Biology

Key Unit Competence
Describe different specialized plant and animal cells and adaptation of tissues.

Learning objectives
By the end of this unit, I should be able to:
–– Define a tissue as a group of cells with similar structure working together for a function.
–– Name the main types of animal and plant tissues.
–– Define an organ as a structure made up of a group of tissues with related
functions working together to perform bodily functions.
–– Explain how epithelial tissues are adapted to perform a diversity of functions in the body.
–– State the advantages and disadvantages of being unicellular.
–– Observe and draw plant and animal tissues as seen under a light microscope.
–– Interpret photomicrographs of plant and animal tissues
–– Acknowledge the relationship between levels of organization
–– Recognize the efficiency shown by multicellular organisms to explore more modes of life that are not available to single celled organisms that show little or no specialization

Introductory activity
Read the following passage and use it to answer the following questions:

In an anthill, there are different groups of termites such as a queen, workers and soldiers. Each group has a specific role to play in the colony. The structure termites of each group is related to their role for example soldiers that protect the colony have mouth parts shaped like a pair of scissors building and a slightly larger abdomen for storing water. The queen is the largest of all and has a role of laying eggs. Workers have mouth parts for cutting and chewing food or soil particles. Some members of workers are in charge of caring for the young while others find food and defend the colony or remove dead members. Their specialization and division of labor bring about efficiency in the colony.
1. Specify the message addressed by the above paragraph.
2. Explain how is the structure of termites related to their functions?
3. What is the significance of specialized tissues in multicellular organisms like plants and animals?

The study of tissues is known as Histology. A tissue is a group of associated, similarly structured cells that perform specialized functions for the survival of the organism. In histology, differentiation is the process by which structures become modified and specialized to perform specific functions. Differentiation is also known as ‘specialization’. In animals, the first type of cells in the developing embryo is stem cells. These are unspecialized cells that go on to form all the different types of cells in adult.

5.1 Specialized plant tissues
Activity 5.1.1
–– Remove an epidermis layer from the ventral side of an onion leaf.
–– Mount it on the slide containing a drop of iodine solution
–– Observe your preparation under a light microscope
–– Draw, label and describe your findings.
–– From your discussion:
1. What is a tissue?
2. What is the role of epidermis in onion?

5.1.1. Plant tissues
Activity 5.1.2
The following figure represents the flow chart of subdivisions of plant tissues. Use it to answer the following questions.

1. How do meristems differ from permanent tissues?
2. Plant tissues are classified into ground tissues and vascular tissues as shown in the figure above. What is meant by the term vascular tissues?
3. How is the structure of the xylem and phloem vessels related to their function?
4. From the flow diagram above, identify three types of ground tissues.

5. Write down short notes on each of the following types of meristems.
a. Apical meristems.
b. Lateral meristems
c. Intercalary meristems
Plant tissues can be divided into two main groups, Meristematic tissues (apical, lateral, and intercalary meristems) and Permanent tissues (ground tissues and vascular tissues).

5.1.2. Meristem tissues
Meristem tissue is a group of cells which retain the ability to divide by mitosis. Meristematic tissues are specialized to carry out specific functions such as reproduction, growth, photosynthesis and replacement of old or damage tissues. The cells making a meristem tissue are small, have a central large nucleus and dense cytoplasm, thin-walled, with no or small vacuole, and no specialized features. The cells are rectangular and closely packed with no intercellular air spaces.

Types of meristematic tissues
Meristematic tissues are subdivided into apical meristems, lateral meristems (cambium) and intercalary meristems

a. Apical meristems
They are located in the root and shoot apex (at the growing points of roots and stems). They are responsible for primary growth, leading to the increase of primary plant body.

b. Lateral Meristems (cambium)
Lateral meristems are in lateral parts of the plant, where they are responsible for Biology Senior Four Student’s Book 87 secondary growth. The cambium gives rise to secondary vascular tissues (secondary
xylem and secondary phloem) in dicotyledonous plants.

c. Intercalary meristems
These are found in the region of permanent tissues like at nodes of monocotyledonous plants (e.g. sugar cane). It allows growth in length to occur between internodes. Functions of meristematic tissues
–– The main function of meristematic tissue is to produce new cells by mitosis.
The cells elongate and differentiate to form new cells for primary growth of shoot and root.
–– Vascular cambium produces new cells to increase the diameter of stems and roots during secondary growth.
–– Cork cambium called (phellogen) produces the outer cork layer called phellem which consists of suberized cells. The cork layer reduces water evaporation from the plant and protects the plant against the entry of pathogens.
–– The intercalary meristems allow growth and increase in length in regions other than the tips.

