Cambridge International AS / Cell Structure / Chapter-1/ Biology A' level

 

Cell Structure

Chapter - 1

A' Level

Q/A

             Q.1. What is cell?

               Ans: A cell is a unit of life. It consists of a mass of living matter called protoplasm.

Q.2. What is protoplasm?

Ans:  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. 

The use of special stains containing silver enabled the Golgi apparatus to be detected for the first time in 1898 by Camillo Golgi. The Golgi apparatus is part of a complex internal sorting and distribution system within the cell.

          The material between the nucleus and the cell surface membrane is known as cytoplasm.                               The cytoplasm is an aqueous material, varying from a fluid to a jelly-like                                           consistency. 

 

Differences between animal and plant cells:

* Plant cells are often larger than animal cells, although cell size varies enormously. Under the light microscope the centriole appears as a small structure close to the nucleus. 

* 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.

          Plant cells are linked to neighbouring cells by means of fine strands of cytoplasm called                           plasmodesmata, which pass through pore-like structures in their walls. 

* 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.

           Plant cells                                                  Animal cells

         1. Have cellulose cell walls.                      1. Do not have cellulose cell walls.

         2. Have chloroplasts.                                 2. Do not have chloroplasts.

          3. Always have a vacuole.                         3. Sometimes have a vacuole.

           4. Have a few different shapes.               4. Have many different shapes.

             Q.4. What does the cell nucleus do?

                Ans: The cell nucleus controls what a cell does, and how it develops.

           Q.5. What are chloroplast and what do they do?

            Ans: Chloroplasts are tiny discs full of green substance. They trap the light energy that                              plants need for making food by photosynthesis.

                  Q.6. Explain what a specialized cell is.

                  Ans: Some cells become specialized muscle cells. These form tissue which can tighten                           up and relax. Nerve cells are specialized and form tissue which can carry messages                          around the body.

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.

https://www.youtube.com/watch?v=Pu2PhjO2HpQ

            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.

Ans: The ribosomes are far too small to have any effect on the light waves. The general rule is that the limit of resolution is about one-half the wavelength of the radiation used to view the specimen. In other words, if an object is any smaller than half the wavelength of the radiation used to view it, it cannot be seen separately from nearby objects. This means that the best resolution that can be obtained using a microscope that uses visible light is 200 nm since the shortest wavelength of visible light is 400 nm. In practice, this corresponds to a maximum useful magnification of about 1500 times. Ribosomes are approximately 25 nm in diameter and can therefore never be seen using light.

 

6. a) Magnification is the number of times larger an image is compared with the real size of the object.

Resolution is the ability to distinguish between two separate points/ the greater the resolution, the greater the detail that can be seen. 

b) Light microscope uses light as a source of radiation.

     Electron microscope uses electrons as a source of radiation.

c) The nucleus is the largest cell organelle.

     Nucleolus gene control the activities of the cell and inheritance; thus the nucleus controls the cell's activities.

d) Chromatin and chromosomes both contain DNA and both found in nucleus.

   Chromatin is the loosely coiled form of chromosomes; Chromatin is the form that exists between cell/ nuclear divisions;

e) nuclear envelope the two membranes, situated closely together, that surroun the nucleus; the envelope is perforated with nuclear pores.

In animals and plants each cell is surrounded by a very thin cell surface membrane.

Rough ER is so called because it is covered with many tiny organelles called ribosomes. 

Smooth ER, so called becasue it lacks ribosomes, has a completely different function. It makes lipids and steroids, oestrogen and testosterone.

Organisms that lack nuclei are called prokaryotes.

Organisms whose cells possess nuclei are called eukaryotes. 

         7. a) nucleolus, ribosome, centriole

             b) lysosomes, rough ER, smooth ER

              c) nucleus, mitochondrion, chloroplast

                 8. a) manufactures lysosomes - Golgi apparatus

                     b) manufactures ribosomes - nucleolus

                      c) site of protein synthesis - ribosome

                       d) can bud off vesicles which form the Golgi body - ER / rough ER

                         e)  can transport newly synthesised protein round the cell - rough ER

                         f) manufactures ATP in animal and plant cells - mitochondrion

                         g) controls the activity of the cell, becasue it contains the DNA - nucleus

                         h) carries out photosynthesis - chloroplast

                          i) can act as a starting point for the growth of spindle microtubules during                                         cell division    - centriole

                                  j) contains chromatin - nucleus

                                k) partially permeable barrier only about 7 nm thick - membrane

                               l) organelle about 25 nm in diameter - ribosome

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

                 9. d. ii) ribosome/messenger RNA

                         iii) nuclear pore

                        iv) ATP