Module 2: Cell Structure

Question 1

Define Magnification

Answer 1

The number of times larger an image appears, compared with the size of the object.

magnification = image size / actual size

Question 2

Define Resolution

Answer 2

The clarity of an image; the higher the resolution, the clearer the image.

Question 3

Why are light microscopes good?

Answer 3

• cheap
• easy to use
• portable and able to use in the field as well as in labs
• able to use to study whole living specimens

Question 4

How do light microscopes work?

Answer 4

Light passes through a condenser lens then through the specimen

The light is then focused through the objective lens and then through the eyepiece lens

Light microscopes have several objective lenses that can be rotated to five different magnifications. Usually it
is x4, x10, x40, x100 (oil immersion lens)

The eyepiece lens is usually x10 and magnifies the image more.

Total magnification is eyepiece magnification x objective magnification

Question 5

State the resolution and magnification that can be achieved by a light microscope

Answer 5

Magnification ‐ most light microscopes can magnify up to x1500 maximum. (There is actually little limit to how much they can magnify but after x1500, the image becomes blurred due to low resolution.

Resolution ‐ the maximum resolving power using light is 200nm . This means that is 2 objects are closer than 200nm they will be seen as 1 object. The limit is due to the size of the wavelength of light ‐ 2 objects can only be distinguished if light waves can pass between them.

Question 6

Explain the need for staining samples in light microscopy

Answer 6

Some of these can be seen directly but many need to be stained as a lot of biological material is not coloured so you can’t see details.

Staining ‐ these are coloured chemicals that bind to chemicals on or in the specimen and allow it to be seen. Some cells bind to specific cell structures e.g. acetic orcein stains DNA dark red.

Sectioning ‐ specimens are embedded in wax and then thin sections are cut without distorting the structure. This is very useful for making sections of soft tissue like brain or lung.

Question 7

What is an eyepiece graticule?

Answer 7

A microscope eyepiece can be fitted with a graticule.

This is transparent with a ruler etched on it.

When looking at the specimen the eyepiece graticule scale it superimposed onto 
it and the specimen can be measured in eyepiece units (epu).

Eyepiece units are arbitrary (not of fixed length) - they represent different 
lengths at different magnifications.

The eyepiece scale has to be calibrated (its value worked out) for each different 
objective lens.

A stage micrometer (tiny ruler) is placed on the microscope stage. Each division on the ruler is usually 0.01mm (10µm)

The stage micrometer is compared with the eyepiece graticule scale to see how 
many eyepiece units are a length of the the stage micrometer

Then the size of each epu is calculated.

example:
40 epu = 1mm 
 (1000µm).
Therefore 1epu = 1000µm/40epu

Question 8

How do electron microscopes work?

Answer 8

• Electron microscopes generate a beam of electrons.
• Electromagnets are used to focus the electrons onto a specimen (instead of lenses).
• The image produced from the electron beam is projected onto a screen to make a black and white image (electron
micrographs).

Question 9

Why do electron microscopes have a higher resolution than light microscopes?

Answer 9

We can achieve higher resolutions with electron microscopes (than with light microscopes) because electron beams have a shorter wavelength than light so the resolution is greater.

Question 10

Transmission electron microscope (TEM)

Answer 10

Maximum magnification: X 500 000
Maximum resolution: 0.1nm

• A beam of electrons transmitted through the specimen.
• Denser parts absorb more electrons and makes them look darker on the image.

+ Highest resolution

• Can only be used with very thin prepared samples
• Produce 2D images

Question 11

Scanning electron microscope (SEM)

Answer 11

Maximum magnification: X 100 000
Maximum resolution: 10nm

• A beam of electrons is directed onto the sample and the electrons bounce off the surface to give a final 3D image

+ Can be used with 3D (dead) specimens
- Lower resolution (than TEM)

Question 12

Advantages and disadvantages of electron microscopes

Answer 12

Advantages:
+ SEM can produce 3D images that show shapes and cellular or tissue arrangement.
+ High resolution.
+ Can produce very detailed images of structures inside cells.

