R2101 4.3 Describe the movement of water and minerals through the plant

Question 1

What is diffusion?

Answer 1

This is the spread of particles through random motion from regions of higher concentration to lower concentration

Question 2

Example of diffusion in a plant

Answer 2

Movement of gases such as water vapour, carbon dioxide and oxygen into and out of the leaf

When the leaf stomata open oxygen and carbon dioxide naturally flow from higher concentration zones to lower. If a leaf has been photosynthesising oxygen can be exchanged for more carbon dioxide from the atmosphere.

Question 3

Cell wall and cell membrane in Osmosis

Answer 3

• Cell wall is permeable to both water and mineral salts
• Cell membrane is permeable to water but will only allow tiny quantites of minerals to pass in or out. It is selectively permeable.

Question 4

What is Osmosis?

Answer 4

The movement of water from a high water (low solute) concentration to a low water (high solute) concentration across a selectively permeable membrane

Water molecules in a dilute solution have a high ‘water potential’

In a concentrated solution they have a low ‘water potential’

Question 5

Example of osmosis in a plant

Answer 5

Soil water contains small quantites of dissolved inorganic minerals in a large volume of water, A cells protoplasm contains a much smaller amount of water in which larger concentrations of salts and sugars are dissolved.

The water moves from the soil where it is most abundant into the root cells where it tries to dilute the cell solutions.

Question 6

What happens when water enters the cell by osmosis?

Answer 6

As water moves into the cell it swells. This pressure is called turgor pressure and is important in provide support to young plants and non-woody herbaceous plants.

Turgor pressure is also the way new cells enlarge contributing to growth by cell expansion

Question 7

3 distinct a stages of pathway of water through plant

Answer 7

1. Water uptake by roots
2. Movement up the stem in the Xylem
3. Movement across the leaves and loss to the air by transpiration

Question 8

What happens with lack of water uptake or plant is losing water faster than it can be replace?

Answer 8

Water will cease to enter the cells are turgor pressue is lost - plant will wilt.

Question 9

What is plasmolysis?

Answer 9

This is where the cell contents shrink away from the cell wall leading to irreversible damage and cell death

Question 10

How does plasmolysis occur? (2)

Answer 10

1. Is there is a continued loss of water that is not replaced
2. If too much fertiliser is added to the soil and causes root scorch. Water moves out of the cells because solute concentration is greater outside the cells

Question 11

Whats is root pressure?

Answer 11

The pressure with which the epidermis and endodermis push water across the root and into the xylem

Question 12

Movement of water in the roots? (4)

Answer 12

1. Root hairs draw in water by osmosis
2. When epidermal cells are fully turgid water is squeezed into the intercellular spaces between the cortex cells - path of least resistance
3. Water moves across the cortex to the endodermis which controls passage of water into the stele.
4. Water passed through the endodermis to the xylem

Question 13

What is transpiration?

Answer 13

The evaporation of water from plants, generally from their leaves, while their stomata are open for the passage of carbon dioxide and oxygen for photosynthesis

Question 14

Water movement in the stem?

Answer 14

1. Water is sucked up the xylem tissue by transpiration pull
2. As water is lost from the leaves it is replaced by water drawn up the stem
3. Pressure is created in the xylem and water moves up through the stem and leaf petiole by suction as long as the water forms a continuous column.
4. In the leaves the mesophyll cells have a high concentation of sugars becuase of photosynthesis taking place there and so draw water up the xylem by osmosis

Question 15

3 facts about transpiration

Answer 15

1. Lifts water (and dissolved salts in xylem) to all parts of the plant, up to topmost leaves
2. Has a significant cooling affect on leaves exposed to full sunlight, water vapour carries heat with it as it evaporates
3. A forest temperate-zone broad leave trees transpires about 30,000 litres of water per acre per day

Question 16

Water movement in leaves

Answer 16

1. When water reaches the leaves it is distributed in the fine network of viens
2. It flows between leaf cells and also passes from cell to cell by osmosis
3. Eventually water evaporates from the cell surfaces into the air spaces of the leaf mesophyll.
4. Water vapor diffuses into the air via stomata - there is lower humidity in the surrounding air than in the leaf
5. Loss of water vapour from the leaf is called transpiration

