P age |1

5 Structures and Functions of Animal

Tissues and Cell Modifications
Module 5

Objective:
At the end of this lesson, students should be able to:

 classify different cell types (plant/animal tissue) and specify the functions of each
 describe some cell modifications that lead to adaptation to carry out specialized functions
(e.g., microvilli, root hair)

5.1 Introduction
As discussed in Module 2, all organisms are made up of cell. In living systems of
biology, cells are fundamental as atoms are to chemistry. Right now, many different types of
cells are working for you. Muscle cell contraction makes your eyes move as you read this
sentence. The nerve cells carry the translated signals of this page to your brain by passing them
to other nerve cells. You make cell connections like these that solidify memories and permit
learning to occur as you study. The following are different types of plant and animal cells.

Parenchyma cells (Figure 5.1) perform most of the

metabolic functions of a plant, synthesizing and storing
various organic products. As an example, within the
chloroplasts of parenchyma cells in the leaf, photosynthesis
occurs. The fleshy tissue of many fruits is composed mainly
of parenchyma cells. Colorless plastids of some in stems and
roots store starch. And most of parenchyma cells keep the
ability to divide and differentiate into other types of plant
Figure 5.1 Parenchyma Cells
cells under particular conditions – for example, during wound
repair. A single parenchyma cell could even grow an entire plant from it.

Collenchyma cells (Figure 5.2) are grouped in

strands and help support young parts of the plant shoot.
These have thicker primary walls than parenchyma cells and
are generally elongated. These cells provide flexible support
without restraining growth.

Figure 5.2 Collenchyma Cells

 P age |2

Sclerenchyma cells (Figure 5.3) also function as

supporting elements in the plant, although they are much
more rigid than collenchyma cells. These cells are so
specialized for support that many are dead at functional
maturity, but they produce secondary walls before the
protoplast (the living part of the cell) dies. In some cases for
hundreds of years, the rigid walls remain as a “skeleton” that
supports the plant. Sclereids and fibers are two types of
sclerenchyma cells, and are specialized entirely for support
Figure 5.3 Sclerenchyma Cells and strengthening.

Tracheids and vessel elements (Figure 5.4) are the

two types of water – conducting cells of xylem. These are
tubular, elongated cells that are dead at functional
maturity.

Tracheids are long, thin cells with tapered ends. The water
can move from cell to cell mainly through the pits, where
it does not have to cross thick secondary walls.

Vessel elements are generally wider, shorter, thinner

walled, and less tapered than tracheids. These form long
pipes which are called vessels by aligning end to end.
Vessels are visible with the naked eye, in some cases.
Water can flow freely through the vessels since the end
walls of vessel elements have perforation plates.
Figure 5.4 Tracheids and Vessel
The secondary walls of these two are hardened
Elements of Xylem
with lignin. This provides support and prevents collapse
under the tension of water transport.

Sugar – conducting cells of the phloem are

alive at functional maturity unlike the water –
conducting cells of xylem. Narrow cells called sieve
cells are used to transport sugar and other organic
nutrients in seedless vascular plants and
gymnosperms. Sieve tubes consist of chains of cells
that are called sieve – tube elements. Sieve plates
are end walls between sieve – tube elements, which
has pores facilitating the flow of fluid from cell to
cell along the sieve tube. A nonconducting cell
called companion cell, is connected to the sieve –
Figure 5.5 Sieve – tube Elements, tube element. In some plants, the companion cells in
Sieve plates, Companion Cell of leaves also help load sugars into the sieve-tube
Phloem elements, which then transport the sugars to other
parts of the plant.
 P age |3

Epithelial Tissue, or epithelia, (singular, epithelium), as

shown in Figure 5.6, occur as sheets of cells, covering the outside
of the body and line organs and cavities within the body.
Epithelial cells are closely packed, often with tight junctions,
hence, they function as a barrier against mechanical injury,
pathogens, and fluid loss. These also form active interfaces with
the environment. For example, the epithelium that lines the nasal
passages is crucial for olfaction, the sense of smell. Different cell
shapes and arrangements correlate with distinct functions.

