BPSC Lecture 13

What can diffuse freely across plasma membrane

Water, O₂, and CO₂

Diffusion

A difference in concentration that moves substances slowly over very short distances

Osmosis

Diffusion of water across a semi-permeable membrane via water potential (higher to lower)

Plasmolysis

Cell surrounded by a concentrated salt solution → Water diffuses out and cell becomes plasmolyzed

Cell surrounded by distilled water → Water diffuses in, cell becomes turgid

Transpiration

Loss of water vapor from plants (leaves)

Side effect of stomata opening

Water potential varies over the day

How do plants reduce water loss from transpiration?

Cuticle: Prevents evaporation from epidermal cells

Dense covering of hairs in some species

Opening + Closing stomata

Opening and closing of stomata process

Caused by changes in turgor pressure in the guard cells

1. Molecular pumps in plasma membrane pump K+ into cells (active transport)

2. Decreases water potential of guard cells

3. Water diffuses into guard cells

4. Increased turgor pressure

5. Guard cells open

Cohesion-Tension theory:

How water travels from the roots all the way up to the leaves through the xylem

1. Cohesion: Water sticks to water (polarity)

   1. Pulls the water up the xylem

2. Adhesion: Water sticks to the xylem walls

   1. Water attracted to xylem walls

   2. Helps water fight against gravity

3. Tension: Water is pulled upward

   1. Transpiration creates negative pressure in the xylem, pulling water up in a continuous column from root to leaf

Pressure Flow Hypothesis:

Long distance transport in Phloem: Sources load sugar into phloem, sinks receive sugar

• Source to Sink transport

1. Phloem loading (at the source)

   1. Sugar is loaded into the phloem

   2. Increases solute concentration inside the phloem = lower water potential

2. Water enters the phloem from the xylem 

   1. Phloem lower water potential → water enters from xylem via osmosis

   2. Increased water amount creates high pressure

3. Pressure pushes sugar toward the sink

4. Phloem unloading

Water potential throughout the cell

How does water enter the root:

• Diffusion driven by differences in water potential of the root and the soil

• Roots have lower water potential compared to soil

• Root pressure: water diffuses into root, pushing the water column up the xylem

Embolism

An air bubble in the Xylem

• Water cannot move past bubble

• Caused by Low tension reaching a critical water potential

• Air entry into Xylem

• Freezing

Apoplastic vs Symplastic

Symplastic loading: Going building to building

• Sugars travels through connected cells via plasmodesmata

• Companion cells → sieve-tube element

Apoplastic: Going around each building

• Sugars travel outside cells first

• Water moves from Xylem to mesophyll and is eventually lost due to transpiration

• Photosynthate moves from mesophyll into phloem and then to sinks in other parts of the plant