Biological Membranes Flashcards
What do phospholipids consist of
Two fatty acid molecules bonded to a glycerol which is bonded to a phosphate group.
What are the hydrophilic and hydrophobic parts of the phospholipids
The hydrophobic region is the fatty acids and the hydrophilic region consists of the phosphate groups.
What happens when phospholipids are put in water
As they contain both hydrophobic and hydrophilic regions, when in water they arrange themselves in a phospholipid bilayer. Where the hydrophilic phosphate groups can interact with the water molecules but the hydrophobic fatty acid tails cluster together away from the water in the centre.
What does the phospholipid bilayer make up
It makes up all cell membranes - including the cell surface membrane and membranes surrounding organelles such as lysosomes
Functions of cell membranes (1) (barrier)
Act as a barrier between the internal contents of the cell and the environment, or between the contents of an organelle and the cytoplasm.
Functions of cell membranes (2) (separation)
Membranes can separate one part of an organelle from another part
Functions of cell membranes (3) (reactions)
Membranes can be locations for chemical reactions for example some of the stages in respiration.
Functions of cell membranes (4) (indicating)
Membranes are involved in cell signalling
Functions of membranes (5) (permeability)
Membranes are partially permeable, allowing some substances to pass but acting as a barrier to others.
Why is the cell membrane partially permeable
Because the membrane contains a hydrophobic centre, the hydrophobic molecules such as steroid hormones can easily pass through the cell membrane. However, the hydrophobic centre prevents hydrophilic molecules from easily passing through. (As they are polar and can’t easily pass through the non-polar region of the membrane)
What is an exception to the permeability of membranes and why. And what is the downside
Water molecules can easily pass through the cell membrane despite being polar as they are very small. However this occurs at a very slow rate.
Functions of cell membranes (6) (electrical)
They can allow electrical signals to pass along them
Functions of cell membranes (7) (enzymes)
They provide attachment sites for enzymes and other molecules involved in metabolism.
Functions of cell membranes (8)
They provide a surface for attachment (ribosomes on the RER)
Provide a surface for pigments (chlorophyll in thylakoid membranes of chloroplasts).
What does the cell surface membrane consist of
It is based on the phospholipid bilayer, however it contains a large number of protein molecules too as well as cholesterol.
What is the structure of cholesterol and why does this allow interaction with the cell membrane
They have a polar hydrophilic group at one end, this group attracts to the polar head group of the phospholipid molecules.
The rest of the molecule is non polar and hydrophobic so attracts to the non-polar fatty acids in the phospholipids.
What is cholesterols first role and what is the effect
Because cholesterol interacts with phospholipids it increases the strength of the cell surface membrane. Making the membrane more stable and less likely to get damaged by other molecules involved the membrane.
What is the second role of cholesterol in the membrane
It controls the fluidity of the membrane by reducing the sideways movement of phospholipids.
What does cholesterol help the membrane to be in different conditions
Prevents the membrane from becoming too fluid in warm conditions and too rigid under cool conditions.
What is the third function of the cholesterol in membranes
By packing the spaces between phospholipids, cholesterol helps to reduce the movement of water soluble chemicals across the cell surface membranes
What is the structure of the cell surface membrane known as
The fluid mosaic model
Why is it called the fluid mosaic model
Fluid - as the phospholipid molecules can move around within each layer, so the membrane is flexible and can change shape.
Mosaic - as the membrane is studded with protein molecules of different sizes and the arrangement of these proteins varies.
What are intrinsic proteins
Proteins that are fully embedded in the membrane from one side to the other
Because they are embedded in the membrane what is a property of the amino acids in the proteins
They have hydrophobic amino acids on the outside surface of the protein. These hydrophobic amino acids can interact with the fatty acid tails in the phospholipid bilayer keeping the intrinsic proteins in place.
What are protein channels
A type of intrinsic proteins with a hydrophilic channel running through the centre, the channel is lined with hydrophilic amino acids and is filled with water molecules.
What is the purpose of protein Chanel’s
They allow water soluble molecules and ions to diffuse down a concentration gradient through the membrane.
What are carrier proteins
A type of intrinsic proteins that can change their shape or position to transfer molecules or ions through the membrane. They are involved in active transport.
What are extrinsic proteins
They don’t span the membrane and are instead found on one side of the membrane or the other side.
What are the main functions of extrinsic proteins
They play a structural role within the molecule whereas others act as enzymes, some act as receptors for other molecules such as hormones too.
What are glycoproteins and where can they be found
Proteins that also contain a carbohydrate molecule attached and they can be found on membranes.
what are the roles of glycoproteins
Some allow cells to attach to each other forming tissues (cell adhesion)
Some play a role in the immune system presenting antigens to T cells
Others act as receptors for hormones and when the hormones binds a response is caused in the cell.
What are glycolipids and where can they be found
Carbohydrates can be found attached to phospholipid molecules too, these are glycolipids. They can also be found on cell membranes.
What are the functions of glycolipids
They are used when cells come in contact with each other, and the glycolipids on the surface of one cell can be recognised by another cell determining whether cells come into contact.
They can act as antigens (e.g in determining blood group)
How does temperature affect membranes
As the phospholipids in the membrane are constantly moving. When the temperature is increased the phospholipids with have more kinetic energy and will move more. As a result then membrane becomes more fluid and begins to lose its structure.
How does temperature affect membranes (2) Further increase
If the temperature continues to increase then the membrane will break down completely eventually.
The loss of structure increases the permeability of the membrane making it easier for substances to pass through it.
How does temperature affect membranes (3)
As well, as this carrier and channel proteins in the membrane will be denatured at higher temperatures. These proteins are involved in transport across membranes, so membrane permeability is affected.
What are many organic solvents
They are less polar than water or non polar
What do many concentrated organic solvents do
They dissolve membranes, disrupting cells
What do less concentrated organic solvents do
They will not dissolve membranes however they can still cause damage.
What can Non-polar alcohol molecules do
They can enter the cell membrane and their presence disrupts the membrane
What happens when the membrane is disrupted
It becomes more fluid and permeable.
