Eukaryotic cells contain many membrane bound organelles and all cells including prokaryotes are surrounded by a plasma membrane
Plasma Membrane - flexible and self sealing
As a result they can maintain all their own conditions - pH, enzymes and other substances can be different in each organelle - therefore many different reactions can take place
There may be a single phospholipid bilayer (cell membrane and vesicles, vacuoles and some organelles) or there may be a double bilayer (nucleus, mitochondria & chloroplasts)
Plasma Membrane - 3 functions :
To control the transport of substances into and out of the cell or organelles - allow certain molecules to pass through (they are partially permeable) - plants and prokaryotes = cell wall as well which is thicker and freely permeable - large pores
To act as a receptor site to recognise chemicals that need to the enter the cell or organelle
To separate off the cell from the environment and the different reactions of the cell from each other by forming the organelles - different concentrations can be maintained on either side of the membrane
Membranes and made up of phospholipids
In water they form a two-layered structure called a bilayer where hydrophobic tails point inwards and the hydrophilic heads point out into the water
Membranes are partially permeable - smaller and less polar = faster diffusion rate across a cell membrane
Small, non-polar molecules - oxygen, co2 rapidly diffuse across a phospholipid bilayer
Small, polar molecules such as water also diffuse across but much slower
Charged particles such as Na+ and K+ are unlikely to diffuse across even if they are small
Large, polar, water-soluble molecules are therefore highly unlikely to be able to pass through the phospholipids
Intrinsic Proteins
span across the whole membrane and are used for transport
Protein carriers
Hydrophilic protein channels
Extrinsic Proteins
Sit on the inner or outer surface and do not go the whole way through the membrane
Provide structural support, binding cells together
Form recognition sites (also glycoproteins)
Act as receptor sites on cell surface for chemicals (hormones, etc) to bind to - how cells detect chemicals from other cells
Enzymes
Glycoproteins
Short, branching carbohydrate chains are attached to proteins in the membrane
These glycoproteins act as receptors for hormones and neurotransmitters or antigens
Glycolipids
A carbohydrate bonded to a phospholipid
They act as recognition sites
They also form hydrogen bonds with the water surrounding the membrane which helps to maintain the stability of the membrane and help cells the attach to one another, so forming tissues
Cholesterol
Cholesterol molecules sit between the hydrophobic tails in the phospholipid membrane
They restrict the movement of molecules in the membrane
The more cholesterol the less fluid and more rigid the membrane is. This is especially important at body temperature, when a membrane would become too fluid and break apart without cholesterol.
Fluid refers to the fact that all the different molecules can move around
Mosaic because there are different types of molecules - the proteins move sideways within the phospholipid bilayer
The fluidity means that the cells are able to change their shape. The ease with which they do this is dependent on the number of phospholipids with unsaturated fatty acids in the phospholipids.
3.2.3 Transport Across Cell Membranes
Eukaryotic cells contain many membrane bound organelles and all cells including prokaryotes are surrounded by a plasma membrane
Plasma Membrane - flexible and self sealing
As a result they can maintain all their own conditions - pH, enzymes and other substances can be different in each organelle - therefore many different reactions can take place
There may be a single phospholipid bilayer (cell membrane and vesicles, vacuoles and some organelles) or there may be a double bilayer (nucleus, mitochondria & chloroplasts)
Plasma Membrane - 3 functions :
To control the transport of substances into and out of the cell or organelles - allow certain molecules to pass through (they are partially permeable) - plants and prokaryotes = cell wall as well which is thicker and freely permeable - large pores
To act as a receptor site to recognise chemicals that need to the enter the cell or organelle
To separate off the cell from the environment and the different reactions of the cell from each other by forming the organelles - different concentrations can be maintained on either side of the membrane
Membranes and made up of phospholipids
In water they form a two-layered structure called a bilayer where hydrophobic tails point inwards and the hydrophilic heads point out into the water
Membranes are partially permeable - smaller and less polar = faster diffusion rate across a cell membrane
Small, non-polar molecules - oxygen, co2 rapidly diffuse across a phospholipid bilayer
Small, polar molecules such as water also diffuse across but much slower
Charged particles such as Na+ and K+ are unlikely to diffuse across even if they are small
Large, polar, water-soluble molecules are therefore highly unlikely to be able to pass through the phospholipids
Intrinsic Proteins
span across the whole membrane and are used for transport
Protein carriers
Hydrophilic protein channels
Extrinsic Proteins
Sit on the inner or outer surface and do not go the whole way through the membrane
Provide structural support, binding cells together
Form recognition sites (also glycoproteins)
Act as receptor sites on cell surface for chemicals (hormones, etc) to bind to - how cells detect chemicals from other cells
Enzymes
Glycoproteins
Short, branching carbohydrate chains are attached to proteins in the membrane
These glycoproteins act as receptors for hormones and neurotransmitters or antigens
Glycolipids
A carbohydrate bonded to a phospholipid
They act as recognition sites
They also form hydrogen bonds with the water surrounding the membrane which helps to maintain the stability of the membrane and help cells the attach to one another, so forming tissues
Cholesterol
Cholesterol molecules sit between the hydrophobic tails in the phospholipid membrane
They restrict the movement of molecules in the membrane
The more cholesterol the less fluid and more rigid the membrane is. This is especially important at body temperature, when a membrane would become too fluid and break apart without cholesterol.
Fluid refers to the fact that all the different molecules can move around
Mosaic because there are different types of molecules - the proteins move sideways within the phospholipid bilayer
The fluidity means that the cells are able to change their shape. The ease with which they do this is dependent on the number of phospholipids with unsaturated fatty acids in the phospholipids.