BIOL 1020U Lecture Notes - Lecture 7: Brown Algae, Cell Adhesion, Red Algae
Being Multicellular Lecture
Chapter 28
Textbook Reference: Chapter 28 in Biology How Life Works (2nd ed.)
Sections 28-1 to 28-5; pp. 578-592)
Simple Multicellularity
Most organisms represented in the 119 branches of the seven super-kingdoms
are unicellular
Only 36 branches show some level of multicellularity such as:
1. Filaments
2. Hollow balls
3. Sheets of little differentiated cells
•Properties of multicellular organisms include:
1. Adhesion molecules – cause adjacent cells to stick together – little
communication or transfer between cells – little differentiated of
specialized cell types each cell can do what it needs to survive but is
still stuck to its neighbor
2. Most cells retain full range of external functions including reproduction
special cells that produce tissues used for reproduction – each cell
still reproducing itself
3. Every cell in contact with external environment can sense signals
from environment – humans have internal cells that are not directly in
contact with the external environment – but their external cells can
sense env.
What is coenocytic organization?
2 algal groups, 3 protozoan groups and 1 fungal group – show coenocytic organization
Multiple nuclear division w/o cytokinesis
Very large cells w/ multiple nuclei result
Why Did Simple Multicellularity Evolve?
Possible Selective Advantages:
oPredator avoidance: helps organisms avoid protozoan predators
Example: Green algae/predatory protist -
1
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oMaintain position: ability to maintain position on surface or in water better
than single-celled relatives
oCo-ordination of movements: for different functions
Example: ciliary movements – efficient for feeding
Complex Multicellularity
Complex multicellularity evolved at least six
separate times in different eukaryotic
groups:
oAnimals
oVascular plants
oRed algae
oBrown algae
oTwice in fungi
Evolved from simple multicellular organisms
most taxonomic groups did not give rise to
complex descendants
List a few types of complex multicellular
organisms: land plants, animals, fungi
Complex multicellular vs. simple multicellular
characteristics:
Complex multicellular Simple multicellular
Highly developed mechanisms for cell
adhesion
Cell adhesion
Specialized structures for cell communication N/A
Complex tissue and organ differentiation
(regulatory genes)
N/A
Small subset of cells contribute to reproduction Cells retain full range of function
*presence of both interior and exterior cells; 3d
organization
Every cell in contact with external
environment
Constraints on Cellular Size, Shape and Function
Relying on diffusion for key metabolic processes tends to keep size of organisms
small
oWhy?
Exerts strong constraint on size, shape, and function of cells – diffusion
happens very slowly – does not get to internal cells of complex organisms
fast enough
Constrains how eukaryotic cells function oxygen
•Most complex multicellular organisms can achieve large size by getting around
the constraints of diffusion
•How?
2
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Document Summary
Textbook reference: chapter 28 in biology how life works (2nd ed. ) Most organisms represented in the 119 branches of the seven super-kingdoms are unicellular. 2 algal groups, 3 protozoan groups and 1 fungal group show coenocytic organization. Possible selective advantages: predator avoidance: helps organisms avoid protozoan predators. 1: maintain position: ability to maintain position on surface or in water better than single-celled relatives, co-ordination of movements: for different functions. Example: ciliary movements efficient for feeding. Complex multicellularity evolved at least six separate times in different eukaryotic groups: animals, vascular plants, red algae, brown algae, twice in fungi. Evolved from simple multicellular organisms most taxonomic groups did not give rise to complex descendants. List a few types of complex multicellular organisms: land plants, animals, fungi. Small subset of cells contribute to reproduction cells retain full range of function. *presence of both interior and exterior cells; 3d organization.
