hlw_ch28

CHAPTER SUMMARY

28.1 COMPLEX MULTICELLULARITY AROSE SEVERAL TIMES IN EVOLUTION.

Bacteria are unicellular or form simple multicellular structures.
Eukaryotes are unicellular or multicellular and exhibit both simple and complex multicellularity.
Simple multicellularity involves the adhesion of cells with little cell differentiation. Complex multicellularity involves cell adhesion, cell signaling, and differentiation and specialization among cells.
Complex multicellular organisms have evolved independently at least six times: in animals, vascular plants, red algae, brown algae (the kelps), and at least twice in the fungi.

28.2 IN COMPLEX MULTICELLULAR ORGANISMS, BULK TRANSPORT OF MOLECULES CIRCUMVENTS THE LIMITATIONS OF DIFFUSION.

Diffusion is the random motion of molecules, with net movement occurring from regions of higher to regions of lower concentration. It generally acts over small distances, placing limits on the size of multicellular organisms.
Bulk transport is an active process that allows multicellular organisms to nourish cells located far from the external environment, thereby circumventing the constraints imposed by diffusion.

28.3 COMPLEX MULTICELLULARITY DEPENDS ON CELL ADHESION, COMMUNICATION, AND A GENETIC PROGRAM FOR DEVELOPMENT.

Animal and plant cells are organized into tissues characterized by specific molecular attachments between cells.
Choanoflagellates express some of the same proteins that permit cell adhesion in animals, even though choanoflagellates are unicellular. Experiments suggest that choanoflagellates use these proteins to capture bacteria, not for cell adhesion.
Gap junctions in animals and plasmodesmata in plants allow cells to communicate with each other in a targeted fashion.
Complex multicellularity involves the genetic programming of cells so that they differentiate in space.
A number of gene families known to play developmental roles in multicellular organisms also occur in their unicellular relatives, where at least some of them play a role in life cycle differentiation.

28.4 PLANTS AND ANIMALS EVOLVED MULTICELLULARITY INDEPENDENTLY OF EACH OTHER AND SOLVED SIMILAR PROBLEMS WITH DIFFERENT SETS OF GENES.

The cell wall characteristic of plant cells provides structural and mechanical support, but does not allow plant cells to move.
The cell wall of plants has led to distinct solutions to the problems of cell adhesion, cell communication, and development. For example, plants grow by the activity of meristems, populations of actively dividing cells at the tips of stems and roots.
Animal cells do not have cell walls, allowing cell movement that is not possible in plants. For example, during animal development, embryos undergo gastrulation, a process in which cells migrate inward to form a layered structure called a gastrula.

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28.5 THE EVOLUTION OF LARGE AND COMPLEX MULTICELLULAR ORGANISMS, WHICH REQUIRED ABUNDANT OXYGEN, IS RECORDED IN THE FOSSIL RECORD.

Oxygen may be required for the evolution of complex multicellularity because of its chemical properties and its abundance.
The evolution of complex multicellularity in the fossil record correlates with increases in atmospheric oxygen that occurred 580-560 million years ago.
Complex multicellular organisms evolved later on land, as ancestral plants evolved the capacity to photosynthesize surrounded by air rather than water.
Evolutionary-developmental biology, or evo-devo, is a field of research that looks at both individual development and evolutionary patterns in an attempt to understand the developmental changes that allowed organisms to diversify and adapt to changing environments.

Self-Assessment Question 1

Describe differences between simple and complex multicellularity.

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Model Answer:

In simple multicellularity, adhesive molecules cause adjacent cells to stick together, but there is relatively little communication or transfer of resources between cells. There is also little differentiation of cell types; most of the cells in simple multicellular organisms retain a full range of functions. In simple multicellular organisms every cell is in direct contact with the external environment during phases of the life cycle when the cells must acquire nutrients. Complex multicellularity, like simple multicellularity, also involves adhesion, but the mechanisms for adhesion are highly developed. Cells in complex multicellular organisms communicate with each other through signaling mechanisms that allow coordinated growth and cell differentiation. Complex multicellular organisms employ bulk transport to distribute nutrients, rather than relying on diffusion, allowing the organisms to grow to a greater size.

Self-Assessment Question 2

Describe the phylogenetic distribution of complex multicellularity.

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Model Answer:

Complex multicellularity is found in three of the seven eukaryotic superkingdoms, and is thought to have evolved independently at least six times.

Self-Assessment Question 3

Explain how diffusion limits the size of organisms.

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Model Answer:

Diffusion—the random motion of molecules, with net movement from areas of high concentration to low concentration—limits the size (thickness) of organisms because it is only effective over small distances. For example, if a cell or tissue relies on diffusion of oxygen, its thickness is limited by the concentration difference in oxygen between the cell or tissue and its environment. In shallow water, this allows organisms to reach a thickness of 1mm to 1cm.

Self-Assessment Question 4

Explain how multicellular organisms get around the size limits imposed by diffusion.

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Model Answer:

Plants and animals get around the size limits imposed by diffusion by actively pumping nutrients and other molecules to all parts of the organism through bulk transport. Other organisms, like sponges or jellyfish, grow large by adopting shapes and structures that allow metabolically active cells to remain in close proximity to the environment.

Self-Assessment Question 5

Describe a key difference between multicellular plants and animals.

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Model Answer:

Multicellular plants have cell walls around their cells, whereas animals do not. This restricts the plant cells from moving.

Self-Assessment Question 6

Describe environmental changes recorded by the sedimentary rocks that contain the oldest fossils of large active animals.

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Model Answer:

The study of sedimentary rocks shows that abundant oxygen came to exist about 575–555 million years ago. This corresponds to the first fossil records of complex multicellular organisms.