5.1.3 Permanent tissues
Permanent tissues consist of two groups of tissues such as: ground and vascular tissues.

5.1.4. Ground tissues
The ground or fundamental tissues are plant tissues which function in storage, metabolism and support. There are three types of ground tissues: parenchyma, collenchyma and sclerenchyma tissues.

5.1.5. Parenchyma tissues
Parenchyma is a soft plant tissue made up of thin-walled cells that forms the greater part of leaves, stem pith, roots, and fruit pulp. They are the main sites for physiological and biochemical processes in the plants including photosynthesis, protein synthesis and storage of starch and mineral ions. Parenchyma tissues can be found in epidermis, mesophyll, endodermis, pericycle, aerenchyma and secretory cells.

Characteristics
–– Parenchyma tissues consist of large living cells, with relatively thin wall containing cellulose, pectin and hemicellulose.
–– Parenchyma tissues consist of cells, usually having a large central vacuole. They are often partially separated from each other.
–– Spongy cells present intercellular spaces that intervene in gaseous exchange and transpiration through stoma. They are usually stuffed with plastids.
–– Parenchyma tissues consist of cells with polygonal and spherical shapes in the leaf. They form the mesophyll, and are located between upper and lower epidermises. They are responsible for photosynthesis.

Functions of parenchyma tissues
–– In the leaves, parenchyma tissues form the mesophyll and are sites for photosynthesis, gaseous exchange and transpiration.
–– They store food substances such as starch, proteins and lipids
–– They can be modified to form specialized cells to carry out other function in epidermis, endodermis, pericycle, parenchyma, and secretory cells.

Adaptations of parenchyma for its function
–– Parenchyma tissues are made of unspecialized cells with variety of functions:
–– Parenchyma can become specialized to carry out specific functions e.g. mesophyll has cells with many chloroplasts, and aerenchyma which has air spaces. All of these adaptations help in photosynthesis and gas exchange.
–– They have isodiametric cells and function as packing tissue and storage tissue.
–– Cells are loosely packed with many large intercellular spaces. This permits diffusion of gases.
–– They have thin cellulose cell wall which is permeable so that it permits passage of materials.
–– The walls are transparent and permit entry of light in photosynthesis cells.
–– Large cells with large vacuoles provides space for storage of substances, where the entry of water causes vacuole to expand and cells become turgid
–– Leucoplasts act as storage of starch while chromoplasts present in some cells
e.g. in petals attract insects for pollination.

5.1.6. Collenchyma tissues
Their cells are elongated with irregularly thickened cell walls that provide structural support, particularly in growing shoots and leaves. Their thick cell walls are composed of cellulose and pectin. These cells are often found under the epidermis, or the outer layer of cells in young stems and in leaf veins.

5.1.7. Sclerenchyma tissues
Sclerenchyma is found in hard parts of the plant body. They are very common in roots, stems, leaves and petioles. They may be present in patches, groups or layers. The cells of the sclerenchyma are dead, they are elongated, narrow, and thick walled and lignified. They are pointed at both ends where it gives strength, rigidity and flexibility to the plant body. They consist of fibres and sclereids. Fibres are long,
narrow, thick and liquefied cells usually tapering at both ends. Sclereids cells are normally short with very thick walls, irregular and not tapering at the ends.

Table 5.1: Comparison between collenchyma and sclerenchyma tissues

5.1.8. Vascular tissues
The vascular tissue system consists of two kinds of conducting tissues: the xylem responsible for conduction of water and dissolved mineral nutrients, and the phloem responsible for conduction of elaborated food.

a. Xylem
The xylem tissues are made of dead cells which have the cell walls removed at the end of the cells, forming tubes through which the water and dissolved mineral ions can flow. Xylem vessels are involved in the movement of water through a plant - from its roots to its leaves via the stem. During this process water is absorbed from the soil through root hair cells, moves by osmosis from root cell to root cell until it
reaches the xylem, and finally it is transported through the xylem vessels up the stem and then to the leaves.
Xylem vessels are hollow tubes or lumen with a thick strengthened cellulose cell wall. The hollow tubes act like pipes allowing water and dissolved minerals to flow through them. They develop from cylindrical cells arranged end to end, in which the cytoplasm dies and the cell walls between adjoining cells breaks down leaving a dead empty tube. The cell walls in xylem vessels contain a substance called lignin
which strengthens the cells and gives structural support.

