Inverse Square Law

- If you know the distance between the light source and photosynthesising plant, you can work out the light intensity using the inverse square law. 

- Inverse Square Law =
= Light intensity is inversely proportional to the square of the distance to the source.

- This states that if the distance to the light source is doubled, the light intensity decreases by the square of the distance. 
→ If distance is doubled → x2
→ Light intensity = of light intensity

Examples:

- The distance of a plant from a light source is 0.1 m. The relative light intensity is 100. 
- If the distance of the light source increases to 0.2 m, what's the relative light intensity?
→ The distance has doubled, so…

If the distance was 0.3 m…
→ The distance trebled, so…

If the distance was 0.5 m…
→ The distance has increased by 5 times…

e.g. at 2 m i = 0.4, workout i at 8 m

Plant Structure

Leaf Structure

Functions:

- Waxy cuticle: To help prevent water loss. 
- Upper Epidermis: Thin and transparent to allow more light through to chloroplasts. 
- Palisade Mesophyll: This type of mesophyll contains a large number of chloroplasts, which allow it to absorb more light.
- Spongy Mesophyll: This type of mesophyll has numerous air spaces that allow oxygen to diffuse through the leaf and increase its surface area.
- Stomata: Holes in the leaf to allow CO₂ to diffuse in and O₂ to diffuse out. 
- Guard cells: To open and close the holes and to prevent water loss. 
- Vascular bundles: Strong veins supporting and holding the leaf.

Adaptations:

- Most leaves are broad, giving them a large surface area for absorbing sunlight and containing more chloroplasts.
- Most leaves are thin to ensure CO₂ has a short diffusion distance from the air outside the leaf to the photosynthesising cells.

Plant Roots

- Plants use their roots to absorb water and mineral ions from the soil.
- Roots also anchor the plant in the soil.
- The term "root tip" refers to the very end of the root. Numerous root hair cells reside behind these roots. These are the cells that absorb water and mineral ions from the soil.

Root Hair Cells:

Functions:

- Large surface area for absorbing water.
- No cuticle, so absorption is easier.
- A large permanent vacuole that can hold lots of water. 
- Close to vascular bundles, quickly carry water to the rest of the plant.
- A thinner cell wall on the root hair compared to the rest of the cell reduces the distance for water to travel.

Fertilisers:

- Fertilisers contain mineral ions, which can be added to soil to ensure plants have enough of each mineral. 
- Minerals are added for the healthy growth of a plant.

Mineral Ions:

- Minerals are in low concentrations in the soil compared to inside plant cells. 
- Active transport is required to absorb these.
- To provide the ATP for this, root hair cells have many mitochondria.

- The root hair cells absorb water for photosynthesis from the soil.

Mineral Ions 

Four Mineral Ions:
- Nitrates - most important
- Magnesium - most important
- Phosphates 
- Potassium 

Nitrates:

- These contain nitrogen.
- The plant needs nitrogen to make amino acids. Amino acids make up proteins, which are used for growth.
- A plant lacking nitrates will have poor growth and yellow leaves. 

Phosphates:

- These contain phosphorus. 
- Plants use phosphates in respiration and in the synthesis of DNA and cell membranes.
- A plant lacking phosphate will have poor root growth and discoloured leaves.

Potassium Compounds:

- It contains potassium.
- The production of enzymes essential for photosynthesis and respiration depends on it.
- Plants deficient in potassium exhibit poor growth, fewer flowers, and discoloured leaves.

Magnesium Compounds:

- Magnesium is necessary for the production of chlorophyll, without which a plant cannot perform photosynthesis.
- A plant lacking magnesium will have yellow leaves.

Stomata

- Stomata are holes/pores in the lower epidermis of the leaf.
- When the stomata are open, gases can pass in and out of leaves.
- E.g. During the day, CO₂ will need to diffuse into the leaf through stomata so it can be used for photosynthesis. 

Structure:

- Stomata technically don’t have a structure as they’re just holes.
- Guard cells are located on either side of a stoma. These cells have the ability to alter the size of the stomata and determine whether they are open or closed.
- Stomata must be opened to let CO₂ in for photosynthesis.

