This posts addresses one of the commonest misconceptions you encounter as a biology teacher and it concerns a mistaken belief about the function of the roots of a plant.
The roots anchor the plant in the ground and so prevent it toppling over due to wind. But their main function is to do with the absorption of materials from the soil into the cells of the plant. The question is what exactly is taken up in the roots?
Well most people remember that water is absorbed in the roots by osmosis. The best candidates will remember the microscopic root hair cells in the root that massively increase the surface area for the uptake of water. This absorbed water is transported into the xylem tissue in the centre of the root and then moved up the plant to the leaves by transpiration pull.
Roots also absorb mineral ions from the soil by active transport. Active transport is the process where energy from respiration in the cell is used to pump material across the cell membrane against the concentration gradient. Mineral ions absorbed included nitrate ions (needed to make amino acids and proteins), magnesium ions (needed to make chlorophyll) and phosphate ions (needed to make DNA)
So where is the common misconception? This all seems sensible and fairly straightforward. Roots absorb water by osmosis and mineral ions by active transport.
Whenever root function is tested in exams, many candidates get in a pickle as they confuse mineral ions (nitrate, phosphate, magnesium, potassium) with food molecules. Plants do NOT absorb food molecules through their roots. There are very few food molecules such as glucose, amino acids, and lipids in soil. If there were, more animals would eat soil as a source of nutrition…… Plants do not need to absorb food molecules of course: the big idea you learn is that plants can make their own food molecules in the leaves in the process of photosynthesis.
So in your exam, if you ever find yourself writing anything that suggests that plants take in food through their roots, stop, take a deep breath, cross it all out and count yourself lucky you have prevented yourself from one horror answer at least!
The previous posts have explained the processes involved in sexual reproduction in plants. But many species of plant can also reproduce asexually and this post is going to explain how and why this might occur… Now this is not a topic that is so exciting that it keeps many GCSE students awake at night but there is some good biology in here so pay attention!
Asexual reproduction is the term used for any reproductive strategy that produces genetically identical offspring. The term for a group of genetically identical organisms is a clone and so asexual reproduction is also called cloning. In animals and plants, asexual reproduction only involves one type of cell division, mitosis.
Sexual reproduction on the other hand always introduces genetic variation into the offspring. In the majority of cases, it involves the formation of two haploid gametes (produced in a specialised type of cell division called meiosis) which then fuse together in fertilisation to form a zygote.
This post is not the correct place to discuss the advantages and disadvantages of the two types of reproduction. But sexual reproduction comes at a cost for an organism: in plants this cost is the energy spent making attractive petals and scent to attract insect pollinators, the cost of wasting millions of pollen grains just to ensure some are transferred, the cost of making sweet tasty fruits for animals to eat. I am writing this the day after Valentine’s Day in the UK which illustrates the courtship costs for animals quite well…..
Asexual Reproduction in Plants:
Plants have evolved a variety of asexual strategies shown in the picture above. We only really need to consider one for your iGCSE exam and that is runners. Some plants, the classic example is the strawberry grow long horizontal stems outwards from the parent plant. When this “runner” touches the ground, root development is switched on and a new plant starts to grow upwards. When the runner dies back, you are left with two genetically identical plants, hence Asexual Reproduction.
(This diagram is a little misleading…. The runner is not the name of the offspring plant, it is the long horizontal structure growing outwards just above the soil from the parent plant)
If you are really interested in learning more about asexual strategies in plants, well you should probably get out more… But you could study how tubers (such as in potato) and bulbs (such as the onion) allow plants both to over-winter and also produce clones.
Artificial Methods of Asexual Reproduction in Plants
This is quite weird if you think about it….. A human can cause asexual reproduction in many species of plant by “taking a cutting“. As the name suggests this involves cutting off a small part of the plant (including a leaf and part of the stalk) and then sticking it into soil to grow a new plant. The only type of cell division in this process is mitosis and so the plant produced from the cutting will be a clone of its parent (genetically identical).
This is an example of artificial asexual reproduction in plants. It is a useful strategy for gardeners as it allows you to produce lots of new plants for your garden without shelling out hard-earned cash at the garden centre….
Cuttings work much better if in between taking the cutting and planting it in a small pot of compost, the cut end of the stalk is dipped in a mixture of chemicals calling a “rooting powder”
Rooting powder contains a mixture of plant growth substances (sometimes incorrectly called hormones) that can switch on the genetic programme of root production.
