Unit 2 - Biology 2

B2.1 Cells and simple cell transport

All living things are made up of cells. The structures of different types of cells are related to their functions. To get into or out of cells, dissolved substances have to cross the cell membranes.

Candidates should use their skills, knowledge and understanding to:

■ relate the structure of different types of cells to their function.

B2.1.1 Cells and cell structure

a) Most human and animal cells have the following parts:

■ a nucleus, which controls the activities of the cell

■ cytoplasm, in which most of the chemical reactions take place

■ a cell membrane, which controls the passage of substances into and out of the cell

■ mitochondria, which is where most energy is released in respiration

■ ribosomes, which is where protein synthesis occurs.

b) Plant and algal cells also have a cell wall made of cellulose, which strengthens the cell. Plant cells often have:

■ chloroplasts, which absorb light energy to make food

■ a permanent vacuole filled with cell sap.

c) A bacterial cell consists of cytoplasm and a membrane surrounded by a cell wall; the genes are not in a distinct nucleus.

d) Yeast is a single-celled organism. Yeast cells have a nucleus, cytoplasm and a membrane surrounded by a cell wall.

e) Cells may be specialised to carry out a particular function.

B2.1.2 Dissolved substances

a) Dissolved substances can move into and out of cells by diffusion.

b) Diffusion is the spreading of the particles of a gas, or of any substance in solution, resulting in a net movement from a region where they are of a higher concentration to a region with a lower concentration. The greater the difference in concentration, the faster the rate of diffusion.

c) Oxygen required for respiration passes through cell membranes by diffusion.

Suggested ideas for practical work to develop skills and understanding include the following:

■ observation of cells under a microscope, eg sprouting mung beans to show root hair cells

■ computer simulations to model the relative size of different cells, organelles and molecules

■ computer simulations to model the process of diffusion

■ making model cells

■ diffusion of ammonium hydroxide in a glass tube using litmus as the indicator

■ investigate how temperature affects the rate of diffusion of glucose through Visking tubing.

B2.2 Tissues, organs and organ systems

The cells of multicellular organisms may differentiate and become adapted for specific functions. Tissues are aggregations of similar cells; organs are aggregations of tissues performing specific physiological functions. Organs are organised into organ systems, which work together to form organisms.

B2.2.1 Animal organs

a) Large multicellular organisms develop systems for exchanging materials. During the development of a multicellular organism, cells differentiate so that they can perform different functions.

b) A tissue is a group of cells with similar structure and function. Examples of tissues include:

■ muscular tissue, which can contract to bring about movement

■ glandular tissue, which can produce substances such as enzymes and hormones

■ epithelial tissue, which covers some parts of the body.

c) Organs are made of tissues. One organ may contain several tissues. The stomach is an organ that contains:

■ muscular tissue, to churn the contents

■ glandular tissue, to produce digestive juices

■ epithelial tissue, to cover the outside and the inside of the stomach.

d) Organ systems are groups of organs that perform a particular function. The digestive system is one example of a system in which humans and other mammals exchange substances with the environment.

The digestive system includes:

■ glands, such as the pancreas and salivary glands, which produce digestive juices

■ the stomach and small intestine, where digestion occurs

■ the liver, which produces bile

■ the small intestine, where the absorption of soluble food occurs

■ the large intestine, where water is absorbed from the undigested food, producing faeces.

B2.2.2 Plant organs

a) Plant organs include stems, roots and leaves.

b) Examples of plant tissues include:

■ epidermal tissues, which cover the plant

■ mesophyll, which carries out photosynthesis

■ xylem and phloem, which transport substances around the plant.

B2.3 Photosynthesis

Green plants and algae use light energy to make their own food. They obtain the raw materials they need to make this food from the air and the soil. The conditions in which plants are grown can be changed to promote growth.

Candidates should use their skills, knowledge and understanding to:

■ interpret data showing how factors affect the rate of photosynthesis

■ evaluate the benefits of artificially manipulating the environment in which plants are grown.

B2.3.1 Photosynthesis

a) Photosynthesis is summarised by the equation: light energy carbon dioxide + water glucose + oxygen

b) During photosynthesis:

■ light energy is absorbed by a green substance called chlorophyll, which is found in chloroplasts in some plant cells and algae

■ this energy is used by converting carbon dioxide (from the air) and water (from the soil) into sugar (glucose)

■ oxygen is released as a by-product.

c) The rate of photosynthesis may be limited by:

■ shortage of light

■ low temperature

■ shortage of carbon dioxide.

d) Light, temperature and the availability of carbon dioxide interact and in practice any one of them may be the factor that limits photosynthesis.

e) The glucose produced in photosynthesis may be converted into insoluble starch for storage. Plant cells use some of the glucose produced during photosynthesis for respiration.

f) Some glucose in plants and algae is used:

■ to produce fat or oil for storage

■ to produce cellulose, which strengthens the cell wall

■ to produce proteins.

g) To produce proteins, plants also use nitrate ions that are absorbed from the soil.

