Tissues, cells and molecular studies

1 Module 1 Tissues, cells and molecular studies In this module you will be able to integrate your knowledge of the use of the light microscope with an understanding of cell structure. The difference between plant and animal cells is highlighted and you will understand how specialised cells make up tissues and organ systems. You will then further integrate this knowledge by discussing the causes, prevention and treatment of cancer, which is uncontrolled cell growth, and other diseases that affect the tissues of the body. Unit 1 The compound microscope and cell structure 2 Historical developments 2 Cell structure 3 The structure and function of cells 4 The difference between plant and animal cells 12 Diffusion and osmosis 13 Unit 2 Cell division: mitosis 14 Mitosis 14 Why mitosis is important 14 Unit 3 Tissues, organs, systems and organisms 19 Tissues 19 Organs 19 Systems 19 Organisms 19 Blood types and blood transfusion 23 Defence against infections 25 Unit 4 Cancer and other diseases 26 Cancer 26 Osteoporosis 27 Alzheimer s disease 27 Module 1 Questions 29

2 Historical developments A microscope is an instrument that is used to see objects that are too small to be seen with the human eye. There was no single inventor of the microscope. microscope to look at a piece of cork. A compound microscope uses more than one lens. In 1674 Van Leeuwenhoek built a simple microscope and examined small living organisms in a drop of pond water. The arrival of microscopes allowed scientists to start to understand the structure of the cell. In 1823 Robert Brown, a Scottish botanist, saw spherical objects in every plant cell he examined. He called this structure the nucleus. In 1838 Matthias Schleiden, a German botanist realised that all plant tissues are composed of cells and stated that the cell was the basic unit of all plants. In 1839 Theodor Schwann, a German zoologist, studied animal cells under the microscope. He realised that all plants and animals consist of cells and nucleus. In 1855 Rudolf Virchow, a German biologist, discovered that cells always multiply by cell division and stated that every cell is derived from pre-existing living cells. Eyepiece (ocular) Contains lenses to magnify the specimen under observation Tube Holds lenses of eyepiece and objectives at the correct working distance apart Coarse adjustment screw Moves tube up and down to locate the object being observed Fine adjustment screw Is used for final focus to get a clear image Arm Joins the base and stage to the tube and supports the adjustment screws Rotating nosepiece Holds objectives with low- and high-power lenses on rotating disc Objectives Lenses of objectives help to enlarge the image (a) Short one gives a lower magnification (4 ) (b) Longer ones give a higher magnification (10 and 40 ) Table Supports slide over opening which transmits light from mirror or electric light source below Clamp Holds slide firmly in position for observation Diaphragm and condenser Controls the amount of light directed into and passing through the object; focuses light rays from mirror onto the object Mirror or electric light source Reflects light upward through the condenser towards the opening in the stage Base It is firm and supports the weight of the microscope Figure 1.1 A compound microscope

3 The microscope illustrated on page 2 is called a compound microscope because it has two lenses to magnify the size of the object being looked at. The compound microscope is much more sophisticated than the simple one built by Van Leeuwenhoek in 1674. The compound microscope is used to examine wet mounts of cells to understand their structure and to identify their organelles. An organelle is a The electron microscope was developed in the 1940s and soon used in the study of cells, allowing far more detail to be seen than was possible with a light microscope. Looking at cells Revise the preparation of a wet mount and, using the diagram in Figure 1.1, revise how to set up and use a microscope. are looking at through a microscope. You can do this in the following way: Cell structure All living organisms are made up of cells. The cell is the basic unit of Light microscopes can only magnify cells a few hundred times. Under this cell. You can only see the following parts of a cell under a light microscope: the cell wall, the cell membrane, the cytoplasm, the nucleus, vacuoles and chloroplasts. To magnify structures such as cells many thousands of times, biologists use a transmission electron microscope. This microscope uses electrons instead of light rays. This allow us to see much more detail in cells. The electron microscope takes photographs that are called electron micrographs, times. In both these types of microscope, electrons pass through the object. In the scanning transmission electron microscope, electrons are focused in a beam that scans across the object. This is how the three-dimensional image is produced. When cells are looked at under an electron microscope, you do not only see the basic cell structures that you can see under a compound light microscope. You can see all of the tiny organelles that a cell contains.

