All organisms have the potential to reproduce. Reproduction is one of the 7 characteristics of life (8 if you study EdExcel IGCSE Biology…..) and of course it means the ability to produce new individuals. But over the 4 billion or so years life has been around on the planet, evolution has developed a myriad ways of producing new individuals. So as biologists are simple folk (well all the ones I work with are….), it makes sense to group different reproductive strategies together to make it easier to understand.
The major distinction between different ways of reproducing is to divide them into asexual and sexual reproduction. This works fairly well, although it seems to be a subject many GCSE students don’t understand too well. So here goes….
The first big idea to dispel is that the number of parents involved determines whether reproduction is sexual or asexual. Too often I am told that asexual involves one parent, sexual two. But although this works in most cases, sexual reproduction can happen with only one parent, for example in flowering plants that self-pollinate. So we need a different way of deciding whether reproduction is sexual or asexual.
And in fact a clear distinction does exist and it is to do with genetics. If the offspring produced are genetically identical to the parent (i.e. a clone) then it is an asexual form of reproduction. If the offspring produced are genetically different to the parents, then it is sexual.
Often sexual reproduction involves the process of fertilisation. This allows two parents to each contribute half their genetic material to their offspring thus generating individuals with new and unique genetic make ups. These specialised cells that contain half the genetic material are called gametes and as you all know, they are made by a special type of cell division called meiosis. Meiosis is vital for sexual reproduction as it produces cells that are haploid (one member of each pair of chromosomes) and all genetically unique.
So this diagram shows two humans each producing gametes by meiosis. The parents on the left will have 23 pairs of chromosomes and so each haploid gamete will have 23 individual chromosomes. Fertilisation restores the diploid number. Mitosis is then used to turn this single cell, the zygote into a multicellular embryo and then indeed into a new individual. (You will remember mitosis is a type of cell division that always produces genetically identical daughter cells)
The examples of asexual reproduction on the left all involve only this second type of cell division, mitosis. There are no gametes, no fertilisation and hence no genetic variation. The simplest type of asexual reproduction is shown as (A) and this is called binary fission. A single-celled organism can divide in two to produce two genetically identical daughter cells. Hydra (B) are a simple type of animal and they reproduce by budding. A new individual just grows off the side and when it is big enough, it drops off….. And many plants can reproduce asexually using a technique called vegetative propagation. The sweet potato plant in (C) can produce several offspring plants from each potato but as they are all clones of each other, this is definitely asexual reproduction.
Students do get confused with this topic so please ask me a question using the comment feature below the post. Keep revising hard!
I am slightly wary about writing about the male and female reproductive systems. Not because I get embarrassed with this topic (5 terms of human dissection at medical school removed any squeamishness about body parts….) But rather that I worry that the school’s internet filters might start blocking my website if the wrong words appear. But you don’t know until you try, so here goes…..
Male Reproductive System
I will start with the male reproductive system as males are simpler than females in many, many ways… The male reproductive system has three functions:
- to produce the male gametes, sperm cells, at a prodigious rate
- to make the male sex hormone testosterone
- to act as a delivery system to ensure sperm cells are carried into the female reproductive tract in conditions that will allow them to fertilise an egg
The first two functions listed above happen in the testis. There are cells in the testis that secrete the hormone testosterone into the blood from puberty onwards. Testosterone switches on secondary sexual characteristics in the male (body hair growth, muscle development, change in pitch of voice etc.) after puberty. The main part of the testis is made up of very long coiled tubules called seminiferous tubules in which the sperm cells are made.
Humans have over a hundred meters of seminiferous tubules in total in both testes and this allows sperm cells to made at a very fast rate. Even though it takes around 75 days to make an individual sperm cell, the testes make them at a rate of around 85 million sperm cells per day. The epididymis is found next to the testis in the scrotum and is a coiled tube in which sperm cells continue to develop and mature. Sperm are stored here too in readiness for ejaculation.
Everyone knows that in humans the testes are found outside the body cavity in order to keep them cool. Sperm production happens at a maximal rate 3 degrees below core body temperature and having testes outside the body keeps them at this temperature.
The vas deferens is a tube lined with smooth muscle that carries sperm cells away from the testis for ejaculation. As you can see it loops around the back of the bladder, before joining up with the urethra just below the bladder. The urethra is the tube that carries urine away from the bladder but can also carry semen once the vas deferens has joined with it.
There are three accessory glands in the male system (prostate gland, seminal vesicle and the Cowpers‘ glands) These glands produce the fluid that when mixed with the sperm cells is called semen. Semen contains a sugar fructose to provide energy for the sperm cells to swim. It is slightly alkaline to neutralise the acidity in the vagina and also contains mucus to make the fluid easy to move along the tubes.
The sperm cells only acquire the ability to swim when in the epididymis and only become totally mature and able to fertilise the egg right next to the egg cell in the female tract.
