Rough ER is a system of membranous sacs; these sacs give the rough ER a very large surface area due to their folded construction. It derives its name due to the fact it is coated with numerous ribosomes, which line the outer surface of its membrane, the large surface area allows more ribosomes to be attached. Rough ER is mainly involved with the production of proteins destined for export from the cell. The proteins are produced by the ribosomes coating the outer membrane and once the protein leaves the ribosome it enters into the lumen of the rough ER. Here the protein is folded into its final conformation, and can also be modified if required. It is then passed onto the smooth ER where it is transported.
Smooth ER is tubular in appearance and does not contain ribosomes; hence is smooth. Because of the lack of ribosomes smooth ER is not involved in protein synthesis, instead it is concerned with lipid and membrane synthesis. In most cells the smooth ER serves primarily as a central packaging and discharge site for the molecules that are transported from the rough ER. These newly synthesized proteins and lipids are enclosed in a membrane layer and these ‘parcels’ bud off to become transport vesicles, moving on to the Golgi apparatus for further processing of the cargo. Smooth ER is ideally made for the job of transport as the membrane used to transport the newly synthesized products can be easily produced and rapidly replaces the transport vesicle.
Figure 2 (www.molecure.com)
Golgi Apparatus
Golgi bodies (apparatus) are closely associated with the ER and can be considered as the final packaging location for the proteins and lipids. Each Golgi body consists of flattened membrane sacs; within these sacs enzymes prepare the proteins and lipids for shipment to specific locations. Vesicles form at the end of the Golgi body where the membrane begins to bulge. These vesicles then break away, via exocytosis, for the transport of these proteins and lipids to their final location. Within the Golgi body the proteins and lipids are labelled with sequences of molecules, which tell the body where these products should be delivered. The number of Golgi bodies in a cell varies with most cells having only one but more specialised cells used for the production of protein may have hundreds.
Figure 3 (www.molecure.com)
Mitochondria
Mitochondria are the energy organelles or ‘power houses’ of the cell; they extract energy from the nutrients in food and transform it into a useable form to energize cellular activities. Each mitochondrion is enclosed in two phospholipid bilayers called the inner and outer mitochondrial membranes. The outer membrane is smooth while the inner membrane is folded into convolutions called cristae. The cristae are
responsible for providing the mitochondrion with a large surface area that enhances the productivity of respiration. The two membranes divide the mitochondria into two internal compartments. The space located between the outer and inner mitochondrial membranes is called the intermembrane space. The space enclosed by the inner membrane is termed the mitochondrial matrix; it is here that the metabolic steps of cellular respiration occur.
Figure 4 (Human Physiology From Cells to Systems)
Ribosomes.
Ribosomes are involved in the assembly of proteins according to cell’s genetic instructions. They are most predominant in cells that have high rates of protein synthesis. A functional ribosome consists of two non-membranous subunits; these subunits are constructed in the nucleolus of the cell, which join together in the presence of messenger RNA (ribonucleic acid). It is these ribosomes that give the ER its rough appearance although not all ribosomes are attached to the ER. Free ribosomes are suspended in the cytoplasm and are involved in the synthesis of proteins that remain in the cytoplasm. Although they produce proteins destined for different fates the ribosomes are structurally identical and interchangeable. A cell can adjust the numbers of free and bound ribosomes to meet its needs at any particular time.
Figure 5 (www. Molecularexpressions.com)
Lysosomes
Sometimes known as the ‘suicide bags’ of the cell and are used in the removal of various unwanted cellular debris and foreign material, such as bacteria. Lysosomes are vacuoles filled with an enzyme (hydrolytic), these enzymes are specially made for the Lysosomes in the rough ER and work only at low ph levels (acidic). The Lysosomes are produced in the Golgi apparatus and from here are dispersed throughout the cytoplasm. The process for removing unwanted cellular debris (autophagy) takes place in two stages. Firstly the ER donates a membrane; this membrane then surrounds the old organelle. Secondly a lysosome fuses with this membrane to form an autophagic vacuole. The lysosome then safely releases its enzyme into the vacuole and destroys the organelle. The process for the removal of foreign material from the cell is called phagocytosis. The unwanted invader is brought into the interior of the lysosome by endocytosis; this is done by the lysosome membrane dipping inwards and forming a poach around the invader. The membrane then seals, forming a small intercellular membrane bound vesicle with the contents trapped inside. Enzymes are then released and the invader is ingested.
Figure 6 (www.molecularexpressions.com)
The Basic Structure Of A Human Cell And Plant Cell
As with the human cell a plant cell also performs specialized tasks within the plant. In this section we will look at both a basic human cell (cheek cells) and a plant cell (onion epidermis cells) under a light microscope, showing a low power map and high power drawing for both cells.
To enable the viewing of the cell make up under the light microscope the cells have to be stained, the procedure for this is outlined below.
- Cheek cells: take a sterile cotton bud and gently move the bud over the inside of the cheek on one side of the mouth and along the lower side of the gum.
- Smear the cotton bud over a small area of a clean microscope slide.
- Place 1 to 2 drops of 1% methylene blue from a dropper pipette onto the smear and cover with a coverslip, take care to prevent excess air bubbles
- Observe the smear under the low power magnification of a light microscope. When the cells are in focus, increase the power of the objective to achieve maximum magnification and resolution.
- After the cells have been observed, immerse the slide and cover slip into a container of disinfectant.
- Onion epidermis cells: take a thin slice from the onion and place this onto a clean microscope slide.
- Place 1 to 2 drops of iodine solution from a dropper pipette onto the onion cells and cover carefully with a coverslip.
- Proceed as above for cheek cells.
Low Power Map of the stained Cheek Cells (mag x40)
High Power Drawing of the stained Cheek Cells (mag x400)
Low Power Map of the stained Onion Cells (mag x40)
High Power Drawing of the stained Onion Cells (mag x400)
The table below outlines the general differences between the human cell and the plant cell observed under the light microscope.
From the table above we can see the two main differences between the human and plant cells.
- Vacuole, A membrane bound sac that plays a role in intercellular digestion and the release of cellular waste products, small in the animal cell but large in the plant cell. In the plant cell the vacuole plays a part in the turgor pressure of the cell, when the plant is well watered the water collects in the cell vacuoles producing rigidity; if the water pressure in the vacuole is reduced the plant wilts.
- Cell wall, this structure is made of a substance called cellulose and provides the cells with both protection and rigidity, it’s this structure that gives the plant the ability to remain upright (not present in animal cells).
Bibliography
Human Physiology From Cells to Systems second edition
Lauralee Sherwood
West Publishing Company
United States 1993
Course Notes
Collette Lawlor 2003