Plant tissues, organs and systems

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Plant Tissues and Their Functions

Epidermal tissues cover the plant, protecting it from damage and water loss; the upper epidermis is thin and transparent to let light reach the palisade cells.
Palisade mesophyll is made of column-shaped cells tightly packed with chloroplasts to absorb as much light as possible for photosynthesis.
Spongy mesophyll has internal air spaces that increase the surface area for diffusion of gases, mainly carbon dioxide, into and out of the leaf.
Xylem and phloem are transport tissues found in vascular bundles; xylem carries water and minerals, phloem carries dissolved sugars and amino acids.
Meristem tissue is found at the growing tips of shoots and roots and is responsible for producing new cells for growth.
Guard cells surround stomata and control their opening and closing, balancing gas exchange with water loss.

The Leaf as a Plant Organ

A leaf is a plant organ adapted for photosynthesis; it has a large surface area to absorb light and carbon dioxide.
The leaf is thin so that carbon dioxide can diffuse quickly to the palisade mesophyll cells.
A waxy cuticle covers the leaf, reducing water loss without blocking sunlight.
The upper epidermis is transparent, allowing light to pass through to the palisade layer beneath.
Palisade cells are at the top of the leaf, maximising light absorption by chloroplasts.
Stomata in the lower epidermis allow carbon dioxide to diffuse in and oxygen to diffuse out; they also allow water vapour to escape.
A network of veins (vascular bundles) transports water to the leaf cells and carbohydrates away from the leaf.

Root Hair Cells and Water Uptake

Root hair cells are single-celled extensions of epidermis cells in the root; they grow between soil particles.
The long hair-like projection increases the surface area to volume ratio, increasing the rate of absorption of water and mineral ions.
Water enters root hair cells by osmosis because the soil water has a higher water potential than the cytoplasm.
Mineral ions are taken up by active transport, which requires energy from respiration.
Water passes through the root cortex and into the xylem vessels, then travels up to the leaves.
The pathway of water is: root hair cell → root cortex cells → xylem → leaf mesophyll cells.

A root hair cell: its long extension increases the surface area for absorbing water.

Xylem and Phloem Structure and Function

Xylem vessels transport water and dissolved mineral ions from the roots to the stem and leaves.
Xylem cells are dead and hollow, with no top and bottom walls, forming continuous tubes for water movement.
Xylem walls are strengthened with lignin, which supports the plant and helps withstand the pressure of water movement.
Phloem vessels transport dissolved sugars (mainly sucrose) and amino acids from photosynthesising leaves to other parts of the plant.
Phloem cells are living and joined end-to-end; their end walls have pores called sieve plates that allow substances to flow through.
Movement in xylem is only one direction (roots to leaves), while phloem transport can occur in different directions.

Transpiration and the Transpiration Stream

Transpiration is the loss of water vapour from the parts of the plant above ground, mainly through the stomata.
Water evaporates from the surfaces of the spongy mesophyll cells into the air spaces, then diffuses out through the stomata.
The transpiration stream is the continuous flow of water from the roots to the leaves through the xylem.
Water molecules in the xylem are held together by cohesion, forming an unbroken column; transpiration pulls this column upwards.
Transpiration helps transport mineral ions, keeps cells turgid for support, supplies water for photosynthesis, and cools the plant.
If the rate of transpiration increases, water is pulled up the xylem faster.

The generation of the transpiration stream

Factors Affecting the Rate of Transpiration

Temperature: higher temperature increases the rate of transpiration because evaporation is faster.
Humidity: higher humidity decreases the rate of transpiration because there is a smaller water potential gradient between the leaf and the air.
Air movement: increased air flow increases the rate of transpiration by removing water vapour from around the leaf, maintaining a steep diffusion gradient.
Light intensity: higher light intensity increases the rate of transpiration because more stomata are open to allow carbon dioxide in for photosynthesis.
If any of these factors are low, the opposite effect on transpiration rate is seen.

