3.8 Plant Cell Organisation

Question 1

Why is diffusion harder in multicellular organisms than single-celled organisms

Answer 1

Single-celled prokaryotes are only one cell big - therefore substances can diffuse straight into and out of them across their partially permeable cell membrane. In multicellular organisms, such as animals and plants, diffusion across the outer surface is more difficult because some cells are deep inside the organism - it’s a long way from them to the outside environment and so specialised exchange surfaces are needed.

Question 2

Why does carbon dioxide move into leaves through diffusion?

Answer 2

When the plant is photosynthesising it uses up lots of carbon dioxide, so there’s hardly any inside the lead. This makes more carbon dioxide move into the leaf by diffusion (from an area of higher concentration to an area of lower concentration).

Question 3

Describe how carbon dioxide diffuses into leaves

Answer 3

In plants, the lower epidermis later is scattered with stomata. The stomata are there to let gases like carbon dioxide and oxygen diffuse in and out. The gases diffuse between air spaces inside the leaf and the plant’s cells through the cell’s partially permeable membrane.

Question 4

What is stomata in leaves for?

Answer 4

The stomata are there to let gases like carbon dioxide and oxygen diffuse in and out. They also allow water vapour to escape, which is known as transpiration

Question 5

Where do the gases in leaves diffuse in and out from?

Answer 5

The gases diffuse between air spaces inside the leaf and the plant’s cells through the cells’ partially permeable outer membrane.

Question 6

Where is the stomata in the structure of the leaf?

Answer 6

Lower epidermis

Question 7

Describe what happens after carbon dioxide diffuses into leaves

Answer 7

The carbon dioxide moves up the leaf and enters the spongy mesophyll tissue. It has lots of air gaps between the cells so that the gas can easily diffuse through to the next layer which is the palisade mesophyll layer.

Question 8

What is distinctive about the palisade mesophyll layer in leaves?

Answer 8

It contains a lot of chloroplasts as this is where most of the photosynthesis happens

Question 9

What is above the palisade mesophyll layer in leaves?

Answer 9

Upper epidermis - ‘epidermal tissue’

Question 10

What is distinctive about the upper epidermis layer in leaves?

Answer 10

The cells are almost transparent as the sunlight needs to pass through them to get to the chloroplasts in the palisade mesophyll layer

Question 11

Once photosynthesis has produced the sugar molecules, how are they carried to the rest of the plant?

Answer 11

Through the phloem

Question 12

What does the xylem do in leaves?

Answer 12

Brings water up from the roots for the palisade cells to use in photosynthesis

Question 13

How do leaves prevent water loss? (2)

Answer 13

• By having a waxy cuticle on top - a thin layer of lipids that the water can’t get through.
• In the lower epidermis layer, the leaves keep the stomata open for as short a time as possible so that they can maximise carbon dioxide absorption but minimise water loss. Each stoma is formed by the gap in between 2 guard cells. When the plant has lots of water, so it doesn’t have to worry about conserving it, the guard cells will be well hydrated (turgid). This makes the gap between them larger, allowing more carbon dioxide to flow through. However, when the plant is short of water, the guard cells will lose water due to osmosis and they’ll become flaccid. This closes the stomata, meaning the plant no longer takes in carbon dioxide and conserves it’s water vapour.

Question 14

How is each stoma formed in a leaf?

Answer 14

Each stoma is formed from the gap between two guard cells

Question 15

Describe the 3 adaptations that guard cells in leaves have

Answer 15

• Each stoma is formed by the gap in between 2 guard cells. When the plant has lots of water, so it doesn’t have to worry about conserving it, the guard cells will be well hydrated (turgid). This makes the gap between them larger, allowing more carbon dioxide to flow through. However, when the plant is short of water, the guard cells will lose water due to osmosis and they’ll become flaccid. This closes the stomata, meaning the plant no longer takes in carbon dioxide and conserves it’s water vapour
• They are sensitive to light, so they close at nighttime when photosynthesis isn’t taking place and they don’t need carbon dioxide.
• They are on the lower surface of leaves as the lower side is more shaded, which makes it cooler and means less water will evaporate.

Question 16

Where is meristem tissue in a plant found?

Answer 16

In the growing tips of the roots and shoots

Question 17

What is the purpose of meristem tissue in plants?

Answer 17

They can differentiate into many different cell types so that the plant can grow - plant stem cells.

Question 18

The four levels of plant organisation are:

Answer 18

cells -> tissues -> organs -> organ systems

Question 19

The leaf is an example of an:

Answer 19

Organ

Question 20

The root, stem, and leaves, together make up an:

Answer 20

Organ system

Question 21

The palisade mesophyll layer is an example of a:

Answer 21

Tissue

Question 22

Which structures help to minimise water loss? (2)

Answer 22

• Waxy cuticle
• Guard cells which can close stomata

Question 23

At night time, when photosynthesis can’t take place, are the stomata normally open or closed?

Answer 23

Closed

Question 24

Describe how changes in the concentration of ions inside the guard cells help to open and close the stomata

Answer 24

• In response to stimuli like light, potassium ions are pumped into guard cells. This increases the solute concentration of the guard cells, which decreases the concentration of water molecules. Water then moves into the guard cells by osmosis. This makes the guard cells turgid and the stoma opens.
• When potassium ions leave the guard cells, the concentration of water molecules in the cell increases. Water then moves out by osmosis, the guard cells become flaccid and the stoma closes.

Question 25

What happens if a cell is very short of water?

Answer 25

If a plant’s really short of water, the cytoplasm inside its cells starts to shrink and the membrane pulls away from the cell wall. A cell in this condition is said to be plasmolysed.

Question 26

Describe how you would prepare a plant to observe the stomata in it under a microscope (3)

Answer 26

1) Paint two thin layers of clear nail varnish onto the leaf you want to look at. Leave the varnish to dry between each coat.
2) Put a piece of clear sticky tape over the top of the painted leaf and use it to peel the varnish off slowly. The varnish will have an impression of the leaf’s surface.
3) Stick the tape with the varnish onto a microscope slide.

Question 27

Describe how you would observe the structure of xylem and phloem in thin sections of a plant’s stem.

Answer 27

If a stem is left upright in a beaker of eosin dye, the dye will travel up the stem, staining the xylem red. A thin section of the stem can then be taken and viewed on a slide under a microscope.