Plants

Question 1

Translocation

Answer 1

the transportation of food in phloem from the source to the sink

Question 2

Xylem

Answer 2

files of dead cells, arranged end to end; long hollow tubes extending from the root to the leaves and transport water and dissolved minerals

• No plasma membranes are present in mature xylem vessels, so water can move freely.
• Pores in the outer cellulose cell wall conduct water out of the xylem vessel into cell walls of adjacent leaf cells.

Question 3

what thickens and supports the walls of xylem?

Answer 3

lignin

Question 5

Lignin

Answer 5

are hard and rigid substances (comes from latin word meaning “wood”). Lignin may be deposited in different ways, giving rise to annular, ring-shaped, helical, spiral or pitted xylem vessels, which are all structures that can resist strong inward pressures.

Question 6

Phloem

Answer 6

consists of a coulomb of sieve tubes and companion cells.

Sieve tubes are separated by sieve plates.

Question 7

Companion cells

Answer 7

narrow and a thin wall with abundance of cytoplasm and a nucleus. This is to keep the sieve tubes alive (as they provide nutrients) and contain mitochondria, which provides energy for translocation.

Question 8

Vascular bundle

Answer 8

xylem and phloem are group together to form the vascular bundles

Question 9

cambium

Answer 9

cells can divide to give rise to new xylem and phloem tissues, hence thickening of the stem

Question 10

pith

Answer 10

is vascular bundles that are arranged in a ring surrounding a central region called pith which serves as a storage

Question 11

cortex

Answer 11

is the region between the vascular bundles and epidermis is the cortex, which also serves as a storage tissue for food

Question 12

epidermis

Answer 12

cells are covered with a layer of wax called cuticle which prevents excessive loss of water from the stem

Question 13

transpiration

Answer 13

the loss of water vapour from the stems and leaves of plants

• It is the inevitable consequence of gas exchange in the leaf, which is the primary organ of photosynthesis in plants.
• Carbon dioxide is absorbed and used and oxygen produced as a waste and excreted through pores called stomata.
• Exchange of these two gases must take place to sustain photosynthesis, which requires a large moist surface, provided by the spongy (large surface area) mesophyll.

Question 14

how to plants try to minimise water leakage in transpiration

Answer 14

by placing pairs of guard cells at the stomata that regulate the aperture of the stomata. In addition, the epidermis of most plant leaves secretes wax to form a waterproof coating to the leaf (waxy cuticle), preventing excessive transpiration but also gas exchange, which is why the stomata are important.

Question 15

environmental factors that affect transpiration

Answer 15

wind speed — using a fan (varying velocity by different distances or rotation speed) and anemometer to measure speed: in still air, humidity builds up around the leaf, reducing the concentration gradient of water vapour and therefore reducing transpiration. Moderate wind velocities reduce or prevent this but high velocities can cause stomata to close.

humidity — using a plastic bag, mist spray, electronic hygrometer: water diffuses out of the leaf when there is a concentration gradient between the humid air spaces inside the leaf and the air outside. As atmospheric humidity is reduced, the concentration gradient gets steeper and transpiration is faster.

light intensity — referring back to heat

temperature — higher temperature (using heat lamp and infrared thermometer; summer/winter):

• increase evaporation from mesophyll cells, hence increasing transpiration rate,
• increases rate of diffusion through air spaces.
• in very high temperatures, the stomata may close

water supply

Question 16

practical experiments showing:

adhesion

cohesion

Answer 16

• Water’s adhesion can be illustrated by water moving up in a paper towel.
• Porous pots can be used to model evaporation from leaves. Water fills pores within the pot demonstrating adhesion to the clay molecules within the pot. As the water is drawn into the pot, cohesion causes water molecules to be drawn up the glass tubing.
• Capillary tubes dipped into water (with dye) and mercury visualises the contrast in adhesion and cohesion levels. Mercury has no adhesion to the glass nor cohesion between mercury atoms, which makes mercury not climb the glass.

Question 17

measuring transpiration rates

Answer 17

It is indirectly measured by the rate of water uptake using a potometer. As the plant transpires it draws water out of the horizontal capillary tube to replace the losses. A bubble in the capillary tube marks the zero point and can be pushed back by the reservoir to reset the bubble. As the bubble moves up, time and distance are noted.

Because the capillary tube is narrow (similar to xylem), small losses of water form the plant give measurable movements of the air bubble. Repeated measurements of the distance moved in one minute ensure reliable results.

