Basrah University Pharmacy College. Human Biology. Cells: The Basic Units of Life Dr. Rawaa Salim Hameed

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1 Basrah University Pharmacy College Human Biology Cells: The Basic Units of Life

2 Cells: The Basic Units of Life All organisms are composed of one (e.g. bacteria) or more cells (e.g. human body contains about a trillion cells). Cells are the basic structural and functional unit of life. The study of cells is called cytology. Cell theory Cell theory consists of three principles: All living things are made up of cells. Cells are the structural and functional unit of an organism. All cells come from pre-existing cells through cell division. Types of cells All living cells can be divided into two groups: 1- Prokaryotic (also spelled procaryotic ): They have their hereditary material in the form of a single long strand of DNA, no separate nucleus, fewer organelles in the cytoplasm and a cell wall like plant e.g. bacteria. Figure 1: Main parts of prokaryotic cell 2- Eukaryotic (also spelled eucaryotic): They have the hereditary material in the form of chromosomes, a nucleus surrounded by a nuclear membrane and many organelles in the cytoplasm e.g. animals, plants and fungi. 1

3 Plants and animals All animals and plants are made of cells. Animal cells and plant cells have features in common, such as a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes. Plant cells differ from animals in the following:- Cell wall: The defensive walls of the plant, they are made of polysaccharide called cellulose. Animal cells do not have a cell wall. Vacuole: Animal cells have one or more small vacuoles whereas plant cells have one large central vacuole that can take up to 90% of cell volume. In plant cells, the function of vacuoles is to store water and maintain turgidity of the cell. Vacuoles in animal cells store water, ions and waste. Chloroplasts: They are present in plant cells; these are packages of green pigment chlorophyll that is used to capture the light energy in the process of photosynthesis. Plants are able to take in simple chemicals (water and salts from the soil and carbon dioxide from the air) and use them to synthesize complex molecules. Plants are therefore autotrophs (they can produce their own food from the substances available in their surroundings). Animals take in complex chemicals and break them down into simpler substances that can then be used by the body, this process called digestion animals are therefore heterotrophs. Figure 2: Plant and animal cells Cell shapes and sizes There are various shapes of cells. For example, it can be a cube like cell, cubical cells which is seen in cubical epithelium tissue, another shape is long column like, the goblet cell. Nerve cells have long projections that help them carry electrical messages to other 2

4 cells. Smooth muscle cells have spindle shape and skeletal muscle cells have rectangular shape. As the shapes are variable, sizes are also variable. Some large cells e.g. an egg such as frogspawn can be seen with the unaided eye. Most cells though are much smaller and so we need a microscope to be able to examine them. Light microscope allows us to see objects as small as 200 nanometres (nm). In order to see the parts of cell that are smaller than this we need to use an electron microscopes. This type of microscope is big, expensive and uses beams of electrons instead of light and it allows us to observe objects that are only 0.2 nm. Figure 3: cell shapes Cells structures A cell examined under the light microscope will reveal an outer cell membrane enclosing an inner fluid, jelly like area called the cytoplasm. Within the cytoplasm are many parts that do different functions these parts are called organelles (small organs). Cytoplasm is about 70% water in which salts are dissolved to form a solution. There are also suspended within it large molecules, such as proteins which cannot dissolve in the water. A cell consists of three main parts: nucleus, cell membrane and cytoplasm. The nucleus The nucleus is a large organelle that may or may not be centrally within the cytoplasm. It is enveloped in a double membrane that has nuclear pores to permit the two-way traffic of large molecules. When a special type of stain (hematoxylin) is put on to a cell the nucleus soaks up the color. The areas within the nucleus that become colored are described as chromatin. When the cell divides into two, chromatin coils up into rod like chromosomes. Chemically chromatin (and therefore chromosome) is made up of large 3

5 protein molecules and DNA (deoxyribonucleic acid). It is DNA together with RNA (ribonucleic acid) that controls the production of proteins by cells. The nucleic acids are the inheritance factors of cells. Most nuclei contain at least one nucleolus (plural, nucleoli), this is made up of RNA and aids in the production of structures called ribosomes. Figure 4: The nucleus Endoplasmic reticulum (ER) The endoplasmic reticulum is a system of double-membraned tubular canals running throughout the cytoplasm. Some of these membranes are dotted with extremely small granular particles called ribosomes. This membrane with ribosomes is described as rough endoplasmic reticulum and is the site of protein synthesis. The membranes without ribosomes are described as smooth endoplasmic reticulum, it is suggested that here fatty substances are synthesized as are some hormones. It is may also be true that dangerous chemicals are destroyed (detoxified) by enzymes located on these membranes. Figure 5: Endoplasmic reticulum Golgi bodies (or Golgi apparatus) Golgi bodies were first discovered by the Italian scientist Camillo Golgi, who noted that these membranous sacs increased in size and filled up when a cell produced secretions. These packages of chemicals then snip off from the main Golgi apparatus and migrate to 4

