Chapter 1
Cells and Basic Tissues

Summary

In this chapter, the learning outcomes are:

• To understand the essential functions required to sustain life – MRS GREN
• To be able to identify the structure and function of animal cells and tissues
• To be able to recognize the diversity of animal cells and tissues in existence

What is biology?

Biology is the study of life and living organisms

What is life?

In order to be considered as a living organism, an organism must be able to perform all the following essential functions of life:

• Movement – the organism is capable of moving itself or a part of itself.
• Reproduction – the organism is capable of reproducing itself so that the species doesn’t die out.
• Sensitivity – to stimuli in its surroundings in order to avoid life-threatening occurrences in the environment.
• Growth – sustained growth from within by a process which involves the intake of new materials from the outside and their incorporation into the internal structure of the organism.
• Release of energy from respiration – in a controlled manner and in a form usable by the organism. The process of respiration releases energy from food to sustain life.
• Excretion – the removal of the waste products of metabolism from the organism.
• Nutrition – taking in food materials which provide energy to maintain life and growth.

The cell is the simplest functional unit of all tissues and has the ability to perform individually all the essential life functions. Organisms may be single-celled or multi-celled. Within the multicellular organisms, the constituent cells show a wide range of specialisations. Cells can be viewed as the building blocks of the body, and so the following can be said:

• Cells form…
• Tissues, and tissues form…
• Organs, and organs… join together to form systems within the body.
• Systems have a specific function to perform in the living organism.

The diversity of cells

Cells are not all identical (Fig. 1.1) but all have the same basic structure. Each component of a cell is known as an organelle:

Fig. 1.1 Diversity of cells from their basic form.

• Cell membrane – the surrounding membrane of the cell which encloses the cytoplasm and is only 0.00001 mm in thickness. The cell membrane is the outer boundary that controls all exchanges between the cell and its surrounding environment. The cell membrane allows certain chemicals to pass in and out of the cell either by diffusion, osmosis or active transport. The cell membrane is described as being selectively permeable.
• Nucleus – acts as the cell’s brain and controls the cell’s activities and there is usually only one nucleus in the cell. The nucleus also contains the chromosomes.
• Cytoplasm – a jelly-like material that supports the organelles within the cell. It contains enzymes and many other chemicals that aid cell metabolism.
• Chromosomes – rod-shaped components that contain the hereditary information of the organism. Chromosomes contain deoxyribonucleic acid (DNA) which controls the characteristics that an organism inherits from its parents.
• Mitochondria – the energy-producing organelles where cell respiration takes place.
• Endoplasmic reticulum (ER) – is a series of tubules acting as a transport and packaging system. ER may be rough ER or smooth ER. Rough ER has ribosomes attached to it where proteins are synthesized. This protein can be used by the cell to synthesize enzymes and hormones. Smooth ER has no ribosomes and is used to synthesize and transport lipids (fats) and steroids made within the body.
• Ribosomes – build proteins within the cell which are then joined to form amino acids which are essential to growth. Ribosomes contain ribonucleic acid (RNA).
• Centrosome – an area found near the nucleus and made up of two centrioles. Centrioles are important during cell division and the formation of the cilia and flagella of certain cells (the slender projecting hairs for movement of single-celled organisms). Centrioles can only be seen during cell division; otherwise, a dark area known as the centrosome is observed.
• Lysosomes – are dark round bodies containing enzymes responsible for splitting complex chemical compounds into simpler ones (known as lysis, meaning ‘to break up’) followed by digestion. They also destroy worn-out organelles within the cell. These destructive enzymes are packaged in an area of the cell called the Golgi complex or Golgi body.
• Peroxisomes – are similar to lysosomes but they contain a different type of enzyme that breaks down toxic materials in the cell. Peroxisomes are good at breaking down fatty acids, alcohol and hydrogen peroxide made during digestion.

The organelles listed earlier are common to virtually all cells, but the shape, form and contents of individual cells show much variation. The structural characteristics of a particular cell are closely related to its functions (Fig. 1.2).

Fig. 1.2 Basic cell structure.

Types of cell found in the animal body

• Epithelial cells – have a shape and form that make them most suitable for lining the surface of the body and the organs and cavities within it.
• Glandular cells – are responsible for producing some kind of secretion, for example, mucus, to lubricate between tissues.
• Osteoblasts – produce bone tissue.
• Erythrocytes (red blood cells) – have a shape designed to hold the red pigment haemoglobin which conveys oxygen around the body. In order to do this, they are one of the few cells in the body which no longer contain a nucleus.
• Nerve cells – or neurones have slender armlike processes which will transmit electrical impulses through the nervous system to reach the whole body.
• Muscle cells – are also capable of electrical activity accompanied by a muscle contraction for body movement.
• Sperm cells – are the male sex cells. They have a tail for swimming and only contain half the amount of chromosomes.
• Ova – are the female sex cells. They contain only half the amount of chromosomes of other cells in the body.

Cells

No matter what the type of cell found in an organism, cells have needs that must be met in order to survive:

• Food for energy
• Water (body fluid) to hydrate the cells
• Oxygen to all cells
• A suitable temperature in which to live

Animal Tissues

Tissues are a collection of cells and their products which have a common fundamental function and in which one particular type of cell predominates:

• Epithelial tissue – forms a protective layer both inside and on the surface of the body. Examples of this tissue are the skin, glands and linings of the various body systems.
• Connective tissue – supports body tissues and acts as a transport system to move materials vital to tissue cells around the body. Examples of this tissue are:
  • loose connective tissue which surrounds organs
  • dense connective tissue which has great strength and is found as tendons and ligaments
  • blood which transports essential nutrients, gases, waste products, hormones and enzymes to and from all body cells
  • cartilage and bone which provide shape and protection for organs and allow movement.
• Muscular tissue – are concerned with movement of the skeleton, the organ systems and the heart.
• Nervous tissue – is concerned with transporting messages to tissues and connecting the body as a whole for the required response.

Epithelial tissue

This tissue covers all surfaces of the body, both inside and out, whether it is a surface, a cavity or a tube. It is made up of a diverse group of tissues which are involved in a wide range of activities such as secretion of a special fluid, protection and absorption.

Depending on their function, the cells of this tissue will have varied shape, structure and thickness. Epithelial tissues are classified according to appearance:

• Number of layers – a single layer of these cells is called simple epithelium; more than one layer is called stratified epithelium.
• Shape of the cells involved.
• Specialisations, such as tiny hairs called cilia or special thickened surface tissue called keratin, which covers the nose and pads of the feet.
• Glandular – means that it is involved in secretion. Secretions which go directly into the bloodstream are called hormones and are produced by glands of the endocrine or ductless system. Some secretions are produced by glands that have ducts. The secretion is released through the duct onto...