Suggested ideas for practical work to develop skills and understanding include the following:

■ investigating the need for chlorophyll for photosynthesis with variegated leaves

■ taking thin slices of potato and apple and adding iodine to observe under the microscope

■ investigate the effects of light, temperature and carbon dioxide levels (using Cabomba, algal balls or leaf discs from brassicas) on the rate of photosynthesis

■ computer simulations to model the rate of photosynthesis in different conditions

■ the use of sensors to investigate the effect of carbon dioxide and light levels on the rate of photosynthesis and the release of oxygen.

■ light intensity

■ temperature

■ carbon dioxide concentration.

B2.4 Organisms and their environment

Living organisms form communities, and we need to understand the relationships within and between these communities. These relationships are affected by external influences.

Candidates should use their skills, knowledge and understanding to:

■ suggest reasons for the distribution of living organisms in a particular habitat

■ evaluate methods used to collect environmental data, and consider the validity of the method and the reproducibility of the data as evidence for

environmental change.

B2.4.1 Distribution of organisms

a) Physical factors that may affect organisms are:

■ temperature

■ availability of nutrients

■ amount of light

■ availability of water

■ availability of oxygen and carbon dioxide.

b) Quantitative data on the distribution of organisms can be obtained by:

■ random sampling with quadrats

■ sampling along a transect.

Suggested ideas for practical work to develop skills and understanding include the following:

■ investigative fieldwork involving sampling techniques and the use of quadrats and transects; which might include, on a local scale, the:

– patterns of grass growth under trees

– distribution of daisy and dandelion plants in a field

– distribution of lichens or moss on trees, walls and other surfaces

– distribution of the alga Pleurococcus on trees, walls and other surfaces

– leaf size in plants growing on or climbing against walls, including height and effect of aspect

■ analysing the measurement of specific abiotic factors in relation to the distribution of organisms

■ the study of hay infusions

■ the use of sensors to measure environmental conditions in a fieldwork context.

Candidates should understand:

■ the terms mean, median and mode

■ that sample size is related to both validity and reproducibility.

B2.5 Proteins – their functions and uses

Proteins have many functions, both inside and outside the cells of living organisms. Proteins, as enzymes, are now used widely in the home and in industry.

Candidates should use their skills, knowledge and understanding to:

■ evaluate the advantages and disadvantages of using enzymes in the home and in industry.

B2.5.1 Proteins

a) Protein molecules are made up of long chains of amino acids. These long chains are folded to produce a specific shape that enables other molecules to fit into the protein. Proteins act as:

■ structural components of tissues such as muscles

■ hormones

■ antibodies

■ catalysts.

b) Catalysts increase the rate of chemical reactions. Biological catalysts are called enzymes. Enzymes are proteins.

B2.5.2 Enzymes

a) The shape of an enzyme is vital for the enzyme’s function. High temperatures change the shape.

b) Different enzymes work best at different pH values.

c) Some enzymes work outside the body cells. The digestive enzymes are produced by specialised cells in glands and in the lining of the gut. The enzymes then pass out of the cells into the gut where they come into contact with food molecules. They catalyse the breakdown of large molecules into smaller molecules.

d) The enzyme amylase is produced in the salivary glands, the pancreas and the small intestine. This enzyme catalyses the breakdown of starch into sugars in the mouth and small intestine.

e) Protease enzymes are produced by the stomach, the pancreas and the small intestine. These enzymes catalyse the breakdown of proteins into amino acids in the stomach and the small intestine.

f) Lipase enzymes are produced by the pancreas and small intestine. These enzymes catalyse the breakdown of lipids (fats and oils) into fatty acids and glycerol in the small intestine.

g) The stomach also produces hydrochloric acid. The enzymes in the stomach work most effectively in these acid conditions.

h) The liver produces bile, which is stored in the gall bladder before being released into the small intestine. Bile neutralises the acid that was added to food in the stomach. This provides alkaline conditions in which enzymes in the small intestine work most effectively.

i) Some microorganisms produce enzymes that pass out of the cells. These enzymes have many uses in the home and in industry.

In the home:

■ biological detergents may contain protein-digesting and fat-digesting enzymes (proteases and lipases)

■ biological detergents are more effective at low temperatures than other types of detergents.

