KS4 Science (Combined)

Module 1

Teacher 1 – Building Blocks of Understanding

Core declarative knowledge: What should students know?

  • Recognise/draw simple diagrams to model the difference between solids, liquids and gases.
  • Know that physical changes which differ from chemical changes because the material recovers its original properties if the change is reversed.
  • Recognise/draw simple diagrams to model the difference between solids, liquids and gases.
  • Explain the differences in density between the different states of matter in terms of the arrangement of atoms or molecules. Describe how, when substances change.
  • Know that physical changes which differ from chemical changes because the material recovers its original properties if the change is reversed.
  • Know that molecules of a gas are in constant random motion. The temperature of the gas is related to the average kinetic energy of the molecules.
  • Know that changing the temperature of a gas, held at constant volume, changes the pressure exerted by the gas.
  • Define internal energy
  • Know that heating changes the energy stored within the system by increasing the energy of the particles that make up the system. This either raises the temperature of the system or produces a change of state.
  • Know that the specific heat capacity of a substance is the amount of energy required to raise the temperature of one kilogram of the substance by one degree Celsius.
  • Know the energy needed for a substance to change state is called latent heat. When a change of state occurs, the energy supplied changes the energy stored (internal energy) but not the temperature.
  • Know the specific latent heat of a substance is the amount of energy required to change the state of one kilogram of the substance with no change in temperature.
  • Know that pure elements and compounds melt and boil at specific temperatures. Melting point and boiling point data can be used to distinguish pure substances from mixtures.
  • The basic structure of an atom is a positively charged nucleus composed of both protons and neutrons surrounded by negatively charged electrons.
  • The radius of a nucleus is less than 1/10 000 of the radius of an atom. Most of the mass of an atom is concentrated in the nucleus.
  • Know the electrons are arranged at different distances from the nucleus (different energy levels). The electron arrangements may change with the absorption of electromagnetic radiation (move further from the nucleus; a higher energy level) or by the emission of electromagnetic radiation (move closer to the nucleus; a lower energy level).
  • Know the total number of protons and neutrons in an atom is called its mass number and the number of protons is called the atomic (proton) number.
  • Know that atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element.
  • Describe the differences between an electron and light microscope.
  • Describe the differences between resolution and magnification.
  • Know that an electron microscope has much higher magnification and resolving power than a light microscope.
    This means that it can be used to study cells in much finer detail. This has enabled biologists to see and understand many more sub-cellular structures.
  • Recall the different organelles in
    plant and animal cells and describe their functions.
  • Know that bacterial cells (Prokaryotic cells) are much smaller in comparison to plant and animal cells (Eukaryotes cells). They have cytoplasm and a cell membrane surrounded by a cell wall. The genetic material is not enclosed in a nucleus. It is a single DNA loop and there may be one or more small rings of DNA called plasmids.
  • Know that diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration.

Core procedural knowledge: What should students be able to do?

  • Use appropriate apparatus to make and record the measurements needed to determine the densities of regular and irregular solid objects and liquids. Volume should be determined from the dimensions of regularly shaped objects, and by a displacement technique for irregularly shaped objects.
  • Dimensions to be measured using appropriate apparatus such as a ruler, micrometer or Vernier callipers.
  • Use and rearrange the density equation.
  • Use and rearrange the equation ∆ E = m c ∆ θ
  • Use and rearrange the equation E = m L
  • Use melting point and boiling point data to distinguish pure from impure substances.
  • Recognise and convert standard form.
  • Calculate the number of neutrons in an atom from the mass number and atomic (proton) number
  • Use prefixes centi, milli, micro and nano.
  • Use and rearrange the equation magnification = size of image
    size of real object.
  • Recognise and convert standard form.
  • Use estimations and explain when they should be used to judge the relative size or area of sub-cellular structures.
  • Recognise, draw and interpret diagrams that model diffusion.

Links to prior learning (to be made explicit and tested)

  • Conservation of material and of mass, and reversibility, in melting, freezing, evaporation, sublimation, condensation, dissolving similarities and differences, including density differences, between solids, liquids and gases
  • Brownian motion in gases diffusion in liquids and gases
  • The difference between chemical and physical changes.
  • Particle model the differences in arrangements, in motion and in closeness of particles explaining changes of state, shape and density.
  • Energy in matter changes with temperature in motion and spacing of particles internal energy stored in materials.