b. Phloem
Phloem vessels are involved in translocation of elaborated substances. Dissolved sugars, produced during photosynthesis, and other soluble food molecules are moved from the leaves to growing tissues such as the tips of the roots and shoots and storage tissues such as in the roots. In contrast to xylem, phloem consists of columns of living cells. The cell walls of these cells do not completely break down,
but instead form small holes at the ends of the cell. The ends of the cell are referred to as sieve plates. The connection of phloem cells effectively forms a tube which allows dissolved sugars to be transported.

Phloem tubes carry food substances like sugar and amino acids produced in leaves during photosynthesis to every part of the plant. The movement of food substances through the plant is called translocation.

Table 5.2: Comparison between Xylem and Phloem tissues

Self-assessment 5.1
1. State where in a flowering plant you would find:
a. Lateral meristem
b. Intercalary meristem
c. Apical meristem
2. Give characteristics of meristematic cells.
3. What do you understand by the following terms?
a. Differentiation
b. Cambium
c. Wood
d. Meristem
4. Differentiate between Collenchyma and sclerenchyma
5. State the main structures (components) that make up a xylem and phloem tissues.
6. Explain how the structure of Parenchyma and Xylem tissues are suitable to their functions.
7. The diagram below shows a longitudinal section of two cells of phloem tissue in a plant stem.

a. Name the cells labelled A and B on the diagram.
b. State the function of phloem in a plant.

5.2 Animal tissues
Activity 5.2.1
Conduct a research by using different sources of information to find out the structures and the main functions of the following four groups of animal tissues: epithelial, connective, muscular and nervous tissues. There are four basic types of animal tissues such as epithelial tissue, muscle tissue, nervous tissue, and connective tissue.

5.2.1. Epithelial tissue
Epithelial tissue consists of closely packed cells arranged in single or multilayered sheets. It is made up of layers of tightly packed cells that form the external surfaces of the body and cover the outer and the inner surfaces of the organs. Some are specialized to form glandular tissues (glands). The epithelium lining the inside of the heart, blood vessels and lymph vessels is referred to as endothelium. Two criteria for classifying epithelia are: the number of cell layers and the shape of cells on the free surface. The following are the types of epithelium tissues:

a. Simple cuboidal epithelium
This is a tissue with cells that are cubical in shape. Cuboidal cells are specialized for secretion and they make up the epithelia of kidney tubules and many glands including salivary glands, and thyroid gland.

b. Simple squamous epithelium
It is thin,leakyand functions in the exchange of material by diffusion. This type of epithelium lines blood vessels and the air sacs of lungs, where diffusion of nutrients and gases is critical.

c. Simple columnar epithelium
These are columnar in shape with free surface containing extensions of micro villi. It lines the intestines. This epithelium secretes digestive juices for the final stages of digestion and absorbs nutrients to blood stream.


d. Pseudo-stratified ciliated columnar epithelium
It forms a mucous membrane that lines the nasal passages of many vertebrates. The beating cilia move the film of mucus along the surface.

e. Stratified squamous epithelium
Itv regenerates rapidly by cell division near the basal lamina. The new cells are pushed outward to , replacecells that are sloughed off. This epithelium is commonly found on surfaces subject to abrasion, such as the outer skin and lining of the esophagus, anus, and vagina.

Figure 5.11 e) Stratified squamous epithelium.

f. Transitional epithelium
In this type of stratified epithelium, the surface cells change their shape from round to squamous. Transitional epithelium lines urinary bladder. When the bladder is empty, the surface cells are rounded. As the bladder fills urine, these cells become flattened. Transitional epithelium enables the bladder to fill and stretch without tearing the lining.

g. Stratified columnar epithelium
It is a rare type of the epithelial tissue composed of column shaped cells arranged in multiple layers. They are found in the conjunctiva or the eye, in parts of the pharynx, anus, uterus, the male urethra and vas deferens.

h. Stratified cuboidal epithelium
It is a type of epithelial tissue composed of multiple layers of cube-shaped cells. Only the most superficial layer is made up of cuboidal cells and the other layers can be cells of other type. It has several locations in the body including sweat gland ducts, egg-producing vesicles and ovaries.