I hope you find this post useful. It probably holds the record for the dullest site anywhere on the World Wide Web…. Typing this has made me feel sleepy, so I am going to lie down…..
Please add comments/questions or tweet me if anything is unclear.
In the previous post, I looked at the flower structure of both insect and wind pollinated flowers and explained the process of pollination. Now we need to ask “What happens next….?”
When a flower has been pollinated, there will be pollen grains that have landed on the stigma. These might be from a different species of plant in which case, nothing will happen. If they are from a different individual of the same species of plant, then triggered by sugary chemicals on the stigma the pollen grain starts to grow a tube called a pollen tube (imaginative people these plant biologists…..) which grows down through the style.
Pollen tube growth through the style is a complex process and the exact mechanism by which the pollen tube “knows” in which direction to grow is not completely understood. But it does know and grows down through the style and enters the ovule through a tiny opening in the ovule called the micropyle.
Now this is where it gets complicated …… but luckily for your exam, you don’t need to learn about the weird way plants undergo fertilisation. But to give you a taste, the male gamete which is a nucleus in the pollen grain divides on the way down the pollen tube to form two sperm nuclei (see diagram above). Each of these nuclei will fertilise a different nucleus in the ovule, hence the name double fertilisation.
But let’s keep it simple. There is a haploid female gamete called an egg cell inside the ovule and one of the haploid sperm nuclei from the pollen grain will fuse with it in the process of fertilisation. This produces a diploid cell called the zygote that will later develop into the embryo plant.
Quick reminder: Haploid is a term that refers to a cell that only has one member of each pair of chromosomes. Gametes are haploid cells and when two gametes fuse they produce a cell with pairs of chromosomes and this cell is described as Diploid.
The egg cell inside the ovule is now fertilised. It has a full set of chromosomes and is now called a zygote. So what happens to the structures in the flower…? After fertilisation the petals, sepals, stamens, stigma and style all dry out and wither. The ovary develops into a structure called the fruit and inside the fruit, each ovule develops into a seed.
Seeds are tough structures that have evolved to allow the embryo plant to undergo a period of dormancy before the seed germinates. The function of fruit is seed dispersal. It is vital for the parent plant that its offspring do not start to grow right next to themselves as they will be in direct competition with the parent for water and minerals from the soil and for sunlight. For animals it is easy for the parent to get rid of their offspring – they simply kick them out of the nest or send them to boarding school to get them out of the house…. Plants need to rely on more ingenious strategies….
In some plants the fruit has evolved to disperse the seed using the wind. Sycamore seeds have a propellor blade to slow down their fall from the tree. Dandelions give each seed a tiny parachute and can be carried for many miles in the wind.
But animals are more commonly used as couriers to get the seeds away from the parent plant. The fruit may be sweet and attractive to eat; the fruit may have hooks or barbs to get stuck to the animals body. Many seeds are dispersed by animals in a wide variety of ways…
It is really important not to get confused between the role of animals as pollinators of flowers and their separate role in seed dispersal. Keep these two processes (pollination and seed dispersal) clearly separated in your notes and in your mind. In the stress of the exam, candidates often get muddled and so write nonsense….. This is something to avoid if possible!
PMG tip: organise your notes on plant reproduction into the following subheadings to keep things separate.
- Flower Structure
- Seed Dispersal
Most of the work you do on transport in plants concerns the movement of water and minerals from the roots to the leaves of the plants in xylem vessels. (see previous post on xylem transport) You should understand what transpiration is, and how the properties of water allow a transpiration pull provide the energy to move large volumes of water up the xylem in the plant.
But what about the second plant transport tissue phloem? How does it differ from xylem in both structure and function?
Well structurally the tissues are very different. Xylem vessels are large, dead, empty thick-walled cells with cell walls strengthened with lignin. The transport cells in phloem are called sieve-tubes. Phloem sieve tubes are living cells with thin cell walls.
In xylem vessels the end walls break down completely but in phloem sieve tubes, the end walls are filled with many holes forming a structure called a sieve plate. Each phloem sieve tube has a smaller cell called a companion cell alongside and both these two cells must be alive for phloem transport to occur.
What is transported in phloem sieve tubes?
Plants transport the products of photosynthesis (food molecules) up and down the stem in phloem. The main carbohydrate transported is sucrose but phloem cells also contain lots of amino acids and a few other sugars.
