Bio Ch 1 1.03 TB - Biology PDF

Nucleolus The nucleolus appears as a darkly stained, rounded structure in the nucleus (Figure 1.23). As mentioned earlier, one or more may be present, although one is most common. Its function is to make ribosomes using the information in its own DNA. It contains a core of DNA from one or more chro...

Nucleolus The nucleolus appears as a darkly stained, rounded structure in the nucleus (Figure 1.23). As mentioned earlier, one or more may be present, although one is most common. Its function is to make ribosomes using the information in its own DNA. It contains a core of DNA from one or more chromosomes which contain the genes that code for ribosomal RNA (rRNA), the form of RNA used in the manufacture of ribosomes. It also contains genes for making tRNA. Around the core are less dense regions where the ribosomal subunits are assembled, combining the rRNA with ribosomal proteins imported from the cytoplasm. The more ribosomes a cell makes, the larger its nucleolus. The different parts of the nucleolus only come together during the manufacture of ribosomes. They separate when, as during nuclear division, ribosome synthesis ceases. The nucleolus as a structure then disappears Endoplasmic reticulum When cells were first seen with the electron microscope, biologists were amazed to see so much detailed structure. The existence of much of this had not been suspected. This was particularly true of the **endoplasmic reticulum** (**ER**) (Figures 1.23, 1.24 and 1.28). The membranes of the ER form flattened compartments called sacs or cisternae. Processes can take place inside the cisternae separated from the cytoplasm. Molecules, particularly proteins, can be transported through the ER separate from the rest of the cytoplasm. The ER is continuous with the outer membrane of the nuclear envelope (Figures 1.19 and 1.21). Rough endoplasmic reticulum There are two types of ER: rough ER (RER) and smooth ER (SER). RER is so called because it is covered with many tiny organelles called ribosomes (described later). These are just visible as black dots\ in Figures 1.23 and 1.24. Ribosomes are the sites of protein synthesis (Chapter 6). They can be found free in the cytoplasm as well as on the RER. Smooth endoplasmic reticulum SER has a smooth appearance because it lacks ribosomes. It has a completely different function to RER. It makes lipids and steroids, such as cholesterol and the reproductive hormones oestrogen and testosterone. SER is also a major storage site for calcium ions. This explains why it is abundant in muscle cells, where calcium ions are involved in muscle contraction (Chapter 15, Section 15.3, Muscle contraction). In the liver, SER is involved in drug metabolism. Ribosomes **Ribosomes** are very small and are not visible with a light microscope. At very high magnifications using an electron microscope they can be seen to consist\ of two subunits: a large and a small subunit. The sizes of structures this small are often quoted in S units (Svedberg units). S units are a measure of how rapidly substances sediment in a high speed centrifuge (an ultracentrifuge). The faster they sediment, the higher the S number. Eukaryotic ribosomes are 80S ribosomes. The ribosomes of prokaryotes are 70S ribosomes, so are slightly smaller. Mitochondria and chloroplasts contain 70S ribosomes, revealing their prokaryotic origins (see the sections on mitochondria and chloroplasts). Ribosomes are made of roughly equal amounts by mass of ribosomal RNA (rRNA) and protein. Their three-dimensional structure has now been worked out (Figure 1.25). Ribosomes allow all the interacting molecules involved in protein synthesis, such as mRNA, tRNA, amino acids and regulatory proteins, to gather together in one place (Chapter 6, Section 6.5, Protein synthesis) Cell surface membrane The cell surface membrane is extremely thin (about\ 7 nm). However, at very high magnifications it can be seen to have three layers -- two dark (heavily stained) layers surrounding a narrow, pale interior (Figure 1.22). The membrane is partially permeable and controls exchange between the cell and its environment The nuclear envelope The nucleus is surrounded by two membranes, forming the **nuclear envelope**. The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum (Figures 1.19 and 1.21). The nuclear envelope has many small pores called **nuclear pores**. These allow and control exchange between the nucleus and the cytoplasm. Examples of substances leaving the nucleus through the pores are messenger RNA (mRNA), transfer RNA (tRNA) and ribosomes for protein synthesis. Examples of substances entering through the nuclear pores are proteins (to help make ribosomes), nucleotides, ATP (adenosine triphosphate) and some hormones such as thyroid hormone T3. Cell walls and plasmodesmata With a light microscope, individual plant cells are more easily seen than animal cells. This is because they are usually larger and, unlike animal cells, are surrounded by a **cell wall**. Note that the cell wall is an extra structure which is outside the cell surface membrane. The wall is relatively rigid because it contains fibres\ of cellulose, a polysaccharide which strengthens the wall. The cell wall gives the cell a definite shape. It prevents the cell from bursting when water enters by osmosis, allowing large pressures to develop inside the cell (Chapter 4, Section 4.5, Movement of substances across membranes). Cell walls may be reinforced with extra cellulose or with a hard material called lignin for extra strength (Chapter 7). Cell walls are freely permeable, allowing free movement of molecules and ions through to the cell surface membrane. Plant cells are linked to neighbouring cells by means of pores containing fine strands of cytoplasm.\ These structures are called **plasmodesmata** (singular: **plasmodesma**). They are lined with the cell surface membrane. Movement through the pores is thought to be controlled by the structure of the pores.