Teacher 2 – Building Blocks of Understanding

Core declarative knowledge: What should students know?

  • Know that before the discovery of protons, neutrons and electrons, scientists attempted to classify the elements by arranging them in order of their atomic weights.
  • Describe the steps in the development of the periodic table.
  • Know that the electrons in an atom occupy the lowest available energy levels (innermost available shells).
  • Know the electronic structure of an atom can
    be represented by numbers or by a diagram.
  • Explain how the position of an element in the periodic table is related to the arrangement of electrons in its atoms and hence to its atomic number.
  • Predict possible reactions and probable reactivity of elements from their positions in the periodic table.
  • Know the elements in Group 1 of the periodic table are known as the alkali metals and have characteristic properties because of the single electron in their outer shell.
  • Know that in Group 1, the reactivity of the elements increases going down the group.
  • Know that the elements in Group 7 of the periodic table are known as the halogens and have similar reactions because they all have seven electrons in their outer shell.
  • Know that rhe halogens are non-metals and consist of molecules made of pairs of atoms.
  • Know that in Group 7, the further down the group an element is the higher its relative molecular mass, melting point and boiling point.
  • Know that in Group 7, the reactivity of the elements decreases going down the group.
  • Explain how properties of the elements in Group 0 depend on the outer shell of electrons of the atoms.
  • Know that the boiling points of the noble gases increase with increasing
    relative atomic mass (going down the group).
  • State how the number of electrons in the outer shell affects reactivity.
  • Explain how the number of electrons in the outer shell affects reactivity.
  • Know that all substances are made of atoms. An atom is the smallest part of an element that can exist.
  • Know the definitions for atoms, elements and compounds.
  • Know relative formula mass (Mr ) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.
  • (HT) Know that the number of atoms, molecules or ions in a mole of a given substance is the Avogadro constant. The value of the Avogadro constant is 6.02 x 1023 per mole.
  • Calculate the masses of substances shown in a balanced symbol equation.
  • Calculate the masses of reactants and products from the balanced symbol equation and the mass of a given reactant or product.
  • Know that the balancing numbers in a symbol equation can be calculated from the masses of reactants and products by converting the
    masses in grams to amounts in moles and converting the numbers of moles to simple whole number ratios.
  • Know that in a chemical reaction involving two reactants, it is common to use an excess of one of the reactants to ensure that all of the other
    reactant is used.
  • Know that the reactant that is completely used up is called the limiting reactant because it limits the amount of products.
  • Explain the effect of a limiting quantity of a reactant on the amount of products it is possible to obtain in terms of amounts in moles or masses in grams.
  • Know that many chemical reactions take place in solutions. The concentration of a solution can be measured in mass per given volume of solution, eg grams per dm3 (g/dm3 ).
  • (HT only) Explain how the mass of a solute and the volume of a solution is related to the concentration of the solution.

Core procedural knowledge: What should students be able to do?

  • Explain how testing a prediction can support or refute a new scientific idea.
  • Draw electronic structures of atoms
  • Visualise and represent 2D and 3D forms including two dimensional representations of 3D objects.
  • Compare the reactivity of group 1 to group 7.
  • Predict the reactions of elements in a group based on their group
  • Use the periodic table to find information about elements
  • Represent word equations or equations using symbols and formulae.
  • Calculate the percentage by mass of an elements in a compound.
  • Calculate the Mr of a compound.
  • Use and rearrange the equation moles = mass/Mr
  • Substitute numerical values into algebraic equations using appropriate units for physical quantities.
  • Convert moles into whole number ratios
  • Calculate the mass of solute in a given volume of solution of known concentration in terms of mass per given volume of solution

Links to prior learning (to be made explicit and tested)

  • A simple (Dalton) atomic model differences between atoms, elements and compounds chemical symbols and formulae for elements and compounds
  • Conservation of mass changes of state and chemical reactions.
  • The varying physical and chemical properties of different elements
  • The principles underpinning the Mendeleev Periodic Table
  • The Periodic Table: periods and groups; metals and non-metals how patterns in reactions can be predicted with reference to the Periodic Table
  • The properties of metals and non-metals
  • The chemical properties of metal and non-metal oxides with respect to acidity.
Module 2

Teacher 1 – Transport of Larger Distances

Core declarative knowledge: What should students know?

