Membranes - The Structure and Function of Membranes Flashcards
What is the primary role of membranes in cells?
Membranes separate cells from their environment and organelles from the cytosol.
State 4 functions of plasma membranes
- separates cell contents from the outside (barrier between cells and its environment)
- controls the passage of materials in and out
- allow cell communication
- recognition of other cells
State 4 functions of membranes inside cells
- isolates DNA
- can be the site of chemical reactions
- compartmentalises reactions
- ribosomes attach
- protection from lysosomes
What is compartmentalisation?
Compartmentalisation is the formation of membrane-bound areas within cells, allowing incompatible reactions to occur in separate areas with specific conditions.
Why is compartmentalisation useful in cells?
- incompatible reactions occur within the cell
- isolates reactions making them more efficient
- allows reaction conditions to be maintained (e.g., chemical gradients)
- protects cell components
Define partially permeable
A membrane that allows some substances to cross but not others
Define cell signalling
A complex system of intercellular communication
Define phospholipid bilayer
- arrangement of phospholipids found in cell membranes
- the hydrophilic phosphate heads form the inner and outer surface of a membrane ,
- sandwiching the fatty acid tails to form a hydrophobic core
What is the structure of membranes in cells?
All membranes have a phospholipid bilayer with hydrophilic phosphate heads on the outer surfaces and hydrophobic fatty acid tails in the core.
Why are phospholipid bilayers ideal for membranes?
Their hydrophilic heads interact with aqueous environments inside and outside the cell, while the hydrophobic core provides a barrier.
How were membranes first visualized, and what did it confirm?
Electron microscopy in the 1950s showed membranes as two black parallel lines, supporting the lipid bilayer theory.
Who proposed the fluid mosaic model, and when?
American scientists Singer and Nicolson proposed it in 1972, building on earlier lipid bilayer models.
Why is the model called the “fluid mosaic model”?
The phospholipids move fluidly within the bilayer, allowing flexibility, and the embedded proteins vary in size, shape, and position like tiles in a mosaic.
What does the fluid mosaic model describe about membrane structure?
It explains the dynamic and flexible nature of membranes, with a lipid bilayer hosting various proteins in specific positions.
Why is the fluid mosaic model important today?
It forms the foundation of our current understanding of membrane structure and function.
Why are membranes referred to as having a ‘fluid-mosaic structure’ ?
the phospholipids are able to move freely relative to each other (fluid)
the proteins embedded in the bilayer vary in shape, size, and position (mosaic)
What do plasma membranes contain, and how do they vary?
Plasma membranes contain specific proteins and lipids, with types and numbers unique to each cell type.
Define glycoprotein
Extrinsic membrane proteins with attached carbohydrate molecules of varying lengths and shapes
What is the role of glycoproteins?
They act as recognition sites for chemicals, such as hormones.
Define glycolipid
Cell-surface membrane lipids with attached carbohydrate molecules of varying lengths and shapes
What is the role of glycolipids?
Glycolipids act as recognition sites, such as for cholera toxins.
Act as antigens that can be recognised by the immune system
Form hydrogen bond with water molecules to stabilise membrane
How are phospholipid molecules oriented in membranes?
Hydrophobic tails point inwards, and hydrophilic heads point outwards.
What are intrinsic proteins?
Transmembrane proteins that are embedded through both layers of a membrane.
How are intrinsic proteins kept in place?
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.
What are extrinsic proteins?
Peripheral proteins that are present in one side of the bilayer.
Where are extrinsic proteins found, and why is this?
They can be present in either layer and some move between layers. This is because they have hydrophophilic R-groups on their outer surface and interact with the polar heads of the phospholipids or with intrinsic proteins.
Why are membrane components important?
They play vital roles in the membrane’s function and in the specific functions of the cells or organelles they are part of.
Define phospholipid
Modified triglycerides, where one fatty acid has been replaced with a phosphate group
What are channel proteins?
Membrane proteins that provide a hydrophilic channel through a membrane
What are receptor proteins?
Extrinstic glycopoteins that bind chemical signals, triggering a response by the cell
What are carrier proteins?
Membrane proteins that play a part in the transport of substances through a membrane
What is the approximate width of an animal cell plasma membrane?
7nm
What are the functions of cholesterol in cell surface membranes?
regulates fluidity and adds stability to membranes
prevents phospholipids from crystallising
What are the functions of phospholipids in cell surface membranes?
forms basic structure
acts as partially permeable barrier
allows the passage of lipid soluble molecules
prevents water soluble molecules from moving across
What are the functions of carrier proteins in cell surface membranes?
have an important role in both passive movement of polar molecules and ions down a concentration gradient
this involves the shape of the protein changing
What are the functions of channel proteins in cell surface membranes?
provide a hydrophillic channel that allows passive movement of polar molecules and ions down a concentration gradient
What are the functions of extrinsic proteins in cell surface membranes?
Can attach to the cytoskeleton
Define cell adhesion
The process by which cells make contacts with each other
Describe 2 ways in which cholesterol regulates membrane fluidity
- hydrophillic end of cholesterol interact with phosphate heads
- hydrophobic end of cholesterol interacts with fatty acid tails
- pulling the phospholipids together
- prevents membranes from becoming too solid and crystallising by preventing the phospholipids from getting too close together
What 3 factors affect membrane structure?
Temperature
Solvents
Cholesterol
How does an increase in temperature affect cell membranes?
increases membrane fluidity
increases membrane permeability
carrier and channel proteins are denatured at higher temperatures
membrane loses its structure
Why does an increase in temperature increase membrane permeability?
more temperature means phospholipids move more
because they have a higher kinetic energy
makes it easier for things to pass through
How do solvents affect cell membranes?
membrane loses structure
- increases permeability
Why do solvents increase membrane permeability?
solvents dissolve phospholipids
Give an example of a solvent that can dissolve phospholipids
Ethanol, Methanol etc.
How can a colorimeter show the permeability of membranes?
more permeable means more betalain leaks out
- more betalain means less light is absorbed by the detector
