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Chapter 3 - Cell CHAPTER 3 - Cell 3.1 CELL STRUCTURE The cell is the fundamental unit of life. All organisms, whatever their type or size, are composed of cells. The modern theory of cellular organization states that: 1. All living organisms are composed of cells 2. All new cells are derived from other cells. 3. Cells contain the hereditary material of an organism which is passed from parent to daughter cells. 4. All metabolic processes take place within cells. 3.2 Cell membranes (plasmalemma) Both the plant and the animal cell have a cell membrane surrounding the cytoplasm. The membrane consists of an inner and an outer layer, with two layers of fat molecules, known as phospholipids, between them. There is no living cell that is not surrounded by a cell membrane, and most cells have membranes inside them too. The membrane around the outside of a cell is called the cell surface membrane. Fig. 3.1, 3.2, 3.3 shows the structure of a cell surface membrane. This representation of its structure is called the fluid mosaic model. Structure of the Cell membranes A cell membrane is formed from a phospholipids bilayer. Interspersed amongst the phospholipids molecules are cholesterol molecules. Cholesterol molecules help to make the membrane more fluid at low temperatures and less fluid at high temperatures. Floating amongst the phospholipids and cholesterol molecules are many globular protein molecules, many of which span from one side to the other. These proteins tend to be arranged with hydrophilic parts of their chains (that is parts containing amino acids with hydrophilic R groups) on the outer surfaces of the membrane, and hydrophobic parts within the membrane amongst the hydrophobic tails of the lipids. Many of these proteins act as pores or transporters, allowing substances to pass from one side of the membrane to the other. Most of the protein molecules and many of the lipid molecules have short carbohydrate chains attached to them. These molecules are called glycoproteins and glycolipids. The carbohydrate chains are all on the outer surface of the membrane. Glycolipids and glycoproteins help to stabilise membrane structure by forming hydrogen bonds with water molecules outside the membrane. Why this structure is called a fluid mosaic? All of these molecules are in constant motion, vibrating and bumping into each other and changing place within layer. So the membrane behaves rather like a fluid-although it does not flow away into its surroundings! The mosaic part of the name refers to the mosaic pattern protein molecules which you would see if you looked down on the surface on the membrane. Membranes inside cells have a very similar structure to cell surface membranes. However, the relative proportions of the different kinds of molecules can vary considerable. For example, the membranes around mitochondria have hardly any carbohydrate attached to their lipids and proteins. Function The cell membrane is part of the living cell and is selectively permeable allowing only certain substances to enter and leave the cell. This prevents the cell from poisoning itself since harmful substances are prevented from entering the cell. 36 Chapter 3 - Cell Fig 3.1 Fig 3.2 Fig 3.3 37 Chapter 3 - Cell 3.3 Cytoplasm The term cytoplasm is often used to refer the background material inside cell, within which all the organelle such as mitochondria and ribosome’s are found. Cytoplasm is mostly water, with a variety of other molecules dissolved or suspend in it. Many of these are proteins, especially enzymes. These include simple ions such as sodium, phosphates and chlorides, organic molecules such as amino acids, ATP and nucleotides, and storage material such as oil droplets. Many important biochemical processes, including glycolysis, occur within the cytoplasm. It is not static but capable of mass flow, which is called cytoplasmic streaming. 3.4 Ribosomes Ribosomes appear as small black dots. They are usually in clusters called polyribosomes. Some ribosomes are found free in the cytoplasm, while others are attached to the outer surfaces of the membranes of the rough endoplasmic reticulum. Ribosomes are also found inside mitochondria and chloroplasts. The function of ribosomes is to provide a platform on which protein synthesis takes place and to help with several stages of this process. 