#6 Summary of Cell structure

The basic unit of life, the cell, can be seen clearly only with the aid of microscopes.

The light microscope uses light as a source of radiation, whereas the electron microscope uses electrons.

• The electron microscope has greater resolution (allows more detail to be seen) than the light microscope, because electrons have a shorter wavelength than light.

• With a light microscope, cells may be measured using an eyepiece graticule and a stage micrometer.

• Using the formula A= I/M, the actual size of an object (A) or its magnification (M) can be found if its observed (image) size (I) is measured and A or M, as appropriate, is known.

• All cells are surrounded by a partially permeable cell surface membrane that controls exchange between the cell and its environment.

• All cells contain genetic material in the form of DNA, and ribosomes for protein synthesis.

• The simplest cells are prokaryotic cells, which are thought to have evolved before, and given rise to the much more complex and much larger eukaryotic cells.

• Prokaryotic cells lack a true nucleus and have smaller ribosomes than eukaryotic cells. They also lack membrane-bound organelles. Their DNA is circular and lies naked in the cytoplasm.

• All eukaryotic cells possess a nucleus containing one or more nucleoli and DNA. The DNA is linear and bound to proteins to form chromatin. The cytoplasm contains many membrane-bound organelles providing separate compartments for specialised activities (division of labour). Organelles include endoplasmic reticulum (ER), 80S ribosomes, mitochondria, Golgi apparatus and lysosomes. Animal cells also contain centrioles. Plant cells also contain chloroplasts, often have a large, permanent, central vacuole and have a cell wall containing cellulose. In eukaryotes, cells may be further organised into tissues, organs and systems.

1. Multiple - Choice Test 

1 Which type of membrane would be present in the largest quantity in a prokaryotic cell?

A    cell surface membrane
B    mitochondrial cristae
C    nuclear envelope
D    smooth endoplasmic reticulum

2 Which type of cell would contain the greatest relative numbers of mitochondria?

A    bacterial cell
B    mesophyll cell
C    muscle cell
D    parenchyma cell

3 In a cell that is specialised for secreting protein, which of the following would be present in relatively large amounts?

A    cell surface membrane
B    Golgi vesicles
C    lysosomes
D    smooth endoplasmic reticulum

4 Which structure could be described as a microtubule-organising centre?

A    centriole
B    Golgi apparatus
C    nucleus
D    spindle

5 What are microtubules made of?

A    cellulose
B    DNA
C    lipid
D    protein

6 Which structure could be found in a plant cell but not in a prokaryotic cell?

A    20 nm ribosomes
B    cell surface membrane
C    circular DNA
D    thylakoid

7 Which organelle makes lysosomes?

A    Golgi apparatus
B    nucleus
C    ribosome
D    smooth endoplasmic reticulum

8 A protein that is to be secreted from a cell would pass through a sequence of cell organelles in the following order:

A    Golgi apparatus → rough endoplasmic reticulum → secretory vesicle
B    Golgi apparatus → secretory vesicle → rough endoplasmic reticulum
C    rough endoplasmic reticulum → Golgi apparatus → secretory vesicle
D    secretory vesicle → Golgi apparatus → rough endoplasmic reticulum

9 A scientist calibrating an eyepiece graticule would notice what change when switching from a low-power lens to a high-power lens?

A    The eyepiece units would appear closer together.
B    The eyepiece units would appear further apart.
C    The stage micrometer units would appear closer together.
D    The stage micrometer units would appear further apart.

10 What explains the fact that an increase in the voltage used in a transmission electron microscope results in an increase in the resolution obtained?

A    The electromagnetic lenses function more efficiently.
B    Increasing the voltage increases the magnification.
C    The electron beam can penetrate the specimen more easily.
D    The wavelength of the electrons is shortened.

Answers to Multiple choice test

1  A
2  C
3  B
4  A
5  D
6  D
7  A
8  C
9  D
10 D 

2. End-of-chapter questions

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 centriole
B chloroplast
C Golgi apparatus
D cell surface membrane

4. Copy an complete the following table, which compares light microscopes with electron microscopes. Some boxes have been filled in for you.

Feature	Light microscope	Electron microscope
Source of radiation
Wavelength of radiation		About 0.005 nm
Maximum resolution		0.5 nm in practice
Lenses	Glass
Specimen		Non-leaving or dead
Stains	Coloured dyes
image	coloured

5. List ten structures you could find in an electron micrograph of an animal cell which would be absent form the cell of a bacterium.

