Enzymes. Enzymes are protein molecules, with complex shapes which are important for their activity: part of an enzyme molecule is its.

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1 Enzymes The lock and key theory is a simple model to explain the action of enzymes. Lock and key theory Enzymes are protein molecules, with complex shapes which are important for their activity: part of an enzyme molecule is its active site active site a substrate molecule fi ts into the active site, like a key fi ts into a lock reaction other substances do not fi t, so are not substrates for the enzyme if the shape of an active site changes, the enzyme is denatured and no longer works. enzyme molecule substrate molecule substrate molecule fits the active site products released Effect of temperature The rate of chemical reactions increases as the temperature increases. This happens with enzyme-catalysed reactions, but: there is an optimum temperature at which there is a maximum reaction rate at higher temperatures the active site changes shape, reducing the reaction rate. Rate of reaction optimum temperature the enzyme works at its maximum rate rate increases as the rate of collisions between the substrate active site increases heating begins to change the shape of the active site enzyme is denatured Temperature ( C) Effect of ph The ph of an enzyme s surroundings affects the shape of the active site. This means: enzymes have an optimum ph at which there is a maximum reaction rate below and above the optimum ph, the bonds in the enzyme are affected, so the shape of the active site changes. Rate of reaction enzyme is denatured 4 5 optimum ph (different for different enzymes) enzyme is denatured ph Bile is made in the liver, stored in the gall bladder and released into the small intestine. Explain its role in the digestion of fats. (3 marks) Bile is alkaline, so it neutralises hydrochloric acid made by the stomach. It also emulsifies fat, breaking it into small droplets. This increases the surface area of the fat. The alkaline conditions and the large surface area increase the rate that lipases break down fat. Partly digested food is acidic as it reaches the small intestine. The optimum ph for the lipase is ph (alkaline conditions). 1 Proteases digest proteins in the stomach. Suggest the optimum ph of these proteases, and justify your answer. 2 Explain the importance of the active site for enzyme activity at different temperatures and ph values. 19

2 Neurones The nervous system allows humans to react to their surroundings and to coordinate their behaviour. Neurones are cells in the nervous system. Types of neurones There are three main types of neurones: sensory neurones carry electrical impulses from receptors to the central nervous system (CNS) relay neurones, found in the CNS, carry impulses from sensory neurones to motor neurones motor neurones carry impulses from the CNS to effectors, such as muscles and glands. Relay neurone Sensory neurone Relay neurone The electrical impulse jumps from one gap in the myelin sheath to the next, speeding up the rate of transmission. Sensory neurone skin receptor cells direction of impulse dendron dendrites collect impulses from receptor cells cell body carry impulses long distances dendrites collect impulses from sensory neurones terminals pass impulses to other neurones fatty myelin sheath Insulates the neurone. The electrical impulse cannot cross the fatty myelin sheath. The diagram shows the structure of a motor neurone. Explain how this cell is adapted to its function in the nervous system. The is long so it can transmit impulses over long distances. The dendrites can receive impulses from other neurones. The terminals can connect with effectors such as muscles and glands. The fatty myelin sheath insulates the from other neurones, and the electrical impulses can jump quickly from one gap to another. myelin sheath cell body terminals dendrite 1 Compare the roles of sensory, relay and motor neurones in the nervous system. (3 marks) 54 cell body terminals pass impulses to other neurones by The insulation provided ps sto th the myelin shea mly impulses travelling rando s. ne uro into neighbouring ne Nerve impulses travel from dendrites towards the cell body, and from the cell body towards the terminals. All types of neurone have an, but: motor neurones and relay neurones do not have a dendron. 2 Explain how the structure of a sensory neurone is related to its function.

3 Transpiration Transpiration is the movement of water from roots to leaves, where it evaporates into the air. Transpiration stream The transpiration stream is the continuous movement of water through a plant. It is driven by the evaporation of water from the leaves. The transpiration stream also carries dissolved mineral ions. You can revise Water is lost by evaporation from the leaves, mainly through the stomata. Water travels through the stem and into the leaves. Water enters the xylem and moves from the roots to the stem. adaptations of root hair cells and xylem tissue on page 5. Stomata The stomata are openings on the surface of a leaf. They are found mainly on the lower surface: most evaporation from a leaf happens through its stomata the size of the opening in a stoma is controlled by its guard cells the stomata control gas exchange and water loss. Water enters root hair cells by osmosis. Mineral ions enter by active tgransport. When guard cells take in water by osmosis, they become turgid swollen which causes the stoma to open. vacuole chloroplast cell wall stoma Four leaves were cut from a plant. Their cut ends were sealed with grease. The leaves were covered in grease in the following ways. A neither surface was greased B the upper surface was greased C the lower surface was greased D both surfaces were greased. The mass of each leaf was measured. They were left for the same time before measuring their mass again. The table shows the results. also affect the Environmental factors page 31). rate of transpiration (see 1 Describe how water moves into a plant and escapes from the leaf. 30 When guard cells lose water, they become flaccid and the stoma closes. stoma open Leaf A B C D stoma closed % loss in mass Explain these results. The loss in mass is due to evaporation of water. Leaves A and B lost the most water vapour because it could still escape through the stomata on the lower surface. Leaves C and D lost very little water because their stomata were covered. 2 Guard cells become turgid in the daytime when it is light, and flaccid when it is dark at night. Explain the effect of this change on the rate of transpiration.