5.2.2. Main characteristics of epithelial tissues

a. Polarity
All epithelia have a free surface and a lower attached basal surface that differ in structure and function. For this reason, epithelium is described as showing polarity.

b. Supported by connective tissue
All epithelia are supported by connective tissue. For instance, deep to the basal lamina is reticular lamina, anextracellular material containing collagen protein fiber which forms the basement membrane. The basement membrane reinforces the epithelium and helps it to resist stretching and tearing.
c. They are avascular; have no blood vessel in them. Nutrients and gases are supplied by blood through the connective tissue by simple diffusion

c. Regeneration
Epithelium have a high regenerative capacity and can reproduce rapidly as long as they receive adequate nutrition.

Functions of epithelium
–– Epithelium forms a protective layer: The epithelium of the skin protects the body from mechanical damage, entry of pathogens, ultraviolet rays and dehydration. Epithelium lining the respiratory air passages secretes mucus which traps inhaled dust particles and microbes.
–– The ciliated epithelium cells have cilia that propel the mucus and trapped particles to the throat.
–– Glandular tissues secrete the digestive enzymes, hormones, mucus, sweat and sebum.
–– Acts as a barrier and regulates movement of substances across kidney
–– Some epithelial cells can divide mitotically producing new cells to replace damaged or dead cells.
–– Some epithelial cells such as taste buds and retina cells are specialized to form sensory receptors.

5.2.3 Muscular tissues
Muscle tissues consist of elongated cells held together by connective tissue. Muscle cells are highly specialized in that they are able to shorten to a half or even a third of their resting length by the process of contraction. The contraction is caused by two types of fibrous proteins: myosin and actin.

Muscles in the body provide the necessary force for the motion and they convert chemical energy into kinetic or mechanical energy. There are three types of muscle tissue:
–– Smooth muscle which is found in the inner linings of organs;
–– Skeletal or striated muscle, which is attached to bone and helps in movement
of the body;
–– Cardiac muscle which is found only in the heart.
Smooth and cardiac muscles are involuntary muscles whereas skeletal muscles are called voluntary muscles because they are under voluntary (conscious) control.

a. Smooth Muscle
Smooth muscle is also called unstriated, unstriped, involuntary or visceral muscle.
It is found in the walls of the hollow internal organs such as blood vessels, intestinal tract, urinary bladder, and uterus. Smooth muscles have the following features;
–– It is under control of the autonomic nervous system; they cannot be controlled consciously, so they are also called involuntarily muscle.They do not have striations.
–– Smooth muscle cells contract slowly and rhythmically

b. Cardiac tissue
Cardiac tissue (figure 5.10 a) is found in the walls of the heart and it is under control of the autonomic nervous system. Cardiac muscle has the flowing basic features.
–– It contracts and relaxes continuously.
–– It is branched and connected to other cardiac muscle fibers through intercalateddiscs(Figure 5.16 b), which are reinforced membranes that hold the cells together during contractions. These interconnections or intercalated discs between the fibers ensure a rapid and uniform spread of excitation throughout the wall of the heart which in turn ensures a synchronous contraction.
–– They are myogenic (their contraction originate from within the heart itself ).

c. Skeletal Muscle
Skeletal muscle is also called striated, striped, or voluntary. They are attached to bone, and are responsible for body movements and body posture. There are approximately 639 skeletal muscles in the human body.
Characteristics of skeletal muscles:
–– They are under control of voluntary nervous system

–– They are attached to bone and this is the reason why they are called skeletal muscles.
–– They are made of elongated and cylindrical muscle fibres
–– They appear under microscope to have alternate light and dark bonds and this is why they are called striated muscles.
–– Their muscle fibres are multinucleated (many nuclei per cell)
–– These muscle cells also contain light and dark stripes called striations

General functions of muscle
The main function of muscle is its contribution to motion, where body movements such as walking, breathing, and speaking, as well as movements associated with digestion and the flow of fluids take place. Muscles contribute to the heat production, maintenance of posture and body support and communication through facial expression, writing and speech.

5.2.4. Nervous tissue
Nervous tissue is composed principally of densely packed cells called the nerve cells (neurons) that together form the nervous system including the brain and spinal cord. Neurons are specialized for transmitting electrical nerve impulses.

