Organisation of Cells
Organisation of cells
Inquiry Question
How are cells arranged in a multicellular organism?
Comparing Unicellular, Colonial and Multicellular Organisms
Investigating structures at the level of the cell and organelle.
Relating structure of cells and cell specialisation to function.
Compare - Estimate, measure or note how things are similar or different.
Organism Types
Organisms can exist as:
Single cell (unicellular).
Single cells working together (colonial).
Organism made up of many cells (multicellular).
Similarities between each of these types of organisms include:
They are all composed of cells.
These cells all possess cell membranes, cytosol, ribosomes and some sort of genetic material.
Differences occur in the number of cells present in the organism, the types of cells and the organisation of the cells.
Unicellular Organisms
Consist of a single cell.
Mostly prokaryotes (and some eukaryotes).
One cell carries out all the functions to sustain life.
Functions are carried out within the cell.
Microscopic size, surface area to volume ratio limits size.
Short lifespan due to energetically expensive workload for one cell.
Mostly asexual, clonal reproduction.
Whole organism is involved in reproduction.
Colonial Organisms
Many cells.
Eukaryotes.
Individual animals (e.g. zooids) work together to perform functions to sustain the colony.
Functions are carried out by individuals (zooids) with specific roles within the colony.
Usually macroscopic.
Long lifespan, as work and energy costs are shared by cells within the colony.
Mostly asexual, clonal reproduction; sexual reproduction is present in some species.
Usually specific zooids are responsible for reproduction.
Multicellular Organisms
Many cells.
Eukaryotes.
Cells are specialised to perform specific functions required by the organism.
Functions are carried out at cellular, tissue, organ and system level (some simple multicellular organisms function only at the cell or tissue level).
Macroscopic size; increasing the number of cells allows increased body size.
Long lifespan, as work is efficiently divided between specialised cells.
Mostly sexual reproduction.
Only cells specialised for reproduction will reproduce (gametes).
Unicellular Organisms Details
Unicellular organisms consist of a single cell that carries out all functions necessary for survival and reproduction.
Example: Paramecium, bacterial cell.
Colonial Organisms Details
A colonial organism is a special form of multicellular organism that consists of many individuals living together.
There are two types of colonies: facultative colonies and obligate colonies.
Facultative colonies are usually independent organisms that come together to form complex social structures to increase the chance of survival.
Obligate colonies consist of individuals called zooids that vary in form and carry out specific functions for the organism to survive.
Multicellular Organisms Requirements
For an organism to be considered truly multicellular:
It must have multiple cells.
Its non-reproductive cells must have identical DNA.
Its cells must be connected and must communicate and cooperate to function as a single organism.
It must have different cells that are specialised to carry out specific functions, one of which must be reproduction.
Its cells must be dependent on each other for survival.
Evolution of Multicellularity
Multicellularity is thought to have evolved several times.
The colonial theory is the most likely explanation for the evolution of multicellular organisms.
Despite the success of prokaryotes, multicellular organisms continue to evolve and thrive, suggesting that being multicellular with specialised cells has advantages for survival, reproduction and evolution.
Advantages of Multicellularity
Cell specialisation is energy efficient.
Longer lifespans.
Increased genetic diversity through sexual reproduction and genetic recombination.
Genetic diversity allows populations to adapt to changing environments.
Organisms are less vulnerable to short-term environmental changes.
Increased size and mobility helps organisms find ideal conditions and avoid predators and negative stimuli.
Organisms can perform more complex functions.
Disadvantages of Multicellularity
More energy is required for survival and reproduction.
Cells cannot function independently.
It takes longer for populations to evolve and adapt.
Advantages of Multicellularity Explained
Multicellularity is energy efficient because specialised cells do not waste energy trying to complete all the functions necessary for life.
Multicellular organisms have longer lifespans than unicellular organisms because they are more energetically efficient.
Sexual reproduction and genetic recombination increases genetic diversity over generations compared with asexual, clonal reproduction in unicellular organisms. Genetic diversity allows populations to adapt to changing environments.
Multicellular organisms are less vulnerable to short-term changes in their environment. They have more systems to cope with change, and cell death does not necessarily affect the survival of the organism.
Multicellular organisms can grow significantly larger than unicellular organisms. Unicellular organisms must be small to obtain nutrients and remove waste efficiently by diffusion.
Increased size and specialisation of limbs means multicellular organisms are more mobile and therefore more efficient at locating resources and avoiding predators and other negative stimuli.
Multicellular organisms can perform more functions than unicellular organisms.
Disadvantages of Multicellularity Explained
Having more cells means more energy is required for survival.
The cells cannot function independently; they are dependent on the whole organism for survival.
More energy is required for reproduction; most animals need to find a mate to reproduce, and most plants need to produce and disperse gametes.
Populations of multicellular organisms take much longer to evolve and adapt to long-term changes in their environment, because they have much longer generation times than unicellular organisms.
Autotrophic Cells
Diagram shows autotrophic cells including Cyanobacteria, Chlamydomonas (unicellular green algae), and Photosynthetic plant cell.
Prokaryotes vs Eukaryotes and Unicellular vs Colonial vs Multicellular
The diagram shows the difference between prokaryotes (cyanobacteria) and eukaryotes (Chlamydomonas, plant, Synura).
Prokaryotes do not have membrane bound organelles and photoautotrophs have bacteriochlorophylls on lamellae (infoldings of the cell membrane) for photosynthesis while eukaryotes have chloroplasts which are membrane bound organelles that carry out photosynthesis.
The diagram also shows differences in structures between unicellular, colonial and multicellular organisms.