3.5 Endoplasmic reticulum ‘ Endoplasmic’ means ‘inside the cytoplasm’, and ‘reticulum’ means ‘network’. The endoplasmic reticulum (often abbreviated to ER) is a network of membranes running through the cytoplasm of every cell. The membranes enclose spaces called cisternae (Fig.3.7), which form an interconnecting channel throughout the cytoplasm. Some parts of the endoplasmic reticulum have ribosomes attached to the cytoplasmic side of the membranes. This is called rough endoplasmic reticulum (or RER). These ribosomes synthesise proteins, which are to be secreted from the cell, to form lysosomes or to become part of the cell surface membrane. As these proteins are being made, some of them are passed through pores in the endoplasmic reticulum membrane into the cisternae, while others become part of the membrane itself, The cisternae then break into small vesicles and carry these proteins to the Golgi apparatus (also sometimes called the Golgi body) (Fig.3.7) Other parts of the endoplasmic reticulum have no ribosomes attached. These are called smooth endoplasmic reticulum (or SER). The cisternae of SER tend to be more tubular, in contrast to the flattened sacs of RER. SER has many different functions, including the synthesis of cholesterol and of steroid hormones such as testosterone, and the breaking down of toxins such as drugs. The functions of the ER may thus be summarized as: 1. Providing a large surface area for chemical 2. Providing a pathway for the transport of materials through the cell 3. Producing proteins, especially enzymes (rough ER) 4. Producing lipids and steroids (smooth ER) 5. Collecting and sorting synthesized material. 6. Providing a structural skeleton to maintain cellular shape (e.g. the smooth ER of a rod cell from the retina of the eye). 38 Chapter 3 - Cell Fig 3.7 3.6 Golgi apparatus The Golgi apparatus (or Golgi body) is a stack of curved cisternae with several smaller vesicles entering and leaving it (Fig. 3.8 and 3.9). Vesicles containing newly synthesized proteins break off from the rough endoplasmic reticulum, and travel towards the Golgi apparatus where they fuse with its convex face. Here the proteins are ‘finished off’ and packaged before being exported from the cell. They may, for example, have carbohydrates added to them to form glycoproteins. The proteins are concentrated within the Golgi cisternae; in pancreatic cells secreting insulin, for example, the insulin is concentrated so much in the Golgi that it crystallizes. When the protein is ready, small vesicles break away from the concave face of the Golgi apparatus and move towards the surface of the cell. They fuse with the cell surface membrane and release their contents to the outside. The membranes of the vesicles, which were originally part of the rough endoplasmic reticulum membrane, become incorporated in the cell surface membrane. Not all the proteins dealt with in the Golgi are destined for export. In animal cells some of the vesicles released from the convex face become lysosomes. Plant cells do not have lysosomes. In plant cells, some of the Golgi vesicles take proteins to the large vacuole where they are released into the cell sap. In plant cells the Golgi is also involved in processing carbohydrates. The polysaccharides, which will make up the matrix (background material) of the cell wall are first made in the endoplasmic reticulum, then assembled in the Golgi apparatus, then carried to the cell surface inside secretory vesicles. The whole process, from endoplasmic reticulum to cell surface, can take as little as 20 minutes. Fig 3.8 39 Chapter 3 - Cell Fig3.9 40 Chapter 3 - Cell 3.7 Lysosomes Lysosomes are tiny vesicles found in most animal cells but not in plant cells. They are surrounded by a single membrane. They have no structure inside them, but simply contain a variety of hydrolytic (digestive) enzymes in solution (Fig 3.10). Lysosomes are formed as buds which break away from the Golgi apparatus. Their main function is to fuse with other vesicles in the cell which contain something which needs to be digested, for example a bacterium which has been brought into the cell by phagocytosis or a worn-out mitochondrion which needs to be destroyed. The enzymes in the lysosome then digest the contents of this vesicle, producing soluble substances which can be absorbed into the cytoplasm. In plant cells these functions are carried out by enzymes in the vacuole. Fig 3.10 3.8 Microbodies (peroxisomes) Microbodies are small spherical membrane-bound bodies. Apart from being slightly granular, they have no internal structure. They contain a number of metabolically important enzymes, in particular the enzyme catalase, which catalyses the breakdown of hydrogen peroxide. Hence these Microbodies are sometimes called peroxisomes. Hydrogen peroxide is a potentially toxic by-product of many biochemical reactions within organisms. Peroxisomes containing catalase are therefore particularly numerous in actively metabolizing cells like those of the liver. 3.9 Storage 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 the vacuoles of plant cells. It may also occur in insoluble form, as grains or granules, within cells or organelles. Starch grains occur within chloroplasts and the cytoplasm of plant cells. Starch may also be stored in specialized leucoplasts called amyloplasts. 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. 