Biology, SS 2,Week: 7
Topic: Feeding in Amoeba, Hydra and Man
Amoeba Feeding
Amoeba feeds on microscopic organisms such as single-celled algae and bacteria. When the amoeba encounters a suitable organism, the cytoplasm flows round the prey and engulfs it, with a drop of water, in a food vacuole. The cytoplasm secretes enzymes into the food vacuole. The enzymes digest the soft parts of the prey and the soluble products are absorbed back into the cytoplasm. Any undissolved residue is left behind as the amoeba flows on.
Feeding in Hydra
Hydra feeds on a variety of small aquatic animals, such as Daphnia and Cyclops, which it catches by means of lots of tiny stinging cells on its tentacles. Scattered over the outer layer tentacles are a great many of these stinging cells called cnidoblasts. Smaller numbers occur on the main body. Each cnidoblast contains a capsule (nematocyst) from which projects a small “trigger” called a cnidocil. Inside each capsule is a tiny hollow thread. It is inverted like a finger of a glove which is pushed into the hand part. These threads are the food-catching apparatus.
Feeding in Mammals
Mammals may eat animals, vegetation, or a mixture of the two. Different words are used to describe mammals and the type of feeding:
- Carnivore: A mammal that eats other animals (flesh or meat eaters).
- Herbivore: A mammal that eats plants.
- Omnivore: A mammal that can eat both plants and meat.
When mammals eat, the food is first broken down into smaller pieces in the mouth. The teeth carry out this mechanical process. There is a row of teeth in the top jaw and a row of teeth in the bottom jaw. When the jaws are moved, the teeth met in different ways so that the food the animal eats is crushed.
Dentition
Dentition refers to the number, arrangement and conformation of teeth in an organism.
Types of Dentition
There are two main types of dentition. These are:
- Homodont dentition: In this type of dentition, the organisms have the same type of teeth. No set of teeth is specialised for any function. All the teeth are of the same shape, size and functions. Examples of homodont dentition are found in fishes, amphibians and reptiles.
- Heterodont dentition: In this type of dentition, the organisms possess teeth of different shapes, sizes and functions. Examples of organisms having heterodont dentition are mammals, e.g. rabbits, man, dog, cattle, etc.
There are different kinds of teeth in the mouths of mammals. These are incisors, canines, premolars and molars. The type of teeth possessed by an animal is closely related to the type of food it eats.
Mammals again have two sets of teeth. These are milk teeth and permanent teeth
- Milk Teeth: This is the set of teeth possessed by the young ones (i.e. children in the case of human) and it is made up of the incisor, canine and premolar teeth (i.e. without the molar). Milk teeth later fall off to be replaced by the permanent teeth.
- Permanent Teeth: This is the set of teeth possessed by adult mammals and is usually four types. They remain till old age and may number up to 32 in man.
Types of Teeth
- The incisors are at the front of the mouth.
- The canines are behind the incisors on each side (although they are missing in some mammals).
- The premolars are at the sides of the mouth.
- The molars are right at the back of the mouth, on each side.
Structure of a Tooth
A typical tooth such as the canine or incisor is made up of three regions which are the crown, the neck and the root.
- The crown is the part of the tooth that is above the gum
- The root is the part of the tooth that is embedded in the socket of the gum
- The neck is the narrow junction between the crown and the root
Incisors and canine have one root each while premolars and molars may have two or three roots each.
- Enamel is white in colour. It is the hardest substance in the human body and covers the outer portion of the crown. It is made up of mineral salts (of calcium and magnesium) and keratin. It can withstand high pressure.
- Cement is the layer present covering the root portion of the tooth. It is made up of mineral salts and water and is almost as hard as bone.
- Periodontal membrane or ligament – It consists of fibres which extend across the cement and anchor the tooth in the bony socket. They also allow a certain degree of movement while chewing thereby acting as shock absorbers.
- Dentine is yellow and is a bone-like material which is present along the full height of the tooth. It is enclosed by the enamel in the crown portion and cement in the root portion. Dentine can also be composed of living cells which show divisions with new cells being added to it regularly.