  • Factors which affect the rate of diffusion are: • the difference in concentrations (concentration gradient) • the temperature • the surface area of the membrane.
  • A single-celled organism has a relatively large surface area to volume ratio.
  • Know that water may move across cell membranes via osmosis.
  • Know that osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
  • Describe and explain the effect of a range of concentrations of salt or sugar solutions on the mass of plant tissue.
  • Know that substances from a more dilute solution to a more concentrated solution (against a concentration gradient).
  • Know that active transport allows sugar molecules to be absorbed from lower concentrations in the gut into the blood which has a higher sugar concentration.
  • Describe the function of stem cells in embryos, in adult animals and in the meristems in plants.
  • Know that stem cells from adult bone marrow can form many types of cells including blood cells
  • Know that meristem tissue in plants can differentiate into any type of plant cell, throughout the life of the plant.
  • Cells divide in a series of stages called the cell cycle. Students should be able to describe the stages of the cell cycle, including mitosis.
  • Describe and explain three overall stages of the cell cycle but do not need to know the different phases of the mitosis stage.
  • Know that cell division by mitosis is important in the growth and development of multicellular organisms.
  • Know that meiosis leads to non-identical cells being formed while mitosis leads to identical cells being formed.
  • Sexual reproduction involves the joining (fusion) of male and female gametes: • sperm and egg cells in animals • pollen and egg cells in flowering plants.
  • Know that in sexual reproduction there is mixing of genetic information which leads to variety in the offspring.
  • Describe the difference between longitudinal and transverse waves.
  • Describe wave motion in terms of their amplitude, wavelength, frequency and period.
  • Define the wavelength of a wave.
  • Define the frequency of a wave.
  • Know that electromagnetic waves are transverse waves that transfer energy from the source of the waves to an absorber.
  • Know that electromagnetic waves form a continuous spectrum and all types of electromagnetic wave travel at the same velocity through a vacuum (space) or air.
  • Know that the waves that form the electromagnetic spectrum are grouped in terms of their wavelength and their frequency.
  • Going from long to short wavelength (or from low to high frequency) the groups are: radio, microwave, infrared, visible light (red to violet), ultraviolet, Xrays and gamma rays
  • (HT only) Know that when radio waves are absorbed they may create an alternating current with the same frequency as the radio wave itself, so radio waves can themselves induce oscillations in an electrical circuit.
  • (HT only) Know that radio waves can be produced by oscillations in electrical circuits.
  • (HT only) Know that some effects, for example refraction, are due to the difference in velocity of the waves in different substances.
  • (HT only) Students should be able to use wave front diagrams to explain refraction in terms of the change of speed that happens when a wave travels from one medium to a different medium.
  • Know that electromagnetic waves have many practical applications.
  • (HT only) Give brief explanations why each type of electromagnetic wave is suitable for the practical application.
  • Know how different electromagnetic waves can be used.
  • Know that ultraviolet waves, X-rays and gamma rays can have hazardous effects on human body tissue.
  • Know that the effects depend on the type of radiation and the size of the dose. Radiation dose (in sieverts) is a measure of the risk of harm resulting from an exposure of the body to the radiation.

Core procedural knowledge: What should students be able to do?

  • Calculate and compare surface area to volume ratios.
  • Calculate percentage gain and loss of mass of plant tissue.
  • Recognise, draw and interpret diagrams that model osmosis.
  • Plot, draw and interpret appropriate graphs of concentration v the change in mass.
  • Compare active transport to diffusion and osmosis.
  • Evaluate the practical risks and benefits, as well as social and ethical issues, of the use of stem cells in medical research and treatments.
  • Compare mitosis to meiosis.
  • Describe evidence that, for both ripples on a water surface and sound waves in air, it is the wave and not the water or air itself that travels.
  • Use and rearrange the equation period = 1/frequency
  • Use and rearrange the equation wave speed = frequency × wavelength
  • Compare the properties of different parts of the electromagnetic spectrum.
  • Draw ray diagrams demonstrating refraction.
  • Convert 1000 millisieverts (mSv) = 1 sievert (Sv)
  • Draw conclusions from given data about the risks and consequences of exposure to radiation.
  • Compare the properties and uses of different electromagnetic waves
  • Draw conclusions from given data about the risks and consequences of exposure to radiation.

