Biology
Cell Structure
Chapter - 1
Q/A
Q.1. What is protoplasm?
Ans: A cell is a unit of life. It consists of a mass of living matter called protoplasm. Protoplasm is a complex jelly-like substance in which endless chemical activities are carried out that ensure the life of a cell.
Q.2. What is cell biology?
Ans: The study of cells has given rise to an important branch of biology known as cell biology.
Q.3. Name the structures that animal and plant cells have in common, those found in only plant cells, and those found only in animal cells.
Ans: In animals and plants each cell is surrounded by a very thin cell surface membrane. This is also sometimes referred to as the plasma membrane.
Many of the cell contents are colourless and transparent so they need to be seen. Each cell has a nucleus, which is a relatively large structure that stains intensely and is therefore very conspicuous. The deeply staining material in the nucleus is called chromatin and is a mass of loosely coiled threads.
Mitochondria are only just visible, but films of living cells, taken with the aid of a light microscope, have shown that they can move about, change shape and divide.
Each cell has a nucleus, which is a relatively large structure that stains intensely and is therefore very conspicuous. The deeply staining material in the nucleus is called chromatin and is a mass of loosely coiled threads.
Differences between animal and plant cells:
* Plant cells are often larger than animal cells, although cell size varies enormously.
* With a light microscope, individual plant cells are more easily seen than animal cells, because they are usually larger and unlike animal cells, surrounded by a cell wall outside the cell surface membrane.
* Vacuoles are not confined to plant cells; animal cells may have small vacuoles, such as phagocytic vacuoles, although these are not usually permanent structures.
Chloroplasts are found in the green parts of plant.
Units of measurement:
one-thousandth = 0.001 mm
one millionth = 0.000 001 micrometre
one thousand millionth = 0.000 000 001 nanometre
Calculating the magnification of a photograph or image:
Magnification is the number of times larger an image is, than the real size of the object.
Magnification = observed size of the image/ actual size
M = I / A
Here, I = observed size of the image
A = actual size
Suppose we want to know the magnification of the plant cell labelled in the figure. If we know its actual length we can calculate its magnification using the formula
Step - 1:
Measure the length in mm of the cell in the photograph using a ruler. It is about 60 mm.
Step - 2:
Convert mm to μm
1 mm = 1000 μm
60 mm = 60 X 1000 μm
= 60,000 μm
Step - 3:
Use the equation to calculate the magnification.
magnification, M = image size, I / actual size, A
= 60,000 μm / 80 μm
= x 750
Q.4. a) Calculate the magnification of the drawing of the animal cell in the figure -1.
b) Calculate the actual length of the chloroplast labeled in the figure - 2.
The structure of the chloroplast as seen with the electron microscope is visible. Chloroplasts tend to have an elongated shape and a diameter of about 3 to 10 micro m.
The second stage of photosynthesis ( the light-independent stage) uses the energy and reducing power generated during the first stage to convert CO2 into sugars. This requires a cycle of enzyme-controlled reactions called the Calvin cycle and takes place in solution in the stroma. The sugars made may be stored in the form of starch grains in the stroma. The lipid droplets also seen in the stroma as black spheres in electron micrographs are reserves of lipid for making membranes or from the breakdown of membranes in the chloroplast.
Q.5. What is resolution?
Ans: resolution is the ability to distinguish between two objects very close together; the higher the resolution of an image, the greater the detail that can be seen.
Q.6. What is magnification?
Ans: Magnification is the number of times greater that an image is than the actual object;
magnification = image size / actual size of the object
Q.7. Explain why ribosomes are not visible using a light microscope.
1. Which one of the following cell structures can be seen with a light microscope?
A. mitochondrion
B. ribosome
C. rough ER
D. smooth ER
2. The use of electrons as a source of radiation in the electron microscope allows high resolution to be achieved because electrons:
A. are negatively charged
B. can be focused using electromagnets
C. have a very short wavelength
D. travel at the speed of light
3. Which one of the following structures is found in animal cells, but not in plant cells?
A. cell surface membrane
B. centriole
C. chloroplast
D. Golgi body
Mitotic Cell Cycle
Chapter-5
Learning Objectives:
a. Describe the structure of chromosomes
b. Describe the cell cycle- the cycle of events by which body cells grow to a certain size and then divide into two
c. explain how a nucleus divides into two genetically identical nuclei by mitosis.
d. prepare and observe a root tip squash in order to see stages of mitosis with a light microscope
e. explain the significance of mitosis
f. explain the significance of telomeres
g. explain the significance of stem cells
h. outline how uncontrolled cell division can lead to cancer.
