Gas Exchange - Flowering plants

What is the role of diffusion in gas exchange?

What is the role of diffusion in gas exchange?

Diffusion in Gas Exchange

• Diffusion is the movement of molecules from a region of its higher concentration to a region of its lower concentration

• Molecules move down a concentration gradient, as a result of their random movement

Diffusion across the cell membrane

• Diffusion is the process by which gas exchange occurs

• Single celled organisms, such as amoeba, can exchange gases sufficiently by simple diffusion through the cell membrane

Gas exchange in single-celled organisms (such as amoeba) occurs through diffusion

• Multicellular organisms (such as plants and animals), however, have exchange surfaces and organ systems that maximise the exchange of materials

  • Gills are the gas exchange organs in fish, lungs in humans and leaves or roots in plants

• These organs increase the efficiency of exchange in a number of ways:

  • Having a large surface area to increase the rate of transport

  • A short diffusion distance for substances to move across. This short distance is created because the barrier that separates two regions is as thin as possible

• In addition, animals have:

  • Gas exchange surfaces that are well ventilated to maintain steep concentration gradients

What is gas exchange in relation to respiration and photosynthesis?

Gas Exchange: Photosynthesis & Respiration

• The processes of respiration and photosynthesis both rely on the exchange of oxygen and carbon dioxide

Gas exchange during respiration

• All living cells respire

• The process requires the uptake of oxygen and the release of carbon dioxide

The respiration equation

• The oxygen diffuses down the concentration gradient from a high concentration (outside the leaf) to a low concentration (inside the leaf)

  • The cells use oxygen in respiration so the concentration is always low inside the respiring cells

• The carbon dioxide diffuses down the concentration gradient from a high concentration (inside the leaf) to a low concentration (outside the leaf)

In respiration, carbon dioxide diffuses out of the leaf and oxygen diffuses into the leaf

Gas exchange during photosynthesis

• Plant cells photosynthesize when there is enough sunlight

• The process requires the uptake of carbon dioxide and the release of oxygen

The photosynthesis equation

• The carbon dioxide diffuses down the concentration gradient from a region of high concentration (outside the leaf) to a region of low concentration (inside the leaf)

  • The cells use carbon dioxide in photosynthesis so the concentration is always low inside the photosynthesizing cells

• The oxygen diffuses down the concentration gradient from a high concentration (inside the leaf) to a low concentration (outside the leaf)

In photosynthesis, carbon dioxide diffuses into the leaf and oxygen diffuses out of the leaf

How is the structure of the leaf adapted for gas exchange?

Diagram showing Cross-section of Plant Leaf

ADAPTATIONS FOR GAS EXCHANGE:

FEATURE	ADAPTATION
STOMATA	Opens to allow the diffusion of Carbon dioxide into the leaf for Photosynthesis, and the diffusion of waste products of Oxygen and Water vapour out of the leaf - gas exchange
GUARD CELLS AROUND STOMATA	Controls the opening and closing of Stomata depending on the time of day to enable gas exchange - e.g, Guard cells become turgid during daytime to open Stomata and allow diffusion of Carbon Dioxide into the leaf for Photosynthesis (due to presence of sunlight)
THIN	Minimises the diffusion length of Carbon Dioxide into the leaf (for Photosynthesis), and Oxygen and Water (waste product of Photosynthesis) out of the leaf, therefore increasing the rate of gas exchange
FLATTENED SHAPE	Larger surface area to volume ratio increases the rate of absorption of sunlight and Carbon Dioxide for use in Photosynthesis, therefore increasing rate of gas exchange
INTERNAL AIR SPACES IN SPONGY MESOPHYL	Larger internal surface area to volume ratio for absorption of Carbon Dioxide into the leaf (for Photosynthesis), therefore increasing the rate of gas exchange

What is the role of the stomata in gas excahange?

Stomata

• Stomata are spaces found between two guard cells predominantly on the lower epidermis of the leaf

• The guard cells are responsible for the opening and closing of the stomatal pore which controls gas exchange and water loss

• Stomata open when water moves (by osmosis) into the guard cells causing them to become turgid

  • This allows gases to diffuse in and out of the leaf through the stomatal pore

  • Stomata tend to open when there is plenty of water and sunlight

• Stomata close when the guard cells lose water (by osmosis) to the neighbouring epidermal cells and they become flaccid

  • This prevents any diffusion into or out of the leaf

  • Stomata tend to close due to low water availability or low sunlight

The guard cells control the opening and closing of the stomata

How does respiration continue during the day and night, but the net exchange of carbon dioxide and oxygen depends on the intensity of light?

Gas Exchange: Day & Night

• Plants can only photosynthesize when they have access to light, however, cells respire all the time

• This means that gas exchange in plants varies throughout a 24 hour period

• During the daytime plants both respire and photosynthesize

  • The rate of photosynthesis tends to be higher than the rate of respiration (unless there is a low light intensity)

  • Therefore there is net diffusion of carbon dioxide into the plant and net diffusion of oxygen out of the plant during the day

• During the nighttime, plants only respire

  • This means that there is a net movement of oxygen into the plant and net diffusion of carbon dioxide out of the plant during the night time

• At low light intensities, the rate of photosynthesis is equal to the rate of respiration

  • This means that there is no net movement of oxygen or carbon dioxide in either direction

Plants photosynthesize and respire during the day but only respire at night time

How do you investigate the effect of light on net gas exchange from a leaf, using hydrogen-carbonate indicator?

HYDROGENCARBONATE INDICATOR: Indicator of Carbon Dioxide Concentration in a Solution

COLOUR AND CARBON DIOXIDE LEVELS:

COLOUR	CARBON DIOXIDE CONCENTRATION
YELLOW	High
RED	Atmospheric
PURPLE	Low

INVESTIGATING EFFECT OF LIGHT ON GAS EXCHANGE

Diagram showing the Apparatus to Investigate the Effect of Light on Gas Exchange

METHOD: Place pondweed in a beaker of Hydrogen Carbonate indicator Place light source at a set distance to adjust light intensity (the smaller the distance between the light source and plant, the higher the light intensity) Allow plant to adapt to adjusted environment for a few minutes Observe and record the colour of indicator Change light intensity and repeat process

RESULT: When light source is placed close to the plant, high light intensity will allow Photosynthesis to occur due to the presence of light, therefore increasing the rate of Photosynthesis involving the absorption of Carbon Dioxide (produced as a waste product of Respiration) Given that the rate of Photosynthesis is greater than the rate of Respiration, the concentration of Carbon Dioxide in solution will decrease, causing the indicator to turn Purple When light source is placed distant to the plant, low light intensity will prevent Photosynthesis to occur due to the absence of light, therefore decreasing the rate of Photosynthesis involving the absorption of Carbon Dioxide (produced as a waste product of Respiration) As the rate of Respiration will therefore be greater than the rate of Photosynthesis, the concentration of Carbon Dioxide in solution will increase, causing the indicator to turn Yellow