41 Chapter 3 - Cell 3.13 Mitochondria Mitochondria are quite large organelles which can be seen with a good light microscope. To see detail in their structure, an electron microscope is needed; Mitochondria are very variable in size and shape. It has two membranes, separated by an intermembrane space. The outer membrane is relatively smooth, but the inner membrane is folded to form projections called cristae. Between the cristae is the matrix, which fills the rest of the space inside the mitochondrion. The matrix also contains ribosomes and DNA, which are used to make some of the mitochondrion’s own proteins. Mitochondria are the site of the aerobic stages of respiration, Krebs cycle and oxidative phosphorylation. Fig 3.15 3.14 Nucleus The nucleus is the part of the cell, which contains DNA. Within the nucleus (Fig 3.16 and 3.17) there is often a particularly darkly staining region called the nucleolus. Here, ribosomal RNA is made by transcription from DNA. The small and large subunits of ribosomes are assembled in the nucleolus. They leave the nucleus through pores (Fig 3.16) before being assembled into complete ribosomes in the cytoplasm. The nucleus is surrounded by a double membrane. The content of the nucleus is known as the nucleoplasm. It contains two important structures: the nucleolus and the chromosomes. The nucleolus consists mainly of nucleic acids. The chromosomes are the carriers of hereditary characteristics which are carried from one generation to the next. Before the cell divides these chromosomes are visible as a network of threads known as the chromatin network. The functions of a nucleus are: 1. To contain the genetic material of a cell in the form of chromosomes. 2. To act as a control centre for the activities of a cell 3. To carry the instructions for the synthesis of proteins in the nuclear DNA. 4. To be involved in the production of ribosomes and RNA. 5. In cell division. 42 Chapter 3 - Cell Fig 3.16 Fig 3.17 3.15 Nuclear envelope The nuclear envelope is made up of two membranes, with a narrow space between them. The outer of these two membranes links up with the endoplasmic 43 Chapter 3 - Cell reticulum. Indeed, in some cells it is exactly like a piece of rough endoplasmic reticulum complete with attached ribosomes. These two membranes have many gaps in them which are called nuclear pores. The gaps are relatively large much bigger than the protein pores in the cell surface membrane. They are large enough to allow partially assembled ribosomes from the nucleolus to pass through, as well as messenger RNA on its way out of the nucleus and enzymes such as DNA polymerase on their way in. 3.16 Cell wall The cell wall only occurs in plant cells. It lies outside the cell membrane. It consists of insoluble cellulose and has three layers: the primary cell wall closest to the cytoplasm, the middle lamella, and an outside secondary cell wall. Pits occur in the cell wall which allows the exchange of substances with adjacent cells through fine protoplasmic threads called plasmodesmata. Although small molecules and ions can pass easily through cell walls, plant cells need a faster and more reliable way of allowing larger substances to move between adjacent cells. This is done through plasmodesmata. Function The cell wall protects the contents of the plant cell. It give rigidity to the plant cell. 44 Chapter 3 - Cell 3.18 Vacuole A vacuole is a membrane-bound organelle that usually contains liquid, All cells have vacuoles, but plant cells differ from animal cells in that their vacuoles are very large, permanent, and usually occupy a position fairly near the centre of the cell, In a mature plant cell, up to 90% of its volume may be taken up by the vacuole, The membrane surrounding a plant cell vacuole is often known as the tonoplast. The fluid inside the vacuole is known as cell sap. Plant cell vacuoles contain many different substances in solution in water; these include sugars, storage proteins, pigments (coloured substances) and enzymes. Functions the colours of some flower petals are caused by pigments held inside vacuoles in their cells, Some plants store sucrose in their vacuoles, either temporarily or for much longer periods; the sugar which we obtain from sugar beet, sugar cane and many fruits comes from vacuoles, In many plants the vacuoles perform the same functions as lysosomes in animal cells and contain digestive enzymes. In unicellular animals, example, Amoeba, the vacuoles are known as contractile vacuoles and function as excretory organs. Food ingested by Amoeba forms a hollow called a phagosome or food vacuole. Vacuoles thus store water and organic and inorganic substances. They maintain the rigidity of the cell and give mechanical support by exerting an outward pressure on the cell wall of the plant cell (turgor pressure). They assist in intracellular support. T 3.19 Plastids Plastids are found only in plant cells, not in animal cells. Plastids are organelles surrounded by two membranes. In this they are similar to mitochondria. Another similarity with mitochondria is that they contain DNA and ribosomes, which 45 Chapter 3 - Cell suggest that, like mitochondria, they have probably evolved from what, were originally symbiotic prokaryotes. Three types can be distinguished – The chromoplast such as