Lecture 4 Plant Physiology: Background (Ch2&3 book)

Chapter 2 (p.30-34 in Ricklefs)

Osmosis: if cell has high concentration of ions and other solutes, water will move from surrounding environment into the cell.

Osmotic potential: the force that an aqueous solution (any solution containing H2O) attracts water by osmosis. Plants use osmotic potential to get water from the soil into their own cells.

How does water a plant get water?
- Explained using the cohesion-tension theory
- The osmotic potential in roots draws water from soil into xylem elements
- Xylem elements are cells connected end to end that conducts water and provides support (think of these cells as water pipes)
- Tension pulls water from xylem into veins of leaves
- Water evaporates and vapor diffuses out of leaf

Transpiration: when water evaporates from leaf into the air
- Leaf have small openings at the surface called stomata (stomates)
- Plants can reduce water loss by closing the stomates

Salt balance and water balance
- Plants and animals must maintain levels of salt and water
- High salt concentrations can change the way proteins interact and disrupt cell function
- If salt concentrations in soil are high, plants pump excel salt back into the soil
- Terrestrial animals are less vulnerable to water loss than plants
- Animals that don’t have access to fresh water have glands that secrete excess salt

Chapter 3 (p.38-47 in Ricklefs)

Sunlight is essential to life on earth. Plants, algae, and some bacteria can absorb the light from the sun and turn it into energy (via photosynthesis).

Key terms associated with light:
- Visible spectrum: portion of electromagnetic radiation that can be seen by human eye
- Photon: basic unit of light, a small particle-like unit
- Irradiance: intensity of the light of all wavelengths on a surface

Light of wavelengths between 400 and 700 nm (nanometers) can be used by photosynthetic organisms.

Chlorophyll cells captures the light energy in photosynthesis. Chlorophyll cells absorb (take in) red and violet light, but they also reflect green and blue light. This is why leaves are mostly green in color.

1. Chlorophyll molecules (located in a chloroplast in a plant cell) absorb photons (light) and release electrons.
2. Cell passes the electrons through a series of reaction that generate ATP (high energy compound).
3. Cell uses energy from step 2 to make glucose. When they make glucose they also release oxygen as a waste product.

Steps 1 – 3 are often called the light reactions because they depend on light energy.

In one of the chemical reactions in step 3, the cell will combine CO2 and a ribulose bisphosphate (RuBP, a sugar) to produce a three carbon molecule (this is called C3 photosynthesis). The Calvin-Benson cycle takes place after C3 photosynthesis and makes glucose.

But, if CO2 levels are low in the cell (e.g., when the stomata are closed to prevent water loss) the cell will add oxygen to RuBP instead and start reactions that use energy and reverse light reactions (wasteful). This is called photorespiration.

Plants maintain high CO2 levels in their cells to avoid photorespiration.

Some plants that grow in hot climates use C4 photosynthesis. This adds an extra step to photosynthesis that creates a four-carbon molecule (oxaloacetic acid, OAA) that likes CO2. Plants that do this are called C4 plants. The cells of C4 plants are arranged differently than C3 plants (see diagram p.44 in Ricklefs). C4 plants (e.g., corn) have a higher CO2 concentration and are highly productive during hot seasons.

CAM plants use the same biochemical pathways as C4 plants but they go through the Calvin-Benson cycle during the day and CO2 assimilation at night.

Some plants have dense hairs and spines on their surface to protect the surfaces from direct sunlight and reduce water loss.

It is difficult for plants and algae to get enough carbon for photosynthesis. Dissolved CO2 from the atmosphere combines with water to form bicarbonate ions (HCO3-). Carbon dioxide and bicarbonate enter the cells of aquatic plants. The plants can use bicarbonate for photosynthesis, but it isn’t as efficient as CO2.

Oxygen isn’t very soluble in water. Below the surface of the water, no oxygen is produced by photosynthesis. These habitats can become depleted of dissolved oxygen. Habitats that are devoid of oxygen are called anaerobic or anoxic.

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