AS Biology Term 2

Question 1

Describe a chromosome and describe the structure of chromosomes

Answer 1

• Chromosomes are thread-like structure that is made up of two identical chromatids held together by a narrow region called the centromere. Each chromatid contains one DNA molecule.
• Centromeres consist of two kinetochores
• As DNA is very long and has to be packed in a small area, DNA is wound around histone proteins which prevent knots from forming.
• The combination of DNA and proteins is called chromatin. Eight histone proteins with DNA coiled around is known as a nucleosome.

Question 2

Describe the Cell Cycle

Answer 2

G1 Phase: Gap between cell division and S Phase.Cells make RNA, proteins and enzymes needed for growth.

S Phase: Synthesis of DNA, occurs when signal to divide is received. DNA replicates so that each chromosome consists of two chromatids.

G2 Phase: Gap after S phase and before cell division. Cell continues to grow, new DNA is checked, errors are repaired. Preparation of process of division.

M Phase: Nuclear division, mitosis. Growth stops temporarily, nucleus divides into two.

Question 3

Describe the stages of mitosis

Answer 3

Early Prophase:
- Chromosomes begin to appear as chromatin coils up, becoming shorter and thicker.

Late Prophase:

• Nuclear envelope and nucleolus ‘disappears’
• Chromosomes are seen to consist of two identical chromatids.
• Centrosomes move to opposite ends of nucleus where they form the poles of spindle.

Metaphase:
- Chromosomes line up across equator of spindle and are attached by their centromeres to the spindle.

Anaphase:
- Chromatids move to opposite poles, centromere first, pulled by spindle microtuble

Telophase:

• Nucleolus and nuclear envelope reform and remains of spindle break down.
• Chromatids reach poles of spindle and uncoil again.

Cytokinesis:
- Cytoplasm and cell divide into two by constriction from the edges of the cell.

Question 4

Explain the biological significance of mitosis

Answer 4

Growth: Daughter cells are genetically identical. Allows growth from unicellular zygote to multicellular organisms.

Replacement of cells/Repair of tissue: Dead cells must be replaced by identical cells.

Asexual Reproduction: Production of new individuals of a species that are genetically identical to a single parent organism.

Immune Response: Cloning of B and T lymphocytes during immune response.

Question 5

Discuss the significance of telomeres

Answer 5

• In the replication of DNA, the ends of the molecules are not included as the copying enzyme cannot run to the end of the DNA and complete the replication. This means that vital information would be lost eventually leading to cell death.
• Telomeres are added onto the ends of the chromosomes by the enzyme telomerase and consist of repeated base sequences.
• Some cells do not ‘top up’ their telomeres at each division causing telomeres to shorten at each division. This could be mechanism for ageing.

Question 6

Define and describe stem cells

Answer 6

• Stem cells are cells that can divide an unlimited number of times and when it divides, it has the potential to remain a stem cell or develop into a specialised cell.
• Totipotent stem cells have the highest potency and can produce any type of cell
• Pluripotent stem cells have a lesser potency so can produce a more limited range of cells.
• Multipotent stem cells have the lowest potency and can only produce a few cells.
• Stem cell therapy is an introduction of an adult stem cell into damaged tissue to treat disease or injury.

Question 7

Define cancer and describe the process of cancer

Answer 7

• Cancer is a result of uncontrolled mitosis. Cancerous cells divide repeatedly forming a tumour, an irregular mass of cells that usually have an abnormal change in shape
• Benign tumours are not cancerous and do not spread from the site of origin. Malignant tumours spread throughout the body. They interfere with the normal functioning of the cell.

1. Carcinogens cause mutations. Oncogenes are transformed by carcinogens.
2. Cancerous cells do not respond to signals from other cells and so continue to divide.
3. Mitosis
4. Cancerous cells not removed by immune system
5. Rapid Mitosis
6. Tumour gets bigger and cells are visibly different.
7. Tumour is supplied with blood and lymph vessels. Tumour cells spread in blood and lymph to other parts of the body
8. Metastasis: Tumour cells invade other tissue, secondary cancers form throughout the body.

Question 8

List the steps of DNA replication and discuss the different theories of DNA replication

Answer 8

Semi Conservative Replication: Correct Theory
1. DNA double helix unwinds and unzips as hydrogen bonds between bases break.

