General Science for Grade 7 PDF

Science is research-based knowledge, practice-based skill and inquiry-based approach to investigate and learn further through scientific method. The key activities involved in scientific method are observation, experimentation and deduction. Technology is application of scientific knowledge for solv...

Science is research-based knowledge, practice-based skill and inquiry-based approach to investigate and learn further through scientific method. The key activities involved in scientific method are observation, experimentation and deduction. Technology is application of scientific knowledge for solving daily life problems, making arduous works easy and improving quality of life. In order to meet the globally recognized standards, we need to broaden our educating horizons and adopt scientific method to learn science. The inquiries and activities of scientific method involve observations, experimentations and conclusions. Hypothesis, theory and principle, etc., are the developmental stages in scientific research. General Science for Grade 7 is the second annual course of studies designed in Single National Curricula for Elementary Level Students. This manuscript is an attempt to be proved as the best Textbook for the said course of studies. Use of scientific method in the teaching-learning approach equipped therein the textbook makes it entirely different from the textbooks in the subjects of humanities group. It has been developed as a source of information and tools for applying scientific method in research based studies. Students’ interest has been engaged in process based learning, in-depth studies and inquiries for making observations and conclusions through experiments, child-centered and teacher-guided activities and in doing science and technology. Acknowledgment Honourable Prof. Muhammad Ali Shahid, Director Technical (Rtd.), Punjab Textbook Board and Prof. Javed Mohsin Malik, Principal (Rtd.), Federal Ministry of Education are acknowledged with thanks for using their ideas and content given in PCTB General Science Textbooks. Appeal Valuable suggestions for the improvement of this book are cordially invited. (Author)  Food is the How basic need of does a plant every cell of all the obtain food and What are organisms transport it to vascular bundles every cell of its and their func on body? in plants? 01 Plant Systems Students' Learning Outcomes After studying this chapter, students will be able to: Explain the root and shoot system in plants. Explain that the structure of leaves is adapted Label different parts of leaf, stem and root to the process of photosynthesis. (external and internal structure). Describe the process of respiration and write Predict the role of xylem and phloem in word equations for it. Compare and contrast transport of water and food in plants by the processes of photosynthesis and observing the cross section of the stem. respiration. Define the process of photosynthesis and Investigate the phenomena of transpiration derive word equations for it. and its importance in a plant (wind, Know that plants require minerals to maintain temperature, light, humidity affecting rate of healthy growth and life processes (limited to transpiration in plants). magnesium to make chlorophyll and nitrates Explore and apply natural raise of water to make proteins). based on the principle of transpiration. VOCABULARY Vascular Root hairs bundles Mesophyll cells Chlorophyll Stomata Transpiration A system consisting Cells which are rich Green coloured Outgrowths of Tiny openings in Loss or removal of of xylem and in chloroplast and pigment in epidermal cells in plant leaves for extra water from phloem for specialized for chloroplast which plant roots which gaseous exchange aerial parts of conduction of preparation of food absorbs light absorb water and and removal of plants water and transport during photosynthesis nutrients from the extra water of food in plants photosynthesis soil Recall what you have learnt We have learnt about: Organs and organ systems in plants (Figure 1.1) Root and shoot system in plants Xylem and phloem Chloroplasts Leaf Stem Root Dermal tissue Vascular tissue Ground tissue Figure 1.1 Organs and organ systems in plants Activity 1.1 Assessment Write 'C' against the correct and 'I' against the incorrect statement in the middle column. Also correct the incorrect statement and write it in the next column. Correct/Incorrect C/I Correct statement Leaves in plants absorb water and nutrients from soil. Photosynthesis takes place in the underground parts of the plants. Xylem tissue conducts water from roots to leaves. Phloem transports food from leaves to other parts of the plant. In this chapter we will learn the following systems in plants: Root and shoot system Photosynthesis Respiration Transpiration 2 1.1 ROOT AND SHOOT SYSTEM IN PLANTS 1.1.1 Root System Roots are the underground parts of the plants forming a system specialized for absorption of water and minerals from the soil (Figure 1.2). Secondary root Primary root Tertiary root Root hairs Region of maturation Region of elongation Region of cell division Root cap Figure 1.2 Parts of root Recall Zygote is the