The Ultrastructure of Eukaryotic Cells
Membrane-Bound Organelles
- Most of the organelles in a eukaryotic cell are covered in a membrane. This ensures
that the organelle is kept separate from the rest of the cell.
- They are a feature of eukaryotic cells; prokaryotic cells do not have them.
- Electron microscopes have enabled scientists to ascertain the structure of eukaryotic
cells and their organelles by building up a 3D picture of them.
ORGANELLE STRUCTURE FUNCTION
A roughly spherical structure The outer and inner nuclear
membranes can fuse together, at
It is surrounded by a nuclear envelope. which point, dissolved substances and
ribosomes can pass through.
Nucleus
When DNA is wound around histone
proteins, it is known as chromatin. Controls the activities of the cell, stores
When the cell is not dividing, chromatin the organism’s genome, transmits
is spread out. When the cell is about to genetic information and provides
divide, it coils tightly into chromosomes. instructions for protein synthesis.
The nucleus is surrounded by a double Separates the contents of the nucleus
membrane known as the nuclear from the rest of the cell.
Nuclear envelope which contains pores.
Envelope The pores allow substances such as
mRNA and ribosomes to leave the
nucleus.
The nucleolus doesn’t have a Makes ribosomes.
Nucleolus membrane around it. It contains RNA.
It is a system of membranes, RER is the intracellular transport
Rough containing fluid-filled cavities system: the cisternae form channels
Endoplasmic (cisternae), that are continuous with the that transport substances from part of
Reticulum nuclear membrane. the cell to another.
(RER) It is coated in ribosomes RER provides a large surface area for
ribosomes.
This is a system of membranes, SER contains enzymes that catalyse
containing fluid-filled cavities reactions involved with lipid
(cisternae), that are continuous with the metabolism like:
nuclear membrane. - Synthesis of cholesterol
Smooth - Synthesis of lipids needed by
Endoplasmic There are no ribosomes on its surface the cell
Reticulum - Synthesis of steroid hormones
(SER) It is also involved in the absorption,
synthesis and transport of lipids from
the gut.
, This consists of a stack of Proteins are modified by:
membrane-bound flattened sacs. - Adding sugar molecules to
Secretory vesicles bring materials to make glycoproteins
and from the Golgi apparatus. - Adding lipid molecules to make
lipoproteins
- Being folded into their 3D
Golgi Apparatus shape
The proteins are then packaged into
vesicles and then stored in the cell or
moved to the plasma membrane where
they are incorporated into it or
exported outside of the cell.
These may be spherical, rod-shaped or Mitochondria are the site of ATP (
branched, and are 2-5 micrometres energy currency ) production during
long. aerobic respiration.
They are surrounded by two They are self-replicating, so more can
membranes with a fluid-filled space be made if the cell's energy needs
between them. The inner membrane is increase.
Mitochondria highly folded into cristae. The cristae
increase the surface area of the They are abundant in cells where
internal membrane, giving it a greater much metabolic activity takes place, for
area for the proteins and enzymes example in liver cells and at synapses
needed for respiration. between neurons where
neurotransmitters are synthesised and
The inner part of the mitochondrion is a released.
fluid-filled matrix.
These are large organelles, 4-10 Chloroplasts are the site of
micrometres long. photosynthesis.
They are found only in plant cells and
in some protoctists. The first stage of photosynthesis is
when light energy is trapped by
They are surrounded by a double chlorophyll and used to make ATP.
membrane or envelope. The inner occurs in the grana. Water is also split
membrane is continuous with stacks of to supply hydrogen.
Chloroplasts flattened membrane sacs called
thylakoids (resembling piles of plates), The second stage, when hydrogen
which contain chlorophyll. Each stack reduces carbon dioxide, using energy
or pile of thylakoids is called a granum ( from ATP, to make carbohydrates,
plural: grana). The fluid-filled matrix is occurs in the stroma.
called the stroma.
Chloroplasts are abundant in leaf cells,
Chloroplasts contain loops of DNA and particularly the palisade mesophyll
starch grains. layer.