2. Two extra phosphates are added to nucleotides that are found in the nucleus, activating the nucleotides, enabling them to take part in the reaction.
3. The activated nucleotides pair up with their complementary bases on the old DNA strand. DNA polymerase links the sugar and innermost phosphate groups of nucleotides. Extra phosphates break off and move into nucleus. New DNA consists of half old DNA and half new DNA

Conservative Replication: One completely new double helix is made from the other one.

Dispersive Replication: Each new DNA molecule is made from old bits and new bits randomly scattered.

Question 9

Define the terms gene, mutations and alleles

Answer 9

Gene: A part of DNA that codes for a protein. DNA consists of many genes.

Mutation: A change in the nucleotide sequence of a gene leading to altered proteins.

Alleles: Variants of a gene which arise through the process of mutation.

Question 10

Describe sickle cell anaemia

Answer 10

• It is a mutation that affects the shape of the haemoglobin.
• Haemoglobin is made up of two alpha globin chains and two beta globin chains.
• The dominant HbA allele contains Glu and not Val amino acid while the recessive HbS allele contains a Val instead of a Glu.
• It is a substitution mutation type.

Question 11

Describe how DNA controls the activities of a cell

Answer 11

• All chemical reactions are controlled by enzymes which are proteins.
• DNA codes for proteins and hence determines protein structure.
• Each sequence of three bases stands for one amino acid

Question 12

Explain the process of protein synthesis

Answer 12

• Transcription is the first stage. It occurs in the nucleus.
  1. DNA molecule unwinds and unzips as hydrogen bonds between bases break. Free RNA molecules are activated and bond to exposed bases of one strand. RNA polymerase bonds sugar phosphate backbone together forming mRNA or messenger RNA that leaves the nucleus through the nuclear pore.

2. tRNA(transport RNA) molecules, which have a base triplet called an anticodon on one end and a site to bind with an amino acid on the other end, bind with a particular amino acid using ATP and an enzyme.

• The next stage is translocation where DNA code is translated into an amino acid sequence.
  3. mRNA binds to the small subunit of a ribosome which is made up of rRNA (ribosomal RNA) and protein. 6 bases of the mRNA(or two codons) are exposed to the large subunit where a tRNA with the complementary anticodon hydrogen bonds to the codon. AUG is always the first codon, UAC is the anticodon, and methanoine is the first protein.

4. A second tRNA molecule binds to the next codon. The closeness of the tRNA molecules form a peptide bond between the amino acids, which is catalysed by peptidyl transferase.
5. Ribosome moves along mRNA. As a third tRNA binds to the next codon, the first tRNA leaves. The polypeptide chain grows until a STOP codon is reached.

Question 13

Describe the different cells that make up a plant

Answer 13

Epidermis: Continuous layer on outside of the plant, one cell thick. Covered in waxy cuticle in stems and leaves which is waterproof. In leaves it has stomata which are pores that allow gas exchange.

Parenchyma: Thin-walled cells used as packing tissue. They are very metabolically active. It forms the cortex in roots and stems and the pith in stems.

Collenchyma: Modified parenchyma with extra cellulose in the corners of the cells providing extra strength.

Endodermis: Surrounds vascular tissue in roots and stems and is one cell thick.

Mesophyll: Specialised parenchyma cells that contain extra chloroplasts for photosynthesis. Palisade mesophyll is columnar and contains more chloroplasts, spongy mesophyll is spongy in appearance and has large air spaces.

Vascular Tissue: Contains xylem and phloem tissue. Xylem is made of xylem vessel elements and reinforced with lignin. Phloem is made of sieve tube elements which have sieve plates on the end walls.

Question 14

Define and describe the process of transpiration

Answer 14

• It is the loss of water vapour from the walls of mesophyll cells into the environment through stomata.
• Movement of water through a plant is known as the transpiration stream as water is being pulled through the plant as a result of transpiration and evaporation.

Question 15

Describe the factors affecting transpiration

Answer 15

• Humidity: High humidity decreases steepness of water potential gradient as difference in water potentials is less. This means transpiration rate is lower.
• Wind Speed and temperature: Transpiration increases as wind speed or temperature increases.
• Light Intensity: Stomata open during day and close at night as stomata opens to allow CO2 in which is used in photosynthesis which requires light and so occurs during the day. Stomata hence remain open during the day inevitably leading to water loss.