first cell formed by the fusion of reproductive cells (male and female gametes) during sexual reproduction of plants and animals. The initial stage of development of multicellular organisms formed by the cell divisions and growth of zygote is called embryo. The initial root which grows from radicle of the embryo is the primary root or tap root. Its further branches form secondary and tertiary root system. The tips of roots and shoots in plants consist of tissues formed by the cells called meristematic tissues. These cells are specialized for growth. These cells increase in number by rapid cell division. Root tip is covered by hard and protective structure called root cap. It protects the newly born soft cells from being damaged and environmental stresses. It enables root tip to grow through soil. More and more cells are formed by rapid cell divisions in this region causing the root to grow deep into the soil and elongate the root behind. The region of maturation has root hairs. Root hairs are thread like outgrowths of epidermal cells which increase the surface area for absorption of water. Plants, like grasses and strawberry have thread like roots spread in soil but not growing deep. Such roots are called fibrous roots (Figure 1.3). 3  Fibrous Taproot Root Figure 1.3 Taproot and fibrous roots Internal structure of root The main parts in the internal structure of root are epidermis, cortex, endodermis vascular bundles, i.e., xylem and phloem (Figure 1.4). The arrangement of these parts are shown in a transverse section (TS) of the root. (Figure 1.5). Phloem Xylem Cortex pharenchyma cells Root hair Root hair Epidermis Xylem Cortex Endodermis Pericycle Phloem Pith Metaxylem Protoxylem Apical Meristem Figure 1.4 Structures inside the root Figure 1.5 Internal structure of root Epidermis Epidermis is an outermost layer of thin walled cells having unicellular outgrowths called root hairs. Epidermis provides protection and helps in absorption of water and minerals from the soil. Cortex Inside the epidermis, many layers of thin walled and rounded cells form cortex. Water and minerals absorbed by root hairs in epidermis are transported towards xylem through cortex for onward conduction to leaves. Endodermis The innermost layer of cortex which consists of barrel shaped cells is called endodermis. It 4 regulates the movement of water and hormones, etc., into and out of the vascular system. Vascular system Xylem and phloem form vascular system. Xylem conducts water and minerals absorbed from roots to leaves through stem. Phloem transports food from leaves to roots and other parts of the plants. Inquiry 1.1 Teacher Guide Facilitate students conduct an interactive discussion of the following: Absorption of water and nutrients (minerals, etc.) by root hairs from soil and their diffusion towards xylem present in the centre of root. Conduction of water and dissolved minerals by xylem tissue from roots to leaves through stem. Water Epidermis Cortex Endodermis Xylem Recall what you have learnt about diffusion. Do water and dissolved minerals (nutrients) move from soil to xylem tissue in the roots following the principle of diffusion through membranes of the cells in epidermis, cortex and endodermis, etc.? Activity 1.2 Examining internal structure of root epidermis (with root hairs) Teacher Guide cortex (parenchyma) Facilitate students: endodermis (with Casparian strip) pericycle Take some herbaceous plant. phloem (sieve and companion cell) xylem (vessel element) Cut the transverse section (TS) of its root. Prepare a glass slide of the TS. Examine the TS under microscope. Identify different parts of the root. Draw an explanatory sketch of the TS. 1.1.2 Shoot System The areal parts of the plants such as stem, its branches, leaves, flowers and fruit, etc., specialized for different functions make the shoot system. Here we will discuss stem and leaves. 5  Epidermal hair Stem Cuticle Stem is usually the aerial part of the plant. It Epidermis Collenchyma provides support to its branches, leaves, Chlorenchyma flowers and fruit, etc. Vascular bundles Parenchyma Endodermis (xylem and phloem) run through stem to Bundle Cap leaves for transport of material within the Phloem plant body. A sketch of internal structure of Cambium stem (Figure 1.6) is self-explanatory with Xylem regards to the arrangement of different tissues in the plants. Pith Leaf Figure 1.6 Internal structure of stem Stem and its branches bear leaves. Leaves are the organs which are well exposed to light and well Margin adapted for preparation of food during photosynthesis. A leaf is provided with the stalk called petiole. Its flat green part is called blade Midrib or lamina, which has many veins. The middle Veins strong vein is called midrib (Figure 1.7). Different shaped cells, as shown in the cross Petiol section of a leaf (Figure 1.8), perform different functions. Figure 1.7 Leaf Palisade Mesophyll cells Xylem Cuticle Spongy mesophyll Air space cells Phloem Lower epidermis Cuticle Guard cell Low water vapor Stomatal pore Figure 1.8 Internal structure of leaf 6 Epidermal cells are tile-like and forming protective layers (upper epidermis and lower epidermis). Palisade mesophyll cells