Science Class 08 part-II NCERT

7. Conservation of plants and animals
1. Satpura National Park is the first Reserve Forest of India.
8. Cells
1. A white blood cell (WBC) in human blood is another example of a single cell which can change its shape.
2. Basic components of a cell are cell membrane, cytoplasm and nucleus. Cytoplasm and nucleus are enclosed within the cell membrane, also called the plasma membrane. Plasma membrane is porous
1. Various other components of cells are present in the cytoplasm. These are mitochondria, golgi bodies, ribosomes
2. Nucleus contains thread-like structures called chromosomes. These carry genes and help in inheritance or transfer of characters.
3. The chromosomes can be seen only when the cell divides
4. Entire content of a living cell is known as protoplasm. It includes the cytoplasm and the nucleus.
3. All organisms other than bacteria and blue green algae are called eukaryotes.
4. Plant cells differ from animal cells in having an additional layer around the cell membrane termed cell wall
5. Colored bodies called plastids are found in the plant cells only. Green plastids containing chlorophyll are called chloroplasts
6. Plant cell has a big central vacuole unlike a number of small vacuoles in animal cells
7. Muscle cells are spindle-shaped to bring about movement of body by contracting and relaxing. Nerve cells are long and branched to receive the stimuli and transmit message to brain that sends the response
9. Reproduction -
1. In many animals fertilisation takes place outside the body of the female, in water
2. Cloning is the production of an exact copy of a cell, any other living part, or a complete organism. Dolly the sheep, first mammal to be cloned.
3. Amoeba reproduces by dividing itself into two. This type of asexual reproduction is called binary fission
10. Adolescence
1. The changes which occur at adolescence are controlled by hormones. Hormones are secretions of endocrine glands which pour them directly into the blood stream.
2. Reproductive life of a woman lasts from menarche to menopause
3. When a sperm containing X chromosome fertilizes the egg, the zygote would have two X chromosomes and develop into a female child. If the sperm contributes a Y chromosome to the egg (ovum) at fertilisation, the zygote would develop into a male child.
4. Adrenal glands secrete hormones which maintain the correct salt balance in the blood. Adrenals also produce the hormone adrenalin. Pituitary gland secretes hormones which include growth hormone and hormones that make other glands such as testes, ovaries, thyroids and adrenals, secrete hormones.
5. If the water in which tadpoles are growing does not contain sufficient iodine, the tadpoles cannot become adults.
11. Force
1. Force could be a push or a pull. Force has magnitude as well as direction.
2. Force due to air in a column of the height of the atmosphere and area 15 cm × 15 cm is nearly equal to the force o gravity on an object of mass 225 kg. The reason we are not crushed under this force of gravity is that the pressure inside our bodies is also equal to the atmospheric pressure and balances the pressure from outside.
12. Friction
1. If an object started moving, it would never stop if there were no friction. Friction opposes the relative motion between two surfaces in contact. It acts on both the surfaces
2. Sliding friction comes into play when an object is sliding over another. Sliding friction is smaller than static friction. When one body rolls over another body, rolling friction comes into play. Rolling friction is smaller than sliding friction. (rolling < sliding < static)
3. Fluid friction can be minimized by giving suitable shapes to bodies moving in fluids.
13. Sound
1. In humans, sound is produced by the voice box or the larynx. Two vocal cords, are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for the passage of air. When the lungs force air through the slit, the vocal cords vibrate, producing sound.
2. Vibrating objects produce sound and it is carried in all directions in a medium. The medium could be a gas, a liquid or a solid.
1. Speed at which sound propagates (or travels from its source) is directly influenced by both the medium through which it travels and the factors affecting the medium, such as altitude, humidity and temperature for gases like air.
2. Fastest in solids than liquids; worst in air. Best in diamonds.
3. Sound cannot travel through a vacuum. A vacuum is an area without any air, like space. So sound cannot travel through space because there is no matter for the vibrations to work in.
4. The frequency determines the shrillness or pitch of a sound. If the frequency of vibration is higher we say that the sound is shrill and has a higher pitch. If the frequency of vibration is lower, we say that the sound has a lower pitch.
5. Usually the voice of a woman has a higher frequency than that of a man (also means man's pitch is low). A bird makes a high-pitched sound whereas a lion makes a low-pitched roar. However, the roar of a lion is very loud while the sound of the bird is quite feeble.
6. For human ear, the range of audible frequencies is roughly from 20 to 20,000 Hz. Dog whistles and ultrasound equipment works at >20,000 Hz.