Question 16

List xerophytic adaptations

Answer 16

• deep tap roots
• thick waxy cuticle
• leaves reduced to spines
• sunken stomata
• rolled leaf using hinge cells
• hairs which trap a layer of moist air

Question 17

Describe the structure of xylem tissue

Answer 17

• Made of dead cells that join end to end to form tubes. Walls are strengthened by lignin.
• Vessel elements and tracheids are involved with the transport of water
• Schelerenchyma fibres are dead, elongated cells with lignified walls that give support to the plant. Parenchyma cells are also present.
• Xylem vessels are formed when lignin is laid down in normal plant cells until the contents of the cell dies leaving a large lumen.
• No lignin is placed where there is plasmodesmata leaving pits in the xylem wall.
• End walls break down leaving a continuous tube

Question 18

Describe the term mass flow and explain how mass flow is aided by cohesion and adhesion

Answer 18

• When water is removed from xylem vessels in the leaf, hydrostatic pressure decreases. This means that the hydrostatic pressure at the top of the xylem is less than the hydrostatic pressure at the bottom of the xylem vessel causing water to move up in continuous columns. Lignin prevents walls from collapsing under tension. This is called mass flow.
• Cohesion, or attraction between water molecules, allow water to continue movement as a continuous column. This is also helped by adhesion, or attraction of water molecules to other substances such as cellulose or lignin.

Question 19

Describe the term root pressure

Answer 19

• Hydrostatic pressure is increased at the bottom of the xylem vessel by the active secretion of solutes into the water in the xylem vessels in the root.
• This decreases water potential in the xylem and draws in water from surrounding root cells, increasing hydrostatic pressure causing water to move up the xylem.

Question 20

Describe how water moves from a root hair into the xylem

Answer 20

• Water can either cross the cortex using the symplastic or the apoplastic pathway
• Symplastic pathway: Water travels through protoplast of cells (cytoplasm or vacuole) by osmosis. They move through plasmodesmata
• Apoplastic Pathway: Water soaks into cellulose cell walls of cortical cells and move from cell wall to cell wall either directly or through intracellular spaces.
• At the endodermis, the apoplastic pathway is blocked by a thick, waxy, waterproof band of suberin called the Casparian Strip. Water can only enter endodermis through the cytoplasm of endodermal cells.
• As endodermal cells get older, suberin deposits get more extensive, except in passage cells.

Question 21

Describe how water moves from the soil into the root hairs

Answer 21

• Water is absorbed into root hairs through osmosis
• Root hair cells contain large amounts inorganic ions, decreasing water potential in root hairs, creating a water potential gradient.
• Surface area for absorption is increased by large amounts of root hair cells.

Question 22

Describe the term translocation

Answer 22

• In plants, it is the movement of assimilates from a source to a sink through the phloem.
• Assimilates are transported through the sieve tube elements.

Question 23

Describe sieve tubes and sieve plates

Answer 23

• Sieve tubes are made of living cells that are joined end to end. The area where two end walls meet, a sieve plate is formed.
• A sieve plate is made up of the walls of both elements perforated by large pores.
• Amount of cytoplasm is small and only forms a thin layer lining on the inside wall of the cell.
• There is no nucleus or ribosomes.

Question 24

Describe companion cells

Answer 24

• At least one companion cell is attached to each sieve tube element.
• They have more mitochondria and larger ribosomes and so they are more metabolically active.
• Many plasmodesmata pass through the cell walls between the companion cells and the sieve tube elements.

Question 25

Describe how the movement of assimilates in the phloem occurs

Answer 25

• Pressure difference is produced by the active loading of sucrose into sieve elements at a source.
• Loading a high concentration of sucrose into a sieve element decreases water potential and water moves into the sap.
• This causes a build up in pressure causing a pressure difference allowing mass flow to occur.

Question 26

Describe the loading of the phloem

Answer 26

• When loading sucrose into the phloem, an active H+ pump, driven by ATP, pumps H+ ions out of the cell into the cell wall creating a high H+ concentration in the wall.
• A H+-sucrose co-transporter molecule allows H+ ions to move back down a concentration gradient, carrying sucrose molecules against the concentration gradient into the companion cell.
• Sucrose molecules then move into the sieve tube through the plasmodesmata.

