SB6: Plant Structures and Their Functions

What is photosynthesis?

A chemical endothermic reaction in photosynthetic organisms that convert light energy into chemical energy (glucose).

What is the word equation for photosynthesis?

Carbon Dioxide + Water → Glucose + Oxygen

What is the symbol equation for photosynthesis?

6CO2 + 6H2O → C6H12O6 + 6O2

Why is photosynthesis important?

Provides oxygen to animals for respiration; the glucose that is produced makes up the plant's biomass to be transferred up the food chain; used in respiration to release energy

Where does photosynthesis occur?

Chloroplasts, in plant cells

What is chlorophyll?

A green pigment found in the chloroplasts of plants that absorb light.

Describe the two main stages of photosynthesis?

Chlorophyll absorbs light energy which is used to split water into oxygen gas (waste product) and hydrogen ions. Carbon dioxide combines with hydrogen ions to form glucose.

What factors limit the rate of photosynthesis?

Light intensity, CO2 concentration, and temperature.

How does light intensity affect the rate of photosynthesis?

Rate of photosynthesis is directly proportional to light intensity; increase of LI means increased rate of photosynthesis.

Why does the rate of photosynthesis eventually plateau even if light intensity continues to increase?

Another factor is limiting the rate of photosynthesis (e.g. temperature or CO2 conc.)

How does temperature affect the rate of photosynthesis?

High temp means more kinetic energy for enzymes which increases rate of reaction; optimum temperature is 25°C; if temperature is too high, enzymes will denature, leading to a decrease in rate of photosynthesis

How does CO2 concentration affect the rate of photosynthesis?

Rate of photosynthesis is directly proportional to CO2 concentration; increase in CO2 conc. means increased rate of photosynthesis.

How can you measure the effect of light intensity on the rate of photosynthesis?

Using a light meter or using inverse square law

What is the inverse square law?

The Inverse Square Law says that the light intensity is inversely proportional to the distance squared (LI \= 1/distance^2)

How can pondweed be used to measure the effect of light intensity on the rate of photosynthesis?

Connect an empty gas syringe to a conical flask containing pondweed in water. Place a source of white light at a specific distance away from the pondweed. Allow the pondweed to photosynthesise for a certain amount of time. Oxygen will start to collect in the gas syringe. Repeat the experiment with the light source at different distances.

How can you calculate the rate of oxygen production?

Volume of oxygen produced / Time taken.

What is the independent variable?

The variable that the experimenters deliberately change

What is the dependent variable?

The variable that changes BECAUSE the independent variable has changed (the variable that is measured).

What is the control variable?

Variables that are deliberately kept the same.

What are the control variables in the experiment investigating the effect of light intensity on the rate of photosynthesis?

The control variables are the temperature and the CO2 concentration.

Why do plants require water?

Photosynthesis; Maintenance of structure; Cooling effect

Why do plants require mineral ions?

For growth e.g. nitrates are required to produce proteins

Which structure in plants is adapted for the uptake of water and mineral ions?

Root hair cells

How is water transported into root hairs?

Lower concentration of water in root hair cells than in the soil; Water diffuses down its concentration gradient into root hair cells by osmosis

How are minerals transported into root hairs?

Lower concentration of mineral ions in the soil than in the root; Root hair cells take up mineral ions by active transport

How are plant roots adapted to absorb water and mineral ions?

Plant roots contain millions of tiny root hair cells which have long hairs that extend from the cell body, increasing surface area for absorption of water and mineral ions; Has many mitochondria which produces ATP for active transport of mineral ions.

What is the function of the xylem?

Transports water and dissolved minerals from the roots to the leaves via the transpiration stream.

How has the xylem adapted to its function?

Xylem tubes made of dead cells laid end-to-end to form a long, continuous hollow column; No end walls, provides little resistance to passage of water; Thick cell wall strengthened by lignin for support.

What is the function of the phloem?

Transports products from photosynthesis (glucose) up and down the stem from photosynthetic tissues (e.g. mature green leaves) to non-photosynthetic tissues (e.g. developing seeds) via translocation

How has the phloem adapted to its function?

Sieve tube elements are long, thin cells laid end-to-end that contain no nucleus and little cytoplasm that enables sugars to flow. Companion cells (adjacent to sieve tubes) contain dense cytoplasm, mitochondria and nucleus. Provides energy for processes in both cell types.

What is transpiration?

The loss of water vapour through the stomata of leaves by diffusion and evaporation.

What is the process of transpiration?

Water evaporates from mesophyll cell surfaces and diffuses out of the stomata; Water molecules (that have cohesive properties) are drawn up the xylem vessels to replace the water loss; this causes more water to be absorbed from the soil into the root hair cells.

What are stomata?

Small pores on the lower epidermis of a leaf which allows gas exchange.

What are guard cells?

They surround the stomata and change shape to control the size of the stomata.

How do guard cells control the size of stomata?

Through open stomata water enters guard cells. They swell and become turgid. They bend and draw away from each other, opening the stomata. Through closed stomata water leaves guard cells. They become flaccid, closing the stomata.

What factors affect the rate of transpiration?

Temperature, humidity, air flow, light intensity

How does high light intensity affect the rate of transpiration?

