Topic 1a - Cell Structure and Cell Division

Question 1

What is a eukaryotic cell and what sub cellular structures do they contain? (3)

Answer 1

• Plant and animal cells are eukaryotic cells.
• Eukaryotic cells are complex cells
• They have a cell membrane, cytoplasm and genetic material enclosed in a nucleus.

Question 2

What is a prokaryotic cell and what do they contain? (6)

Answer 2

• Bacterial cells (prokaryotic cells) are much smaller in comparison to eukaryotic cells.
• They have cytoplasm and a cell membrane surrounded by a cell wall.
• The genetic material is not enclosed in a nucleus.
• It is a single DNA loop and there may be one or more small rings of DNA called plasmids.
• Some bacterias may also have a flagellum.
• Prokaryotes are single celled organisms.

Question 3

Which sub-cellular structures do animal cells contain? (5)

Answer 3

• a nucleus
• cytoplasm
• a cell membrane
• mitochondria
• ribosomes.

Question 4

Which additional sub-cellular structures do plant cells have and what is their cell made of? (3)

Answer 4

• chloroplasts
• a permanent vacuole filled with cell sap
• cell wall made of cellulose, which strengthens the cell

Question 5

What is the function of a nucleus?

Answer 5

Contains genetic material that controls the activities of the cell.

Question 6

What is the function of a cell membrane?

Answer 6

Holds the cell together and controls what goes in and out of the cell.

Question 7

What is the function of the cytoplasm?

Answer 7

A gel-like substance where most of the chemical reactions happen. It contains enzymes that control these chemical reactions.

Question 8

What is the function of a ribosome?

Answer 8

These are where proteins are made in the cell.

Question 9

What is the function of a mitochondria?

Answer 9

These are where most of the reactions for aerobic respiration take place. Respiration transfers energy the cell needs to work.

Question 10

What is the function of a permanent vacuole?

Answer 10

Contains cell sap, a weak solution of sugar and salts.

Question 11

What is the function of a chloroplast?

Answer 11

This is where photosynthesis occurs, which makes food for the plant. They contain a green substance called chlorophyll, which absorbs the light needed for photosynthesis.

Question 12

What is the function of the cell wall?

Answer 12

A rigid structure made of cellulose. It supports and strengthens the cell. The cells of algae (e.g. seaweed) also have a rigid cell wall.

Question 13

What is the function of a plasmid?

Answer 13

Circular DNA - can be involved in genetic engineering.

Question 14

What is the function of a circular loop of DNA?

Answer 14

Loose within the cytoplasm - not constrained in a nucleus.

Question 15

What is the function of a flagellum?

Answer 15

Some bacteria have this and it allows them to move as it propels them along.

Question 16

How can you estimate the size of a sub-cellular structure?

Answer 16

By comparing its size to that of the cell.
E.g, if the cell is 60μm wide and you can roughly fit the nucleus 8 times across the cell, then you can assume the width of the nucleus is 60/8.

Question 17

How can you estimate the area of a sub-cellular structure?

Answer 17

By comparing it to a regular shape.
E.g, a mitochondrion is a similar shape to a rectangle, so you would use the same formula to find the area - area = length x width

Question 18

What is the method of how to prepare a slide for a light microscope (specifically onion cells) - required practical 1?

Answer 18

1. Add a drop of water to the middle of a clean slide.
2. Cut up an onion and separate it out into layers. Use tweezers to peel off some epidermal tissue from the bottom of one of the layers.
3. Using tweezers, place the epidermal tissue into the water on the slide.
4. Add a drop of iodine solution. Iodine solution is a stain. Stains are used to highlight objects in a cell by adding colour to them.
5. Place a cover slip ( a square of thin, transparent plastic or glass) on top. To do this, stand the cover 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.

Question 19

What is the 6 step method of how to use a light microscope to observe the specimen (specifically onion cells) - required practical 1?

Answer 19

1. Start by clipping the slide you’ve prepared on 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 move the stage up to just below the objective lens.
4. Look down the eyepiece. Use the coarse adjustment knob to move the stage downwards 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 20

What measures should you ensure to take when making a drawing of your specimen from under the microscope? (8)

Answer 20

• Use a pencil with a sharp point
• Draw with clear, unbroken lines
• No colouring or shading
• If drawing cells, sub cellular structures should be drawn in proportion to each other
• Drawing must take up over half the space provided
• Include a title of what you were observing
• Label the important features of your drawing using straight, uncrossed lines
• Work out and write down the magnification of your drawing

Question 21

What are specialised cells?

Answer 21

One cell that performs a specific function.
A cell’s structure helps it to carry out its function - so depending on what job it does, specialised cells can look very different to typical cells.

Question 22

What are 3 specialised cells in animals?