Question 18

Water and mineral uptake in roots

Answer 18

Plants absorb water and mineral ions (e.g. potassium, phosphate, nitrate) from the soil using their root hairs (greater surface area by branching of roots; long and narrow). The hairs grow from epidermis cells (outer layer cells). The concentration of ions in the soil is usually much lower than inside root cells, so they are absorbed by active transport. Root hair cells have mitochondria and protein pumps in their plasma membranes. Most roots only absorb mineral ions if they have a supply of oxygen, because they produce ATP for active transport, by aerobic cell respiration. As a result of active transport, the cytoplasm of root cells (and cell sap) has a higher solute concentration than the water in the soil and, therefore, absorb water from the soil by osmosis (prevents water leakage).

Question 19

adaptions of plants to saline soils

Answer 19

Saline solids are found in coastal habitats and in arid areas where water moves up in soil and evaporates leaving dissolved ions at the surface. In saline soils the concentration of ions such as Na+ and Cl- is so high that most plants are unable to grow, except some specially adapted plants called halophytes.

To prevent water moving by osmosis from halophytes to saline soils the solute concentration inside the plant must be higher than in the saline soil. This cannot be done simply by raising the Na+ concentration because high concentration of this ion can have adverse effects on cell activities such as protein synthesis. High concentration of other solutes such as sugars or K+ are maintained in the cytoplasm instead. However, concentrations of Na+ and Cl- above those of the saline soil can be maintained in the vacuoles of cells as metabolic aviaries do not occur there.

Halophytes use a range of methods to get rid of excess Na+ and Cl- such as active transport back into the soil, excretion form special glands in the leaf, and accumulating the ion in certain leaves and then shedding them. Many halophytes also have adaptions for water conservation similar to those of xerophytes (succulents: plants adapted to store water).

Question 20

Adaptions of plants in deserts

Answer 20

Xerophytes are plants adapted to dry habitats like deserts.

For example, the “giant cactus” conserves water by reducing transpiration:

• vertical stems to absorb sunlight early and late in the day but not at midday when light is most intense
• very thick waxy cuticle covering the stem
• spines instead of leaves to reduce the surface area of for transpiration
• CAM physiology, which involves opening stomata during the cool nights instead of in the intense heat of the day
• roots spread out widely, but only penetrate a short distance into the soil; form new roots quickly when rain falls after a drought; the concentration of salts in the root cells of cacti is relatively high; all these adaptations enable cacti to absorb water rapidly during periods of brief or light rainfall

The leaf of Ammophila arenaria has

• small air spaces in the mesophyll than other plants
• hairs on the underside of the leaf
• few stomata, that are sunk in pits
• cells that can change shape to make the leaf roll up, with the lower epidermis and stomata on the inside

Question 21

annotate the stem of a plant

Answer 21

Question 22

annotate root

Answer 22

Question 23

function of xylem

Answer 23

• Xylem is a tissue in plants that provides support and transports water: long tubular structures, with strong side walls and very few cross walls.
• The main movement in xylem is from the roots to the leaves, to replace water losses from transpiration. This flow of water is called transpiration stream.
• Water polarity - cohesion and adhesion - allow transport. Water adheres to cellulose in plant walls. As water transpires from mesophyll cells walls in the leaf, more water is drawn through narrow cellulose-lined pores in leaf cell walls from the nearest xylem vessels to replace it, generating the tension. The result of these properties is capillary action (tension) that causes the water to move (be pulled) up the leaves. (experiment: capillary action; water adheres to glass)

Tension can be transmitted from one water molecule to the next because of the cohesive property of water molecules that results from hydrogen bonding. The tension generated in the leaves is transmitted all the way down the columns of water in xylem vessels to the roots. (water traveling up paper, which cellulose cells)

At times of maximum transpiration the pressures in xylem vessels can be extremely low and the side walls have to very strong to prevent inward collapse. Depositing more cellulose and impregnating the walls with lignin makes the cell walls much harder — woody.

The first xylem formed by a shoot or root tip is primary xylem. The walls of primary xylem vessels are thickened in a helical or annular (ring-shaped) pattern, which allows the vessel to elongate as the root or shoot grows in length.

Question 24

success in plant reproduction depends on three different processes:

Answer 24

pollination, fertilisation, seed dispersal

Question 25

factors needed for germination

Answer 25

water (hydration), oxygen (cell respiration) and warmth (enzymes)

Question 26

advantages of micropropagation (breeding) of plants

Answer 26

Micropropagation can do this with very small pieces of tissue taken from the shoot apex of a plant.

+ New varieties can be bulked up much more quickly than by previous methods of propagation.

+ Virus-free strains of existing variety can be produced because cells in the shoot apex normally do not contain viruses that reduce plant growth even if other cells in a plant do contain them.

+ Large numbers of rare plants such as orchids can be produced, reducing the cost to people who want to buy them and making it unnecessary to take them from wild habitats.