Cell Structure

Question 1

What is the description of a cell membrane?

Answer 1

The membrane found on the
surface of animal cells and just
inside the cell wall of plant cells
and prokaryotic cells. It’s made
mainly of lipids and protein.

Question 2

What is the function of a cell membrane?

Answer 2

Regulates the movement of
substances into and out of the cell.
It also has receptor molecules on
it, which allow it to respond to
chemicals like hormones.

Question 3

Whats the description of a cell wall?

Answer 3

A rigid structure that surrounds
plant cells. It’s made mainly of
the carbohydrate cellulose.

Question 4

What is the function of a cell wall?

Answer 4

Supports plant cells.

Question 5

Whats the description of a nucleus?

Answer 5

A large organelle surrounded
by a nuclear envelope (double
membrane), which contains
many pores. The nucleus
contains chromatin (which is
made from DNA and proteins)
and a structure called the
nucleolus.

Question 6

Whats the function of a nucleus?

Answer 6

The nucleus controls the cell’s
activities (by controlling the
transcription of DNA. DNA contains
instructions to make proteins
The pores allow substances (e.g. RNA) to
move between the nucleus and
the cytoplasm. The nucleolus
makes ribosomes

Question 7

Whats the description of a lyosome?

Answer 7

A round organelle surrounded
by a membrane, with no clear
internal structure.

Question 8

What is the function of a lyosome?

Answer 8

Contains digestive enzymes.
These are kept separate from
the cytoplasm by the surrounding
membrane, and can be used to
digest invading cells or to
break down worn out
components of the cell

Question 9

Whats the description of a ribosome?

Answer 9

A very small organelle that
either floats free in the
cytoplasm or is attached to the
rough endoplasmic reticulum.
It’s made up of proteins and
RNA. It’s not surrounded by a membrane.

Question 10

Whats the function of a ribosome?

Answer 10

The site where proteins

are made.

Question 11

Whats the description of Rough endoplasmic reticulum?

Answer 11

A system of membranes enclosing
a fluid-filled space. The surface is
covered with ribosomes.

Question 12

Whats the function of the rer?

Answer 12

Folds and processes proteins
that have been made at the
ribosomes.

Question 13

Whats the description of ser?

Answer 13

Similar to rough endoplasmic

reticulum, but with no ribosomes.

Question 14

What is the function of ser?

Answer 14

Synthesises and

processes lipids.

Question 15

What is the description of a vesical?

Answer 15

A small fluid-filled sac in
the cytoplasm, surrounded
by a membrane.

Question 16

What is the function of a vesical?

Answer 16

Transports substances in and
out of the cell (via the plasma
membrane) and between
organelles. Some are formed
by the Golgi apparatus or the
endoplasmic reticulum, while
others are formed at the cell
surface.

Question 17

Whats the description of a golgi?

Answer 17

A group of fluid-filled,
membrane-bound, flattened sacs.
Vesicles are often seen at the
edges of the sacs.

Question 18

What is the function of a golgi?

Answer 18

It processes and packages
new lipids and proteins.
It also makes lysosomes

Question 19

Whats the description of a mitochondrion?

Answer 19

They’re usually oval-shaped.
They have a double membrane
— the inner one is folded to
form structures called cristae.
Inside is the matrix, which
contains enzymes involved in
respiration.

Question 20

Whats the function of a mitocondrion?

Answer 20

The site of aerobic respiration,
where ATP is produced. They’re
found in large numbers in cells
that are very active and require
a lot of energy.

Question 21

Whats the description of a chloroplast?

Answer 21

A small, flattened structure found
in plant cells. It’s surrounded by a
double membrane, and also
has membranes inside called
thylakoid membranes. These
membranes are stacked up in
some parts of the chloroplast to
form grana. Grana are linked
together by lamellae — thin, flat
pieces of thylakoid membrane.

Question 22

Whats the function of a chloroplast?

Answer 22

The site where photosynthesis
takes place. Some parts of
photosynthesis happen in the
grana, and other parts happen in
the stroma (a thick fluid found in
chloroplasts).

Question 23

Whats the description of a centriole?

Answer 23

Small, hollow cylinders,
made of microtubules (tiny
protein cylinders). Found in
animal cells, but only some
plant cells.

Question 24

What is the function of a centriole?

Answer 24

Involved with the separation
of chromosomes during cell
division

Question 25

Whats the description of cilia?

Answer 25

Small, hair-like structures found
on the surface membrane of
some animal cells. In cross-
section, they have an outer
membrane and a ring of nine
pairs of protein microtubules
inside, with two microtubules
in the middle.

Question 26

What is the function of cilia?

Answer 26

The microtubules allow the cilia
to move. This movement is used
by the cell to move substances
along the cell surface.

Question 27

Whats the description of flagella?

Answer 27

Flagella on eukaryotic cells are
like cilia but longer. They stick
out from the cell surface and
are surrounded by the plasma
membrane. Inside they’re like
cilia too — two microtubules
in the centre and nine pairs
around the edge.