Stomata and Guard Cells

Stomata are pores mainly found on the underside of the leaf; they allow gas exchange and water loss.
Guard cells control the opening and closing of stomata by changing shape as they gain or lose water.
When guard cells are turgid (full of water), the stomata are open, allowing carbon dioxide in and water vapour out.
When guard cells lose water and become flaccid, the stomata close, reducing water loss.
Stomata open during the day to allow carbon dioxide in for photosynthesis and close at night to conserve water.
The daily rhythm of stomatal opening and closing continues even in constant light or darkness.

Translocation

Translocation is the transport of dissolved sugars (mainly sucrose) and amino acids in the phloem, from regions of production to regions of storage or use.
Translocation is an active process that requires energy.
Substances are transported from source (where they are made) to sink (where they are stored or used).
In winter, phloem may transport dissolved food from storage organs to other parts of the plant for respiration.
In spring, storage organs are the source and growing areas are the sinks.
In summer, leaves are the source and roots are the sinks, storing sucrose as starch.

Translocation through the phloem

Investigating Transpiration

A potometer is used to measure the rate of transpiration by measuring water uptake.
A bubble potometer measures the distance an air bubble moves in a set time; the further it moves, the faster the transpiration rate.
A mass potometer measures the change in mass of a plant as water evaporates from the leaves and stem.
When setting up a potometer, cut the shoot underwater to prevent air entering the xylem, and ensure all joints are airtight using Vaseline.
Dry the leaves before starting because wet leaves affect the results.
Calculate the rate of transpiration by dividing the distance the bubble travelled by the time period.
Control variables such as temperature, humidity and wind speed to ensure a fair test.

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1.Which option correctly describes the organisational structure of a leaf?
Easy
- ACell
- BTissue
- COrgan
- DOrgan system
2.Which feature of palisade mesophyll cells makes them adapted for efficient photosynthesis?
Medium
- AThey are transparent to allow light to pass through.
- BThey have a thick waxy coating.
- CThey contain lots of mitochondria.
- DThey contain lots of chloroplasts.
3.Which substance is transported in the phloem?
Easy
- AWater
- BDissolved sugars
- CMineral ions
- DCarbon dioxide
4.Which of the statements A to D does not refer to the xylem?
Medium
- ATiny pores allow water and mineral ions to enter.
- BComposed of tubes of elongated living cells.
- CStrengthened by lignin.
- DDead cells form a hollow tube.
5.What is the predominant process by which water is lost from the leaf?
Medium
- ADiffusion
- BEvaporation
- CTranslocation
- DPhotosynthesis
6.What would happen to the transpiration rate of a plant during the night time?
Easy
- ATranspiration increases
- BTranspiration remains constant
- CTranspiration stops
- DTranspiration decreases
7.What is the name given to the flow of water from the roots to the leaves via the xylem vessel?
Medium
- ATranslocation
- BTranspiration stream
- COsmosis
- DActive transport
8.Which of the following are functions of transpiration in plants? (select all that apply)
Medium
- ATransporting mineral ions
- BProviding water to keep cells turgid
- CProducing glucose for the plant
- DKeeping the leaves cool
- EAbsorbing carbon dioxide from the air

