cell biology

Question 1

what is the difference between a eukaryote and a prokaryote?

Answer 1

eukaryotic cells: animal cells, plant cells, and fungi. contain their genetic material (DNA) enclosed in a nucleus. they also have a cell membrane and a cytoplasm. cell walls are in plants and fungi, but not in animals. chloroplasts (in plants), mitochondria and ribosomes.

prokaryotic cells: bacteria (single-celled organism). genetic material is not enclosed in a nucleus (either in one long loop, or in several smaller loops - plasmids). these are much smaller than eukaryotes. they have a cell membrane, a cytoplasm and a bacterial cell wall. ribosomes. no chloroplast/mitochondria.

Question 2

compare the size of a eukaryote to a prokaryote:

Answer 2

eukaryotes (5-100 micrometres) are much larger than prokaryotes (0.2-2 micrometres).

Question 3

what is meant by ‘order of magnitude’?

Answer 3

1 order of magnitude = 10x.
- a pineapple is 10x larger than a lemon, so it’s one order of magnitude larger than a lemon.

Question 4

what is the function of the nucleus?

Answer 4

to enclose genetic material.

Question 5

what is the function of the cytoplasm?

Answer 5

watery solution where chemical reactions take place. contains many of the cell organelles.

Question 6

what is the function of the cell membrane?

Answer 6

partially-permeable layer. controls the molecules that can enter and leave the cell.

Question 7

what is the function of mitochondria?

Answer 7

where aerobic respiration takes place. produces most of the energy for the cell’s chemical reactions to take place.

Question 8

what is the function of ribosomes?

Answer 8

the sites of protein synthesis - proteins are incredibly important in cells. they provide support and strength, and can act as enzymes, among many other roles.

Question 9

what are the three plant cell structures that aren’t in animal cells?

Answer 9

1. chloroplasts - contain chlorophyll, and are the sites of photosynthesis.
2. cell wall - made of a chemical called cellulose, which strengthens the cell.
3. permanent vacuole - full of a fluid called cell sap. because it’s packed with this fluid, it gives the cell shape and stores excess nutrients (e.g. water, sugar, amino acids)

Question 10

what is a ‘specialised cell’?

Answer 10

a cell that has adaptations to help it carry out a particular function. when cells become specialised, it’s called ‘differentiation’.

Question 11

how is a sperm cell adapted to its function?

Answer 11

• the job of a sperm cell is to join with an ovum (fertilisation). the genetic information of the sperm and ovum combine.
• contains genetic info in nucleus, but only half the amount of a normal adult cell.
• has a long flagellum (tail) and is streamlined, making the swim to the ovum easier.
• packed full of mitochondria, providing the energy needed for swimming.
• contains many digestive enzymes, allowing it to penetrate the ovum much easier.

Question 12

how is a neurone adapted to its function?

Answer 12

• sends electrical impulses around the body.
• has a long axon, carrying electrical impulses from one area of the body to another.
• a myelin sheath insulates the axon and speeds up the transmission of electrical impulses.
• has long dendrites, increasing the surface area so that other neurones can connect more easily.

Question 13

how is a muscle cell adapted to its function?

Answer 13

• responsible for all movements under voluntary control.
• can contract (get shorter), as they contain protein fibres, which can shorten for contraction.
• packed full of mitochondria to provide energy for muscle contraction.
• muscle cells work together to form muscle tissue.

Question 14

how is a root hair cell adapted to its function?

Answer 14

• absorb water and nutrients from the soil through osmosis.
• covered in hairs, which increase its surface area, meaning it can absorb water and dissolved more quickly and effectively.
• do not contain chloroplasts, as they’re underground and can’t photosynthesise.

Question 15

how is a xylem cell adapted to its function?

Answer 15

• transport water and nutrients from the roots to the stems and leaves (TRANSPIRATION)
• found in the plant stem, long tubes.
• very thick walls containing lignin. provides support to the plant, but also causes the xylem cells to die (but this allows free travel of water).
• end walls between the cells have broken down, allowing water to pass through.
• no subcellular structures, making it easier for water and minerals to flow.

Question 16

how is a phloem cell adapted to its function?

Answer 16

• moves food substances that the plant has produced by photosynthesis to where they are needed (TRANSLOCATION)
• made up of sieve tubes, which are specialised for transport of nutrients (e.g. sugars, amino acids) and lack nuclei
• made up of living cells (contrary to xylem tubes)
• companion cells contain lots of mitochondria to provide energy for the transport

Question 17

describe cell differentiation in animals:

Answer 17

• happens during an organism’s development. they start as one cell (a zygote), which undergoes mitosis to split into many daughter cells, which form an embryo. once part of an embryo, cells continue to undergo mitosis, and also begin to differentiate into specialised cells.
• it’s rare for cell differentiation to occur in an adult animal. it only occurs when replacing cells and repairing tissues.

Question 18

describe cell differentiation in plants, and what is an advantage of it?

Answer 18

many plant cells keep their ability to differentiate throughout their life, as their roots and buds contain meristem tissue. this is why plants are always able to create new tissue (e.g. if you cut off a part from the stem of a plant, it’ll re-grow).

• we could clone a rare plant to prevent it from going extinct, or we could produce cloned crop plants for farmers.

Question 19

what are the limitations of light microscopes?

Answer 19

• limited magnification (can’t view sub-nucleus structures)
• have limited resolution (the image is blurred, and we can’t see fine details)

electron microscopes have a much greater magnification and resolution than light microscopes. biologists can study cells in more detail, and understand sub-cellular structures better.