4 leucoplast Golgi apparatus cell wall cell membrane dictyosome vesicle cytoplasm mitochondrion ribosomes chloroplast vacuole ER nuclear pore chromatin network nucleolus nuclear membrane Figure 1.2 below shows diagrams of a plant cell and an animal cell. Golgi apparatus nucleus cell membrane lysosome vesicle ER centriole nuclear pore chromatin network nucleolus nuclear membrane cytoplasm mitochondrion ribosome on ER pinocytic invagination nucleus Figure 1.2 Diagrams of a plant cell (left) and an animal cell (right) The structure and function of cells protoplasm. The protoplasm consists of the nucleus and the cytoplasm. The nucleus is surrounded by a double unit membrane and has a more jelly-like consistency than the cytoplasm. The cytoplasm is that part of the protoplasm that surrounds the nucleus. The cytoplasm has many organelles suspended in it. All cells are surrounded by a plasma membrane, which is an integral part of the living cell. The diagram below shows the components of a cell. Components of a cell Cell wall only in plant cells non-living Nucleus Protoplasm living part of a cell Cytoplasm cell membrane (plasma membrane or plasmalemma) cytosol (medium in which organelles are suspended) plastids (e.g. chloroplasts plant cells only) mitochondria lysosomes vacuoles Golgi body endoplasmic reticulum ribosomes centrioles (mainly in animal cells)

5 The cell membrane The cell membrane is also called the plasma membrane or the consisting of two dark lines on either side of a narrow, light interior. Figure 1.3 Micrograph of the cell membrane glycolipid glycoprotein protein cholesterol Figure 1.4 The fluid mosaic model outside inside carbohydrate part of glycoprotein phospholipid transport protein phospholipid The cell membrane is differentially permeable or selectively permeable. This means that it only allows certain substances to pass in or out of the cell. The structure of the cell membrane is explained best using the The membrane is made up of a double layer, called a bilayer, of thousands of phospholipid molecules. These are the two dark lines that A phospholipid molecule has a head that faces the outside and a tail that The tails of the phospholipid molecules point inwards and face each Large protein molecules are found in among the phospholipid bilayer. Carbohydrates can attach themselves to the lipids in the membrane to form glycolipids. Carbohydrates can also attach themselves to the The cell membrane is dynamic. That is, it is not static because the phospholipids and the proteins can move about within it. As the proteins move, they form channels that allow selected substances to move into and out of the membrane. This is differential or selective permeability.

6 The membranes that surround the nucleus, the endoplasmic reticulum and the Golgi apparatus have basically the same structure as the plasma membrane. Functions of the cell membrane The cell membrane: keeps the contents of the cell together controls the exchange of substances that pass into the cell and out of the cell. Nucleus Most cells have a nucleus. The nucleus is the largest organelle in the cell. A Figure 1.5 The cell nucleus The nucleus consists of four main parts: 1 nucleolus nuclear plasma nuclear membrane chromatin material nuclear pore endoplasmic reticulum The outer layer is continuous with the reticulum. The many nuclear pores allow substances to enter or leave the nucleus. 2 The nucleoplasm is a jelly-like substance that contains the chromatin network. 3 The chromatin network is made up of DNA. This throughout the nucleoplasm. Each individual thread is a chromosome. 4 The nucleolus is a darker structure in the nucleus, which is made up of protein and RNA and is not bound by a membrane. The function of the nucleus The nucleus controls the activities of the cell. The nucleus determines when and what proteins the cell will make and in this way controls all the cell s activities. The chromosomes of the nucleus are the carriers of heredity because they carry genes. The nuclear membrane The nuclear membrane is also known as the nuclear envelope. This is the double membrane that encloses the nucleus. This membrane separates the contents of the nucleus from the cytosol. The cytosol is the liquid medium of the cell. The nuclear membrane is particularly important for separating the DNA in the nucleus from the rest of the cell. The space between the two layers of the nuclear membrane is called the perinuclear space and is between 20 and 100 nm wide. The outer membrane is continuous with the rough endoplasmic reticulum.

7 The cell wall All plant cells are surrounded by a cell wall. The cell wall is thick, rigid cellulose, which is a type of to form a very strong covering to the cell. There are spaces between these large molecules. Figure 1.6 The cell wall of a plant The cell wall of a fully mature cell consists of three layers: 1 The middle lamella. This is the layer next to the cell membrane and is made up of pectin. 2 The primary cell wall expands inside the middle lamella, so it is found between the middle lamella and the plasma membrane. It consists of 3 The secondary wall. This is the layer closest to the cell membrane and woody substance, called lignin. Certain areas of the cell wall are thinner, forming tiny openings or pores, that allow strands of cytoplasm to pass from one cell to another. These strands of cytoplasm are called. cell membrane secondary cell wall cytoplasm plasmodesmata (thin areas of the cell wall through which cytoplasm passes from one cell to another) primary cell wall middle lamella Figure 1.7 A part of a plant cell showing the three layers of the cell wall.