Figure 5.6 Epithelial Tissue

Connective tissue (Figure 5.7) consisting of a sparse population of cells scattered

through an extracellular matrix, holds many tissues and organs together and in place. The matrix
generally consists of a web of fibers embedded in a liquid, jellylike, or solid foundation. Within
the matrix are numerous cells called fibroblasts, which secrete fiber proteins, and macrophages,
which engulf foreign particles and any cell debris. Connective tissue fibers are of three kinds:
Collagenous fibers provide strength and flexibility, reticular fibers join connective tissue to
adjacent tissues, and elastic fibers make tissues elastic. If you pinch a fold of tissue on the back
of your hand, the collagenous and reticular fibers prevent the skin from being pulled far from the
bone, whereas the elastic fibers restore the skin to its original shape when you release your grip.

Figure 5.7 Connective Tissue

The muscle tissue (Figure 5.8) is responsible for nearly all types of body movement. All
muscle cells consist of filaments containing the proteins actin and myosin, which together enable
muscles to contract. Skeletal, sooth, and cardiac muscles are the three types of muscle tissue in
the vertebrate body.

Figure 5.8 Muscle Tissue

 P age |4

Nervous tissue functions in the receipt, processing, and transmission of information.

Nervous tissue contains neurons, or nerve cells, which transmit nerve impulses, as well as
support cells called glial cells, or simply glia. In many animals, a concentration of nervous tissue
forms a brain, the information – processing center.

Figure 5.9 Nervous Tissue

The cells are the simplest collection of matter that can live in the hierarchy of biological
organization. Plant and animal bodies are cooperatives of many kinds of specialized cells that
could not survive for long on their own. In this module, we will focus our discussion in the
structure and function of animal tissues.

5.2 Hierarchical Organization of Body Plans

Cells form a working animal body through their emergent properties, which arise from
successive levels of structural and functional organizations.

Emergent properties are novel properties at each level of organization that are absent
from the preceding level. These are due to the arrangement and interactions of parts as
complexity increases. For example, although photosynthesis occurs in an intact chloroplast, it
will not take place in a disorganized test-tube mixture of chlorophyll and other chloroplast
molecules. The coordinated processes of photo-synthesis require a specific organization of these
molecules in the chloroplast. Isolated components of living systems, serving as the objects of
study in a reductionist approach to biology, lack a number of significant properties that emerge at
higher levels of organization.

Emergent properties are not unique to life. A box of bicycle parts will not transport you
anywhere, unless arranged in a certain way, enabling you to pedal to your chosen destination.
However, compared to nonliving examples, biological systems are far more complex, making the
emergent properties of life especially challenging to study.

Cells are organized into tissues, groups of cells with a similar appearance and a common
function. Different types of tissues are further organized into functional units called organs.
Groups of organs work together, and provide an additional level of organization and
coordination, that is, the organ system. And these systems make up a multicellular organism,
as shown in Figure 5.10.

Figure 5.10 Structural Organizations of Animals

 P age |5

Just as viewing the body’s organization from cells to multicellular organisms reveals emergent
properties. The specialized and complex organ systems of animals are built from a limited set of
cell and tissue types. For example, lungs and blood vessels have different functions but are lined
by tissues that are of the same basic type and that therefore share many properties.

Structure and Function

 At each level of the biological hierarchy, we find a correlation of structure and function.
Consider a leaf that is thin, flat – shaped. This maximizes the capture of sunlight by
chloroplasts.
 More generally, analyzing a biological structure provides us clues about what it does and
how it works.
 Conversely, knowing the function of something provides insight into its structure and
organization.
 Many examples from the animal kingdom show a
correlation between structure and function.
 For example, the hummingbird’s anatomy allows
the wings to rotate at the shoulder, so
hummingbirds have the ability that is unique
among birds, to fly backward or hover in place.
The birds can also extend their long, slender beaks
into flowers and feed on nectar while hovering, as
shown in the Figure 5.11. Figure 5.11 A Humming Bird

Can you identify other examples of plants or animal organisms having unique
structures for a specific function?

5.3 Exploring the Structure and Function of Animal Tissues

An animal’s size and shape are fundamental aspects of form that significantly affect the
way the animal interacts with its environment. Although we may refer to size and shape as
elements of a “body plan” or “design,” this does not imply a process of conscious invention. The
body plan of an animal is the result of a pattern of development programmed by the genome,
itself the product of millions of years of evolution.

