3.2.1 Cell structure

Question 1

What are prokaryotes and eukaryotes ?

Answer 1

Prokaryotic organisms are prokaryotic cells (single-cell organisms) and
eukaryotic organisms are made of eukaryotic cells.

Question 2

What are the difference between eukaryote and fungal cells ?

Answer 2

Fungal cells are also very similar but have have two key differences:
○ Their cell walls are made of chitin and not cellulose
○ They don’t have chloroplasts because they don’t photosynthesise

Question 3

Describe the cell membrane and its function

Answer 3

This membrane is found on the surface of animal cells and just inside the cell wall of
other cells. It’s made mostly of lipids and protein.
The cell membrane regulates the movement of substances into and out of the cell.
Receptor molecules on the membrane allow it to respond to chemicals like hormones.

Question 4

What are components of the cell membrane ?

Answer 4

Glycoprotein - protein attached to carbohydrate
Glycolipid - lipid attached to carbohydrate
Peripheral/external protein
Internal/integral protein
Cholesterol
Protein channel
Phospholipid bilayer

Question 5

Describe the nucleus and its function

Answer 5

This is the largest organelle that is
surrounded by a nuclear envelope which
contains many pores. The nucleus contains
chromosomes made from protein-bound
linear DNA and one or more structures
called nucleolus.
The nucleus controls cell activity through
transcription of DNA - which contain
instructions to make protein. The pores
allow substances (i.e RNA) to move
between the nucleus and the cytoplasm.
The nucleolus makes ribosomes.

Question 6

What does the nucleus consist of ?

Answer 6

Endoplasmic reticulum
Nucleolus
Chromatin
Nucleoplasm
Nuclear pore
Nuclear envelope

Question 7

Describe the mitochondria and the function

Answer 7

This is an oval or rod-shaped organelle that has a double membrane. The inner
membrane is folded to form structures called cristae. Inside is the matrix, which
contains enzymes involved in respiration.
Mitochondrion is the site of aerobic respiration where ATP is produced. They are found
in large numbers in very active cells and require a lot of energy

Question 8

What does the mitochondria consist of ?

Answer 8

ATP synthase particles
Intermembrane space
Matrix
Cristae
Ribosome
Granules
Inner membrane
Outer membrane
DNA

Question 9

Describe the chloroplast and its function

Answer 9

Chloroplast is a small, flattened structure
found in plant and algal cells. Surrounded
by a double membrane with membranes
inside called thylakoid membranes. These
stack up to form grana. Grana are linked
together by lamellae which are thin, flat
pieces of thylakoid membrane.
Chloroplast is the site where
photosynthesis takes place. Some parts of
that process happen in the grana, and
others happen in the stroma, which is a
thick liquid found in chloroplasts.

Question 10

What does the chloroplast consist of ?

Answer 10

Outer membrane
Inner membrane
Intermembrane space
Stroma
Thylakoid
Granum ( stack of thylakoids )

Question 11

Describe the golgi apparatus and its function

Answer 11

A series of fluid filled, flattened membrane sacs. Vesicles are often seen at the edges
of the sacs.
The golgi apparatus (or golgi body) processes and packages new lipids and proteins. It
also makes lysosomes

Question 12

Describe the golgi vesicle and its function

Answer 12

This is a small fluid-filled sac (see diagram above) found in the cytoplasm. It is
surrounded by a membrane and is produced by the Golgi apparatus.
The vesicle stored lipids and proteins made by the Golgi apparatus and then transports
them out of the cell through the cell-surface membrane.

Question 13

Describe the lysosome and its structure

Answer 13

These are small, round organelles surrounded by a membrane and with no clear
internal structure. Lysosome is a type of Golgi vesicle.
They contain the digestive enzymes lysozymes. The enzymes are kept separate from
the cytoplasm by the membrane and can be used to digest invading cells or break
down worn out components of the cell wall.

Question 14

Describe the ribosome and its function

Answer 14

The smallest and most numerous of the
cell organelles - ribosomes either float free
in the cytoplasm or are attached to the
rough endoplasmic reticulum.
It’s made up of proteins and RNA, is not
surrounded by a membrane and is the site
where proteins are made.

Question 15

Describe the RER rough endoplasmic reticulum and its function

Answer 15

This is a system of membranes that enclose a fluid-filled space. The surface is covered
with ribosomes.
RER folds and processes the proteins that have been produced at the ribosomes.

