Biotic and Abiotic Factors, Grade 10: Light and Temperature Investigations

Two plant pots on a lab bench, each with five bean seedlings: on the left short, sturdy seedlings with open green leaves grown in light, on the right tall, thin, pale yellow seedlings grown in the dark

This page covers the second part of the Grade 10 topic Biosphere to ecosystems: the biotic and abiotic factors in an ecosystem. It has two classroom investigations of abiotic factors, the effect of light on seedlings and the effect of temperature on germination, each set out as a recipe.

Subject Life Sciences
Grade and term Grade 10, Term 3
Topic Biosphere to ecosystems, part 2 of 3: biotic and abiotic factors
Practicals The effect of light on seedling growth, and the effect of temperature on germination. The fieldwork for this topic is on the study an ecosystem page
Time Each investigation takes about 30 minutes to set up and a few minutes a day to check. Light: 14 days. Temperature: 7 days

Practical 1: the effect of light on seedling growth

Aim

To find out how light affects the growth of bean seedlings.

What you need

  • 10 dry bean seeds, such as sugar beans, soaked in water overnight
  • 2 plant pots or yoghurt tubs of the same size with holes in the bottom, filled with the same potting soil
  • A dark cupboard, or a large cardboard box to cover one pot, in the same room as a sunny windowsill
  • A 25 ml measuring cylinder for watering
  • A 30 cm ruler
  • A thermometer to check that both places are at the same temperature
  • A marker for labelling the pots

Method

  1. Label the pots Light and Dark.
  2. Plant 5 soaked seeds in each pot, 2 cm deep and evenly spaced.
  3. Water each pot with 20 ml of water.
  4. Stand the Light pot on the sunny windowsill. Put the Dark pot in the cupboard or under the box.
  5. Check the temperature in both places with the thermometer. They should be within 2 °C of each other. If not, move the Light pot back from the glass or to a bright spot out of direct sun.
  6. Water each pot with 20 ml of water every second day. Work quickly at the Dark pot so it gets as little light as possible.
  7. On day 7 and day 14, measure the height of every seedling from the soil to the tip with the ruler, and note its colour and leaves.
  8. Work out the average height of the seedlings in each pot.

What you should see

Pot Height Colour Leaves
Light Shorter, with thick, sturdy stems Green Open and spread out
Dark Taller, with long, thin, weak stems Pale yellow or white Small and not opened out

Why it happens

A plant needs light to make chlorophyll, the green pigment used in photosynthesis. In the dark the seedlings cannot make chlorophyll, so they stay pale. Their stems grow long and thin as if searching for light. This is called etiolation. Without light they cannot make more food by photosynthesis, so all their growth comes from the food stored in the seed.

Variables

Variable In this investigation
Independent (what you change) Light: light or dark
Dependent (what you measure) Height of the seedlings, their colour and their leaves
Controlled (kept the same) Type and number of seeds, the same soil and pot size, the volume of water, the temperature

Safety

  • Do not eat the seeds. Raw beans can cause food poisoning.
  • Wash your hands after handling soil.
  • Wipe up spilt water and soil straight away.

Practical 2: the effect of temperature on germination

Aim

To find out how temperature affects the germination of bean seeds.

What you need

  • 30 dry bean seeds, soaked in water overnight
  • 3 petri dishes with lids
  • Paper towel, cut to fit the dishes
  • A 10 ml measuring cylinder
  • 3 thermometers
  • A fridge, a place at room temperature, and a warm place at about 27 °C, such as an incubator or a warm cupboard
  • A marker for labelling the dishes

Method

Diagram of three petri dishes of soaked bean seeds on damp paper towel, one in a fridge at about 5 °C, one at room temperature of about 20 °C and one in a warm place at about 27 °C

  1. Label the dishes Fridge, Room and Warm.
  2. Put 3 layers of paper towel in each dish and wet each with 10 ml of water.
  3. Place 10 soaked seeds on the paper towel in each dish and put the lids on.
  4. Put one dish in the fridge, one at room temperature and one in the warm place, each with a thermometer beside it. Record the temperature of each place.
  5. Keep all three dishes in the dark. The fridge and an incubator are dark already. Put the Room dish in a closed drawer or box.
  6. Every day for 7 days, count how many seeds have germinated. A seed has germinated when its root has come out of the seed coat.
  7. Add 2 ml of water to any dish where the paper towel has started to dry out.
  8. Work out the percentage of seeds that germinated in each dish on day 7.

What you should see

Dish Seeds germinated after 7 days
Fridge, about 5 °C None, or almost none
Room, about 20 °C Most of them, within a few days
Warm, about 27 °C Most of them, and the fastest

Why it happens

A seed needs water, oxygen and a suitable temperature to germinate. Germination is controlled by enzymes, which break down the food stored in the seed so the young plant can use it to grow. Enzymes work slowly in the cold, so in the fridge the seeds do not germinate. As the temperature rises towards the best temperature for the enzymes, germination becomes faster. This is why seeds sown in cold soil in winter come up slowly or not at all.