A typical neuron has three main parts: Cell body, dentrites and axon.

a. The cell body or soma
–– It is the main part from which, extensions derive (Axon and Dendron).
–– It is made of a great spherical nucleus, granular cytoplasm and controls
all nerve cell activities.

b. Dendrites (Dendron when single): small branches attached to the cell body and receive nerve impulse from other neurons

c. Axon or cylindrax:
–– It is the thinner nerve fibre that carries messages away from the cell body and can be as long as 1 m. In some neurones, the axons have a fatty myelin sheath formed by Schwann cells which wrap themselves around the axon to increase the speed of impulse transmission.

5.2.5. Connective tissues
Connective tissue is made up of many different types of cells that are all involved in structure and support of the body. Bone, blood, fat, and cartilage are all connective tissues. Connective tissues can be densely packed together, as bone cells are or loosely packed, as adipose tissue (fat cells) are. A connective tissue is made up of a variety of cells embedded in a large amount of intracellular substance called matrix
and fibers which are non-living products of the cells.

a. Common functions of connecting tissues:
–– Connective tissues protect and support the body and internal organs.
–– They act as connecting systems, binding all other tissues together.
–– They also form surrounding sheaths to separate the various organs.

b. Cells of connective tissue
The specialized cells of the various connective tissues produce the extracellular matrix. The names of the cells end with suffixes that identify the cell functions as blasts, cytes, or clasts. Blasts create the matrix, cytes maintain it, and clasts break it down for remodeling. For example: Fibroblasts are cells that form fibrous connective tissue and fibrocytes maintain it, chondroblasts form cartilage and chondrocytes
maintain it, and osteoblasts form bone, osteocytes maintain it, and osteoclasts break it down

Adipose, or fat cells, also called Adipocytes, contain large amounts of lipid. The lipid pushes the rest of the cell contents to the periphery, so that each cell appears to contain a large and centrally located lipid droplet with a thin layer of cytoplasm around it. Adipose cells are rare in some connective tissue types like cartilage but they are abundant in others like loose connective tissue, and they are predominant in adipose tissue.

Mast cells are commonly found beneath membranes in loose connective tissue and along small blood vessels of organs. They contain chemicals such as heparin, histamine and proteolytic enzymes. These substances are released in response to injury such as trauma and infection and play important roles in inflammation.

White blood cells continuously move from blood vessels into connective tissues. The rate of movement increases dramatically in response to injury or infection. In addition, accumulations of lymphocytes, a type of white blood cell, are common in some connective tissues, such as in the connective tissue beneath the epithelial lining of certain parts of the digestive system.

Macrophages are found in some connective tissue types. They are derived from monocytes, a white blood cell type. Macrophages are either fixed and do not move through the connective tissue in which they are found or are wandering macrophages and move by amoeboid movement through the connective tissue. Macrophages phagocyte foreign or injured cells, and they play a major role in providing protection against infections.

Note that there are three structural major components of the extracellular matrix of connective tissue such as fluid, ground substance consisting of non-fibrous protein and other molecules and protein fibers. The structure of the matrix gives connective tissue types most of their functional characteristics, such as the ability of bones and cartilage to bear weight, tendons and ligaments to withstand tension, and dermis of the skin to withstand punctures, abrasions, and other abuses.

c. Fiber connective tissues
Another type of connective tissues consists of fibers. Fibers are of different types including:
–– Connective tissue fibers: which are made of protein and are of three kinds:
collagenous, elastic and reticular fibers.
–– Collagenous fibers: These provide strength combined with flexibility. They are made up of collagen, probably the most abundant protein in the animal kingdom.
–– Elastic fibers: These are easily stretched but are also resilient, snapping back to their original length when tension is released. Shaped as long threads, elastic fibers are made of a protein called elastin.
–– Reticular fibers: These are thin collagen fibers coated with glycoprotein. They are very short, thin fibers that branch to form a network and appear different microscopically from other collagen fibers. Reticular fibers are not as strong as most collagen fibers, but networks of reticular fibers fill space between tissues and organs.

d. Loose connective tissue
This is also called areola connective tissue and is the most widespread connective tissue in all animal tissues. It binds epithelial tissues to underlying tissues and functions as packing material, holding organs in place. Loose Connective tissue has the following main components;

–– Fibers: collagen, elastic and reticular.
–– Cells; fibroblasts and macrophages. Fibroblasts secrete the protein ingredients of the extracellular fibers. Macrophages are cells that roam the maze of fibers, engulfing both foreign particles and the debris of dead cells by phagocytosis.

