The Fundamental Unit of Life
5.1-Cell-
•Cells are basic building units of all the organisms that are in our surroundings.
.•Discovered by Robert Hooke in the year 1665.
•He used primitive microscope for the observation.
•Cell is a Latin word that has the meaning ‘ a little room’.
•Types of organisms-
a)Unicellular=organisms which are made up of single cell.
Example-Amoeba, Paramoecium, Bacteria
b)Multicellular=group of many cells in a single body which perform different functions to form
various body parts.
Example-Fungi, Plants, Animals
5.2- Structural organisation of a cell-
1-Plasma membrane or cell membrane-
•outermost covering of the cell.
• The plasma membrane is flexible and made up of lipids and proteins.
•It separates the cell from an external environment.
•It allows the entry and exit of some materials in and out of the cell so-called a selectively permeable membrane. Materials like carbon dioxide or oxygen move across the membrane through the process of diffusion.
•Diffusion – It is a process of movement of materials from the region of high concentration to that of low concentration.
•Osmosis – It is a process by which water moves through such a selectively permeable membrane.
Three conditions can be possible:
Hypotonic solution- Surrounding of a cell has a higher water concentration than that of the cell, so the cell will gain water. This solution is called a hypotonic solution. In this process, a cell is likely to swell up.
Isotonic solution- Medium has exactly the same water concentration as that of the cell, so there will be no movement of water across the membrane. Such a solution is called an isotonic solution. In this, the cell with stay the same size.
Hypotonic solution – If the medium has a lower concentration of water than the cell. This time water leaves the cell in a larger amount as comparing the entered rate and cell shrinks.
2-Cell wall-
•Present in plant cell only.
•It is additional rigid covering outside the plasma membrane.
•It is made up of cellulose which provides structural strength to plants.
•Plasmolysis is a process when a living plant cell loses water through osmosis there is shrinkage
or contraction of the contents of the cell away from the cell wall.
3- Nucleus-
• It has a double layer covering which is called a nuclear membrane, which has pores that allowed the transfer of material from inside the nucleus to its outside.
•It contains Chromosomes which are visible as rod-shaped structures only when the cell is about to divide. It contains information of Inheritance of characters from parent to their offsprings. Chromosomes are composed of DNA and protein.DNA contains genes.
•The nucleus plays important role in-
– Cellular reproduction.
-Determining way the cell will develop and what form it will exhibit at maturity.
•Prokaryotes- Cells which are lack in a nuclear membrane and cytoplasmic organelles.
Example- Bacteria
•Eukaryotes- Cells in which nuclear membrane and cytoplasmic organelles are present.
4-Cytoplasm-
•It is the fluid content inside the plasma membrane.
•It contains many specialised cell organelles which perform a specific function. These cell organelles are enclosed by membranes.
Prokaryotes-absent
Eukaryotes-present
5-Cell organelles-
•There are membrane-bound little structures within cells to separate different chemical activities from each other.
•This is a feature of eukaryotic cells.
•Some of these organelles are visible only with an electron microscope.
•Important examples of cell organelles which we have to discuss are:
1-Endoplasmic Reticulum(ER)
2-Golgi apparatus
3-Lysosomes
4-Mitochondria
5-Plastids
6-Vacuoles
Let us discuss it one by one-
5.1-Endoplasmic Reticulum(ER)
-It is a large network of membrane-bound tubes and sheets.
-It is round or long tubules or oblong bags.
-Two types of ER:
a)Rough endoplasmic reticulum(RER)-It has rough surface because ribosomes are
attached to its surface. Ribosomes are sites of protein manufacture which sent to various parts of a cell depending on need using ER.
b)Smooth endoplasmic reticulum(SER)- It helps in the manufacture of fat molecules or lipids. Some of these lipids help in building the cell membrane.
– Functions-
•ER is to serve as channels for the transport of materials between various. regions of the cytoplasm.
• It provides a surface for some of the biochemical activities of the cell.
• Detoxifying many poisons and drugs.
5.2-Golgi bodies
-Described by-Camillo Golgi.
-It consists of a system of membrane-bound flattened sacs arranged in parallel to each other in a stack for cisterns.
-Function-
- Storage, Modification and packaging of products in vesicles.
- Conversion of simple sugar into complex sugar.
- Formation of lysosomes.
5.3-Lysosomes-
-Kind of waste disposal system of the cell.
-Function-
•Digest any foreign materials to keep the cell clean by breaking complex substance into a simpler form.
5.3 Mitochondria-
– They are also known as a powerhouse of the cell.
-It has two membranes covering:
- Outer membrane.-porous
- Inner membrane-deeply folded which increases the surface area of ATP- generating chemical reactions.
- ATP from mitochondria which release energy for chemical reactions.
-Mitochondria are able to make some of the protein.
5.4 Plastids
– They are present in plant cells only.
-They are of two types-
• Chloroplasts-
- Coloured plastids.
- Contain chlorophyll which makes photosynthesis in plants possible.
- Contain yellow or orange pigments.
•Leucoplasts-
- White colourless plastids.
- Starch, oil and proteins are stored.
5.5-Vacuoles-
They are storage sacs for solid or liquid contents.
Plants-Small sized vacuoles.
Animals-Large sized vacuoles.
Cell division-
Two types of cell division-
•Mitosis- Most of the cells divide for growth. Mother cell divides to form two identical daughters having the same number of chromosomes as that of the mother cell.
•Meiosis- Cells of reproductive organs in plants and animals divide to form gametes which after fertilization give rise to offsprings.
NCERT SOLUTION
Page-59
1-Who discovered cells, and how?
Answer:
The cell was discovered by Robert Hooke in the year 1665 with the help of a primitive microscope. He observed that shape of cork is similar to that of honeycomb with tiny blocks in it. These blocks were named as cells.
