9. HL Plant Biology

9.1 Transport in the xylem

Lesson One: The pathway of water through a plant


Objectives: Understand how water flows through a plant from the root hairs up to the stomata.


  • Transpiration: the loss of water from a plant, mostly through the leaves
  • Transpiration pull: the negative pressure in the xylem, resulting from transpiration, that helps to draw water up the plant stem.
  • Root pressure: the positive pressure in the roots, resulting from the active absorption of mineral ions and the subsequent entry of water by osmosis. This helps to push water up the stem, in the xylem
  • Vasculature: the xylem and the phloem. Vascular plants contain the xylem and phloem vessels.
  • Stomata: Small holes in the underside of a leaf, which allow water vapour to escape and facilitate the exchange of gases between the atmosphere and the spongy mesophyll.
  • Spongy mesophyll: a layer of tissue inside a leaf which contains air spaces to facilitate gas exchange, and also the loss of water vapour through stomata.


The flow of water through a plant

Watet moves continuously up through the body of the plant, and out into the atmosphere. This water loss is called transpiration and it is continuous and unavoidable  –  although plants may reduce the amount by closing their stomata.

water movement

Q.) Why is it unavoidable?

A) Because small holes in the underside of a leaf, called stomata, open to  allow gas exchange. This gaseous exchange is necessary to expel oxygen (a product of photosynthesis), and to absorb carbon dioxide ( a reactant of photosynthesis). It is impossible to let these gases exchange, without letting water vapour out.

Transpiration is driven by physical forces:

  • The loss of water vapour through the leaves, causes a suction force called ‘transpiration pull’. This works like a drinking straw, and is possible because of the forces of cohesion (sticking together) between water molecules
  • The roots use energy to pump in ions into their root hair cells. Water follows by a process called osmosis. This leads to a positive root pressure, forcing water up the xylem
  • Water sticks to the sides of the xylem, and climbs up inside of the walls on it’s own to some extent. These are called forces of adhesion (sticking to surfaces).

Movement of water in the roots, from the root hair to the xylem follows two main pathways:

  • The symplast movement (memory trigger – the simplest way) through the cytoplasm. Even though adjacent cells are surrounded by a cell wall, small strands of cytoplasm called ‘plasmodesmata’ connect these cells.
  • The apoplast movement through the cell walls – these are permeable to water and so water may pass from one cell wall into the adjacent cells cell wall. This pathway is closed near to the xylem by an impermeable strip called the casparian strip. This forces water to ultimate flow through the symplast pathway to enter the xylem vessels, and it is thought this allows the plant more control over the contents of the xylem.


in the figure, the blue line represents the symplast pathway, and the red line; the apoplast pathway.


Figure 2: A potometer measures transpiration.


image credit: everything maths.za

image credit: everything maths.za

  • the bubble moves along the capillary tube according to the transpiration of the plant.
  • the moment of the bubble represents a volume of water taken up by the plant
  • this is almost the same as the water loss (transpiration of the plant). It is not quite the same as the plant uses some of the water for photosynthesis.

Activity: Online lab on potometers


Lesson two

Some plants are especially adapted to reduce transpiration to a minimum. These are called xerophytes, and include the cactus family.

Exercises from textbook on adaptations of xerophytes

Task: Discuss how these adaptations may reduce transpiration?


image credit: bbc.co.uk


Lesson three

Objective: practice recognition of vascular bundles from slide images.


  • cambium: an undifferentiated tissue which is located between and gives rise to new xylem and phloem cells. 
  • vascular tissue: xylem and phloem vessels

Introduction: Read 415-416. Take a photo of the image using your iPad, and label the parts isomer sketchup.

This labelled photo below, of a vascular bundle; gives you an idea.

image credit: M. in HL

image credit: M. in HL


Work through the exercises on concepts 6, 7, 9, 11, and 13.


You should be confident in identify vascular tissues in the leaves, roots, and stems, of dicotyledonous plants.

 9.2 Transport in the phloem

Lesson one: How do substances move in the phloem?

Objectives: to understand how sugars and amino acids are loaded into the phloem, moved along the phloem, and unloaded from the phloem


  • translocation: the movement of sugars and amino acids in the phloem
  • phloem sieve tubes: the vessels through which translocation occurs (called sieve tubes because they contain sieve like structures).
  • companion cells: smaller cells which are located adjacently to phloem sieve tubes, and aid in the unloading and loading of sugars into the phloem.

Translocation refers to the movement of sugars and amino acids in the phloem

aphidAn aphid is an insect that feeds on the sap (phloem content), of a plant. It inserts specialised tubular mouthparts called a stylet.

In experiments, biologists have found that if they remove the mouthparts of an aphid, leaving them still attached to the plant;

the sap continues to flow out of the plant.

Discussion: what does this mean?


Movement in the phloem

In order to understand the movement of sap in the phloem, the process can be divided into:

  • 1) Phloem loading, where the sucrose is transported from a source tissue, through the companion cells and into the phloem sieve tubes.
  • 2) Mass flow, where the sucrose moves along the phloem as a solution
  • 3) Phloem unloading, there the sucrose is transported from out of the phloem sieve tubes, through the companion cells and into a destination tissue.

