Topic 6.2: The transport system

6.2.1 Draw and label a diagram of the heart showing the four chambers, associated blood vessels, valves and the route of blood through the heart

6.2.2 State that the coronary arteries supply heart muscle with oxygen and nutrients

The heart is a muscle itself, so note that the walls of the heart are supplied with oxygenated blood via coronary arteries. These arteries and the capillaries they serve, deliver to the muscle fibres of the heart the oxygen and the nutrients essential for the pumping action.

6.2.3 Explain the action of the heart in terms of collecting blood, pumping blood, and opening and closing of valves

The cavity of the heart is divided into four chambers, with those on the right side of the heart completely separate from those on the left. The two upper chambers are called the artia and these receive blood into the heart.

The lower chambers are thick-walled ventricles, with the muscular wall of the left ventricle much thicker than that of the right ventricle. However, the volumes of the right and left sides (the quantities of blood they contain) are identical. The ventricles pump blood out of the heart.

The valves of the heart prevent backflow of the blood, thereby maintaining the direction of flow through the heart. The artio-ventricular valves are large valves, positioned to prevent backflow from ventricles to artia. The edges of these valves are supported by tendons anchored to the muscle walls of the ventricles below. A different type of valve separates the ventricles from the pulmonary artery and the aorta. These are called the semi-lunar valves.

6.2.4 Outline the control of the heartbeat in terms of myogenic muscle contraction, the role of the pacemaker, nerves, the medulla of the brain and epinephrine (adrenaline)

The heart normally beats about 75 times per minute - approximately 0.8 seconds per beat. In each beat, the heart muscle contracts strongly, followed by a period of relaxation.

The atrium contacts pushing blood past the bicuspid valve into the ventricle, Atrium muscle relax. Ventricle muscles contract, causing blood pressure to close the bicuspid valve and open the semilunar valves, forcing blood into the aorta. Ventricle and atrium muscle relax, while the pressure of blood in the aorta causes the semilunar valves to shut. Blood flows into the atrium and opens the bicuspid valve as it starts to flow into the ventricle.

The heart beats rhythmically throughout life, without rest, apart from the momentary relaxation between beats. Even more remarkably, the origin of each beat is within the heart itself - we say the heart beat is myogenic in origin.

Beats originate in a strucuture in the muscle of the wall of the right atrium, called the pacemaker. Special muscle fibre radiate out from the pacemaker, conducting the impulse to the muscles of both artia, triggering contraction. Then a second excitation is passed onto the ventricles causing a ventricular contraction.

The heart receives impulses from a control centre in the hindbrain (medulla), via two nerves. One nerve, when stimulated, triggers speeding up of the heart rate, and the other nerve triggers a slowing down of the heart. Since these nerves have opposite effect, they are antagonistic.

The hormone adrenaline is secreted by the adrenal glands and carried in the blood, causes the pacemaker to increase the heart rate.

6.2.5 Explain the relationship between the structure and function of arteries, capillaries and veins

There are three types of vessels in the circulation system.

Arteries
- Carry blood away from the heart
- Carry blood at high pressures
- Have a narrow lumen
- Have a thick middle layer to help the pulse flow
- Have a outer layer to prevent rupture

Capillaries
- Are a fine networks of tiny tubes linking arteries and veins.
- Involved with material and gas exchange with the surrounding body tissues
- Blood pressure is relatively low
- Small diametre
- Single cell to allow easier diffusion
- Contains pores to aid the transport of materials

Veins
- Carry blood to the heart
- Carry blood at low pressure
- Very wide lumen
- Have valves to prevent blooding pooling
- Thin surrounding tissue as blood do not go in rhythmic pulse.

6.2.6 State that blood is composed of plasma, erythrocytes, leucocytes (phagocytes and lymphocytes) and platelets

Blood is a special tissue consisting of a liquid medium called plasma in which are suspended red cells or erythrocytes, white cells or luecocytes, and platelets. The plasma is the medium for exchange of substances between cells and tissues, the erythrocytes are involved in transport of respiratory gases.