6. Advice on answering question 6: If you are asked to distinguish between two things, it is likely that it is because they have certain things in common and that they may even be confused with each other. In your answer it is helpful where relevant to point out similarities a well as differences. Remember that for organelles there may be differences in both structure and function.

Distinguish between the following pairs of terms:

a magnification and resolution
b light microscope and electron microscope
c nucleus and nucleolus
d chromarin and chromosome
e membrane and envelope
f smootth ER and rough ER
g prokaryote and eukaryote
h tissue and organ (include one example of each in an animal and in a plant)
i xylem and phloem
j epidermis and epithelium
k palisade mesophyll and spongy mesophyll

7 List:
a three organelles each lacking a boundary membrane
b three organelles each bounded by a single membrane
c three organelles each bounded by two membranes (an envelope).

8 Identify each cell structure or organelle from its description below.

a  manufactures lysosomes
b manufactures ribosomes
c site of protein synthesis
d can bud off vesicles which form the Golgi apparatus
e can transport newly synthesised protein round the cell
f manufactures ATP in animal and plant cells
g controls the activity of the cell, because it contains the DNA
h carries out photosynthesis
i can act as a starting point for the growth of spindle microtubules during cell division
j contains chromatin
k partially permeable barrier only about 7 nm thick

1 organelle about 25 nm in diameter

9 The electron micrograph shows part of a secretory cell from the pancreas. The secretory   vesicles are Golgi vesicles and appear as dark round structures. The magnification is x 8 000.

a Copy and complete the table. Use a ruler to help you find the actual sizes of the structures.    Give your answers in micro metres.

  

    

Structure	Observed  diameter  (measured   with  ruler)	Actual size
maximum diameter of a Golgi vesicle
maximum diameter of  nucleus
maximum length of the labelled  mitochondrion

 [9]

b Make a fully labelled drawing of representative parts of the cell. You do not have to draw everything, but enough to show the structures of the main organelles. Use a full page of plain paper and a sharp pencil. Use Figures 1.16 and 1.17 in this book and the simplified  diagram in d below to help you identify the structures.                                                                    [14]

c The mitochondria     in pancreatic    cells are  mostly   sausage-shaped    in three  dimensions.    Explain   why  some  of the mitochondria    in the  EM  appear   roughly   circular.                                                                                                                          [1]

d The figure below shows a diagram based on an electron micrograph of a secretory cell from the  pancreas.

This type of cell is specialised   for  secreting (exporting)  proteins.  Some of the proteins are digestive enzymes of the pancreatic  juice. The cell is very active, requiring a lot  of energy.  The arrows  show the route  taken by the protein molecules.

i Describe  briefly  what  is happening  at each of the stages A, B, C and D.          [8] 

ii  Name one molecule or structure  which leaves the nucleus by route  E.          [1]

iii  Through which structure  must the molecule or structure you named in ii pass to get  through the nuclear  envelope?                                                                                         [1] 

iv  Name the molecule  which leaves the mitochondrion in order to provide  energy for this  cell.                                                                                                                                            [1]  

[Total:  35]

10 One technique used to investigate the activity of cell organelles is called differential centrifugation. In this technique, a tissue is homogenised (ground in a blender),    placed in tubes and spun in a centrifuge. This makes organelles sediment (settle) to the bottom of the tubes. The larger the organelles, the faster they sediment. By repeating the process at faster speeds, the organelles can be separated from each other according to size. Some liver tissue was  treated in this way to separate ribosomes, nuclei and mitochondria. The centrifuge was spun at  1000g, 10 000g or  100 000g ('g' is gravitational  force).

a    In which   of the  three  sediments -   1000 g, 10 000  g or  100 000 g -  would you expect to  find the following?

i  ribosomes

ii  nuclei

iii mitochondria

b Liver tissue contains many lysosomes. Suggest why this makes it difficult to study  mitochondria using the differential centrifugation technique.

Answers to end of chapter questions

1   A
2   C
3   A
4

5   nucleus; (smooth) endoplasmic reticulum; rough endoplasmic reticulum; 25 nm/larger/80S ribosomes; linear/non-circular DNA; chromatin/chromosome(s); lysosome(s); Golgi apparatus; mitochondrion/mitochondria; centriole(s); vacuole(s); microvilli; cilium/cilia; nucleolus/nucleoli; nuclear envelope; nuclear pore(s);

