B2 SPEC
Slides do conjunto (20)
-
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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
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.
-
- 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.
-
- 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.
-
- 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
-
- 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 respiration
- results 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.
-
- a) In body cells the chromosomes are normally found
- 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
-
- 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.
-
- 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.
-
- 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.