|Eukaryotic Cell structure
A Rough endoplasmic reticulum (RER)
C Glycogen granules
G Nuclear envelope
I Golgi apparatus
J Vacuole (e.g. lysosome, food vacuole)
K Smooth endoplasmic reticulum (SER)
L Nuclear pore
M Nucleoplasm / Nucleus
N Cytosol ('cytoplasm')
P Cell surface membrane
Process A is endocytosis
Process B is exocytosis
The diagram above is of a generalised animal cell (but is perhaps most similar to a locomoting fibroblast or white
blood cell (leukocyte)). The front-end of the cell (top) is flattened into an advancing lamellipod, which creeps and
ripples forwards; whilst the tail-end (bottom) is a retracting uropod. MF, microfilament; NE, nuclear envelope; NP,
Another diagram of a generalised animal cell is shown below:
The cell is made-up of 'little organs' or organelles. The cell consists of a cell surface membrane (plasma membrane or
plasmalemma) which acts as the cell's 'skin' and as the cell border the membrane determines what substances can enter
(import) and leave the cell (export) and which substances cannot pass. Inside the cell surface membrane the cell is
composed of two principle parts: the cytoplasm and the nucleus. The nucleus is the control centre of the cell and
behaves like a computer and stores information that the cell needs in DNA molecules. The cytoplasm is made-up of all
The Cell Surface Membrane
The cell surface membrane is a fluid double-layer of phospholipid molecules (A) that jostle about to form a 'lipid sea' and
protein 'islands' (C, F and G) float around in this 'sea'. Click here for a more detailed description of the membrane and an
explanation of some of its functions.
Many of the organelles of the cell are made-up of similar membranes (so-called 'unit membrane'). These include the
nuclear envelope (which is a double layer of two membranes) the rough endoplasmic reticulum (RER), the smooth
endoplasmic reticulum (SER), vesicles and vacuoles and mitochondria.
The structure of the endoplasmic reticulum is shown below.
The RER consists of interconnected membranous sacs (cisternae) - unit membrane enclosing a fluid-filled lumen.
The function of the RER is the synthesis, storage and transport of proteins around the cell. The proteins are
manufactured by the ribosomes, 10 nm diameter particles that stud the outside the RER cisternae. The nuclear
envelope is continuous with the RER and is really a specialised part of it and is sometimes also studied with
ribosomes on its cytoplasmic surface. The RER is continuous with the smooth endoplasmic reticulum (SER) - a
network of branching membranous tubes that may fill much of the cytoplasm. The SER is responsible for the
synthesis, storage and transport of lipids and carbohydrates and also the storage of calcium ions.
Golgi Apparatus, Vesicles, Vacuoles and Lysosomes
The Golgi apparatus (Golgi body or Golgi complex) is the cell's 'post office'. It consists of stacks of 4-8 fluid-filled
membranous disc-like sacs (cisternae, singular cisterna). A mammalian cell typically has 40-100 such stacks.
The Golgi apparatus above has been sectioned down the middle. One face (the cis) face points towards the
nucleus and RER (the top face in this diagram). Fluid-filled spherical membranous globules, called vesicles,
bud-off from the RER, carrying synthesised proteins as cargo. These vesicles travel to the cis-face of the Golgi
complex and fuse with it, delivering their protein cargo (along with the lipids from the membrane of the vesicle).
Inside the Golgi cisternae the proteins (and lipids) are sorted and labeled by attaching carbohydrate chains
(chains of sugar molecules bonded together) to the proteins (a process called glycosylayion). The proteins
and their attached carbohydrate chains may also be sulphated - sulphur may be added to them, giving them a
negative charge. They may also be phosphorylated (by addition of phosphate). These carbohydrates may be
required for the final function of the protein (now a glycoprotein, protein + carbohydrate chain = glycoprotein) or
they may serve as address labels.
Once sorted, the proteins are packaged into different types of vesicles which are labeled so that the cell knows
what to do with them. These vesicles bud-off from the trans-face of the Golgi complex (the face pointing away
from the cell nucleus and pointing outward toward the cell membrane - the bottom face in the diagram above).
Some of these vesicles are secreted - they move to the cell surface membrane and fuse with it and release their
contents outside the cell in a process called exocytosis. Other vesicles are stored until needed, for example,
vesicles of neurotransmitter are stored in the axon terminal of a nerve cell (neuron or neurone) until the nerve
passes a signal and then the neurotransmitter is secreted by exocytosis so that the neurotransmitter can pass
the signal on to the next cell. Plasma B cells are white blood cells (lymphocytes) that synthesise and secrete
proteins called antibodies that fight infection. These cells have a number of prominent Golgi complexes and as
soon as vesicles bud-off, containing mature antibody, they are immediately exported (constitutive exocytosis) so
that the cell churns out as much antibody as quickly as possible.
Some vesicles become lysosomes (a special type of vesicle or a small vacuole - a vacuole is essentially a
large vesicle). The proteins carried by lysosomes are hydrolytic enzymes, such as proteases (enzymes that
break-up proteins) and the contents are also acidic. Lysosomes engulf old and worn out organelles and break
them down (autophagy, lit. 'eating self'). They also digest and break-down 'food items' absorbed during
phagocytosis. In single-celled organisms this is a normal feeding process - the cells engulf food items, such as
bacteria and other smaller cells, in a process called phagocytosis. Phagocytosis is a type of endocytosis,
which is the reverse of exocytosis - a pocket forms in the cell membrane enclosing the food item and then this
invagination pinches off as a food / phagocytic vacuole which enters the cytoplasm. Lysosomes will then fuse
with the food vacuole, forming a phagolysosome, in order to digest the food item. Useful nutrients enter the
cytosol and indigestible remains are excreted by exocytosis. Lysosomes budding from the Golgi are called
primary lysosomes. Secondary lysosomes are phagolysosomes in which the food stuff is at least partly digested.
Lysosomes are also important in apoptosis or programmed-cell death in which a cell self-destructs. This is
important in the disposal of old and worn-out cells and cells that are no longer required during development. In
apoptosis the lysosomes release their proteases into the cytosol, destroying the cell and its organelles. These
proteases include members of the cathepsin protease family.
Lysosomes occur only rarely in plant cells, since their function is taken over by the main vacuole (though
sometimes this vacuole breaks-up into smaller vacuoles, which possibly forms those lysosomes that have been
observed in plant cells).
Proteins targeted for lysosomes carry the phosphorylated sugar mannose-6-phosphate as an address tag,
which the Golgi adds to them. In this way the attached carbohydrates act as signal sequences. The Golgi also
synthesises many of the carbohydrates. For example, it synthesises gylcosaminoglycans (GAGs) which are
chains of a repeating disaccharide (in this case each disaccharide is a six-carbon hexose sugar or hexuronic
acid and a hexosamine) which are added to proteins to make proteoglycans which are exported in vesicles and
secreted to form the extracellular matrix (ECM).
You may be wondering how do the proteins move from the cis-cisterna of the Golgi, where they arrive, to the
trans-cisterna where they are exported in vesicles. The answer appears to be cisternal maturation - vesicles
bud from the trans-cisterna until it is completely dispersed, then the next cisterna in-line becomes the
trans-cisterna. Since fusion of RER vesicles forms new cis-cisternae, the stack remains, but in a state of
dynamic flux, with a cis-cisterna eventually becoming the trans-cisterna as it matures - a constant cycling like a
conveyor belt. As the cisternae mature, their contents change as enzymes are added or removed.