Endoplasmic Reticulm (ER)
The endoplasmic reticulm (ER) is an elaborate system of membranes found throughout the cell, forming a cytoplasmic skeleton. It is an extension of outer nuclear membrane with which it is continuous. The membranes form a series of sheets, which enclose flattened sacs called cisternae. Where the membranes are linked with ribosomes they are called rough endoplasmic reticulm. The rough ER are involved with the synthesis of protein. Where the membranes lack ribosomes they are called smooth endoplasmic reticulm. The smooth ER makes lipids and steroids (e.g. cholesterol and reproductive hormones). Overall, the functions of the ER are as follows:
- Provides a large surface area for chemical reactions.
- Provides a pathway for the transport of materials through the cell.
- Provides lipids and steroids (smooth ER).
- Collects and stores synthesized material.
- Provides a structural skeleton to maintain cellular shape. (e.g. the smooth ER of a rod cell form the retina of the eye).
Cell Membrane
The cells main function is to serve as a boundary between the cell and its environment. It is a functional organelle, which allows some substances to pass freely in and out of the membrane. Yet others may be excluded at one moment only to pass freely across the membrane on another occasion. On account of the membrane’s ability to permit different substances to pass across it at different rates, it is said to be differentially permeable. The cell membrane is made up of almost two chemical groups – proteins and phospholipids. The precise arrangement of these chemicals is less certain. They are either in a protein-phospholipid sandwich or the fluid-mosaic model. The proteins in the membrane have a number of functions. Apart from giving structural support they have very specific, varying from cell to cell. It is this specifically, which allows cells to be recognised by other agents in the body, e.g. enzymes, hormones and antibodies. In the fluid-mosaic model it is thought probable that the proteins also assist the active transport of materials across the membrane.
Ribosomes
Ribosomes are small cytoplasmic granules found in all cells. They are around 20nm in diameter in eukaryotic cells (80S type) but slightly smaller in prokaryotic ones (70S). They occur either singly or in groups called polysomes. They may be associated with endoplasmic reticulm or occur freely within the cytoplasm. Despite their small size, their enormous numbers mean that they can account for up to 20% of the mass of a cell. Ribosomes are made up of relatively small RNA molecules and protein. They are important in protein synthesis.
The Golgi Body
The Golgi apparatus has a similar structure to the smooth ER but is more compact. It is composed of stacks of flattened sacs made of membranes. The sacs are fluid-filled and pinch off smaller membranous sacs, called vesicles, at their ends. There is normally only one Golgi apparatus in each animal cell but in plant cells there may be a large number of stacks known as dictyosomes. Its position and size varies form cell to cell but it is well developed in secretory cells and neurones and is small in muscle cells. This suggests that the Golgi apparatus plays some role in the production of secretary material. In particular, it is thought to perform the following functions.
- Produces glyco-proteins such as mucin required in secretions by adding the carbohydrate part to the protein.
- Produces secretary enzymes, e.g. the digestive enzymes of the pancreas.
- Secretes carbohydrates such as those involved in the production of new cell walls.
- Transports and stores lipids. Also, forms lysosomes.
Microtubules
Microtubules occur widely throughout living cells. They are slender, unbranched tubes about 20nm in diameter and up to several micros in length. Their functions are as follows. To provide an internal skeleton (Cytoskeleton). for cells and so help determine their shape. They may aid transport within cells by providing routes along which materials move. They form a framework along which the cellulose cell wall of plants is laid down. They are the major components of cilia and flagella where they contribute to their movement. They are found in the spindle during cell division and within the centrioles from which the spindle is formed.
The following organelles are all featured in an animal cell but not in a plant cell.
Lysosomes
Lysosomes are similar in size in size to spherical mitochondria, being 0.2-0.5μm in diameter. Lysosomes are bounded by a single membrane, but unlike mitochondria they have no cristae or internal structure. They contain a large number of enzymes, mostly hydrolases, in acid solution. They isolate these enzymes from the remainder of the cell. By so doing they prevent them from acting upon other chemicals and organelles within the cell. The functions of lysosomes are as follows.
- They digest material, which the cell consumes from the environment.
- They digest parts of the cell, such as worn-out organelles, in a similar way to that described above.
- They release their enzymes outside the cell (exocytosis) in order to break down other cells.