Links to prior learning (to be made explicit and tested)

  • Cells as the fundamental unit of living organisms, including how to observe, interpret and record cell structure using a light microscope, the functions of the cell wall, cell membrane, cytoplasm, nucleus, vacuole, mitochondria and chloroplasts
  • The similarities and differences between plant and animal cells the role of diffusion in the movement of materials in and between cells the structural adaptations of some unicellular organisms the hierarchical organisation of multicellular organisms: from cells to tissues to organs to systems to organisms.
  • Reproduction in humans (as an example of a mammal), including the structure and function of the male and female reproductive systems, menstrual cycle (without details of hormones), gametes, fertilisation, gestation and birth, to include the effect of maternal lifestyle on the foetus through the placenta
  • Reproduction in plants, including flower structure, wind and insect pollination, fertilisation, seed and fruit formation and dispersal, including quantitative investigation of some dispersal mechanisms.
  • Waves on water as undulations which travel through water with transverse motion; these waves can be reflected, and add or cancel – superposition.
  • Frequencies of sound waves, measured in hertz (Hz); echoes, reflection and absorption of sound.
  • Sound needs a medium to travel, the speed of sound in air, in water, in solids.
  • Sound produced by vibrations of objects, in loudspeakers, detected by their effects on microphone diaphragm and the ear drum; sound waves are longitudinal.
  • Auditory range of humans and animals.
  • The similarities and differences between light waves and waves in matter light waves travelling through a vacuum; speed of light the transmission of light through materials: absorption, diffuse scattering and specular reflection at a surface.
  • Use of ray model to explain imaging in mirrors, the pinhole camera, the refraction of light and action of convex lens in focusing (qualitative); the human eye light transferring energy from source to absorber leading to chemical and electrical effects; photo-sensitive material in the retina and in cameras colours and the different frequencies of light, white light and prisms (qualitative only); differential colour effects in absorption and diffuse reflection.

Teacher 2 – Interactions Over Small & Large Distances

Core declarative knowledge: What should students know?

  • Know scalar quantities have magnitude only.
  • Know scalar quantities have magnitude only.
  • Know vector quantities have magnitude and an associated direction.
  • Know a vector quantity may be represented by an arrow. The length of the arrow represents the magnitude, and the direction of the arrow the direction of the vector quantity.
  • Know that a number of forces acting on an object may be replaced by a single force that has the same effect as all the original forces acting together. This single force is called the resultant force.
  • (HT only) Know that a single force can be resolved into two components acting at right angles to each other. The two component forces together have the same effect as the single force
  • Know that the weight of an object depends on the gravitational field strength at the point where the object is.
  • Define weight and mass.
  • Describe examples of the forces acting on an isolated object or system.
  • Know that a single force can be resolved into two components acting at right angles to each other. The two component forces
    together have the same effect as the single force.
  • Know that when a force causes an object to move through a distance work is done on the object. So a force does work on an object when the force causes a displacement of the object.
  • Know that work done against the frictional forces acting on an object causes a rise in the temperature of the object.
  • Know that when an object gains height, mass or is put in a stronger gravity field it gains GPE
  • Describe the difference between elastic deformation and inelastic deformation caused by stretching forces.
  • Know that the extension of an elastic object, such as a spring, is directly proportional to the force applied, provided that the limit of
    proportionality is not exceeded.
  • Recall that metals form positive ions and non-metals form negative ions.
  • Explain why ions form the ions they do based on gaining or losing electrons.
  • Know that the charge on the ions produced by metals in Groups 1 and 2 and by non-metals in Groups 6 and 7 relates to the group number of the element in the periodic table.
  • Know that an ionic compound is a giant structure of ions. Ionic compounds are held together by strong electrostatic forces of attraction between oppositely charged ions. These forces act in all directions in the
    lattice and this is called ionic bonding.
  • Know that when atoms share pairs of electrons, they form covalent bonds. These bonds between atoms are strong.
  • Know that covalently bonded substances may consist of small molecules. Some covalently bonded substances have very large molecules,
    such as polymers. Some covalently bonded substances have giant covalent structures, such as diamond and silicon dioxide.
  • Know metals consist of giant structures of atoms arranged in a regular
    Pattern.
  • Know the electrons in the outer shell of metal atoms are delocalised and so are free to move through the whole structure. The sharing of delocalised electrons gives rise to strong metallic bonds.
  • Know the region around a magnet where a force acts on another magnet or on a magnetic material (iron, steel, cobalt and nickel) is called the magnetic field.
  • Know the direction of the magnetic field at any point is given by the direction of the force that would act on another north pole placed at
    that point. The direction of a magnetic field line is from the north (seeking) pole of a magnet to the south(seeking) pole of the magnet.
  • Know that when a current flows through a conducting wire a magnetic field is produced around the wire. The strength of the magnetic field depends on the current through the wire and the distance from the wire.
  • Know that when a conductor carrying a current is placed in a magnetic field
    the magnet producing the field and the conductor exert a force on
    each other. This is called the motor effect.
    Recall factors that affect the size of conduction.
  • Know a coil of wire carrying a current in a magnetic field tends to rotate.
    This is the basis of an electric motor.
  • Explain how the force on a conductor in a magnetic field causes the rotation of the coil in an electric motor.
  • If an object moves along a straight line, the distance travelled can be represented by a distance–time graph. The speed of an object can be calculated from the gradient of its distance–time graph.

