1. Cell-
ü Word cell came from Latin word cella, meaning small room.
ü It is the basic structural, functional and biological unit of all known living organisms.
ü A cell is the smallest unit of life that can replicate independently, and cells are often called the building blocks of life.
ü There are billions and trillions of cells in a tall tree or a large animal.
ü A human body also has trillions of cells which have different shapes, sizes and functions.
2. Discovery Of Cell-
ü Robert Hooke discovered cells in 1665.
ü Hooke saw honeycomb like structure when he observed thin slices of cork under a microscope.
ü He noticed that each box was separated from the other by a partition or a wall.
ü He was the one to name each of these boxes as cells.
ü Scientists could study cells of living organisms only when improved microscopes with high magnification were invented – 150 years after the Hooke first observed the cells.
ü In 1830s, two German scientists called Matthias Schleiden and Theodor Schwann proposed the Cell Theory which stated-
n All living organisms are made of cells.
n Cells are basic structural or functional units of living organisms.
n All cells are born out of pre-existing cells through cell division.
3. Multicellular Organisms And Unicellular Organisms-
ü They are made up of more than one cell. For Example, Mango Tree and Frog.
ü They are made up of a single cell. For Example, Amoeba and Paramecium.
ü Multicellular organisms with billions of cells also start their life as a single cell.
ü The fertilized egg multiplies to form more cells as the organism develops.
ü Single-celled organisms also perform all the necessary life functions like multicellular organisms, including -
n ingestion and digestion of food, respiration, excretion, reproduction and growth.
ü The only difference is that while a single cell performs all the functions in the unicellular organisms, multicellular organisms have a specialized group of cells to perform different functions. The specialized cells form tissues, which in turn form organs.
4. Different Shapes Of Cells-
Figure 01- Amoeba and paramecium
ü Amoeba, which is a single-celled organism, does not have a definite shape.
ü Its shape keeps changing as it moves or feeds, protruding parts of its body to form 'pseudo' (meaning 'false') 'podia' (meaning 'feet').
ü These projects are known as pseudopodia.
ü Humans have millions of cells, such as white blood cells (found in blood), cells that make up muscles, and cells that form nerves.
ü Their shapes vary according to the functions they perform.
ü Most cells are round, spherical or elongated. Some are spindle-shaped which are long and pointed at both ends. Nerve cells or neurons are quite long and are branched out to receive and transfer messages.
Figure 02- Different shapes of human cells
ü Shapes of the cells are maintained by a covering called Cell Membrane or Plasma Membrane.
ü Bacterial cells and plant cells have an additional rigid covering called a Cell Wall.
ü The size of the cells may vary from a millionth of a meter (known as a micron) to a few centimetres but most of them cannot be seen with the naked eye. We need microscopes to see these microscopic cells.
ü It is not necessary that the size of cells of a large animal (such as an elephant) will be larger than cells of a small animal (such as a rat).
ü The size of the cell depends on the function it performs.
ü For example, nerve cells of a rat and nerve cells of an elephant perform the same functions and hence, are of the same size.
5. Peel An Onion -
Experiment-
ü Place a small piece of the dry and thin onion peel in a drop of water on a glass slide.
ü Put a drop of methylene blue solution on this thin layer and place a coverslip on it (while ensuring that no air bubbles get trapped in the coverslip).
ü Place the slide under the microscope. Onion peel cells under microscope.
Figure 03- Onion peel cells under microscope
Inference-
ü The boundary of the onion cell is a cell membrane, which is covered by another thick covering called the cell wall.
ü The dense round body in the centre of the cell is called the nucleus.
ü The jelly-like substance between the cell membrane and the nucleus is called the cytoplasm.
6. An Animal Cell-
Experiment-
ü Scrape the inside of your cheek lightly with a clean toothpick.
ü Place it in a drop of water on a glass slide.
ü Add a drop of iodine solution or methylene blue solution and place a coverslip on it.
ü Place the slide under the microscope.
Figure 04- Cheek cells
Inference-
ü Cell membrane (Cell wall is absent in animal cells),
ü Nucleus, and Cytoplasm.
