Cell Biology

Question 1

How are sperm cells specialised?

Answer 1

Long tail and streamlined so they can swim to the ovum.
Packed full of mitochondria to provide the energy needed for swimming.
Contain enzymes in the head which allow them to digest their way through the outer layer of the ovum.

Question 2

How are nerve cells specialised?

Answer 2

The function is to send electrical impulses around the body.
The long axon carries the electrical impulses from one part of the body to another.
Axon is covered in myelin, which insulates the axon and speeds up the transmission of nerve impulses.
The end of the axon has synapses. These are junctions which allow the impulse to pass from one nerve cell to another.
Dendrites increase the surface area so that other nerve cells can connect more easily.

Question 3

How are muscle cells specialised?

Answer 3

They can contract, so the protein fibres shorten, decreasing the length of the cell.
They are also packed full of mitochondria to provide energy for muscle contraction.

Question 4

Root hair cell specialisation

Answer 4

The hairs increase the surface area of the root, so it can absorb more water and minerals.
They do not contain chloroplasts because they are underground, no sunlight to carry out photosynthesis.

Question 5

How are xylem cells specialised to carry out their function?

Answer 5

Found in the plant stem. Form long tubes. These tubes carry water and dissolved minerals from the roots to the leaves.
They have very thick walls containing lignin. This provides support to the plant.
The lignin causes the xylem cells to die.
The end walls between the cells have completely broken down. This means that the cells form a long tube. So water and dissolved minerals can flow easily.

No internal structure

Question 6

Required Practical 1: Microscopes

How to use an optical microscope to view a prepared slide:

Answer 6

Add a drop of water to the middle of the slide.
Cut up an onion into layers.
Use tweezers to peel off some epidermal tissue from the bottom of one of the layers.
Use the tweezers, place the epidermal tissue into the water on the slide.
Add a drop of iodine solution. The staining highlights the objects in the cell by adding colour.
Place a cover slip and try not to get any air bubbles in there or it will obstruct the view.

Light Microscope:
Clip the slide onto the stage.
Select the lowest-powered objective lens.
Use the coarse focus knob to move the stage up just below the objective lens.
Look down the eyepiece.
Use the coarse focus knob to move the stage downwards until the image is roughly in focus.
Adjust the focus with the fine focus knob until you get a clear image of what’s in the slide.
If a greater magnification is needed, swap to a higher powered objective lens and refocus.

Question 7

Light vs electron microscopes.

Answer 7

Light microscopes have a limited magnification and limited resolution.
Limited resolution makes a blurred image. So even if we increase magnification, the image will still be difficult to see fine detail.

Electron microscopes have a much greater magnification and resolution.

Question 8

How to calculate the magnification of a microscope.

Answer 8

Size of image/ size of real object

Question 9

How do bacteria divide?

Answer 9

Simple cell division- so one bacterial cell splits into two bacterial cells.
This is called binary fission.
Bacteria can carry out binary fission once every 20 minutes as long as they have enough nutrients and the temperature is suitable.

Question 10

How do you calculate the number of bacteria after a given time?

Answer 10

number of bacteria = 2^n

n is the number of rounds of division

Question 11

How to culture bacteria

Answer 11

First sterilise all Petri dishes, bacterial nutrient broth and agar. This kills any unwanted microorganisms and prevents contamination.
Bacteria are normally transferred into the culture using an inoculating loop. To sterilise the loop before using it, we pass it through a Bunsen burner flame.
Next we attach the lid of the Petri dish using adhesive tape. This stops the lid from falling off and any unwanted microorganisms entering.
We then place the agar plate upside down into an incubator. This stops moisture from dripping down onto the bacteria and disrupting the colonies.
Incubate at 25 degrees in school labs. This reduces the chances that harmful bacteria will grow.

Question 12

Required Practical 2: Effect of antibiotic on bacterial growth.

Answer 12

Clean the bench with disinfectant solution. This kills any microorganisms that could contaminate our culture.
Sterilise an inoculating loop by passing it through a Bunsen burner flame.
Open a sterile agar plate near a Bunsen burner flame. The flame kills any bacteria in the air.
Now use the loop to spread the chosen bacteria evenly over the plate.
Place sterile filter paper discs containing antibiotic onto the plate.
Incubate the plate at 25 degrees.
The bacteria have formed a layer on the surface of the agar gel.
Around the discs, we have a region where bacteria has not grown- zone of inhibition.
To calculate the effect of the antibiotic, we find the area of the zone of inhibition.

area = pi r squared

Question 13

Describe mitosis- the cell cycle

Answer 13

First stage
The DNA replicates to form two copies of each chromosome.
The cell also grows and copies its internal structure such as mitochondria and ribosomes.
Second stage
Now mitosis takes place.
One set of chromosomes is pulled to each end of the cell. The nucleus also divides.
Final stage
The cytoplasm and the cell membrane divide to form two identical cells.

Question 14

What are the functions of mitosis?

Answer 14

Essential for the growth and development of multicellular organisms.

Mitosis takes place when an organism repairs itself.

Mitosis happens during asexual reproduction.

Question 14

What are the functions of mitosis?

Answer 14

Essential for the growth and development of multicellular organisms.

Mitosis takes place when an organism repairs itself.

Mitosis happens during asexual reproduction.

Question 15

What is an embryonic stem cell?

Answer 15

An undifferentiated cell which can give rise to more cells of the same type and can differentiate to form other types of cells.

Question 16

Stem cells in the bone marrow:

Answer 16

They can differentiate to form cells found in the blood e.g. red blood cells, white blood cells and platelets.
These are adult stem cells, so they can’t differentiate into anything else.

Question 17

Bone marrow transplants.