6 the outer cell membrane where they discharge to the outside. The packages are called vacuoles or if they are extremely small, vesicles. Figure 6: Golgi apparatus Lysosomes Lysosomes are vacuoles that probably snip-off from Golgi apparatus. They contain enzymes (lysozymes) that break down the cell material itself by a process of selfdigestion or autolysis. They are therefore a danger to the life of the cell if released. The plasma membrane (cell membrane) The plasma membrane encloses all the cell components; it is a double structure like the endoplasmic reticulum. It regulates what enters and exits the cell. The currently accepted model for the structure of the plasma membrane, called the fluid mosaic model. According to the fluid mosaic model, the plasma membrane is a mosaic of componentsprimarily, phospholipids, cholesterol, and proteins-that move freely and fluidly in the plane of the membrane. The principal components of the plasma membrane are:- 1- Phospholipid bilayer: (two layers of phospholipids): Phospholipids are lipids with a phosphate group attached to them. The phospholipids have one head and two tails. The head is polar and hydrophilic, or water-loving. The tails are nonpolar and hydrophobic, or water-fearing. 2- Proteins: There are many different types of proteins associated with the phospholipid bilayer. Some lie in just one of the phospholipids layers peripheral (extrinsic proteins) and some span both layers integral (intrinsic proteins). 5

7 3- Carbohydrate: groups that are attached to some of the lipids and proteins. Figure 7: The fluid mosaic model Mitochondria A mitochondrion is a fluid-filled tubular structure surrounded by double membrane. The inner membrane is folded into projections called cristae, and it is on these cristae that the energy-producing enzymes are located. Mitochondria are frequently described as the powerhouses of cell because here cellular respiration takes place and energy stored in the molecule ATP is produced. It is called cellular respiration because the cell take up the oxygen that we respire (breath in) and the glucose produced by digestion of the food that we eat and convert it to carbon dioxide and water in a chemical reaction that gives off energy. It is summarized by this equation:- Glucose + oxygen carbon dioxide + water Figure 8: Mitochondrion 6

8 The movement-related parts (microfilaments, centrioles and cilia) Within the cytoplasm there would still be ions and organic molecules, plus a network of protein fibres that help maintain the shape of the cell and secure some organelles in specific positions. This network of protein fibres is known as the cytoskeleton. The cytoskeleton is the framework of the cell which forms the structural supporting component. There are three types of fibres within the cytoskeleton: microfilaments, intermediate filaments, and microtubules. Microfilaments Microfilaments are the thinnest part of the cytoskeleton, are used to give shape to the cell and support all of its internal parts. They made of the proteins actin and myosin. Actin works with myosin to produce muscle movements and cell division. Figure 1: Arrangement of actin and myosin filaments Microtubules Microtubules are small hollow tubes made of proteins called tubulin. Microtubules are the largest element of the cytoskeleton. They are the structural elements of centrioles, cilia, and flagella, they help the cell resist compression, provide a track along which vesicles move through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. Centrioles Centrioles are found in animal cells but not in plant cells. They are extremely small tubules (microtubules) located in pairs near the nucleus. In the cross section, centrioles have a bundles of microtubules arranged in threes in a circle nine pairs (9+0 pattern). Centrioles lie at the right angles to one another and as a cell begins to undergo division one of the 7

9 pair moves around to the opposite side of the nucleus from the other centriole. Fibres spread out from each centriole towards the centre of the cell and in some way act to move the chromosomes in cell division (mitosis or meiosis). Figure 2: The 9+0 pattern of microtubules in the centrioles Cilia and Flagella Cilia are short, hair-like projections from cell membranes. They are lining the cells of the respiratory tract that trap particulate matter and prevents them getting into our lungs. Flagella (singular = flagellum) are similar to cilia but they are longer, hair-like structures that extend from the plasma membrane and are used to move an entire cell. Both cilia and flagella have a bundles of microtubules arranged in nine pairs and an extra central pair of tubules (9+2 pattern). Figure 3: The 9+2 pattern of microtubules in a cilia or flagella 8