8 The functions of the cell wall The cell wall: is rigid and thick and so gives shape to plant cells and gives strength and rigidity to the cell is fully permeable to most substances because of the relatively large contains plasmodesmata that allow the cytoplasm of one cell to connect with the cytoplasm in the next cell through pores in thin areas of the wall. The cytoplasm The cytoplasm consists of all the organelles in the cell suspended in the cellular inclusions like fat globules and starch grains, solutes like salt and sugar and gases such as O 2 and CO 2. The chloroplasts Chloroplasts are oval structures that are found in the cells of plants and algae. A chloroplast is a plastid. A chloroplast is about 3-10 µm in diameter. Their number varies from one to about 100 per cell. Chloroplasts are only present in those parts of a plant that are exposed to light and which are green in colour. stroma oil droplet granal lamellae containing chlorophyll lamella Figure 1.8 Micrograph and drawing of a chloroplast inner membrane outer membrane The structure of chloroplasts A chloroplast is surrounded by a double membrane: an inner membrane and an outer membrane. the enzymes that are necessary for the dark phase of photosynthesis. Photosynthesis is the process that plants use to turn the energy of sunlight into food. lamellae

9 single lamellae called intergrana lamellae. The lamellae contain the green pigment called chlorophyll and other photosynthetic pigments. Chlorophyll is the chemical that absorbs the sunlight that is used to make food for the plant in the process of photosynthesis. The function of chloroplasts Photosynthesis takes place in the chloroplasts. Chlorophyll traps light energy, which is used by plants to convert CO 2 and water into the chemical potential energy found in food. CO 2 is used in the process of photosynthesis and O 2 is released as a by-product. Thus the CO 2 and O 2 balance on the Earth is maintained by photosynthesising plants. Other plastids are: Leucoplasts, which do not contain any pigment and which occur in cells that store food in insoluble forms, such as starch, oil and protein granules. Chromoplasts, which contain any pigment except green and contain large amounts of carotenoids. They are responsible for the colour of tomatoes, Ribosomes Ribosomes are very small spherical structures that are found in plant and animal cells. Each ribosome is approximately 15-20 nm in diameter. Ribosomes are made up of proteins and RNA. At very high and a larger subunit. nucleus or the endoplasmic reticulum. They are also found in mitochondria and in chloroplasts. The function of ribosomes Ribosomes are the sites of protein synthesis. During this process, amino polypeptide chains, which then form proteins. The vacuole consists mainly of water and solutes such as sugars and salts. Vacuoles are most prominent in mature plant cells, where they can occupy as much as 80-90% of the total cell volume. The vacuoles that are found in animal cells and in young plant cells are usually small.

10 The function of vacuoles Vacuoles maintain the turgidity of plant cells. This means that the water in the vacuole puts pressure on the cytoplasm and the cell wall, which makes the cell swell up. Vacuoles may also contain pigments, especially anthocyanins in the cell dark red colour. contractile vacuoles control the amount of water in the cell. In the same organism vacuoles called food vacuoles are responsible for food storage and digestion. Knowledge of the following organelles is not asked for in the Grade 10 syllabus, but it is useful to know about them because it will help you to understand the structure and function of cells. The mitochondria and animal cells. Mitochondria are rod/sausage-shaped organelles about 1 µm long. The mitochondrion is surrounded by two membranes: an outer smooth membrane and an inner membrane. The inner membrane is folded cristae, which project into the interior of the organelle. These cristae increase the surface area of the membranes in the mitochondrion. The solution surrounding the cristae is called the matrix. The surface of the inner membrane is covered with granules that contain the enzymes that are used for aerobic respiration in the cell. The function of mitochondria The main function of mitochondria is to carry out the process of aerobic respiration, in which energy is released by breaking down food in the presence of oxygen. As a result of respiration, cells store energy in triphosphate about this later. The number of mitochondria in the cytoplasm can vary from 50 to 2 500 depending on the type of cell. The more active a cell, the more mitochondria it has. For example, muscle cells need large amounts of energy and so have large numbers of mitochondria. Mitochondria are called the powerhouse of the cell. outer membrane inner membrane crista matrix ribosomes Figure 1.9 Micrograph and drawing of a mitochondrion