Answer 22

sperm cells, nerve cells and muscle cells

Question 23

What are 3 specialised cells in plants?

Answer 23

root hair cells, xylem and phloem cells

Question 24

What is the function of a sperm cell + what is it specialised for?

Answer 24

• specialised for sexual reproduction
• function is to carry the genetic information from the male through the female reproductive tract to fertilise the egg cell (female gamete) in sexual reproduction

Question 25

How is the sperm cell specialised? (4)

Answer 25

• the head is streamlined for faster movement and to penetrate the egg cell
• the acrosome (in the head) contains enzymes to break down the outer lining of the egg cell
• the middle section contains lots of mitochondria to release large amounts of energy to fuel the flagellum (tail)
• the flagellum rotates + turns to propel the sperm cell along the reproductive tract towards the egg in the fallopian tube of the female

Question 26

What is the function of a nerve cell, and what are the types?

Answer 26

• specialised for rapid signalling
• function is to carry electrical impulses through the body to the brain
• there are 3 types of neurone; sensory, relay and motor

Question 27

How is the nerve cell specialised? (3)

Answer 27

• the dendrites extend from the cell body and make a network of connections with other neurones around the body
• the axon is a long extension from the main cell body which stretches through the peripheral nervous system, e.g along your arms and legs
• the myelin sheath is a layer of fatty insulation around the axon - ensures that the electrical impulse can be carried for long distances

Question 28

What is the function of a muscle cell?

Answer 28

• specialised for contraction
• function is to enable movement of the body through its connection to the skeletal system

Question 29

How is the muscle cell specialised? (3)

Answer 29

• filament bundles are made up of thick and thin filaments (band of proteins) - they slide over one another to become shorter and contract
• many muscle cells are connected together in a tissue and can exert more force to move the skeleton + body
• have lots of mitochondria to release energy for muscle contraction

Question 30

What is the function of a root hair cell?

Answer 30

• function is to absorb water and dissolved minerals from the ground water in the soil, surrounding the plant roots

Question 31

How is the root hair cell specialised? (3)

Answer 31

• cytoplasm doesn’t contain chloroplasts as the root hair cell is underground and not exposed to sunlight, so chloroplasts would take up unnecessary space as this cell doesn’t photosynthesise
• cell membrane is semi-permeable so it allows mineral ions to pass into the cell
• has a long extension, ‘hairs’ which increases the SA available for absorption

Question 32

What is the function of a xylem cell? (3)

Answer 32

• function is to transport water and dissolved minerals within the plant
• carries water from the roots to the leaves and shoots in one direction only
• also acts to support the plant in an upright position

Question 33

How is the xylem cell specialised? (2)

Answer 33

• it isn’t living - doesn’t contain any cytoplasm or nucleus, instead the cells from a hollow tube to hold the water for transport
• the cell wall + cell membrane don’t exist on the ends, so the cells connect together in a hollow tube-like structure

Question 34

What is the function of a phloem cell?

Answer 34

• function is to transport the dissolved sugars produced in the leaves around the plant, to where they are needed
• dissolved sugars are moved in both directions through the phloem tubes

Question 35

How is the phloem cell specialised? (2)

Answer 35

• companion cells alongside the phloem tube have lots of mitochondria and release energy for the active transport of the dissolved sugars
• phloem cells join together and form sieve tubes - sieve tube plate is the end of the cell wall which contains lots of holes, they allow continuous flow of sugars throughout the plant

Question 36

What is Cell Differentiation?

Answer 36

The process by which a cell changes to become specialised for its job. As a cell differentiates it acquires different sub-cellular structures to enable it to carry out a certain function. It has become a specialised cell.

Question 37

When do most animal cells differentiate?

Answer 37

Most types of animal cell differentiate at an early stage.

Question 38

When do most plant cells differentiate?

Answer 38

Many types of plant cells retain the ability to differentiate throughout life.

Question 39

In mature animals, what is cell division usually used restricted to?

Answer 39

In mature animals, cell division is mainly restricted to repair and replacement.

Question 40

How do light microscopes work, how powerful can they be, and what can you see?

Answer 40

• pass light through glass lenses to magnify specimens
• most powerful light microscope = 2000x - magnification
• can see animal cells, plant cells, and bacteria cells, dead or alive

Question 41

What is resolution/how is it measured?

Answer 41

Measured by distance between 2 points that can be seen distinctly.

Question 42

How has electron microscopy increased understanding of sub-cellular structures?

Answer 42

An electron microscope has much higher magnification and resolving power than a light microscope. This means that it can be used to study cells in much finer detail. This has enabled biologists to see and understand many more sub-cellular structures.