Plus de questions

9.Stomata are found in much greater concentration on the upper surface of a typical leaf.
Easy
True or false?
10.Xylem cells are living cells that contain cytoplasm and organelles.
Easy
True or false?
11.Match each plant tissue to its main function.
Medium
- Xylem
- Phloem
- Palisade mesophyll
- Guard cells
- Transports water and mineral ions
- Transports dissolved sugars and amino acids
- Absorbs light for photosynthesis
- Open and close stomata
12.Place the parts of the water pathway through a plant in the correct order, starting from where water enters the plant.
Medium
- Root hair cell
- Root cortex cells
- Xylem
- Leaf mesophyll cells
13.Which of the following are plant organs that form part of the plant organ system for transport? (select all that apply)
Medium
- ARoots
- BStem
- CLeaves
- DXylem vessels
- EPalisade mesophyll cells
14.Which of the following statements about phloem tissue are correct? (select all that apply)
Medium
- AIt is made of living cells.
- BIt transports water and mineral ions from the roots.
- CIt contains sieve plates in the end cell walls.
- DIt transports dissolved sugars and amino acids.
- EIt is strengthened by lignin.
15.Which of the following are adaptations of xerophytic plants to reduce water loss? (select all that apply)
Hard
- ASunken stomata
- BLarge, thin leaves
- CA thick waxy cuticle
- DHairs surrounding the stomata
- EMany stomata on the upper leaf surface
16.Match each leaf structure to its function.
Medium
- Waxy cuticle
- Palisade mesophyll
- Spongy mesophyll
- Guard cells
- Prevents water from evaporating from the leaf
- Contains many chloroplasts for light absorption
- Contains air spaces for gas diffusion
- Open and close the stomata
17.Match each environmental factor to its effect on the rate of transpiration.
Medium
- Increase in light intensity
- Increase in temperature
- Increase in humidity
- Increase in air movement
- Increases transpiration rate
- Decreases transpiration rate
18.Place the stages of translocation in the correct order, starting from where sucrose is made.
Medium
- Sucrose is made in the leaves.
- Sucrose is loaded into the phloem.
- Sucrose is transported through the phloem.
- Sucrose is delivered to the roots for storage or use.
19.Place the steps of using a bubble potometer to measure the rate of transpiration in the correct order.
Medium
- Cut a shoot underwater.
- Assemble the apparatus underwater and seal with Vaseline.
- Dry the leaves of the shoot.
- Allow a single air bubble to form.
- Record the starting and finishing position of the bubble.
20.Transpiration is the loss of water vapour from the parts of the plant that are above ground, such as the leaves, stem and flowers.
Easy
True or false?
21.In the phloem, transport always occurs in one direction only, from the roots to the leaves.
Medium
True or false?
22.Lignin is deposited in the cell walls of xylem cells, causing them to die and become hollow.
Easy
True or false?
23.Which conclusion is best supported by the results?
Medium
- AIncreasing light intensity increases the rate of transpiration.
- BIncreasing light intensity decreases the rate of transpiration.
- CLight intensity has no effect on the rate of transpiration.
- DTranspiration only occurs in the dark.
24.A student counts 45 stomata in a 0.25 mm² area of the lower leaf surface. What is the stomatal density in stomata per mm²?
Medium
- A11.25
- B45
- C180
- D1125
25.A plant is moved from a warm, dry, windy room into a cool, humid, still room. Which of the following statements about the new conditions are correct? (select all that apply)
Hard
- AThe lower temperature reduces the kinetic energy of water molecules, decreasing the transpiration rate
- BThe higher humidity reduces the water potential gradient between the leaf and the air, decreasing the transpiration rate
- CThe still air removes water vapour from around the stomata more quickly, increasing the transpiration rate
- DThe reduced air movement means water vapour is not carried away from the stomata as quickly, decreasing the transpiration rate
- EThe higher humidity increases the rate of diffusion of water vapour out of the stomata
26.Match each plant adaptation to the condition it helps the plant cope with.
Medium
- Sunken stomata
- Needle-shaped leaves
- Thick waxy cuticle
- Hairs surrounding stomata
- Large water-storing stem
- Reduces water loss by trapping humid air near the stomata
