(S.J. Singer and G.L. Nicolson (1972)); this is illustrated in figure 2.
The membrane is a lipid bilayer made up of phospholipids. Each phospholipid contains a hydrophilic head composed of glycerol and a phosphate group and 2 hydrophobic tails made up of fatty acids. The hydrophilic heads are orientated towards the aqueous phase, whilst the hydrophilic tails face away from this phase.
Embedded within this bilayer are intrinsic and extrinsic (peripheral) proteins, channel proteins, cholesterol, glycolipids, and glycoproteins. Each of these carries out a function, for example cholesterol makes the membrane more stable and prevents it from solidifying when the body temperature falls. While glycoproteins trigger action potentials in order to excrete particular enzymes.
Small molecules like water, oxygen and carbon dioxide pass freely across the membrane. However larger molecules such as amino acids must be carefully managed [5]. Transport of chemicals across the cell surface membrane happens through diffusion down a concentration gradient, facilitated diffusion, endocytosis, exocytosis and active transport. Transport of water across the membrane occurs by osmosis. The cell surface membrane must efficiently control this transport into and out of the cell as it is fundamental to many biological processes including conduction of nerve cell action potentials and muscle contraction.
The cell membrane must also facilitate electrical signalling between cells using hormones and neurotransmitters [2].
It is also important to remember that internal membranes are in place as well as the surface membrane, for example the nuclear envelope. These define the boundaries of organelles and can provide a matrix on which chemical reactions can occur [4].
The nucleus
The nucleus is often described as the most important site of the cell as it contains all the necessary genetic material for a cell to work efficiently. It contains the genome which is encoded in DNA. The DNA associates with proteins called histones to form chromatin fibres which can be condensed or dispersed [2]. The chromatin plays a key role during cell division; it becomes distributed into pairs of chromosomes which attach to the mitotic spindle before the cells divide.
Figure 3. The structure of a nucleus Source:
The nucleus is surrounded by a double membrane separated by inter membrane space. The inner membrane defines the nucleus’ shape.
At certain points the two membranes join to form nuclear pores, these allow the transfer of substances which cannot pass through the bilayer. After transcription in protein synthesis mRNA leaves the nucleus using these pores.
Deep within the nucleus is a dark staining area called the nucleolus. The nucleolus takes a main role in the biosynthesis of rRNA and the assembly of ribosomes [3]. Abundance of nucleoli is seen in cells which produce vast amounts of protein, for example embryonic and secretory cells [2].
Although by definition a mammalian cell must contain a nucleus, not every animal cell does throughout its entire life. For example erythrocytes are anucleate when mature. The reason for this is to improve efficiency, having no nucleus provides more space for haemoglobin, meaning they can carry more oxygen to sites around the body.
As can be seen in figure 3, the nuclear membrane is continuous at points with the endoplasmic recticulum to allow rapid cell signalling and communication during protein processing [3].
Endoplasmic recticulum
The endoplasmic recticulum is a series of single flattened sacs enclosed by a membrane. As shown by figure 4, two distinct regions can be recognised in most cells: the rough endoplasmic recticulum and the smooth endoplasmic recticulum. The only small difference is that the rough endoplasmic recticulum has ribosomes lined on its surface whereas the smooth does not.
The ribosomes on the rough endoplasmic recticulum are the site where amino acids are made into proteins. The newly made proteins are then stored in the cavity and packaged at the ends. The membrane closes forming vesicles which travel to the golgi.
Figure 4. The structure of the endoplasmic recticulum Source:
The rough endoplasmic recticulum also adds carbohydrates to membrane proteins through a process called gylcosylation [2].
On the other hand, the function of the smooth endoplasmic recticulum varies from tissue to tissue [4]. In the ovaries, testes and adrenal glands it is where steroid hormones are made, in the liver it is the site of detoxication of foreign chemicals [4]. However its most extensive function is the storage and release of calcium ions. Many stimuli can cause the release of calcium into the cytosol which then triggers several cell processes.
Golgi apparatus
The golgi apparatus is a series of single curved sacs enclosed by a membrane, each sac is progressively smaller than the previous. It has a characteristic exterior in which one face appears convex (cis), whilst the other is concave (trans), this is illustrated in figure 5 below.
The golgi has a main function of modifying and packaging proteins for secretion.
Whilst the golgi is thought to be an extension of the endoplasmic recticulum, in reality it does not communicate directly with it.
Figure 5. The structure the golgi apparatus Source:
Small vesicles known as transport vesicles move proteins to the cis site from the endoplasmic recticulum, whilst secretory vesicles bud off the golgi from the trans face and migrate the proteins to the plasma membrane. At the membrane, proteins leave via exocytosis.
The source of energy for these processes is the mitochondria.
Mitochondria
Mitochondria are surrounded by a double membrane and are easily recognisable due to the extensive folding of the inner membrane into projections called cristae, shown in figure 6. The central section contains a jelly like substance called the matrix. Within the matrix are ribosomes and DNA (as a loop).
Mitochondria generate energy through oxidative phosphorylation. The link reaction and the krebs cycle occur in the matrix, and the electron transport chain is located on the cristae.
The abundance of mitochondria within a cell is thereby dependent on the amount of energy needed for a metabolic process. For example muscle cells require ample mitochondria in order to supply energy in the form of ATP for contraction.
Figure 6. The structure a mitochondria Source:
The mitochondria are normally suitably located near to ATP utilization, for example in the heart muscle; the mitochondria are closely situated to the myofibrils [2].
Other organelles
There are several smaller organelles present in a mammalian cell including: lysosomes, centrioles and a cytoskeleton.
Lysosomes contain digestive enzymes and are enclosed by a single membrane. Lysosomes are particularly apparent in cells that digest and destroy other cells, such as macrophages [4]. A common lysosome is a peroxisome; its function is to breakdown lipids within the bilayer.
Centrioles are two hollow tubes, which are arranged at right angles to one another, each centriole is made of nine bundles of microtubules. They are self-replicating organelles which help organise cell division [5].
The shape of a cell is maintained by a variety of protein filaments known as the cytoskeleton. The protein filaments come in 3 main forms: actin, intermediate and microtubules [2].
Summary
Although all cells that make up a single organism contain the same genetic material in their nuclei, not all cells are identical. Cells become specialised due to their reading of the DNA fingerprint, which in turn effects to proteins made and thereby the functioning of the cell.
It is vital to remember that cell components are part of a much more complex system. Life comes in an array of sizes, from single-celled bacteria to multi cellular organisms. Single-celled organisms rely on just one cell to carry out all functions, whereas a multicellular organism requires a larger extent of organisation to survive. Even though some cells are specialised to carry out a function they are limited to what they can do alone. As a result, cells must work as coherent units through cell signalling and communication for life to exist.
Bibliography
- Lodish (2008) Molecular Cell Biology, Sixth edition (Date accessed: 06.10.12)
- Pocock (2006) Human physiology, third edition (Date accessed: 06.10.12)
- Evans, Manson (2008) Cell Biology and Genetics, third edition (Date accessed: 08.10.12)
- Bolsover (2003) Cell Biology, second edition (Date accessed: 09.10.12)
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Molecular expressions, animal cell structure [Internet] (Last modified: 26.05.05) Available at: (Accessed 10.09.12)
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Difference between, cytoplasm and cytosol [Internet] (Last modified: 2012) Available at: (Accessed: 10.09.12)
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