The mechanism by which these sugars are moved around the plant is less well understood than for water movement in xylem. Translocation is an active process and requires energy from respiration in the cells. It is possible that a bulk flow exists as shown in the diagram below, but this mechanism cannot be the whole story….. Can you think why? Post a comment if you want to explain some of the difficulties with this theory…..
You should understand how bacteria can be genetically modified to produce useful proteins (e.g.human insulin, human growth hormone, vaccine antigens etc.) – see previous post if you are unsure. In this post, I will try to explain how transgenic plants can be made and indeed what kind of genes might be added to plants to benefit humans. Genetically modifying a plant will be a more complex process since plants are multicellular and so many millions of cells need to be genetically altered.
Luckily a vector exists that can transfer genes into many varieties of plant. Agrobacterium tumefaciens is a species of bacterium that contains a plasmid that can be transferred into plant cells. This plasmid (called the Ti plasmid) can be cut open with a restriction enzyme and a new gene inserted with DNA ligase.
The Agrobacterium that have been genetically modified will then infect the plant tissue and then when this plant tissue is cultivated using the micropropagation techniques you learned about with cauliflower, a whole GM plant can be produced.
Genes can also be inserted into plants by a gene gun. A gene gun literally fires “bullets” made of tiny particles of gold that have been coated with the required DNA and while Agrobacterium does not infect all species of plant, the gene gun can work to get foreign DNA into any plant species.
What are the potential advantages of GM crops?
Resistance to Herbicide: some crop plants have been altered so they contain a gene that makes them resistant to a particular herbicide. This means a farmer can spray the herbicide on his crop without risk of harming the crop plant.
Resistance to Frost: some GM plants have a gene from a species of Arctic fish that codes for an “antifreeze” chemical in the fish blood. Plants that contain this gene will be frost-resistant and so produce can be transported in refrigerated containers without damaging the plant cells.
Golden Rice: rice plants have been altered so they contain genes that make the molecule beta-carotene. This is the orange pigment found in carrots and is a precursor for making vitamin A. So in populations who rely on rice as a staple component of their diet, the rice will be more nutritious and so prevent the night-blindness associated with vitamin A deficiency.
Antibody production: plants may also be genetically modified to produce antibodies for treatment of human disease.
The specification has a section called “Variety of Living Organisms”. In this section, candidates are asked to learn about the features of the Five Kingdoms of living things and certain examples are mentioned. This model of grouping organisms states that all living things can be allocated to one of these five groups:
- Bacteria (Monera)
- Protoctists (Protista)
It is disappointing that Viruses are added as a sixth group in this section of the syllabus. Viruses are not classified as a Kingdom of living things as they are not made of cells and have no metabolism.
Bacteria are small, single celled organisms that are made of a fundamentally different kind of cell to all the other Kingdoms. Bacterial cells are described as being prokaryotic: they are smaller than other cells, have no nucleus and no membrane-bound organelles (such as mitochondria or chloroplasts). Bacteria cells have a cell wall containing a cell membrane but their cell wall does not contain any cellulose. Instead the bacterial cell wall is made mostly of a molecule called proteoglycan a molecule is only found in bacterial cells.
Bacteria cells contain DNA (all living things use this molecule as their genetic material) but the key idea is that bacterial DNA is not contained inside a nucleus. Bacterial DNA is in the form of a single circular ring that just floats around in the cytoplasm of the cell. This circular ring of DNA is sometimes called the bacterial chromosome (but I dislike this term as the DNA molecule in bacteria is not wrapped around a scaffold of protein as in eukaryote cells). Some bacteria contain small additional rings of DNA that are called plasmids. These plasmids can be transferred from one bacterial cell to another, and can also be used as a vector in genetic engineering.
Examples of bacteria mentioned in the specification are Lactobacillus bulgaris a rod–shaped bacterium used in the production of yoghurt and Pneumococcus, a spherical bacterium that is the pathogen that causes the infectious disease pneumonia.
Remember that some bacteria are autotrophic and can carry out photosynthesis but most feed by absorbing material through their cell walls.
Animals by definition are multicellular organisms. Animal cells do not have a cell wall and do not contain chloroplasts and so cannot photosynthesise. Animals are often able to move from place to place and have a nervous system. Animal cells can store carbohydrate in liver and muscle cells in the form of a storage polysaccharide called glycogen.