Question 43

How do electron microscopes work and how powerful can they be? (3)

Answer 43

• shoots electrons through the specimen, in a vacuum chamber, and builds an image based off of how many pass through
• can magnify over 500,000 x
• most samples need a thin coating of metal before being put in the vacuum chamber

Question 44

What are the negatives of an electron microscope? (2)

Answer 44

• have to kill the specimen to see it
• very big and expensive to buy/operate

Question 45

What is the formula to work out magnification?

Answer 45

magnification = size of image/size of real object

Question 46

How do prokaryotic cells/bacteria replicate? (3)

Answer 46

• Bacteria multiply by simple cell division (binary fission).
• They can divide as often as once every 20 minutes if they have enough nutrients and a suitable temperature.
• If conditions become unfavourable, the cells will stop dividing and eventually begin to die.

Question 47

What is the process of binary fission (4 steps)?

Answer 47

1. Circular DNA + Plasmids replicate.
2. Cells get bigger + circular DNA strands move to opposite ‘poles’ (ends) of the cell.
3. Cytoplasm begins to divide + new cell walls begin to form.
4. Cytoplasm divides and 2 daughter cells are produced. Each daughter cell has one copy the circular DNA, but can have a variable number of copies of plasmids.

Question 48

Where can Bacteria be grown?

Answer 48

Bacteria can be grown in culture mediums such as; a nutrient broth solution or as colonies on an agar gel plate.

Question 49

What are culture mediums?

Answer 49

Bacteria, and some other microorganisms, are grown in a culture medium, which contains the carbohydrates, minerals, proteins and vitamins they need to grow.

Question 50

What are uncontaminated cultures of microorganisms required for?

Answer 50

Uncontaminated cultures of microorganisms are required for investigating the action of disinfectants and antibiotics.

Question 51

What are the techniques used to prepare an uncontaminated culture?

Answer 51

Aseptic techniques.

Question 52

Why must petri dishes and culture media be sterilised before use?

Answer 52

To kill any unwanted microorganisms that may be lurking on them.

Question 53

Why must inoculating loops used to transfer microorganisms to the media be sterilised by passing them through a flame?

Answer 53

All contaminants on the wire are incinerated.

Question 54

Why must the lid of the Petri dish be secured with adhesive tape and stored upside down? (2)

Answer 54

• To stop microorganisms from the air getting in - tape
• To stop drops of condensation falling onto the agar surface. - upside down

Question 55

Why, in school laboratories, should cultures generally be incubated at 25°C?

Answer 55

Because harmful pathogens are more likely to grow above this temperature.

Question 56

What is the 6 step method for Required Practical 2 - culturing bacteria?

Answer 56

1. To make an agar plate, hot agar jelly is poured into shallow round plastic dishes called Petri dishes. When the jelly’s cooled and set, inoculating loops (wire loops) can be used to transfer microorganisms to the culture medium. Alternatively, a sterile dropping pipette and spreader can be used to get an even covering of bacteria. The microorganisms then multiply.
2. Place paper discs soaked in different types (or different concentrations) of antibiotics on an agar plate that has an even covering of bacteria. Leave some space between the discs.
3. The antibiotic should diffuse (soak) into the agar jelly. Antibiotic-resistant bacteria (i.e. bacteria that aren’t affected by the antibiotic) will continue to grow on the agar around the paper discs, but non-resistant strains will die. A clear area will be left where the bacteria have died — this is called an inhibition zone.
4. Make sure you use a control. This is a paper disc that has not been soaked in an antibiotic. Instead, soak it in sterile water. You can then be sure that any difference between the growth of the bacteria around the control disc and around one of the antibiotic dises is due to the effect of the antibiotic alone (and not something weird in the paper, for example).
5. Leave the plate for 48 hours at 25°C.
6. The more effective the antibiotic is against the bacteria, the larger the inhibition zone will be.

Question 57

How would you calculate cross-sectional areas of colonies or clear areas around colonies?

Answer 57

Use a ruler to measure the diameter of the section you need to find the area of, and then use the formula πr², to work out the area.

Question 58

What is the mean division time?

Answer 58

Average time it takes a cell to divide.

Question 59

If given the mean division time, what can you calculate from this?

Answer 59

• Can calculate the number of bacteria in a population after a certain time.

Question 60

In the following example, use mean division time to answer the question.
A bacterial cell has a mean division time of 30 minutes. How many cells will it have produced in 2.5 hours?

Answer 60

1. Make sure both times are in the same units;

2.5 hours x 60 = 150 minutes

2. Divide the total time that the bacteria are producing cells by the mean division time. This gives you the number of divisions;

150 minutes/30 minutes = 5 divisions

3. Multiply 2 by itself for the number of divisions to find the number of cells;

2(to the power of 5) = 2 × 2 × 2 × 2 × 2 = 32 cells

Question 61

What are chromosomes and where are they found?

