Biology, B4

Gateway OCR, B4- its a green world

HideShow resource information


Leaves are designed for making food by photosynthesis

They have:

  • Waxy cuticle
  • upper epidermis
  • palisade layer
  • spongy mesophyll layer
  • chloroplasts
  • vein
  • air spaces
  • lower epidermis
  • stoma (plural= stomata)
  • Guard cell
1 of 37

Adaptations of a leaf

  • Broad- large surface area exposed to light
  • thin- carbon dioxide and water vapour only have to travel a short distance
  • there are air spaces in the spongy mesophyll layer- allows gases to move through cells easily, also have a very big internal surface area to volume ratio
  • contain lots of chlorophyll found in chloroplasts in the palisade layer so they get most light
  • upper epidermis is transparent allowing most light to the palisade layer
  • lower epidermis has holes called stomata, let gases in and out and allow transpiration to happen
  • have a network of veins to deliver water and nutrient also help support the leaf
2 of 37

Leaf Palisade cells

  • are designed for photosynthesis
  • contain lots of chloroplasts for photosynthesis
  • tall shape means have alot of surface area for absorbing CO2
  • tall shape also means good chance of light hitting chloroplasts before reaching the bottem of the cell
3 of 37

Plants exchanging gases

plants exchange gases by diffusion

Photosynthesis- plants use up CO2 and produce O2

Respiration- Plants use O2 and produce CO2

gases move through plants by diffusion:

Diffusion is the passive movement of particles from an area of higher concentration to an area of lower concentration

4 of 37

Diffusion of gases

Photosynthesis- Co2 moves into leaf by diffusion and O2 is used by respiration but rest diffuses out through the stomata

At night:

  • no photosynthesis
  • CO2 is made and oxygen is used up
  • CO2 diffuses out and oxygen in

Water vapour escapes from the leaf by diffusion because there is lots of water inside leaf and not much outside- transpiration

5 of 37


Osmosis is the movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower concentration

  • partially permeable= very small holes, only tiny molecules like water can get through
  • water molecules pass both ways as they move randomly
  • more water molecules on one side than the other there's a steady net flow of water into the region with fewer water molecules
  • strong sugar solution becomes more dilute
  • osmosis is a type of diffusion
6 of 37

Turgor pressure

  • turgid= when plant is well watered, draws water in by osmosis and it becomes plump and swollen
  • turgor pressure= contents of cell push against cell wall, helps support plant
  • flaccid= when no water in soil and plant begins to wilt as they lose their turgor pressure
  • plasmolysed= when plant is really short of water and cytoplasm starts to shrink and the membrane pulls away from cell wall- doesn't lose shape as has an inelastic cell wall
7 of 37

Animal cells

Don't have an inelastic cell wall

  • lysis= when animal cell has too much water and it bursts
  • crenation= loses too much water and shrivels up

animal cells must keep water in cells constant

8 of 37

Root Hairs

Take in water by osmosis

  • plant roots have lots of microscopic 'hairs' which branch out from roots
  • these give plant a big surface area for absorbing water from soil
  • usually a higher concentration of water in soil than inside plant so water is drawn in by osmosis
9 of 37


= loss of water from the plant

  • caused by evaporation and diffusion
  • creates a slight shortage of water in leaves so water is drawn up from rest of plant through xylem vessels
  • means more water drawn up through the roots
  • constant transpiration stream through the plant
  • It is the side effect of the way the leaves are adapted for photosynthesis
10 of 37

Benefits of transpiration

  • constant stream of water keeps plant cool
  • provides plant constant supply of water for photosynthesis
  • creates turgor pressure in plants which helps support the plant and prevents wilting
  • minerals needed can be brought in along with water
11 of 37

Transpiration Rate

Light intensity- brighter light, greater rate

  • stomata close when its darker and photosynthesis doesn't happen, stomata closed= no water escapes

Temperature- warmer, faster rate

  • water particles have more energy for evaporation and diffusion

Air movement- lots of air movement, rate quicker

  • very still- water vapour surrounds leaf so not much concentration difference. If windy water swept away maintaining low concentration

Air Humidity- air around leaf dry, happens quicker

  • air is humid- a lot of water already in it so not much difference so diffusion is slower, happens faster when bigger difference
12 of 37

Balancing water loss

Have adaptations to reduce water loss especially in areas of little water

  • waxy cuticle covering upper epidermis, helps keep it waterproof
  • most stomata found on the lower surface where its darker and cooler which slows down diffusion
  • bigger the stomata and more leaf has the more water loss, so plants in hot climates have fewer and smaller stomata
13 of 37

Transportation systems in plants

Phloem tubes= food

  • made of living cells with perforated end plates to allow things to flow through
  • transport food substances
  • travel in both directions
  • movement of food substances= translocation

Xylem tubes= water up

  • made of dead cells with no end walls and a lumen down the middle
  • thick sided walls are strong and stiff giving support
  • carry water and minerals from roots up to leaves
  • transpiration stream
14 of 37

