From Cells to Systems (VCE SSCE Biology): Revision Notes

From Cells to Systems

Introduction to biological organisation

Complex living things like plants and animals are made up of many specialised cells working together. These cells cannot survive on their own - they must cooperate with other cells to meet the needs of the whole organism. This cooperation is achieved through organisation into increasingly complex structures.

Key definitions:

• Organism: A living thing made up of one or more cells
• Cell: The smallest functional unit of a living organism
• Specialised cells: Cells which serve a unique, particular function
• Tissue: A cluster of cells which perform a shared function
• Organ: The combination of tissues and cells into a distinct structure that performs a specific function
• System: A collection of organs and tissues that perform specific functions necessary for survival

Levels of biological organization

Complex multicellular organisms arrange their cells into four levels of increasing complexity:

1. Cells - Individual specialised cells
2. Tissues - Groups of similar cells working together
3. Organs - Multiple tissues combined into recognisable structures
4. Systems - Collections of organs working together for vital functions

This hierarchical arrangement allows organisms to carry out complex processes that individual cells cannot perform alone. Each cell requires nutrients, oxygen, waste removal, and stable conditions (temperature, pH). Meeting these needs becomes increasingly challenging as organisms grow larger and more complex, requiring greater coordination between cells.

Why organisms need this organization

As organisms increase in size and complexity, individual cells can no longer directly exchange materials with the environment. Greater cooperation and coordination becomes essential for survival. Some simple organisms (like jellyfish and corals) only require tissues for their survival needs, but more complex organisms need the full hierarchy of organisation.

Organisation of plants

Plant cells are organised into specialised tissues, distinct organs, and two major systems that allow them to survive and reproduce.

Plant cells

Vascular plants are plants that contain vascular tissue responsible for transporting water and minerals throughout the plant. Non-vascular plants (such as moss) lack this tissue and only require simpler tissues to function.

The major types of vascular plant cells include:

Cell type	Function
Parenchyma cells	The major cells of plants, responsible for photosynthesis and other metabolic activity
Sclerenchyma cells	Provide support to the plant
Collenchyma cells	Provide support to the plant
Xylem cells	Responsible for the transport of water and minerals from the roots to the leaves
Phloem cells	Responsible for the transport of sugars and other nutrients throughout the plant

Key definitions:

• Vascular tissues: Conducting tissues that transport water and nutrients throughout a plant. An encompassing term for xylem tissue and phloem tissue

Plant tissues

Plant cells combine to form three main types of tissue:

1. Dermal tissue: A single layer of cells covering the outside of the plant. It secretes a waxy film called a cuticle that forms a physical barrier between the plant and its environment, reducing water loss and physical damage.
2. Ground tissues: Make up the majority of the plant's interior and carry out metabolic functions like photosynthesis and storage.
3. Vascular tissues: Run through the ground tissue, carrying water and nutrients throughout the plant.

Plant organs

Plants have five major organs, each performing specific functions:

Organ	Function
Leaves	Sites of gas exchange and responsible for photosynthesis. Usually organised to increase sunlight exposure
Flowers	The sexual reproductive organs of flowering plants (angiosperms). After fertilisation of male and female gametes in pollen, seeds develop and the ovary grows into a fruit
Fruits	Grown from a flower after fertilisation. Protect seeds and are often specialised to attract animals that help with seed dispersal
Stems	Support the leaves, flowers, and fruits, as well as transport water and nutrients between roots and shoots
Roots	Absorb and store water and nutrients from the soil. Also anchor the plant to the ground and provide structural support

Plant systems

Vascular plants have two major organ systems:

1. Root system: Typically underground. Absorbs water and nutrients from the soil and provides the plant with support and structure.
2. Shoot system: Made up of:
   • Reproductive sections: Flowers and fruit
   • Non-reproductive sections: Leaves and stems

The shoot system is the above-ground portion of the plant responsible for photosynthesis, reproduction, and connecting different parts of the plant.

Organisation of animals

Animal cells combine to form four major tissue types, which are organised into organs and ultimately into complex systems.

Animal cells

Complex animals contain hundreds of different specialised cell types, each responsible for specific functions. Common examples include skin cells, muscle cells, blood cells, nerve cells, and fat cells.

