Plant Biology Notes

Plants (Autotrophs)

• Nutrient and gas requirements.
• Transport systems.

Content Structure

The content will cover:

• Plants: General structure, gas exchange through stomata, xylem and phloem structure, transpiration-tension-cohesion, and translocation.
• Animals: Gas exchange and respiratory systems, digestion, and elimination of waste.
• Comparisons between plants and animals.

Organisation of Cells

Inquiry question: How are cells arranged in a multicellular organism?

• Comparing Unicellular, Colonial, and Multicellular Organisms:
  • Investigating structures at the level of the cell and organelle.
  • Relating cell structure and specialisation to function.
• Tissues, Organs, and Systems:
  • Investigating the structure and function of tissues, organs, and systems.
  • Relating these functions to cell differentiation and specialisation.
• Hierarchical Structural Organisation:
  • Justifying the hierarchical structural organisation of organelles, cells, tissues, organs, systems, and organisms.

Nutrient and Gas Requirements

Inquiry question: What is the difference in nutrient and gas requirements between autotrophs and heterotrophs?

• Structure of Autotrophs:
  • Examining a variety of materials, such as dissected plant materials and microscopic structures.
  • Using imaging technologies to determine plant structure.
• Function of Structures in a Plant:
  • Tracing the development and movement of the products of photosynthesis.
• Gas Exchange Structures:
  • Investigating gas exchange structures in animals and plants through primary and secondary data collection.
  • Microscopic structures: alveoli in mammals and leaf structure in plants.
  • Macroscopic structures: respiratory systems in a range of animals.
• Secondary-Sourced Information:
  • Interpreting information to evaluate processes, claims, and conclusions about plant structure and function.
    • Photosynthesis.
    • Transpiration-cohesion-tension theory.
• Mammalian Digestive System:
  • Tracing the digestion of foods, including physical and chemical digestion.
  • Absorption of nutrients, minerals, and water.
  • Elimination of solid waste.
• Comparison:
  • Comparing the nutrient and gas requirements of autotrophs and heterotrophs.

Transport

Inquiry question: How does the composition of the transport medium change as it moves around an organism?

• Transport Systems:
  • Investigating transport systems in animals and plants by comparing structures and components using physical and digital models.
    • Macroscopic structures in plants and animals.
    • Microscopic samples of blood, the cardiovascular system, and plant vascular systems.
• Gas Exchange:
  • Investigating the exchange of gases between the internal and external environments of plants and animals.
• Comparison of Transport Systems:
  • Comparing the structures and function of transport systems in animals and plants.
    • Vascular systems in plants and animals.
    • Open and closed transport systems in animals.
• Changes in Transport Medium:
  • Comparing the changes in the composition of the transport medium as it moves around an organism.

Overall Plant Structure

• Draw a large plant, noting where water is absorbed, where glucose is made, and where gas exchange occurs, indicating which gases are going in which direction.
• Color code substances for photosynthesis and respiration.

Photosynthesis

• Glucose is made in the leaves.
• Phloem:
  • Glucose travels from the leaves to the rest of the plant (up and down).
• Xylem:
  • Water moves up the plant to the leaves, where it is used in photosynthesis (up only).
  • H_2O
• Stomates (pores):
  • Allow O2 and CO2 exchange.
  • Surrounded by guard cells.

Gas Exchange in Plants

• Gas exchange occurs in plants through photosynthesis.

Plant Transport Systems

• Transport systems in plants are continuous through roots, stems, and leaves.

• Vascular tissues (xylem and phloem) are tubular pathways through which fluids travel.

• Plant cells:

  • Root Hair Cell
  • Stoma Guard Cell

• Substances:

  • Water + Minerals
  • Glucose

• Systems:

  • shoot system
  • root system

• Exchange:

  • Water from Soil to Root Hair Cell
  • Water + Minerals from Xylem
  • Glucose to Phloem

Stomata

• Leaves possess pores in the epidermis called stomata, through which oxygen and carbon dioxide move into and out of the plant.
• Stomata are mostly found on the underside of leaves.
• Stomata are bordered by two bean-shaped guard cells, which contain chloroplasts.
• Stomata open and close to balance gas exchange with water loss.
  • When guard cells fill with water, they become turgid and bend outwards, opening the pore for gas exchange.
  • When guard cells lose water, they straighten and close, preventing gas exchange and water loss.
• Stomata tend to open during the day and close at night.
• Different plants have stomata arranged differently depending on their climate.
  • Eucalyptus leaves hang vertically to minimise exposure to the midday sun and have stomata on both sides of the leaf.

Leaf Structure

• Cuticle
• Upper Epidermis
• Palisade Mesophyll
• Spongy Mesophyll
• Lower Epidermis
• Guard Cells
• Stoma: CO2 enters, O2 exits
• Sunlight

Gas Exchange Structures

• Most gas exchange occurs through the stomata.
• Some plants have lenticels for gas exchange along woody parts, like branches or the trunk.
• Diffusion through lenticels is relatively slow.