4 Viral diseases Pathogens are microorganisms that cause infectious disease. Viruses are a type of pathogen. Viruses A virus consists of genetic material (DNA or RNA) surrounded by a coat of proteins and lipids. e r than Viruses are much smaller than bacteria. bacterium teria may releaseviruses: toxins ons) that make us feel areill.much e types of bacteria invade exist and nd destroy body cells. virus fungi Viruses take over a body Fungi are cell's DNA, eukaryotic smaller thancausing cells the cell to make toxins or causing organisms. reproduce inside cells, causing cell damage when new viruses are released from cells. damage reproduce rapidly in the body can infect plants or animals are spread by direct contact, water or air. Colds and flu are viral diseases. Virus life cycle virus enters the cell through the cell membrane cell produces more viral DNA or RNA, and viral proteins protist Protists are eukaryotic organisms. Many new are viruses are assembled free-living but someinside the cell are pathogens. new viruses leave or burst out of the cell Human viral diseases The table shows some information about two human viral diseases. Measles virus How it is spread Symptoms of infection HIV sexual contact inhalation of droplets from exchange of body fluids such as blood, sneezes and coughs e.g. when drug users share needles flu-like illness at the start fever and a red skin rash attacks body s immune cells unless it is can be fatal if complications controlled with antiretroviral drugs occur (so most young children AIDS (late stage HIV infection) occurs are vaccinated against when the body s immune system is measles) too badly damaged to deal with other infections or cancers Tobacco mosaic virus, TMV, is a plant pathogen. Describe the symptoms of infection by TMV. The leaves get a distinctive mottled or mosaic pattern of discoloration. This reduces the rate of photosynthesis, so the growth of the plant is affected. 1 Give two ways in which viruses can be spread. 34 TMV affects tomato plants and many other species of plants, not just tobacco plants. The virus can spread by contact from cell to cell, and plant to plant. It can also travel through the phloem. 2 Explain how viruses damage their host cells. e The host cell is th s. ct fe in cell the virus

5 Papers 1 & 2 Investigating microbial cultures Practical skills You can measure the effect of antiseptics and antibiotics by measuring zones of inhibition. The effects of antiseptics Aims To investigate the effect of antiseptics on bacterial growth using agar plates. Apparatus Petri dish with bacteria spread over nutrient agar filter paper discs, about 4 mm in diameter three different antiseptics forceps adhesive tape permanent marker pen Bunsen burner and heat-resistant mat Methods 1 Turn the Petri dish upside down and mark the bottom into three equal segments. 2 Mark a spot in the middle of each segment. Number each segment. Write your name and the date on the bottom. 3 Add a different antiseptic to each disc. Record what you added to each segment. 4 Partly lift the Petri dish and use the forceps to place a disc over each spot. 5 Use adhesive tape to secure the lid. 6 Incubate the plate for 2 days at 25 C. Results Measure the diameter of the clear zone around each paper disc: measure in millimetres, mm make two measurements at 90 to each other, and calculate the mean diameter calculate the area of each clear zone, giving your answers in standard form in metres squared, m 2. You could also investigate the effects of antibiotics on bacterial growth. Your teacher or school technician will prepare the Petri dish for you. They will: add a few drops of a bacterial culture to the agar on the Petri dish use a sterile glass spreader to spread the bacteria over the agar. Certain strains of E. coli are suitable to use. Make sure you wipe the bench with a disinfectant solution, then dry it with clean paper towels before starting. Before you use the forceps, briefly pass the end of the forceps through a Bunsen burner flame to sterilise them. The area of a circle You can calculate the area of a circle using: area = πr 2 where r = radius. Divide the diameter by 2 to fi nd the radius. Remember that 1 mm = m. DEEP PAGE 8 A student investigates the effect of three different antiseptics on the growth of bacteria. The table shows her results. Diameter of the clear zone (mm) Antiseptic Measurement 1 Measurement 2 Mean value Area (m 2 ) A B C The values for antiseptic A are already shown. Include these when you make your conc

6 Papers 1 & 2 Specialised animal cells Animal cells may be specialised to carry out a particular function. Ciliated cells Cilia are tiny hair-like structures on the surface of some cells. They can move in organised ways to push substances past the surface of the cell. cilia ribosomes mitochondrion cell membrane cytoplasm Ciliated cells line the trachea or windpipe. These sweep mucus away from the lungs, carrying away dirt and bacteria. Differentiation As an organism develops, cells differentiate to form different specialised cells: most types of animal cell differentiate at an early stage cells acquire different structures so muscle they can carry out a particular function cells cell division in mature animals is mainly for repair and replacement. fibrils that You can revise: contract the structures in generalised animal cells on page 2 stem cells on page 10. sperm cell Sperm cells Nerve cells Sperm cells carry the male parent s genetic information to the female acrosome parent s egg cell. They may have to travel a relatively long way, then tail get through the surface of the egg cell. Nerve cells carry electrical impulses from one part of the body to another, which may be a long way apart. Nerve cells often connect with one another. A sperm cell has: a tail, and many mitochondria to release the energy needed for the tail to move enzymes in its acrosome to digest the outer layers of an egg cell. tail for swimming A typical nerve cell has: a long to connect distant parts tiny finger-like dendrites that make connections with other nerve cells. The diagram shows muscle cells. Describe, using information shown, how they are specialised for their function. protein fibres Muscle cells also con carries electrical tain many impulsesmitochondria. These supply the energy needed to keep the muscles working. The protein fibres slide over each other, making the muscle cells contract.secretes sperm (produces hormones) cell tail for swimming 4 to cells movein the trachea and Different specialised cells have different shapes. Explain how the shapescontracts of ciliated parts of the body sperm cells are related to their functions. nerve cell long fibre swims to an egg to fertilise it