Core procedural knowledge: What should students be able to do?

  • Calculate the resultant of two forces that act in a straight line.
  • Use vector diagrams to illustrate resolution of forces, equilibrium situations and determine the resultant of two forces, to include both magnitude and direction (scale drawings only).
  • Use the calculation weight = mass × gravitational field strength
  • Use vector diagrams to illustrate resolution of forces, equilibrium situations and determine the resultant of two forces, to include both magnitude and direction (scale drawings only).
  • Draw vector diagrams.
  • Use the calculation work done = force × distance
  • Use and rearrange the equation g . p . e . = mass × gravitational field strength × height
  • Recall and apply the equation force = spring constant × extension.
  • Relate the number of electrons in the outer shell of an atom to the ionic charge
  • Draw dot and cross diagrams for ionic compounds formed by metals in Groups 1 and 2 with non-metals in Groups 6 and 7.
  • Deduce that a compound is ionic from a diagram of its structure in one of the specified forms
  • Draw dot and cross diagrams for covalent compounds
  • Describe the limitations of using dot and cross, ball and stick, two and three-dimensional diagrams to represent molecules or giant structures.
  • Recognise substances as metallic giant structures from diagrams showing their bonding
  • Describe how to plot the magnetic field pattern of a magnet using a compass
  • Draw the magnetic field pattern of a bar magnet showing how strength and direction change from one point to another
  • Explain how the behaviour of a magnetic compass is related to evidence that the core of the Earth must be magnetic.
  • Describe how the magnetic effect of a current can be demonstrated
  • Draw the magnetic field pattern for a straight wire carrying a current and for a solenoid (showing the direction of the field)
  • Show that Fleming’s left-hand rule represents the relative orientation of the force, the current in the conductor and the magnetic field.
  • Use and rearrange force = magnetic flux density × current × length
  • Draw distance–time graphs from measurements and extract and interpret lines and slopes of distance–time graphs, translating information between graphical and numerical form.

Links to prior learning (to be made explicit and tested)

  • Forces as pushes or pulls, arising from the interaction between two objects
  • Using force arrows in diagrams, adding forces in one dimension, balanced and unbalanced forces
  • Moment as the turning effect of a force forces: associated with deforming objects; stretching and squashing – springs; with rubbing and friction between surfaces, with pushing things out of the way; resistance to motion of air and water
  • Forces measured in newtons, measurements of stretch or compression as force is changed force-extension linear relation; Hooke’s Law as a special case
  • Work done and energy changes on deformation
  • Non-contact forces: gravity forces acting at a distance on Earth and in space, forces between magnets and forces due to static electricity.
  • Forces being needed to cause objects to stop or start moving, or to change their speed or direction of motion (qualitative only)
  • They change depending on direction of force and its size.
  • The varying physical and chemical properties of different elements
  • The principles underpinning the Mendeleev Periodic Table
  • The Periodic Table: periods and groups; metals and non-metals how patterns in reactions can be predicted with reference to the Periodic Table
  • The properties of metals and non-metals the chemical properties of metal and non-metal oxides with respect to acidity.
Module 3

Teacher 1 – Interactions With The Environment

Core declarative knowledge: What should students know?