e. Fibrous connective tissue
Fibrous Connective tissue is dense with collagenous fibers. The fibers form parallel bundles, which maximize non-elastic strength. Fibrous Connective tissue is found in tendons, which attach muscles to bones, and ligaments, which connect bones at joint.

f. Adipose tissue
Adipose tissue is a specialized form of loose connective tissue that stores fats in adipose cells distributed throughout its matrix. Adipose tissue consists of adipocytes, or fat cells, which contain large amounts of lipid. Unlike other connective tissue types, adipose tissue is composed of large cells and a small amount of extracellular matrix that consists of loosely arranged collagen and reticular fibers with some scattered elastic fibers. Blood vessels form a network in the extracellular matrix. The fat cells
are usually arranged in clusters or lobules separated from one another by loose connective tissue. Adipose tissue functions as:

–– An insulator against heat loss
–– A protective tissue to delicate internal organs
–– A site of energy storage in the form of fat.

g. Bone and Cartilage tissue
Cartilage has an abundance of collagenous fibers embedded in a rubbery matrix made of a protein-carbohydrate complex called chondroitin sulfate. Cartilage is composed of specialized cells, called chondrocytes, surrounded by a gelatinous matrix of collagen, a tough protein. The cartilage surface is covered by a membrane known as the perichondrium. There are three types of cartilage (hyaline cartilage, yellow elastic and white fibrous cartilage.)

–– Hyaline cartilage is semi-transparent and is often stained light blue or pink in tissue sections. It is extremely very strong but very flexible and elastic. Hyaline cartilage occurs in the trachea, larynx, tip of the nose, connection between the ribs and the breastbone; and at the ends of bones where they form joints. It also forms much of the fetal skeleton.
–– Elastic cartilage is similar to hyaline cartilage, but in addition to the collagenous fibers.The matrix of the elastic cartilage also contains an abundant network of branched elastic fibers. This type of cartilage is found in the lobe of the ears, the epiglottis and in the parts of the larynx. They provide flexibility and elastic support.
–– Fibro-cartilage(White fibrous cartilage) is an extremely tough tissue. It is found as discs between the vertebrae, bones, anterior joint between the two halves of pelvic girdle and at points where tendons inserted on bones near hyaline cartilage. It resists compression and absorbs shock in some joints.

Bone tissue
This is a firmer and denser material that has the following features:
–– Hard and compact
–– Has many collagen fibres
–– Its matrix has inorganic salts such is calcium carbonate and calcium phosphate
–– Has few cells located in the lacunae in the matrix
–– Has osteoblasts as mature and non-dividing cells
–– Have a harversian canal
–– Consists of irregular cylinder with layer of matrix call lamellae The following are the main functions of bone tissue:
–– Structural support of the body
–– Protection of internal organs, heart and lungs.
–– Attachment of the muscles to effect movement
–– Production of blood cells

h. Blood tissue
Blood is a flowing made up of particles suspended in a fluid composed of fluid called plasma, and several kinds of cells. Within the blood plasma, there are erythrocytes (red blood cells), leukocytes (white blood cells), thrombocytes (platelets) and other substances. Blood performs the following important functions:

Transport
–– Blood transports absorbed substances such as glucose, amino acids, mineral ions and vitamins from the small intestine.
–– Blood transports the respiratory gases (Oxygen and Carbon dioxide).
–– Blood transports the excretory wastes such as urea, uric acid to excretory organs to be removed out of the body.
–– Blood transports hormones e.g. insulin from pancreas to the liver where it is stored.

Homeostasis
Na+ affects the water potential of the blood and regulates the diffusion of water between blood and tissues. Hydrogen carbonates help to maintain the pH of the blood.

Protection
–– Leucocytes such as neutrophils and macrophages engulf pathogens e.g. bacteria
–– B-lymphocytes produce antibodies to destroy pathogens or to neutralize toxins.
–– T-lymphocytes destroy infected cells.
–– Platelets, fibrinogen and prothrombin play an important role in blood clotting to reduce blood loss and the entry of pathogens.