Animal tissues

In animals, cells are grouped into four main tissue types:

Tissue type	Function
Muscle tissue	Contracts to exert a force. The three major types are skeletal, cardiac, and smooth muscle tissue
Nervous tissue	Detects stimuli and transmits electrical signals. Composed of neurons that detect stimuli and carry signals, plus glia that assist neuron signal transmission
Connective tissue	Connects and supports other tissues and organ structures of the body
Epithelial tissue	Forms the external and internal layers of the body. These tissues assist in protection, secretion, and absorption

Types of muscle tissue

There are three distinct types of muscle tissue in animals:

1. Skeletal muscle tissue:
   • Attached to bones via connective tissues called tendons
   • Under voluntary (conscious) control
   • Contracts to assist movement
2. Cardiac muscle tissue:
   • Found entirely in the heart
   • Contracts to pump blood around the body
   • Under involuntary (unconscious) control
3. Smooth muscle tissue:

   • Found in the linings of hollow organs and structures (stomach, intestines, blood vessels)

   • Contracts slowly to change the shape and size of these structures

   • Under involuntary control

     

Key definitions:

• Muscle tissue: Collection of animal cells that are capable of contraction. Includes skeletal, cardiac, and smooth muscle
• Nervous tissue: Collection of animal cells that sense stimuli and initiate responses. Also known as nerve tissue
• Connective tissue: Collection of many different animal cells that bind and support the other major tissue types
• Epithelial tissue: One of the basic tissue types in animals that line the outer surface of organs and blood vessels

Animal organs

Animal organs are easily recognisable structures inside the body that perform specialised tasks. Major human organs include:

• Brain
• Heart
• Lungs
• Liver
• Stomach
• Pancreas
• Kidneys
• Intestines
• Bladder

Animal systems

Complex animals have multiple systems that perform vital tasks essential for survival. The 11 major systems in humans are:

The 11 major human body systems

System	Function	Example organs/tissues
Digestive system	Breaks down and processes food to be used by the body. Eliminates food waste that is not digested via egestion	Stomach, liver, pancreas, small intestine, large intestine
Excretory system	Removes waste substances from the blood via excretion. Also assists in controlling water balance in the body	Kidneys, bladder, lungs
Endocrine system	Responsible for the production and secretion of hormones which control and regulate bodily processes	Hypothalamus, pituitary gland, thyroid gland, thymus, pancreas, adrenal glands
Skeletal system	Provides an internal structure to support the body. Works together with the muscular system to enable movement	Bones, joints, cartilage
Muscular system	Responsible for contractions in skeletal muscle, the heart, and other contractions throughout the body. Works together with the skeletal system to enable movement	Skeletal muscles, cardiac muscles, tendons
Integumentary system	Protects internal body structures from the external environment	Skin, hair, nails, subcutaneous fat
Nervous system	Detects and processes sensory information to activate responses in the body	Brain, spinal cord, peripheral nerves
Immune system	Defends the body against infection. Contains the lymphatic system which transports lymph fluid around the body	Bone marrow, spleen, thymus, tonsils, lymph nodes
Respiratory system	Responsible for removing carbon dioxide from the body and delivering oxygen to the blood via respiration	Lungs, nasal passage, trachea
Blood circulatory system	Delivers oxygen and nutrients to the tissues of the body via blood	Heart, blood vessels (arteries, veins, and capillaries)
Reproductive system	In males, regulates the production of sperm and certain hormones. In females, regulates the production of egg cells, certain hormones, ovulation, and nurturing offspring during development. In both sexes, coordinates action for reproduction	Testes, penis, ovaries, vagina, uterus, mammary glands

Key definitions:

• Hormone: A signalling molecule released from endocrine glands that regulates the growth or activity of target cells

Gas exchange in animals

All animals need to absorb oxygen for aerobic cellular respiration and expel carbon dioxide to prevent toxic buildup. This process is known as respiration and is typically carried out by the respiratory and blood circulatory systems.

Very simple animals (such as sponges, coral, and flatworms) can exchange gases directly with the environment. They do not require a respiratory system for gas exchange or a blood circulatory system to transport gases throughout their body.

Complex animals are often too large to passively exchange gases with the environment, and therefore require more advanced respiratory and blood circulatory systems:

• Humans: The lungs carry out gas exchange, whilst the trachea carries gas between the lungs and the environment.
• Insects: Air enters through tiny holes called spiracles, which attach to a network of internal tubes called trachea. This network, combined with an insect's open blood circulatory system, allows oxygen to reach all cells of the body.
• Fish: Fish have extremely efficient gas exchange systems to compensate for the low oxygen content of water (about 1% O₂ compared to 21% in air). Their gills, located on the sides of their throat, are made of gill arches composed of specialised rows of filaments, which are made of tightly packed lamellae. Water taken in through the mouth is pushed out between the gill arches and lamellae. The high surface area of the lamellae allows oxygen and carbon dioxide to quickly diffuse in and out of the gills. Oxygen is then transported around the body via the blood circulatory system.

Comparing plant and animal organization

Both plants and animals follow the same hierarchical pattern of organization, but with different specific structures:

Plants: Cell → Tissue (e.g. ground tissue) → Organ (e.g. leaf) → System (e.g. shoot system) → Organism

Animals: Cell → Tissue (e.g. nerve tissue) → Organ (e.g. brain) → System (e.g. nervous system) → Organism

This organisation allows both plants and animals to coordinate sophisticated functions that individual cells cannot perform alone. With increased organisation, cells, tissues, organs, and systems are able to perform specific and vital body functions necessary for the survival of the whole organism.

Remember!