Plant Physiology: Photosynthesis, Respiration & Nutrition

• Plant physiology is the study of how a plant's lifeprocesses work inside it.
• A plant works like a tiny food factory: leaves are the kitchen, roots are the pumps, stem holds the delivery pipes.
• Leaves make food; roots absorb water and minerals; xylem and phloem transport them.
• Four big life-jobs: photosynthesis (making food), respiration (getting energy), nutrition (minerals), and transport.
• Plants that make their own food are autotrophs; those that depend on others are heterotrophs.

Plant physiology is the study of how a plant's lifeprocesses work inside it.

A plant works like a tiny food factory: leaves are the kitchen, roots are the pumps, stem holds the delivery pipes.

Leaves make food; roots absorb water and minerals; xylem and phloem transport them.

Plants that make their own food are autotrophs; those that depend on others are heterotrophs.

• Photosynthesis = making food using light (photo = light, synthesis = making).
• Green plants use sunlight, CO2 and water to make glucose and release oxygen.
• Word equation: 6CO2 + 6H2O + light → C6H12O6 + 6O2.
• The main product is glucose (the food); oxygen is only a by-product.
• The released oxygen comes from the splitting of water, not from CO2.
• CO2 enters through tiny leaf-pores called stomata; water comes from the roots.
• Main site of photosynthesis is the mesophyll cells of the leaf.
• Carbon dioxide — Air (through stomata)
• Water — Soil (absorbed by roots)
• Sunlight — The Sun
• Chlorophyll — Green leaf cells

Photosynthesis = making food using light (photo = light, synthesis = making).

Green plants use sunlight, CO2 and water to make glucose and release oxygen.

The main product is glucose (the food); oxygen is only a by-product.

The released oxygen comes from the splitting of water, not from CO2.

CO2 enters through tiny leaf-pores called stomata; water comes from the roots.

• Chlorophyll is the green pigment that traps sunlight; it sits inside chloroplasts.
• Chlorophyll contains the metal magnesium (Mg) at its centre.
• Chlorophyll a is the chief (main) pigment of photosynthesis.
• Chlorophyll b and carotenoids are accessory pigments that capture extra light.
• Chlorophyll absorbs light best in the blue and red regions of the spectrum.
• Green light is reflected (not absorbed), so leaves look green.
• Chlorophyll is used to trap and convert light energy for making food.
• Chlorophyll a — Chief pigment
• Chlorophyll b — Accessory pigment
• Carotenoids — Accessory (yellow-orange)

Chlorophyll is the green pigment that traps sunlight; it sits inside chloroplasts.

Chlorophyll contains the metal magnesium (Mg) at its centre.

Chlorophyll a is the chief (main) pigment of photosynthesis.

Chlorophyll b and carotenoids are accessory pigments that capture extra light.

Chlorophyll absorbs light best in the blue and red regions of the spectrum.

Green light is reflected (not absorbed), so leaves look green.

• Photosynthesis runs in two stages inside the chloroplast.
• Light reaction happens on the thylakoid membranes (grana) and needs light.
• In the light reaction, water is split (photolysis), releasing O2 and storing energy as ATP and NADPH.
• Dark reaction (Calvin cycle) happens in the stroma and fixes CO2 into glucose.
• It is called 'dark' because it does not directly need light, not because it needs darkness.
• Two photosystems: PS I (P700) and PS II (P680); watersplitting is linked with PS II.
• Place — Thylakoid (grana) — Stroma
• Needs light — Yes — Not directly
• Output — ATP, NADPH, O2 — Glucose

Photosynthesis runs in two stages inside the chloroplast.

Light reaction happens on the thylakoid membranes (grana) and needs light.

In the light reaction, water is split (photolysis), releasing O2 and storing energy as ATP and NADPH.

Dark reaction (Calvin cycle) happens in the stroma and fixes CO2 into glucose.

It is called 'dark' because it does not directly need light, not because it needs darkness.

Two photosystems: PS I (P700) and PS II (P680); watersplitting is linked with PS II.