Question 27

Describe the mammalian cardiovascular system

Answer 27

• Consists of a pump, the heart, and a interconnecting system of tubes called blood vessels. It is a closed blood system as blood never leaves the vessels.
• Sytemic circulation is blood that leaves the left ventricle through the aorta, travels through the body, and comes back to the vena cava. Pulmonary circulation is blood that leaves the right ventricle through the pulmonary artery and returns to the pulmonary vein.
• As blood travels through the heart twice in one circuit, it is a double circulatory system.

Question 28

Describe the arteries

Answer 28

Arteries:

• Transport blood swiftly, and at high pressures, away from the heart to tissues.
• Made up of:
  1. Endothelium called the Tunica Intima: squamous epithilium. Very smooth minimising friction with flowing blood.
  2. Middle Layer called the Tunica Media: contains smooth muscle, collagen and elastic fibres.
  3. Outer Layer called tunica externa: contains elastic fibres and collagen fibres
• Elasticity of arteries allows them to ‘give’ preventing them from bursting. After pressure drops, artery walls recoil, giving blood a push and slightly increasing pressure.
• Branch into arterioles as they approach capillaries which have a higher proportion of smooth muscle.
• Blood pressure drops from 120mmHg in aorta to 85mmHg as it enters the arterioles (16kPa to 11.3kPa) and drops further to 35mmHg or 4.7 kPa as it leaves the arterioles.

Question 29

Describe capillaries

Answer 29

• Form a network throughout every tissue in the body except for the cornea and cartilage. They are used for rapid transfer of substances between cells and blood and so must be as close as possible to cells.
• Approximately 7 μm in diameter, about same size as RBC. Walls are made up of singular endothelial cells.
• Blood pressure enters at about 35mmHg and leaves at 10mmHg (4.7kPa to 1.3kPa).

Question 30

Describe Veins

Answer 30

• Veins take blood back to the heart and so blood pressure is very low at about 5mmHg or less.
• Has same 3 layers as arteries but there is no need for thick walls.
• Blood movement through veins occur due to contraction of muscles around the vein which squeeze on the vein temporarily increasing pressure.
• Semi-lunar valves are used to maintain unidirectional flow which prevent the backflow of blood.

Question 31

Describe blood plasma and tissue fluid

Answer 31

• blood consists of cells floating a pale yellow liquid called plasma which is mainly water but has other substances, such as proteins and hormones dissolved in it.
• Some plasma leaks out into intracellular spaces through gaps between the cells in the capillary walls. This liquid is known as tissue fluid.
• Tissue fluid has less protein molecules than blood plasma as protein is too big to fit through the holes in the capillary endothelium.
• If blood pressure is too high, too much fluid accumulates in the tissues forming an oedema. Arterioles reduce pressure to prevent this from happening.

Question 32

Describe lymph and the lymphatic system

Answer 32

• This is tissue fluid that has entered lymph vessels or lymphatics.
• They contain valves which allow tissue fluid in but stop it from flowing out. They are big enough for proteins to be removed.
• Lymphatics join to form lymph vessels which transport lymph to the sub-clavian veins.
• Movement of lymph is caused by contractions of muscles around the vessels. They also have smooth muscle in their walls.
• There are intervals called lymph nodes which are involved in protection against disease.

Question 33

Describe red and white blood cells

Answer 33

Red Blood Cells:
- Also known as erythrocytes. Red colour is caused by haemoglobin pigment. The function of haemoglobin is to transport oxygen from lungs to respiring tissues.
- first RBC’s are formed in the liver but is later made by bone marrow after birth. RBC’s usually rupture inside the spleen.
White Blood Cells:
- Also known as leucocytes are made in bone marrow but have a nucleus and are usually larger than RBC’s.
- Phagocytes are WBC’s that destory invading microorganisms by phagocytosis.
- Lymphocytes secrete antibodies which attach to and destroy invading cells.

Question 34

Describe haemoglobin and the haemoglobin disassociation curve

Answer 34

• Haemoglobin contains 4 haem groups which can each combine with one O2 molecule.
• The haemoglobin disassociation curve shows the effect on the partial pressure or concentration of oxygen and the saturation of haemoglobin with oxygen.
• At low partial pressures, saturation of haemoglobin is low and at high partial pressures, saturation of haemoglobin is high.
• The curve is S-shaped due to the fact that when an oxygen molecule combines with haemoglobin, the shape of the haemoglobin is slightly distorted, making it easier for a second oxygen to enter. This is the same for the third and fourth oxygen molecules.
• This means that small changes in the partial pressure cause a very large change in haemoglobin saturation.