Question 16

Describe the smooth endoplasmic reticulum and its function

Answer 16

Very similar to rough endoplasmic reticulum, except that no ribosomes are present.
SER synthesises and processes lipids

Question 17

Describe the cell wall and its function

Answer 17

The cell wall is a rigid structure that surrounds cells in plants, algae and fungi. It’s
made mainly of the carbohydrate cellulose in plant and algae cells. In fungi, the cell
wall is made of chitin.
Its primary function is to support cells and prevent them from changing shape

Question 18

Describe the cell vaccuole and its function

Answer 18

A membrane bound organelle found in the
cytoplasm of plant cells. It contains a weak
solution of sugars and salts called cell sap.
The membrane surrounding plant cell
vacuoles is called the tonoplast.
They help to maintain pressure inside the
cell and keep it rigid.
The vacuole is also involved in isolating
unwanted chemicals in the cell.

Question 19

What does the cytoplasm in prokaryotes contain ?

Answer 19

The cytoplasm in prokaryotic cells doesn’t have membrane-bound organelles. It does
contain ribosomes and they are smaller than those in a eukaryotic cell.

Question 20

Describe the structure of the plasma membrane and structure of prokaryotes

Answer 20

This membrane is made mostly from lipids and proteins - just like a eukaryotic cell. It
controls movement of substances into and out of the cell.

Question 21

Describe the structure and function of DNA in prokaryotes

Answer 21

Unlike eukaryotic cells, a prokaryotic cell doesn’t have a nucleus. The DNA is always
circular and floats free in the cytoplasm as a long, coiled strand and is not attached to
any proteins to form chromatin.

Question 22

Describe the structure and function of plasmid in prokaryotes

Answer 22

These are small circles of DNA that are not part of the main DNA molecule. They
contain genes (for things like antibiotic resistance) and can be passed between cells.
They are not always present in prokaryotic cells and some cells can have several of
them.

Question 23

Describe the structure and function of the mesosome in prokaryotes

Answer 23

A tightly-folded region of the cell membrane containing all the membrane- bound
proteins required for respiration and photosynthesis. Can also be associated with the
nucleoid (DNA).

Question 24

Describe the function and structure of cell wall in prokaryotes

Answer 24

This is made from a polymer called murein (not cellulose), which is a glycoprotein - a
protein with a carbohydrate attached.
There are two kinds of cell wall, which can be distinguished by a Gram stain: Gram
positive bacteria have a thick cell wall and stain purple, while Gram negative bacteria
have a thin cell wall with an outer lipid layer and stain pink.

Question 25

Describe the function and structure of capsule in prokaryotes

Answer 25

This is a thick polysaccharide layer outside of the cell wall, like the glycocalyx of
eukaryotes. Used for sticking cells together, as a food reserve, as protection against
desiccation and chemicals, and as protection against phagocytosis.

Question 26

Describe the function and structure of the flagellum in prokaryotes

Answer 26

A rigid rotating helical-shaped tail used to make the cell move. The motor is embedded
in the cell membrane and is driven by a H+ gradient across the membrane. Clockwise
rotation drives the cell forwards, while anticlockwise rotation causes a chaotic spin.
This is the only known example of a rotating motor in nature. Not all prokaryotic cells
have a flagellum but some cells have more than one.

Question 27

What is a virus ?

Answer 27

A virus is just nucleic acid surrounded by protein - they are not alive

Question 28

Which is larger a bacteria or a virus?

Answer 28

Bacteria is larger

Question 29

Describe the structure of a virus

Answer 29

Envelope protein
Envelope
Viral genome in nucleocapsid
Viral tegument surriunding the nucleocapsid

Question 30

Describe the structure and function of nucleic acid in a virus

Answer 30

Viruses contain a core genetic material made up of either DNA or RNA. Viruses are
classified according to the type of nucleic acid they contain.

Question 31

Describe the structure of the capisid/nucleocapsid in virus

Answer 31

This is a protein coat that surrounds the core of the virus.

Question 32

Describe the function of envelope protein /attachment protein of virus

Answer 32

These are also referred to as attachment proteins and they stick out from the edge to
let the virus cling to a suitable host cell.

Question 33

Define magnification

Answer 33

Magnification is how much bigger the image
appears than the specimen sample is.

Question 34

What is the equation for magnification ?