- Reduces surface area for transpiration
- Prevents water evaporating from the leaf surface
- Protects the stomata from external air currents
- Stores water for use in dry periods
27.The opening and closing of stomata follows a daily rhythm, so they continue to open and close even when a plant is kept in constant darkness.
Easy
True or false?
28.Place the steps in the correct order to measure the rate of transpiration using a bubble potometer.
Medium
- Cut a shoot underwater and place it in the tube
- Seal all joints with Vaseline to make the apparatus airtight
- Dry the leaves of the shoot
- Allow a single air bubble to form in the capillary tube
- Record the starting position of the air bubble
- Record the end position of the air bubble after a set time
- Calculate the rate by dividing the distance moved by the time taken
29.What conclusion is best supported by these results?
Medium
- AIncreasing light intensity increases the rate of transpiration
- BIncreasing light intensity decreases the rate of transpiration
- CLight intensity has no effect on the rate of transpiration
- DTranspiration rate is independent of stomatal opening
30.A plant is placed in a sealed transparent bag and the air inside is kept at 95% humidity. What effect does this have on the rate of transpiration compared with normal room air?
Hard
- AIt increases, because water vapour diffuses out faster
- BIt decreases, because the water potential gradient between the leaf and the air is reduced
- CIt stays the same, because humidity does not affect transpiration
- DIt stops completely, because stomata close in humid air
31.A student sets up a bubble potometer to measure the rate of transpiration. Why is the plant shoot cut underwater before being placed in the apparatus?
Medium
- ATo stop air bubbles entering the xylem and blocking the water column
- BTo allow the plant to absorb extra water before the experiment starts
- CTo kill any bacteria on the cut surface of the stem
- DTo make the leaves lose water more quickly
32.In a bubble potometer investigation, the leaves of the shoot are carefully dried before the experiment begins. Why is this step carried out?
Medium
- AWet leaves would affect the results because water on the surface would evaporate
- BWet leaves stop the air bubble from forming in the capillary tube
- CWet leaves would cause the xylem vessels to close up
- DWet leaves make the plant photosynthesise too quickly
33.A bubble potometer records the movement of an air bubble along a capillary tube. What does the distance moved by the bubble directly measure?
Medium
- AThe volume of water taken up by the shoot
- BThe volume of carbon dioxide absorbed by the leaves
- CThe mass of sugar transported in the phloem
- DThe number of stomata on the leaf surface
34.Two students set up identical bubble potometers with plant shoots. Student A places a fan blowing air over the shoot. Student B leaves the shoot in still air. All other conditions are the same. What is the expected result?
Medium
- AThe bubble moves further in Student A's potometer over the same time period
- BThe bubble moves further in Student B's potometer over the same time period
- CThe bubble moves the same distance in both potometers
- DNo bubble forms in Student A's potometer
35.A potometer is set up to investigate the effect of humidity on transpiration. A plant shoot is placed inside a clear plastic bag and water is sprayed inside before sealing. The bubble moves 6 mm in 10 minutes. The bag is removed and the shoot is left in the same room with no bag. The bubble now moves 18 mm in 10 minutes. Which conclusion is best supported?
Hard
- AHigh humidity decreases the rate of transpiration
- BHigh humidity increases the rate of transpiration
- CHigh humidity has no effect on the rate of transpiration
- DHigh humidity stops transpiration completely
36.A student uses a potometer to measure water uptake by a leafy shoot. The air bubble moves 24 mm in 8 minutes. What is the rate of water uptake?
Medium
- A3 mm per minute
- B0.33 mm per minute
- C192 mm per minute
- D16 mm per minute
37.A student investigates transpiration using a potometer. The bubble moves 15 mm in 5 minutes when a lamp is 20 cm from the plant. The lamp is moved to 10 cm from the plant and the bubble moves 30 mm in 5 minutes. Which statement best explains the difference?
Hard