Disadvantages:

• Extremely expensive
• Electron beams are deflected by molecules in the air ‐ samples have to be placed in a vacuum and so specimens must be dead

Question 13

Compare and contrast light microscopes and electron microscopes

Answer 13

INSERT PICTURE

Question 14

What is the structure and function of a nucleus?

Answer 14

Structure:
The nucleus is surrounded by nuclear envelope (double membrane) studded with pores to allow mRNA and nucleotides through.

Contains genetic material in form of chromatin.

Contains nucleolus (ribosome factory)

Function:
Controls cell activities

Stores the organism’s genome.

Provides instructions for protein synthesis.

Question 15

What is the structure and function of a nucleolus?

Answer 15

Structure:
The nucleolus does not have a membrane around it. It contains RNA.

Function:
This is where ribosomes are made.

Question 16

What is the structure and function of a nuclear envelope?

Answer 16

Structure:
The nuclear envelope consist of a double membrane which is wrapped around the nucleus.

Function:
The nuclear envelope separates the contents of the nucleus from the rest of the cell.

In some regions the inner and outer membranes fuse together. At these points some dissolved substances and ribosomes can pass through.

Question 17

What is the structure and function of Smooth Endoplasmic Reticulum?

Answer 17

Structure:
A system of membrane bound sacs, containing cisternae.

There are no ribosomes on its surface.

Function:
SER contains enzymes that catalyse reactions involved with lipid synthesis, such as:
- synthesis of cholesterol 
- synthesis of lipids/phospholipids
- synthesis of steroid hormones 

It is involved with absorption, synthesis and transport of lipids from the gut.

Question 18

What is the structure and function of Rough Endoplasmic Reticulum?

Answer 18

Structure:
A system of membrane bound sacs, containing cisternae.

It is studded with ribosomes.

Function:
It provides a large SA for ribosomes, which assemble amino acids into proteins. These proteins then actively pass through the membrane into the cisternae and are transported to the golgi apparatus for modification and packaging.

Question 19

What is the structure and function of the golgi apparatus?

Answer 19

Structure:
Consists of a stack of membrane-bound flattened sacs.

Function:
To modify and package proteins for secretion in vesicles.

To form lysosomes.

Question 20

What is the structure and function of mitochondria?

Answer 20

Structure:
mitochondria are formed from 2 membranes separated by a narrow inter-membrane space.

the inner membrane is folded to increase its surface area into extensions called cristae. The central part is called the matrix.

Function:
aerobic respirations take place in the mitochondria and releases chemical energy in the form of ATP.w

Question 21

What is the structure and function of chloroplasts?

Answer 21

Structure:

• Green organelle with double-membrane (envelope)
• Inside is a colourless matrix – the stroma
• Contains flattened membrane sacs called thylakoids.
• These stack together to form a granum (pl. grana).
• The grana can be interconnected by tubular extensions called intergranal lamella
• Chlorophyll molecules are found on the thylakoid membranes and in the intergranal membranes

Function:
Chloroplasts are the site of photosynthesis.

Question 22

What is the structure and function of lysosomes?

Answer 22

Structure:
These are small bags formed by the golgi apparatus. Each is surrounded by a single membrane.
They contain powerful digestive enzymes.
They are abundant in phagocytic

Function:
Lysosomes keep powerful digestive enzymes away from the rest of the cell.
Lysosomes can engulf old cell organelles and foreign matter, digest them and return the digested components to the cell for reuse.

Question 23

What is the structure and function of centrioles?

Answer 23

Structure:
Centrioles consist of two bundles of microtubules at right angles to each other. The microtubules are made of tubulin protein subunits and are arranged to form a cylinder.

Animal cells only

No membrane

Function:
Before a cell divides, the spindle, made of threads of tubulin, forms from the centriole. Chromosomes attach to the spindle and motor proteins walk along the tubulin threads, pulling the chromosomes to opposite ends of the cell.

Question 24

What is the structure and function of vacuole?