Where phloem and xylem vessels are

run alongside each other in vascular bundles

Root cross section picture:

Stem cross section picture:

Leaf cross section picture:

15 of 37

Minerals needed for growth


  • contain nitrogen for making amino acids and proteins and cell growth
  • if not enough: stunted and yellow older leaves


  • making DNA and cell membranes also respiration and growth
  • without: poor root growth, purple older leaves


  • needed for photosynthesis and respiration
  • not enough: poorer flower or fruit growth and discoloured leaves

Magnessium- in small amounts

  • needed for making chlorophyll, without- yellow older leaves
16 of 37

Root hairs and minerals

  • root hairs give plant big surface area for absorbing minerals
  • concentration of minerals in soil is usually pretty low, normally higher in the root hair cell
  • normal diffusion doesn't explain absorption of minerals
  • they would go the other way if followed normal rules
  • active transport is responsible
  • active transport uses energy from respiration to help the plant pull minerals into the root hair against the concentration gradient- essential for growth
17 of 37

Pyramids of numbers

  • Each bar on a pyramid of numbers shows the number of organisms at that stage of the food chain
  • A typical pyramid of numbers= as go up a tropic (feeding) level, the number of organisms goes down
  • this is because it takes a lot of food from the level below to keep one animal alive
18 of 37

Pyramids of Biomass

  • each bar on a pyramid of biomass shows the mass of living material at that stage of the food chain
  • practically always the right shape
  • Biomass and energy are still decreasing as you go up the levels but e.g. one tree has a very big biomass and can use alot of the suns energy
19 of 37


  • energy from the sun is the source of energy for most things
  • plants use a small percentage of light for photosynthesis this energy then works its way through the food web as animals eat the plants
  • the energy lost at each level is used for staying alive i.e respiration
  • most of energy is lost as heat- especially mammals and birds who have to keep their bodies at constant temperature
  • material and energy are also lost from the food chain during egestion
  • This is why hardly ever get food chains with more than 5 tropic levels as so much energy is lost at each stage
  • material and energy are both lost at each stage of the food chain
  • this explains why you get biomass pyramids
20 of 37

Energy flow data

  • numbers show amount of energy available to the next level
  • you can work out how much energy has been lost by taking away the energy that is available to the next level from the energy that was available to the previous level
  • can also calculate efficiency of energy transfer
  • Efficiency= energy available to the next level divided by energy that was available to the previous level (times 100 for percentage)
21 of 37


different ways to release energy stored in biomass- eating it, feeding to livestock, growing the seeds and using it as fuel


  • fast growing trees- burning fast growing trees doesn't contribute to CO2 as are replaced and are planted especially for the purpose
  • fermenting biomass using bacteria or yeast- breaking down by anaerobic respiration, use micro-organisms to make biogas from plant and animal waste in a fermenter called a digester- the biogas can be burned to release energy for heating

developing biofuels is a good idea because:

  • are renewable
  • reduces air pollution
  • can be energy self reliant
22 of 37

Intensive farming

= trying to produce as much food as possible for your land, plants or animals

farmers do this in different ways, but it all involves reducing the energy losses e.g

  • herbicides to kill weeds- means more of suns energy goes to crops and not competing plants
  • pesticides to kill insects that eat crops- makes sure no energy is transferred into a different food chain
  • animals are battery farmed- kept close to each other in small pens so can't move around, stops wasting energy in movement and stops them wasting energy keeping warm

Intensive farming allows us to produce a lot of food from less and less land which means a huge variety of top quality foods can be made all year round at cheap prices

23 of 37

Intensive farming and the environment

Methods are efficient but raise ethical dilemmas and damage the environment by making it polluted, unattractive, devoid of wildlife

  • removal of hedges to make big fields destroys natural habitats of wild creatures and increases soil erosion
  • careless use of fertilisers pollutes rivers and lakes (eutrophication)
  • pesticides disturb food chains
  • lots of people think battery farming of animals like battery hens is cruel
24 of 37

Pesticides disturb food chains

  • pesticides are sprayed onto crops to kill creatures that damage them but also kill harmless animals like bees and beetles
  • can cause a shortage of food for animals in the food chain
  • pesticides can be toxic to creatures that aren't pests and there's a risk they could harm humans
  • otters where almost wiped out by pesticide DDT as the pesticides got into rivers 
  • DDT can't be excreted so it accumulates along the food chain and the otter ends up with most of the DDT collected by the other animals
25 of 37

Biological control

Biological control= using living things instead of chemicals to control a pest, could use a predator, a parasite or a disease e.g.:

  • aphids eat roses and vegetables, so ladybirds (predators of aphids) are released to keep numbers down
  • certain types of wasps and flies produce larvae which develop on a host insect and eventually kill the insect host- parasite
  • Myxomatosis is a disease that kills rabbits, the virus was released in australia when the rabbit population grew out of control
26 of 37

Advantages and disadantages of biological control


  • the predator, parasite or disease usually only affects the pest animal so other  creatures aren't killed
  • no chemicals are used so there is less pollution, disruption of food chains and risk of people eating food thats been sprayed