Answer 17

Leakaemia is a cancer of the bone marrow.
First the patient’s existing bone marrow is destroyed using radiation.
The patient then receives a transplant of bone marrow from a donor.
The stem cells in the bone marrow now divide and form new bone marrow.
They also differentiate and form blood cells.

Question 18

What is the problem with bone marrow transplants?

Answer 18

The donor has to be compatible or the white blood cells produced by the donated bone marrow could attack the patient’s body.

There is a risk that viruses can pass from the donor to the patient.

Question 19

How are stem cells used in therapeutic cloning?

Answer 19

An embryo is produced with the same genes as the patient.
Stem cells from the embryo can be transplanted into the patient without being rejected by the patient’s immune system.
Once inside the patient, the stem cells can now differentiate to replace cells which have stopped working correctly.

Question 20

What is therapeutic cloning useful for?

Answer 20

Diabetes, paralysis and more.

Some people have ethical or religious objections to this procedure.

Question 21

Plant stem cells?

Answer 21

Roots and buds contain meristem tissue.

These stem cells can differentiate into any type of plant tissue, at any point in the life of the plant.

Question 22

What could meristem tissue be used for?

Answer 22

Could clone a rare plant to stop it from going extinct.
We could produce cloned crop plants for farmers.
This can be done quickly and cheaply.

Question 23

What is the diffusion?

Answer 23

The spreading out of particles resulting in a net movement from an area of higher concentration to an area of lower concentration.

Question 24

How does oxygen move in and out of cells through diffusion?

Answer 24

Cells need oxygen for respiration which is carried out by the mitochondria.
Cells are surrounded by a high concentration of oxygen. This because oxygen is transported in the bloodstream from the lungs.
There is a lower concentration of oxygen inside the cell.
So the oxygen molecules move into the cell by diffusion from an area of high concentration to an area of lower concentration.

Question 25

How does carbon dioxide move in and out of cells through diffusion?

Answer 25

Co2 is a waste gas produced in respiration.
We have a higher concentration of carbon dioxide inside the cell than the outside.
The carbon dioxide moves out of the cell by diffusion.

Question 26

How does urea move in and out of cells through diffusion?

Answer 26

Urea is a waste product produced inside cells. It diffuses out of the cells into the blood plasma and is excreted by the kidneys.

Question 27

What affects the rate of diffusion?

Answer 27

The greater the concentration gradient, the faster diffusion takes place.
The higher the temperature, the greater the rate of diffusion. The particles have more kinetic energy so they move faster.
The larger the surface area of the cell membrane, the greater the rate of diffusion.

Question 28

What is osmosis?

Answer 28

The movement of water molecules from a dilute solution to a concentrated solution through a partially permeable membrane.

Question 29

What happens if we place an animal cell in water?

Answer 29

The cytoplasm is a relatively concentrated solution, so it contains a relatively low concentration of water.
So if we place this cell in water, osmosis will take place.
Water will move from outside the cell to inside the cell.
In an animal cell, the water moving in will cause the cell to expand and even burst.

Question 30

What if we place an animal cell in a very concentrated solution?

Answer 30

Then water will move out of the cell by osmosis the cell will shrink.

Question 31

What if we place a plant cell in water?

Answer 31

Then water will into the cell by osmosis and the cell will expand.
The cell wall prevents the plant cell from bursting. Instead the cell becomes swollen or turgid.

Question 32

What is we place a plant cell into a concentrated solution?

Answer 32

Then water moves out of the plant cell by osmosis. This causes the cell to shrink or become flaccid.

Question 33

Required Practical 3: Effects of osmosis on plant tissue.

Answer 33

First we peel the potato since the potato skin can affect osmosis.
Use a cork borer to produce three cylinders of potato. Using a cork borer makes all of the cylinders the same diameter.
Use a scalpel to trim the cylinders to the same length (around 3cm).
Anything shorter could prevent us from measuring the effect of osmosis.
Measure the length of each cylinder using a ruler and the mass of each cylinder using a balance.
Now place each cylinder into a test tube . Add 10cm^3 of 0.5 molar sugar solution to the first test tube.
Add 10cm^3 of 0.25 molar sugar solution to the second test tube and 10cm^3 of distilled water to the third test tube.
Distilled water contains no dissolved substances, so it won’t affect the rate of osmosis.
Leave the potato cylinders overnight to allow osmosis to take place.
Next remove the potato cylinders and gently roll them on paper towel to remove any surface moisture.
Don’t press on the cylinders because you don’t want to force water out of them.
Measure the length and the mass of the cylinders again.
Calculate the percentage change in length and mass.

(change in value/original value) x 100

Question 34

What is active transport?

Answer 34

Moves substances from a more dilute solution to a more concentrated solution which is against the concentration gradient. This requires energy from respiration.

Question 35

What is the difference between diffusion and active transport?

Answer 35

Diffusion: particles move down the concentration gradient. Active transport: Particles are moved against the concentration gradient.
Diffusion: does not require energy from respiration. Active transport: requires energy from respiration.

Question 36

Describe active transport in the lumen of the small intestine.

Answer 36

In the lumen, we can find the molecules produced when food is digested e.g. sugars.
The concentration of sugars in the lumen is lower than the concentration inside the cells. These sugars can’t diffuse into the cells. Instead they are carried in by active transport.
Once inside the cell, the sugars can the be transported into the bloodstream and carried around the body. The mitochondria in the cell carry out respiration to provide the energy needed for active transport.

Question 37

Describe active transport in the root hair cell.

Answer 37

The root hair cell transports ions such as magnesium into the plant from the soil.
Plants need magnesium to make chlorophyll in the leaves.
The concentration of ions in the soil is lower than concentration the in the cell.
The ions in the soil are moved across the cell membrane, against the concentration gradient.
This requires energy from the mitochondria.
These ions are then transported to the xylem vessels and moved to the leaf.