In industry:

■ proteases are used to ‘pre-digest’ the protein in some baby foods

■ carbohydrases are used to convert starch into sugar syrup

■ isomerase is used to convert glucose syrup into fructose syrup, which is much sweeter and therefore can be used in smaller quantities in slimming foods.

j) In industry, enzymes are used to bring about reactions at normal temperatures and pressures that would otherwise require expensive, energy-demanding equipment. However, most enzymes are denatured at high temperatures and many are costly to produce.

Suggested ideas for practical work to develop skills and understanding include the following:

■ design an investigation to find the optimum temperature for biological and non-biological washing powders to remove stains from cotton and other materials

■ investigate the action of enzymes using catalase at different concentrations and measuring the rate at which oxygen is given off from different foods, eg liver, potato, celery and apple

■ plan and carry out an investigation into enzyme action using the reaction between starch and amylase at different temperatures, pH and concentrations

■ using small pieces of cooked sausage, use 2% pepsin and 0.01M HCl in water baths at different temperatures to estimate the rate of digestion. This can also be carried out with 2% trypsin and 0.1M NaOH. The concentration of both enzymes can be varied

■ using computer simulations of enzymes to model their action in varying conditions of pH, temperature and concentration.

B2.6 Aerobic and anaerobic respiration

Respiration in cells can take place aerobically or anaerobically. The energy released is used in a variety of ways. The human body needs to react to the increased demand for energy during exercise.

Candidates should use their skills, knowledge and understanding to:

■ interpret the data relating to the effects of exercise on the human body.

B2.6.1 Aerobic respiration

a) The chemical reactions inside cells are controlled by enzymes.

b) During aerobic respiration (respiration that uses oxygen) chemical reactions occur that:

■ use glucose (a sugar) and oxygen

■ release energy.

c) Aerobic respiration takes place continuously in both plants and animals.

d) Most of the reactions in aerobic respiration take place inside mitochondria.

e) Aerobic respiration is summarised by the equation: glucose + oxygen ➞ carbon dioxide + water (+ energy)

f) Energy that is released during respiration is used by the organism. The energy may be used:

■ to build larger molecules from smaller ones

■ in animals, to enable muscles to contract

■ in mammals and birds, to maintain a steady body temperature in colder surroundings

■ in plants, to build up sugars, nitrates and other nutrients into amino acids which are then built up into proteins.

g) During exercise a number of changes take place:

■ the heart rate increases

■ the rate and depth of breathing increases.

h) These changes increase the blood flow to the muscles and so increase the supply of sugar and oxygen and increase the rate of removal of

carbon dioxide.

i) Muscles store glucose as glycogen, which can then be converted back to glucose for use during exercise.

B2.6.2 Anaerobic respiration

a) During exercise, if insufficient oxygen is reaching the muscles they use anaerobic respiration to obtain energy.

b) Anaerobic respiration is the incomplete breakdown of glucose and produces lactic acid.

c) As the breakdown of glucose is incomplete, much less energy is released than during aerobic respiration. Anaerobic respirationresults in an oxygen debt that has to be repaid in order to oxidise lactic acid to carbon dioxide and water.

d) If muscles are subjected to long periods of vigorous activity they become fatigued, ie they stop contracting efficiently. One cause of muscle fatigue is the build-up of lactic acid in the muscles. Blood flowing through the muscles removes the lactic acid.

Suggested ideas for practical work to develop skills and understanding include the following:

■ investigating the rate of respiration in yeast using carbon dioxide sensors and dataloggers

■ investigating the effect of exercise on pulse rate, either physically or using pulse sensors and dataloggers

■ investigating the link between exercise and breathing rate with a breathing sensor

■ investigating holding masses at arm’s length and timing how long it takes the muscles to fatigue

■ designing an investigation using force meters and dataloggers to find the relationship between the amount of force exerted by a muscle and muscle fatigue.

B2.7 Cell division and inheritance

Characteristics are passed on from one generation to the next in both plants and animals. Simple genetic diagrams can be used to show this. There are ethical considerations in treating genetic disorders.

Candidates should use their skills, knowledge and understanding to:

■ explain why Mendel proposed the idea of separately inherited factors and why the importance of this discovery was not recognised until after his death

■ interpret genetic diagrams, including family trees

■ construct genetic diagrams of monohybrid crosses and predict the outcomes of monohybrid crosses and be able to use the terms homozygous, heterozygous, phenotype and genotype

■ predict and/or explain the outcome of crosses between individuals for each possible combination of dominant and recessive alleles of the same gene

■ make informed judgements about the social and ethical issues concerning the use of stem cells from embryos in medical research and treatments

■ make informed judgements about the economic, social and ethical issues concerning embryo screening.

