Leaves Leaf Structure Epidermal tissue system chloroplasts only

Leaves

Leaf Structure • Epidermal tissue system – chloroplasts only in guard cells • Ground tissue system – mesophyll • Vascular tissue system • Leaf adaptations – sun and shade leaves – xerophytes

Photosynthesis sunlight, water CO 2 Dark reactions (Calvin cycle) Light reactions Oxygen (O 2) Glucose (C 6 H 12 O 6) (carbohydrates)

Start with the Big Picture • It’s all about energy – potential vs kinetic energy • Nearly all energy used on earth comes from the sun • Nearly all energy used on earth ends up as heat • Metabolism – the way living things manage energy

Laws of Thermodynamics • Energy can neither be created nor destroyed but it can be converted from one form to another • In any transfer of energy there is always loss of useful energy, usually as heat

What makes chemical reactions happen? • Chemical bonds have potential energy • When the bonds are broken, the energy is released • Cells transfer energy from one molecule to another, often as a proton (H+) or electron (e-) • Enzymes are proteins that speed up chemical reactions • ATP has high-energy phosphate bonds • ATP bonds come from glucose • Glucose comes from. . .

Metabolism summary • Photosynthesis – light reactions – dark reactions • Respiration – glycolysis – Krebs cycle – electron transport – fermentation Solar energy captured in chemical bonds Chemical bond energy is released to do work

Photosynthesis CO 2 sunlight, water Thylakoid membrane Dark reactions (Calvin cycle) ATP, NADPH Light reactions Chloroplast stroma oxygen Glucose (C 6 H 12 O 6) (carbohydrates)

Movie

Light Reactions of Photosynthesis • Chlorophyll is a pigment in the thylakoid membrane • It can absorb light energy • The absorbed energy knocks an electron out Movie 1 of the chlorophyll molecule • This electron is transported from one molecule to another • The energy is used to make ATP Movie 2 • The original electron is replaced with one from a molecule of water, releasing O 2

A good summary. . .

Dark Reactions (Calvin cycle) • Occurs in the stroma of the chloroplast • CO 2 diffuses into spongy mesophyll cells • RUBISCO is the enzyme that fixes the CO 2 into a carbon compound – it’s the most abundant protein on earth – a very complicated enzyme!

Rubisco

Calvin cycle Movie

The atmosphere contains oxygen because of photosynthesis

Oxygen causes a problem for RUBISCO • RUBISCO binds O 2 as well as CO 2 – the substrates compete for the active site • O 2 results in no new sugar • Some plants have evolved a way to avoid this wasteful reaction

Some C 4 plants

ATP <

Water molecules Waterbox <

Companion cells Sieve cells <

End of Show

Dicot leaf surface Monocot leaf surface

Palisade parenchyma Xylem Stoma Lower epidermis Phloem Upper epidermis Spongy parenchyma

Sun leaf and shade leaf sections Genetic vs environmental differences in leaves

Xerophytes are plants that grow in dry conditions

What are the adaptations to dry conditions?

Leaf cross section Which tissue holds the water?

Net or pinnate venation (dicots) Parallel venation (monocots)

Typical horizontal leaf Typical vertical leaf