Plant Transport Systems and Structures

• Investigate transport systems in animals and plants by comparing structures and components using physical and digital models, including but not limited to:
  • Macroscopic structures in plants and animals
  • Microscopic samples of blood, the cardiovascular system and plant vascular systems
• Investigate the exchange of gases between the internal and external environments of plants and animals
• Compare the structures and function of transport systems in animals and plants, including but not limited to:
  • Vascular systems in plants and animals
  • Open and closed transport systems in animals
• Compare the changes in the composition of the transport medium as it moves around an organism

Vascular Systems

• Xylem: Transpiration occurs.
• Phloem: Translocation occurs.

From inside of vascular bundle to outside:

1. Xylem
2. Cambium
3. Phloem
4. Phloem fibres (sclerenchyma)

Vascular Bundle

• STEM
  • XYLEM
  • PHLOEM
• ROOT
  • PHLOEM
  • XYLEM
  • -SCLERENCHYMA
• LEAF
  • XYLEM
  • PHLOEM

Microscope Image of Vascular Tissue

Longitudinal Diagrams

• Xylem
  • Meta-xylem
  • Proto-xylem
  • Spiral
  • Annular
  • Pit
  • Lignin
  • Cell Wall
• Phloem
  • Sieve Plate

Celery Practical

1. Soak celery in food dye
2. Cut a THIN cross section and observe the vascular bundles.
3. View a longitudinal section of the xylem.
4. View prepared stems- TS and LS.

Sclerenchyma

• “Fibres” is a nice simple way of remembering those sclerenchyma cells.

Plant Tissue

• Epidermis
• Collenchyma
• Parenchyma
• Cortex
• Pith (parenchyma)
• Phloem
• Xylem

More Plant Tissues

• Cortex
• Phloem fibers
• Phloem
• Metaxylem
• Protoxylem
• Pith

Spongy and Palisade Mesophyll

• Upper Epidermis
• Palisade Parenchyma
• Spongy Parenchyma
• Lower Epidermis

Transport Systems

• Water + Minerals from Xylem
• Glucose to Phloem
• Systems:
  • shoot system
  • root system

Transpiration & Translocation

• Plants are tall (mostly).
• Scientists knew that water moved in the xylem and sucrose moved in the phloem, but it took nearly 200 years to determine HOW this movement occurred.
• Considerable force was required to move water up AGAINST gravity.

Transpiration- TTC theory

• Transpiration: the pull from above. Energy from sunlight causes water molecules to become a gas and move into the air (through stomata).
• Tension: the loss of water through transpiration creates tension (a negative pressure or pull) - like sucking on a straw.
• Cohesion: the natural attraction of water molecules to each other. Water molecules that are higher up attract molecules below them to fill the space left by transpiration.
• Adhesion: the attraction of water molecules to different types of molecules, such as cellulose and lignin in the xylem wall.
• Capillarity: the fact that this all happens in a narrow tube also helps to drag the water up.
• Extra points:
  • Water moves into the roots through osmosis.
  • Mineral ions also enter, but by diffusion.
  • The water moving across into the root cell exerts a pressure on the water in front of it - this is called root pressure.

Movement of Water up Stems

• The main force that pulls water up the stem is transpiration.
• Transpiration is the evaporation of water from the leaves.
• When stomata are open, water vapour molecules diffuse out of the air spaces.
• The water is replaced by water evaporating from the cell walls of the mesophyll cells.
• Water from the mesophyll cells is then replaced by water in the xylem by the apoplastic or symplastic pathways.
• In symplastic pathways it occurs:
  • Mesophyll cells lose water to air spaces
  • Cells now have lower water potential so water enters by osmosis
  • The neighboring cells lose water lowering their water potential
  • The neighbouring cells then take water from the cells next to them
  • This establishes a water potential gradient to pull the water from the xylem
• The two main factors that cause water movement up the xylem are cohesion tension and root pressure
• Cohesion Tension operates:
  • Water evaporates from leaves due to transpiration
  • Water molecules form hydrogen bonds between them this is cohesion
  • Water form continuous pathway across the mesophyll cells and down the xylem
  • As water evaporates from the mesophyll cells in the cells in leaves into the air spaces beneath the stomata molecules of water are brought up
  • Water is pulled up the xylem due to transpiration pull
  • Transpiration pull put the xylem under tension

Evidence of the Cohesion Tension Theory

• During the day transpiration is at its greatest so more tension in the xylem
• This causes the trunk to shrink
• At night transpiration is at its lowest so little tension in the xylem
• Diameter then increases at night
• If Xylem vessel is broken and air enters it
• The tree can no longer draw up water as no continuous column
• If broken air is drawn in
• Transpiration pull is a passive process so doesn't require metabolic energy
• As the xylem is dead it can form series of continuous unbroken tubes from root to leaves
• These tubes are essential to the cohesion tension theory
• Energy is needed for transpiration and it comes from the heat of the sun

Graphing Activity

Experiment- how different factors affect transpiration rate.