Activity 5.2.3
You are provided with photomicrographs or slides of different plant and animal tissue. Study them carefully and answer questions that follow. Identify the different tissues provided and where they are located. One of the images is a blood smear. Draw a well labeled diagram of this tissue

5.3 Levels of organization: cell, tissue, organ and system
Activity 5.3
Visit a classroom block, administration block or any building in school which is constructed with bricks and use it to answer the following questions.
1. What is the smallest unit or component of the classroom block?
2. How are bricks arranged?
3. Do you think the brick has other smaller particles in it?
4. How many bricks does a classroom block have?
5. How are walls, classrooms, washrooms and other apartments of the block formed?
6. Arrange the following in their ascending order of size (from the smallest to the largest); whole block, wall, a brick, a room, course (a line of bricks).
7. Relate the above arrangement of a building to levels of organization in multicellular organisms, beginning with a cell and ending with an organism The human body is organized into structural and functional levels of increasing complexity. Each higher level incorporates the structures and functions of the previous level. The simplest is the cells, organized into tissues, organs, and organ systems. All of the levels of organization of the human body are represented in the following figure.

5.3.1 Cells
The smallest structural and functional living units of living things are cells. There are many different types of human cells, though they all have certain similarities. Each type of cell is made of chemicals and carries out specific chemical reactions.

5.3.2 Tissues
A tissue is a group of cells with similar structure and function. There are four groups of tissues (Epithelial tissues, Connective tissues, Muscle tissues, Nerve tissue)

5.3.3 Organs
An organ is a group of tissues precisely arranged so as to accomplish specific functions. Examples of organs are the kidneys, individual bones, the liver, lungs, and stomach. The kidneys contain several kinds of epithelial or surface tissues, for their work of absorption. The stomach is lined with epithelial tissue that secretes gastric juice for digestion. Smooth muscle tissue in the wall of the stomach contracts to mix food with gastric juice and propel it to the small intestine. Nerve tissue carries impulses that increase or decrease the contractions of the stomach.

5.3.4 Organ systems
An organ system is a group of organs that all contribute to a particular function. Examples are the urinary system, digestive system, and respiratory system. For example, the urinary system consists of the kidneys, ureters, urinary bladder, and urethra. These organs all contribute to the formation and elimination of urine.
The Human body has 11 organ systems: circulatory, digestive, endocrine, and
excretory (urinary), the lymphatic, integumentary, muscular, nervous, reproductive,
respiratory, and skeletal systems.
Table 5.4: Major organ systems of the human body

Self-assessment 5.3
1. Answer by true or false
a. Organic chemicals are often very complex and always contain the element carbon only.
b. A tissue is a group of cells with similar structure and function.
c. Integumentary organ system plays the role in protection of the human
body from injury and fluid loss.
d. An organ system is a group of organs that all contribute to a particular function.
2. Explain why the cell as level of organization of human body is said to be:
a. Basic unit of human body
b. Structural unit of human body
c. Functional unit of human body

5.4 Advantages and disadvantages of being Unicellular or Multicellular
Activity 5.4
Discuss the advantages and disadvantages of an organism being unicellular or Multicellular

5.4.1 Advantages of unicellular organisms
–– Unicellular organisms need fewer nutrients and can survive in unfavorable conditions.
–– Some of the organisms can generate energy through photosynthesis.
–– Sometimes different bacteria work together to work to their advantages.
–– Unicellular organisms can multiply quickly and have less energy/resource demands.

5.4.2. Disadvantages of unicellular organisms
Unicellular organisms only have one cell that is used to function their entire being. This is a disadvantage compared to multicellular organisms, which have many cells and function more easily and properly.

5.4.3 Advantages of a multicellular state of an organism
–– Multicellular organism usually has a wider range of functions because of the aggregation of different types of cells.
–– Multicellular organisms have many more necessities and can only survive in certain conditions.
–– Multicellular organisms such as animals are unable to make their own food so
they survive by eating living things such as vegetables, fruits, and meat. They can also eat things that are produced by other living things such as eggs, milk, and honey.

Self-assessment 5.4
1. Give the advantages and disadvantages of being Unicellular organisms.
2. Describe how unicellular organisms perform their functions.

End of unit assessment 5
1. Which type of tissue forms glands?
a. Epithelial
b. Connective
c. Nervous
d. Muscles
2. What are the four types of animal tissues?
a. Epithelial, squamous, muscular, connective
b. Epithelial, connective, muscular, cardiac
c. Connective, muscular, epithelial, nervous
d. Cuboidal, ciliated, glandular, columnar
3. Which type of the tissues form glands
a. Epithelial
b. Connective
c. Nervous
d. Muscle
4. Describe how epithelial tissues have adapted to their functions
5. Describe the three main functions of the bloo
6. Complete the following table by filling in the examples of the respective tissues: