Organisation of Cells (HSC SSCE Biology): Revision Notes

Comparing Different Types of Organisms

Introduction to organismal diversity

Living organisms on Earth display remarkable diversity in their structure and appearance. Despite these obvious differences, at the cellular level, organisms share many fundamental similarities. Each cell must obtain nutrients, carry out metabolic processes, and dispose of waste products effectively.

Three types of organismal organization

Organisms can be classified into three main categories based on their cellular organization:

1. Unicellular organisms - composed of a single cell
2. Colonial organisms - groups of identical cells working together
3. Multicellular organisms - many different specialized cells functioning as one organism

Similarities across all organism types

All three organizational types share fundamental characteristics:

• Composed of cells as their basic unit
• Cells possess cell membranes, cytosol, ribosomes, and genetic material
• Cells must carry out essential life processes

Key differences between organism types

Unicellular organisms

Unicellular organisms contain only one cell that performs all necessary life functions. This single cell can be either prokaryotic (like bacteria) or eukaryotic (like Paramecium).

Prokaryotic unicellular organisms

Prokaryotes are the oldest form of life on Earth, existing for approximately $3.5$ to $4$ billion years. These organisms consist of a single prokaryotic cell without membrane-bound organelles or a nucleus.

Key characteristics include:

• Simple cellular structure
• No specialized organelles
• Limited number of metabolic reactions occurring simultaneously
• Highly adaptable - remain the most widespread organisms on Earth

Eukaryotic unicellular organisms

Unicellular eukaryotes possess specialized organelles that allow them to carry out life processes more efficiently than prokaryotes. These membrane-bound compartments enable different metabolic reactions to occur in separate locations within the cell.

Surface area to volume ratio in unicellular organisms

Colonial organisms

Colonial organisms consist of groups of identical single-celled organisms living together as a colony. Each individual cell can carry out all functions necessary for life independently, yet the colony provides advantages through cooperation.

Volvox - a colonial algae

Volvox serves as an excellent example of colonial organization. This organism was first observed by Anton van Leeuwenhoek (1632-1723), who crafted his own microscope lenses and made precise observations of microscopic life.

Despite showing simple specialization, Volvox lacks true tissues and organs, so it is classified as a unicellular colony rather than a multicellular organism.

Choanoflagellates

These eukaryotic organisms can exist either as free-living single cells or grouped in colonies. Each cell has:

• An ovoid or spherical cell body
• A single collared flagellum

Multicellular organisms

Multicellular organisms are composed of many different types of cells. Similar cells group together to perform specialized functions that work together for the efficient operation of the entire organism.

Key characteristics of multicellular organisms

Unlike unicellular and colonial organisms, specialized cells in multicellular organisms:

• Cannot survive independently
• Depend on other cell types to perform functions they cannot
• Work together in a coordinated manner

Addressing the surface area challenge

Multicellular organisms are larger overall, giving them a smaller total SA:V. This creates a challenge because passive transport alone cannot meet all cellular needs. This problem is solved through functional organization:

Organization at the cellular level:

• Large organisms contain numerous small cells
• Each individual cell maintains its own large SA:V
• This arrangement increases the efficiency of diffusion and osmosis in each cell

Organization into tissues:

• Cells don't simply clump together randomly
• Similar cells organize into groups called tissues
• Examples: blood tissue and skin tissue in animals; photosynthetic tissue and epidermal tissue in plants

Organization into organs and systems:

• Small multicellular organisms may still rely primarily on diffusion and osmosis
• Large multicellular organisms organize tissues into organs and organ systems
• Specialized systems develop for efficient nutrient uptake (digestive system) and gas exchange (respiratory system)

Division of labour in multicellular organisms

Division of labour occurs when different cell types (tissues) become structurally suited to carry out different functions. This specialization increases overall efficiency:

• Some cells obtain nutrients
• Other tissues enable movement
• Some cells support growth
• Specialized cells handle waste excretion

Development and specialization

Young cells, called embryonic cells, begin with similar structures. In early development:

• Their primary function is cell division through mitosis
• They require protection and nutrients to grow
• As they mature, they develop structural changes enabling specialized functions
• Different cells become specialized for fighting infection, storing nutrients, processing information, secreting hormones, or providing protection

Investigation 4.1

A practical investigation to compare cells of unicellular, colonial and multicellular organisms

This practical investigation allows you to observe and compare the features of different cell types from the three organizational categories.

