2.40B-2.45B Plant Gas Exchange
Gas exchange surface-✔✔ Tissue allowing gases (e.g. oxygen, carbon
dioxide, water vapor) to pass between internal/external environments of
plants/animals. They have a high surface area to volume ratio so large
amounts of gas can diffuse across.
Stomata (plural for stoma)-✔✔ Pores usually found in the leaf’s lower
epidermis. They allow carbon dioxide to diffuse into the leaf to reach
photosynthetic tissue, and allow oxygen as well as water vapor to diffuse
out simultaneously. This movement of gases in opposite directions is called
gas exchange. The upper epidermis has less or no stomata, because the
lower/underside of the leaf has more shade as opposed to the upper
epidermis which is frequently exposed to sunlight: ↓ sunlight exposure → ↓
heat exposure → ↓ water evaporation → ↓ vapor loss by transpiration.
Guard cells-✔✔ Pair of specialized cells surrounding a stoma in the leaf’s
epidermis. Guard cells can change shape to open or close the stoma:
Bright light → Guard cells take in water by osmosis (so stoma usually open
in day time when there’s lots of light) → plump/turgid guard cells → guard
cells expand, bend outwards (become like a curved banana surface) →
they stretch open stoma to enable gas exchange.
Less light (e.g. nighttime) → Guard cells lose water by osmosis → Flaccid
guard cells → Guard cells shrink, bend inwards/become straight, and
collapse against one another thus closing the stomatal pore → Closed
stoma → Gases cannot diffuse in and out the leaf.
Guard cells only close in the dark when no carbon dioxide is needed for
photosynthesis.
Transpiration-✔✔ Process of loss/evaporation of water vapor through the
plant’s stomata. When stomata open to let in carbon dioxide, water on the
surface of spongy and palisade mesophyll cells evaporates as well as
diffuses out the leaf. This evaporation occurs in aerial (in/exposed to air)
plant organs such as leafs, stems, flowers, etc. Meanwhile, roots take up
the same volume of water from the soil too.
, Leaf adaptations for gaseous exchange and photosynthesis-✔✔ Flat →
Large surface area to volume ratio → maximized light absorption
Thin leaf and thin cell walls → ↓ diffusion distance for gases
Has chlorophyll → Absorbs sunlight to transfer light energy into chemical
energy.
Network of veins (xylem and phloem) → supports leaf and transports water,
mineral ions, as well as sucrose to other plant/leaf parts.
Many stomata → Allows carbon dioxide to diffuse in and oxygen as well as
water vapor to diffuse out, allows movement of gases into and out of air
spaces to maintain steep concentration gradients for faster diffusion and
gas exchange.
Waxy cuticle → Reduces water loss from leaves by transpiration and
protects leaf from pathogens without blocking out sunlight.
Thin/transparent upper epidermis → more light reaches palisade cells.
Palisade cells at top of leaf → near light source → more light absorbed →
faster rate of photosynthesis
Spongy mesophyll → air spaces allow efficient gas exchange and diffusion,
allows gas to move around loosely-packed mesophyll cells.
Palisade cells have many chloroplasts/chlorophyll → absorbs all available
light and maximizes photosynthesis. Palisades are also packed close
together so lots of light energy can be absorbed.
Petioles can turn/change position → can follow sunlight → maximized
sunlight absorption
Plants in drier conditions have less stomata (only stomata on underside of
leaf) → less water loss
Stomatal openings’ size can be controlled → Levels of water loss from the
leaf can be limited and/or controlled as well.
Vascular bundles’ tissues have thick cell walls to support the plant’s stem
as well as leaf.
Moist air → Gases can dissolve → Easier movement into and out of cells.
Close contact between cells and air spaces → ↓ diffusion distance
Gas exchange surface-✔✔ Tissue allowing gases (e.g. oxygen, carbon
dioxide, water vapor) to pass between internal/external environments of
plants/animals. They have a high surface area to volume ratio so large
amounts of gas can diffuse across.
Stomata (plural for stoma)-✔✔ Pores usually found in the leaf’s lower
epidermis. They allow carbon dioxide to diffuse into the leaf to reach
photosynthetic tissue, and allow oxygen as well as water vapor to diffuse
out simultaneously. This movement of gases in opposite directions is called
gas exchange. The upper epidermis has less or no stomata, because the
lower/underside of the leaf has more shade as opposed to the upper
epidermis which is frequently exposed to sunlight: ↓ sunlight exposure → ↓
heat exposure → ↓ water evaporation → ↓ vapor loss by transpiration.
Guard cells-✔✔ Pair of specialized cells surrounding a stoma in the leaf’s
epidermis. Guard cells can change shape to open or close the stoma:
Bright light → Guard cells take in water by osmosis (so stoma usually open
in day time when there’s lots of light) → plump/turgid guard cells → guard
cells expand, bend outwards (become like a curved banana surface) →
they stretch open stoma to enable gas exchange.
Less light (e.g. nighttime) → Guard cells lose water by osmosis → Flaccid
guard cells → Guard cells shrink, bend inwards/become straight, and
collapse against one another thus closing the stomatal pore → Closed
stoma → Gases cannot diffuse in and out the leaf.
Guard cells only close in the dark when no carbon dioxide is needed for
photosynthesis.
Transpiration-✔✔ Process of loss/evaporation of water vapor through the
plant’s stomata. When stomata open to let in carbon dioxide, water on the
surface of spongy and palisade mesophyll cells evaporates as well as
diffuses out the leaf. This evaporation occurs in aerial (in/exposed to air)
plant organs such as leafs, stems, flowers, etc. Meanwhile, roots take up
the same volume of water from the soil too.
, Leaf adaptations for gaseous exchange and photosynthesis-✔✔ Flat →
Large surface area to volume ratio → maximized light absorption
Thin leaf and thin cell walls → ↓ diffusion distance for gases
Has chlorophyll → Absorbs sunlight to transfer light energy into chemical
energy.
Network of veins (xylem and phloem) → supports leaf and transports water,
mineral ions, as well as sucrose to other plant/leaf parts.
Many stomata → Allows carbon dioxide to diffuse in and oxygen as well as
water vapor to diffuse out, allows movement of gases into and out of air
spaces to maintain steep concentration gradients for faster diffusion and
gas exchange.
Waxy cuticle → Reduces water loss from leaves by transpiration and
protects leaf from pathogens without blocking out sunlight.
Thin/transparent upper epidermis → more light reaches palisade cells.
Palisade cells at top of leaf → near light source → more light absorbed →
faster rate of photosynthesis
Spongy mesophyll → air spaces allow efficient gas exchange and diffusion,
allows gas to move around loosely-packed mesophyll cells.
Palisade cells have many chloroplasts/chlorophyll → absorbs all available
light and maximizes photosynthesis. Palisades are also packed close
together so lots of light energy can be absorbed.
Petioles can turn/change position → can follow sunlight → maximized
sunlight absorption
Plants in drier conditions have less stomata (only stomata on underside of
leaf) → less water loss
Stomatal openings’ size can be controlled → Levels of water loss from the
leaf can be limited and/or controlled as well.
Vascular bundles’ tissues have thick cell walls to support the plant’s stem
as well as leaf.
Moist air → Gases can dissolve → Easier movement into and out of cells.
Close contact between cells and air spaces → ↓ diffusion distance