Data analysis:

• How do the following factors affect transpiration rate? Wind, Heat, humidity, Light?

Translocation

• Translocation: the movement of sugars in the phloem can happen in any direction, however, it always moves: sugar source → sugar sink.
• Glucose, sucrose, starch: (what’s the difference?)
• Phloem movement is an ACTIVE process, meaning cells must be ALIVE.
• It’s known as the source-path-sink system.

Translocation Source to Sink Model

• Sucrose is loaded into the phloem vessels from nearby cells (that have been photosynthesising) AGAINST the concentration gradient (by active transport using ATP). When the sucrose moves in, water follows it by osmosis.
• Sucrose is removed from the phloem into the plant cells that require it by active transport also
• At the source, there is a large amount of solute and water which creates high pressure. At the sink there is a low amount of solute and water, which creates a low pressure.
• This pressure flow drives the movement of sugars from where they are made to where they are needed

Parts of Phloem

• sieve tube
• sieve tube element cell
• companion cell
• phloem
• pores
• sieve plate
• plasmodesmata
• xylem

Translocation Source to Sink Model

• Companion cells use active transport to move sugars from the source cell (leaf) into the phloem.
• Water from the xylem moves toward the high solute concentration in that part of the phloem, creating high pressure.
• The contents of the phloem flow toward low pressure areas where sugar is required (sink).

Symplastic and apoplastic loading

• In the leaves, sucrose is made in the process of photosynthesis.
• Amino acids may be synthesised in the leaf using nitrogen brought up in the xylem.
• Both these products plus other mineral nutrients from the xylem are 'loaded' into the phloem in the leaves.
• There are two theories as to how this may occur (symplastic loading and apoplastic loading), although it is likely that both methods are used by plants
  • Symplastic loading: sugars and other nutrients move in the cytoplasm from the mesophyll cells to the sieve elements through plasmodesmata. This theory requires there to be many plasmodesmata between leaf cells. In some plants many have been found, but not in all.
  • Apoplastic loading: sugars and other nutrients move along a pathway through the cell walls until they reach the sieve element. They then cross the cell membrane to enter the phloem tube. Materials would pass into the sieve element by active transport.
• As the sugars enter the phloem, the concentration of phloem sap increases and the osmotic pressure at this 'source' end of the phloem tube is high.

Phloem unloading at the sink

• Once loaded at the source, the materials flow towards a sink.
• A sink is a region of the plant where sugars and other nutrients are actively being removed from the phloem.
• It might be in the roots, stem, flowers or storage areas of the plant.
• As the sugars are actively taken out from the phloem, water flows out with them.

Phloem loading Model

• Apoplastic pathway (through cell wall)
• Symplastic pathway (through cytoplasm)
• plasma membrane
• plasmodesma
• cell wall (apoplast)

Phloem movement

• Phloem sap moves from source to sink at rates that are too fast to occur by cytoplasmic streaming.
• One model for phloem movement is the pressure-flow (bulk flow) hypothesis.
• Phloem sap moves by bulk flow, which creates a pressure (hence the term "pressure-flow").
• The key elements in this model
  • Loading sugar into the phloem increases the solute concentration inside the sieve-tube cells. This causes the sieve-tubes to take up water by osmosis.
  • The water uptake creates a hydrostatic pressure that forces the sap to move along the tube, just as pressure pushes water through a hose.
  • The pressure gradient in the sieve tube is reinforced by the active unloading of sugar and consequent loss of water by osmosis,at the sink (e.g. root cell).
  • Xylem recycles the water from sink to source.

Measuring Phloem Flow

• Experiments investigating flow of phloem often use aphids.
• Aphids feed on phloem sap (left) and act as natural phloem probes.
• When the mouthparts (stylet) of an aphid penetrate a sieve-tube cell, the pressure in the sieve-tube force-feeds the aphid.
• While the aphid feeds, it can be severed from its stylet, which remains in place in the phloem.
• The stylet serves as a tiny tap that exudes sap.
• Using different aphids, the rate of flow of this sap can be measured at different locations on the plant.

Photosynthesis and Radioisotopes

• In module 1 we learnt about photosynthesis. But how did scientists work out how the atoms were rearranged during the process of photosynthesis?

Balanced Equation

• carbon dioxide + water -> glucose + oxygen
• 6CO2 + 6H2O -> C6H{12}O6 + 6O2

Tracing Photosynthesis

• Radioisotopes are radioactive versions of atoms.
• Their radioactivity means these atoms can be followed and tracked.
• Hence radioisotopes can be used to determine where reactant atoms are placed in products during photosynthesis.

