Chapter 26

How do plants and animals differ in the ways their cells adhere, communicate, and differentiate during development?

All multicellular organisms have molecules that promote adhesion between cells and communication between cells, but plants and animals have largely distinct sets of molecules for these functions. Plants and animals also exhibit distinct patterns of development: stem cells called meristems remain active for life in plants, whereas tissues and organs differentiate early on in animal development.

Which of the statements is true?

Cadherins on the surface of animal cells permit cells to adhere to other cells.

Imagine a hypothetical habitat where there are many simple and complex multicellular organisms of roughly the same size. A new predator is introduced into the habitat that feeds by taking bites of the organisms, rather than consuming them entirely. Given this, which of the statements is most likely?

Complex multicellular organisms will suffer more, because vital differentiated cells that the rest of the organism requires will be lost.

How does diffusion limit the size of organisms?

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

Consider the image. At which point on the diagram shown did complex multicellular organisms first appear?

E. 600 million years ago

Which arose first: animals with many cells or cell adhesion molecules?

Evidence from the single-celled protists most closely related to animals shows that at least some of the proteins that govern cell attachment in animals were present in their unicellular ancestors. These proteins probably had distinct functions in these cells. Cell adhesion molecules continued to evolve along with complex multicellularity in animals.

How does evo-devo research help us to understand how animals have diversified to form so many distinct species?

In evo-devo research, biologists compare the developmental pathways that regulate growth and differentiation of body types. By studying differences in the genetic regulation of development between species, biologists can understand how specific mutations have given rise to distinct animal morphologies. Evo-devo also helps us to understand the genetic basis of plant diversity.

A marine biologist discovers a new species of green algae, which, under the microscope, appears to be multinuclear. Which of the options is likely true of these algae?

It is coenocytic.

Which of the statements describes the effect of diffusion on organismal size?

It limits the size of organismal cells.

What is a key difference in structure between the cells of multicellular plants and the cells of multicellular animals?

One key difference is that plants have cell walls around their cells, whereas animals do not. Cell walls restrict plant cells from moving, which in turn determines many features of plants' development and function.

Multiple lines of evidence indicate that complex multicellularity evolved at least six different times. Which of the superkingdoms includes organisms that exhibit complex multicellularity?

Opisthokonta, Archaeplastida, Stramenopila

Why is bulk flow a necessary condition of complex multicellularity?

Organisms exhibiting complex multicellularity are large and have cells/tissues that are not in contact with their environment. Systems that can overcome the limits of diffusion must evolve for basic physiologic processes to occur.

How do multicellular organisms get around the size limits imposed by diffusion?

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 flow. Bulk flow allows key molecules to be transported over distances much greater than those possible that can be covered by diffusion alone, making larger organisms possible. Other organisms, such as sponges or jellyfish, grow large by adopting shapes and structures that allow metabolically active cells to remain in close proximity to the environment.

How do simple multicellular organisms differ from complex multicellular organisms?

Simple multicellular organisms do not have differentiated cells and tissues, whereas complex multicellular organisms do have differentiated cells and tissues. Moreover, in simple multicellular organisms, all or nearly all cells are in direct contact with the environment. In complex multicellular organisms, most cells are completely surrounded by other cells. As a result, cells in complex multicellular organisms communicate with one another through signaling mechanisms that allow coordinated growth and cell differentiation.

How do the cells within your body become differentiated? Select all that apply.

The cells' pattern of gene expression is regulated. The cells are in different local environments.

Sulfate, nitrate, and ferric iron are readily available, exist in appreciable amounts, and act as terminal electron acceptors in anaerobic respiratory pathways. If multiple electron acceptors existed before 575 million years ago, why do we not see evidence of complex multicellularity earlier in Earth's history?

The characteristics necessary for complex multicellularity did not evolve until 575 million years ago.

All cells in your body derive from a single fertilized egg, yet your body contains many different types of cells. How can we explain this phenomenon?

The differentiation of distinct cell types, tissues, and organs in animals reflects a developmental program in which cells receive molecular signals from surrounding cells and the environment. These signals alter the expression patterns of genes and the activities of proteins, thereby governing the different developmental fates of the cells in our bodies.

The lion's mane jellyfish can be up to 8 feet wide. In spite of its large size, it has no structures for bulk flow to deliver oxygen to its cells. How can it be so large and yet lack a mechanism for bulk flow?

The jellyfish is filled with tissue made of structural cells that are not metabolically active. Therefore, a mechanism to transport oxygen to its interior is not necessary.

Which environmental change(s) are recorded by the sedimentary rocks that contain the oldest fossils of large active animals?

The study of sedimentary rocks shows that abundant oxygen came to exist about 580‒560 million years ago. This corresponds in time to the first fossil records of complex multicellular organisms.

What is a major difference between plants and animals?

Unlike animal cells, plant cells have walls.

The acquisition of traits necessary for complex multicellularity occurred independently in plants and animals, but took place in a specific order. In what order did these processes evolve?

cell adhesion molecules, a mechanism of communication, bulk flow

Which of the terms describes a multinucleated organism made up of a single cell?

coenocytic

Which of the terms describes the process by which a fertilized egg develops into a multicellular organism with many different cell types, each with different structures and functions?

development

Consider the image. Which of the traits appeared at point 3 on the phylogeny?

gap junctions

An inhibitor of mitosis would most likely have the greatest effect on the _____ of a plant.

meristems

Choanoflagellates are:

protists

Choanoflagellates are single-celled eukaryotes that feed on bacteria. In the presence of their food bacteria, some choanoflagellates develop multicellular structures in which the individual choanoflagellate cells are attached to one another. Based on this observation, it may be assumed that:

the bacterial cells, or a chemical released by the bacteria, bind to a receptor on the plasma membrane of the choanoflagellate cells.

Diffusion is best defined as:

the random motion of individual molecules, with net movement from areas of higher concentration to areas of lower concentration.

Which of the options is in the correct order of development zones, starting from the tip of a growing plant root?

zone of cell division, zone of cell expansion, zone of cell differentiation