Answer 61

Chromosomes are found in the nucleus of the cell. They are long threads of deoxyribonucleic acid (DNA) which are made up of genes.

Question 62

What do chromosomes contain?

Answer 62

Each chromosome carries many genes. Most people have many of the same genes. However, a small number of genes make up the unique combination which codes for our individual physical features and characteristics.

Question 63

Order the items related to chromosomes from biggest to smallest.

Answer 63

Cell - Nucleus - Chromosomes - DNA - Genes

Question 64

How many chromosomes are there in a normal body cell?

Answer 64

• 46
• 23 pairs

Question 65

What is a gene?

Answer 65

A small section of DNA in a chromosome. Each gene codes for a particular sequence of amino acids in order to make a specific protein.

Question 66

Why does cell division take place?

Answer 66

Cell division tales place to replace damaged cells and for growth of the organism.

Question 67

What is Mitosis?

Answer 67

Mitosis is a type of cell division that produces two identical copies of the parent cell.

Question 68

Why does mitosis occur?

Answer 68

Mitosis occurs to replace damaged cells. Identical cells produced from mitosis can replace damaged cells and contribute to growth of an organism.

Question 69

Describe the 3 stages in the cell cycle.

Answer 69

• Stage 1 - Interphase - longest stage.
• DNA / Chromosomes replicate / duplicate.
• Mitochondria / ribosomes / sub-cellular structures replicate.
• Stage 2 - Mitosis.
• One set of chromosomes is pulled / moved to each end of the cell.
• Stage 3 - Cytokinesis.
• The cytoplasm and cell membrane divides to form two cells.

Question 70

When is cell division by mitosis important?

Answer 70

Cell division by mitosis is important in the growth and development of multicellular organisms.

Question 71

If asked to, how would you work out the length of time in a stage of the cell cycle?

Answer 71

length of time in stage = observed number of cells in a particular stage/total number of cells observed x total length of cell cycle

Question 72

What is a stem cell?

Answer 72

A stem cell is an undifferentiated cell of an organism which is capable of giving rise to many more cells of the same type, and from which certain other cells can arise from differentiation.

Question 73

What is the function of stem cells in embryos?

Answer 73

The cells on the inside layer of a very early embryo can make all of the cell types needed in your body. They are called embryonic stem cells.

Question 74

What can you do to embryonic stem cells that you can’t do to adult stem cells?

Answer 74

Stem cells from human embryos can be cloned and made to differentiate into most different types of human cells. E.g, red blood cells, and nerve cells. Whereas adult stem cells are much more limited in terms of which types of cell they can make.

Question 75

What is the function of stem cells in adult animals?

Answer 75

Adult stem cells are more limited than embryonic stem cells in the types of cell they can make, and are only found in certain places. E.g, Stem cells from adult bone marrow can only form many types of cells including blood cells.

Question 76

What is the function of meristems in plants?

Answer 76

Plant stem cells are called meristems and are found at the growing tips of the roots and the shoots. Meristem tissue in plants can differentiate into any type of plant cell, throughout the life of the plant.

Question 77

What is the difference between adult stem cells in animals and plants?

Answer 77

The big difference between adult stem cells in humans and plants is that the stem cells in adult plants can still make every type of plant cell.

Question 78

What are some uses of plant stem cells?

Answer 78

Plant stem cells can be used to make clones, identical copies of the parent plant, quickly and economically. E.g, popular house plants quickly and cheaply, conserving endangered species, or making clones of plants that have been genetically modified to deal with environmental stresses

Question 79

Treatments with stem cells, might be able to help what conditions and how?

Answer 79

Treatment with stem cells may be able to help conditions such as;
* diabetes - by replacing the insulin producing cells in the pancreas
* paralysis by spinal injuries - by regrowing spinal nerves

Question 80

What is therapeutic cloning and how is it useful for medical treatment?

Answer 80

In therapeutic cloning an embryo is produced with the same genes as the patient. Stem cells from the embryo are not rejected by the patient’s body so they may be used for medical treatment.

Question 81

How would therapeutic cloning work, to cure type 1 diabetes (for example)?(5)

Answer 81

• A nucleus is removed from the skin cell of a patient with type 1 diabetes and implanted into an egg cell from a donor.
• The egg cell develops into an embryo.
• Stem cells from the embryo are removed and cultured in a Petri dish.
• The stem cells are cultured to differentiate into pancreatic cells that can produce insulin.
• These stem cells can then be injected into the patient with type 1 diabetes, and they will now have insulin-producing pancreatic cells.

Question 82

What are some objections to using embryonic stem cells in medicine?

Answer 82

The use of stem cells has potential risks such as transfer of viral infection, and some people have ethical or religious objections.