Question 28

Whats the function of flagella?

Answer 28

The microtubules contract
to make the flagellum move.
Flagella are used like outboard
motors to propel cells forward
(e.g. when a sperm cell swims).

Question 29

How is protein produced in a cell?

Answer 29

1) Proteins are made at the ribosomes.
2) The ribosomes on the rough endoplasmic
reticulum (ER) make proteins that are
excreted or attached to the cell membrane.
The free ribosomes in the cytoplasm make
proteins that stay in the cytoplasm.
3) New proteins produced at the rough ER
are folded and processed (e.g. sugar chains
are added) in the rough ER.
4) Then they’re transported from the
ER to the Golgi apparatus in vesicles.
5) At the Golgi apparatus, the proteins may
undergo further processing (e.g. sugar
chains are trimmed or more are added).
6) The proteins enter more vesicles to be
transported around the cell.
E.g. glycoproteins (found in mucus) move
to the cell surface and are secreted.

Question 30

Whats the cytoskeleton?

Answer 30

1) The organelles in cells are surrounded by the cytoplasm. The cytoplasm is more than just
a solution of chemicals though — it’s got a network of protein threads running through it.
These protein threads are called the cytoskeleton.
2) In eukaryotic cells the protein threads are arranged as microfilaments (small solid strands)
and microtubules (tiny protein cylinders).

Question 31

What are 4 functions of the cytoskeleton

Answer 31

The cytoskeleton has four main functions:
1) The microtubules and microfilaments support the
cell’s organelles, keeping them in position.
2) They also help to strengthen the cell and maintain its shape.
3) As well as this, they’re responsible for the movement of materials
within the cell. For example, the movement of chromosomes
when they separate during cell division depends on contraction
of microtubules in the spindle
4) The proteins of the cytoskeleton can also cause the cell to move.
For example, the movement of cilia and flagella is caused by
the cytoskeletal protein filaments that run through them. So in
the case of single cells that have a flagellum (e.g. sperm cells),
the cytoskeleton propels the whole cell.
The cytoskeleton is dynamic (constantly changing), which allows it
to respond to changes in the cell and carry out its functions.

Question 32

Whats the difference between prokaryotes and eukaryotes?

Answer 32

PROKARYOTES
Extremely small cells (less than 2 µm diameter)
DNA is circular
No nucleus — DNA free in cytoplasm
Cell wall made of a polysaccharide, but not cellulose or chitin
Few organelles and no membrane-bound organelles, e.g. no mitochondria
Flagella (when present) made of the protein flagellin, arranged in a helix
Small ribosomes
EUKARYOTES
Larger cells (about 10-100 µm diameter)
DNA is linear
Nucleus present — DNA is inside nucleus
No cell wall (in animals), cellulose cell wall (in plants) or chitin cell wall (in fungi)
Many organelles — mitochondria and other membrane-bound organelles present
Flagella (when present) made of microtubule
proteins arranged in a ‘9 + 2’ formation
Larger ribosomes

Question 33

What is magnification?

Answer 33

Magnification is how much bigger the image is than the specimen
(the sample you’re looking at).

Question 34

What is resolution?

Answer 34

Resolution is how detailed the image is. More specifically, it’s how well a microscope
distinguishes between two points that are close together. If a microscope lens can’t
separate two objects, then increasing the magnification won’t help.

Question 35

How to work out magnification

Answer 35

magnification = image size/object size

Question 36

Whats a light microscope?

Answer 36

1) Light microscopes use light
2) They have a lower resolution than electron microscopes — they have a maximum resolution
of about 0.2 micrometres (µm). So they’re usually used to look at whole cells or tissues.
3) The maximum useful magnification of a light microscope is about × 1500.

Question 37

Whats a laser scanning confocal microscope?

Answer 37

1) Laser Scanning Confocal Microscopes use laser beams (intense beams of
light) to scan a specimen, which is usually tagged with a fluorescent dye.
2) The laser causes the dye to fluoresce — give off light. This light is then
focused through a pinhole onto a detector. The detector is hooked up
to a computer, which generates an image. The image can be 3D.
3) The pinhole means that any out-of-focus light is blocked,
so these microscopes produce a much clearer image
than a normal light microscope.
4) They can be used to look at objects at different depths in thick specimens.

Question 38

Whats a electron microscope?

Answer 38

Electron microscopes use electrons instead of light to form an image.
They have a higher resolution than light microscopes so give more detailed
images. There are two kinds of electron microscope:

Question 39

What is a transmission electron microscope?

Answer 39

Transmission electron microscope (TEM) — use electromagnets to focus
a beam of electrons, which is then transmitted through the specimen.
Denser parts of the specimen absorb more electrons, which makes them
look darker on the image you end up with. TEMs are good because
they provide high resolution images (so they can be used to look at a
range of organelles) but they can only be used on thin specimens.
A TEM image of a mitochondrion is shown above on the right.

Question 40

Whats a scanning electron microscope?

