Relationships Between Plants And Animals Aqa Essay Definition

Plant/Animal Relationships

By BBG Staff   |   December 1, 1994

Plants and animals evolved together, so it is not surprising that there are many complex plant/animal relationships. This process of interdependent evolution of two or more species is called coevolution. Some relationships are beneficial to both parties, while others have a clear benefit for one at the expense, or even death, of the other. Four important plant/animal interactions are explored here: plant/herbivore, plant/pollinator, plant/disperser, and other examples of mutualism.

Plant/Herbivore Relationships

Herbivory is an interaction in which a plant or portions of the plant are consumed by an animal. At the microscopic scale, herbivory includes the bacteria and fungi that cause disease as they feed on plant tissue. Microbes that break down dead plant tissue are also specialized herbivores. Browsers and grazers, from aphids and caterpillars to deer and bison, are more familiar herbivores. Even insects and animals that eat seeds are considered herbivores.

More: Brooklyn Botanic Garden is a habitat where herons hunt for crayfish, monarchs feed on milkweed, and woodpeckers nest and forage for insects.

Some herbivores consume entire plants, or enough to kill them. Others only eat a portion of the plant, and so the plant can recover. The plant/herbivore relationship traditionally has been seen as lopsided, with the animal as the beneficiary and the plant as the loser. Current research, however, is revealing that herbivory has some potential benefits to plants. One example is canopy grazing by insects, which allows more light to penetrate into the lower layers of the forest. Gypsy moth grazing on canopy trees in some areas of Virginia's Blue Ridge Mountains, for instance, has resulted in more light penetration and therefore a more diverse and productive ground layer.

Herbivores and Their Food Plants

Bison, sheep, and other grazers - Succulent forbs, grasses, grass-like plants
Deer and other ungulate browsers - Leaves and twigs of woody plants such as willows, arborvitaes, yews
Beaver - Tree bark, young shoots, leaves
Rodents - Succulent forbs, grasses, grass-like plants
Rabbits - Succulent forbs, grasses, bark
Voles - Roots, bark
Caterpillars - Leaves; in some cases, of specific species
Monarch butterfly - Milkweeds
Gypsy moth - Oaks and other hardwoods
Aphids - Plant juices; in some cases, of specific species
Many birds - Seeds and fruits
Locusts - All plants; seeds, leaves, and stems

Plants and Their Pollinators

Pollination is the transfer of the pollen from one flower to the stigma, or female reproductive organ, of another, which results in fertilization and, ultimately, the formation of seeds. The earliest plants were pollinated by wind, and for some modern plants this is still the most expedient method. Many trees, all grasses, and plants with inconspicuous flowers are designed for wind pollination. Bright, showy flowers evolved for another purpose—to attract a pollinator.

Many plants depend on animals for pollination. Insects, birds, even bats are important for perpetuating plants. The flowers of these plants evolved in concert with their pollinators, and their form reflects the form and habits of their pollinators. Bee-pollinated plants are often irregular in shape, with a lip that acts as a landing pad to facilitate the bee's entry into the flower. Butterfly-pollinated flowers are often broad and flat, like helicopter pads. The flowers of many plants are brightly colored to attract their insect pollinators, and many offer nectar as an enticement. Hummingbirds, with their long beaks, pollinate tubular flowers. Bats require open flowers with room for their wings, such as those of the saguaro cactus. In the tropics, birds and bats take the place of insects as pollinators. Hummingbirds and honeycreepers, for example, have distinctive beaks that have evolved to exploit flowers. Often, a beak may be so specialized that it is only effective on a small group of flowers.

The pollinators, in turn, have evolved to take advantage of the flowers. A successful pollinator typically has good color vision, a good memory for finding flowers, and a proboscis, or tongue, for attaining nectar.

Animal pollination has obvious advantages for plants. Many pollinators cover great distances, which insures genetic diversity through outcrossing, or the transfer of pollen to unrelated individuals. The pollinator benefits as well by gaining access to a source of food. The relationship of pollinator plant is an example of mutualism.