• Rate of photosynthesis = how fast a plant makes food; it is not fixed.
• Four main factors: light intensity, CO2 amount, temperature, and water.
• More light raises the rate, but only up to a point, then it plateaus (levels off).
• Blackman's Law of Limiting Factors: the rate is set by the factor in shortest supply.
• That scarce factor is called the limiting factor.
• Very high temperature damages enzymes and lowers the rate.
• Light intensity — Rises, then plateaus
• CO2 level — Rises with more CO2
• Temperature — Optimum, then falls
• Water — Shortage lowers rate

Rate of photosynthesis = how fast a plant makes food; it is not fixed.

More light raises the rate, but only up to a point, then it plateaus (levels off).

Blackman's Law of Limiting Factors: the rate is set by the factor in shortest supply.

That scarce factor is called the limiting factor.

Very high temperature damages enzymes and lowers the rate.

• Respiration is the controlled breakdown of glucose to release energy as ATP.
• Equation: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy.
• Respiration is almost the reverse of photosynthesis.
• Plants respire day and night, but photosynthesise only in light.
• Aerobic respiration uses oxygen and gives a lot of energy.
• Anaerobic respiration (fermentation) works without oxygen and gives little energy.
• Respiration occurs in the mitochondria, the 'powerhouse of the cell'.
• Energy — Stored — Released
• Oxygen — Released — Used up
• When — Daytime only — Day and night
• Site — Chloroplast — Mitochondria

Respiration is the controlled breakdown of glucose to release energy as ATP.

Plants respire day and night, but photosynthesise only in light.

Aerobic respiration uses oxygen and gives a lot of energy.

Respiration occurs in the mitochondria, the 'powerhouse of the cell'.

• In daytime, plants photosynthesise and release oxygen.
• At night there is no photosynthesis, but respiration continues.
• So at night plants release carbon dioxide and use up oxygen.
• This is why it is advised not to sleep under trees at night – CO2 builds up.
• Photosynthesis using the invisible (UV) part of sunlight is done by some bacteria.

In daytime, plants photosynthesise and release oxygen.

At night there is no photosynthesis, but respiration continues.

So at night plants release carbon dioxide and use up oxygen.

This is why it is advised not to sleep under trees at night – CO2 builds up.

Photosynthesis using the invisible (UV) part of sunlight is done by some bacteria.

• Plants need two things: home-made sugar and mineral nutrients from outside.
• Carbon comes from air (CO2); mineral salts come mainly from the soil.
• Macronutrients (needed in large amounts): N, P, K, Ca, Mg, S.
• Micronutrients (needed in tiny amounts): Fe, Mn, Zn, Cu, B, Mo, Cl.
• Nitrogen (N) is needed for proteins and chlorophyll; phosphorus (P) stimulates early growth and roots.
• Potassium (K) helps overall growth and stomatal function.
• Roots absorb nutrients dissolved in water.
• Nitrogen (N) — Proteins, chlorophyll
• Phosphorus (P) — Early growth, roots
• Potassium (K) — Growth, stomata
• Magnesium (Mg) — Chlorophyll core

Plants need two things: home-made sugar and mineral nutrients from outside.

Carbon comes from air (CO2); mineral salts come mainly from the soil.

Macronutrients (needed in large amounts): N, P, K, Ca, Mg, S.

Micronutrients (needed in tiny amounts): Fe, Mn, Zn, Cu, B, Mo, Cl.

Nitrogen (N) is needed for proteins and chlorophyll; phosphorus (P) stimulates early growth and roots.

Potassium (K) helps overall growth and stomatal function.

• Plants cannot use free N2 gas; they take nitrogen as nitrates (NO3-) and ammonium (NH4+).
• Nitrogen fixation by Rhizobium bacteria in legume root nodules converts N2 into usable form.
• Insectivorous plants (e.g. pitcher plant, Venus flytrap) trap insects to get nitrogen.
• They grow in soil deficient in nitrogen.
• Mycorrhiza is a root-fungus partnership; the plant gets better water and mineral absorption.
• The fungus in turn gets sugar (food) from the plant.

Plants cannot use free N2 gas; they take nitrogen as nitrates (NO3-) and ammonium (NH4+).