The penis is an organ that contains erectile tissue that can fill with blood to allow the penis to fit into the female vagina for ejaculation.
The iGCSE specification says that all living organisms share the following basic characteristics and then lists 8 bullet points. This seems unnecessarily unhelpful because every student in the whole word learns MRS GREN for the 7 characteristics of life…
Make sure you understand the exact meaning of each of the following terms:
Not all organisms Move from place to place of course and lots of things move that are not alive. So that doesn’t make me think that this is a good way to start the whole study of Biology. It is true that all living things, without exception, Respire. “Respiration is a series of chemical reactions that happens inside cells in which food molecules are oxidised to release energy for the cell” – good definition that…. Sensitivity means the ability to detect and respond to changes in the environment. Mammals do this through their nervous and hormonal systems, plants through plant growth substances such as auxin. Growth either involves a cell getting larger or in multicellular organisms, the two processes of cell division and cell specialisation. All living things have the potential to Reproduce, to create new individuals of their species. Excretion is the removal of waste molecules (e.g. carbon dioxide, urea) that have been made inside cells. Nutrition means either obtaining food molecules by eating another organism or if you are a plant, and I guess none of you are, by making your own food molecules through photosynthesis.
The people who wrote the specification have added “they control their internal conditions” to the list. This is actually a better characteristic of life than many above as it is a universal feature of all life. The term for this process is Homeostasis – the ability to regulate and control the internal environment.
It is a shame that two of the best ways to decide whether something is alive have been left off the list. All living things on earth are made of cells. Some organisms are unicellular (Paramecium for example) but many are made of many cells. And all living organisms have the molecule DNA as their genetic material. If you get a question on this in the exam, it’s probably better to talk about the 8 characteristics of life the examiner likes… That’s exams for you!
This video illustrates clearly why you have to be careful using YouTube to find information for revision. It is produced by a company in Australia and is clearly presented, includes the right level of detail for GCSE in the UK and is easy to follow. But….
The video reinforces one of the commonest areas of confusion in this topic by its choice of images to accompany the text. In the section when the voice over is describing the role of animals in seed-dispersal, it has an image of a bee feeding on pollen in a flower. This is NOT seed dispersal!
Later in the video when the voice over talks about wind-pollinated flowers, there is an image of the seeds of a dandelion being blown by the wind. This is NOT pollination!
It might seem like a small point but when you have marked exam questions on this topic for 20 years and seen many students confuse these two separate processes, it starts to take on more significance. So please watch YouTube for science videos – there are some great resources on there…. But be critical and remember, just because it is on a video, it doesn’t mean it is correct!
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.
Sexual reproduction in plants is a topic that some students find difficult at iGCSE. Perhaps it is the plethora of jargon terms, perhaps there are one or two complex ideas to master, perhaps it is just because some people just aren’t interested in plants (more fool them…) Anyway this post will attempt to cover all the main ideas needed for an A* understanding…. Flower structure The flower is the reproductive organ of the plant. A key difference in reproductive biology between animals and plants is that the majority of plants are hermaphrodite. Hermaphrodite means “an organism able to produce both male and female gametes”.
The male gamete in plants is a nucleus found inside a pollen grain, the female gamete is an egg cell nucleus found inside a structure called an ovule in the ovary of the flower. The male parts of the flower are called stamens. Stamens are made up of the pollen-producting anther supported on a stalk called the filament. The female part of the flower is called the carpel. The carpel is made of a stigma (adapted for receiving pollen), a thin style and a swelling at the base called the ovary. Inside the ovary are one or more smaller structures called ovules that contain the egg cells, ova that are the female gamete. The flower also has petals, often brightly coloured and scented to attract insects and that are protective structures that cover the flower when it is still a bud.
Pollination is the “transfer of pollen from the anther to the stigma“. Please learn this definition! A few plants (such as the garden pea) self-pollinate. This means pollen grains from one flower stick to the stigma of the same flower. If you want to irritate me in class, tell me that this is a type of asexual reproduction because only one parent is involved. I will respond with a strange facial grimace and a low guttural growl…… You have been warned. A plant that self-pollinates is still undergoing sexual reproduction as it is making gametes (by meiosis so every gamete will be genetically different) and then fertilising them in a random process. The offspring of self-pollination will still be genetically different from each other but the total genetic variation will be less than if DNA from two different individuals is used.
The vast majority of plants cross-pollinate. They transfer pollen from the anther of one flower to the stigma of a flower on a different plant. Cross-pollination can be brought about by a variety of mechanisms but the commonest two in the UK are by insects and by the wind.
The diagrams above all show flowers that are pollinated by insects. But look at this flower.
The stamens are large and hang outside the flower. The petals are green and small. The stigma is also hanging outside the flower and is large and feathery. This is a flower of a grass plant and typical of a wind-pollinated species.