Question 17

Factors that affect rate of transpiration? (4)

Answer 17

1, Relative humidity
2. Temperature
3. Light
4. Wind speed

Question 18

How does humidity affect rate of transpiration? (3)

Answer 18

1. If air is very humid the rate of diffusion of water vapour will reduced and the rate of transpiration will decrease.
2. Moving air around the leaf reduces humidity and so will increase rate of transpiration,
3. Wind breaks help reduce windspeed and the risk of plants drying out

Question 19

How does wind affect transpiration?(2)

Answer 19

1.When there is no breeze the air around the leaf becomes increasingly humid thus decreasing rate of transpiration
2.When a breeze is present the humid air is carried away and replaced by drier air,

Question 20

How does temperature affect transpiration? (4)

Answer 20

1. The rate as which water in the leaf evaporates and diffuse determines the transpiration rate.
2. As temperature increases, diffusion is speeded up and transpiration increases
3. At high temperatures though the stomata will close and the transpiation will cease
4. At 30C the plant may transpire three times as fast as it does at 20C

Question 21

How does light affect rate of transpiration and how to control the rate of it by horticulural means (5)

Answer 21

1. Light stimulates opening of the stomata
2. Plants transpire more rapidly in the light than the dark
3. Light can also warm the leaf, increasing transpiration
4. Shading may be used in greenhouses to reduce temperature and light levels, thus reducing transpiration rate and water loss
5. Cuttings are kept out of direct sunlight to reduce transpiration until they develop roots and are able to take up water more efficiently

Question 22

Ways plants reduce water loss (6)

Somata close when water absorbed by roots < water lost by transpiration

Answer 22

1. Smaller leaves - reduced surface area and stomata sunk into leaf e.g.* Pinus sylvestris*
2. Thick waxy cuticles that act as barriers to water loss e.g. Ilex aquifolium
3. Epidermal hairs that reduce water loss by increasing the layer of still air across the leaf surface e.g. Stachys byzantia
4. Grey or silver leaves that refect light e.g. many Mediterranean plants
5. Thickened leaves that act as water storage organs e.g. Jade plant Crassula ovata
6. Cacti such as Opuntia have leaves modified as spines and whole plant photosynthesises

Question 23

Pathway fo water through the plants Xylem (6)

Answer 23

1. Water molecules run in unbroken chains through the xylem in the leaves, stems and roots
2. This transpiration pull operates through the plant to move water upwards against gravity
3. Water is pushed through root pressure and pulled upward by transpiration
4. The plant draws water (and minerals dissolved in the soil water) from the soil (diffusion, osmosis)
5. Water moves up through the xylem tissue in the root into the stem, then into the leaves
6. It is lost to the atmosphere from the stomata through transpiration

Question 24

What are essential nutrients?

Answer 24

Inorganic substances necessary for the plant to grow and develop

Question 25

What is active transport?

Answer 25

The movement of a substance into a cell across the cell membrane against a concentration gradient. It requires energy and substances are taken up selectively

Question 26

Process of mineral uptake? (4)

Answer 26

1. Mineral are dissolved in water and absorbed by root
2. At endodermis nutrients have to cross cell membrane. Since concentration of nutrients inside the cell is almost always greater than the concentration in the soil water uptake is against the concentation gradient
3. Nutrients cannot therefore enter the cell by simple diffusion. There are taken in by Active Transport which is selective.
4. Most mineral uptake it done by the roots, therefore damage to the surface roots (e.g. by hoeing) can reduce this process.

Question 27

Movement of sugars in the plant (4)

Answer 27

1. Phloem is responsible for the transport fo sugars
2. Unlike Xylem where direction of flow is always from the root to the leaves, sugars can flow in the phloem both up and down the plant as needed
3. Moving sugars is an active process that requires energy
4. The companion cells which accompany each sieve tube cell is thought to control this process