R2101 - Photosynthesis, respiration & the movement of water & minerals through the plant

Question 1

4.1 State the equation for photosynthesis in words
and state the necessity for chlorophyll and light.

Answer 1

Carbon Dioxide + Water = Sugars + Oxygen
By using light energy as a fuel and chlorophyll as a catalyst.

Question 2

4.1 List the environmental factors that affect the rate
of photosynthesis. (5)

Answer 2

• Carbon dioxide - constant supply
• Light: Intensity (amount present), quality/*PAR and duration (length of time) (*Photosynthetically Active Radiation)
• Temperature: Effects the rate of photosynthesis - optimum 25–36°C (varies with plant species)
• Water: Correct amount to maintain leaf turgidity and retain fully open stomata for carbon dioxide movement into the leaf.
• Mineral nutrients: Iron, magnesium and nitrogen produce chlorophyll - deficiency in these will lead to loss of photosynthetic ability.

Question 3

4.1 Describe the ‘Law of Limiting Factors’.

Answer 3

• A limiting factor = anything in short supply that will limit the rate of a process, i.e. photosynthesis.
• Photosynthesis has three limiting factors: Light (intensity, quality & duration), Carbon Dioxide concentration, Temperature
• In a process influenced by more than one factor, the rate of the process will be limited by the factor that is in lowest supply.
• E.g not enough CO2 - optimising light intensity and/or temperature will have no effect as there is still not enough CO2

Question 4

4.1 Describe how growers can optimise the conditions for photosynthesis.

Answer 4

Light - maximise and control. Light levels can be raised by using artificial lighting of the correct wavelength.

Carbon dioxide - levels can be artificially raised in enclosed environments like greenhouses.

Temperature - keeping plants at the optimum temperature will improve the rate of photosynthesis – keep within the right temperature range for the specific plant.

Question 5

4.2 What is respiration?

Answer 5

• Process in which the sugars the plant has made through photosynthesis are used to make energy to live.
• We respire just as plants do – we breathe in oxygen to burn the food we have eaten to provide energy.
• Respiration continues day and night, in all of the living cells of the plant.
• Important enough sugars are produced during the day to keep the plant going through the night.

Question 6

4.2 State the equation for aerobic respiration in words.

Answer 6

sugars (glucose) + oxygen –> carbon dioxide + water + energy (high yield)

Aerobic respiration is with oxygen.

Question 7

4.2 State the equation for anaerobic respiration in words.

Answer 7

sugars (glucose) –> ethanol + carbon dioxide + energy (low yield)

Anaerobic respiration is without oxygen.

Question 8

4.2 List the factors that affect the rate of respiration.

Answer 8

• oxygen
• temperature

Question 9

4.2 How do temperature and oxygen affect the rate of aerobic respiration?

Answer 9

• Rate of respiration is dependent on temperature – too low or too high and it slows down. The optimum is different for different plants.
• A lack of oxygen leads to anaerobic respiration, using up the plant’s sugar stores quickly, only providing a low yield of energy. Ethanol produced is toxic to the plant.

Question 10

4.2 Describe the significance of anaerobic and aerobic respiration in horticultural situations: waterlogging

Answer 10

• Anaerobic respiration occurs when roots become waterlogged
• Lack of oxygen and consequent ethanol production can result in tissue damage and root death.

Question 11

4.2 Describe the significance of anaerobic and aerobic respiration in horticultural situations: propagation

Answer 11

• Propagation requires a high rate of respiration e.g rooting cuttings or seed germination
• Higher temperatures enable a higher rate of respiration and cell division, thus enabling growth to occur
• Lower rates of respiration are also useful to help keep plant material from growing.
• Cool storage helps reduce respiration and keeps plant material fresh.
• Cuttings stored at at low temperatures root more readily later

Question 12

4.2 Describe the significance of anaerobic and aerobic respiration in horticultural situations: produce storage

Answer 12

• Sometimes anaerobic conditions can be advantageous.
• Controlled atmosphere storage - oxygen, carbon dioxide and temperature levels can be controlled to slow ripening and extend storage times greatly. E.g. apples
• Produce packaging can provide a ‘modified atmosphere’ - low oxygen and high carbon dioxide reducing respiration
• Storing produce in low temperatures reduces the rate of respiration, increasing shelf life

Question 13

4.2 Describe the significance of anaerobic and aerobic respiration in horticultural situations: seed storage

Answer 13

• Sometimes anaerobic conditions can be advantageous.
• Viability of stored seeds can be greatly increased stored in a ‘modified atmosphere’.
• Sealed airtight packets: oxygen is removed, carbon dioxide levels rise then respiration rate is reduced and germination is inhibited.

Question 14

4.3 What is diffusion?

Answer 14

Liquid or gas movement from area of high concentration to an area of lower concentration. (apoplast route)

Question 15

4.3 What is osmosis ?

Answer 15

Movement of water from high water concentration to a low concentration across a semi-permeable membrane. (symplast route)

Question 16

4.3 Identify examples of diffusion in plants, to include: transpiration and gaseous exchange.

Answer 16

Examples of diffusion in plants includes the movement of gases such as water vapour (transpiration), carbon dioxide and oxygen (photosynthesis and respiration) into and out of the leaf.

Question 17

4.3 Identify examples of osmosis in plants, to include: water uptake into cells, turgor, and cell expansion.

(long answer to make into separate cards)

Answer 17

Osmosis is a specific type of diffusion that only ever involves the movement of water.

Water moves into the cell by osmosis through the semi-permeable membrane.

When water moves into the cell by osmosis it swells like a balloon. This inner pressure, caused by the water pushing outwards, is called turgor pressure.