  • It slower than pesticides
  • won't kill all the pests just one type
  • takes more management and planning and workers might need training or educating
  • control organisms can drive out native species or become pests themselves
27 of 37


= where plants are grown without soil


  • less space so less land required
  • no soil preparation or weeding
  • plants can be grown in areas of poor soil
  • avoids pests that live in soil
  • mineral levels can be controlled


  • expensive to set up and run
  • need to use specially formulated soluble nutrients
  • growers need to be skilled and trained
  • no soil to anchor roots so plants need support
28 of 37

Organic farming techniques

  • organic fertilisers (manure and compost) recycles nutrients left in plant and animal waste
  • crop rotation- growing a cycle of different crops, stops build up of pests and disease for one crop and stops nutrients running out, legume plants replace nitrates
  • weeding- physically removing weeds
  • varying seed planting times- sowing later or earlier avoids the major pests
  • biological control
29 of 37

Advantages and Disadvantages of organic

  • takes up more space
  • more labour intensive but this provides jobs but makes food more expensive
  • uses fewer chemicals
  • can't grow as much food but Europe over produces food anyway
  • better for the environment
  • for a farm to be classed as organic it must comply with guidelines of ethical treatment of animals
30 of 37


Living things are made of materials taken from the surrounding world

when they die and decompose or release waste the elements are put back where they originally came from

these elements are used by plants and cycle repeats

nearly all decomposition is done by soil bacteria and fungi

happens everywhere in nature and also in compost heaps and sewage works

all the important elements are recycled, carbon, hydrogen, nitrogen and oxygen

31 of 37

Rate of decay

depends on 3 main things

  • temperature- a warm temperature makes things decay faster because it speeds up respiration in decomposers
  • Moisture- things decay faster when they're moist because decomposers need water
  • Oxygen- decay is faster when there's oxygen available as decomposers can respire aerobically providing more energy

These factors cause decomposers to reproduce quicker so there will be more of them

32 of 37

Food preservation

  • Canning- airtight can to keep decomposers out, after canning tin is heated at high temperatures to kill any micro-organisms
  • Cooling- slows down decay as it slows down respiration in the micro organisms
  • Freezing- micro-organisms cannot respire or reproduce
  • Drying- micro-organisms need water
  • adding salt- if there's a high concentration of salt decomposers will lose water by osmosis
  • adding vinegar- vinegar is acidic and the low pH inhibits the enzymes inside the micro-organisms
33 of 37

Detritivores and Saprophytes

Detritivores and Saprophytes are organisms that are important in decay

Detritivore- feed on dead and decaying material

  • e.g. earthworms, maggots and woodlice
  • they break decaying material into smaller pieces increasing the surface area for smaller decomposers to work on which speeds up decay

Saprophytes- feed on decaying material by extracellular digestion

  • feed by secreting digestive enzymes on to the material outside of their cells
  • enzymes break down the material into smaller bits which can be absorbed by the saprophyte
  • most are bacteria or fungi
34 of 37

Carbon cycle

  • only one arrow going down- whole thing powered by photosynthesis
  • In photosynthesis plants convert carbon from CO2 in the air into sugars and plants can now incorporate this carbon into carbohydrates, fats and proteins
  • eating passes carbon compounds in the plant along animals in a food chain
  • both plant and animal respiration puts CO2 back into the air
  • plants and animals die and decay and are turned into useful products
  • when plant and animals decay they are broken down by bacteria and fungi which release co2 back into the air by respiration
  • some useful plant and animal products e.g. wood and fossil fuels are burned releasing CO2 back into air

another way of recycling carbon= sea, marine organisms make shells made of carbonates when they die the shells fall to ocean floor and form limestone rocks

35 of 37

The nitrogen cycle

  • atmosphere= 78% nitrogen, very unreactive and can't be used directly
  • nitrogen needed for making proteins for growth
  • plants get it from soil, so nitrogen in air has to be turned into nitrates before plants can use it
  • decomposers break down proteins in rotting plants and animals and urea in animal waste into ammonia
  • Nitrogen fixation- is process of turning N2 from the air into nitrogen compounds
    • lightening- so much energy in bolt of lightening that's its enough to make nitrogen react with oxygen
    • nitrogen fixing bacteria in soil and roots
36 of 37

Types of Bacteria in the nitrogen cycle

Decomposers= decompose proteins and urea and turn them into ammonia

Nitrifying Bacteria= turn ammonia in decaying matter into nitrates

Nitrogen fixing bacteria= turn atmospheric N2 into nitrogen compounds

Denitrifying bacteria= turn nitrates back into N2 gas

Some nitrogen fixing bacteria live in the soil in nodules on the roots of legume plants

The plants have a mutualistic relationship with the bacteria- bacteria get food from the plant and the plant gets nitrogen compounds from the bacteria to make proteins

37 of 37


No comments have yet been made

Similar Biology resources:

See all Biology resources »