2. Why is the cell called the structural and functional unit of life?
Answer:
It is called a structural and functional unit because all the organisms in our surrounding are made up of basic structural unit which we call cell. Cells on combining form tissues, then tissue combines to form organisms.
Page- 61
1-How do substances like CO2 and water move in and out of the cell? Discuss.
Answer:
CO2move in and out of the cell by the process of diffusion in which movement takes place from higher concentration to the lower concentration. While water moves by the process osmosis from the region of high concentration to low concentration.
2- Why is the plasma membrane called a selectively permeable membrane?
Answer:
Plasma membrane allows only selective materials to pass through them so they are called as a selectively permeable membrane.
Page: 63
1- Fill in the gaps in the following table illustrating differences between prokaryotic and eukaryotic cells.
| Prokaryotic Cell | Eukaryotic Cell |
| 1. Size: Generally small (1-10 μm)1 μm = 10-6m
2. Nuclear region: ______________________________ ______________________________ and known as ___________________ 3. Chromosome: single 4. Membrane-bound cell organelles absent. |
1. Size: Generally large (5-100 μm)
2. Nuclear region: well defined and surrounded by a nuclear membrane.
3. More than one chromosome. 4. ______________________________ ______________________________ ______________________________ |
Answer:
| Prokaryotic Cell | Eukaryotic Cell |
| 1. Size: Generally small (1-10 μm)1 μm = 10-6m
2. The nuclear region: Poorly defined as the nuclear membrane is absent and known as the nucleoid. 3. Chromosome: single 4. Membrane-bound cell organelles absent. |
1. Size: Generally large (5-100 μm)
2. Nuclear region: well defined and surrounded by a nuclear membrane.
3. More than one chromosome. 4. Cell organelles are present |
Page 65
1- Can you name the two organelles we have studied that contain their own genetic material?
Answer:
Mitochondria and Plastids are two organelles which have their own genetic material.
2- If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?
Answer:
Lysosomes may burst and the enzymes digest their own cell.
3-Why are lysosomes known as suicide bags?
Answer:
When the cell gets damaged, lysosomes may burst and enzymes digest their own cell. So they are called suicide bags.
4-Where are proteins synthesised inside the cell?
Answer:
Ribosomes are responsible for protein synthesis.
Exercise
1- Make a comparison and write down ways in which plant cells are different from animal cells.
Answer:
| Animal cell | Plant cell | |
| •Small in size. | •Larger in size as compared to the animal cell. | |
| •Cell wall is absent. | •Cell wall is covered with a plasma membrane. | |
| •Temporary vacuoles are present. | •Permanent vacuoles are present. | |
| •Plastids are absent | •Plastids are present. | |
2- How is prokaryotic cell different from a eukaryotic cell?
Answer:
| Prokaryotic Cell | Eukaryotic Cell |
| 1. Size: Generally small (1-10 μm)1 μm = 10-6m
2. The nuclear membrane is absent. 3. There is a single chromosome. 4. Membrane-bound cell organelles absent. |
1. Size: Generally large (5-100 μm)
2. The nucleus is surrounded by a nuclear membrane. 3. There are more than one chromosomes. 4. Cell organelles are present |
3-What would happen if the plasma membrane ruptures or breaks down?
Answer:
The plasma membrane is a semi-permeable membrane which allows some materials to pass through them through the process of osmosis and diffusion. If this membrane breaks down, the process of osmosis and diffusion will not take place and materials will freely move in and out of the cell.
4. What would happen to the life of a cell if there was no Golgi apparatus?
Answer:
The cell will be not able to –
•store, modify and pack products in vesicles.
•convert simple sugar into complex sugar.
•Form lysosomes.
5. Which organelle is known as the powerhouse of the cell? Why?
Answer:
Mitochondria is the powerhouse of the cell because it releases the energy in the form of ATP(Adenosine triphosphate) which is necessary for various essential chemical activities.
6. Where do the lipids and proteins constituting the cell membrane get synthesised?
Answer:
Lipids and proteins are synthesised in the Endoplasmic Reticulum.
7-How amoeba obtain its food?
Answer:
Amoeba has a flexible plasma membrane which creates pseudopodia (false feet ). This process of taking food through pseudopodia is called endocytosis. Pseudopodia trap the food by forming a vacuole around it. After trapping food, a cell will secret enzymes that digest the food.
8-what is osmosis?
Answer:
Osmosis is a process of movement of water from high concentration to low concentration through a semi-permeable membrane.
9- Carry out the following osmosis experiment:
Take four peeled potato halves and scoop each one out to make potato cups. One
of these potato cups should be made from a boiled potato. Put each potato cup in
a trough containing water. Now,
(a) Keep cup A empty
(b) Put one teaspoon sugar in cup B
(c) Put one teaspoon salt in cup C
(d) Put one teaspoon sugar in the boiled potato cup D.
Keep these for two hours. Then observe the four potato cups and answer the
following:
Explain why
1-Explain water gathers in the hollow portion of B and C
Answer:
Water concentrations are different in both region. Concentration is higher in surrounding of potato comparatively so that process of osmosis takes place and water gathers in hollowed portion of B and C.
2-Why is potato A necessary for this experiment?
Answer:
It is necessary because by this we can find out that potato cavity is not affecting water movement.
3-Explain why water does not gather in the hollowed-out portions of A and D
Answer:
Cup A is empty so that there are no two regions, so the process of osmosis is not possible and in result cup A is hollow. Cup D has dead plasma membrane so osmosis does not occur.
10. Which type of cell division is required for growth and repair of the body and which type is involved in the formation of gametes?
Answer:
•Type of cell division that is involved in the growth and repair of body –Mitosis.
•A type of cell division which results in the formation of gametes.- Meiosis