- In the light, guard cells absorb water through osmosis, swell, and become turgid.
- However, the uneven swelling of the cells occurs because the guard cells have a thicker cell wall on the inside.
- This means that when guard cells are turgid, the stomata open.

- During the night, guard cells absorb water through osmosis.
- Cells are no longer full of water. 
- The stomata close, and the cells move towards each other.

- CO₂ enters through stomata and diffuses into leaf cells, where it is used for photosynthesis.
- O₂ diffuses out of stomata.

Transpiration + Translocation

Transpiration

- Because stomata have to be open during the day to let CO₂ in for photosynthesis, transpiration will occur.
- "Transpiration" is the loss of water vapour from a leaf.
- Water evapourates from the spongy mesophyll cells within the leaf. When water evapourates, it forms water vapour, which diffuses out of the leaf via the stomata.
- Water enters the plant through the roots. It travels up the stem in vascular bundles. Some is used in photosynthesis. Most will evaporate and diffuse out the stomata.

Steps:

1) Water moves into the root hair cell by osmosis from the soil.
2) Water moves across the root from cell to cell by osmosis.
3) Water enters the vascular bundle and ascends the plant to the leaves.
4) Water evapourates from spongy cells and enters 'air spaces' in the lower epidermis of the leaf during transpiration.
5) Water vapour diffuses out of the leaf via the stomata.

- The water lost from the leaf must be replaced.
- The spongy mesophyll cells have lost water, so they have a low ψ. Therefore, water moves from the xylem—high ψ—into the spongy mesophyll cells by osmosis.
- As water exits the leaf's xylem, it draws more water up the stem's xylem behind it. The roots are absorbing water to replace it.
- Cohesion pulls the water molecules upward as they move up the xylem in the stem and into the leaf.
- Thus, water moves through the xylem vessels in a continuous transpiration stream. From root → stem → leaf. 

Advantages:

- Water evaporation can cool a plant; transpiration loses about 97% of the water a plant absorbs.
- To use water in photosynthesis, leaves must receive it.
- Water must enter the cells to make them turgid, supporting the plant and keeping it upright.
- Water carries dissolved minerals in it.

Transpiration Stream:

- As water is continuously lost from the leaves, this water must be replaced by water from the veins.
- The roots take in water to replace the water from the veins.
- There is a stream of water constantly moving through the plant.

Stomatal Density:

- Some plants have a lot more stomata than others.
- You can count how many stomata there are in a set size area of a leaf (1 mm²) and then compare it with other plants.
- A plant with a higher stomatal density will transpire faster.

Adaptations

Marram Grass:

- If plants lose too much water, they will wilt and die.
- Marram grass is an example of a plant that has evolved well to minimise water loss through transpiration.
- It lives on sand dunes. Sand lets water drain away easily, so the marram grass roots often cannot absorb much water.
- To reduce water loss, plants have rolled leaves with a thick waxy cuticle, stomata sunk into pits, and leaf hairs that trap water vapor inside the leaf.

General:

- Transpiration is a consequence of gas exchange.
- Although most stomata are found on the underside of leaves, there can be some stomata on the upper epidermis.
- Water lilies have most of their stomata on the upper epidermis because the lower surface is in contact with the water.

How does it work? (recap in more detail)

1) The water vapour lost from the leaves is replaced by water in the capillary tube.
2) Movement of the air bubble towards the plant is measured at the end of the set time. e.g., 5 minutes using scale.
3) To repeat the experiment, use the reservoir to refill the capillary tube and reset the bubble.
4) To study the effect of different environmental factors on the rate of water uptake, place the whole apparatus under different sets of conditions but only change 1 variable at a time.

- A potometer can only measure water uptake (not transpiration rate). The plant uses some of the water it takes up from the capillary tube for photosynthesis, resulting in turgid cells. Transpiration does not lose all of the water the plant takes up from the capillary tube.

Investing Transpiration:

- You can use a potometer to investigate how different factors affect the rate of transpiration.
→ Wind using a fan
→ Sunlight using a bright/dark room
→ Maintain humidity levels by covering the plant with a transparent plastic bag.
→ Temperature by heating/cooling plant

Plant Transport

Plants have two different transport systems:

- A vascular bundle gathers the xylem and phloem tubes.

