2.3 - Living Processes in Multicellular Organisms

Multicellular organisms contain various cells that differ in size, shape and arrangement. Each type of cell has a specialised structure that is related to its specific function.

Specialised Cells in Humans

Epithelial Cell

Structure: Thin and flat.
Function: Covers organ surfaces such as the digestive tract.

Muscle Cell

Structure: Arranged as multinuclear striated fibres.
Function: Contracts and relaxes to generate movement.

Nerve Cell

Structure: Long and thin.
Function: Sends nerve impulses.

Red Blood Cell

Structure: Has no nucleus and is shaped as a biconcave disc.
Function: Optimises the transport of oxygen.

White Blood Cell

Structure: Can change shape.
Function: Destroys pathogens.

Sperm Cell

Structure: Has a long tail that enables it to swim towards the ovum in the Fallopian tube. Its head carries a set of chromosomes from the male.
Function: Fertilises the ovum.

Specialised Cells in Plants

Palisade Mesophyll Cell

Structure: Long cylindrical cells arranged vertically and close together; contains a high density of chloroplasts.
Function: Maximises the absorption of sunlight for photosynthesis.

Spongy Mesophyll Cell

Structure: Loosely arranged with large air spaces between cells.
Function: The large air spaces allow gaseous exchange from the inside of the leaf to the palisade mesophyll cells.

Guard Cell

Structure: Modified epidermal cell with a cell wall that is thicker on the inner side.
Function: Controls the opening and closing of the stoma. The stoma allows the exchange of oxygen and carbon dioxide.

Root Hair Cell

Structure: Has a long projection that increases surface area.
Function: Absorbs water and mineral salts.

Xylem Vessel

Structure: A long, hollow vessel joined from end to end.
Function: Transports water and mineral salts from the roots to all parts of the plant.

Sieve Tube Element

Structure: A long cylindrical tube arranged from end to end.
Function: Transports organic substances from the leaves to storage organs such as fruits.

Tissue Organisation in Humans

A tissue is a group of cells that have the same structure and function and are arranged together to carry out a specific function. Human tissues are classified as epithelial tissue, muscle tissue, nerve tissue and connective tissue.

Epithelial Tissue

Covers the outer surface of the body and hollow surfaces such as the digestive tract and respiratory tract.
Epithelial tissue on the skin protects against infections, injuries, chemicals and dehydration.
Epithelial tissue lining the trachea has hair-like projections called cilia.

Muscle Tissue

Smooth muscle is found in the digestive tract, blood vessels, urinary tract and reproductive tract. Its contraction and relaxation enable involuntary activities such as peristalsis.
Skeletal muscle is found in the legs and arms. It contracts and relaxes in controlled movement to move bones and limbs.
Cardiac muscle forms the heart wall and contracts involuntarily to pump blood throughout the body.

Nerve Tissue

Made of neurons. Each neuron has a cell body and nerve fibres called dendrites and an axon.
Detects stimuli and sends electrical signals called nerve impulses to muscles or glands.
Regulates and controls body activities.

Connective Tissue

Loose connective tissue links epithelial tissue to the tissue below it and keeps organs in position.
Fibrous connective tissue forms tendons, which connect bones to muscles, and ligaments, which connect bones to bones.
Cartilage encloses bone ends and prevents bones from wearing out.
Bone forms the body frame and protects internal organs.
Adipose tissue stores fat beneath the skin dermis and on the surface of major organs.
Blood tissue functions in regulation, transportation and protection.

Tissue Organisation in Plants

Plant tissues are organised into meristem tissue and permanent tissue. Permanent tissue consists of dermal tissue, ground tissue and vascular tissue.

Meristem Tissue

Includes apical meristem tissue and lateral meristem tissue.

Permanent Tissue

Dermal tissue forms the outer covering of plants.
Parenchyma tissue stores starch, protein and water and can carry out photosynthesis.
Collenchyma tissue supports young, non-woody stems of herbaceous plants.
Sclerenchyma tissue provides support and mechanical strength to mature parts of plants.
Xylem tissue transports water and mineral salts from the roots to other parts of the plant. Its lignified wall provides support and mechanical strength.
Phloem tissue transports organic substances such as sucrose from the leaves to all parts of the plant.

Density of Certain Cell Components and Specialised Cell Functions

The density of a cell component is related to the specific function of the cell.

Sperm cell: A high density of mitochondria supplies energy to swim towards the uterus and Fallopian tube to fertilise the secondary oocyte.
Flight muscle cell in insects and birds: A high density of mitochondria supplies energy for contraction and relaxation during movement and flight.
Plant meristem cell: A high density of mitochondria supplies energy for active cell division to produce new cells.
Palisade mesophyll cell and spongy mesophyll cell: A high density of chloroplasts absorbs more sunlight for photosynthesis.
Pancreatic cell: A high density of rough endoplasmic reticulum and Golgi apparatus increases the synthesis and secretion of digestive enzymes.
Goblet cell in the intestinal epithelium and respiratory tract: A high density of rough endoplasmic reticulum and Golgi apparatus produces mucus.
Liver cell: A high density of smooth endoplasmic reticulum and Golgi apparatus carries out carbohydrate metabolism and detoxification of drugs and poisons.

Effects of Deficiency, Absence or Failure of Cell Components

Failure or dysfunction of mitochondria can cause stunted growth, weak muscles, and hearing and vision problems.
Tay-Sachs disease is a hereditary disease caused by the failure of enzyme production in lysosomes. It causes stunted growth and mental retardation.