High light intensity. greater number of stomata open to allow gas exchange for photosynthesis; Rate of PS increases, more water taken from soil up the xylem; More water vapour diffuses out of the stomata; Rate of transpiration increases.

How does low light intensity affect the rate of transpiration?

Lower light intensity, less stomata is open, slower rate of transpiration.

How does temperature affect the rate of transpiration?

Temperature increases, more KE for water molecules so rate of diffusion increases; Rate of PS increases, more water taken from soil up the xylem; More water diffuses out of the stomata; Rate of transpiration increases.

How does air movement affect the rate of transpiration?

Air movement increases, high conc. gradient between air spaces in the leaf and atmosphere; Increased rate of diffusion of water molecules out of the stomata; Rate of transpiration increases.

What apparatus is used to measure the rate of transpiration?

Potometer

What is assumed when measuring the rate of transpiration using a potometer?

Rate of water uptake ≈ Rate of transpiration

How can the rate of transpiration be calculated using a potometer

Rate of transpiration \= distance moved by bubble/time taken

What is translocation?

The movement of sugars (e.g. sucrose, amino acids, etc.) up and down a plant via the phloem. Requires ATP.

How are leaves adapted for photosynthesis and gas exchange?

Leaves are Broad, large surface area - to absorb light; Thin - short diffusion pathway for gases can diffuses in and out; Many chloroplasts - absorb sunlight; Xylem and phloem - transport water/mineral ions/sugars to the parts of the plant that need them

How are the tissues of leaves adapted for photosynthesis and gas exchange?

Palisade mesophyll layer receives most light, contains highest conc. of chloroplasts. Upper epidermis is transparent, allows light to reach palisade layer. Spongy mesophyll layer has air spaces which increases the rate of diffusion. Lower epidermis contains many stomata for gas exchange. Epidermal tissues covered with a waxy cuticle which helps reduce water loss by evaporation.

How are plants adapted to live in extreme, hot conditions?

Small leaves/spines: reduces SA for water loss; Thick waxy cuticle: reduces evaporation; Thick stem: provides storage for water; Shallow but widespread roots: Large SA for water absorption; Stomata sunken in pits and leaves curled: reduces airflow, lowering conc. gradient and reducing water loss by evaporation; Stomata close to prevent water loss.

What are plant tropisms?

The growth of a plant in response to a stimulus

What is positive tropism?

The growth of a plant towards a stimulus.

What is negative tropism?

The growth of a plant away from a stimulus

What is phototropism?

A plant's growth in response to light.

What is gravitropism?

A plant's growth in response to gravity

What are auxins?

Auxins are plant hormones which control growth at the tips of shoots and roots. They cause cell elongation.

How do auxins behave differently in plant shoots and plant roots?

In plant shoots, auxins stimulate growth; In plant roots, auxins inhibit growth.

How do auxins move through the plant?

They diffuse through the plant in solution

Are plant shoots positively or negatively phototropic?

Positively phototropic, they grow TOWARDS the light stimulus.

Why are plant shoots positively phototrophic?

Shoot tips are exposed to light; auxins accumulate on the shaded part of the shoot; cells on the shaded side began to elongate unevenly; shoot tip bends towards the light.

Are plant shoots positively or negatively gravitropic?

Negatively gravitropic, plant grows upwards away from gravity.

Why are plant shoots negatively gravitropic?

Shoot placed horizontally; auxins accumulate on lower side of the shoot; lower side's cells elongate; shoot bends upwards away from gravity.

Are plant roots positively or negatively phototropic?

Negatively phototropic, plant roots grow away from the light.

Why are plant roots negatively phototropic?

Root exposed to light; on shaded side auxins accumulate; cell elongation on shaded side is inhibitted; root grows away from the light.

Are plant roots positively or negatively gravitropic?

Positively gravitropic, plant roots grow towards gravity.

Why are plant roots positively gravitropic?

Root placed horizontally; auxins accumulate on lower side of the root; inhibition of cell elongation on lower side; root grows downwards towards gravity.

How do plant tropisms increase the chance of survival?

Enables plants to respond to their environment; Shoot growth towards the light maximises light absorption; Root growth downwards increases uptake of water and minerals from soil and enables anchorage of plant body to the ground.

How can auxins be used in commercial plant cultivation?

Rooting powder - contains auxins that stimulate the growth of roots in cuttings (a part of a plant that has been cut off); Selective weed killer - target and alter growth in patterns in broad-leaved plants (most weeds), killing them.

What are gibberellins?

Gibberellins are a type of plant hormone involved in seed germination.

How are gibberellins used in commercial plant cultivation?

Initiates seed germination at all times of the year when they naturally wouldn't; Triggers flowering in plants under irregular conditions; Increases fruit size by reducing flower formation; Produces seedless fruits by stimulating growth of fruit from unpollinated flowers.

How do gibberellins stimulate seed germination?

In the presence of water, gibberellins break seed dormancy initiating seed germination.

How can ethene be used in commercial plant cultivation?

Stimulates enzymes that control fruit ripening; Enables fruits t be picked when they are unripe and less easily damageable; They can be stimulated to ripen during transportation to shops.