Chapter 5 Asexual reproduction

advertisement

1 tto

Asexual reproduction

1

1 Introduction

Reproduction is the process by which an organism produces new individuals

(offspring - nakomelingen). Cell division, which we studied in the last chapter, is the basis of reproduction.

In humans and many other organisms reproduction involves two individuals, the male and the female. We call this sexual reproduction.

However, some organisms can reproduce on their own without another individual. We call this asexual reproduction. In this chapter, we shall look at some of the methods by which organisms reproduce asexually.

2 Splitting in two

Paramecium

is a single-celled organism which lives in ponds and puddles. It reproduces by splitting in two. We call this binary fission

(

fission

means 'splitting' and binary means 'two'). The process is just like cell division described before: first the

nucleus divides into two, and then the cell splits across the middle as shown in Fig 1. The two new paramecia then grow, and after a day or so each of them may split again.

The nucleus is the control centre of the cell because it holds the

Figure 1. Paramecium reproduces asexually

by dividing into two chromosomes. In chromosomes all the characteristics of the organism are described. When a cell divides it copies the nucleus and its chromosomes so that the new cell will receive the exact same characteristics. As a result, binary fission leads to two identical new cells, with the same characteristics. bacteria dividing paramecia dividing

2

questions:

1.

When it is warm and plenty of food is available,

Paramecium

may split once every 24 hours. Dividing at this rate, how many

paramecia

could be formed from a single

paramecium

after seven days?

2.

Why is it important that the nucleus is copied and passed on to the new cells?

3.

Explain that asexual binary fission leads to identical offspring.

3 Budding

Yeast reproduces by budding. The cell sends out a small outgrowth which gets larger and eventually breaks away from the parent cell (Fig 2a). Meanwhile the nucleus divides into two. One of the two resulting nuclei stays in the parent cell, and the other one moves into the bud.

Sometimes the new cell starts budding before it has broken away from the parent cell, thus giving rise to a chain of cells as shown in Fig 2 b.

Yeast cells budding

4 Spores

Figure 2. Yeast cells budding.

A mushroom is also a fungus, but a very different one from yeast. Mushrooms, like many other fungi, reproduce asexually by forming millions of

spores. A spore is a tiny round cell enclosed within a thick protective wall. The wall enables the spore to withstand unfavourable conditions such as frost (vorst) and drought (droogte).

Figure 3. The spore forming body of a mushroom or toadstool.

A spore is formed by cell division and receives the same nucleus and chromosomes as its parent. All offspring are therefore identical.

The spore is like a tiny speck of dust and is so light that it can float through the air over a large distance. If it lands in a suitable (gunstige) place it breaks open and gives rise to a new fungus.

Spores are formed by these three main groups of organisms: fungi (Fig.

3), mosses (mossen) and ferns (varens).

3

questions:

4.

As well as providing a method of reproduction, spores enable the species to grow many new individuals over a large area.

Name two characteristic of spore formers that enable them to do this.

5.

Some serious diseases of crop plants (landbouwgewassen) such as wheat

(graan) are caused by fungi. Once they get into the crop they are very difficult to get rid of. Why do you think it is difficult to get the fungi out of the soil?

5 Tubers

A tuber is a swollen part of the stem of a plant located under the ground. It is mainly filled with starch.

In plants, chloroplasts play an important role. In the chloroplasts, plants make glucose from water, sunlight and carbon dioxide. The plant needs glucose as building blocks to grow and it gives the plant energy (respiration).

Figure 4. A potato tuber.

During the season, the leaves of a potato plant produce more glucose than the plant needs. The extra glucose is stored as starch in tubers.

Normally, plants grow stems and branches that reach out of the soil.

Potato plants, however, grow many underground braches as well. The ends of these branches become thick when they are filled with starch. These thick branch endings are called tubers: the potato’s (Fig. 4). In a potato cell, the starch is stored in starch grains. You’ll study starch grains in experiment 1.

When the potato plant dies the potato’s stay alive under the ground.

When the winter has passed, a new plant will grow from each potato. In spring (lente), the potatoes start to form shoots (uitschieters, stengels) and roots (wortels). The nutrients necessary for growth and energy come from the starch stored in the potato. Starch is turned into glucose which is respired to give energy or turned into proteins, fats or carbohydrates which serve as building blocks.