ü There are some other organelles (or components of cells) too. These include mitochondria, ribosomes, and golgi bodies.
7. Organelles Found In Both Plant And Animal Cells Are-
ü Endoplasmic Reticulum- It is a network of canals made up of membranes that encloses a fluid-filled lumen. It is of two types viz-
n Smooth Endoplasmic Reticulum -It does not have any ribosomes and hence, look smooth. It plays a key role in synthesis of lipids.
n Rough Endoplasmic Reticulum - It is lined with ribosomes and hence, look rough. It plays a key role in synthesis of protein as ribosome are attached to it.
ü Ribosomes- They are protein factory of cells.
n Ribosomes are spherical bodies made up of ribonucleic acid (RNA) and protein enzyme. They do not have membranes and are present separately in cytoplasm.
n Ribosomes are the sites where the protein synthesis takes place.
ü Golgi Bodies- Smooth, flattened sac-like structures called Cisternae stack together in parallel rows to form Golgi bodies.
n Golgi Cis face receives protein from Endoplasmic reticulum and modifies, packages and stores it.
n It also dispatches proteins in vesicles to various destinations.
n The Golgi apparatus are responsible for the secretion of enzymes, hormones and proteins.
ü Mitochondria- They are rod-shaped organelles with a double membrane.
n The outer membrane is smooth while the inner membrane folds over many times to form cristae. Cristae increase the surface area of the inner membrane by several times.
n Matrix is the fluid inside the mitochondria.
n Mitochondria act as energy production sites and are hence, known as the Powerhouses of the Cell.
Glucose + Oxygen → Carbon Dioxide + Water + Energy (ATP)
ü Lysosome - A single membrane organelle
n They are sac-like structures surrounded by single-layered membranes.
n They contain powerful digestive enzymes that can break down all organic material.
n It acts as a mini digestive system within the cell.
n Lysosomes digest damaged cells and a variety of extra- and intra-cellular material. Since they remove cell organelles that are worn out or are not functioning properly and may even digest the entire cells, they are also known as Suicidal Bags.
ü Vacuoles- They are organelles enclosed by a membrane and filled with fluid. Plant cells usually have a large vacuole filled with a liquid called ‘cell sap’.
n Cell Sap contains dissolved sugar and salts.
n Animal cells may or may not have vacuoles. If they do have vacuoles, they are much smaller than the ones found in plant cells.
n Vacuoles in plant cells keep the cells firm or turgid. They store various substances (including waste products of the cell).
n Vacuoles in animal cells store food, water, sugar, minerals and waste products of the cell. In Amoeba, vacuoles that contain food particles are also referred to as food vacuoles.
ü Plastids- They are also type of organelles that are found only in plant cells.
n With double-layered membrane, these organelles are found in cytoplasm of the cells.
n They are of two types with respect to the colour of the pigment they contain viz-
u Leucoplasts: These colourless organelles store starch or other plant nutrients. For Example, Starch stores in potato cells. Leucoplast are of different types :
u Amyloplast: stores starch
u Elaioplast: stores fat
u Proteinoplast/ Aleuroplast: stores protein
u Chromoplasts: These contain different-coloured pigments. Most important type of chromoplasts is chloroplast which contain green-coloured pigments called Chlorophyll.
Note: Chloroplasts are the sites where photosynthesis takes place. Here, carbon dioxide and water combine in the presence of energy from the sunlight to produce food. Hence, chloroplasts help in synthesis of food by green plants.
CO2 + H2O → Glucose + Oxygen
(reaction occur in the presence of chlorophyll + sunlight)
Figure 05- Plant and animal cell
Table 01- Animal vs. Plant cell
8. Osmosis-
ü When the solvent moves from the area of dilute solution to the area of concentrated solution through a semipermeable membrane in order to equalize the concentration level of both the solutions, the process is known as osmosis. For example, roots absorb water from the soil through osmosis.
9. Diffusion-
ü When the particles of any material move from a region of higher concentration to that of lower concentration until the equilibrium is reached, the process is known as diffusion. For example, when you spray perfume, its fragrance spreads in air through diffusion.
