Structure and Functions of Plant and Animal Cells PDF

Biology Biology 06 Structure and functions of the plant and animal cell 6.1 Basic unit of life In 1665, Robert Hooke observed a section of a c...

Biology Biology 06 Structure and functions of the plant and animal cell 6.1 Basic unit of life In 1665, Robert Hooke observed a section of a cork using a microscope prepared by him. He discovered a structure like chambers in a beehive and he named them as cells. Figure 6.1- Robert Hooke, the microscope and the cells in a section of the cork Schleiden, Schwann and Radolf Virchow introduced the cell theory, based on the facts revealed by observing different live tissues through the microscope. The contents of the cell theory are as follows. y The structural and functional unit of life is the cell. y All organisms are made up of one or more cells. y New cells are formed from pre-existing cells. 6.2 Concept of the cell The cell is the smallest structural unit of the organization of the living body. The organisms composed of a single cell are called unicellular organisms and those of many cells are called multicellular organisms. Cells perform different functions in the body' 110 For free distribution For example - The transportation of oxygen is done by red blood cells. Transmission of impulses is done by neurons. Accordingly, the smallest bio unit that is adapted to perform a particular function is the cell. So it is clear that the structural and functional unit of life is the cell. The cells differ from one another from their shape, size and function. Except few occasions, mostly cells are not visible to the naked eye. Therefore they have to be observed using the light microscope. 6.3 Structure of cells Let’s do the following activities (01 and 02) to study the structure of animal cells and plant cells. To study the animal cells we will observe cheek cells and for the plant cells, let's take onion epidermal cells, as these cells are easily obtained. Activity 01 Study of animal cells (cheek cells) Materials Required - Sample of cheek cells, Glass slide, Cover slip, Microscope, Water Method Wash the mouth and scrape the inner side of the cheek using a yoghurt spoon. Obtain a clean glass slide and put a drop of water and transfer the specimen on to the slide. Cover the specimen using a cover slip without trapping any air bubbles and observe through the light microscope. The appearance of stained cheek cells through the light microscope Figure 6.2 (a) Figure 6.2 (b) 111 For free distribution  Activity 02 Study of plant cells (onion peel cells) Materials Required - Onion peel, Watch glass, Glass slide, Cover slip, Paint brush, Microscope, Water Method Cut an onion and obtain an inner fleshy tissue as shown in the diagram. Remove a peel from inner or outer surface of it and transfer it on to a watch glass containing water. Put a water drop on to a clean glass slide and transfer the specimen on to the slide using a paint brush. Cover it with a cover slip without trapping any air bubbles and observe it through the microscope. The appearance of stained onion peel Figure 6.3 (a) cells through the light microscope Figure 6.3 (b) Typical cell The small structures present within the cell to perform different functions are known as organelles.The types of organelles and the number of them differ according to the function performed by the cell. The cell prepared by including all the organelles is known as the typical cell. In the living world such cells do not exist. But cells with a certain number of organelles of the typical cell can be found in living organisms. Cell membrane Nucleus Cytoplasm nucleus vacuole Cell wall Cytoplasm Figure 6.4 (a) -Animal cells Figure 6.4 (b) - Plant cells (appearance through a light microscope) 112 For free distribution All animal cells are covered by a plasma membrane or a cell membrane. It is a live semi permeable membrane as well as a selective permeable membrane. There is a centralized nucleus in an animal cell. The cytoplasm is a gelatinous material. The outer covering of the plant cell is the cell wall. It is made up of cellulose. Inner to the cell wall is the plasma membrane. At the center of plant cell is a large vacuole. Generally there are no such vacuoles in animal cells. Animal cells as well as plant cells possess different organelles that perform different functions. Most of the above organelles cannot be observed through the light microscope. Therefore the electron microscope should be used. Below are the typical plant and animal cells created based on electron microscopic information. Ribosome Nuclear Rough Endoplasmic Nucleus envelope Reticulum Nucleolus Chloroplast Plasma Cell wall membrane Mitochondrion Cell sap Vacuole Tonoplast Golgi complex Figure 6.5 - Typical plant cell created using electron microscopic information 113 For free distribution  Smooth Endoplasmic Reticulum Cytoplasm Ribosome Golgi Complex Centriole Plasma membrane Nucleolus Nuclear Nucleus Envelope Rough Endoplasmic Reticulum Mitochondrion Figure 6.6 -Typical animal cell created using electron microscopic information