- AMore stomata are open in brighter light, so more water vapour diffuses out
- BThe lamp heats the xylem so water moves faster through the tube by active transport
- CBrighter light causes the stomata to close, trapping water vapour inside the leaf
- DThe lamp causes the guard cells to become flaccid and open the stomata
38.A student sets up a potometer and finds that the air bubble does not move at all over 30 minutes. Which of the following is the most likely explanation?
Hard
- AThere is a leak in the potometer apparatus
- BThe plant is photosynthesising too quickly
- CThe room temperature is too low
- DThe plant has too many leaves
39.A student counts stomata on the lower surface of a leaf using a microscope. In a field of view measuring 0.2 mm², there are 18 stomata. What is the stomatal density?
Medium
- A90 stomata per mm²
- B3.6 stomata per mm²
- C0.9 stomata per mm²
- D360 stomata per mm²
40.A plant species has a very high number of stomata per mm² of leaf surface. Which statement best explains why this could be a disadvantage in a hot, dry environment?
Hard
- AMore stomata means more water vapour can diffuse out, increasing water loss
- BMore stomata means less carbon dioxide can enter the leaf for photosynthesis
- CMore stomata means the leaf cannot absorb enough light energy
- DMore stomata means the guard cells cannot open and close
41.Name the cells that control the opening and closing of stomata.
Easy
42.A leaf surface has 60 stomata in a 0.5 mm² area. Calculate the number of stomata per mm².
Medium
43.Complete the sentence using the words from the box: Xylem, Sugars, Phloem, Mineral ions, Translocation, Water, Leaves, Transpiration, Roots, Photosynthesis.
Easy
The ____ transports food substances such as dissolved ____ made in the ____ to the rest of the plant; this process is called ____.
44.Complete the sentences using the words from the box: Evaporation, Diffusion, Active transport, Osmosis, Translocation, Transpiration.
Easy
Water moves into the cytoplasm of root hair cells by the process of ____. Mineral ions move into the cytoplasm of root hair cells by the process of ____.
45.A potometer records that an air bubble moves 30 mm in 5 minutes. Calculate the rate of water uptake in mm per minute.
Medium
46.A leaf surface has 80 stomata in a 0.4 mm² area. Calculate the number of stomata per mm².
Medium
47.Complete the sentence about the pathway of water through a plant.
Medium
Water enters the root hair cell by osmosis, then passes through the root cortex cells into the ____, and is carried up to the leaf ____ cells.
48.Complete the sentence about the plant transport tissues.
Medium
The ____ transports water and mineral ions from the roots to the leaves, while the ____ transports dissolved sugars and amino acids around the plant.
49.Name the process by which water enters root hair cells from the soil.
Medium
50.Name the process by which mineral ions enter root hair cells.
Medium
51.Calculate the rate of transpiration when the lamp is 10 cm from the plant, in mm per minute.
Medium
52.Calculate the rate of transpiration when the lamp is 5 cm from the plant, in mm per minute.
Medium
53.Describe how the structure of a leaf is adapted for efficient photosynthesis and gas exchange.
Medium
54.Explain how the structure of xylem tissue is adapted for its function.
Hard
55.Explain how the structure of a root hair cell is adapted for the efficient uptake of water and mineral ions.
Hard
56.Explain how the structure of the xylem is adapted for its function.
Hard
57.Explain why increasing light intensity increases the rate of transpiration.
Hard
58.A leaf surface has 60 stomata in a 0.5 mm² area. Calculate the number of stomata per mm².
Medium
- A108 stomata/mm²
- B144 stomata/mm²
- C120 stomata/mm²
- D132 stomata/mm²
59.A potometer records that an air bubble moves 30 mm in 5 minutes. Calculate the rate of water uptake in mm per minute.
Medium
- A5 mm/min
- B6 mm/min
- C8 mm/min
- D7 mm/min
60.A leaf surface has 80 stomata in a 0.4 mm² area. Calculate the number of stomata per mm².
Medium
- A180 stomata/mm²
- B200 stomata/mm²
- C220 stomata/mm²
- D240 stomata/mm²
61.Calculate the rate of transpiration when the lamp is 10 cm from the plant, in mm per minute.
Medium
- A3 mm/min
- B2 mm/min
- C4 mm/min
- D1 mm/min
62.Calculate the rate of transpiration when the lamp is 5 cm from the plant, in mm per minute.
Medium
- A5 mm/min
- B4 mm/min
- C3 mm/min
- D6 mm/min