are elongated, spongy mesophyll cells are irregularly shaped having air spaces among them. Their function is to prepare food. Xylem tissue consists of tubular shaped cells used to conduct water. Phloem cells are also tubular in shape and used for transport of food. There are openings in the lower epidermis called stomata (sing. stoma). Each pore is surounded by a pair of guard cells. They allow exchange of carbon dioxide and oxygen. They also diffuse out Phloem tissues water vapour. carry food up from leaf 1.1.3 Role of Xylem and Phloem Food made Evaporation in transport of water and food by the leaf of water from leaves Water and minerals diffuse from soil to Stem (Transpiration) Water and roots and are conducted towards leaves minerals from the soil through stem by xylem and the food Xylem tissues Leaf prepared in leaves is transported to carry water Phloem tissues and dissolved carry food down different parts of the plant through minerals up from leaf from roots phloem (Figure 1.9). Roots Soil Do you know? Figure 1.9 Transportation of materials in plants ¡ Recall what you have learnt about solute, solvent, solution and diffusion of particles. ¡ The quantity of a solute dissolved in a given quantity of solution is termed as concentration. ¡ 1L sugar solution containing 3 teaspoonful of sugar dissolved in it, is more concentrated as compared to 1L sugar solution containing 1 teaspoonful of sugar dissolved in it. ¡ Solvent is the component of a solution which is present comparatively in large quantity. ¡ Water in the air is solute not solvent. Conduction of water and dissolved minerals water Movement of material particles from the area where they are more to the area where they are less is called diffusion. The diffusion through membranes (like cell water water membranes) is called osmosis. As the concentration of water water and dissolved minerals is more in soil as Transpiration water compared to the root cells, they move in the root tissue (osmosis). The xylem is a tissue in plants which forms a water system of pipelines (xylem vessels) from roots to water leaves through stem for the conduction of water, only in one direction (Figure 1.10). Figure 1.10 Conduction of water in plants Transport of food (Translocation) Food prepared by photosynthesis in leaves (carbohydrates, i.e., glucose, sucrose, etc.) enters the phloem sieve tube elements in dissolved form through companion cells. It is then transported 7  Xylem Phloem to all other parts of plant body through phloem Companion (Figure 1.11). cell Water Xylem tissue consists of dead cells joined end to end Leaf cell with no end walls to form a continuous tube. Phloem is composed of living cells called sieve tubes, companion cells and phloem parenchyma. Parenchyma is a Steve-tube elements type permanent tissue forming major part of ground tissue in plants. The main function of parenchyma is to Root cell store and assimilate food. Water Companion cell Stomata Figure 1.11 Transport of food in plants A large number of stomata (Figure 1.12) in lower epidermis of the leaf helps in absorption of carbon dioxide and release of surplus oxygen in the environment. Stomata Stoma open Stoma closed Stoma Figure 1.12 Stomata in leaves Distribution of chloroplast in mesophyll tissue for maximum absorption of sunlight, regular supply of water through xylem tissue and carbon dioxide through stomata are the features of leaf that facilitate photosynthesis. 1.2 PHOTOSYNTHESIS Photosynthesis is defined as the process during which carbon dioxide and water combine in the presence of sunlight and chlorophyll to form glucose (food) and oxygen. Carbon dioxide + Water + Sunlight energy Food (Glucose) + Oxygen The process of photosynthesis depends on the following factors: Light Photosynthesis requires energy for combining carbon dioxide and water to form glucose (food). Sunlight provides this energy. The rate of photosynthesis depends upon the intensity of light. There is more light intensity at noon, so, the rate of photosynthesis is fast at the noon. In the evening or morning, the rate of photosynthesis is slower due to less light intensity. Photosynthesis 8 stops at night, because, there is no sunlight at night. Light consists of seven colours but chlorophyll absorbs blue and red light. Carbon Dioxide Air contains 0.35 to 0.4 % carbon dioxide. If there is more amount of carbon dioxide in the environment, the rate of photosynthesis will be fast, but once the carbon dioxide concentration reaches a certain level, there is no further increase in the rate of photosynthesis. Less quantity of carbon dioxide in an environment slows down the process of photosynthesis. Water Besides carbon dioxide, water is also an essential requirement for photosynthesis. If it is not available as much as require, the rate of photosynthesis will be affected and plant will produce less food. Chlorophyll The process of photosynthesis cannot take place without chlorophyll. It is only the chlorophyll which absorbs sunlight and makes it usable in the photosynthesis process. Chlorophyll is present in chloroplasts which are present in photosynthetic cells (mesophyll). Temperature Like many other chemical reactions, photosynthesis is also affected by temperature. The suitable temperature for this process is 25 – 35°C. At very high or very low temperatures, the process of 1.2.1 Adaptations in leaf structure for photosynthesis 1. The blades of leaves are flat and absorb maximum light required for photosynthesis. 