6.2.7 State that the following are transported by the blood: nutrients, oxygen, carbon dioxide, hormones, antibodies, urea and heat

The blood circulation has roles in the body's defense against diseases as well as being the all-important transport system of the body. Nutrients from digestion, oxygen and carbon dioxide, urea, hormones and antibodies are all transported. In the tissues of the body, exchange between the blood and cells of the tissues occurs from the capillaries, the walls of which are permeable and highly "leaky".

Tissue respiration - gas exchange
Hydration - water to all the tissues
Nutrition - Nutrients (sugars, amino acids, lipids, vitamins) and inorganic ions to all cells.
Excretion - Waste product urea to kidneys
Development and co-ordination - Hormones from endocrine glands to target organs
Temperature regulation - distribution of heat
Defense against disease - Antibodies are circulated in the blood system

Topic 6.1: Digestion

6.1.1 Explain why digestion of large food molecules is essential.

Most food is solid and in the form of large complex molecules which are insoluble and chemically inert (not readily stable)

Large molecules need to be broken down into smaller molecules that can be readily absorbed across membranes and into cells. Small molecules can be reassembled into new products (e.g. amino acids can be reassembled to form new proteins).

6.1.2 Explain the need for enzymes in digestion

Enzymes are biological catalysts that speeds up the rate of reaction. It allows reactions to take place at body temperatures. They are specific for a certain type of reaction though based on the lock and key model.

6.1.3 State the source, substrate, products and optimum pH conditions for one amylase, one protease and one lipase

Amylase

Example - Salivary amylase
Source - Salivary glands
Substrate - Starch
Product - Maltose
Optimum pH - 7

Protease

Example - Pepsin
Source - Gastric juice
Substrate - Protein
Product - Short polypeptides
Optimum pH - 2

Lipase

Example - Pancreatic juice
Source - Pancreas
Substrate - Lipids (triglyceride)
Product - Glycerol and fatty acids
Optimum pH - 8

6.1.4 Draw and label a diagram of the digestive system

6.1.5 Outline the function of the stomach, small intestine and large intestine

Stomach

Present in the wall of the stomach are millions of tiny pits called gastric glands which secrete the components of gastric juice. This juice includes hydrochloric acid - sufficiently acidic to create an environment of pH 1.5-2.0, which is the optimum pH for protein digestion by the protease enzymes of the gastric juice. These proteases, of which pepsi is one, are formed in cells of gastric glands and secreted in an inactive state. The hydrochloric acid then activates them, and kills most of the in coming bacteria in the food.

The whole stomach lining is supplied with goblet cells that secrete mucus. Mucus bathes the interior lining of the stomach, forming an effective barrier to both the hydrochloric acid and the protease of the gastric juices, preventing autolysis (self-digestion) of the stomach wall.

As the food is mixed with gastric juice and churned by muscle action it becomes a semi-liquid called chyme. The churning action of the stomach is an important part of the mechanical digestion process. A typical meal may spend up to four hours in the stomach.

Small intestines

Food enters the first part of the small intestine (known as the duodenum) a little at a time. Here the chyme meets bile from the bile duct, and the pancreatic juice from the pancreas. Bile is strongly alkaline and neutralises the acidity of the chyme. It also lowers the surface tension of large fat globules, causing them to break into tiny droplets, a process called emulsification. This speeds digestion by the enzyme lipase later on. Bile itself contains no enzyme.

All these enzymes act as the chyme, bile and pancreatic juice are mixed together by a churning action (a form of peristalsis) called segmentation.

6.1.6 Distinguish between absorption and assimilation

Absorption can be defined as the movement of particles or dissolved substances across a membrane

Assimilation can be defined as the conversion of nutrients into fluids or solid parts of an organism.

6.1.7 Explain how the structure of the villus is related to its role in absorption and transport of the products of digestion

There are different structure of the villus which aid its role in absorption.

Micro-villi is present to provide a huge surface area for absorption.

Epithelium cells has a single layer of small cells, packed with mitochondria - the source of ATP for active uptake across the plasma membrane.

Protein pumps in the plasma membrane of the epithelial cells can actively transport nutrients across the plasma membrane into the villi

There is also a big network of capillaries which proved a large surface area for uptake of amino acids, monosaccharides, and fatty acids and glycerol into blood circulation.

Lacteal is a branch of the lymphatic system into which triglycerides (combined with protein) pass for transport to body cells

The mucus from goblet cells in the epithelium can lubricate movement of digested food among the villi and protects plasma membrane of epithelial cells.