Question 20

what is the equation for the magnification of an object?

Answer 20

magnification = size of image / size of real object

Question 21

describe bacterial division (binary fission):

Answer 21

• bacteria multiply by simple cell division (one bacterial cell splits into two bacterial cells).
• bacteria can carry out binary fission every 20 minutes, as long as they have enough nutrients and the temperature is suitable.
• bacteria can be grown in a nutrient broth solution, or as colonies on an agar gel plate.
• uncontaminated cultures of organisms such as bacteria are required for investigating the action of disinfectants and antibiotics.

Question 22

what equation would you use to answer this question:

‘under ideal conditions, a type of bacterium divides every twenty minutes. calculate the number of bacteria present after three hours.’

Answer 22

number of bacteria = 2^n

e.g.
3 hours = 180 minutes
180/20 = 9
number of bacteria = 2^9 = 512
512 = 5.12 x 10^2

Question 23

why do organisms (especially young organisms) need a continuous supply of new cells?

Answer 23

a continuous supply of new cells enables organisms to grow, develop, and repair.

• good for young organisms for growth
• older organisms still have to replace skin cells, which fall off our bodies

Question 24

describe the cycle of mitosis:

Answer 24

1. increase the numbers of sub-cellular structures (mitochondria, ribosomes)
2. DNA is replicated
3. DNA is condensed into chromosomes (which are basically coiled up packages of genetic material). these chromosomes contain a large number of genes, and control the development of different characteristics (e.g. eye colour). humans have 23 pairs of chromosomes.
4. Each chromosome is duplicated, but stays attached to the original chromosome, forming an x-shape. Each half is called an arm. Each arm has the same DNA.
5. All 46 of the cell’s chromosomes line up at the centre of the cell, and fibres from either side of the cell attach to their respective halves of the chromosomes, and pull these halves to opposite sides of the cell (opposite poles).
6. the cell membrane and cytoplasm pull apart, forming two daughter cells. all the cells are identical (the daughters and the parent)

Question 25

what is another word for the last stage, division?

Answer 25

cytokinesis.

Question 26

what is an embryonic stem cell?

Answer 26

an undifferentiated cell in an embryo that can differentiate to form practically any type of animal cell.

Question 27

what is an adult stem cell?

Answer 27

an undifferentiated cell that is found in adults. however, these can only differentiate into certain types of related cells.

e.g. stem cells in bone marrow can only differentiate into different blood cells.

Question 28

describe a bone marrow transplant:

Answer 28

• leukaemia is a cancer of the bone marrow
• 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 29

what are problems with bone marrow transplants?

Answer 29

• the donor must be compatible with the patient, otherwise the white blood cells produced by the donated blood marrow can attack the patient’s body.
• there is a risk that viruses can be passed from the donor to the patient.

Question 30

describe stem cells in meristem tissue:

Answer 30

meristem tissue in plants can differentiate into any type of plant cell, throughout the life of the plant.

Question 31

describe the uses of meristem tissue:

Answer 31

• can produce clones of plants quickly and economically.
• rare species can be cloned to protect from extinction.
• crop plants with special features such as disease resistance can be cloned to produce large numbers of identical plants for farmers.

Question 32

what is therapeutic cloning?

Answer 32

an embryo is produced with the same genes as the patient. stem cells from the embryo can then be transplanted into the patient without affecting their immune system. once inside the patient, the stem cells can then differentiate to replace cells that have stopped working correctly.

Question 33

what are the advantages and disadvantages of therapeutic cloning?

Answer 33

• can be used for a whole range of conditions (e.g. diabetes, paralysis)
• some people have ethical/religious objections to this procedure
• could transfer a viral infection

Question 34

describe diffusion:

Answer 34

the spreading out of particles resulting in a net movement from an area of high concentration to an area of low concentration (moving down the concentration gradient)

• oxygen and carbon dioxide diffuse in and out of cells for gas exchange.
• the waste product urea moves out of cells and into the blood plasma for excretion in the kidney.

Question 35

what affects the rate of diffusion?

Answer 35

• a greater concentration gradient means that diffusion will take place faster
• the higher the temperature, the greater the rate of diffusion (particles have more kinetic energy)
• a larger surface area means that diffusion can occur more quickly

Question 36

why is surface area: volume ratio a problem for multi-cellular, large organisms?

Answer 36

their surface area is simply too small for their volume. cells on the surface get enough oxygen through diffusion, however not enough oxygen can diffuse to the cells in the centre of the organism, as they’re too far away from the surface.

Question 37

what adaptations do small intestines and lungs in mammals, gills in fish, and leaves and roots in plants have for exchanging materials?

Answer 37

• give the gills a massive surface area
• thin membrane (providing a short, diffusion pathway)
• good blood supply, ensuring blood is quickly taken away, so the concentration gradient is always high (in animals)
• being ventilated (in animals, for gaseous exchange) - maintains a steep concentration gradient.

Question 38

describe osmosis:

Answer 38

the diffusion of water from a dilute solution (lots of water) to a concentrated solution (not much water) through a partially permeable membrane.

Question 39

describe active transport:

Answer 39

moves substances from a more dilute solution to a more concentrated solution (against the concentration gradient). this requires energy from respiration.

Question 40

how does active transport benefit plants?

Answer 40

allows mineral ions to be absorbed into plant root hairs from very dilute solutions in the soil. plants require ions for healthy growth.

Question 41

how does active transport benefit humans?

Answer 41

allows sugar molecules to be absorbed from lower concentrations in the gut into the blood, which has a higher sugar concentration. sugar molecules are used for cell respiration.