<< Chapter < Page Chapter >> Page >

The ciliates, which include Paramecium and Tetrahymena , are a group of protists 10 to 3,000 micrometers in length that are covered in rows, tufts, or spirals of tiny cilia. By beating their cilia synchronously or in waves, ciliates can coordinate directed movements and ingest food particles. Certain ciliates have fused cilia-based structures that function like paddles, funnels, or fins. Ciliates also are surrounded by a pellicle, providing protection without compromising agility. The genus Paramecium includes protists that have organized their cilia into a plate-like primitive mouth, called an oral groove, which is used to capture and digest bacteria ( [link] ). Food captured in the oral groove enters a food vacuole, where it combines with digestive enzymes. Waste particles are expelled by an exocytic vesicle that fuses at a specific region on the cell membrane, called the anal pore. In addition to a vacuole-based digestive system, Paramecium also uses contractile vacuoles , which are osmoregulatory vesicles that fill with water as it enters the cell by osmosis and then contract to squeeze water from the cell.

The illustration on the left shows a shoe-shaped Paramecium. Short, hair-like cilia cover the outside of the cell. Inside are food vacuoles, a large macronucleus, and a small micronucleus. The Paramecium has two star-shaped contractile vacuoles. The mouth pore is an indentation located just where the foot narrows. A small opening called the anal pore is located at the wide end of the cell. The micrograph on the right is a Paramecium, which is about 50 microns across and 150 microns long.
Paramecium has a primitive mouth (called an oral groove) to ingest food, and an anal pore to excrete it. Contractile vacuoles allow the organism to excrete excess water. Cilia enable the organism to move. (credit “paramecium micrograph”: modification of work by NIH; scale-bar data from Matt Russell)

Watch the video of the contractile vacuole of Paramecium expelling water to keep the cell osmotically balanced.

Paramecium has two nuclei, a macronucleus and a micronucleus, in each cell. The micronucleus is essential for sexual reproduction, whereas the macronucleus directs asexual binary fission and all other biological functions. The process of sexual reproduction in Paramecium underscores the importance of the micronucleus to these protists. Paramecium and most other ciliates reproduce sexually by conjugation. This process begins when two different mating types of Paramecium make physical contact and join with a cytoplasmic bridge ( [link] ). The diploid micronucleus in each cell then undergoes meiosis to produce four haploid micronuclei. Three of these degenerate in each cell, leaving one micronucleus that then undergoes mitosis, generating two haploid micronuclei. The cells each exchange one of these haploid nuclei and move away from each other. A similar process occurs in bacteria that have plasmids. Fusion of the haploid micronuclei generates a completely novel diploid pre-micronucleus in each conjugative cell. This pre-micronucleus undergoes three rounds of mitosis to produce eight copies, and the original macronucleus disintegrates. Four of the eight pre-micronuclei become full-fledged micronuclei, whereas the other four perform multiple rounds of DNA replication and go on to become new macronuclei. Two cell divisions then yield four new Paramecia from each original conjugative cell.

Art connection

The illustration shows the life cycle of Paramecium. The cycle begins when two different mating types form a cytoplasmic bridge, becoming a conjugate pair. Each Paramecium has a macronucleus and a micronucleus. The micronuclei undergo meiosis, resulting in four haploid micronuclei in each parent cell. Three of these micronuclei disintegrate. The remaining micronuclei divide once by mitosis, resulting in two micronuclei per cell. The parent cells swap one of these micronuclei. The two haploid micronuclei then fuse, forming a diploid micronucleus. The micronucleus undergoes three rounds of mitosis, resulting in eight micronuclei. The original macronucleus dissolves, and four of the micronuclei become macronuclei. Two rounds of cell division result in four daughter cell per each parent cell, each with one macronucleus and one micronucleus.
The complex process of sexual reproduction in Paramecium creates eight daughter cells from two original cells. Each cell has a macronucleus and a micronucleus. During sexual reproduction, the macronucleus dissolves and is replaced by a micronucleus. (credit “micrograph”: modification of work by Ian Sutton; scale-bar data from Matt Russell)

Which of the following statements about Paramecium sexual reproduction is false?

  1. The macronuclei are derived from micronuclei.
  2. Both mitosis and meiosis occur during sexual reproduction.
  3. The conjugate pair swaps macronucleii.
  4. Each parent produces four daughter cells.

Questions & Answers

what is demand of supply
music Reply
What is the meaning of supply of labour
Anthonia Reply
what is production?
Elizabeth Reply
Production is basically the creation of goods and services to satisfy human wants
Anthonia
under what condition will demand curve slope upward from left to right instead of normally sloping downward from left to right
Atama Reply
how i can calculate elasticity?
Tewekel Reply
What is real wages
Emmanuella Reply
what are the concept of cost
Tabitha Reply
what is the difference between want and choice
Grace Reply
Want is a desire to have something while choice is the ability to select or choose a perticular good or services you desire to have at a perticular point in time.
Dalton
substitutes and complements
Amman Reply
Substitute are goods that can replace another good but complements goods that can be combined together
nkanyiso
account for persistent increase in lnflation
niwahereza Reply
what is opportunity cost
Adebowale Reply
opportunity cost reffered to as alternative foregone when choice is made
niwahereza
government measures to control inflation?
Formu Reply
control populationk growth rate by using family planning to reduce faster increase of people than job creation
niwahereza
how do we calculate the firm's profit maximizing output in the short run given marginal cost ,average cost and average variable costs?
Nomuhle Reply
what is employment?
Ahmarh Reply
what is meant by broadening the tax base?
Fiona Reply
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, General biology i lecture. OpenStax CNX. Aug 25, 2015 Download for free at https://legacy.cnx.org/content/col11869/1.1
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'General biology i lecture' conversation and receive update notifications?

Ask