Answer 40

Scanning electron microscope (SEM) — scan a beam of electrons across
the specimen. This knocks off electrons from the specimen, which are
gathered in a cathode ray tube to form an image. The images produced
show the surface of the specimen and can be 3D. But they give lower
resolution images than TEMs. Here’s an SEM image of a mitochondrion.

Question 41

comparison light microscope TEM SEM

Answer 41

light microscope
maximum resolution 0.2 µm
maximum magnification × 1500
TEM
maximum resolution 0.0002 µm
maximum magnification can be more than × 1 000 000
SEM
maximum resolution 0.002 µm
maximum magnification usually less than × 500 000

Question 42

Why do we need to use stain?

Answer 42

In light microscopes and TEMs, the beam of light (or electrons) passes through the object being viewed.
An image is produced because some parts of the object absorb more light (or electrons) than others.
2) Sometimes the object being viewed is completely transparent. This makes the whole thing look white
because the light rays (or electrons) just pass straight through.
3) To get round this, the object can be stained:

Question 43

How to stain in light microscope?

Answer 43

Staining samples for light microscopes:
• For the light microscope, this means using
some kind of dye. Common stains include
methylene blue and eosin.
• The stain is taken up by some parts of the
object more than others — the contrast
makes the different parts show up.
• Different stains are used to make different things
show up. For example, eosin is used to stain cell
cytoplasms. Methylene blue stains DNA.
• More than one stain can be used at once.

Question 44

How to stain e- microscope?

Answer 44

Staining samples for electron microscopes:
For the electron microscope, objects are dipped in a solution of heavy
metals (like lead). The metal ions scatter the electrons, again creating
contrast — some parts of the object show up darker than others.

Electron micrograph images are always
black and white even when stained,
but colour can be added artificially
after the image has been made.

Question 45

How to prepare a microscope slide dry mount?

Answer 45

Dry mount
Your specimen needs to let light through it for you to be able to
see it clearly under the microscope. So if you’ve got quite a thick
specimen, you’ll need to take a thin slice to use on your slide.
• Use tweezers to pick up your specimen and
put it in the middle of a clean slide.
• Pop a cover slip (a square of thin, transparent plastic or glass) on top

Question 46

How to prepare a microscope slide wet mount?

Answer 46

Wet mount
• Start by pipetting a small drop of water onto the slide.
Then use tweezers to place your specimen on top of the water drop.
• To put the cover slip on, stand the slip upright on the slide, next to the water
droplet. Then carefully tilt and lower it so it covers the specimen. Try not to
get any air bubbles under there — they’ll obstruct your view of the specimen.
• Once the cover slip is in position, you can add a stain. Put a drop of stain next
to one edge of the cover slip. Then put a bit of paper towel next to the opposite
edge. The stain will get drawn under the slip, across the specimen.

Question 47

Here’s How to Use a Light Microscope…

Answer 47

1) Start by clipping the slide containing the specimen
you want to look at onto the stage.
2) Select the lowest-powered objective lens
(i.e. the one that produces the lowest magnification).
3) Use the coarse adjustment knob to bring the stage up to just
below the objective lens.
4) Look down the eyepiece (which contains the ocular lens).
Use the coarse adjustment knob to move the stage downwards,
away from the objective lens until the image is roughly in focus.
5) Adjust the focus with the fine adjustment knob, until
you get a clear image of what’s on the slide.
6) If you need to see the slide with greater magnification,
swap to a higher-powered objective lens and refocus.

Question 48

What is a Eyepiece Graticule and Stage Micrometer

Answer 48

Eyepiece Graticule and Stage Micrometer
1) Sometimes, you might want to know the size of your specimen.
And that’s where the eyepiece graticule and stage micrometer come in — they’re a bit like rulers.
2) An eyepiece graticule is fitted onto the eyepiece. It’s like a transparent ruler with numbers, but no units.
3) The stage micrometer is placed on the stage — it is a microscope slide with an accurate scale (it has units)
and it’s used to work out the value of the divisions on the eyepiece graticule at a particular magnification.
4) This means that when you take the stage micrometer away and replace it with the slide containing your
specimen, you’ll be able to measure the size of the specimen.

Question 49

How to use a Eyepiece Graticule and Stage Micrometer?

Answer 49

Line up the eyepiece graticule and the stage micrometer.
2) Each division on the stage micrometer is 0.1 mm long.
3) At this magnification,1 division on the stage micrometer
is the same as 4.5 divisions on the eyepiece graticule.
4) To work out the size of 1 division on the eyepiece
graticule, you need to divide 0.1 by 4.5:
1 division on eyepiece graticule = 0.1 ÷ 4.5 = 0.022 mm
5) So if you look at an object under the microscope at
this magnification and it’s 20 eyepiece divisions long,
you know it measures: 20 × 0.022 = 0.44 mm

Remember: at a different magnification, 1 division on the stage micrometer
will be equal to a different number of divisions on the eyepiece graticule —
so the eyepiece graticule will need to be re-calibrated
Eyepiece graticule