The specialized and complex organ systems of animals are built from a limited set of cell
and tissue types. For example, lungs and blood vessels have different functions but are lined by
tissues that are of the same basic type and that therefore share many properties. There are four
main types of animal tissues: epithelial, connective, muscle, and nervous.

Epithelial Tissue
This type of tissue is commonly seen outside the body as coverings or as linings of
organs and cavities. Epithelial tissues are characterized by closely – joined cells with tight
junctions (i.e., a type of cell modification). Being tightly packed, tight junctions serve as barriers
for pathogens, mechanical injuries, and fluid loss.
 P age |6

Cells that make up epithelial tissues can have distinct arrangements, shown in Figure 5.12:

 cuboidal—for secretion
 simple columnar—brick-shaped cells; for secretion and active absorption
 simple squamous—plate-like cells; for exchange of material through diffusion
 stratified squamous—multilayered and regenerates quickly; for protection
 pseudo – stratified columnar—single layer of cells; may just look stacked because of
varying height; for lining of respiratory tract; usually lined with cilia (i.e., a type of cell
modification that sweeps the mucus).

Figure 5.12 Epithelial Tissue

A clearer description of each distinctive arrangement can be read in Campbell Biology,

th
10 Edition, Chapter 40.

Polarity of epithelia

All epithelia are polarized,

meaning that they have two different sides.
The apical surface faces the lumen (cavity)
or outside of the organ and is therefore
exposed to fluid or air. Specialized
projections often cover this surface. For
example, the apical surface of the
epithelium lining the small intestine is
covered with microvilli, projections that Figure 5.13 Small Intestine covered with Microvilli
increase the surface area available for
 P age |7

absorbing nutrients. The opposite side of each epithelium is the basal surface, shown in Figure
5.13.

Connective Tissue
These tissues are composed of the following:

 BLOOD —made up of plasma (i.e., liquid extracellular matrix); contains water, salts, and
dissolved proteins; erythrocytes that carry oxygen (RBC), leukocytes for defense (WBC),
and platelets for blood clotting.
 CONNECTIVE TISSUE PROPER (CTP)—made up of loose connective tissue that is
found in the skin and fibrous connective tissue that is made up of collagenous fibers
found in tendons and ligaments. Adipose tissues are also examples of loose connective
tissues that store fats which functions to insulate the body and store energy.
 CARTILAGE —characterized by collagenous fibers embedded in chondroitin sulfate.
Chondrocytes are the cells that secrete collagen and chondroitin sulfate. Cartilage
functions as cushion between bones.
 BONE —mineralized connective tissue made by bone-forming cells called osteoblasts
which deposit collagen. The matrix of collagen is combined with calcium, magnesium,
and phosphate ions to make the bone hard. Blood vessels and nerves are found at a
central canal surrounded by concentric circles of osteons.

Figure 5.14 illustrates these connective tissues.

Figure 5.14 Connective Tissue

A clearer description of each connective tissue can be observed in Campbell Biology,

th
10 Edition, Chapter 40.
 P age |8

Muscle Tissue
These tissues (Figure 5.15) are composed
of long cells called muscle fibers that allow the
body to move voluntary or involuntary.
Movement of muscles is a response to signals
coming from nerve cells. In vertebrates, these
muscles can be categorized into the following:

 skeletal—striated; voluntary movements

 cardiac—striated with intercalated disk
for synchronized heart contraction;
involuntary
 smooth—not striated; involuntary
Figure 5.15 Muscle Tissue

Nervous Tissue
These tissues are composed of nerve cells called neurons and glial cells that function as
support cells. These neurons sense stimuli and transmit electrical signals throughout the animal
body. Neurons connect to other neurons to send signals. The dendrite is the part of the neuron
that receives impulses from other neurons while the axon is the part where the impulse is
transmitted to other neurons. Nervous tissues can be described as shown in Figure 5.9.

Figure 5.9 Nervous Tissue

 P age |9

References
(1) Teaching Guide for Senior High School General Biology 1 Specialized Subject
Academic – STEM, 2013, Commission on Higher Education (CHED)

(2) Campbell Essential Biology, 4th Edition, 2010, Simon, Eric J.; Reece, Jane B.;
Dickey, Jean L.

(3) Campbell Biology, 10th Edition, Copyright 2014, Pearson Education, Inc., Jane B.
Reece, Lisa A. Urry, Michael L. Cain, et. Al.