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; a statement linking the terms, such as both terms used with reference to microscopy;

b  light microscope uses light as a source of radiation; electron microscope uses electrons as a source of radiation;

c  both organelles/both found in eukaryotic cells; nucleolus is located inside nucleus; nucleus controls cell activity; nucleolus makes ribosomes;

d  chromatin and chromosomes both contain DNA (and protein/histones)/both found in nucleus;
chromatin is the loosely coiled form of chromosomes; chromatin is the form that exists between cell/nuclear divisions; chromosomes are formed just before/during, cell/nuclear division;

e  an envelope consists of two membranes (one just inside/outside the other); a membrane is a thin (partially permeable) barrier found around cells and some organelles; example of at least one organelle surrounded by an envelope is given; membranes found in/around all cells, envelopes only in eukaryotes;

f  both consist of flattened membrane-bound sacs; both found spreading through cytoplasm of
eukaryotic cells; smooth ER lacks ribosomes, rough ER has ribosomes on surface; one function of smooth ER given, e.g. makes lipids/steroids;rough ER transports proteins made by ribosomes
on its surface;

g   prokaryotes have no nucleus, eukaryotes have nucleus; prokaryotes are smaller/simpler;prokaryotes have few organelles, eukaryotes have many organelles, some membrane-bound/ compartmentalisation/more division of labour; eukaryotes evolved from prokaryotes;

h  tissue is a group of cells specialised for a particular function; one animal and one plant example given; organ is a group of tissues specialised for a particular function; one animal and one plant example given;

i both found in plants; both (complex) tissues; both vascular tissues/involved in (long distance)
transport; xylem transports water and mineral salts; phloem transports organic solutes/sugar/sucrose;

j both are tissues; both cover/protect surfaces; epidermis in plants, epithelium in animals; epidermis one cell thick, epithelium one or more cells thick; epidermis may be covered with (waxy) cuticle;

k mesophyll cells are found in leaves; palisade is upper layer/just below (upper) epidermis/above spongy mesophyll/OR spongy mesophyll is below palisade mesophyll; both contain chloroplasts/specialised for photosynthesis; palisade mesophyll cells are column-shaped,
spongy mesophyll cells are irregular/pack together loosely/have large intercellular air spaces;
palisade mesophyll cells have more chloroplasts/more photosynthesis; spongy mesophyll cells allow gas exchange/circulation of carbon dioxide (for photosynthesis);

7 Any three appropriate organelles:
a   e.g. nucleolus; ribosome; centriole;
b   e.g. lysosomes; rough ER; smooth ER; Golgi apparatus;
c nucleus; mitochondrion; chloroplast;

8 a Golgi apparatus:
b nucleolus;
c ribosome;
d ER/rough ER;
e rough ER;
f mitochondrion;
g nucleus;
h chloroplast;
i centriole;
j nucleus;
k membrane;
l ribosome;

Exam-style questions 

9 a 1 mark for each accurately measured ‘observed size’ (to within ± 2 mm) and 1 mark for each            accurately calculated ‘actual size’'
     1 mark for applying the formula A= I/M
     1 mark for measuring in mm and converting mm to μm for each calculation;

      1 mark for rounding up actual size to no more than one decimal place;                             [9]

b quality of drawing:
sharp pencil used; more than half of available space used; clean, continuous lines/not sketchy;
interpretation of structures accurate; representative parts of main organelles drawn, including those
below for which label marks are awarded;                                                                           [5]

labels:
nucleus; nuclear envelope; nuclear pore;
nucleolus; rough ER; ribosome(s);
mitochondrion; crista or cristae; Golgi apparatus;
Golgi vesicle/secretory vesicle;                                                                                     [max. 9]

c mitochondria will appear circular if they are cut, in transverse section/across (the long axis); [1]

d i A protein made on the ribosome is moving into the rough ER;
     B rough ER buds off small vesicles; vesicles fuse to form the Golgi apparatus; (therefore) protein
moves into Golgi apparatus; protein may be modifi ed/processed inside Golgi apparatus;
     C Golgi apparatus buds off Golgi vesicles;
     D Golgi vesicles travel to cell surface membrane; Golgi vesicle(s) fuses with cell surface membrane; protein/enzyme leaves cell; exocytosis/secretion;                                     [max. 8]
ii  ribosome/messenger RNA;                                                                                       [1]
iii  nuclear pore;                                                                                                          [1]
iv ATP;                                                                                                                      [1]
                                                                                                                           [Total: 35]

 10 a i 100 000 g
        ii 1000 g
        iii 10 000 g; [1]
 b lysosomes are, similar in size to/slightly smaller than, mitochondria;
 therefore sediment at same/similar, g force/speed;
 therefore contaminate mitochondrial sample; AW
 therefore cannot be sure whether eff ects
due to mitochondria or lysosomes in any experiments; [4]   [Total: 5]