Class 9 Science

Cell Organelles

  • Mitochondria, endoplasmic reticulum, Golgi complex
  • Lysosome, ribosome, plastids, vacuoles
  • Comparison of plant cell and animal cell
  • Cell division: Mitosis and meiosis

These are small structures inside the cell and are called cell organelles. Endoplasmic reticulum, mitochondria, lysosome, Golgi Body and plastids are the important cell organelles.

Mitochondria

Mitochondrion is a capsule-like structure. It is bound by double membrane. The inner membrane is projected into numerous finger-like structures, called cristae.

Mitochondria are the sites of cellular respiration. After cellular respiration, energy is stored in the form of ATP (Adenosine triphosphate) in mitochondria. Mitochondria have their own DNA and ribosomes and hence mitochondria can make their copy or replicate on their own. As mitochondria are the sites for generation of energy, they are also known as the Powerhouse of the Cell.

Plant cell

Endoplasmic Reticulum

Endoplasmic reticulum is a mesh-like structure which is composed of numerous tubes. It extends from the plasma membrane to the nuclear membrane. There are two kinds of endoplasmic reticulum, viz. smooth ER and rough ER. Rough ER has ribosomes on its surface which give it the rough appearance.

Function of ER

It serves as the transport channel in the cell. Substances are transported from cell membrane to cytoplasm and to nucleus and vice-versa. RER is also the site for some important biochemical processes.

Golgi Complex

Golgi Complex was discovered by Camillo Golgi. It is composed of many sac-like structures which are stacked one above another. Golgi complex is responsible for packaging of various substances in the cell.

Lysosome

Lysosome are small sac-like structures and they are derived from Golgi complex. Lysosome contains digestive enzymes.

Functions of lysosome

The enzymes in the lysosome digest foreign particles and thus destroy them. Sometimes, the lysosome may burst open and its content ends up digesting the contents of the cell. The cell gets killed in the process. Due to this, lysosome is also called the Suicide bag of the cell.

Ribosome

These are tiny dot like structures interspersed in the cytoplasm and also on the surface of Rough ER. Ribosome is responsible for protein synthesis.

Plastids

These are somewhat similar to mitochondria in appearance. Plastids are found in plant cells only. They are of two types, chromoplast and leucoplast. Colourful plastids are called chromomplast and colourless plastids are called leucoplast. Chloroplast is green in colour and is found in green parts of plants. Leucoplast are responsible for storage of substances. Plastids too have their own DNA and ribosome.

Vacuoles

These are fluid filled chambers and are often seen in many cells. Vacuoles are very large in plant cells. A plant cell usually has single but large vacuole. Such a vacuole fills almost the entire space inside the cell. Vacuoles are much smaller and very few in animal cells.

Plant Cell Vs Animal Cell

Plant cellAnimal cell
Cell wall presentCell wall absent
Plastids presentPlastids absent
Vacuole is usually very large and singleVacuoles are much smaller an may be absent

Cell Division

Cell division is necessary for growth and repair. As per the cell theory, any cell is formed from a pre-existing cell. There are two types of cell division, viz. mitosis and meiosis.

Mitosis:

Mitosis

Mitosis takes place in all the cells which are responsible for growth and repair. After mitosis, two daughter cells are formed from a mother cell. The number of chromosomes in daughter cells is same as in mother cell.

Meiosis:

Meiosis

Meiosis takes place during gametogenesis, i.e. formation of gametes. This means that pollen grains and eggs are formed through meiosis. Similarly, sperms and ovules are formed through meiosis. After meiosis, four daughter cells are formed from a mother cell. The number of chromosomes in daughter cells is half the number in mother cell.

Quiz



See Answer

1: (a) Mitochondria, 2: (c) Mitochondria, 3: (d) Ribosomes, 4: (b) Transport, 5: (a) Packaging, 6: (b) Golgi body, 7: (d) Lysosome, 8: (c) Plastids, 9: (b) Two, 10: (d) Four


Matter in Our Surroundings

Anything that has both mass and volume is called matter. You can also say that anything which has mass and which occupies space is called matter.

Is Matter Around Us Pure?

Elements and compounds are pure substances. All other substances are mixtures which means they are not pure substances.

Atoms and Molecules

Read about law of conservation of mass, law of constant proportions and Dalton's atomic theory.

Structure of Atoms

Atom is made of three particles; electron, proton and neutron. These particles are called fundamental particles of an atom or sub atomic particles.

Cell: The Fundamental Unit of Life

A cell is capable of independent existence and can carry out all the functions which are necessary for a living being.

Tissue

A groups of cells which is meant for a specific task is called tissue. Tissues are the first step towards division of labour in complex organisms.

Diversity in Living Organisms

Without proper classification, it would be impossible to study millions of organisms which exist on this earth.

Motion

If an object changes its position with respect to a reference point with elapse of time, the object is said to be in motion.

Force & Laws of Motion

Force has numerous effects. Force can set a stationary body in motion. Force can stop a moving body.

Gravitation

Earth attracts everything towards it by an unseen force of attraction. This force of attraction is known as gravitation or gravitation pull.

Work & Energy

When force is exerted on an object and object is displaced, work is said to be done. It means work is the product of force and displacement.

Sound

Sound is a type of energy. Sound travels in the form of wave from one place to another.

Why Do We Fall Ill

Health is a state of physical, mental and social well being. A condition in which the affected person is unable to carry out normal activities is termed as disease.

Natural Resources

Resources which are obtained from nature are called natural resources. Examples: Air, water, soil, wood, etc.

Improvement in Food Production

Food security is said to exist when all people, at all times, have physical and economic access to sufficient, safe and nutritious food to meet their dietary needs and food preferences for an active and healthy life.