  • Describe cellular respiration as an exothermic reaction which is continuously occurring in living cells.
  • Know that respiration in cells can take place aerobically (using oxygen) or anaerobically (without oxygen), to transfer energy.
  • Compare the processes of aerobic and anaerobic respiration with regard to the need for oxygen, the differing products and the relative amounts of energy transferred.
  • Know that anaerobic respiration in muscles is represented by the equation: glucose lactic acid As the oxidation of glucose is incomplete in anaerobic respiration much less energy is transferred than in aerobic respiration.
  • Know that anaerobic respiration in plant and yeast cells is represented by the equation: glucose ethanol + carbon dioxide Anaerobic respiration in yeast cells is called fermentation and has economic importance in the manufacture of bread and alcoholic drinks.
  • Explain how the small intestine and lungs in mammals, gills in fish, and the roots and leaves in plants, are adapted for exchanging materials. In multicellular organisms, surfaces and organ systems are specialised for exchanging materials.
  • Blood is a tissue consisting of plasma, in which the red blood cells, white blood cells and platelets are suspended. Students should know the functions of each of these blood components.
  • The body contains three different types of blood vessel: • arteries • veins • capillaries.
  • Explain how the structure of these vessels relates to their functions.
  • Know the heart is an organ that pumps blood around the body in a double circulatory system. The right ventricle pumps blood to the lungs where gas exchange takes place. The left ventricle pumps blood around the rest of the body.
  • Describe how the role of stomata and guard cells are to control gas exchange and water loss.
  • Know digestive enzymes convert food into small soluble molecules that can be absorbed into the bloodstream. Carbohydrases break down carbohydrates to simple sugars. Amylase is a carbohydrase which breaks down starch. Proteases break down proteins to amino acids. Lipases break down lipids (fats) to glycerol and fatty acids.
  • Explain how the structure of the nervous system is adapted to its functions. The nervous system enables humans to react to their surroundings and to coordinate their behaviour.
  • Explain how the various structures in a reflex arc – including the sensory neuron, synapse relay neuron and motor neurons – relate to their function.
  • Know that reflex actions are automatic and rapid; they do not involve the conscious part of the brain.
  • Know that the endocrine system is composed of glands which secrete chemicals called hormones directly into the bloodstream. The pituitary gland in the brain is a ‘master gland’ which secretes several hormones into the blood in response to body conditions.
  • (HT only) Explain how glucagon interacts with insulin in a negative feedback cycle to control blood glucose (sugar) levels in the body.
  • Describe different plant cells and transport organs.
  • Explain how the structures of plant tissues are related to their functions.

Core procedural knowledge: What should students be able to do?

  • Recognise the chemical symbols: C6H12O6, O2, CO2 and H2O.
  • Calculate and compare surface area to volume
  • Extract and interpret data from graphs, charts and tables, about the functioning of the nervous system.
  • Translate information about reaction times between numerical and graphical forms.
  • Identify the position of the following on a diagram of the human body: • pituitary gland • pancreas • thyroid • adrenal gland • ovary • testes.
  • Interpret and explain simple diagrams of negative feedback control.

Links to prior learning (to be made explicit and tested)

  • Aerobic and anaerobic respiration in living organisms, including the breakdown of organic molecules to enable all the other chemical processes necessary for life
  • A word summary for aerobic respiration
  • The process of anaerobic respiration in humans and micro-organisms, including fermentation, and a word summary for anaerobic respiration
  • The differences between aerobic and anaerobic respiration in terms of the reactants, the products formed and the implications for the organism.
  • Content of a healthy human diet: carbohydrates, lipids (fats and oils), proteins, vitamins, minerals, dietary fibre and water, and why each is needed
  • Calculations of energy requirements in a healthy daily diet.
  • The consequences of imbalances in the diet, including obesity, starvation and deficiency diseases the tissues and organs of the human digestive system, including adaptations to function and how the digestive system digests food (enzymes simply as biological catalysts)