Answer 34

magnification = size of image/
size of real object

Question 35

Define resolution

Answer 35

The resolution is how detailed the image is.
The better the resolution, the better the
microscope is able to distinguish between
two points that are close together

Question 36

What units does image size and actual size need to be on ?

Answer 36

Image size - mm
Actual size - um

Question 37

How to convert from cm - mm - um -nm ?

Answer 37

cm x10 mm x1000 μm x1000 nm

Question 38

Describe an optical microscope

Answer 38

1. Uses a light to form the image
2. Has a maximum resolution of about
   0.2 micrometres (μm). You cannot
   view organelles that are smaller than
   0.2μm - including ribosomes, the
   endoplasmic reticulum and lysosomes.
   Mitochondria may be visible but not
   clear. You can see the nucleus.
3. Maximum useful magnification of
   optical microscopes is about x 1500

Question 39

Describe electron microscope

Answer 39

1. Uses electrons to form an image.
2. Has a higher resolution than optical
   microscopes to give a more detailed
   image (and can be used to view all
   organelles)
3. Maximum resolution of about 0.0002
   micrometres (μm) - about 1000 times
   higher than optical.
4. Maximum useful magnification of an
   electron microscope is about x
   1,500,000.

Question 40

Describe transmission electron microscope

Answer 40

Transmission electron microscopes (TEMs)
1. Uses electromagnets to focus a beam
of electrons which is transmitted
through a specimen
2. The densest parts of the specimen
absorb more electrons which makes
them look darker
3. TEMs produce a high resolution image
to see internal structure of organelles
4. They can only be used on thin
specimens

Question 41

Describe scanning electron microscopes

Answer 41

1. Scans a beam of electrons over the
   specimen that knocks off electrons
   that are gathered in a cathode ray tube
   to form an image
2. The images show the surface of the
   specimen and can be 3D
3. Good for use on thick specimens
4. SEMs give a lower resolution image
   than TEMs

Question 42

What is the method to view specimens under an optical microscope

Answer 42

1. Pipette a drop of water onto the slide and use tweezers to place a thin section of the
   specimen on the drop
2. Add a drop of stain to highlight objects in the cell (e.g. eosin for cytoplasm, iodine in
   potassium iodine solution for starch grains in plant cells)
3. Add a cover slip (square of clear plastic) to protect the specimen. Stand the slip upright on
   the slide and tilt and lower it until the specimen is cover - making sure not to get any air
   bubbles under.

Question 43

What are the steps in cell fractionation ?

Answer 43

Homogenisation- breaking up cells
Filtration - getting rid of cell debris
Ultracentrifugation - seperating organelles

Question 44

Describe homogenisation

Answer 44

Homogenisation can be done by vibrating the cells or by grinding them up. Both
methods break up the plasma membrane and release the organelles into a solution that
is ice cold to reduce enzyme activity that breaks down organelles. The solution also
needs to be isotonic, which means to have the same concentration of chemicals as the
cells being broken down to avoid damage to the organelles through osmosis. A buffer
solution to prevent pH changes should also be used.

Question 45

Describe filtration

Answer 45

The homogenised cell solution needs to be filtered through gauze so that any large cell
debris like connective tissue is removed. Organelles are much smaller than the debris
so they pass the gauze and don’t get filtered.

Question 46

Describe ultracentrifugation

Answer 46

To separate a particular organelle from your homogenised solution, you use a process
called ultracentrifugation:
1. Cell fragments are poured into a tube and the tube is put into a centrifuge (a
machine which separates material by spinning). The solution is spun at low speed
and the heaviest organelles, like nuclei, will get moved to the bottom by the
centrifugal force. They form a thick sediment called the pellet and the rest of the
organelles stay suspended in fluid above the sediment - this is called the
supernatant.
2. Supernatant is drained off and poured into another tube which is then spun at a
higher speed in the centrifuge. The next heaviest organelles (the mitochondria)
form a pellet and the supernatant is drained off and spun again at even higher
speed.
3. The whole process is repeated at higher and higher speeds until all of the
organelles have been separated. Each time, the pellet at the bottom contains
lighter and lighter organelles.

Question 47

What is the order of mass of organelles in the pellet in ultracentrifugation ?

Answer 47

The order of mass of these organelles (from heaviest to lightest) is usually:
Nuclei
Chloroplasts (if carrying out cell fractionation of plant tissue)
Mitochondria
Lysosomes
Endoplasmic reticulum
Ribosomes