Determining the source of Oxygen in Photosynthesis

• Once scientists had discovered that plants could absorb carbon dioxide (CO2) and release oxygen (O2), it was wrongly assumed that plants must be able to split a carbon dioxide molecule to release oxygen.
• However, in the 1930s, Cornelius van Niel proposed that water was actually the source of the oxygen released by plants - based on his work with purple sulfur bacteria.
• Nearly 20 years later, van Neil's hypothesis was proven correct by experiments to grow algal plant cells using water labelled with an isotope of oxygen, called oxygen-18 (^{18}O).
• The carbon dioxide used was not labelled.
• Van Neil found that the oxygen produced by the plant cells in photosynthesis was labelled with ^{18}O. Further experiments using carbon dioxide labelled with ^{18}O, resulted in oxygen that did not contain any ^{18}O.
• These results showed that water, not carbon dioxide, was the source of the oxygen.

Radioactive carbon tracing

• The most common form of carbon is carbon-12 (^{12}C). This atom has 6 protons and 6 neutrons.
• Carbon-14 (C^{14}) is a radioactive isotope of carbon. Carbon-14 has 6 protons and 8 neutrons. It is unstable, and releases detectable radiation as it undergoes radioactive decay.
• C^{14} behaves chemically in the same way as ^{12}C and so plants can use it in photosynthesis to form compounds that involve carbon, including carbon dioxide (CO2) and glucose (C6H{12}O2).
• Since C^{14} has a relatively long half-life, it can be used to investigate the pathway of carbon dioxide and the intermediate products formed during photosynthesis in plants.
• For example, when the carbon dioxide supplied to some algal plant cells was labelled with C^{14}, the radiation emitted by the C^{14} was able to be detected and monitored using autoradiography.

Scientists who have contributed to our understanding of plant processes

• Photosynthesis contributions:
  • Jean Senebier
  • Nicholas-Theodore de Saussure
• Transpiration contributions:
  • John Joly
  • Henry Horatio Dixon

Jan Baptista van Helmont

• Van Helmont thought that the soil formed all of the plant matter.
• He weighed 90 kg of dried soil and planted a 2.25 kg willow seedling.
• Van Helmont then watered the plant for 5 years.
• He separated the seedling from the soil, dried the soil and weighed both.
• The soil weighed 89.9 kg - a small decrease. The plant weighed 76.1 kg - a large increase.
• Van Helmont concluded that all of the plant matter came from the water.
• Van Helmont's conclusion was incorrect he was testing whether the soil provided the matter for the plant, but concluded that it was the water - nothing else was considered. His conclusion should have been that very little of the plant matter came from the soil.
• The experiment itself was flawed:
  • The mass of lost leaves was not included in his measurements.
  • There was no control to test whether the plant grew just as much if only water was used (as it should have if Van Helmont's conclusion was correct).
• There was no repetition - only one plant was tested.
• There were many inaccuracies - his description of the experiment was incomplete, he did not measure how much water he added, and weighing accurately was impossible as the soil could not be fully separated from the plant.

Joseph Priestley

• Priestley noticed that in an enclosed space a candle would go out and a mouse would die.
• He put a mint plant into the enclosed space with an extinguished candle. He put another mint plant into the space with a mouse.
• Priestley found that when the plant was present in the enclosed space, a candle lit with focused sunlight remained alight and the mouse survived.
• He concluded that the plant restores to the air whatever the candle and the mouse had removed.
• He thought that the candle and the mouse that breathed the air somehow 'injured' the air.
• He also thought that the plant growth itself was responsible for replacing the factors that the mouse and candle had removed.
• Priestley's experiments were well designed and his conclusions matched the knowledge of the time.
• These experiments contributed greatly to the eventual understanding of photosynthesis.

Jan Ingenhousz

• Ingenhousz set up a similar experiment to Priestley, with a candle and plant in the bell jar.
• He covered the bell jar so that no light could enter and left it for a number of days.
• Ingenhousz also submerged a small aquatic plant in water that was exposed to light.
• He then subjected the experimental set-up to darkness.
• The candle did not light. Ingenhousz showed that light is necessary to restore air after it has been 'fouled' by candles or animals. He demonstrated that light is necessary for plants to make oxygen.
• When the aquatic plant was exposed to light, bubbles were formed around the leaves and the green parts of the stem. When it was in darkness, the bubbles stopped.
• Ingenhausz concluded that light was necessary for the plant to produce the gases that 'purify' the air.
• Ingenhousz took Priestley's work even further and is credited with discovering photosynthesis.
• His experiments tested what he set out to test.
• He demonstrated that light and the green sections of the plant are required for the plant to produce the gas oxygen.