1.2 Ultrastructure of cells

Question 1

Light microscopes are limited in resolution by the wavelengths of

Answer 1

visible light (400–700 nm).

Question 2

Electrons have a much shorter wavelength (2-12 pm) therefore

Answer 2

electron microscopes have a much higher resolution

Question 3

Resolution

Answer 3

is defined as the shortest distance between two points that can be distinguished

Question 4

Ultrastructure

Answer 4

is all the structures of a biological specimen that are at least 0.1nm in their smallest dimension

Question 5

Light microscopes allow us to see the…

Answer 5

structure of cells

Question 6

Electron microscopes allow us to see the

Answer 6

ultrastructure of cells, such as these pancreatic exocrine cells

Question 7

Electron microscopes can see viruses (0.1μm diameter) ,

Answer 7

but light microscopes cannot

Question 8

First organisms to evolve on Earth..

Answer 8

(pro “before”, karyotic “nucleus”)

Question 9

Simple cell structure, some with..

Answer 9

pilli and at least one flagellum

Question 10

prokaryotes have a…

Answer 10

cell membrane, cell wall, 70S ribosomes (smaller than in eukaryotes), and a single circular DNA molecule (nucleoid)

No membrane to separate cellular components

Question 11

What are the functions of these prokaryotic cell parts?

Answer 11

Movement – flagellum

Protein synthesis – ribosomes

Attachment, DNA transfer – pili

Protective coating – cell wall

Contains enzymes for metabolism – cytoplasm

Region containing closed-loop DNA – nucleoid

Controls entry and exit of substances – plasma membrane

Question 12

Prokaryotes reproduce asexually using the process of binary fission

Answer 12

Single circular chromosome is replicated semi conservatively [2.7.U1]
Each DNA loops moves to opposite ends of the cell and attach to the membrane
The membrane elongates and pinches off (cytokinesis) forming two separate cells
The two daughter cells are genetically identical (clones)
Binary fission is also used by some organelles in eukaryotes [links to 1.5.U3]

Question 13

Cytoplasm is divided into compartments by

Answer 13

single or double membranes to form organelles

Question 14

There are several advantages in being compartmentalized:

Answer 14

Efficiency of metabolism - enzymes and substrates can localized and much more concentrated

Localised conditions - pH and other such factors can be kept at optimal levels. The optimal pH level for one process in one part of the cell

Toxic / damaging substances can be isolated, e.g. digestive enzymes (that could digest the cell itself) are stored in lysosomes

Numbers and locations of organelles can be changed dependent on the cell’s requirements.

Question 15

Nucleus

Answer 15

Generally spherical with a double membrane
Pores (holes) are present in the membrane
Contains genetic information in the form of chromosomes (DNA and associated histone proteins)
Uncoiled chromosomes are referred to as chromatin – they stain a dark colour and are concentrated at the edges of the nucleus
mRNA is transcribed in the nucleus (prior to use in protein synthesis in the cytoplasm)
mRNA leaves the nucleus via the pores
(DNA is too large to move through the pores)

Question 16

The Mitochondrion (pl. Mitochondria)

Answer 16

Has a double membrane
A smooth outer membrane and a folded inner membrane
The folds are referred to as cristae
Variable in shape
Site of ATP production by aerobic respiration (if fat is used as a source of energy it is digested here)

Question 17

Free ribosomes

Answer 17

80S Ribosomes (approx. 20nm diameter) - larger than the ribosomes found in prokaryotes
No membrane
These appear as dark granules in the cytoplasm
Synthesizes proteins to function in the cytoplasm, for use within the cell, e.g. enzymes

Question 18

The Rough Endoplasmic Reticulum (rER)

Answer 18

The consists of flattened membrane sacs, called cisternae
Often located near to the nucleus
80S Ribosomes are attached to the outside of the cisternae are ribosomes
rER synthesizes proteins which are transported, by vesicles, to the golgi apparatus for modification before secretion outside the cell

Question 19

The Golgi apparatus

Answer 19

this organelle also consists of flattened membrane sacs called cisternae, like rER.
Different to rER:
No attached ribosomes
Often sited close to the plasma membrane
The cisternae are shorter and more curved that those of the rER
The Golgi apparatus processes (modifies) proteins from from the rER. The proteins are then repackaged in vesicles for secretion outside the cell.

Question 20

Vesicles

Answer 20

A single membrane with fluid inside
Very small in size
Used to transport materials inside of a cell

Question 21

Lysosomes

Answer 21

Generally spherical with a single membrane
Formed from Golgi vesicles.
They contain digestive enzymes for breakdown of:
ingested food in vesicles
unwanted/damaged organelles
The cell itself
High concentration of enzymes (a type of protein) cause this organelle to stain heavily and hence appear dark on micrographs

Question 22

Vacuoles

Answer 22

Single membrane with fluid inside
In Plant cells vacuoles are large and permanent, often occupying the majority of the cell volume
In animals vacuoles are smaller and temporary and used for various reasons, e.g. to absorb food and digest it

Question 23

Flagellum (Flagella pl.)

Answer 23

Thin projection (usually singular) from the cell surface.
Contain microtubules
Used to move the cell

Question 24

Cilia

Answer 24

Thin projections from the cell surface.
Contain microtubules
Used to either move the cell or to move the fluids next to the cell

Question 25

Microtubules

Answer 25

Small cylindrical fibres called microtubules
Have a variety of functions, e.g. part of the structure of flagella and they play a role in cell division

Question 26

Centrioles

Answer 26

Consist of two groups of nine triple microtubules
Are mainly found in animal cells, not present in vascular plants or fungi.

Question 27

Chloroplast

Answer 27

Many, but not all, plant cells contain chloroplasts
A double membrane surrounds the chloroplast
Inside are stacks of thylakoids
Each thylakoid is a disc composed of a flattened membrane
The shape of chloroplasts is variable but is usually ovoid
The site of photosynthesis and hence where glucose is produced.
Starch grains maybe present if photosynthesis is happening quickly

Question 28

Cell wall

Answer 28

an extracellular component not an organelle.
secreted by all plant cells (fungi and some protists also secrete cell walls).

Plant cell walls consist mainly of cellulose which is:
Permeable - does not affect transport in and out of the cell
Strong – gives support to the cell and prevent the plasma membrane bursting when under pressure
Hard to digest –resistant to being broken down, therefore lasts along time without the need for replacement/maintenance