carotenoids which give colour to carrots and ripe fruits; the leucoplasts which are organelles for the storage of starch, oil and protein granules; The chloroplast which are essential for photosynthesis. Chloroplast contains green pigments known as chlorophyll. Fig 3.21 shows structure of a chloroplast. They are large organelles, surrounded by a double membrane. Inside the chloroplast is a third system of membranes, forming many tiny flattened sacs called thylakoids. In places these thylakoids are stacked on top of each other to form grana. Grana are linked by extensions of some of the thylakoids, forming long membrane-bound tubes called intergranal lamellae. These entire membranes lie in a matrix called the stroma. The thylakoid membranes contain chlorophyll molecules, which give the whole chloroplast – and the whole leaf – its green colour. Also in these membranes are the molecules involved in the light-dependent reactions of photosynthesis, including photophosphorylation. These reactions involved the ejection of electrons from some of the chlorophyll molecules when light hits them; the electrons are then passed along a chain of carrier molecules in the thylakoid membrane. This process provides energy for synthesising ATP. The stroma contains the enzymes required for the Calvin cycle, in which carbohydrates are made from carbon dioxide and water. The most abundant of these enzymes is ribulose bisphosphate carboxylase, usually known as Rubisco. It is not only the most abundant enzyme in the world, but actually the most abundant protein. Up to one-quarter of the total protein in a leaf is Rubisco. In1993, it was estimated that there were 10kg of Rubisco in the world for every person on Earth. If the plant makes more carbohydrate in photosynthesis than it needs, then some may be temporarily stored as starch inside the chloroplasts. The starch forms granules, which may take, up a large amount of space in the stroma. The stroma also contains lipid droplets, DNA and ribosomes. Fig 3.21 46 Chapter 3 - Cell Centrioles In animal cells there is a specialised patch of cytoplasm called the centrosome. This is found near the nucleus. Inside this ‘patch’ are two cylindrical bodies called centrioles. Function: Centrioles play an important role in cell division. PROKARYOTIC CELLS 3.20 Structure of a prokaryotic cell Prokaryotic cells (pro-‘before’, karyo – ‘nucleus’) were probably the first forms of life on earth. Their hereditary material, DNA, is not enclosed within a nuclear membrane. This absence of a true nucleus only occurs in two groups, the bacteria and blue-green bacteria. There is no membrane-bound organelle within a prokaryotic cell, the structure of which is shown in Fig 3.22 Fig 3.22 Fig 3.22 shows the structure of a bacterium. Like all bacteria it is single-celled and has no nucleus. Such cells are called prokaryotic cells, all bacteria, including bluegreen, are prokaryotes. Plant, animal and fungal cells, which do have nuclei, are eukaryotic cells. Prokaryotic cells are usually smaller than eukaryotic cells, they are similar in size to a mitochondrion or a chloroplast. Indeed, mitochondria and chloroplasts probably were prokaryotic cells, which came to live inside the larger eukaryotic ones many millions of years ago. All prokaryotic cells are surrounded by a cell wall, which gives support and protection to the cell and is made of a variety of polysaccharides. These polysaccharides, however, are very different from those in plant cell walls; prokaryote cell walls do not contain cellulose, for example. Bacterial cell walls contain large amounts of substances known as peptidoglycans, which, as their name suggests, are made up of molecules in which peptides and sugars are combined. These form long, branched, cross-linked chains and make the wall very 47 Chapter 3 - Cell strong. Cell walls are very important to bacteria. They stop them from bursting when they absorb water and help to protect them from invasion by viruses. If you can damage the cell wall you can kill the bacterium. The antibiotic penicillin inhibits the enzymes which help to form the cross-links between the peptidoglycans. Many bacteria have a thick layer of jelly-like material surrounding them called a capsule. The capsule is made of polysaccharides which absorb water to form a slimy material. (Bacterial capsules make up a high proportion of the plaque which can collect on you teeth.) The capsule protects the bacterium from attack by viruses, and from antibodies. For example, the bacterium pneumococcus exits in two forms, one with a capsule and one without. The one with capsule is a dangerous pathogenic (disease-causing) organism, able to infect a person’s lungs and cause severe pneumonia. The one without a capsule is easily destroyed by the immune system, and does not cause disease at all. Beneath the cell wall is a cell surface membrane. This has a very similar structure to that of eukaryotic cells, being made up of a phospholipid bilayer in which protein molecules float. (Some bacteria, known