The examples of animals mentioned in the specification are humans, housefly and mosquito.
The plant kingdom also contains organisms that are multicellular. In contrast to animals, plant cells do photosynthesise and do contain chloroplasts. Plant cells have a cell wall made of the polysaccharide cellulose. Carbohydrates are stored in plant cells in the form of starch and are transported in the phloem as a sugar called sucrose.
The examples of plants mentioned in the specification are maize, peas and clover. Maize is a wind-pollinated flowering plant and peas and clover are interesting because they are leguminous plants. If you remember your work on nitrogen-cycle from E summer, you will know that leguminous plants contain root nodules that contain nitrogen-fixing bacteria.
Fungi are a group of organisms that include moulds, mushrooms, toadstools and yeasts. They are made of cells with a cell wall made of chitin and a nucleus. Fungi do not photosynthesise and do not contain chloroplasts. They feed by secreting digestive enzymes onto the food material they are living on and then absorbing the products of digestion: a process called saprotrophic nutrition. Fungi store carbohydrate in the form of glycogen.
Multicellular fungi such as Mucor are often organised into a mycelium, a mesh of thread-like structures called hyphae. Each hypha is a structure containing many nuclei. Some fungi such as the yeasts used in the brewing and baking industries are single-celled.
This is the least interesting of the 5 Kingdoms (which is saying something…..) Protoctists are all single celled organisms but unlike bacteria they are made of eukaryotic cells: cells with a nucleus and organelles like mitochondria and chloroplasts. Some protoctists like Amoeba share many features with animal cells while others like Chlorella are more plant-like and contain chloroplasts to photosynthesise. Some protoctists are pathogenic for example Plasmodium, the single celled organism that causes the disease Malaria.
An Amoeba cell on the left and some Chlorella cells on the right.
The topic of plant transport can appear quite complicated but you will see from your past paper booklets that the questions examiners tend to set on it are much more straightforward.
The key piece of understanding is to realise that there are two transport systems in plants, learn their names and what they transport.
- Xylem vessels move water and mineral ions from the roots to the leaves.
- Phloem sieve tubes move sugars, notably sucrose, and amino acids around the plant. Both of these molecules are made in photosynthesis in the leaves and so can be transported from the leaves to the areas in the plant where they are needed.
Water is needed for photosynthesis of course in the leaves (remember that rain water cannot enter leaves directly because of the waxy cuticle on the surface of the leaf). All the water that is used in photosynthesis is absorbed in the roots from the soil and moved up the plant in the xylem vessels. Minerals such as nitrate, phosphate and magnesium ions are also required in the leaves for making amino acids, DNA and chlorophyll respectively. These minerals are moved up the plant along with the water in the xylem.
How does water enter the roots from the soil?
Water molecules can only enter root hair cells (and indeed can only cross any cell membrane) by one mechanism and that is OSMOSIS. If you understand the mechanism of osmosis that is great but don’t worry too much about it at this stage. You need to know that osmosis is a net movement of water from a dilute solution to a more concentrated solution across a partially permeable membrane.
How do mineral ions enter the roots from the soil?
Minerals are pumped into the root hair cells from the soil using ACTIVE TRANSPORT. This a process that uses energy from respiration in the cell to move ions against their concentration gradient (so from a lower concentration in the soil to a higher concentration inside the cell cytoplasm.)
What do we know about xylem vessels?
The cells that water and minerals are transported in are called xylem vessels. They have some interesting specialisations for this function. They are dead cells that are empty with no cytoplasm or nucleus. The end walls of these cells break down to provide a continuous unbroken column of water all the way up the plant. The cell walls of xylem vessels are thick and strengthened and waterproofed with a chemical called lignin.
What causes the water to move up the xylem?
Clearly it will take energy from somewhere to move water against gravity all the way up a plant from the roots to the leaves. The key question here is what provides the energy for this movement? There is no pumping of water up the plant and indeed the plant spends no energy at all on water movement. The answer is that it is the heat energy from the sun that evaporates water in the leaves that provides the energy for water movement. When you combine this with the fact that water molecules are “sticky” – they are attracted to their neighbours by a type of weak bond called a hydrogen bond – you can see that the water evaporating into the air spaces in the leaf can pull water molecules up the continuous column of water found in the xylem. The proper adjective for this stickiness is cohesive and you should know the name for the evaporation of water in the leaves (Transpiration)