1. During prophase of mitosis, chromosomes consist of two chromatids. At which stage of the cell cycle is the second chromatid made?a. cytokinesisb. G1c. G2d. S2. Growth of cells and their division are balanced during the cell cycle. Which column shows the consequences that would follow from the two errors shown in the table?3. A cell with four chromosomes undergoes a cell cycle including mitosis. Which diagram correctly shows the changes in chromatid number during interphase?4. Distinguish between the following terms: centrosome, centriole and centromere.5. The diagram shows three cells (labelled A, B, and C) from a root tip which have been stained to show chromosomes.a. identify the stage of mitosis shown by each cell.b. Describe what is happening at each stage.6. a. Diagram 1 below shows a plant cell dividing by mitosis. Only two of the many chromosomes are shown for simplicity.i. What stage of mitosis is shown?ii. Draw prophase for the same cell assuming the cell has only two chromosomes, as in diagram 1.b. Diagram 2 shows the same cell at telephase. The cell is beginning to divide and a new cell wall is forming. spreading out from the middle of the cell. Copy the diagram and add drawings of the chromosomes as they would appear at this stage.
Q.1. What is centrosphere?
Ans: Centrosphere is the differentiated layer of cytoplasm surrounding the centriole within the centrosome.
Q.2. What do you mean by genetic code?
Ans: The genetic code is the set of rules by which information encoded in genetic material (DNA and RNA sequences) is translated into the proteins (amino acid sequences) by living cells. This code is a set of three-letter combinations of nucleotides called codons, each of which corrresponds to a specific amino acid or stop signal.
Q.3. What is germ cell?
Ans: A germ cell is any biological cell that gives rise to the gametes of an organism that reproduces sexually.
Q.4. What is nucleic acid?
Ans: Nucleic acids are the polynucleotides having high molecular weight. The monomeric unit of which is nucleotide.
Q.5. What do you mean by genetic code?
Ans: A unit of heredity which is transferred from a parent to offspring and is held to determine some characteristic of the offspring. The correspondence between specific DNA base sequences and the amino acids they specify is called code.
Q.6. Differences between DNA and RNA are given below-
Physical structure - DNA - Double stranded
RNA - Single stranded
Chemical structure- DNA has two nucleotide strands which consist of its phosphate group, five-carbon sugar (the stable 2-deoxyribose), and four nitrogen-containing nucleobases: adenine, thymine, cytosine, and guanine.
RNA - Like DNA, RNA is composed of its phosphate group, five-carbon sugar (the less stable ribose), and 4 nitrogen-containing nucleobases: adenine, uracil (not thymine), guanine, and cytosine.
Types- DNA no types
RNA 5 types - tRNA, rRNA, mRNA, gRNA, and minor RNA
Replication - DNA occurs
- RNA does not occur.
Location - DNA is found in the nucleus, with a small amount of DNA also present in mitochondria.
RNA forms in the nucleolus, and then moves to specialized regions of the cytoplasm depending on the type of RNA formed.
Function- DNA replicates and stores genetic information. It is a blueprint for all genetic information contained within an organism.
RNA converts the genetic information contained within DNA to a format used to build proteins, and then moves it to ribosomal protein factories.
Q. 7. Which orgenelle is called the protein making factory of cell?
Ans: Ribosome is the cell organelle which is termed as the protein factory of the cell. It gathers amino acids and assembles them into protein chains to make almost anything the cell needs. A strand of ribonucleic acid, or messenger RNA, is the template. The process through which proteins are formed with the help of mRNA and tRNA is called translation.
Q.8. The difference between transcription and translation
1) transcription is the first step of gene expression process.
a) Translation is the second step of gene expression process.
2) Synthesises RNA copies of the genetic instructions written in the genome.
b) Synthesises proteins from RNA which are copied from genes.
3) Template is the genes in the genome.
c) Template is the mRNA.
4) Occurs in the nucleus.
d) Occurs in the cytoplasm.
5) RNA polymerase are the enzymes.
e) Ribosomes are enzymes.
Q.9. What is codon?