Q. What is the significance of cell wall in plants?
Ans.- A cell wall is the outer thick layer in plant cells that protect the cell membrane. Since plants cannot move from their place, this cell wall serves to protect their cells from the possible damage by temperature variations, high wind speed, atmospheric moisture etc.
10. Prokaryote-
ü Bacterial cells and cells of blue-green algae do not have well-defined nuclei (plural of nucleus) like those of multi-cellular organisms.
ü The cells of such organisms have nuclear materials without the nuclear membrane. Such cells are called prokaryotic cells where 'pro' stands for 'primitive' and 'karyon' stands for 'nucleus'.
ü Organisms with prokaryotic cells are known as prokaryotes.
11. Eukaryotes-
ü Plant and animal cells with well-organised nucleus with nuclear membrane are called eukaryotic cells.
ü 'Eu' stands for 'true' while 'karyon' stands for 'nucleus'.
ü Organisms with eukaryotic cells are known as Eukaryotes.
Table 02- Prokaryotes vs. Eukaryotes
NCERT PRACTICE QUESTIONS
1. Indicate whether the following statements are True (T) or False (F).
(a) Unicellular organisms have one-celled body. (T/F)
(b) Muscle cells are branched. (T/F)
(c) The basic living unit of an organism is an organ. (T/F)
(d) Amoeba has irregular shape. (T/F)
Ans.- a)True b) False c) False d) True
2. Make a sketch of the human nerve cell. What function do nerve cells perform?
Ans.-
3. Write short notes on the following.
(a) Cytoplasm (b) Nucleus of a cell
Ans.(a) Cytoplasm- The cytoplasm is a fluid that contains all the cell organelles such as the Mitochondria, Ribosomes, Golgi bodies, etc. It is present between the nucleus and the plasma membrane. It helps in the exchange of materials between cell organelles. It is made up of eighty percent water and is usually clear and colourless.
(b) The nucleus of a cell- The Nucleus is a double-membrane bound cell organelle present in eukaryotic cells. It contains the DNA, the genetic material. It is the command centre of the cell and is spherical in shape. It has the following components- Nuclear membrane, Nucleolus and Chromosomes. Nuclear membrane- It has is a double-layered membrane. It separates the contents of the nucleus from the cytoplasm and acts as a wall. It has pores that allow the transfer of certain substances in and out of the cell. Nucleolus- It is a small dense spherical body. It is not bound by any membrane. Chromosome- These are thread-like structures that carry genes. All the necessary information required for the transfer of characteristics from the parents to the offspring are stored in the genes. Inheritance of characteristics is possible only because of chromosomes.
4. Which part of the cell contains organelles?
Ans.- Cytoplasm- It is a clear and colorless fluid that contains organelles like Mitochondria, Ribosomes, Golgi bodies.
5. Make sketches of animal and plant cells. State three differences between them.
Ans.-
6. State the difference between eukaryotes and prokaryotes.
7. Where are chromosomes found in a cell? State their function.
Ans.- Chromosomes are thread-like structures present in the nucleus that carry genes. All the necessary information required for the transfer of characteristics from the parents to the offspring are stored in the genes. Inheritance of characteristics is possible only because of chromosomes.
8. ‘Cells are the basic structural units of living organisms’. Explain.
Ans.- Various components of plants and animals are constituted by cells. It is the smallest unit of life and is capable of all living functions. They are the building blocks of life. That is the reason why cells are referred to as ‘the basic structural and functional blocks of life’.
9. Explain why chloroplasts are found only in plant cells.
Ans.- Chloroplasts are plastids required for the food making process, called photosynthesis, and thus they are only present in plant cells.
10. Complete the crossword with the help of clues given below.
Across
1. This is necessary for photosynthesis.
3. Term for component present in the cytoplasm.
6. The living substance in the cell.
8. Units of inheritance present on the chromosomes.
Down
1. Green plastids.
2. Formed by collection of tissues.
4. It separates the contents of the cell from the surrounding medium.
5. Empty structure in the cytoplasm.
7. A group of cells.
Ans.-
Junior Science Topic - Cell Structure And Function. Chapter notes, video lectures, power point presentation and NCERT exercise solution. (Class 8, CBSE syllabus, chapter 08)
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