2. Thin blades of leaves make the light and carbon dioxide reach the internal parts easily. 3. Thick layer of palisade mesophyll containing large number of chloroplasts just beneath the upper epidermis can make maximum absorption of light to make food for the plant. 4. Maximum air spaces among spongy mesophyll near the lower epidermis provide an easy passage for carbon dioxide to diffuse into cells containing chloroplasts to facilitate photosynthesis. 5. Numerous stomata in the lower epidermis absorb maximum carbon dioxide from the air needed for photosynthesis. 6. Division of vascular bundles into small branches spread throughout the leaf makes an easy and maximum water supply needed for photosynthesis. Do you know? ¡ Photosynthesis takes place mainly in leaves; but, in some plants it occurs in green stems also. The process actually occurs inside the chloroplasts in the cells of leaves and stem. 9  Scientific Investigation Teacher Guide Facilitate students to prove that photosynthesis occurs in the leaves Background Information Glucose made during photosynthesis is converted into starch which is stored in different parts of the plant including leaves. The presence of starch in a leaf proves that photosynthesis takes place in the leaf. The presence of starch in the leaf can be checked by iodine test. Iodine Test Iodine solution is yellow in colour. When one or two drops of iodine solution are added in starch solution, the colour of iodine solution changes to blue. 1. Fill a beaker (500mL) half with water, heat it till boiling and dip a green leaf in boiling water for 2 to 3 minutes. 2. Fill a test tube with alcohol up to its half. Remove the leaf from boiling water and dip it in alcohol. 3. Keep the test tube containing the leaf in boiling water for a few minutes, so that chlorophyll dissolves in alcohol. 1 2 3 4. Remove the leaf from alcohol and again dip it in boiling water so that it becomes soft. 5. Place the leaf in a dish and spread it. 4 5 6. Pour a few drops of iodine solution on the leaf with the help of a dropper. 7. You will see that the colour of the iodine solution becomes blue. This will prove that the starch is present in the leaf. 10 Mineral nutrition in plants Minerals are the chemical substances required by an organism for growth and other vital functions. Carbon, hydrogen, oxygen, nitrogen, magnesium, etc. are the nutrients required by plants in large quantities. Carbon, hydrogen and oxygen are obtained from carbon dioxide and water. The other nutrients like magnesium and nitrogen are obtained from the soil dissolved in water. Magnesium Magnesium is required by plants for the formation of chlorophyll. It is helpful for the functioning of enzymes to produce carbohydrates and fats. Its deficiency causes poor growth, yellowing and wilting of leaves. Nitrogen Nitrogen in the form of its compounds, (nitrates) dissolved in soil water, is required by plants for making chlorophyll and amino acids. Amino acids form proteins. Chlorophyll is necessary for photosynthesis. Proteins are necessary for growth, repair and other developments and defensive functions. Deficiency in nitrogen makes the leaves pale green or yellow. It affects the rate of photosynthesis and growth in plants. 1.3 RESPIRATION IN PLANTS Respiration is defined as the process during which glucose (food) reacts with oxygen to produce carbon dioxide, water and energy required for survival of life. Food (Glucose) + Oxygen Carbon dioxide + Water + Chemical energy Respiration takes place in all the cells of Light living bodies. The mitochondria which are found in the cells perform the process of Energy respiration. For this reason, mitochondria (Figure 1.13) are called power house of Chloroplast the cell. Photosynthesis Importance of Respiration CO2 + H2O VS Glucose + O2 Respiration is the only process which Cellular respiration provides energy in usable form by living Mitochondria things. This process continues all the time in all kinds of living organisms. Stopping of respiration means death of the organism. In plants, the oxygen required for respiration comes from photosynthesis. Energy Figure 1.13 Comparison between photosynthesis and respiration 11  Aa CBb c Informative ¡ Plants absorb carbon dioxide and release oxygen during day time, whereas, at night, they absorb oxygen and release carbon dioxide. ¡ In plants, exchange of gases takes place through stomata, whereas, in the animals this work is performed by the lungs. 1.3.1 Comparison and Contrast between Photosynthesis and Respiration 1. Photosynthesis takes place in green plants, algae and some bacteria, whereas, respiration takes place in all the living things. 2. Photosynthesis takes place in chloroplasts, whereas, respiration takes place in mitochondria. 3. Photosynthesis uses sunlight energy to prepare food, whereas, respiration releases energy from food which is used for growth and performing all other body functions. 