Cells PDF

4 UNIT 1 CELLS CELL STRUCTURE animal cell The basic building block of living organisms is the cell. The huma...

4 UNIT 1 CELLS CELL STRUCTURE animal cell The basic building block of living organisms is the cell. The human body is nucleus cell membrane composed of countless millions of cells. There are many different types of cell, which are specialised so they can carry out particular functions in the body. Despite these differences, certain features are the same in most cells. Figure 1.1 shows the structures present in a typical animal cell. The living material that makes up a cell is called cytoplasm. It has a texture rather like sloppy jelly, in other words, somewhere between a solid and a liquid. Unlike a jelly, it is not made of one substance; rather, it is a complex material made of many different structures called organelles. You cannot mitochondria see many of these structures under an ordinary light microscope. An electron microscope has a much higher magnification and can show the details of cytoplasm these parts of the cell (Figure 1.2). 10 µm 1 The largest organelle in the cell is the nucleus. Nearly all cells have a nucleus, (1 µm = mm) 1000 with a few exceptions, such as red blood cells. The nucleus controls the activities of the cell. It contains chromosomes (46 in human cells) which ▲▲ Figure 1.1 The structure of a generalised animal carry the genetic material or genes. Genes control the activities in the cell by cell, as seen through a light microscope. determining which proteins the cell can make (see below). One very important group of proteins found in cells is enzymes (see Chapter 3). Enzymes control chemical reactions that take place in the cytoplasm. All cells are surrounded by a cell membrane, mitochondria sometimes called the cell surface membrane to distinguish it from other membranes inside the cell. This is a thin layer like a ‘skin’ on the surface of the cell. It forms a boundary between the cytoplasm of the cell and the outside. However, it is not a complete barrier. Some chemicals can pass into the cell and others can pass out of it. We say that the membrane is partially permeable. In fact, as you will see, the membrane can actively control the movement of some substances. Because of this, it is also described as selectively permeable. There are many other membranes inside a cell. Throughout the cytoplasm, there is a network of membranes called the endoplasmic reticulum (ER). In places, the endoplasmic reticulum is covered with tiny granules called ribosomes. endoplasmic reticulum These are the organelles where proteins are made or synthesised (see ‘The stages of protein synthesis’ later in this chapter). The spaces between the membranes of the endoplasmic reticulum act as a transportation system, sending nucleus protein to the part of the cell where it is needed. ◀ Figure 1.2 The higher magnification of the electron microscope lets us see more detail in the cell. This photograph shows a small part of a liver cell, with the nucleus in the bottom left-hand corner. The colours are not real – they have been added later to show up the different structures. You can just make out the ribosomes on the endoplasmic reticulum.  UNIT 1 CELLS 5 One organelle that is found in the cytoplasm of nearly all living cells is the mitochondrion (plural mitochondria). In cells that need a lot of energy, such as muscle or nerve cells, there are many mitochondria. This gives us a clue as to their role. They perform some of the reactions of respiration, releasing energy that the cell can use (see Chapter 5). Most of the energy from respiration is released in the mitochondria. Figure 1.3 shows some cells from the lining of a human cheek. They were obtained by gently rubbing a cotton swab on the inside of a person’s mouth 10 mm and transferring the cells to a slide. They are stained with a dye to show them ▲▲ Figure 1.3 Cells from the inside lining of a more clearly. How many different organelles can you identify? human cheek. The cells are stained to show up the details of structures such as the CHROMOSOMES, GENES AND DNA nucleus. The chemical that is the basis of inheritance is deoxyribonucleic acid or DNA. DNA is usually found in the nucleus of a cell, in structures called chromosomes (Figure 1.4). A section of DNA that determines a particular feature is called a gene. Genes determine a person’s characteristics by instructing cells to produce particular proteins (see below). cell nucleus (contains chromosomes) DNA gene (length of DNA) body (made of cells) chromosome (contains DNA) ▲▲ Figure 1.4 Our genetic make-up. Each chromosome contains one DNA molecule. The DNA is folded and coiled so that it can be packed into a small space. The DNA is coiled around proteins called histones (Figure 1.5). supercoiled DNA/histones chromosome DNA coiled around histones histones DNA ▲▲ Figure 1.5 The structure of a chromosome.