Answer 24

Structure:
The vacuole is surrounded by a membrane called the tonoplast and contains fluid.

Function:
It is filled with water and solutes to maintain cell stability, because when fill it pushes against the cell wall, making it turgid.

If all the plant cells are turgid then this helps to support the plant.

Question 25

What is the structure and function of cilia?

Answer 25

• occur in large numbers on ciliated epithelial tissue
• less than 10 micrometres long
• beat to and fro to move substances across cell surfaces
• move mucus across airway linings e.g. trachea, bronchi etc
• move egg down follopian tubes
• rare in plants

Question 26

What is the cytoskeleton?

Answer 26

The cytoskeleton is the network of protein fibres within a cell that 
gives structure and shape to the cell and also moves organelles 
around inside the cell. It is made of 3 parts: microtubules, actin microfilaments, intermediate filaments.

Question 27

What are microtubules?

Answer 27

• main part of the cytoskeleton
• hollow cylinders of tubulin (protein) about 25nm diameter
• maintain cell shape
• connect organelles to each other to keep them in place
• may be used to move microbes through liquid or waft liquid 
 past a cell (cilia and flagella)
• microtubule motors (proteins found on the microtubules) use 
 ATP (to release energy) to move organelles or cell contents 
 along the fibres.
   - this is how chromosomes are moved in mitosis (spindle 
 fibres are microtubules made by centrioles)
   - how vesicles move from the ER to the Golgi apparatus

Question 28

What is the structure and function of flagella?

Answer 28

• tail like structures which can help move whole cells e.g. sperm or 
 whole organisms
• just 1 or 2 on a cell
• move by a motor protein on the microtubules pushing against 
 each other, making the flagella bend and causing a whip like 
 motion which propels the cell.
• Some bacteria (prokaryotes) have flagella but their internal 
structure is different - their flagellum spins to propel them

Question 29

What are intermediate filaments?

Answer 29

• thick cables made of keratin
• 10nm diameter
• maintain cell shape
• anchor nucleus and organelles

Question 30

What are actin microfilaments?

Answer 30

• two intertwinned strands of actin
• 7nm diameter
• maintain cell shape
• cause muscle contraction
• involved in cell division (cytokinesis)
• cause certain type of movement called (pseudopodia) seen by 
amoeba and some white blood cells

Question 31

Describe the structure and function of plant cell walls

Answer 31

• made of cellulose (carbohydrate polymer)
• forms a grid of strands which make the wall strong
• supports the cell because of the pressure from the liquid in the 
 cell - keeps it rigid and so helps to support the plant

Question 32

Outline the interrelationship between the organelles involved in the production and secretion of proteins

Answer 32

1) The gene that has the coded instructions for a protein is transcribed into a length of RNA, called messenger RNA (mRNA).
2) Many copies of this mRNA are made and they leave the nucleus through a nuclear pore to the ribosomes.
3) mRNA attaches to a ribosome. The instructions are translated and insulin molecules are assembled.
4) Insulin molecules pass into the cisternae and are pinched off from the RER and move via microtubules to the Golgi apparatus.
5) Vesicles fuse with golgi .
6) Golgi processes and packages insulin molecules ready for release.
7) Packaged insulin molecules are pinched off in vesicles from the golgi and move towards the plasma membrane.
8) Vesicle fuses with plasma membrane
9) Plasma membrane opens to release insulin molecules outside.

Question 33

Compare and contrast the structure of prokaryotic cells and eukaryotic cells

Answer 33

Similarities:
- Plasma membrane, cytoplasm, ribosomes, DNA and RNA.

Differences:
- Prokaryotic cells are much smaller, less developed cytoskeleton, do not have a nucleus, do not have membrane bound organelles, wall made of peptidoglycan and not cellulose , naked DNA that floats free in the cytoplasm.

Some prokaryotic cells also have:

• A protective waxy capsule surrounding their cell wall
• Small loops of DNA called plasmids
• Flagella
• Pili - small hair-like projections that enable the bacteria to adhere to host cells and allow the passage of plasmid DNA from one cell to another.