There are similarities and differences between animal and plant cells. The table 6.1 contains the differences between animal and plant cells. Table 6.1- Differences between animal cells and plant cells Animal Cell Plant Cell y Cell wall absent y Cell wall present y Large content of it contains y Cytoplasm is pushed towards cytoplasm periphery y A large vacuole is absent. y A large central vacuole or few (Some times few small vacuoles vacuoles may present may present) y Chloroplasts absent y Chloroplasts present 114 For free distribution 6.4 Cell organelles and structures present in a cell Every organelle and structures present in a cell perform a specific function. Therefore the cell shows division of labour. y Cell wall The outer most covering of the plant cell is the cell wall. It is a dead structure. The main constituent of it is cellulose. The main functions of the cell wall are , to maintain the shape of the cell, support and protection of Figure 6'7 the cell. y Plasma Membrane (Cell membrane) Plasma membrane is present interior to the cell wall of plant cells.The boundary of the animal cell is the plasma membrane. It is made up of phospholipids and proteins. Plasma membrane is a semi permeable membrane. The main function of it is to enclose the cell, allow entry of water, ions, some molecules and thereby control the entry and exit of materials into and out of the cell. Figure 6.8 Plasma membrane is also known as cell membrane. Cytoplasm The gelatinous liquid part of the cell excluding organelles is known as the cytoplasm. Inorganic and organic substances are present in it. The functions of the cytoplasm are to maintain a shape to the cell, bear cell organelles and carryout different metabolic processes. The structures submerged in the cytoplasm are named as organnells. some organelles are surrounded by cell membranes. Eg.- mitochondrion, nucleus, endoplasmic reticulum, golgi complex. ² Nucleus Nucleus is the main organelle in a cell. It is surrounded by a nuclear envelope. One or two nucleolus and the chromatin body are present inside the nucleus. During cell division, the chromatin body converts into chromosome. The functions of chromosomes are the storage of genetic material and transfer inherited characters from generation to Figure 6.9 generation. 115 For free distribution The number of chromosomes is specific to a species. Eg : There are 46 chromosomes in a human being.There are 26 chromosomes in a frog There are 24 chromosomes in a paddy plant. The main function of the nucleus is the control of life activities of the cell. ² Mitochondrion It is an oval or rod shaped, membrane bounded organelle. Aerobic respiratory reactions take place within the mitochondrion to release energy. So it is known as the power house of the cell. The energy produced within the mitochondrion is used for the metabolic activities of the cell. Figure 6.10 ² Golgi Complex Membrane bounded sacs stacked on top of the other with associated secretory vesicles are collectively known as golgi complex.The functions of golgi complex is the production of secretory substances, packaging and secretion. Figure 6.11 ² Ribosome They are small organelles without a membrane. It is made up of a large subunit and a small subunit. They can be found freely in the cytoplasm or attached to Endoplasmic Reticulum. The function of ribosome is providing place for the protein synthesis. Figure 6.12 116 For free distribution ² Endoplasmic reticulum It is an inter membranous network made up of flat or tubular sacs within the cytoplasm. Endoplasmic reticulum is of two types. They are rough endoplasmic reticulum and smooth endoplasmic reticulum. Rough endoplasmic reticulum Rough endoplasmic Endoplasmic reticulum become rough due reticulem to ribosomes attahed to the membrane. The function of it is the transportation of proteins within the cell. Smooth endoplasmic reticulum Ribosome It is a network of tubular sacs without Ribosomes Smooth endoplasmic on the membrane. Synthesis of Lipids, steroids reticulem and to transport them within the cell are the Figure 6.13 functions of it. ² Vacuole It is a fluid filled large organelle found in plant cells which is surrounded by a membrane. The membrane that surrounds the vacuole is known as tonoplast. The fluid contained in it is known as the cell sap. Water, sugar, ions and pigments store within the vacuole. Generally no vacuoles are found and sometimes small vacoules may present in animal cells. Contractile vacuoles can be found in unicellular organisms. Maintenance of water balance, support and provision of colour Figure 6.14 to the cell by the pigments within it are the functions of the vacuole. Activity 03 Materials Required - permanent slides, electron micrographs of organelles, A microscope Method y Identify cells and organelles by observing the permanent slides through the light microscope with the help of your science teacher. y Observe and study the nature of organelles using electron micrographs. 