Nitrogen fixation by Rhizobium bacteria in legume root nodules converts N2 into usable form.

Insectivorous plants (e.g. pitcher plant, Venus flytrap) trap insects to get nitrogen.

Mycorrhiza is a root-fungus partnership; the plant gets better water and mineral absorption.

The fungus in turn gets sugar (food) from the plant.

• Plants have two transport tissues: xylem and phloem.
• Xylem carries water and minerals upward from roots to leaves.
• Phloem carries food (sugar) from leaves to all parts (translocation).
• Water enters root cells from the soil by osmosis (water moving toward more solute).
• Water enters by the physical process of diffusion/osmosis, needing no energy.
• After cutting cucumber and adding salt, water comes out due to osmosis (exosmosis).
• Xylem — Water, minerals — Roots to leaves (up)
• Phloem — Food (sugar) — Leaves to all parts

Plants have two transport tissues: xylem and phloem.

Xylem carries water and minerals upward from roots to leaves.

Phloem carries food (sugar) from leaves to all parts (translocation).

Water enters root cells from the soil by osmosis (water moving toward more solute).

Water enters by the physical process of diffusion/osmosis, needing no energy.

After cutting cucumber and adding salt, water comes out due to osmosis (exosmosis).

• Transpiration is the loss of water as vapour from leaves through stomata.
• It creates a suction pull (transpiration pull) that lifts water up tall trees.
• This pull, with cohesion of water, lets water reach great heights in trees.
• Stomata are tiny pores guarded by two guard cells.
• Stomata open when guard cells become turgid (swell with water) and close when flaccid.
• So stomata open or close due to a change in the turgidity of guard cells.

Transpiration is the loss of water as vapour from leaves through stomata.

This pull, with cohesion of water, lets water reach great heights in trees.

Stomata are tiny pores guarded by two guard cells.

Stomata open when guard cells become turgid (swell with water) and close when flaccid.

So stomata open or close due to a change in the turgidity of guard cells.

• Each stoma is bordered by two bean-shaped guard cells.
• When water flows in, guard cells become turgid and bend apart, opening the pore.
• When water leaves, guard cells go flaccid and the pore closes.
• Open stomata allow CO2 in for photosynthesis and water vapour out.
• Stomata usually open in light (day) and close in the dark.

Each stoma is bordered by two bean-shaped guard cells.

When water flows in, guard cells become turgid and bend apart, opening the pore.

When water leaves, guard cells go flaccid and the pore closes.

Open stomata allow CO2 in for photosynthesis and water vapour out.

Stomata usually open in light (day) and close in the dark.

• Plant hormones are chemicals that control growth and responses.
• Auxin promotes cell elongation and bending toward light (phototropism).
• Gibberellin promotes stem growth and seed germination.
• Cytokinin promotes cell division and delays ageing.
• Ethylene is a gas that ripens fruits and causes ageing/leaf fall.
• Abscisic acid (ABA) is the stress hormone that closes stomata and causes dormancy.
• Auxin — Cell elongation, bending
• Gibberellin — Stem growth, germination
• Cytokinin — Cell division
• Ethylene — Fruit ripening
• Abscisic acid — Stomata close, dormancy

Plant hormones are chemicals that control growth and responses.

Auxin promotes cell elongation and bending toward light (phototropism).

Gibberellin promotes stem growth and seed germination.

Cytokinin promotes cell division and delays ageing.

Ethylene is a gas that ripens fruits and causes ageing/leaf fall.

Abscisic acid (ABA) is the stress hormone that closes stomata and causes dormancy.

• Fruit ripening is driven by the hormone ethylene and by respiration.
• Cold lowers the rate of respiration and ethylene action.
• So fruits in a cold chamber stay fresh longer (slower ripening and rotting).
• Low temperature also slows the growth of spoilage microbes.
• This is the basic principle behind cold storage of fruits and vegetables.

Fruit ripening is driven by the hormone ethylene and by respiration.

So fruits in a cold chamber stay fresh longer (slower ripening and rotting).

This is the basic principle behind cold storage of fruits and vegetables.