Cells
Prokaryotes
Means ‘before nucleus’
Have no membrane-bound nucleus
Very little internal organisation
Possess few membrane-bound organelles(no mitochondria or endoplasmic reticulum)
Prokaryotic cells have small ribosomes
Eg: bacteria
Eukaryotes
Means ‘true nucleus’
Have membrane-bound structures
Have helical DNA with histone protein coat held on chromosomes
High level of internal structure-large range of complex organelles that have specific roles in the cell
Capable of forming a mitotic spindle
Eg: plants, animals, fungi
Cell membrane
Also known as the plasma membrane
Found in plants and animals
‘Boundary between the cell and its environment.’
No real strength
Function: controls what enters and leaves the cell. It is described as selectively or partially permeable.
Phospholipid bilayer with protein pores scattered all along it.
Phospholipid bilayer
A phospholipid is a lipid molecule (made from fatty acids and glycerol) with a phosphate group attached. It also
has 2 fatty acid tails attached.
The head is hydrophilic (polar/charged) meaning it is attracted to water whereas the hydrophobic tails (non-
polar) repel water.
The phospholipid bilayer is what gives the membrane its selectively permeable properties.
Fluid mosaic theory
Used to describe the structure of the phospholipid bilayer
Throughout the PLBL, there are proteins and other molecules scattered along the entire length of the membrane
It is fluid because the lipid molecules are in constant sideways motion, although they always keep the bilayer
arrangement, and the proteins are the mosaic.
The proteins can be intrinsic or extrinsic.
Intrinsic proteins which extend the entire length of the bilayer are called transmembrane proteins.
Cholesterol molecules are important in the structure of the membrane and they lie in between the phospholipid
tails. The cholesterol gives stability to the membrane.
Cholesterol increases membrane stability by restricting the sideways movement of the phospholipid molecules
at higher temperatures.
At lower temperatures, the cholesterol acts to maintain membrane fluidity-it stops the phospholipid molecules
from sticking together and therefore allows them to move freely.
Carbohydrates can be attached to the membrane proteins or the lipids-forming glycoproteins or glycolipids.
These are referred to as the glycocalyx and they are located on the outer phospholipid layer only.
, Proteins in the membrane:
o Provide stability
o Can be enzymes-very beneficial to have enzymes in the membrane
As they are kept in the ideal place in terms of substrate availability and pH
They would need to be replaced less often than other enzymes
o Act as adhesion sites where adjacent cells are held together
o Act as receptor sites-usually the glycocalyx-proteins allow specificity-the receptor site and the specific
molecule are complementary in shape to one another. Receptor sites are important in hormone action
and when transporting neurotransmitters between neurons.
o They act as antigens
o Involved in transporting substances across the bilayer. Either through protein channels or protein
carriers. Protein channel crosses the entire length of the membrane and forms a hydrophilic channel
that will allow polar molecules to pass through the bilayer. Channels can be open all the time or can be
open and closed-it is then referred to as a
gated channel.
o Protein carriers transport specific ions or
molecules across the membrane. Molecule
may be charged or need to go against the
concentration gradient. Carrier usually
changes shape to allow the molecule inside.
o Glycocalyx molecules are usually involved in
cell-cell recognition. This is particularly useful
in allowing similar cells to recognize each
other so they can group together to form
tissues. Glycolipids and glycoproteins can
form hydrogen bonds with water molecules
so they can help to stabilise the membrane.
Nucleus
Found in all eukaryotic cells
Spherical in shape and surrounded by a nuclear envelope. This is a double membrane. In between the
membrane is the perinuclear space.
The membrane has pores along it
Nucleoplasm is what is known as the main bulk of the nucleus
The nucleus contains the cells genetic information (DNA), allowing the cell to divide and carry out all its cellular
processes. DNA is isolated inside the nucleus for
protection.
DNA carries the genetic code for protein synthesis.
Protein synthesis does not occur inside the nucleus, so the
DNA code leaves the nucleus through nuclear pores to get
to the cytoplasm.
DNA does not leave the nucleus, but its genetic code is
carried by mRNA (messenger RNA) out of the nucleus.
Each nucleus has 1 or more nucleoli (sing: nucleolus). This is
a dark staining, spherical structure that contains DNA that
codes for rRNA (ribosomal RNA). The rRNA is then used to
make ribosomes. Once made, ribosomes leave the
nucleus.
