5.1 The Cell Surface Membrane Flashcards
What is the purpose of the cell surface membrane?
The cell surface membrane creates an enclosed space separating the internal cell environment from the external environment
What membranes occur inside of the cell?
Intracellular membranes (internal membranes) form compartments within the cell, such as organelles (including the nucleus, mitochondria and RER) and vacuoles
What do membranes form?
Membranes form partially permeable barriers between the cell and its environment, between cytoplasm and organelles and also within organelles
How can substances cross membranes?
Substances can cross membranes by diffusion, facilitated diffusion, osmosis and active transport
Why are membranes important in cell signalling?
Membranes play a role in cell signalling by acting as an interface for communication between cells
What does the fluid mosaic model explain?
Passive and active movement between cells and their surroundings
Cell-to-cell interactions
Cell signalling
Why does the fluid mosaic model describe membranes as fluid?
The phospholipids and proteins can move around via diffusion
The phospholipids mainly move sideways, within their own layers
The many different types of proteins interspersed throughout the bilayer move about within it (a bit like icebergs in the sea) although some may be fixed in position
Why does the fluid mosaic model describe membranes as mosaics?
The scattered pattern produced by the proteins within the phospholipid bilayer looks somewhat like a mosaic when viewed from above
What are the four main components of the fluid mosaic model?
Phospholipids
Cholesterol
Glycoproteins and glycolipids
Transport proteins
What is the basic structure of the membrane?
The phospholipid bilayer
Why do phospholipid bilayers act as a barrier to most water-soluble substances?
The non-polar fatty acid tails prevent polar molecules or ions from passing across the membrane
How can phospholipids be modified to act as signalling molecules?
Phospholipids can be chemically modified to act as signalling molecules by:
Moving within the bilayer to activate other molecules (eg. enzymes)
Being hydrolysed, which releases smaller water-soluble molecules that bind to specific receptors in the cytoplasm
What does cholesterol do at low temperatures
Cholesterol increases the fluidity of the membrane, stopping it from becoming too rigid at low temperatures (allowing cells to survive at lower temperatures)
This occurs because cholesterol stops the phospholipid tails packing too closely together
What does cholesterol do at higher temperatures?
Interaction between cholesterol and phospholipid tails also stabilises the cell membrane at higher temperatures by stopping the membrane from becoming too fluid
Cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising them and causing phospholipids to pack more closely together
Considering cholesterol can impact how closely a membrane is packed, what effect can this have on ions?
The impermeability of the membrane to ions is also affected by cholesterol
What effect does cholesterol have on cell surface membranes overall?
Cholesterol increases the mechanical strength and stability of membranes (without it membranes would break down and cells burst)
Where do glycolipids and glycoproteins exist?
Glycolipids and glycoproteins contain carbohydrate chains that exist on the surface (the periphery/extrinsically), which enables them to act as receptor molecules
The glycolipids and glycoproteins bind with certain substances at the cell’s surface
What are the three main receptor types?
Signalling receptors for hormones and neurotransmitters
Receptors involved in endocytosis
Receptors involved in cell adhesion and stabilisation (as the carbohydrate part can form hydrogen bonds with water molecules surrounding the cell
What are cell markers?
Some glycolipids and glycoproteins act as cell markers or antigens, for cell-to-cell recognition (eg. the ABO blood group antigens are glycolipids and glycoproteins that differ slightly in their carbohydrate chains)
What are transport proteins?
Transport proteins create hydrophilic channels to allow ions and polar molecules to travel through the membrane.
What are the two types of transport proteins?
Channel (pore) proteins
Carrier proteins
Carrier proteins change shape to transport a substance across the membrane
Can a transport protein transport any ion or molecule?
Each transport protein is specific to a particular ion or molecule.
What is the purpose of transport proteins?
Transport proteins allow the cell to control which substances enter or leave the cell.