Topic 6: Human health and physiology

Topic 6 of the IB SL Biology syllabus is the Human health and physiology. IBO recommends to spend 20 hours on this topic.

This topic has 6 sub-chapters: "Digestion", "The transport system", "Defense against infectious defense", "Gas exchange", "Nerves, hormones and homeostasis" and "Reproduction". Each are separated with numerical values in order of mentioned.

These are all SL syllabus statements, it is recommended to bring a Casio Graphical Calculator instead of Texas.

Topic 10.3: Polygenic inheritance

10.3.1 Define polygenic inheritance

Polygenic inheritance refers to a single characteristic that is controlled by more than two genes (also called multifactorial inheritance)

Polygenic inheritance patterns normally (bell-shaped) distribution curve - it shows continuous variation

By increasing the number of genes controlling a trait, the number of phenotype combinations also increase, until the number of phenotypes to which an individual can be assigned are no longer discrete, but continuous

10.3.2 Explain that polygenic inheritance can contribute to continuous variation using two examples, one of which must be human skin colour

Human skin colour

• The colour of human skin is determined by the amount of dark pigment (melanin) it contains
• At least four (possibly more) genes are involved in melanin production; for each gene one allele codes for melanin production, the other does not
• The combination of melanin producing alleles determines the degree of pigmentation, leading to continuous variation

Grain colour in wheat

• Wheat grains vary in colour from white to dark red, depending on the amount of red pigment they contain
• Three genes control the colour and each gene has two alleles (one coding for red pigment, the other coding for no pigment)
• The most frequent combinations have an equal number of "pigment producing" and "no pigment" alleles, whereas combinations of one extreme or the other are relatively rare
• The overall pattern of inheritance shows continuous variation

Topic 10.2: Dihybrid crosses and gene linkage

10.2.1 Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes

A dihybrid cross determines the allele combinations of offspring for two particular genes that are unlinked (not on the same chromosomes)

Because there are two genes with two alleles per gene (multiple alleles not required), there can be up to four different gamete combinations

10.2.2 Distinguish between autosomes and sex chromosomes

Autosomes: Pairs of chromosomes that are identical in appearance (e.g. same size, same gene loci, etc) and are not involved in sex determination

Sex chromosome: Pairs of chromosomes involved in sex determination and are not identical in appearance (e.g. X and Y chromosome in humans)

10.2.3 Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange in alleles

During crossing over in prophase I, non-sister chromatids of a homologous pair may break and reform at points of attachment called chiasmata

As these chromatids break at the same point, any gene loci below the point of the break will be exchanged as a result of recombination

This means that maternal and paternal alleles may be exchanged between the maternal and paternal chromosomes, creating new gene combinations

The further apart two gene loci are on a chromosome, the more likely they are to be exchanged

10.2.4 Define linkage group

A linkage group is a group of genes whose loci are on the same chromosome and therefore do not follow the law of independent assortment

Linkage genes will tend to be inherited together - the only way to separate them is through recombination (via crossing over during synapsis)

10.2.5 Explain an example of a cross linkage between two linked genes

When two genes are linked, they do not follow the expected phenotypic ratio for a dihybrid cross between heterozygous parents

Instead the phenotypic ratio will follow that of a monohybrid cross as the two genes are inherited together

This means that offspring will tend to produce the parental phenotypes

Recombinant phenotypes will only be evident if crossing over occurs in prophase I and would thus be expected to appear in low number (if at all)

An example of a cross between two linked genes is the mating of a grey bodied, normal wing fruit fly with a black bodied, vestigial wing mutant

10.2.6 Identify which of the offspring are recombinants in a dihybrid cross involving linked genes

Recombinants of linked genes are those combinations of genes not found in parents

For example, in a test cross of a heterozygous fruit fly (grey bodied, normal wings) with a homozygous recessive mutant (black bodied, vestigial wings), the recombinants would be the grey bodied, vestigial winged offsprings and the black bodied, normal winged offspring

Linked genes that have undergone recombination can be distinguished from unlinked genes via test cross because the frequency of the recombinant genotype will always be less than would occur for unlinked genes (crossing over does not happen every time)

