Structure and function of plants
A flowering plant has a root system and a shoot system.
- Roots hold the plant in the soil and take in water and minerals. Tiny root hairs give extra surface to drink from.
- Stem holds leaves up to the light. Inside it, the xylem carries water up and the phloem carries food to all parts.
- Leaves make food by photosynthesis: water + carbon dioxide + light gives sugar + oxygen. Tiny holes called stomata let air in and out.
- Flowers, fruits and seeds help the plant to make the next generation.
Why one plant cell can make a whole plant
Every living plant cell has a nucleus with a full copy of the plant's genes. This power is called totipotency (say: toe-TIP-uh-ten-see). Under the right conditions a cell can divide and become roots, shoots and leaves. Plants also grow from meristems, small zones at the tips of roots and shoots where cells keep dividing. Meristem cells are the best starting material for clean, healthy cultures because they are usually free of viruses.
This is the base of tissue culture, which you meet in Plant tissue culture.
Why we need aseptic technique
Aseptic means free from germs. Germs are bacteria, fungi (moulds, yeast) and viruses. A jar of jelly food with sugar, minerals and warmth is perfect for them too. They grow much faster than a plant piece, so they take over the jar. This is called contamination. A contaminated jar looks cloudy, slimy, or fuzzy white, green or black, and it is thrown away safely.
Sources of germs: hands, hair and breath, tools, the table, air, and the plant piece itself.
Basic aseptic operations step by step
- Clean the person: wash hands with soap, tie hair, wear a clean coat, and wipe hands with 70% alcohol.
- Clean the place: wipe the table with 70% alcohol. Best of all, work inside a laminar air flow cabinet, which blows germ-filtered air over the table.
- Sterilise tools: dip forceps and scalpel in alcohol and pass them through a flame, then let them cool. Heating in a steam pressure cooker (autoclave) at 121 °C for 15–20 minutes sterilises jars, jelly food and water.
- Sterilise the plant piece: wash it, dip it briefly in a mild disinfectant such as dilute bleach, then rinse with sterile water.
- Work fast and close: open lids only as long as needed, keep them tilted and never talk over the jars.
- Label and watch: write the date, then check every few days and remove spoiled jars.
Safety: alcohol burns, so keep the flame away from the bottle. Always have an adult or teacher with you.
Try it: a germ-growth test at home
Boil two small slices of potato for 10 minutes to make them clean. Put each in a clean, lidded jar. Touch one slice with a clean spoon that you just washed with hot water. Touch the other with your unwashed finger. Close both and keep them in a warm room. After 4–7 days, compare. The finger jar will show mould first. This is the same reason a plant lab is so careful. (Do not open or smell the mouldy jar; throw it away sealed.)
Key formulas and definitions
- Key term: Totipotency = a living cell can grow into a whole plant
- Key term: Photosynthesis: water + carbon dioxide + light → sugar + oxygen
- Key term: Xylem carries water up; phloem carries food
- Key term: Aseptic = free from germs; contamination = germs entering the culture
- Autoclave: 121 °C, about 15 pounds pressure, 15–20 minutes
- Disinfectant alcohol strength: 70% (v/v)
Worked examples
1. A student sees a jar with white fuzzy threads over the jelly food. What happened and what should be done?
Fungus (mould) got in. It is contamination. The jar must be closed and discarded safely, and the student should check which step of cleaning was missed (tools, hands or table).
2. You need 300 mL of 70% alcohol. How much pure alcohol is needed?
70% of 300 mL = 0.70 × 300 = 210 mL of alcohol. Add water to make a total of 300 mL.
3. A cutting from the tip of a root keeps growing for weeks, but a piece of dry bark does not. Explain.
The root tip has meristem cells that keep dividing and are alive. Dry bark has dead cells that cannot divide. Only living cells show totipotency.
4. Why is a pressure cooker used to sterilise the jelly food but a flame is used for the forceps?
The food and water would spoil or boil away in a flame, so steam heat at 121 °C is used. Forceps are metal and can be flamed quickly. Both kill germs with heat.
Common mistakes
- Thinking aseptic means just "clean-looking". Germs cannot be seen, so we use flame, alcohol and heat.
- Using the tool while it is still hot. Let it cool for a few seconds, or it will burn the plant piece.
- Believing only roots make the plant's food. Leaves make the food; roots take in water.
- Talking, coughing or waving hands over open jars. This drops germs into the culture.