7. Total hearing impairment, which is rare, is usually from birth itself. Partial disability is generally the result of a disease, injury or age.
14. Electric current
1. Some liquids are good conductors of electricity and some are poor conductors. Water from taps, ponds is not pure and a good conductor of electricity. Distilled water is free of salts (no ions) and is a poor conductor.
2. Common salt, when dissolved in distilled water, makes it a good conductor. Most liquids that conduct electricity are solutions of acids, bases and salts
3. Process of depositing a layer of any desired metal on another material by means of electricity is called electroplating. It is one of the most common applications of chemical effects of electric current.
15. Natural phenomena
1. The electrical charges generated by rubbing are static. They do not move by themselves. When charges move, they constitute an electric current. Process of transferring of charge from a charged object to the earth is called earthing.
2. During the development of a thunderstorm, the air currents move upward while the water droplets move downward. These vigorous movements cause separation of charges.
1. Positive charges collect near the upper edges of the clouds and the negative charges accumulate near the lower edges. There is accumulation of positive charges near the ground also
2. When the magnitude of the accumulated charges becomes very large, the air which is normally a poor conductor of electricity, is no longer able to resist their flow. Negative and positive charges meet, producing streaks of bright light and sound
3. During a thunderstorm, safer to use mobile phones and cordless phones than wired phones. Bathing should be avoided during thunderstorms to avoid contact with running water
16. Light
1. Laws of reflection -
1. Angle of incidence is always equal to the angle of reflection.
2. Incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane
17. Image formed by a mirror the left of the object appears on the right and the right appears on the left. This is known as lateral inversion
18. When all the parallel rays reflected from a rough or irregular surface are not parallel, the reflection is known as diffused or irregular reflection.
19. Sunlight, called white light, consists of seven colors. Splitting of light into its constituent colors is known as dispersion.
20. The iris is that part of eye which gives it its distinctive color. The iris controls the amount of light entering into the eye.
21. The retina contains several nerve cells. Sensations felt by the nerve cells are then transmitted to the brain through the optic nerve.
22. There are two kinds of cells–
1. cones, which are sensitive to bright light and
2. rods, which are sensitive to dim light. Cones sense color. At the junction of the optic nerve and the retina, there are no sensory cells, so no vision is possible at that spot. This is called the blind spot.
• Eyes of a crab are quite small but they enable the crab to look all around. So, the crab can sense even if the enemy approaches from behind. Butterflies have large eyes that seem to be made up of thousands of little eyes
• Solar system
1. The time period between one full moon to the next full moon is slightly longer than 29 days. In many calendars this period is called a month
2. Stars are present in the sky during the day-time also. However, they are not visible then because of the bright sunlight. Stars appear to move from east to west.
3. Pole star appears to be stationary from the Earth, because it is situated close to the direction of the axis of rotation of the Earth.
4. Constellation appears to move in the sky from east to west
1. Ursa Major can be seen in summer in early part of the night, Ursa Major is visible throughout the year from most of the northern hemisphere
2. Orion is another well-known constellation that can be seen during winter in the late evenings. Sirius, which is the brightest star in the sky, is located close to Orion
3. Cassiopeia is another prominent constellation in the northern sky. It is visible during winter in the early part of the night
5. Planets
1. Mercury - no satellite of its own
2. Venus - often called a morning or an evening star, rotates from east to west while the Earth rotates from west to east.
3. Earth - From space, the Earth appears blue-green due to the reflection of light from water and landmass on its surface. Axis of rotation of the Earth is not perpendicular to the plane of its orbit. The tilt is responsible for the change of seasons on the Earth
4. Mars - has two small natural satellites.
5. Jupiter - has faint rings and has 67 natural satellites
6. Saturn - definite rings and 62 number of satellites, least dense among all the planets. Its density is less than that of water
7. Uranus - rotates east to west like Venus and has highly tilted rotational axis
6. Equatorial and orbital planes - two planes are inclined to each other at an angle of 23.5º. This means that the ax is of the Earth is inclined to its orbital plane at an angle of 66.5º.
7. Comets - Comets are also members of our solar system. They revolve around the Sun in highly elliptical orbits. However, their period of revolution round the Sun is usually very long. A Comet appears generally as a bright head with a long tail
8. Meteors - A meteor is usually a small object that occasionally enters the earth’s atmosphere. At that time it has a very high speed. The friction due to the atmosphere heats it up. It glows and evaporates quickly. Body that reaches the Earth is called a meteorite.
9. Meteor shower is a celestial event in which a number of meteors are observed to radiate, or originate, from one point in the night sky.