Prokaryotes
Means ‘before nucleus’
Have no membrane-bound nucleus
Very little internal organisation
Possess few membrane-bound organelles(no mitochondria or endoplasmic reticulum)
Prokaryotic cells have small ribosomes
Eg: bacteria
Eukaryotes
Means ‘true nucleus’
Have membrane-bound structures
Have helical DNA with histone protein coat held on chromosomes
High level of internal structure-large range of complex organelles that have specific roles in the cell
Capable of forming a mitotic spindle
Eg: plants, animals, fungi
Cell membrane
Also known as the plasma membrane
Found in plants and animals
‘Boundary between the cell and its environment.’
No real strength
Function: controls what enters and leaves the cell. It is described as selectively or partially permeable.
Phospholipid bilayer with protein pores scattered all along it.
Phospholipid bilayer
A phospholipid is a lipid molecule (made from fatty acids and glycerol) with a phosphate group attached. It also
has 2 fatty acid tails attached.
The head is hydrophilic (polar/charged) meaning it is attracted to water whereas the hydrophobic tails (non-
polar) repel water.
The phospholipid bilayer is what gives the membrane its selectively permeable properties.
Fluid mosaic theory
Used to describe the structure of the phospholipid bilayer
Throughout the PLBL, there are proteins and other molecules scattered along the entire length of the membrane
It is fluid because the lipid molecules are in constant sideways motion, although they always keep the bilayer
arrangement, and the proteins are the mosaic.
The proteins can be intrinsic or extrinsic.
Intrinsic proteins which extend the entire length of the bilayer are called transmembrane proteins.
Cholesterol molecules are important in the structure of the membrane and they lie in between the phospholipid
tails. The cholesterol gives stability to the membrane.
Cholesterol increases membrane stability by restricting the sideways movement of the phospholipid molecules
at higher temperatures.
At lower temperatures, the cholesterol acts to maintain membrane fluidity-it stops the phospholipid molecules
from sticking together and therefore allows them to move freely.
Carbohydrates can be attached to the membrane proteins or the lipids-forming glycoproteins or glycolipids.
These are referred to as the glycocalyx and they are located on the outer phospholipid layer only.
, Proteins in the membrane:
o Provide stability
o Can be enzymes-very beneficial to have enzymes in the membrane
As they are kept in the ideal place in terms of substrate availability and pH
They would need to be replaced less often than other enzymes
o Act as adhesion sites where adjacent cells are held together
o Act as receptor sites-usually the glycocalyx-proteins allow specificity-the receptor site and the specific
molecule are complementary in shape to one another. Receptor sites are important in hormone action
and when transporting neurotransmitters between neurons.
o They act as antigens
o Involved in transporting substances across the bilayer. Either through protein channels or protein
carriers. Protein channel crosses the entire length of the membrane and forms a hydrophilic channel
that will allow polar molecules to pass through the bilayer. Channels can be open all the time or can be
open and closed-it is then referred to as a
gated channel.
o Protein carriers transport specific ions or
molecules across the membrane. Molecule
may be charged or need to go against the
concentration gradient. Carrier usually
changes shape to allow the molecule inside.
o Glycocalyx molecules are usually involved in
cell-cell recognition. This is particularly useful
in allowing similar cells to recognize each
other so they can group together to form
tissues. Glycolipids and glycoproteins can
form hydrogen bonds with water molecules
so they can help to stabilise the membrane.
Nucleus
Found in all eukaryotic cells
Spherical in shape and surrounded by a nuclear envelope. This is a double membrane. In between the
membrane is the perinuclear space.
The membrane has pores along it
Nucleoplasm is what is known as the main bulk of the nucleus
The nucleus contains the cells genetic information (DNA), allowing the cell to divide and carry out all its cellular
processes. DNA is isolated inside the nucleus for
protection.
DNA carries the genetic code for protein synthesis.
Protein synthesis does not occur inside the nucleus, so the
DNA code leaves the nucleus through nuclear pores to get
to the cytoplasm.
DNA does not leave the nucleus, but its genetic code is
carried by mRNA (messenger RNA) out of the nucleus.
Each nucleus has 1 or more nucleoli (sing: nucleolus). This is
a dark staining, spherical structure that contains DNA that
codes for rRNA (ribosomal RNA). The rRNA is then used to
make ribosomes. Once made, ribosomes leave the
nucleus.