Cartes mémo

What is the function of the xylem?

The xylem transports water and mineral ions from the roots to the stem and leaves.

What is the function of the phloem?

The phloem transports dissolved sugars (mainly sucrose) and amino acids from the leaves to the rest of the plant for immediate use or storage.

How is the xylem adapted for its function?

The xylem is composed of hollow tubes strengthened by lignin, and its cells are dead without organelles or cytoplasm, allowing free passage of water.

How is the phloem adapted for its function?

The phloem is made of living cells joined end-to-end with sieve plates (pores in end walls) that allow easy flow of substances; cells have few subcellular structures to aid flow.

What is transpiration?

Transpiration is the loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by diffusion of water vapour through the stomata.

What is translocation?

Translocation is the transport of sucrose and amino acids in the phloem, from regions of production (source) to regions of storage or use (sink).

What is the transpiration stream?

The transpiration stream is the continuous flow of water through the xylem from the roots to the leaves, replacing water lost by transpiration.

How does an increase in temperature affect the rate of transpiration?

An increase in temperature increases the rate of transpiration because it increases the rate of evaporation of water from the mesophyll cells.

How does an increase in humidity affect the rate of transpiration?

An increase in humidity decreases the rate of transpiration because it reduces the water potential gradient between the leaf and the air, slowing diffusion of water vapour out of the stomata.

How does an increase in air movement affect the rate of transpiration?

An increase in air movement increases the rate of transpiration because it removes water vapour from around the leaf, maintaining a steep concentration gradient for diffusion.

How does an increase in light intensity affect the rate of transpiration?

An increase in light intensity increases the rate of transpiration because more stomata are open in bright light to maximise photosynthesis, allowing more water vapour to diffuse out.

What is the role of guard cells?

Guard cells control the opening and closing of stomata to regulate gas exchange and water loss.

How do guard cells open and close stomata?

When guard cells become turgid (gain water by osmosis), the stomata open; when they become flaccid (lose water by osmosis), the stomata close.

Why do stomata close at night?

Stomata close at night when photosynthesis cannot occur to reduce water loss by transpiration and conserve water.

What is the function of the palisade mesophyll?

The palisade mesophyll is column-shaped cells tightly packed with chloroplasts to absorb more light, maximising photosynthesis.

What is the function of the spongy mesophyll?

The spongy mesophyll contains internal air spaces that increase the surface area to volume ratio for the diffusion of gases, mainly carbon dioxide.

What is the function of the epidermis in a leaf?

The upper epidermis is thin and transparent to allow light to enter the palisade mesophyll; the lower epidermis contains guard cells and stomata.

What is the function of the waxy cuticle?

The waxy cuticle is a protective layer on top of the leaf that prevents water from evaporating.

How are root hair cells adapted for water and mineral ion uptake?

Root hair cells have long hair-like projections that increase the surface area for absorption of water by osmosis and mineral ions by active transport.

What is the pathway of water through a plant?

Water moves from root hair cells → root cortex cells → xylem → leaf mesophyll cells.

What is the role of the vascular bundle in a leaf?

The vascular bundle contains xylem and phloem to transport substances to and from the leaf.

What is the function of meristem tissue?

Meristem tissue is found at the growing tips of shoots and roots and is responsible for growth.

How does a potometer measure the rate of transpiration?

A potometer measures the uptake of water by a plant shoot as a measure of the amount of water lost by evaporation, by recording the distance moved by an air bubble over a set time.

Why is transpiration beneficial for a plant?

Transpiration transports mineral ions, provides water to keep cells turgid for support, supplies water to leaf cells for photosynthesis, and helps cool the plant.

What is the difference between xylem and phloem in terms of cell status?

Xylem cells are dead and hollow, while phloem cells are living and retain some subcellular structures.

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