respiratory enzymes denature, ATP production decreases.
what are the effects on membrane function due to temperature increase? 4
Increased gaps in the membrane increase permeability to larger/ hydrophilic molecules and to ions, which pass through the gaps by diffusion, and increased permeability
Diffusion rates increase due to increased KE of the molecules/ions
Facilitated diffusion and active transport cannot occur if carrier proteins have denatured
Active transport and bulk transport cannot occur if insufficient ATP is available.
what are the effects on membrane structure due to temperature decrease? 2
Decreased KE of PLs means they move around less (packing more closely together), so bilayer becomes less fluid and hence less/smaller gaps occur between PLs
Decreased KE decreases respiration rate, so ATP production decreases.
what are the effects on membrane function due to temperature decrease? 3
Decreased gaps in the membrane decrease permeability even to molecules which usually can diffuse through
Diffusion and facilitated diffusion rates decrease due to decreased KE of the molecules/ions
Active transport and bulk transport cannot occur if insufficient ATP is available.
what are the effects on membrane structure due to freezing?
If temperature decreases so much that the membrane and water around it freeze solid, ice crystals form which penetrate into the membrane
what are the effects on membrane function due to freezing? 2
Whilst the membrane and surrounding water are frozen solid, the membrane will have zero permeability and diffusion rates will be zero
BUT if temperature later increases and the frozen membrane thaws, large holes (punctured by ice crystals) appear, causing a massive increase in permeability that results in the leakage of cytoplasm out of cells.
what are the effects on membrane structure due to organic substances such as ethanol? 2
Non‐polar solvent molecules interact with the hydrophobic fatty acid tails of PLs, inserting themselves into the bilayer and disrupting its regular structure
If the solvent is present in significant quantities, the PLs dissolve into the solvent, resulting in the bilayer completely dispersing.
what are the effects on membrane function due to organic substances such as ethanol?
Gaps appear in the bilayer as its structure is disrupted, resulting in increased permeability to larger/ hydrophilic molecules and to ions, which pass through the gaps by diffusion
If the bilayer is dispersing due to PLs dissolving into the solvent, there will be leakage of cytoplasm out of cells.
what are the effects of detergents on membranes?
Detergent molecules have similar properties to phospholipids (i.e. hydrophilic heads and hydrophobic tails), but have a conical rather than a cylindrical shape. If detergent is applied to a membrane, this causes the phospholipids to leave the bilayer and associate with the detergent molecules in spherical structures called micelles.
The hydrophilic part of the detergent molecule is attracted to/by the phosphate heads of the phospholipids and water molecules; the hydrophobic part interacts with the fatty acids tails of the phospholipids, repelling/repelled by water molecules.
Consequently, the membrane structure is significantly disrupted, losing its partial permeability properties and eventually ceasing to exist as a coherent structure altogether.
Molecules that were previously held inside organelles or inside the cell (due to being too large and/or too hydrophilic to pass through the membrane) are now released.
label