Variables

Variable In this investigation
Independent (what you change) Temperature
Dependent (what you measure) The number of seeds that germinate each day
Controlled (kept the same) Type and number of seeds, the volume of water, the paper towel, soaking time, the dishes kept closed, light (all kept dark)

Safety

  • Do not eat the seeds.
  • Keep the dishes away from food in the fridge, and throw the seeds away in a bin at the end.
  • Handle glass thermometers carefully. Tell the teacher at once if one breaks.

Biotic and abiotic factors in brief

An ecosystem has living parts, the biotic factors, and non-living parts, the abiotic factors. They affect each other all the time.

Biotic factors

Group How it gets food Examples
Producers Make their own food by photosynthesis, using light energy Green plants, algae, some bacteria
Consumers Cannot make their own food, so they feed on other organisms Herbivores such as zebra, carnivores such as lions, omnivores such as baboons
Decomposers Break down dead organisms and waste, and return the nutrients to the soil Bacteria and fungi

How energy and nutrients pass between these groups is covered on the energy flow and nutrient cycles page.

Physiographic factors

Diagram of a valley in South Africa with the Sun in the north: the north-facing slope is in sunlight and covered in grass, the south-facing slope is in shade with trees

Factor What it means Effect
Altitude Height above sea level The air gets colder by about 6,5 °C for every 1 000 m you climb, so high places are colder and often have frost
Slope How steep the land is Rain runs off steep slopes quickly, so they have thinner soil and more erosion
Aspect The direction a slope faces South Africa is in the southern hemisphere, so for most of the year the midday Sun is in the northern part of the sky. North-facing slopes get more direct sunlight and are warmer and drier. South-facing slopes are cooler and damper, and often carry denser plants

Soil

Diagram of a jar of soil stirred with water and left to settle, with sand at the bottom, silt above it, clay on top and clear water above the soil

  • Texture depends on the size of the particles. Sand particles are 0,05 to 2 mm across, silt 0,002 to 0,05 mm, and clay less than 0,002 mm. Loam is a mixture.
  • To see the particles in a soil, put about 5 cm of soil in a jar, fill it with water, shake or stir well and leave it to stand for two days. The largest particles settle first, so sand forms the bottom layer, silt settles on top of it and clay settles last, on top.
  • Humus is the dark, decayed remains of plants and animals. It holds water and nutrients and improves the soil's structure.
  • pH affects which minerals plants can take up. Most plants grow best in slightly acid to neutral soil.
  • Water retention capacity is how much water a soil can hold. Clay holds the most and sand the least. Clay has tiny spaces between its particles, so water moves through the tiny spaces slowly and clings to the clay particles. Sand has large spaces, so water drains through quickly.
  • Air content is the space between the particles that is filled with air. Roots and soil organisms need oxygen from it, so waterlogged soil, where water fills the spaces, harms most plants.

Light

Plants need light for photosynthesis. Day length, called the photoperiod, changes with the seasons: days are longer in summer and shorter in winter. Changing day length triggers flowering in many plants, and breeding and migration in many animals.

Temperature

Temperature changes between day and night and between the seasons. Animals respond in different ways:

Response What happens Example
Hibernation Body processes slow down to survive cold weather and food shortage in winter Some bats
Aestivation Body processes slow down to survive heat and drought The African bullfrog and the lungfish, buried in mud until the rain returns
Migration Animals move to another area for the season Barn swallows fly to South Africa for our summer

Water

All organisms need water. It moves through ecosystems in the water cycle, which is explained on the energy flow and nutrient cycles page. Wetlands such as vleis, marshes and pans are important because they:

  • soak up water like a sponge, which reduces floods downstream
  • release water slowly in the dry season and help recharge groundwater
  • filter pollution and hold back sediment, so the water leaving them is cleaner
  • provide habitat for birds, frogs, fish and many plants

Atmospheric gases

Gas Share of dry air Importance
Nitrogen About 78% Needed to make proteins. Plants take it up as nitrates from the soil
Oxygen About 21% Used in respiration
Argon Almost 1% Not used by living things
Carbon dioxide About 0,04%, and rising Used in photosynthesis. It keeps heat in the atmosphere, and burning fossil fuels is increasing it

Wind

  • Carries pollen and seeds, and spreads spores.
  • Increases transpiration, so plants lose water faster on windy days.
  • Blows away dry topsoil and causes erosion.
  • Bends and shapes trees in very windy places.

Free worksheet and marking memo

Related practicals

References