Watch this video, and then follow the notes on mechanisms for movement underneath

Mechanisms for movement

Mechanisms for phloem loading 1)

The sucrose is loaded into the phloem sieve tubes using active transport ( a process requiring energy). This causes the water potential to be reduced (there is less water compared to the sucrose)This causes water to enter by osmosis from the xylem (water moving from where there is more water potential, to where there is less).

Mechanisms for the movement in the phloem 2)

The sucrose solution flows along the phloem by hydrostatic pressure (from high pressure to low pressure)

Mechanisms for the unloading from the phloem 3)

The sucrose is unloaded into from the phloem sieve tubes by active transport (a process requiring energy). This allows water to return to the xylem (moving in the reverse direction as in of phloem loading 1)

Activity: Make a table of sources and sinks of sucrose for a plant 

Xylem and phloem vessels permeate all parts of the plants, including the roots. The phloem is found closer to the perimeter of the stem or roots. This has implications for the practice of ‘ring barking’, or removing the outer layer of bark from a tree. Discuss these implications.

image credit: ptinterest

image credit: ptinterest

ring barking











Activity: Data-based questions on Q481 Q1a,b,c

Lesson two: Radioisotopes of Carbon as tools in studying translocation

Objectives: Know that the movement of sugars in the phloem can be tracked using radioactivity


  • Isotopes: Atoms of an element that may have an alternative atomic mass.
  • Carbon-fixing: A term used to describe how plants take carbon dioxide from the air and turn it into organic compounds such as carbohydrates and proteins.

Carbon has two isotopes, carbon 12 and carbon 14 (that means they have differing atomic mass). Carbon 12 is the more commonly found isotope in atmospheric carbon dioxide, which means that carbon 14 can be used for experimentation:

  • Plants are exposed to radioactive carbon 14 (in the form of carbon dioxide)
  • Radioactive carbon is fixed (this means it is turned into organic compounds) in photosynthesis
  • The movements of radioactive carbon 14 can be traced using a geiger counter (measures radiation) as it moves around the plants.

Page 419 questions 1a-1d

Lesson three and four

Celery stalk lab. Measuring the speed of transpiration in a celery stalk using methylene blue dye.

download the lab instructions: INK RISE EXPT

Plant growth 9.3


Lesson one – What is a tropism?

Essential questions: What controls the way that plants grow? How do plants grow differently to us?


Understand how plants grow including the role of meristemsUnderstand what is meant by a tropism, classify tropisms according to type eg. a positive phototropism means that plants are growing towards a light source.


  • Tropism: A directional growth.
  • Positive tropisms: growing towards something
  • Negative tropisms: growing away from something
  • Gravitropisms: a tropism caused by gravity
  • Hydrotropism: a tropism caused by water
  • Phototropism: a tropism caused by light


Copy this idea: Growth is defined as the increase in the number of cells in an organism. Growth is achieved by cell division in the form of mitosis (this definition does not apply to unicellular organisms).

Introduction activity:

read this passage and write down one question to bring to a higher level meeting


How is growth in plants different to growth in other organisms?

The growth of plants:

  • is indeterminate – cells continue to divide indefinitely (when does our growth stop?)
  • Growth in plants is confined to regions known as meristems 
  • Primary meristems – apical meristems are at the tips of plants (roots and shoots)
  • Secondary meristems are in the middle of the plants and at the sides (branches).
  • There are two kinds of secondary meristems, lateral – involved in growing wider or growing branches eg. axillary which always grow in where a leaf joins to the stem.
  • Intercalary meristem helps plants stretch in the middle and occur mainly in monocots.
  • Meristems contain undifferentiated cells (they are not specialised)
  • Some cells have the power to generate entire new plants (totipotent).
image credit: apical meristems

image credit: biology junction.com

image credit: biology junction

image credit: biology junction









image credit: pearsons

image credit: pearsons


Monocots and Dicots

Are different classes of flowering plants.




HELP I’m LOST!  need help on plant structure? Excellent review on this site, spend between 1-30 minutes catching up on pre-IB knowledge


Consider this: in the movie Deadpool, he cuts off his own hand and regrows it. This is possible for plants because of the meristems. Humans do not have this layer of tissue and although our outer layer of skin can regrow we cannot replace organs.


Three main ideas regarding tropisms:

  • 1) As tropism is a directional growth. Phototropisms – respond to light. Gravitropisms – respond to gravity.
  • 2) Tropisms can be positive  (towards a stimulus) or negative (away from a stimulus)
  • 3) Tropisms are caused by auxins (plant growth hormones), which are made in the apical meristems.


In context: Roots growing down through a rock show show positive gravitropism, Llianas growing up the trunk of a large tree in a rainforest canopy show positive phototropism. Germinating seeds in a dark room show negative gravitropism when their blind shoots (called coleoptiles), grow upwards.

Lesson two: How do tropisms actually work?

