5.1. STRUCTURE AND FUNCTION OF MEMBRANES Flashcards
What are the different types of cell membrane?
-Plasma membrane (AKA cell surface membrane)
-Tonoplast membrane
-Outer mitochondrial membrane
-Inner mitochondrial membrane
-Outer chloroplast membrane
-Nuclear envelope
What type of membrane is the cell surface membrane?
Partially Permeable
What is compartmentalisation?
Formation of separate membrane-bound areas
What does compartmentalisation allow?
-metabolic reaction to be separated
-different environmental conditions in different parts of the cell
-chemical concentration gradients to be made
-protection of cellular components
What does the plasma membrane surround?
The cytoplasm of living cells, physically separating the intracellular components from the extracellular environment
What is the main ability of membranes?
They are flexible (fluid) and able to break and fuse easily
What are the roles of the Cell Surface Membrane?
Environment (homeostasis)
Transport
Cell to Cell Signalling (communication)
Detection of changes in the environment
Site of Chemical Reactions
Anchorage for cytoskeleton/extracellular matrix
Cell to Cell Joining
What does the Cell surface membrane not do?
Does NOT provide support for the cell shape
What are membranes made out of?
Phospholipids
What is the structure of a phospholipid?
-consists of a polar head (hydrophilic) composed of a glycerol and a phosphate molecule
-consists of two non-polar tails (hydrophobic) composed of fatty acid chains (hydrocarbon)
What is the definition of hydrophilic?
Water-loving
What is the definition of hydrophobic?
Water-hating
Why are phospholipids classed as amphipathic?
As they contain both hydrophilic and lipophilic regions
What is the definition of lipophilic?
Fat- loving
How are phospholipids arranged?
-into a bilayer
-hydrophobic tails face inwards and are shielded from the surrounding polar fluids
-hydrophilic heads face outwards into the cytosolic and extracellular fluids
What does bilayer mean?
Two layers
What does a kink in the tails represent?
A double bond - increases fluidity
What are micelles?
Phospholipids form micelles when submerged in water
What is the common structure all phospholipids share?
-polar organic molecule
-phosphate group
-glycerol molecule
-two fatty acid tails
What was the first model for plasma membranes?
By Davson and Danielli in 1935
-when under a membrane, showed a ‘trilaminar’ appearance
-model was described as a lipo-protein sandwich
-dark segments representing two protein layers were artifacts
What is a trilaminar?
3 layers (two dark outer layers and a lighter inner region)
What is an artefact?
defraction of electrons through cell membrane
- shows something that isn’t there
What is the fluid-mosaic model?
-By singer and nicolson
Proteins were embedded within the lipid bilayer and were constantly moving
According to the fluid-mosaic model, how easily can proteins move within the bilayer?
dependent on the number of phospholipids with unsaturated fatty acids
What can often be found within the plasma membrane?
-phospholipids
-integral/transmembrane proteins (channel/carrier)
-peripheral proteins
-cholesterol
-glycoproteins
What are extrinsic membrane proteins?
-AKA peripheral proteins
-only present on one side of the membrane
-often have amino acids with hydrophilic R groups on the outer surface of the protein that will bind to the phospholipid heads/a transmembrane protein
What are glycoproteins?
proteins that contain oligosaccharide chains covalently attached to amino acid R groups (‘glycosylation’)
What are the functions of glycoproteins?
-cell adhesion
-cell signalling
-cell surface markers for cell to cell recognition
-immune responses
-ABO blood groups
What is the structure of a glycoprotein?
The phosphate groups in a phospholipid is replaced by the sugar residue
What is cholesterol?
-a sterol/type of lipid
-modulates membrane fluidity
-maintains membrane integrity
Hydroxyl group of each cholesterol molecule interacts with water molecules (near phospholipid heads)
Why do animal cells not need cell walls?
Cholesterol increases membrane packing which allows the membrane to remain stable and durable without being rigid - allows animal cells to change shape and animals to move
What is a feature of intrinsic membrane proteins?
Some act as enzymes
-i.e. the enzymes used in aerobic respiration in the inner mitochondrial membrane
e.g. ATP Synthase
What is ATP Synthase?
Formation from ATP to ADP and Pi is energetically unfavourable and would normally proceed in the reverse direction - ATP synthase couples with ATP synthesis to overcome this.
