Chapter 5- 5.1- The structure and fuction of membranes Flashcards
What are membranes?
Membranes are the structures that separate the contents of the cell from their environment.
They also separate the different areas within a cells (organelles) from each other and the cytosol.
Some organelles are divided further by internal membranes.
What is the formation of separate membrane-bound areas in a cell called?
Compartmentalisation.
This is vital to a cell as metabolism includes many different and often incompatible reactions.
Containing reactions in a separate parts of the cell allows the specific conditions required for cellular reactions, such as chemical gradients, to be maintained, and protects vital cell components.
What is the membrane structure?
All the membranes in a cell have the same basic structure.
The cell surface membrane which separates the cell from its external environment is known as the plasma membrane (cell surface plasma membrane).
Membranes are formed from the phospholipid bilayer.
The hydrophilic phosphate heads of the phospholipids form both the inner and outer surface of a membrane, sandwiching the fatty acid tails of the phospholipids to form a hydrophobic core inside the membrane.
Cells normally exist in aqueous environments. The inside of cells and organelles are also usually aqueous environments.
Phospholipid bilayers are perfectly suited as membranes because the outer surfaces of the hydrophilic phosphate heads can interact with water.
What is the cell membrane theory?
Membranes were seen for the first time following the invention of electron microscopy, which allowed images to be taken with higher magnification and resolution.
Images taken in the 1950s showed the membranes as two black parallel lines- supporting an earlier theory that membranes were composed of a lipid bilayer.
In 1972 American scientists Singer and Nicolson proposed a model, building upon an earlier lipid-bilayer model, in which proteins occupy various positions in the membrane.
The model is known as the fluid-mosaic model because the phospholipids are free to move within the layer relative to each other (they are fluid), giving the membrane flexibility, and because the proteins embedded in the bilayer vary in shape, size, and position (in the same was as the tiles of a mosaic).
This model forms the basis of our understanding of membrane today.
A closer look at cell membrane components
Plasma membranes contain various proteins and lipids- the type and number of which are particular to each cell type.
The components of plasma membranes play an important role in the functions of the membrane and the cell or organelle they are part of.
- refer to p103.
What are membrane proteins?
Membrane proteins have important roles in the various functions of membranes.
There are two types of protein in the cell-surface membrane- intrinsic and extrinsic proteins.
What are intrinsic proteins?
Instinct proteins, or integral proteins, are trans membrane proteins that are embedded through both layers of a membrane. They have amino acids with hydrophobic R-groups on their external surfaces, which interact with the hydrophobic core of the membrane, keeping them in place.
Channel and carrier proteins are intrinsic proteins. They are both involved in transport across the membrane.
Channel proteins- provide a hydrophilic channel that allows the passive movement of polar molecule and ions down a concentration gradient through membranes. They are held in position by interactions between the hydrophobic core of the membrane and the hydrophobic R-groups on the outside of the proteins.
Carrier proteins- have an important role in both passive transport (down a concentration gradient) into cells. This often involves the shape of the protein changing.
What are glycoproteins?
Glycoproteins are intrinsic proteins.
They are embedded in the cell-surface membrane with attached carbohydrates (sugar) chains of varying lengths and shapes. Glycoproteins play a role in cell adhesion (when cells join together to form tight junctions in certain tissues) and as receptors for chemical signals.
When the chemical binds to the receptor, it elicits a response from the cell. This may cause a direct response or set off a cascade of events inside the cell.
This process is known as cell communication or cell signalling.
Examples include:
- receptors for neurotransmitters such as acetylcholine at nerve cell synapses. The binding of neurotransmitters triggers or prevents an impulse in the next neurone.
- receptors for peptide hormones, including insulin and glucagon, which affect the uptake and storage of glucose by cells.
Some drugs act by binding to cell receptors. For example, beta blockers are used to reduce the response of the heart to stress.
What are glycolipids?
Glycolipids are similar to glycoproteins. They are lipids with attached carbohydrate (sugar) chains. These molecules are called cell markers as self (of the organism) or non-self (of cells belonging to another organism).
What are extrinsic proteins?
Extrinsic proteins or peripheral proteins are present in one side of the bilayer. They normally have hydrophilic R-groups on their outer surfaces and interact with the polar heads of the phospholipids or with intrinsic proteins. They can be present in either layer and some move between layers.
What is cholesterol?
Cholesterol is a lipid with a hydrophilic end and a hydrophobic end, like a phospholipid. It regulates the fluidity of membranes.
Cholesterol molecules are positioned between phospholipids in a membrane bilayer, with the hydrophilic end interacting with the heads and the hydrophobic end interacting with the tails, pulling them together. In this way cholesterol adds stability to membranes without making them too rigid.
The cholesterol molecules prevent the membranes becoming too solid by stopping the phospholipid molecules from grouping too closely and crystallising.
How is the plasma membrane the site of chemical reactions?
Like enzymes, proteins in the membranes forming organelles, or present within organelles, have to be in particular positions for chemical reactions to take place. For example, the electron carriers and the enzyme ATP synthase have to be in the correct positions within the cristae (inner membrane of mitochondrion) for the production of ATP in respiration.
The enzymes of photosynthesis are found on the membrane stacks within the chloroplasts.
