Plants

What are Plants?

Plants are autotrophs — they produce their own food using energy from the sun through a process called photosynthesis. They are essential to life on Earth: they produce the oxygen we breathe, absorb CO2, and form the base of nearly every food chain.

Plants are multicellular eukaryotes with cell walls made of cellulose, chloroplasts for photosynthesis, and a large central vacuole for support.

Photosynthesis

Photosynthesis converts light energy into chemical energy stored as glucose.

Word equation: Carbon dioxide + Water → Glucose + Oxygen (using light energy and chlorophyll)

Where it happens: mainly in leaves, inside organelles called chloroplasts, which contain the green pigment chlorophyll. Chlorophyll absorbs red and blue light and reflects green — which is why plants appear green.

Glucose is used for:

• Respiration — releasing energy for growth and life processes
• Starch — storage in roots and leaves
• Cellulose — building cell walls
• Proteins — combined with nitrates to make amino acids
• Lipids — stored as oils in seeds

Plant Parts and Their Functions

Part	Function
Roots	Absorb water and minerals; anchor the plant; some store food (e.g., carrots)
Root hair cells	Greatly increase surface area for water and mineral absorption
Stem	Supports the plant; transports water (xylem) and food (phloem)
Leaves	Main site of photosynthesis; gas exchange via stomata
Flowers	Sexual reproduction — attract pollinators, produce seeds
Fruit	Surrounds seeds; aids dispersal by animals, wind, or water
Seeds	Contain the embryo and food reserves for germination

Transport in Plants

Plants have two specialised transport systems running through vascular bundles:

Xylem:

• Carries water and dissolved minerals upward from roots to leaves
• Made of hollow, dead cells with lignified walls — forms continuous tubes
• Movement is called transpiration stream
• Driven by water loss from leaves (transpiration pull), root pressure, and osmosis

Phloem:

• Carries dissolved sugars (mainly sucrose) from leaves to all parts of the plant
• Made of living sieve tube cells with companion cells
• Movement is called translocation
• Can move in both directions (up and down)

Transpiration

Transpiration is the loss of water vapour from leaves, mainly through stomata. It is a consequence of keeping stomata open for gas exchange.

Factors that increase transpiration:

• Higher temperature (more evaporation)
• Lower humidity (steeper concentration gradient)
• More wind (removes water vapour from around leaves)
• More light (stomata open wider)

Plants in dry environments have adaptations to reduce transpiration: thick waxy cuticles, fewer stomata, stomata on the underside of leaves, rolled leaves (e.g., marram grass), and spines instead of leaves (cacti).

Stomata and Gas Exchange

Stomata are tiny pores on leaves (mainly the underside) controlled by guard cells.

• Open in light — CO2 enters for photosynthesis; O2 and water vapour exit
• Close in darkness — conserves water when photosynthesis is not occurring
• Close when the plant is dehydrated — guard cells lose water and become flaccid, closing the pore

Gas exchange in leaves:

• CO2 diffuses in (concentration lower inside during photosynthesis)
• O2 diffuses out (produced by photosynthesis)
• At night: O2 diffuses in and CO2 diffuses out (respiration only)

What Plants Need to Grow

Plants require several conditions for healthy growth:

Requirement	Role
Light	Powers photosynthesis
Water	Reactant in photosynthesis; keeps cells turgid
Carbon dioxide	Reactant in photosynthesis
Warmth	Enzymes function optimally at 25–35°C
Nitrogen	Needed to make proteins and chlorophyll
Magnesium	Needed to make chlorophyll (deficiency causes yellow leaves)
Phosphorus	Needed for root development and energy transfer

Worked Example — Mineral Deficiency

A plant has yellow (chlorotic) leaves. What is the likely cause?

Yellow leaves suggest a magnesium deficiency — magnesium is a component of chlorophyll. Without magnesium, less chlorophyll is made, reducing the green colour and the rate of photosynthesis.

Alternatively, nitrogen deficiency causes stunted growth and yellowing because nitrogen is needed to make amino acids (and therefore proteins and chlorophyll).

Key Terms

• Autotroph — an organism that makes its own food (e.g., plants)
• Chlorophyll — green pigment that absorbs light energy for photosynthesis
• Xylem — vascular tissue transporting water and minerals upward
• Phloem — vascular tissue transporting dissolved sugars throughout the plant
• Transpiration — loss of water vapour from leaves through stomata
• Translocation — transport of dissolved sugars through phloem
• Stomata — pores in leaves for gas exchange, controlled by guard cells
• Turgid — a cell swollen with water; gives plants structural support

Common Mistakes

• Confusing xylem (water up) with phloem (sugars in all directions)
• Saying plants only photosynthesise and do not respire — plants respire all the time; they also photosynthesise when light is available
• Thinking transpiration is "breathing" — it is water loss, not gas exchange
• Forgetting that stomata close at night or when the plant is short of water

Tips and Tricks

• Remember: Xylem carries water (X for crossing upward); Phloem carries Phood (glucose)
• Transpiration is faster in hot, dry, windy, bright conditions — remember HDBW (Hot, Dry, Breezy, Well-lit)
• Mineral deficiency symptoms: yellow leaves = magnesium or nitrogen; poor roots = phosphorus
• Stomata open in light (to let in CO2 for photosynthesis) and close in dark (to save water)