Chapter 28 - Being Multicellular

Question 1

What is a multicellular organism?

Answer 1

Organisms whose cells are specialized for specific function, so as a whole the organism can perform a broad range of tasks (as opposed to single cells)

Question 2

When did multicellular organisms first arise?

Answer 2

more than 3 billion years after microorganisms first evolved.

Question 3

True or False: Bacteria and Eukarya have both developed distinct types of tissues.

Answer 3

False. No bacteria have developed macroscopic bodies with differentiated tissues and cells - only eukaryotes have.

Question 4

What is simple multicellularity? What are some examples?

Answer 4

-single cells joined by adhesive molecules, but little communication or transfer between cells
-little cell differentiation
-most retain a full range of function including reproduction
-every cell is in direct contact with the external environment during phases acquiring nutrients
Example: Algae, filamentous fungi, stalklike colonies, slime molds

Question 5

What is a coenocytic organism? What are some examples?

Answer 5

It’s an organism in which the nucleus divides multiple times, but the nuclei are not partitioned into individual cells. Result is large cells, sometimes visible to the naked eye, that have many nuclei.
Examples: organisms on the sea floor

Question 6

Why did simple multicellular organisms evolve?

Answer 6

• Protection against prey
• Advantageous position in environment (physically, like on top of water or anchored to sea floor)
• Assistance in feeding by direction food toward cells

Question 7

What are some important characteristics of complex multicellular organisms?

Answer 7

• only small subset of all cells contributes to reproduction
• cell or tissue loss can be fatal for entire organism
• 3D organization - only some cells have direct contact with environment
• exterior and interior cells
• interior cells don’t receive signals from environment so there must be mechanism for transferring external signals for gene repression or activation

Question 8

How many times did complex multicellularity evolve?

Answer 8

At least 6 separate occasions- twice in fungi, once in green algae, once in red algae, once in brown algae.

Question 9

What is an essential transportation mechanism for complex multicellular organisms?

Answer 9

Diffusion is important, but can only be utilized effectively over short distances or large surface area to low distance. Bulk transport is essential - it is the means by which molecules move through organisms at rates beyond those possible by diffusion across a concentration gradient.

Question 10

What are some examples of bulk transport?

Answer 10

In animals: Oxygen hitches a ride on hemoglobin in red blood cells and is carried through the vascular system to distant sites of respiration. Circulatory system carries nutrients in and waste out.
In plants: vascular tissues transport water upward and downward throughout the plant to provide nutrients

Question 11

True or False: Without bulk transport, animals could possible still be the same size, shape, and function we have today

Answer 11

False. When some cells within an organism are buried deep within tissues with no access to the external environment, bulk transport is required to supply those cells with molecules needed for metabolism.

Question 12

What are some other complex multicellular organisms that rely on bulk transport?

Answer 12

Trees, fungi (filaments)

Question 13

What are 3 general requirements for complex multicellularity?

Answer 13

cell adhesion; cell signaling and communication; genetic interactions supporting cell division and differentiation.

Question 14

What are some cell adhesion mechanisms in animals?

Answer 14

transmembrane proteins/cadherins- cell to cell in epithelia

integrins - cell to matrix attachment

Question 15

What are some cell adhesion mechanisms in plants?

Answer 15

Polysacccharides called pectins

Question 16

How did cell adhesion develop? How was this determined?

Answer 16

The closest protistan relative of animals, Choanoflagellates, are unicellular, but their genes can code for same proteins that promote cell adhesion in animals. They can differentiate to tether themselves to substrates, prey, etc. Supports hypothesis that cell adhesion in animals resulted from redeployment of protein families that evolved to perform other functions before animals diverged from protistans.

Question 17

What are plant and animal cell signaling based off of?

Answer 17

Receptor kinases

Question 18

What sources are cell signals derived from?

Answer 18

prey, environmental conditions, sexual reproduction

Question 19

What kinds of animals have gap junctions, and what function do they serve?

Answer 19

Animals more complex than sponges, and gap junctions are channels that allow ions and signaling molecules to move from one cell to another- targeted communication. Represent important step in complex multicellularity

Question 20

Do plants have gap junctions?

Answer 20

No. Plants have similar mechanisms called plasmodesmata which are thin strands of cytoplasm that extend from one cell to another and permit signaling molecules to pass through.

Question 21

What is the cause of development?

Answer 21

Molecular communication between cells

Question 22

What was the purpose of the innovation of complex multicellularity?

Answer 22

To differentiate cells in space instead of time. Spatial differentiation of cells in multicellular organisms began with the redeployment of genes that regulate cell differentiation in single-celled ancestors. Example: Volvox

Question 23

How does bulk transport play a role in cell differentiation?

Answer 23

It participates in cell signaling, for example releasing hormones into the bloodstream and regulating development in the body

Question 24

True or False: Complex multicellularity evolved from a common multicellular ancestor

Answer 24

False: Complex multicellularity evolved independently in two groups - plants and animals

Question 25

Why do plants and animals differ in cell signaling and differentiation?

Answer 25

Plants have rigid, unmoving cell walls while animal cells can move

Question 26

What is the basic progression of plant cell differentiation and what are some reasons for it?

Answer 26

Basis: cell walls of plants are rigid and cannot engulf particles, and cannot move since they have no pseudopodia and flagella

• plant growth is confined to meristems - populations of actively dividing cells at the tips of stems and roots.
• first cells undergo mitosis at meristem, then begin to expand at a location above that, and then respond to cell signaling that induce differentiation

Question 27

What is the basic outline of plant cell differentiation?

Answer 27

cell division, cell adhesion from meristems, absorption or nutrients and water supplied by adjacent cells, differentation to govern plants as a whole.

Question 28

What is the basic mechanism of animal cell differentiation?

Answer 28

1. fertilized eggs undergo mitosis, forming ball of undifferentiated cells - blastula
2. blastula cells migrate and fold inwards to form hollow gastrula
3. gastrula brings cells into new contact and initiate signaling and gene regulation to form growth and tissue differentiation
4. organs and tissues formed unlike in plant cells through migration and active movement
5. form organism that can actively move and respond to environmental stressors unlike plants.

Question 29

True or False: All 3 ways of the evolution of multicellular organisms can be acquired at random times

Answer 29

False. They must be acquired in a specific order - cells need to stick together first, then communicate, then differentiate

Question 30

What is the evolutionary history of complex multicellularity?

Answer 30

1800 mya - single-celled protists and some multicellular forms
575-555 mya - complex multicellular organisms (found in fossils in Newfoundland)
560-555 mya - more complex animals with distinct head and tail

Question 31

What did the early complex multicellular organisms look like?

Answer 31

frondlike form made up of tubular structures, developed under guidance of regulatory genes in modern sponges, jellyfish

Question 32

What is the main hypothesis for why complex multicellularity did not appear until much later than simple multicellular organisms?

Answer 32

The critical requirement for complex multicellular function - presence of abundant oxygen.
Abundant oxygen only reached required levels about 580-560 mya, which is when complex multicellular organisms came about. The theory is that complex organisms could finally develop an differentiate.

Question 33

How did land plants evolve from water-bound plants?

Answer 33

They originated about 460 mya after marine animals and algae. Ancestral green algae evolved molecular cell-cell adhesion and cell communication. Then they developed plasmodesmata. By 400 mya plants were adapted for bulk transport of water and spread.

Question 34

What is the genetic basis for range of sizes and shapes displayed by complex multicellular organisms?

Answer 34

Gene regulation

Question 35

What is evo-devo?

Answer 35

Evolutionary-developmental biology, which compares genetic programs for growth and development in species found on different branches of phylogenetic trees.