Photosynthesis
Synthesis of carbohydrates using light energy
What sustains photosynthesis?
The exchange of carbon dioxide (raw material) and oxygen (waste product)
What is the primary organ for photosynthesis?
The leaf
Describe the structure of the leaf
Leaf is covered by a waxy cuticle, which causes a very low permeability. Hence, the epidermis has pores in it called stomata
What is the role of the stomata?
Allows carbon dioxide to be absorbed, but in the process, allows water vapour to escape
What is transpiration?
The loss of water vapour from the leaves and stems of plants
Guard cells
Cells found in pairs on either side of a stomata
Role of guard cells
Control the aperture of the stomata, adjusting it from wide open to fully closed. Hence, it can minimize water loss from transpiration.
How do guard cells perform their function?
When guard cells take in water, the turgor pressure increases, causing the cells to swell. The swelling pushes the guard cells outwards, opening the stomata
In what plants are stomata found?
Almost all groups of land plants for at least part of the plant's life cycle
What factors affect rate of transpiration?
Air movement, temperature, light intensity and humidity
Air movement
As air movement increases, transpiration increases. Air currents carry water molecules away from the leaf surface, increasing the concentration gradient (higher concentration inside) and causing more water to diffuse out
Temperature
As temperature increases, transpiration increases up to a certain point, then decreases. An increase in temperature results in an increase in kinetic energy of molecules. Hence, water molecules evaporate out f the leaf at a faster rate. However, if the temperature gets too high, the stomata close to prevent excess water loss, reducing the rate of transpiration.
Light intensity
As the light intensity increases, transpiration increases until it plateaus. Stomata close in the ark to minimize water loss. As light intensity increases, the stomata will open to enable gas exchange for photosynthesis, increasing the rate of transpiration. Once fully open, any increase in light intensity will not affect the rate of transpiration.
Humidity
As humidity increases, transpiration increases. High humidity means that the air surrounding the leaf surface is saturated with water vapor. Hence, transpiration will decrease as the concentration gradient between the inside and the outside of the leaf gets smaller. At a certain level, equilibrium is reached, and there is no net loss of water vapor from the leaves.
Xylem vessels
Structures that transport water and nutrients from roots to stem and leaves
Arrangement of cells in xylem tissue
Formed from long lines of cells that are connected at each end
Mature xylem vessels
Long, continuous, hollow tubes made of non-living cells
Formation of mature xylem vessels
As the xylem vessels develop in a flowering plant, the cell walls between the connected cells degrade an the cell contents are broken down
Contents of xylem wall + their purpose
Thickened with cellulose and strengthened with a polymer called lignin. These make xylem vessels extremely tough so they can withstand very low internal pressures without collapsing
Intermolecular attractions in the xylem
Cohesion and adhesion
Cohesion
Attraction between water molecules
What causes cohesion?
Within water molecules, the oxygen atom has a partial negative charge and the hydrogen atoms have a partial positive charge. As a result, hydrogen bonds form between the positive and negatively charged regions of adjacent water molecules.
Adhesion
Attraction between molecules of a different type
Adhesion in plants
Water molecules are attracted to the hydrophilic surface of the cell walls on the interior of xylem vessels
How is the water lost from transpiration replaced?
When water evaporates from the surface of the leaf during transpiration, more water is drawn from the nearest xylem vessels to replace the water lost by evaporation
Since water molecules adhere to the cell walls of plant cells in the leaf, water moves through the cell walls of the xylem cells into the cells of the leaf
The loss of water from the xylem vessels causes low pressure within the xylem
Transpiration stream
The continuous upwards flow of water in the xylem vessels
Describe the transpiration stream
When water evaporates from the surface of the leaf during transpiration, more water is drawn from the nearest xylem vessel to replace the water lost by evaporation. This occurs because of the adhesion between water molecules and the cell walls of the leaf, whose force draws water through the cell walls
The loss of water creates low pressure within the xylem
The low pressure creates a pulling force throughout the xylem vessel
Adhesion between water and cell walls in the leaf is strong enough to suck water out of the xylem, further reducing its pressure
The column of water created by cohesion and adhesion transmits this pulling force all he way down the stem and to the ends of the xylem in the roots.
This is called transpiration-pull
This allows water to be moved upwards through the plant, against the force of gravity
Energy used in the transpiration stream
The energy needed for the transpiration stream comes from the thermal energy that causes transpiration. Therefore, the plant oes not use any energy for the transpiration pull, which makes it a passive process.
Plant roots
Responsible for the uptake of both water and mineral ions 2. Root hair cells increase the available surface area for these uptakes
Mineral uptake
The concentration of mineral ions is higher in the root cells than in the soil surrounding them (100x or more), creating a concentration gradient
Mineral ions are actively transported into the root cells via protein pumps in their plasma membrane, maintaining the concentration gradient
What does the active transport of mineral ions depend on?
Each mineral ion has a specific protein pump, and active transport can only occur if the ion makes contact with it.
What brings mineral ions in contact with their pumps?
