An example of a hydrophilic pore found
in some proteins (Boyle et al, 1999)
Like carbohydrates, lipids contain carbon, hydrogen and oxygen but far less oxygen. Therefore, there are fewer polar OH groups so lipids have very low solubility in water. Other lipids found in the cell membrane other than phospholipids are glycolipids which have a branching carbohydrate (polysaccharide) molecule. Most polysaccharides are attached to the protein forming glycoproteins. These polysaccharide chains which stick out like antennae only occur on the outside of the membrane. Both glycolipids and glycoproteins contribute to the glycocalyx functioning as receptor sites for chemical signals.
Cell membranes are important for a number of reasons. They separate the contents of cells from their external environment and controlling exchange between the two. They also enable compartments to be formed inside cells in which specialised metabolic pathways can take place. They can also act as receptor sites for chemical signals such as hormones, cell to cell recognition and binding cells together. Phospholipid membranes in chloroplasts and mitochondria keep pigments and carrier molecules in fixed positions.
Transport across membranes is vital in order for the cells to function efficiently. Cells need to obtain certain food supplies for energy such as glucose and oxygen and excrete substances such as enzymes and hormones which are produced within the cell. Due to the hydrophobic nature of the fatty acid chains of the phospholipids, it is difficult for water soluble molecules to penetrate this barrier and so specific transport systems are required such as diffusion.
A modified form of diffusion known as facilitated diffusion allows transport proteins with specific binding sites for certain molecules such as glucose to bind. The protein then changes shape and moves the glucose to the other side of the membrane. The glucose is released and the protein reverts back to its original shape and position in the membrane. These are known as carrier proteins. Another type of protein responsible for facilitated diffusion is the channel protein which are which are fixed shape molecules. It is selective about which ions can pass through and controls the passage of selected charged particles. These are not active processes as no energy is required.
Another form of transport is osmosis. Water diffuses from a region of high concentration to a region of lower concentration through a partially permeable membrane. If a human or animal cell is placed in distilled water, it absorbs water by osmosis and swells up. As cell surface membranes have virtually no physical strength the cell often bursts. Hypotonic plasma causes red blood cells to swell and burst leaving ‘ghosts’ of the membrane. This process is known as haemolysis and is a passive process which requires no energy. (Boyle et al, 1999).
Active transport is the energy consuming transport of molecules or ions across a membrane against a concentration gradient. It is driven by metabolic energy derived from ATP. The mechanism by which this energy is used for transportation is referred to as a ‘pump’ and the most common is the sodium pump. Cells are able to actively pump out sodium ions while potassium ions are actively pumped in. This combined mechanism is called a sodium – potassium pump which controls the cell volume and collects potassium ions for use in cell activities such as protein synthesis. This pump is essentially a protein which spans the cell membrane.
The flexibility of the cell is an important factor involving the bulk transport of materials through membranes, either into cells by endocytosis or out by exocytosis. Endocytosis occurs by invagination of the plasma membrane which forms small vesicles or food vacuoles that become detached and enter the cytosol. Lysosomes fuse with the vacuole membrane and digestive enzymes enter the vacuole and digests the contents. Soluble products such as amino acids and sugars are absorbed into the cytosol while undigested remains are discharged from the cell by exocytosis. (Boyle et al, 1999). The main types of endocytosis are phagocytosis where cells take in solid particles such as bacteria in order to destroy them and pinocytosis – a process identical to phagocytosis but involves fluids.
The endoplasmic reticulum is made up of a complex system of membrane bound flattened sacs or tubules called cisternae. These may be covered with ribosomes forming the rough ER. It is concerned with the transport of proteins which have been synthesised by the ribosomes, Such proteins include digestive enzymes and hormones.
ER with no ribosomes attached is known as smooth ER and is not involved in protein synthesis. It is concerned with lipid metabolism and contains enzymes that detoxifies a variety of organic molecules and acts as a storage site for calcium in skeletal muscle cells. (Boyle et al, 1999)
The ER is a kind of intracellular transport system facilitating movement of materials from one part of the cell to another. In this connection, it is interesting that the nuclear membrane is pierced by tiny pores which are continuous with the ER, thus providing a route by which materials move from nucleus to cytoplasm and vice versa. (Roberts, 1987).
An illustration of membrane bound organelles found in the cell such as
the ER, golgi apparatus and lysosome (Adds et al, 2000)
The golgi complex consists of a stack of flattened cisternae and associated vesicles. The whole organelle is a shifting, flexible structure where vesicles are constantly being added at one side and lost from the other. Generally, vesicles fuse with the forming face (nearest to the nucleus) and leave from the maturing face (nearest to cell surface membrane). It is involved with the synthesis and modifications of proteins, lipids and carbohydrates. Proteins made on the ribosomes are packaged into vesicles by the ER. Some of the vesicles join with the golgi complex and the proteins they contain are modified before they are secreted out of the cell. (Boyle et al, 1999). The golgi complex also plays an important role in the formation of lysosomes.
Lysosomes are membrane bound organelles containing digestive enzymes called lytic enzymes and is bounded by a single membrane. If they were not enclosed by a membranous sac they would attack the other cell organelles. One use of lysosomes is they supply the enzymes which destroy old or surplus organelles. They also digest material taken into the cell. After a white cell has engulfed a bacterium, for example, lysosmes discharge enzymes into the vacuole and digest the organism. This process is called phagocytosis. (Boyle et al, 1999).
Mitochondria are large, individual organelles which occur in large numbers in most cells. Each mitochondrion is bounded by a double membrane with the outer layer being a smooth continuous boundary. The inner membrane is extensively folded in to form partitions called cristae and encloses the mitochondrial matrix. Mitochondria are the sites of the aerobic respiration within cells.
In conclusion, the structure and function of the cell membrane is of great importance in order for the cell to operate efficiently. The most important property of the cell membrane is its partial permeability. Because of this property, the cell membrane plays a key part in determining the composition of the cytoplasm by controlling what gets in and out.
Bibliography
Adds J, Larkom E, (2000) Molecules and Cells
Miller R Nelson Thornes Ltd
Cheltenham
Bailey M & Hirst K (1995) Biology Core
Collins Educational
London
Boyle M, Indge B (1999) Human Biology
Senior K Collins Educational
London
Clegg C.J & (2000) Advanced Biology
Mackean D.G Principles & Application
2nd Edition
John Murray Ltd
Green N.P.O, Stout (1990) Biological Science
G.W, Taylor D.J 2nd Edition
Ebenezer Bayliss &
Son Ltd
Roberts M.B.V (1986) Biology a Functional
Approach
4th Edition
Thomas Nelson &
Son Ltd