Objectives: Understand the mechanism for trophisms

Key words:

  • Plant growth hormones: Regulate the growth of plants. Include auxins (involved in tropisms), giberillins (control how tall plants grow), and cytokinins (promote cell division).
  • PIN3 Proteins. Protein pumps that move auxin within a plant stem to promote tropisms.
  • Expansin. A protein that causes cell walls to elongate.

 Tropisms: Summary of action (phototropism in a stem)

  • Auxins are produced in the tips of stems and roots (IAA is the most abundant auxin, indole-3-acetic acid)
  • Auxins are plant growth hormones. Giberillins and cytokinins are other groups of plant growth hormones
  • If light is detected, special proteins calles PIN3 proteins transport auxin to the shady side of the stem
  • Auxins in the shady side of the cell stimulate proton pumps, which pump acid (H+ ions out of the cells and into the cell walls).
  • This causes the cellulose fibres to break down, allowing for expansion of the cell
  • A protein called expansin, stimulated by the acid conditions, also helps the cell walls to elongate
  • Cells on the shady side elongate, causing the stem to bend towards the light.
  • This is how a phototropism is produced

Consider this yoga stretch (A). The right side of the woman’s body is elongating (right side from the person’s perspective, not ours)., and the person is therefore bending to the left. in this way, when a plant B coleoptile is bending to the left, it is the right side of the plant which is elongating


image credit: yogaoutlet.com

image credit: yogaoutlet.com


image credit: estrellamountain.org

image credit: estrellamountain.org

Auxin in roots (a gravitropism)

In a root the action of auxin is different, it inhibits elongation instead of promoting elongation. Statoliths are organelles inside root cells that accumulate according to gravity on the lower side. Statoliths lead to the production of PIN3 proteins, and therefore auxins will accumulate.

 Micropropagation: A procedure where plants are grown in vivo using tissue from the shoot apex, nutrient gels and growth hormones (culturing apical meristems). This allows many identical copies of a plant to be made quickly. This could be used to protect rare species (eg. orchids), or to spread disease resistant varieties of plants. 

image credit: magazinr.com

image credit: magazinr.com

Genomics (the study of genomes) has allowed for another way to study plant tropisms; by detecting which genes are being expressed.

9.4 Reproduction in plants

imafe credit ptinterest.com

imafe credit ptinterest.com

 Lesson One: The switch to flowering

Objectives: Understand how plants can begin to flower. Understand that the hormone responsible has the opposite effect in spring flowering plants (long day flowering plants), as it does to fall flowering plants (short day flowering plants).

Memory trigger: Pfr (phytochrome far-red) makes flowers ready in spring. Meaning that Pfr promotes flowering in spring flowering plants, and is waiting for a long enough day that a meaningful concentrations can build up in order to promote flowering.

Consider this: If the bee disappeared off the surface of the globe then man would only have four years of life left. No more bees, no more pollination, no more plants, no more animals, no more man. Sounds absurd?

85% of the worlds 250,000 species of flowering plants depend on pollinators for reproduction. The bumble bee is a major pollinator.

Pre-Ib knowledge on flower structure. If you have forgotten, take this quiz.

Activity: Take this online quiz of flower structure – http://www.proprofs.com/quiz-school/quizshow.php?title=plant-quiz-anatomy–flower&q=1

 The switch to Flowering


Plants begin life without flowers. They will grow for a while, without developing any structures for reproduction. This is called the vegetative phase. Suddenly, they will begin producing flowers. This is called the reproductive phase.

What triggers this change?


 Summary of key changes

A long day spring flowering plant like Daisies is sensitive to phytochrome far-red (Pfr) in that it stimulates flowering.

A short day (long night flower) like Poinsettia (Euphorbia p) is sensitive to phytochrome FR in that phytochrome inhibits flowering.

For reference, insect-pollinated flowers often resemble this one:

image credit RHS

image credit RHS

Lesson two: Seed structure

Objective: Recall seed structure. Understand that oxygen, warmth and water are needed for germination.

Key words:

  • Seed. A dormant stage in the life cycle of a plant, an embryonic plant.
  • Testa: seed coat that protects the dormant seed
  • Micropyle: a small hole in the testa that allows water and oxygen to be absorbed
  • Cotyledons: An organ that stores starch, that can be hydrolysed to make sugars in germination. This is why seedlings do not need to photosynthesise.
  • Embryo: the part that contains the new plant. This is the plumule / epicotyl (young stem), and the radicle (young roots).
Seeds often remain viable after long periods in storage. This is a good example of what biologists call dormancy. A seed represents a dormant stage in the life-cycle of a plant. This means it effectively remains in a state of very low metabolism, waiting to germinate. Germination is the first stage in the life-cycle of a plant, when a dormant seed begins to grow into a young plant.


Seed structure.




Lesson three:

Possible lab

Bean dissection lab

Lesson four: Germination experiment design (HL project juniors only)


Choose a possible cause of crop failure, or an idea of something that you think may affect the germination or subsequent growth of  young plants.

Design an experiment to see whether you obtain evidence for or against your cause.

You will need to decide:

  • which seed type to use
  • how to vary the factor you are investigating
  • how to keep other factors constant
  • how to collect your results, including how to assess whether germination has occurred


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