The starch in the potato is enough to make a new plant with leaves. Once the leaves are formed, the new plant can make the all glucose it needs for growth and energy.

4

Since all potatoes are formed by cell division (growth), all new potato plants are identical. Thus, the formation of tubers presents the potato plant with two advantages: asexual reproduction and a way to survive the winter.

Experiment 1 Examining a potato tuber

a You may have heard people talk about the 'eyes' of a potato. They are shown in

Fig 5. Can you see "any 'eyes' on your potato tuber? Each one is a very small bud which is capable of sprouting into a new potato plant.

You will also see little black specks dotted about over the skin of the potato. These are little holes called lenticels which allow air to get through the skin so that the tuber can breathe. b Cut a potato tuber in two. With a knife, scrape away a little of the white pulp and put it on a slide. Add a drop of dilute iodine solution to the pulp, and cover it with a cover slip. c Look at the potato pulp under the microscope. Do you see a lot of dark blue, egg-shaped objects? These are starch grains. d Keep another potato in the window of your laboratory. You may find that eventually leafy shoots start growing out of it (Fig. 6). questions:

6.

By what process is the starch stored in the potato tuber produced?

7.

Where in the cell is the starch stored?

8.

What will the starch be used for after the winter (two purposes)?

9.

Forming spores is sometimes better than making tubers.

Name two advantages of forming spores over forming tubers.

10.

What are the two advantages of potatoes to the potato plant?

11.

What makes potatoes so suitable (geschikt) as a food for humans?

5

6 Bulbs

The most well known bulbs (de bekendste bollen) are the tulip bulb and the onion.

A bulb is really a kind of bud (een knop) in which the leaves are filled with glucose. A regular bud is a collection of leaves wrapped around each other at the side of a branch waiting to grow into a branch with leaves (Fig. 7).

Like a tuber a bulb contains a store of food

(glucose, sugar). However, its structure is more complicated (Fig. 8). The glucose is stored in thick leaves (“fleshy leaves”). In the middle of the bulb, the

Figure 7. A regular bud. young plant is already present (“bud”). At the bottom of the bulb we find the stem to which the leaves are attached.

Each bulb has a number of lateral bulbs

(Fig. 8), which will be filled with glucose during the season. When the parent plant dies in autumn, it will have produced several new bulbs.

In spring, each bulb sprouts into a new

Figure 8. The parts of a bulb. plant. Then, the glucose, present in the fleshy leaves, is used up for growth and energy and the lateral bulbs will be filled with glucose again.

6

Experiment 2 Examining a bulb

a Slice the bulb (daffodil) crossways (Fig

9). b Make a schematic drawing of this cross section indicating the bud, the fleshy leaves, the roots and the stem. c A bulb is capable of producing new bulbs. If you look at a daffodil bulb you can see small lateral bulbs attached to the side of the parent bulb. questions:

Figure 9. How to slice a bulb to see

12.

When you sliced your bulb in half crossways, did you notice that it consists of a series of 'scales' (rokken)? These are actually leaves. In what ways do they differ from normal leaves? Explain the differences.

13.

There are three kinds of leaves in the bulb. The brown ones around it for protection. What are the functions of the other two kinds?

14.

Some plants need to flower in early spring. That is important because if they don’t flower early, the trees around them will take away all the sunlight.

Often these plants form bulbs. How does producing bulbs help them to solve this problem (hoe helpt het maken van bollen hen dit probleem op te lossen).

15.

A bulb will start to grow only if it is first subjected to a period of cold. a) What is the advantage of sprouting áfter a period of cold? b) Describe an experiment to find out how low the temperature has to be before the bulb will produce a shoot.

16.

Suggest three uses that humans make of bulbs. In each case give at least one example.

7

7 Cuttings

If a branch of a plant is cut off and stuck into some soil, with compost or water, roots may grow out of it so that it becomes a new plant. This is called a cutting.

Gardeners often take cuttings from plants which they particularly like because the plants formed this way are identical to the parent plant

(because they are formed by cell division ...). However, not all plants will grow from cuttings.

Experiment 3 Taking cuttings of Peace-in-the-home.

Peace-in-the-home (slaapkamergeluk – Fig.