Diffusion: As minerals are absorbed into the roots, a small concentration gradient is created. Mineral ions diffuse slowly towards the roots
Mass flow: As water carrying the ions rains through the soil, it brings the ions in contact with the pump proteins
Fungi role in mineral uptake
Some ions bind to the surface of soil particles, causing them to move very slowly. Some plants have developed a mutualistic relationship with fungi to overcome this problem.
Fungus grows on the surface of the roots, an even into the cells of the root.
The hyphae of the fungus grows out into the soil and absorbs mineral ions from the surface of soil particles
These ions are then supplied to the roots, allowing them to grow n mineral-deficient soil
In return, most (but not all) plants supply sugars and other nutrients to the fungus
Examples of plants with this mutualistic relationship
Many trees, members of the heather family and in orchids
Water uptake
Occurs by osmosis. However, osmosis is only possible because of the active transport of mineral ions into root cells.
Describe how mineral uptake makes water uptake possible
Mineral ion uptake raises the solute concentration (osmolarity) of the root cells, causing water to move from the soil (lower osmolarity) to the root cells (high osmolarity)
Root cortex
Central region of the root
Two pathways in the root cortex
Apoplastic and symplastic pathways
Apoplastic pathway
Water moves through the spaces in the cellulose cell walls of the root cells
Not osmosis since water is not diffusing through any membranes
Occurs rapidly
Casparian strip + function
A waterproof and that infiltrates the cell wall of the roots cells near the xylem. It forces the water out of the cell walls and into the cytoplasm of the cell, moving from the apoplastic pathway to the symplast pathway
Symplastic pathway
Water moves through the cytoplasm of the root cells
Occurs by osmosis in and out of each cell
Slower tan apoplastic pathway
Volume of water that takes this pathway is lower
Xerophytes
Plants with adaptations to conserve water
What is the purpose of xerophytes adaptations? Give examples of adaptations.
Increase the rate of water uptake from the soil and reduce the rate of water loss by transpiration. Some adaptations include: very few stomata, sunken stomata, hairs surrounding stomata. needle-shaped or small leaves, thickened waxy cuticle
Types of xerophytes
Ephemeral and perennial
Ephemeral xerophytes
Plants with a very short life cycle that is completed in the brief period when water is available after rainfall. They remain dormant as embryos inside a seed until the next rainy period.
Perennial xerophytes
Plants that rely on storage of water in specialized leaves, stems or roots
Examples of xerophytes
Cacti and marram grass
Adaptations of cacti
Leaves reduced in size, consisting of spines that cannot photosynthesize
Epidermis of cactus stems have a thick cuticle and contain stomata, although they are more sparsely distributed
Epidermis contains water storage tissue
Crassulacean acid metabolism (CAM)
Shallow and deep penetrating roots
Function of spines
Reduces leaf surface area, which also reduces water loss
Function of water storage tissue
Allows the stem to expand and contract in volume rapidly, becoming swollen with water after rainfall
Crassulacean acid metabolism (CAM)
A specialized form of photosynthesis, that enables plants to keep their stomata closed during the day.
CO2 is absorbed at night and stored as C4 (malic acid)
CO2 is released from C4 during the day, allowing for photosynthesis even when stomata are closed
Function of CAM
Reduces water loss by evaporation in the heat of the day
Function of shallow and deep penetrating roots
Allow access to all available water
Adaptations of marram grass
Leaves can be rolled up
Stomata are sunken in small pits within the curls of the structure
Has hairs on the inner surface of the leaf, called trichomes
Thick waxy cuticle on the exposed surface
Function of rolled up leaves
Creates a localized environment of water vapour, which reduces the exposure of surfaces to the wind, thus minimizing water loss by evaporation
Function of sunken stomata
Makes them less likely to open and lose water
Function of trichomes
Shield the stomata, slowing or stopping air movement, which reduces water loss
Function of thick cuticle
Reduces evaporation
Halophytes
Plants with adaptations to saline, or salty, conditions
Saline soils + examples
Soils with a high concentration of salts, e.g. coastal salt marshes and land with tides
Adaptations of halophytes
Ability to sequester, or store away, salts within their cell wall or vacuoles
Some can concentrate the salts they absorb in certain leaves, which then fall off the plant
Shed leaves when water is scarce to reduce water loss. The stem becomes green and takes over photosynthesis.
Thick cuticle and multi-layered epidermis
Reduced leaf surface area and sunken stomata
Long, deep roots, which go in search of water
Salt glands excrete salt to prevent build-up
Primary xylem
Visible in cross sections of young stems
Structure of primary xylem
Thin primary wall that is unlignified and freely permeable
Secondary wall is strengthened with lignin, which allows xylem vessel to continue growing in length as the plant grows taller. Lignin structures stretch apart from each other as the xylem grows.
Type of secondary walls
Annular thickening: lignin form rings
Helical thickening: lignin forms spirals
Secondary xylem
Formed when the stem has stopped growing in length
Structure of secondary xylem
Less need for lignin to stretch, so its walls are much more extensively lignified. This makes the secondary xylem stronger but less flexible than the primary xylem.
Drawings of secondary walls