Turgor pressure is very important in providing support for young plants and non woody herbaceous plants. The plant and its individual cells stay upright like an inflated balloon.

Turgor pressure is also the way in which new cells enlarge, contributing to growth by causing the cell to swell (cell expansion) until the cell wall prevents further expansion.

Question 18

4.3 Describe the pathway of water movement from the soil through the plant into the atmosphere.

(long answer to make into separate cards)

Answer 18

Water held around soil particles is drawn in to the root hairs by osmosis.

Water then moves through the roots, around the cells and from cell to cell (osmosis) until it reaches the endodermis which controls the passage of water and minerals into the stele (central core) of the root.

Water passes through the endodermis by osmosis into the xylem tissue.

Water is sucked up the xylem tissue of the stem by a process called transpiration pull.

Water then moves into the xylem of the leaf stem and then into the xylem of the leaf veins.

Water then passes through the leaf cells by osmosis and also between the cells until it reaches the air spaces of the mesophyll.

The water becomes vapour around the spongy mesophyll cells which have lots of gaps around them.

The vapour is lost by transpiration through the stomata of the leaf.

Question 19

4.3 Describe the 3 distinct stages of the pathway of water movement through the plant.

Answer 19

• water uptake from the soil by the roots
• movement up the stem by the xylem
• movement across the leaves and loss to the air by transpiration

Question 20

4.3 What is transpiration?

Answer 20

• loss of water vapour from the plant into the atmosphere by evaporation
• through the stomata of the leaves, across the leaf surface and other plant surfaces

Question 21

4.3 What is transpiration pull?

Answer 21

Transpirational pull: water evaporated through the leaves of plants causes a ‘sucking’ action of water up the xylem tissue.

Question 22

4.3 List the factors that affect the rate of transpiration.

Answer 22

Relative humidity - if the air around the leaf is humid, the rate of diffusion of water vapour will be reduced and the rate of transpiration will decrease.

Temperature - affects the rate at which water from the leaf evaporates causing the level of transpiration to increase or decrease.

Wind speed - moving air around the leaf will reduce the surrounding humidity so will increase the rate of transpiration.

Light - affects the rate of transpiration due to the response of stomata to light levels (photosynthesis)

Question 23

4.3 Describe how the plant may limit water loss. stomatal closure

Answer 23

Stomata are surrounded by two guard cells which change shape to ‘open and close’ in different environmental conditions.

Question 24

4.3 Describe how the plant may limit water loss: hairs (one named plant example)

Answer 24

• many plants have a layer of microscopic hairs covering the leaf surface, making it look grey or silver, and feel slightly furry to the touch
• the hairs shade the leaf surface and reduce air movement so limiting loss of water vapour
• Salvia officinalis

Question 25

4.3 Describe how the plant may limit water loss: thick cuticle (one named plant example)

Answer 25

• Many evergreens have a very thick, waxy cuticle (outer skin) to make it waterproof
• Ilex aquifolium

Question 26

4.3 Describe how the plant may limit water loss: needles (one named plant example)

Answer 26

• Conifer needles have a reduced surface area and stomata sunk into the leaf
• Pinus sylvestris

Question 27

4.3 Describe the uptake and distribution of mineral nutrients in the plant.

(long answer to make into separate cards)

Answer 27

Essential mineral nutrients are necessary for the plant to grow. They are dissolved in soil water and absorbed by the root.

Nutrients have to cross the endodermis cell membrane

The concentration of nutrients inside plant cells is usually greater than the soil water so uptake is against the concentration gradient.

Nutrients can not enter the cell by diffusion, they to be taken in by active transport, which requires energy.

Uptake is selective, the plant only absorbs the minerals it needs and rejects others.

The mineral nutrients are then transported up the xylem to the leaves.

Mineral nutrients are distributed in the phloem to other plant organs such as flowers and fruits. Active transport is needed for this process too.

Phloem is made up of sieve tube cells accompanied by companion cells which control the process of active transport.

Question 28

4.3 What is active transport?

Answer 28

• Movement of a substance into a cell across the cell membrane against a concentration gradient.
• Requires energy
• Substances are taken up selectively

Question 29

4.3 What is phloem tissue?

Answer 29

• Transports sugars made in photosynthesis from the leaves to other plant organs.
• Mineral nutrients are redistributed in the phloem to other plant organs too. Active transport is needed for this process.
• Phloem is made up of sieve tube cells accompanied by companion cells which control the process of active transport.

Question 30

4.3 What is xylem tissue?

Answer 30

Transports water and dissolved mineral nutrients from the roots, up the stem to the leaves and other plant organs.

Question 31

4.3 What are essential mineral nutrients?

Answer 31

Inorganic substances necessary for the plant to grow and develop.

Question 32

4.3 What is a concentration gradient?

Answer 32

• The difference in the concentration of a substance between two areas.
• The bigger the difference, the steeper the concentration gradient and the faster the molecules of a substance will diffuse.

Question 33

4.4 Describe how the internal and external structure of the leaf designed to maximise photosynthesis and minimise transpiration:

(long answer with diagram)

Answer 33

Dicotyledon leaf

Epidermis is covered with a waxy cuticle which acts as waterproofing to minimise transpiration.

Palisade cells, cylindrical in shape in order to get as much light as possible to the chloroplasts where photosynthesis will take place.

Spongy mesophyll layer whose air spaces allow for gases such as CO2, oxygen and water vapour to come and go – they enter and exit through the stoma.

Veins bring water and mineral nutrients through their xylem tissue.

Veins take away sugars and other metabolic products in their phloem tubes.

The leaf itself will be angled in order to get the maximum benefit from sunlight.