Lecture 20 - Multicellularity: Origins and Concepts PDF

Preview

Summary

Lecture notes from Module 5 - Multicellularity. Covers
Multicellularity: Origins and Concepts
• Building the plant body
• Transport in plants
• Building the animal body
• Circulation in animals
• Cell communication & homeostasis...

Description

BIOL10008 Introductory Biology: Life’s Machinery Module – Multicellularity Name: Dr John Golz School of BioSciences Office: Building 184, Room 320 Email: [email protected] Web: https://blogs.unimelb.edu.au/golzlab/

Arabidopsis seed

Marchantia

My research interests: -

Investigating genetic pathways controlling seed development

-

Evolution of plant development Developing plant transformation procedures for agronomically important crops

BIOL10008

Chia Module – Multicellularity

Single-celled vs multicellular organisms

Bacteria

Multicellular algae Plants

Archaea

Fungi Protists (e.g. paramecium)

Animals Images from Wikimedia Commons

BIOL10008

Module – Multicellularity

Multicellularity Module outline

• Multicellularity: Origins and Concepts • Building the plant body • Transport in plants • Building the animal body • Circulation in animals • Cell communication & homeostasis

BIOL10008

Module – Multicellularity

Multicellularity: Origins and Concepts

BIOL10008

Module – Multicellularity

This lecture will look at these big ideas: Evolution Evolution of multicellularity has increased the diversity of lifeforms

Cells Cells differentiate and have specialized functions in multicellular organisms Cell fate is established during during embryogenesis leading to tissues and organ formation

Information Information is transmitted directly between cells as well as between tissues and organs Signalling is either chemical or electrical in nature

Regulation Multicellular organisms must regulate their internal environment

BIOL10008

Module – Multicellularity

End of introduction Go to Part I

BIOL10008

Module – Multicellularity

Part I Evolution and Consequences of Multicellularity

BIOL10008

Module – Multicellularity

Origins of multicellularity Simplified evolutionary tree showing the relationship between various groups of organisms Fossil evidence for eukaryote emergence but not divergence of eukaryotes Molecular clock provides an estimate of the divergence time of eukaryotes

BACTERIA chloroplast

PLANTS

mitochondria

FUNGI eukaryotes

ANIMALS ARCHAEA

2.1 Bya Eukaryotes emerged

Molecular clock: technique used to estimate the time when two or more life forms diverged and is based on the mutations rate of DNA

1.6 Bya

Last common ancestor of plant and animals is likely to have been unicellular

Common ancestor of animals and plants

BIOL10008

Module – Multicellularity

Origins of multicellularity Combining molecular clock data with fossil record suggests multicellularity arose at least 6 times

Cnidaria Anomalocar

Animals

Placozoans Basidiomycetes/ Sponges Ascomycetes Fungi

First multicellular algal fossils (~600Mya – Lantian Biota)

Land plants

Cambrian explosion! (~540Mya – Burgess Shale)

Red algae Slime moulds

Choanoflagellates Green algae Brown algae

First multicellular animal fossils (~600Mya – Ediacaran fauna)

BIOL10008

Single celled common ancestor

Oomycetes Fungi

1.6 Bya

Module – Multicellularity

Diversity of multicellular organisms Brown algae

Placozoans

Simple multicellular organisms that attain large sizes

Simple multicellular organisms that divide by fission

Dictyostelium discoideum - cellular slime mold

Single-cell organisms that aggregate to form a colony with distinct cell types Sadava 11e Fig. 26.18, 10e Fig. 27.18

https://www.youtube.com/watch?v=nQQJNIJbzd8 Developmental Biology Film Series Preservations Project – Lynn Margulis

BIOL10008

Module – Multicellularity

The transition to multicellularity Probably occurred in several steps:

Volvocine green algae Chlamydomonas

BIOL10008

Pandorina

Eudorina

Pleodorina

Volvox

Sadava 11e Fig. 7.17

Module – Multicellularity

The transition to multicellularity Probably occurred in several steps: •

Aggregation of cells into a cluster

Volvocine green algae Chlamydomonas

Pandorina

Eudorina

Single-celled

16-celled cluster

Larger cluster of cells

BIOL10008

Pleodorina

Volvox

Sadava 11e Fig. 7.17

Module – Multicellularity

The transition to multicellularity Probably occurred in several steps: •

Aggregation of cells into a cluster

•

Intercellular communication within the cluster

•

Specialization of some cells within the cluster (cooperation)

Volvocine green algae Chlamydomonas

Pandorina

Eudorina

Pleodorina

Single-celled

16-celled cluster

Larger cluster of cells

Large cluster of cells with somatic and reproductive cells

BIOL10008

Volvox

Sadava 11e Fig. 7.17

Module – Multicellularity

The transition to multicellularity Probably occurred in several steps: •

Aggregation of cells into a cluster

•

Intercellular communication within the cluster

•

Specialization of some cells within the cluster (cooperation)

•

Organization of specialized cells into groups (tissues)

Transition to multicellularity also results in individuals cells losing the ability to live independently

Volvocine green algae Chlamydomonas

Pandorina

Eudorina

Pleodorina

Single-celled

16-celled cluster

Larger cluster of cells

Large cluster of cells with somatic and reproductive cells

BIOL10008

Sadava 11e Fig. 7.17

Volvox

Even larger cluster of cells with somatic and reproductive cells organized into tissues

Module – Multicellularity

What are the consequences of multicellularity?