as Gram-negative bacteria, have another membrane) In photosynthetic bacteria, there may be extensive membrane systems inside the cell, sometimes closely associated with the cell surface membrane. These membrane systems hold the molecules which are involved in capturing light energy. The cytoplasm often contains large numbers of ribosomes. Like the slightly larger ribosomes of eukaryotic cells, these are made of ribosomal RNA and protein and are the sites of protein synthesis. The DNA of bacteria is a single, large, circular molecule. This is unlike the DNA of eukaryotes, which is linear rather than circular and is usually made up of several molecules each of which forms a chromosome. The chromosomes of eukaryotic cells are complex structures involving proteins called histones as well as DNA. Prokaryotic DNA does not form chromosomes, and although it does have proteins associated with it, these are not histones. There is no nuclear envelope in a prokaryotic cell so the DNA lies free in the cytoplasm. Prokaryotic cells do not have a cytoskeleton that is they do not have microtubules or intermediate filaments supporting the structure of the cell. Some prokaryotic cells have a flagellum, which is used for movement. These flagella have no Similarity in structure with those of eukaryotic cells and are unique to prokaryotes both in their structure and the way they work. While eukaryotic flagella throw waves along themselves, bacterial flagella actually rotate like the propellor of a boat. At the base of the flagellum is a true motor with a rotating bearing-the smallest motor known. Table 2-Comparison of prokaryotic and eukaryotic cells (Fig 3.23) Prokaryotic cells Eukaryotic cells No distinct nucleus; only diffuse area(s) of nucleoplasm with no nuclear membrane No chromosomes – circular strands of DNA No membrane – bound organelles such as chloroplasts and mitochondria Ribosomes are smaller Flagella (if present) lack internal 9+2 fibril arrangement A distinct, membrane –bound nucleus Chromosomes present on which DNA is located Chloroplasts and mitochondria may be present Ribosomes are larger Flagella have 9+2 internal fibril arrangement 48 Chapter 3 - Cell No mitosis or meiosis occurs Mitosis and /or meiosis occurs Fig 3.23 3.21 Structure of the eukaryotic cell Eukaryotic cells (eu – ‘true’, Karyo – ‘nucleus’) probably arose a little over 1000 million years ago, nearly 2500 million years after their prokaryotic ancestors. The development of eukaryotic cells from prokaryotic ones involved considerable changes, as can be seen from table 2. The essential change was the development of membrane-bound organelles, such as mitochondria and chloroplast, within the outer plasma membrane of the cell. The presence of membrane-bound organelles confers four advantages: 1. Many metabolic processes involve enzymes being embedded in a membrane. As cells become larger, the proportion of membrane area to cell volume is reduced. This proportion is increased by the presence of organelle membranes. 2. Containing enzymes for a particular metabolic pathway within organelles means that the products of one reaction will always be in close proximity to the next enzyme in the sequence. The rate of metabolic reactions will thereby be increased. 3. The rate of any metabolic pathway inside an organelle can be controlled by regulating the rate at which the membrane surrounding the organelle allows the first reactant to enter. 4. potentially harmful reactants and / or enzymes can be isolated inside an organelle so they won’t damage the rest of the cell Fig 3.24 49 Chapter 3 - Cell Fig 3.25 Difference between plant and animal cells The major difference between plant and animal cells are given in Table 3 Similarities between plant and animal cells Both have a cell membrane surrounding the cell Both have cytoplasm Both contain a nucleus Both contain mitochondria Both contain endoplasmic reticulum Both contain ribosomes Table 3 - Difference between plant and animal cells Plant cells Animal cells Have a cellulose cell wall present (in addition to the cell membrane) Plastids, e.g. chloroplasts and leucoplasts, present in large numbers Mature cells normally have a large single, central vacuole filled with cell sap Tonoplast present around vacuole Cytoplasm normally confined to a thin layer at the edge of the cell Nucleus at edge of the cell Cell wall absent – only a membrane surrounds the cell Plastids absent Vacuoles, e.g. contractile vacuoles, if present, are small and scattered throughout the cell Tonoplast absent Cytoplasm present throughout the cell Nucleus anywhere in the cell but often central 50 Chapter 3 - Cell Lysosomes not normally present Centrioles absent in higher plants Often regular in shape Starch grains used for storage Only some cells are capable of division Few secretion are produced Lysosomes almost always present Centrioles present Often irregular in shape Glycogen granules used for storage Almost all cell are capable of division A wide variety of secretions are produced 51