- Pulp cavity is the innermost region of the teeth and shows the presence of blood vessels and nerve endings. The blood vessels serve to nourish the tooth and the nerves transmit messages of heat, cold and pain to the brain and back. The pulp cavity in the root portion is also referred to as the root canal. The number of root canals may range from 1 to 3 depending on the type of tooth. The blood vessels and the nerves enter the root canal through a small hole at the tip of the root canal.
Dental Formula
The dental formula refers to the numbers and types of teeth present in the mouth of an animal. The numbers and types of teeth present in the jaw of an animal is a reflection of special adaptation of mammalian teeth for feeding.
Teeth of a carnivore – the dog
The teeth in carnivores are well designed for dealing with flesh and bones
Incisors: These are small and designed to meet together to grip the prey and pull the meat apart.
Canines: These are large, sharp and pointed. They may be used for holding and killing the prey, and also for tearing the meat apart.
Premolars and molars: These are powerful teeth that meet together and can crush flesh and bone.
Carnassials: These do not meet, but pass each other, rather like a pair of scissors, so that the meat is cut into pieces and the bones cracked.
Jaw hinge: This only allows up and down movement to provide a firm scissor action. There is no sideways movement.
The dental formula for dog:
Puppy (temporary teeth) not presents at birth – but complete by 60 days.
I – 3/3 C- 1/1 P – 3/3 M – 0/0 = 14 x 2 = 28
Adult (permanent teeth) – emerge at about 4 months of age
I – 3/3 C – 1/1 P – 4/4 M – 2/3 = 21 x 2 = 42
Teeth of a herbivore
The teeth in herbivores are well designed for dealing with plant material. It indicates that in the upper right (or left) half of the jaw there are no incisors or canines (i.e. there is a diastema), three premolars and three molars. In the lower right (or left) half of the jaw are three incisors, one canine, three premolars and three molars
The dental formula for sheep:
Temporary teeth
I – 0/4 C – 0/0 P – 3/3 M – 0/0 = 10 x 2 = 20
Permanent teeth
I – 0/4 C – 0/0 P – 3/3 M – 3/3 = 16 x 2 = 32
In sheep the permanent teeth are not completely erupted until 3.5 – 4 years of age
Teeth of an omnivore – the Human
Incisors: Used for biting – apples, for example.
Canines: Not as large and powerful as those of a dog, although they can be used for tearing.
Premolars and molars: Used for chewing and crushing food. These teeth slide over each other as the jaw moves from side to side.
Jaw hinge: The hinge joint in a Human skull allows some sideways movement, so the bottom jaw can move both up and down and from side to side.
The dental formula for man where I = incisors, C = canines, P = premolars and M = molars:
Man (adult): I – 2/2 C – 1/1 P – 2/2 M – 3/3 = 16 x 2 = 32
During the life span of man, he grows two sets of teeth. This condition is called diphyodont. The teeth, which appear during the infancy, number only 20 and are temporary. They start falling from around the age of 5. They are thus called deciduous teeth (also called milk teeth). In each jaw there are 10(4 incisors, 2 canines and 4 pre-molars).
The permanent teeth take the place of the milk teeth after they fall. The types of teeth remain the same. In addition to these 20 teeth, the permanent set have 12 molars which are present 3 on each side. Thus, a total of 32 teeth are present in the normal human adult.
Dental Care
Keeping your teeth and gums healthy requires good nutrition and regular brushing and flossing. The teeth can therefore be cared for in the following ways:
- Brush your teeth twice a day—in the morning and before bed—and floss once a day. This removes plaque, which can lead to damaged teeth, gums, and surrounding bone.
- Use toothpaste that contains fluoride, which helps prevent tooth decay and cavities. Ask your dentist if you need a mouthwash that contains fluoride or one with ingredients that fight plaque. Look for toothpastes that have been approved by the American Dental Association.
- Avoid foods that contain a lot of sugar. Sugar helps plaque grow.
- Avoid using tobacco products, which can cause gum disease and oral cancer. Exposure to tobacco smoke (secondhand smoke) also may cause gum disease as well as other health problems.
- Practice tongue cleaning. You can use a tongue cleaner or a soft-bristle toothbrush, stroking in a back-to-front direction. Tongue cleaning is particularly important for people who smoke or whose tongues are coated or deeply grooved.
- Schedule regular trips to the dentist based on how often you need exams and cleaning.