Sadava 11e Fig. 7.17

Chlamydomonas Single cell that switches between two cellular phases: - Swimming phase (via flagellum) - Non-swimming phase – associated with cell division

BIOL10008

Volvox Multicellular colony with two cell types (no phase switching) - Outer cells have coordinated flagella movement - Outer cells create an inner space to protect reproductive cells - Big inner cells specialized for reproduction

Module – Multicellularity

What are the consequences of multicellularity for Volvox?

Copepod (2mm)

Chlamydomonas (0.03mm)

Volvox (0.5mm)

Size • Becoming larger shifts the position of Volvox in the ecological system Functional specialization • Cells perform dedicated tasks - e.g. reproduction or motility • Cells work in unison e.g. beating of flagella -> efficient phototaxis

Multicellularity enables cells to dedicate their energy to one task rather than multiple tasks – increased efficiency BIOL10008

Module – Multicellularity

Cell specialization enabled the evolution of multicellular organisms • Cell specialization allows cells to adopt new functions • Integration and co-operation between cells allowing for the development of tissues and organs • Structurally and functionally complex bodies • Creation of a stable internal environment • Increase in size • More efficient gathering of resources and adapting to specific environments

BIOL10008

Module – Multicellularity

End of Part I Complete canvas recap activity before going on to Part II Learning outcomes: Ø Provide an argument suggesting multicellularity arose independently in plants and animals Ø Propose possible steps leading to multicellularity Ø Describe the importance of cell specialization for multicellular organism

BIOL10008

Module – Multicellularity

Part II Building a Multicellular Body

BIOL10008

Module – Multicellularity

Development in animals and plants Embryogenesis – Formation of a multicellular organisms from a zygote

Fertilised egg

gastrula stage

The final body is comprised of many types of specialized cells

Fertilised egg

heart stage

During embryogenesis there are multiple rounds of cell division producing specific cell types in precise patterns along major spatial axes Organization of cells is NOT RANDOM – cell types arranged according to a body plan BIOL10008

Module – Multicellularity

Cell fate establishment •

Embryogenesis can be thought of as a tree of cell divisions

zygote BIOL10008

Module – Multicellularity

Cell fate establishment • • •

Embryogenesis can be thought of as a tree of cell divisions Establishment of cell fate (determination) arises during the early stages of embryogenesis Cells become increasingly more restricted in their fate – change in cell potency

zygote BIOL10008

Determination Cells begin to express different genes

Module – Multicellularity

Cell fate establishment • • • •

Embryogenesis can be thought of as a tree of cell divisions Establishment of cell fate (determination) arises during the early stages of embryogenesis Cells become increasingly more restricted in their fate – change in cell potency Pattern of cell fate is highly ordered and reflects the position of cells in the developing embryo – instructive cues (cytoplasmic factors/cell signalling molecules) Instructive information

zygote BIOL10008

Determination Cells begin to express different genes

Highly ordered pattern of cell fate established

Module – Multicellularity

Morphogenesis – sculpting the body Morphogenesis – process by which cells and tissues organize and arrange themselves to create the final form of the body Division Changing shape (expansion) Moving (not seen in plant embryogenesis) Adhering to one another (not seen in plant embryogenesis) Death (apoptosis)

Example of morphogenesis in the sea urchin

BIOL10008

Module – Multicellularity

Cell differentiation and gene expression Cell differentiation ultimately determines the properties of a cell and is associated with the subset of genes that are active A certain set of genes is expressed in one cell type, but not another (cell-specific genes) – differential gene expression Therefore, the specification of cell fate involves gene regulation epidermal cells

Note: Some essential genes, also called house-keeping genes, are expressed in every cell (e.g. tubulin, histones)

House-keeping Expressed in all cells

Cell specific Expressed in some cells

BIOL10008

Module – Multicellularity

Animal body plans Body plan = General structure of an organism, arrangement of organ systems, integrated functioning of its parts Body plans can be categorized according to symmetry, body cavity structure, segmentation, type of appendages, and type of nervous system Radial symmetry

Bilateral symmetry - bilaterians

Dorsal

Anterior Central axis

Posterior Ventral Sadava 11e Fig. 30.5

Any plane along the central body axis divides the animal into similar halves

BIOL10008

A single plane through the anterior-posterior midline divides the animal into mirror-image halves

Module – Multicellularity

Plant body plan Body plan is modular Aerial structures (shoot)- subterranean structures (roots) have a modular arrangement of organs (phytomers/rhizomers) Plants have a radial arrangement of tissue types

Phytomer Shoot system

Radial

Root system Seedling (immature plant)