B2.7.1 Cell division

a) In body cells the chromosomes are normally foundm in pairs. Body cells divide by mitosis.

b) The chromosomes contain the genetic information.

c) When a body cell divides by mitosis:

■ copies of the genetic material are made

■ then the cell divides once to form two genetically identical body cells.

d) Mitosis occurs during growth or to produce replacement cells.

e) Body cells have two sets of chromosomes; sex cells (gametes) have only one set.

f) Cells in reproductive organs – testes and ovaries in humans – divide to form gametes.

g) The type of cell division in which a cell divides to form gametes is called meiosis.

h) When a cell divides to form gametes:

■ copies of the genetic information are made

■ then the cell divides twice to form four gametes, each with a single set of chromosomes.

i) When gametes join at fertilisation, a single body cell with new pairs of chromosomes is formed. A new individual then develops by this cell repeatedly dividing by mitosis.

j) Most types of animal cells differentiate at an early stage whereas many plant cells retain the ability to differentiate throughout life. In mature animals, cell division is mainly restricted to repair and replacement.

k) Cells from human embryos and adult bone marrow, called stem cells, can be made to differentiate into many different types of cells, eg nerve cells.

l) Human stem cells have the ability to develop into any kind of human cell.

m) Treatment with stem cells may be able to help conditions such as paralysis.

n) The cells of the offspring produced by asexual reproduction are produced by mitosis from the parental cells. They contain the same alleles as the parents.

B2.7.2 Genetic variation

a) Sexual reproduction gives rise to variation because, when gametes fuse, one of each pair of alleles comes from each parent.

b) In human body cells, one of the 23 pairs of chromosomes carries the genes that determine sex. In females the sex chromosomes are the same (XX); in males the sex chromosomes are different (XY).

c) Some characteristics are controlled by a single gene. Each gene may have different forms called alleles.

d) An allele that controls the development of a characteristic when it is present on only one of the chromosomes is a dominant allele.

e) An allele that controls the development of characteristics only if the dominant allele is not present is a recessive allele.

f) Chromosomes are made up of large molecules of DNA (deoxyribo nucleic acid) which has a double helix structure.

g) A gene is a small section of DNA.

h) Each gene codes for a particular combination of amino acids which make a specific protein.

i) Each person (apart from identical twins) has unique DNA. This can be used to identify individuals in a process known as DNA fingerprinting.

B2.7.3 Genetic disorders

a) Some disorders are inherited.

b) Polydactyly – having extra fingers or toes – is caused by a dominant allele of a gene and can therefore be passed on by only one parent who has the disorder.

c) Cystic fibrosis (a disorder of cell membranes) must be inherited from both parents. The parents may be carriers of the disorder without actually having the disorder themselves. It is caused by a recessive allele of a gene and can therefore be passed on by parents, neither of whom has the disorder.

d) Embryos can be screened for the alleles that cause these and other genetic disorders.

Suggested ideas for practical work to develop skills and understanding include the following:

■ observation or preparation and observation of root tip squashes to illustrate chromosomes and mitosis

■ using genetic beads to model mitosis and meiosis and genetic crosses

■ making models of DNA

■ extracting DNA from kiwi fruit.

B2.8 Speciation

Changes in the environment of plants and animals may cause them to die out. The fossil record shows that new organisms arise, flourish, and after a time become extinct. The record also shows changes that lead to the formation of new species.

Candidates should use their skills, knowledge and understanding to:

■ suggest reasons why scientists cannot be certain about how life began on Earth.

B2.8.1 Old and new species

a) Evidence for early forms of life comes from fossils.

b) Fossils are the ‘remains’ of organisms from many years ago, which are found in rocks. Fossils may be formed in various ways:

■ from the hard parts of animals that do not decay easily

■ from parts of organisms that have not decayed because one or more of the conditions needed for decay are absent

■ when parts of the organism are replaced by other materials as they decay

■ as preserved traces of organisms, eg footprints, burrows and rootlet traces.

c) Many early forms of life were soft-bodied, which means that they have left few traces behind. What traces there were have been mainly destroyed by geological activity.

d) We can learn from fossils how much or how little different organisms have changed as life developed on Earth.

e) Extinction may be caused by:

■ changes to the environment over geological time

■ new predators

■ new diseases

■ new, more successful, competitors

■ a single catastrophic event, eg massive volcanic eruptions or collisions with asteroids

■ through the cyclical nature of speciation.

f) New species arise as a result of:

■ isolation – two populations of a species become separated, eg geographically

■ genetic variation – each population has a wide range of alleles that control their characteristics

■ natural selection – in each population, the alleles that control the characteristics which help the organism to survive are selected

■ speciation – the populations become so different that successful interbreeding is no longer possible.