117 For free distribution 6.5 Cell Growth and Cell Division Cell growth Immature cell Growth is a basic feature of organisms. Growth of a cell is the irreversible increase of size or dry mass. But a cell has a maximum limit to grow. Beyond that level the cell will not grow, instead it divides. Mature cell Figure 6.15 Cell division The cell has the ability to grow and multiply its number. Accordingly a cell can multiply into two, four and eight cells. By multiplication new cells are formed. The cells multiply by cell division. The cell division is the process by which new cells are formed by the division of cellular materials. To complete the cell division of an eukaryotic cell, first the nucleus should divide and then the cytoplasm. Chromosomes contain and transfer genetic materials, the inherited characters from generation to generation (Before the division of nucleus). Chromosomes can be seen clearly as in the figures below. Thread-like Cytoplasm chromosomes Nucleus Nucleolus Sister chromatids in a chromosmome The appearance of chromosomes The appearance of in an ordinary cell before the chromosomes in a dividing division of the nucleus cell Figure 6.16 118 For free distribution The number of chromosomes in an ordinary somatic cell of a species is constant. That is specific to a species. Example - There are 46 chromosomes in a chromosomal set of human somatic cells. This is comprised of 23 pairs of chromosomes. The same hereditary information is born by each pair of chromosomes. A pair of chromosomes which contains same hereditary information is called as homologous pair of chromosomes. One of these homologous chromosomes is inherited from father where as the other is from mother. Accordingly a child inherits 46 chromosomes receiving 23 chromosomes from father and 23 chromosomes from mother. The cell division takes place in 2 methods. Mitosis Meiosis Mitosis 2n It is the type of division which multiplies the number of cells by maintaining a 2n constant number of chromosomes in the cells. First the nucleus divides and then the cytoplasm divides to produce two identical daughter cells equal to 2n mother cell. Figure 6.17 Significance of Mitosis ² For the growth of multicellular organisms. ² As an asexual reproduction method. ² Wound healing and cell replacement. 119 For free distribution Meiosis The cell division that makes the 2n number of chromosomes into half as the parent cell is the meiosis. Division 1 Meiosis takes place during formation of gametes (eggs and sperms) in higher 2n organisms. After the gametes being fused, the number of chromosomes n of a species should be maintained, constant. For that the number of Division 2 chromosomes should be halved n during the formation of gametes and become n (haploid). Figure 6.18 Eggs and sperms possess only one chromosome of each pair of chromosomal set. (2n n) When these gametes fuse to form the zygote, the chromosomes become n+n 2n again. Meiosis takes place in 2 stages. The first stage is a meiotic division (reduction division) and the next is a mitosis. During meiosis, structural changes occur in chromosome. Therefore, new variations or new characters appear in organisms and this is a very important phenomena in evolution. Significance of Meiosis y Maintenance of the constant number of chromosomes from generation to generation. y Help in evolution due to variations occur in chromosomes. Differences between Meiosis and Mitosis are mentioned in table 6.2 120 For free distribution  Table 6.2- Differences between meiosis and mitosis Meiosis Mitosis 1. Takes place in two divisions Only one division Takes place in both diploid and haploid 2. Takes place only in diploid cells cells 3. Variations occur Thus changes No variations. The changes in take place in chromosomes chromosome are rare 4. Four daughter cells result at the end Two daughter cells result at the end of of the division the division 5. Daughter cell receives half of the Two daughter cells receive the same chromosomal number of mother cell chromosomal number as the mother cell 6. Daughter cells are different from Daughter cells are similar to mother cell mother cell Summary y The basic structural unit of the organism is the cell. y The structural and functional unit of life is the cell. y New cells are formed from pre-existing cells. y Different functions are performed by different organelles in the cell. y All animal cells are surrounded by the plasma membrane. Generally the nucleus is present at the centre of the cell. The area between nucleus and the plasma membrane is the cytoplasm. There are different organelles present in the cytoplasm. Eg :- Mitochondrion, Golgi complex, Endoplasmic reticulum y Most of the cell organelles are present in both animal and plant cells. But some organelles like cell wall, chloroplast, large central vacuole are present only in plant cells. y The cellular structures that carry genetic information are the chromosomes in the nucleus. y The cell growth is the irreversible increase of dry mass or the size of the cell. y The cell divides at a particular stage during the growth. y The cell division takes place according to two methods. They are Mitosis and Meiosis. 121 For free distribution