Centrioles
Centrioles have the same basic structure as the basal bodies of cilia. They are hollow cylinders about 0.2μm long. They are in the centrosome, part of the cytoplasm. It contains two Centrioles. At cell division they migrate to opposite poles of the cell where they synthesize the microtubules of the spindle. Despite the absence of Centrioles, the cells of higher plants do form spindles.
Glycogen granules
Every cell contains a limited store of food energy. This store may be in the form of soluble material such as the sugar found in vacuoles of plant cells. It may also occur in insoluble form, as grains or granules, with cells or organelles. Glycogen granules occur throughout the cytoplasm of animal cells. They store animal starch or glycogen. Oil or lipid droplets are found within the cytoplasm of both plant and animal cells.
Cillium
Cilia and flagella are almost identical, except that cilia are usually shorter and more numerous. Both are around 0.2μm in diameter; cilia are about 10μm long whereas flagella may be 100μm long. They are found in a limited number of cells but are nevertheless of great importance. They function to either move an entire organism or to move material within an organism.
Pinocytosis
Pinocytosis or ‘cell drinking’ is very similar to phagocytosis except that the vesicles produced, called pinocytic vesicles, are smaller. The process is used for the intake of liquids rather than solids. Even smaller vesicles, called micropinocytic vesicles, may be pinched off in the same way. Both Pinocytosis and phagocytosis are methods by which materials are taken into the cell in bulk. This process is called endocytosis. By contrast, the reverse process, in which materials are removed from cells in bulk, is called exocytosis.
The following organelles are all featured in a Plant cell but not in an animal cell.
Chloroplasts
Chloroplasts are found only in plant cells. Their shape and size vary with species but there is always a double unit membrane surrounding a matrix (stoma) in which are stacks of lamellae (grana). The lamellae hold the chlorophyll in the most suitable position for photosynthesis. The stroma contains numerous starch granules and enzymes for the reduction of carbon dioxide.
Vacuoles
Vacuoles are a fluid-filled sac bounded by a single membrane. Within mature plant cells there is usually one large central vacuole. The single membrane around it is called the tonoplast. It contains a solution of mineral salts, sugars, amino acids, wastes and sometimes also pigments such as anthocyanins. Plant vacuoles serve a variety of functions.
- The sugars and amino acids may serve as a temporary food store.
- The anthocyanins are of various colours and so may colour petals to attract pollinating insects, or fruits to attract animals for dispersal.
- They act as temporary stores for organic wastes, such as tannins. They may accumulate in the vacuoles of leaf cells and are removed when the leaves fall.
- They occasionally contain hydrolytic enzymes and so perform functions similar to those of lysosomes.
- They support herbaceous plants, and herbaceous parts of woody plants by providing an osmotic system, which creates a turgor pressure.
Cell wall
A cell wall is a characteristic feature of plant cells. It consists of cellulose fibres embedded in an amorphous polysaccharide matrix. The matrix is usually composed of polysaccharides, e.g. pectin or lignin. The fibrils may be regular or irregular in arrangement. The main functions of the cell wall are as follows.
- To provide support in herbaceous plants. As water enters the cell osmotically the cell wall resists expansion and an internal pressure is created which provides turgidity for the plant.
- To give direct support to the cell and the plant as a whole by providing mechanical strength. The strength may be increased by the presence of lignin in the matrix between the cellulose fibres.
- To permit the movement of water through and along it and so contribute to the movement of water in the plant as a whole, in particular in the cortex of the root.
- In some cell walls the presence of cutin, suberin or lignin in the matrix makes the cell less permeable to substances. Lignin helps to keep the water within the xylem; cutin in the epidermis of leaves prevents water being lost from the plant and suberin in root endodermal cells prevents movement of water across them, thus concentrating its movement through special passage cells.
- The arrangement of the cellulose fibrils in the cell wall can determine the pattern of growth and hence the overall shape of a cell.
- Occasionally cell walls act as food reserves.
Starch grains
Starch grains occur within chloroplasts and the cytoplasm of plant cells. Starch may also be stored in specialized leucoplasts called amyloplasts. More information on starch grains can be found in the section ‘glycogen granules’ in this essay.
The next two pages, shows diagrams of a plant cell and an animal cell.