BIOL10008

Module – Multicellularity

Post-embryonic growth patterns in animal and plants Determinate growth

Structures arising during embryogenesis

Predetermined body form Increase in size

All organ and tissue types formed

young

old Indeterminate growth Flexible body form Increase in size

Few organ and tissue types formed

BIOL10008

young

old Continuous organ and tissue formation through activity of meristems

Module – Multicellularity

End of Part II Complete canvas recap activity before going on to Part III Learning outcomes: Ø List the key processes occurring during embryogenesis Ø Describe differences in animal and plant development

BIOL10008

Module – Multicellularity

Part III Requirements of Multicellularity

BIOL10008

Module – Multicellularity

Exchange and its relationship to volume Cells must exchange substances with the external environment (O2, CO2, waste, nutrients) Contact with environment (opportunity for exchange) is related to surface area

Length (n) Surface area Volume

(6 x

n2)

(n3)

Surface-area : Volume ratio

1 cm 6 cm2

2 cm 24 cm2

3 cm 54 cm2

1 cm3 6:1

8 cm3 3:1

27 cm3 2:1

1

2

3

As the volume of an object increases – the surface area to volume ratio decreases Consequence –> Less of the interior is exposed to the exterior

BIOL10008

Module – Multicellularity

Multicellular animals face limitations in exchange

nutrients

O2

O2

Problem 1. Surface area to volume ratio of a multicellular organism is small

nutrients

Problem 2. Distance of internal cells to external environment is large

waste CO2

Not drawn to scale

Large surface area to volume ratio

BIOL10008

CO2

waste Small surface area to volume ratio

Consequence: Diffusion with the external environment cannot meet the exchange needs of multicellular organisms Module – Multicellularity

Exchange organs of multicellular animals Human lung

Axolotl gills

Features of exchange organs: • Large surface area - long, flat, folded and branched • Thin surface with small diffusion distances Ensures maximal rate of exchange

See later lectures on gaseous exchange and digestion

BIOL10008

Module – Multicellularity

Multicellular organisms have an internal environment Cells meet their exchange needs through exchange with an internal aqueous environment (extracellular fluid) A barrier is needed to create an internal environment The internal environment is kept stable by homeostasis O2

Nutrients

O2

Nutrients

Barrier

waste

CO2

waste

CO2 Internal environment

Rapid exchanges with the extracellular fluid

BIOL10008

Module – Multicellularity

A circulatory system solves the diffusion limit bu diffusion O2

CO2

O2 CO2

b ul

CO2

k flo w diffusion CO2

O2

lk

flo

w • Movement of extracellular fluids around the body to ensure exchanged substances from exchange organs reach cells of the body (bulk flow) • Maintains high concentration gradients for diffusion

Ensures optimal:

bulk flow

BIOL10008

gaseous exchange, nutrient mobilisation waste removal, intercellular communication

Module – Multicellularity

Large multicellular organisms require complex transport systems O2

To maintain a high level of metabolism large multicellular organism:

CO2

Highly branched internal transport systems

H2O

H2O carbohydrates etc

Rapid movement of exchange substances

Considerable force required to move fluids through these transport systems Active vs passive processes

H2O and dissolved minerals

Sadava, 10e, Fig. 50.2

Circulatory systems of animal involve active mechanisms - pumps

BIOL10008

Sadava, 10e, Fig. 35.1

Transport systems of plants involve passive processes

Module – Multicellularity

Cells in multicellular organisms need to communicate A key requirement for multicellularity is the use of intercellular signalling to coordinate cellular activities and respond to the internal and external environment: Conveying positional information during development Maintain an stable internal environment (homeostasis) Ensuring cells work in unison (beating of the Volvox flagella) Physical/chemical signals arising from the external environment

Sadava 10e Fig. 45.18

BIOL10008

Module – Multicellularity

Intercellular communication can be chemical or electrical Chemical signals

Chemical signals can activate receptors on nearby cells (e.g. ligands)

local

systemic

Or secreted into the bloodstream and activate cells throughout the body (e.g. hormones)

Electrical signals can be passed long distances (1m) very rapidly via neurons to very specific targets

BIOL10008

Module – Multicellularity

Lecture Summary • Multicellularity arose independently in plants and animals several times • Multicellularity enables an organism to increase in size, maintain an internal environment, have specialized cells, form specialized structures and occupy new ecological niches • The body plan of multicellular organism is mapped out during embryogenesis • Formation of tissues and organs of a multicellular organism involves cell specialization and differential gene expression • Multicellular life is associated with the formation of a stable internal environment (homeostasis) • Extensive intercellular signaling and transport systems are also a characteristic feature of multicellular life

BIOL10008

Module – Multicellularity

End of Part III Complete canvas recap activity before going on to the next lecture Learning outcomes: Ø Explain why multicellular organisms have diffusion limitations Ø Describe ways in which multicellular organisms have optimize exchange Ø Propose reasons why intercellular communication is important in multicellular organisms

BIOL10008

Module – Multicellularity...