10) is a plant from which it is particularly easy to take cuttings. You will need a plastic cup with some soil and a pair of scissors. a Cut off a side branch from the Peace-inthe-home plant, cutting cleanly with a sharp pair of scissors. This is your cutting. b Put it in the soil. Cover the plastic cup with sandwich bag.

Figure 10. Peace-in-the-home. c Observe your cutting every week. d After a few weeks several roots will have grown out. If the cutting formed roots itself successfully you wilt have obtained a new Peace-in-the-home plant. You can take it home then. questions:

17.

Why was it important to put a sandwich bag over the cup with the cutting?

18.

Taking cuttings is an artificial way of reproducing plants, used by gardeners.

In what circumstances might this kind of reproduction occur naturally?

19.

Why is it best to take cuttings from branches which do have leaves and do not have any flowers on them?

20.

Some gardeners take cuttings by sticking a branch straight into the soil; others put the branch in water until new roots start growing out and then they stick it in the soil. What are the advantages and disadvantages of each method?

8

8 Runners

These are side branches of some plants that grow along the surface of the soil. Roots grow down from buds

(knoppen) on the runners. These develop into new plants. Strawberry plants reproduce in this way (see

Fig.11).

Figure 11. Strawberry runners.

9 Advantages and disadvantages of asexual reproduction

The most obvious advantage of asexual reproduction is that there is no need for the organism to have a partner. When asexual reproduction occurs by the formation of spores it occurs rapidly and enables the species to spread quickly over a wide area. Spores, tubers and bulbs enable the plant to survive the winter.

The main disadvantage is that the offspring are always exactly like the parent. It is therefore impossible to produce new

kinds

of organisms by this method. In other words asexual reproduction does not give rise to variety

(variatie – verschil). Variety can only be produced by sexual reproduction as we shall see in the next chapter.

10 Importance of asexual reproduction to humans

Suppose a gardener has a plant with a particularly good feature. How could the gardener produce a lot of plants with the same feature? Asexual reproduction produces offspring which are exactly like the parent, so it would be sensible to propagate the plant by asexual means, for example by taking cuttings.

New varieties of useful plants with good features are created by sexual reproduction and then propagated by asexual methods. This is particularly important to farmers and market gardeners who produce commercially important plants such as potatoes on a large scale.

9

questions:

21.

The fern (varen) is a plant which has two methods of asexual reproduction: like the mushroom it can form numerous spores, and it also possesses an underground rhizome

(wortelstok) from which new plants grows up each spring (Fig. 12). A rhizome is a long horizontal root full of starch.

Figure 12. Ferns with a rhizome.

Suggest two advantages which the spores have over the rhizome as a means of reproduction.

22.

Seven methods of asexual reproduction are listed below. In each case, write down the name of

one

organism which

uses

the method.

Binary fission, budding, spore formation, tuber and bulb formation and runner and rhizome formation.

23.

Make a table (6 columns) with in the first column the seven methods of asexual reproduction. Above each subsequent column write down the questions below. Finish off the table by putting crosses in the correct boxes. Label the first column “method of reproduction”.) a the method enables the organism to survive the winter b the method results in two or more of the offspring remaining attached to one another for a while c the method produces two offspring every time reproduction occurs d the method involves the formation of an underground structure containing food e the method enables the species to spread quickly over a wide area

24.

Out of which parts is a plant made up?

For each part explain how it is involved in reproduction (sexual or asexual).

10

Glossary

asexual reproduction binary fission cell division nucleus chromosomes characteristics multiply budding spores offspring identical tuber potato plant branches

11

starch grains proteins fats carbohydrates building blocks lenticels

Bulbs cutting

Runners variety rhizome

12

its internal structure.

Lists

Name the ways of asexual reproduction and examples of organisms that use it.

Name advantages of asexual reproduction.

Name disadvantages of asexual reproduction.

Name the advantages of spore formation.

Name which parts of the plant are involved in each type of asexual reproduction.

Name the parts of a bulb and their functions.

Name the advantages of taking cuttings.

What do you need to explain?

Explain how asexual reproduction leads to identical offspring.

Explain what the difference is between binary fission and budding.

Explain the function of the nucleus.

Explain how each way of asexual reproduction works.

Explain the life cycle of a potato plant.

Explain what the importance is of storing starch/glucose in tubers/bulbs.

Explain what variety is.

13

Download