Membrane-Bound Organelles
- Most of the organelles in a eukaryotic cell are covered in a membrane. This ensures
that the organelle is kept separate from the rest of the cell.
- They are a feature of eukaryotic cells; prokaryotic cells do not have them.
- Electron microscopes have enabled scientists to ascertain the structure of eukaryotic
cells and their organelles by building up a 3D picture of them.
ORGANELLE STRUCTURE FUNCTION
A roughly spherical structure The outer and inner nuclear
membranes can fuse together, at
It is surrounded by a nuclear envelope. which point, dissolved substances and
ribosomes can pass through.
Nucleus
When DNA is wound around histone
proteins, it is known as chromatin. Controls the activities of the cell, stores
When the cell is not dividing, chromatin the organism’s genome, transmits
is spread out. When the cell is about to genetic information and provides
divide, it coils tightly into chromosomes. instructions for protein synthesis.
The nucleus is surrounded by a double Separates the contents of the nucleus
membrane known as the nuclear from the rest of the cell.
Nuclear envelope which contains pores.
Envelope The pores allow substances such as
mRNA and ribosomes to leave the
nucleus.
The nucleolus doesn’t have a Makes ribosomes.
Nucleolus membrane around it. It contains RNA.
It is a system of membranes, RER is the intracellular transport
Rough containing fluid-filled cavities system: the cisternae form channels
Endoplasmic (cisternae), that are continuous with the that transport substances from part of
Reticulum nuclear membrane. the cell to another.
(RER) It is coated in ribosomes RER provides a large surface area for
ribosomes.
This is a system of membranes, SER contains enzymes that catalyse
containing fluid-filled cavities reactions involved with lipid
(cisternae), that are continuous with the metabolism like:
nuclear membrane. - Synthesis of cholesterol
Smooth - Synthesis of lipids needed by
Endoplasmic There are no ribosomes on its surface the cell
Reticulum - Synthesis of steroid hormones
(SER) It is also involved in the absorption,
synthesis and transport of lipids from
the gut.
, This consists of a stack of Proteins are modified by:
membrane-bound flattened sacs. - Adding sugar molecules to
Secretory vesicles bring materials to make glycoproteins
and from the Golgi apparatus. - Adding lipid molecules to make
lipoproteins
- Being folded into their 3D
Golgi Apparatus shape
The proteins are then packaged into
vesicles and then stored in the cell or
moved to the plasma membrane where
they are incorporated into it or
exported outside of the cell.
These may be spherical, rod-shaped or Mitochondria are the site of ATP (
branched, and are 2-5 micrometres energy currency ) production during
long. aerobic respiration.
They are surrounded by two They are self-replicating, so more can
membranes with a fluid-filled space be made if the cell's energy needs
between them. The inner membrane is increase.
Mitochondria highly folded into cristae. The cristae
increase the surface area of the They are abundant in cells where
internal membrane, giving it a greater much metabolic activity takes place, for
area for the proteins and enzymes example in liver cells and at synapses
needed for respiration. between neurons where
neurotransmitters are synthesised and
The inner part of the mitochondrion is a released.
fluid-filled matrix.
These are large organelles, 4-10 Chloroplasts are the site of
micrometres long. photosynthesis.
They are found only in plant cells and
in some protoctists. The first stage of photosynthesis is
when light energy is trapped by
They are surrounded by a double chlorophyll and used to make ATP.
membrane or envelope. The inner occurs in the grana. Water is also split
membrane is continuous with stacks of to supply hydrogen.
Chloroplasts flattened membrane sacs called
thylakoids (resembling piles of plates), The second stage, when hydrogen
which contain chlorophyll. Each stack reduces carbon dioxide, using energy
or pile of thylakoids is called a granum ( from ATP, to make carbohydrates,
plural: grana). The fluid-filled matrix is occurs in the stroma.
called the stroma.
Chloroplasts are abundant in leaf cells,
Chloroplasts contain loops of DNA and particularly the palisade mesophyll
starch grains. layer.