Aim: To observe and compare the features of different types of cells from unicellular, colonial and multicellular organisms

Materials required:

• Light microscope
• Mini-grid
• Chopping board
• Knife or scalpel
• Forceps
• Dissecting needle
• Microscope slides and coverslips
• Pipettes
• Prepared or fresh specimens:
  • Unicellular eukaryotes (Euglena, Amoeba, Paramecium)
  • Colonial organism (Volvox)
  • Plant cells (onion epidermis, Spirogyra, root hair cells)
  • Animal cells (human nerve cells or epithelial cells)
• Labelled diagram of a bacterial cell

Risk assessment

What Are the Hazards?	What Risk Does This Hazard Pose?	How Can You Safely Manage This Risk?
Knife/scalpel	Sharp edges can cause cuts	Use the knife/scalpel with care, hold it by the handle and keep your fingers away from the sharp edge of the blade.
Microscope slides/coverslips	Sharp edges can cause injury if broken	Handle with care. Push gently on the coverslip. Always focus by moving the objective lens away from slide.

Method

1. Observe one specimen from each category provided
2. Draw a labelled diagram of each specimen, including the magnification used
3. Estimate the size of each cell observed
4. Record any other relevant information about the cells

Results

Complete your observations by:

1. Labeling each diagram as either: unicellular prokaryote, unicellular eukaryote, colonial organism, or multicellular organism
2. Completing a summary table with specimen name, cell type, estimated size (in micrometers), and other relevant features

Analysis points

Consider these aspects when analyzing your observations:

• Similarities: What features do all the cells share?
• Differences: How do the cell types differ from each other?
• Complexity: How does the complexity of unicellular organisms compare with multicellular organisms?

Cell structure and specialization

In multicellular organisms, there exists a vast array of cell structures and types. The structure and arrangement of cells directly relate to their specific functions. These specialized cells work together to ensure the organism functions most efficiently.

Forming specialized cells

Differentiation is the process by which cells become specialized to perform particular functions. During differentiation, cells develop suitable structural features that allow them to carry out specific functions, making them structurally different from other cell types and from the embryonic cells from which they originated.

Stem cells - the source of specialized cells

All specialized cells originate from stem cells. These cells are:

• Undifferentiated with no specialized structure or function
• Able to divide many times over a long period
• Capable of becoming specialized into various cell types

Key terminology:

• Cell specialization refers to the particular functions that a cell performs
• Differentiation is the process a stem cell undergoes to become specialized

The differentiation process

During growth and development, cells (except sex cells) constantly divide through mitosis, producing identical copies. All cells in an organism (excluding sex cells) contain the same genetic information in their genes. However:

• Cells don't use all their genetic information
• Different cells develop because only certain genes are 'switched on'
• Which genes activate depends on the cell's location in the organism
• Activated genes control the types of proteins the cell produces
• These proteins determine the cell's structure and therefore its specialized function

Consequences of differentiation

Once differentiated into a particular cell type, specialized cells:

• Lose their capacity to develop into other cell types
• Many lose their ability to divide
• Cannot revert to a stem cell state

Plant cell differentiation

In plants, undifferentiated cells are found in meristematic tissue, located in young growing regions such as root and shoot tips. As these cells divide and mature, they differentiate into specialized cells the plant requires for effective functioning.

Cells working together

While multicellular organisms have significant advantages - including large size and the ability to carry out many varied activities efficiently - their specialized cells cannot survive independently. Each cell type relies on others to perform functions it cannot do itself.

Communication and coordination

Efficient functioning requires well-developed communication and coordination between specialized cells:

In animals:

• Chemical secretions relay messages
• Nerve cells transmit information rapidly throughout the body
• Blood cells bind and transport oxygen
• Heart muscle cells pump blood
• Various cells provide nutrients and remove wastes

In plants:

Relationship between cell structure and function

The structure of a cell directly relates to its specific function. Many examples demonstrate this fundamental biological principle.

Cells involved in substance exchange

Parts of the body involved in exchanging substances with the environment have special structural features to increase their SA:V, enabling more efficient exchange of materials.

Structural modifications include:

1. Flattened cells - such as the squamous epithelium lining air sacs (alveoli) in lungs. This flat shape provides greater SA:V than cube-shaped cells, allowing efficient gas exchange.
2. Elongated cells - such as palisade cells in plant leaves. These long, column-shaped cells have increased SA:V for efficient photosynthesis and gas exchange.
3. Membrane extensions - such as:
   • Microvilli - tiny folds on cells lining the small intestine, dramatically increasing surface area for nutrient absorption
   • Root hairs - extensions of root epidermal cells that increase surface area for water and mineral absorption

Red blood cells - a detailed example

Red blood cells provide an excellent example of how structure relates to function. Their function is to transport oxygen throughout the body in the blood.

Remember!