The unicellular eukaryote Chlamydomonas has flagella for locomotion and a light sensitive spot so it can move towards the light.
Synura forms colonies and has silica scales lying over each of its two chloroplasts.
The photosynthetic plant cell is part of a plant organ (leaf) that is specialised for photosynthesis and the conversion of light energy into chemical energy in organic compounds.
Locomotion and Movement
Compare locomotion and movement in unicellular, colonial and multicellular organisms.
Locomotion Details
Locomotion is the ability to move from one place to another.
Unicellular organisms can move in various ways, e.g. beating cilia, flagella or by pseudopods.
In colonial organisms movement is usually related to the mechanisms used by the individuals that make up the colony.
Multicellular organisms have developed many means of locomotion with movement involving the whole body or some part of the body.
Movement in animals is usually related to finding food, finding a mate or escaping from predators.
Many plant movements are slow, e.g. leaves turning towards the Sun.
Choanoflagellates
Choanoflagellates are thought to be an important link in the evolution of multicellular organisms.
Cell Specialisation
Relating structure of cells and cell specialisation to function
Cell Differentiation and Specialisation
Define cell differentiation and specialisation.
Cell Specialisation and Differentiation Defined
Cell specialisation refers to the particular functions that a cell has.
Cell differentiation is the process that a stem cell goes through to become specialised.
When organisms are growing and developing, their cells, with the exception of the sex cells(sperm and ova), are constantly dividing by the process of mitosis.
In this process of division, identical copies of the original cell are produced.
All cells in the organism, excluding the sex cells, contain the same genetic information in their genes.
The cell does not use all of this information; different cells develop as a result of only certain parts of this genetic information being switched on.
The genetic information that is activated will depend on the location of the undifferentiated cells in the body of the organism.
Genes and Cell Specialisation
For example, cells in the outer layers become skin cells, cells beneath that become muscle cells.
The genes that are activated control the types of proteins produced by the cell.
This in turn determines the particular structure of the cell and therefore its specialised function.
Once they have become specialised to form a particular type of cell, differentiated cells lose their capacity to develop into other types of cells.
Many even lose their ability to divide and give rise to the same type of cells.
Cell Differentiation Process
Differentiation is a process in which cells become more specialised as they mature.
Includes Ectoderm, Mesoderm, Endoderm, Germ cells etc.
Types of Tissues
Summarise the different types of tissues found in mammalian systems.
Epithelial Tissue
Covers many surfaces and linings of body cavities.
Endothelia lines internal cavities.
Acts as a protective layer and as a barrier against infection by microbes or water loss.
Has no direct blood supply.
Cells can have microvilli to increase surface area.
Connective Tissue
Found in many areas of the body and includes bone, cartilage, blood, tendons and ligaments.
A large proportion of connective tissue is an intercellular matrix that contains a network of protein fibres in a semi-liquid ground substance.
Cartilage has no direct blood supply and cells are not in layers with cell shape irregular to round.
Muscle Tissue
Is specialised to convert chemical energy in ATP into mechanical energy for movement.
Smooth muscle forms the walls of many internal cavities.
Cardiac muscle is in the heart.
Striated muscle is connected to bones directly or via a tendon for voluntary motion.
Cells are linked in sheets or elongated bundles.
Nervous Tissue
Consists of neurons and glial cells.
Neurons can transmit a nervous impulse from the dendrite to another neuron or an effector.
Neurons are concentrated in the brain and spinal cord.
Plant Tissues - Cell Differentiation
Includes Phloem cells, Root hair cell, Xylem tissue, Leaf epidermis with guard cells which are products of differentiation of Plant meristematic tissue
Plant Tissues - Types
Summarise the three main types of plant tissues.
Dermal Tissue
Forms the outer protective coating of plant organs.
Example: Epidermis of root and epidermis of leaf.
Vascular Tissue
Transport of materials throughout the plant.
Xylem transports water up the plant from the roots to the leaves.
Phloem translocates sugars.
Ground Tissue
Makes up the remainder of the plant.
Pith is internal to the vascular tissue and cortex is external to the vascular tissue.
Need for Specialised Cells
Explain why multicellular organisms need specialised cells.
Diffusion Limitations
In a multicellular organism each cell must be supplied with oxygen and nutrients and wastes need to be removed.
The process of diffusion is too slow and inefficient to meet the requirements of each cell.
Thus a multicellular organism needs specialised cells with division of labour so particular tasks can be carried out more efficiently and each cell is supplied with oxygen and nutrients and wastes removed.
Stem Cells and Specialised Cells
Stem cells are undifferentiated cells with no specialised structure or function.
Specialised cells are formed when differentiation of the stem cells occurs and they develop suitable structural features that allow them to carry out their specific functions.
The type of cell that is formed is determined by the location of the undifferentiated cells in the organism and the particular genes that are switched on.
Stem cells can be either embryonic or adult stem cells.
In plants, specialised cells are formed by the differentiation of meristematic tissue.
Hierarchical Structure
Justify the hierarchical structural organisation of organelles, cells, tissues, organs, systems and organisms.
Hierarchical Structural Organisation
Organelles: Mitochondria, nucleus
Cells: Cardiac muscle cells
Tissues: Cardiac muscle tissue
Organs: Heart
Organ Systems: Cardiovascular system
Organism: Human
Structural Organisation Flow
Organelles (membrane-bound structures that have specific roles in the cell)
Cells (basic structural and functional unit of living organisms)
Tissues (cells that perform similar functions)
Organs (different tissues grouped together to perform a specific function)
Organ system (organs grouped together to carry out particular function)
Organism (a living thing made up of many interrelated components that work together)