4. The products made during photosynthesis, i.e., glucose and oxygen are the reactants of respiration. 5. The products of respiration, i.e., carbon dioxide and water are the reactants of photosynthesis. 1.4 TRANSPIRATION Plants continuously absorb water from soil through roots. The same is being conducted to leaves where it is used in photosynthesis. The excessive water is removed through stomata and from the aerial parts of the plants. The loss of water from aerial parts of the plants is called transpiration (Figure 1.14). Water evaporates from the leaves Veins carrying water for the leafs Water is drawn up the stem Roots absorb water from the soil Figure 1.14 Transpiration in plants 12  Activity 1.3 Transpiration in plants Take a potted plant and sprinkle water on the soil in the pot, where the plant is Polythene grown. bag Leaf Cover the plant with a transparent polythene bag. Water Tie the mouth of the bag around the base of the stem as shown in the Figure. droplets Place the plant in sunlight for 4–5 hours. You will observe drops of water on the inner side of the polythene bag. Where do these water drops come from? Conclusion:_________________________________________________________________________________________________________ ______________________________________________________________________________________________________________________ Capillary action Very narrow glass tubes are called capillary tubes. Water moves up in the capillary tubes due to interaction between water molecules and surface of the tubes (Figure 1.15). This effect is called capillary action. Water level is lower in wider spaces where surface tension is reduced Water is attracted to the sides of the container Figure 1.15 Capillary action Capillary action in narrow xylem vessels in small plants helps water move up towards leaves. Do you know? ¡ Water filled in a glass tube faces two types of forces. One is the attractive force between the molecules of water (intermolecular force) and the other is the force between the molecules of water and glass surface. ¡ The force between the molecules of water is termed as cohesive force, whereas, the forces between the water molecules and glass surface is termed as adhesive force. ¡ If the adhesive force is greater than the cohesive force, water rises up the tube through capillary action. ¡ In narrow tubes, the adhesive force becomes greater than the cohesive force which makes the water rise up the tubes through capillary action. ¡ Water rise up in xylem vessels is a natural example of capillary action. ¡ In narrow tubes, the adhesive force becomes greater than the cohesive force which makes the water rise up the tubes through capillary action. ¡ Water rise up in xylem vessels is a natural example of capillary action. 13 1.4.1 Natural Raise of Water Based on Principle of Transpiration Transpiration is the water escape from the pores in leaves or stems (stomata). It develops suction force in the xylem bundles, causing the water rise up. Tall trees having more surface area for transpiration cause strong suction force or transpiration pull on water in the xylem bundles. The increased transpiration pull in tall trees causes the plant to absorb more water. Capillary action and transpiration pull are the causes of natural raise of water in the materials. Building materials such as bricks and concrete blocks, etc., when come in contact with moisture, water rises into their pores due to capillary action and makes the material damped. Water is also raised in the pores of paper and plaster. Wicking of paint between the hairs of a paintbrush and movement water through sand are also the examples in this regard. Importance of transpiration Transpiration pulls the water through air spaces in spongy mesophyll and keeps the mesophyll moist, which is essential for the exchange of gases. Effects of evaporation of water from leaves or stems are: cools the surface of the leaves and the surroundings as well, allows the movement of minerals from the soil to different parts of the plant, helps in growth and development. controls the temperature of the plants. 1.4.2 Factors Affecting the Rate of Transpiration Wind, temperature, light and humidity are the main factors that affect the rate of transpiration. Wind The still air surrounding the plant leaves becomes humid and resists the diffusion of water from leaves into the air. It decreases the rate of transpiration. Wind sweeps the humidity away from leaves surroundings and increases the rate of transpiration. Temperature Rise in temperature provides more energy to the water molecules for evaporation from the leaves surfaces, hence, increases the rate of transpiration. Light In sunlight, the stomata remain open for removal of water through them. At night stomata are closed. In this way light also affects the rate of transpiration. Humidity Humid air surrounding the plants contains more Point to ponder amount of water, thus, decreasing the rate of — Gravity does not affect the capillary diffusion of water molecules from plants leaves action. Request your teacher to explain into air. It slows down the transpiration. The rate why? of transpiration is rapid in dry air. 14