Question 35

Describe the ways in which CO2 moves through the blood

Answer 35

• About 85% of CO2 reacts with water and forms carbonic acid, H2CO3, using the enzyme carbonic anhydrase. Carbonic acid disassociates into hydrogen ions and hydrogencarbonate ion. Haemoglobin combine with CO2 to form haemoglobinic acid and in doing so, O2 is deposited. The HCO3- ions dissolve into the plasma.
• About 5% of CO2 simply dissolve into the blood plasma.
• 10% of CO2 combines to -NH2 terminal amine group on the haemoglobin molecule to form carbaminohaemoglobin.
• As blood reaches the lungs, these reactions occur in reverse and CO2 diffuses into the alveoli.

Question 36

Explain the structural adaptations of RBC’s

Answer 36

• Shaped like biconcave disks: Increases SA:Volume ratio allowing oxygen to diffuse quicker into or out of the cell.
• Very Small: About 7μm. Means no haemoglobin molecule is very far from the cell surface membrane, allowing oxygen to be exchanged quickly. Also means it can squeeze through capillaries, bringing oxygen very close to cells.
• Very Flexible: They are bigger than some capillaries so the cells are able to deform so that they can pass through vessels.
• Have no nucleus, mitochondria or endoplasmic reticulum: Lack of organelles increases room for haemoglobin and maximises amount of O2 carries by each cell.

Question 37

Explain how carbon monoxide and high altitude affects oxygen transport

Answer 37

Carbon Monoxide:

• Haemoglobin forms an extremely stable structure (it combines 250 times more readily) than oxygen.
• CO comes from the incomplete combustion of organic compounds.

High Altitude:

• Partial pressure at high altitudes is lower than normal meaning haemoglobin saturation with oxygen is much lower.
• Altitude sickness may be the result if someone gains altitude to quickly.
• As a person becomes acclimitised to high altitudes, after a few months, they may have a higher proportion of red blood cells than normal.
• Those who permanently live at higher altitudes may develop broader chests to accommodate for larger lung capacities, and have larger hearts.

Question 38

Describe the structure of a mammalian heart

Answer 38

• Made of cardiac muscle and consists of 4 chambers. the right and left ventricles and atria. The two sides of the heart are separated by a wall of muscle called the septum.
• Coronary arteries branch from the aorta and provide the heart with its own blood supply.
• The wall of the left ventricle is the thickest as it must provide the greatest force to the blood as blood enters the body from here. It must also withstand the greatest amount of pressure.

Question 39

Describe the cardiac cycle

Answer 39

• The first stage is atrial systole in which the atria contract simultaneously. The pulmonary vein and the vena cava have semilunar valves to prevent the backflow of blood.
• About 0.1 seconds later, the ventricles contract in a stage called ventricular systole as blood is pushed out of the heart. As soon as pressure in the ventricles is greater than the pressure in the atria, the atrioventricular valves close to prevent blood from flowing into the atria. Blood moves into the aorta and pulmonary artery, pushing open the semi-lunar valves.
• Ventricular systole lasts for about 0.3 seconds, the muscles relax and ventricular diastole begins. Semilunar valves prevent backflow of blood into ventricles.
• During diastole, the whole heart relaxes and blood flows into atria from veins. The walls of the atria are easily distended and so some blood trickles into the ventricles. The atrial muscle then contracts and the cycle starts again.

Question 40

Describe how the heart beat is controlled

Answer 40

• Cardiac muscle is myogenic meaning is naturally contracts and relaxes.
  1. There is a patch of muscle tissue in the right atrium wall called the sinoatrial node which automatically contracts and relaxes.
  2. As muscle in SAN contracts, an electrical excitation wave sweeps through the muscle in the atria of the heart causing the atrial walls to contract.
  3. The impulse reaches the atrioventricular node which delays the impulse before it travels down to the ventricles. This means that the ventricles receive the signal after the atria, staggering their contractions.
  4. The impulse moves down through septum of the heart, along fibres called Purkyne tissue. Once impulse reaches the apex of the heart, it sweeps upwards through the ventricle walls and the ventricles contract.
  5. The ventricles then relax and the SAN contracts again, restarting the cycle.