- The plant contains vascular bundles in various arrangements throughout its roots, stems, and leaves.

Factors Affecting Transpiration

1) Temperature:
- As the temperature rises, water evapourates from spongy mesophyll cells more quickly, and water vapour diffuses out of the leaf more quickly. This leads to an increase in water particles' KE and an increase in transpiration.

2) Light Intensity:
- The rate increases until all stomata are open.
- When exposed to light, stomata open and close.
- The higher the light intensity, the more stomata open on the leaf to allow CO₂ in for photosynthesis. More stomata are open, and transpiration increases.

3) Wind Speed:
The wind blows away any water vapour that has diffused out of the leaf, resulting in a low concentration of water vapour outside the leaf and a high concentration of water vapour inside, creating a steep gradient between the interior and exterior of the leaf. Diffusion happens quickly. Water vapour diffuses out of the leaf; from high to low concentrations, transpiration increases.

4) Humidity:
Humidity fills the air with water vapour. When humidity increases, there is not a steep water vapour concentration gradient between inside and outside the leaf; less water vapour diffuses out of it, so transpiration decreases.
→ Transpiration stops when there is the same concentration of water vapour inside and outside the leaf when humid.

- When there are more leaves or stomata, the rate of transpiration also increases.

- Xylem vessels transport water for photosynthesis to leaves.
- Diffusion out of spongy mesophyll cells causes a film of water to cover all surfaces.
- Water evapourates into the air spaces, and the water vapour diffuses out of the leaf via stomata.
- When transpiration increases, the plant's water uptake increases. 

- When the light intensity increases, the rate of transpiration does too. The graph then reaches a plateau as the light intensity no longer serves as the limiting factor and something else takes its place.

- As the temperature rises, the rate of transpiration also increases until it reaches its peak. The leaves' water also evaporates from them as they have more KE; the temperature increases and diffuses out faster as the rate increases and diffuses. 

- As the wind speed rises, it accelerates the removal of water, leading to a greater influx of water into the leaf through osmosis. The graph then plateaus; wind speed is no longer the limiting factor.

- As the humidity increases, the rate of transpiration decreases proportionally; there is a smaller concentration gradient, and this makes the diffusion of water vapour out of the leaf slower.

- The rate of transpiration is linked to the rate of photosynthesis; this is because increasing the light intensity will also increase the rate of photosynthesis → more water is drawn into leaves where photosynthesis occurs, and so the rate of transpiration is faster.

Translocation

- Moving sugar in a leaf means translocation.

Sucrose:

- Transporters convert glucose to sucrose. Sucrose is a disaccharide.
- Plants use it for transport: It's small and soluble, but also it's more energy efficient to transport a disaccharide rather than a monosaccharide (glucose). 

Sap:

- Phloem transports sap, a liquid containing lots of dissolved sucrose, either up or down the plant.
- It also contains other dissolved beneficial substances, such as amino acids.
- Translocation = transport of sap in phloem.
- The sieve plates allow sap to pass through.

Phloem:

- Phloem lacks a nucleus and contains only a small amount of cytoplasm. Companion cells, which are specialised cells, surround them.
- The companion cells regulate the activity of phloem vessel cells, but they lack a hollow structure, preventing sap from passing through them.
- A plant uses active transport to transport sap into the phloem, a process that requires a significant amount of ATP, and the companion cells in this process contain numerous mitochondria.

Phloem Vessel:

- Made of living cells.
- Cell walls don’t completely break down.
- Sieve plates are formed, with small holes in the end walls that allow dissolved sugars to pass through.
- Phloem cells connect to form a tube that facilitates the transportation of dissolved sugars.

Water Effects:

- Healthy plants balance water uptake and loss.
- Plants wilt if they lose water more quickly than they can replenish it.
- Wilting protects the plant as:
→ Leaves droop: Reduces SA for the sun to prevent water loss by evaporation. 
→ Stomata remain closed until they receive water due to insufficient transpiration.
- Plants stay wilted until they get water, the temperature drops, or the sun shines on them.
- Plants can also have too much water:
→ Waterlogged soils contain very little air.
→ Root cells don't receive O₂ for respiration.
→ They die because they lack energy from respiration and decay.