10 Transport between cells and their surroundings Cells are grouped together to form tissues, where the membrane of one cell touches that of another they tend to join up. They glued together by one means or another. Cell junctions are of different sorts:- 1- Tight junctions: proteins from each membrane fuse and thereby seal in the cell contents. 2- Desmosomes: cell to cell links by means of thin filaments. 3- Gap junction: the cells are joined by means of protein channels between the membranes across which substances such as salts, sugars, amino acids, vitamins and water may be transported. Figure 3: The Cell junction Cell membranes do not allow the molecules to pass through them. They are often described as semi-permeable but a better term is selectively permeable. This means that some small and large molecules can pass through the pores whilst other large and small molecules cannot. Cell membranes have very important job because they must prevent potential poisons from entering the cell but allow foods, water and oxygen to enter. The methods of passing through cell membranes can be summarised as follows:- 1- Endocytosis 2- Exocytosis 3- Diffusion 4- Osmosis 5- Active transport 6- Facilitated transport 1- Endocytosis: Large molecules or other materials can enter the cell by this method. Liquid can be packaged into a vacuole or vesicle which is then taken into the cell. This cell drinking is called pinocytosis. Solids can also be taken up in this way. Cell eating is called 9

11 phagocytosis. It is method used by white blood cells when they eat up harmful microbes that may invade our bodies. 2- Exocytosis: There are many substances that must exit from a cell. For example, the secretions produced by Golgi apparatus need to leave the cell if they are going to do their job. Exocytosis is the reverse of Endocytosis. The secretions or enzymes are packaged into vacuoles and then moved towards the cell membrane where they are discharged. Figure 4: Endocytosis and Exocytosis 3- Diffusion: The molecules are move from an area of high concentration to an area of low concentration. This movement from high concentration to low concentration is called movement down a diffusion gradient or a concentration gradient. 4- Osmosis: This is the diffusion of water across a selectively permeable membrane from an area of high concentration of water to an area of low concentration. Only the water molecules are able to cross the membrane and any large molecules being held back. The effects of osmosis depend upon the difference in the concentration between solutions on other side of the membrane. Solutions have an osmotic effect on our tissues that is dependent upon the strength of the solution compared with that of the body fluids: Isotonic solutions: are those equal strength to our body fluids. They therefore cause neither shrinking nor swelling of cells and tissue. Hypotonic solutions: contain less dissolved material and more water than the body fluids. This water passes into the cells by osmosis and causes them to swell up and eventually burst. Hypertonic solutions: contain more dissolved materials and less water than the body fluids. Water therefore leaves the cell by osmosis and causes them to shrink. 10

12 Figure 5: Isotonic, Hypotonic and Hypertonic solutions Active transport: use energy to pump materials into the cell against the concentration gradient. For example, the thyroid gland cells collect iodine from the blood. Facilitated transport: this is a sort of faster diffusion. It takes place along special protein pathways in the cell membrane across which chemicals can pass more quickly. These protein carriers take glucose and amino acids into cell quicker than would be expected for normal diffusion processes. Figure 6: The Facilitated transport Cell division In cell division, the cell that is dividing is called the "parent" cell. The parent cell divides into two or more "daughter" cells. The process then repeats in what is called the cell cycle. Depending on the type of cell, there are two ways cells divide-mitosis and meiosis. Each of these methods of cell division has special characteristics. One of the key differences in 11

13 mitosis is a single cell divides into two cells that are replicas of each other and have the same number of chromosomes. This type of cell division is good for basic growth, repair, and maintenance. In meiosis a cell divides into two cells that have half the number of chromosomes. Reducing the number of chromosomes by half is important for sexual reproduction and provides for genetic diversity. For convenience, the events can be separated into phases (interphase, prophase, metaphase, anaphase and telophase). In real life it is really one continuous process taken about minutes. Figure 6: The Cell division 12

14 The characteristics of living organisms Plants and animals show characteristics that suggest they are alive. There are seven major characteristics of living organisms as shown in the table below: Features Animals Plants Movement Very obvious Not very obvious Irritability (sensitivity) Obviously able to respond to such Little obvious response to stimuli as light, temperature, changes, vibration and touch stimulation over a short period of time Respiration Obvious organs associated with No obvious breathing action breathing nor any organ associated with respiration Nutrition Heterotrophic method Autotrophic method Growth Cell number increase more rapidly in some areas of the body than the other. The skin for example is in a state of The increase number of cells is called growth. Cells divide by mitosis. continuous repair or growth Excretion There are obvious excretory organ There are no obvious organ of (kidney, skin, lung) and obvious excretion or obvious excretory excretory products (urine, faeces, products except for water and water and carbon dioxide) carbon dioxide. Reproduction The ovary and testis are the The flowers are the reproductive organs of mammals. Egg reproduction organs of the and sperm are produced by meiosis plant. The sex cells are produced by meiosis 13