• For example:
• Heterozygous test cross of unlinked genes = 1:1:1:1 phenotypic ratio
• Heterozygous test cross of linked genes = 1:1:0.1:0.1 pehnotypic ratio (uncommon phenotypes are recombinants)

Topic 10.1: Meiosis

10.1.1 Describe the behaviour of the chromosomes in the phases of meiosis

Interphase: Cell growth and DNA replication (duplication of DNA creates sister chromatid chromosome)


Meiosis I

• Prophase I: DNA supercoils and chromosomes condense, nuclear membrane dissolves, homologous pairs form bivalents, crossing over occurs
• Metaphase I: Spindle fibres from centrioles (at poles) attach to centromeres of bivalent, bivalents line up along the equator of the cell
• Anaphase I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
• Telophase I: Chromosomes decondense, nuclear membranes may reform, cell divide (cytokinesis) forming two haploid daughters cells

Interkinesis: An optional rest period between meiosis I and meiosis II, no DNA replication occurs in this stage

Meiosis II

• Prophase II: Chromosomes condense, nuclear membrane dissolves (if reformed), centrioles move to opposite poles (perpendicular to previous poles)
• Metaphase II: Spindle fibres form centrioles attach to centromeres of chromosomes, chromosomes line up along the equator of the cell
• Anaphase II: Spindle fibres contract and split the chromosome into sister chromatids, chromatids (now called chromosomes) move to opposite poles
• Telophase II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis), resulting in four haploid daughter cells

10.1.2 Outline the formation of chiasmata in the process of crossing over

Crossing over involves the exchange of segments of DNA between homologous chromosomes during Prophase I of meiosis

The process of crossing over occurs as follows

• Homologous chromosomes become connected in a process called synapsis, forming a bivalent (or tetrad)
• Non-sister chromatids break and recombine with their homologous partner, effectively exchanging genetic material (crossing over)
• The non-sister chromatids remain connected in an X-shaped structure and the positions of attachment are called chiasmata

Chiasmata hold homologous chromosomes together as a bivalent until anaphase I

As a result of crossing over, chromatids may consist of a combination of DNA derived from both homologous - these are called recombinant

10.1.3 Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I

During anaphase I, homologous chromosomes separate, such that each resultant daughter (and subsequent gametes) contains a chromosome of either maternal or paternal origin

The orientation of these homologous in metaphase I is random, such that there is an equal probability of the daughter cell having either the maternal or paternal chromosome

As humans have a haploid number of 23 chromosomes, this means that there is 2²³ potential gamete combinations (over 8 million combinations)

Crossing over in prophase I results in entirely new chromosome combinations, as recombination through gene exchange produces wholly original chromosomes containing both maternal and paternal DNA, resulting in near infinite genetic variability

Other sources of genetic variation include random fertilisations, DNA mutations, chromosome mutations and non-disjunction

10.1.4 State Mendal's law of independent assortment

Gregor Mendal was a 19th century Moravian monk who demonstrated that the inheritance of traits (i.e. genes) followed particular laws:

• Law of segregation: Each herediatary characteristics is controlled by two alleles, which segregate and pass into different reproductive cells (gametes)
• Law of independent assortment: The separation of alleles for one gene will occur independently of the separation of alleles for another gene
• According to the law of independent assortment, different allele combinations should always be equally impossible
• However this law only holds for genes that are on different chromosomes - the law of independent assortment does not apply to linked genes

10.1.5 Explain the relationship between Mendal's law of independent assortment and meiosis

The law of independent assortment relates to the random orientation of homologous chromosomes in metaphase I of meiosis

Because the orientation of a homologous pair is random, and does not affect the orientation of any other homologous pair, any one of a pair of alleles on a chromosome has an equal chance of being paired with, or separated from, any one of a pair of alleles on another chromosome

This means the inheritance of two different traits will occur independently of each other (provided the genes aren't linked)

Topic 10: Genetics

Topic 10 of the IB HL Biology syllabus is the Genetics. IBO recommends to spend 6 hours on this topic.

This topic has 3 sub-chapters: "Meiosis", "Dihybrid crosses and gene linkage" and "Polygenic inheritance". Each are separated with numerical values in order of mentioned.

These are all HL syllabus statements, it is recommended to bring a Casio Graphical Calculator instead of Texas.