Imperiled Pollinators

All is not well in the realm of pollinators. The age-old relationships between plants and pollinators is threatened, especially in urbanized and agricultural regions. Habitat destruction and fragmentation, pesticide abuse, and disease all have taken their toll on pollinators.

As more land is cleared for human habitation, bees, butterflies, bats, and birds are left homeless. Our gardens offer little to sustain them. They need a constant source of nectar and pollen throughout the entire season. The few flowering plants most people grow will not suffice.

A related problem is fragmentation of plant communities. Plants must be pollinated in order to set seed for the next generation. Without pollinators, no seed is set and the plants eventually die out, leading to local extinction. Isolated patches of forest, grassland, or desert are particularly vulnerable. A small patch may not sustain enough pollinators, or may be too far from other patches for pollinators to travel. As a result, plants do not reproduce.

Pesticides have also reduced pollinator populations. Bees are often killed by chemicals applied to eliminate other pests. Honeybees are being destroyed by diseases and parasitic mites. The crisis is not just affecting native ecosystems. Fruit trees and many other food crops depend on pollination for production. We stand to lose over three quarters of our edible crops if we lose pollinators.

What can be done? Encourage pollinators by planting a diverse mixture of adult and larval food plants in your garden. Erect bat and bird houses, as well as bee hives. Reduce or eliminate pesticide use. Help restore native plant communities not only in your yard, but also in parks and along roadways, and connect them through corridors to preserves and other natural areas.

Plants and Their Dispersers

No two plants can occupy the same spot. In order to have room to grow, seeds must be dispersed away from the parent plant. Seed dispersal is accomplished by a variety of means, including wind, water, and animals. Animal dispersal is accomplished by two different methods: ingestion and hitch-hiking. Animals consume a wide variety of fruits, and in so doing disperse the seeds in their droppings. Many seeds benefit not only from the dispersal, but the trip through the intestine as well. Digestive acids scarify seeds, helping them to break out of thick seed coats.

Some seeds are armed with hooks and barbs that enable them to lodge in the fur of animals that brush past them. Beggar's ticks and bur marigold are two examples. Eventually, the seeds are rubbed or scratched off, and may find a suitable spot on which to germinate and grow. People are important for dispersing plants, too. The common weed plantain was called "white man's footsteps" by Native Americans because wherever settlers walked, the plantain came in the mud on their shoes.

Some Animals and the Plants They Disperse

Ants - Many wildflowers, such as trilliums, bloodroot, violets
Birds - Fleshy fruits and grains, such as baneberry, viburnums, mountain ash
Clark's Nutcracker - Whitebark pine
Woodpeckers - Poison Ivy
Mammals - Fruits, grains, nuts, berries
Squirrel - Nuts, such as those of oaks, hickories, pines
Fox - Berries, such as blackberry, grapes
Humans - Weeds such as plantain, dandelion, lamb's-quarters
Reptiles - Fleshy fruits, especially berries such as strawberry, groundcherry, jack-in-the-pulpit

Mutualism

Mutualism is an obligate interaction between organisms that requires contributions from both organisms and in which both benefit. There are many examples in nature. Pollination and dispersal, discussed above, are mutualistic because both plant and pollinator or disperser benefit from the relationship. The relationship between mycorrhizal fungi and many higher plants is another common example of mutualism. The bodies of the fungi, called hyphae, live on or in the tissues of plants, and make nutrients available for the plants to absorb. The plants provide the fungi with amino acids and other complex compounds. One of the most celebrated examples is the orchids. Whereas some plants may support as many as 100 different fungi, orchids have quite specific mycorrhizal associations. Different plant communities have different mycorrhizal associations. The microflora of a grassland is different from that of a forest. These differences, at least in part, may influence the distribution of plant communities.

The Lovely Lady-slipper

The reason lady-slipper orchids are so hard to grow in a garden is that the needs of both the orchid and its fungus must be attended to. The growing conditions in the garden must duplicate exactly those in the orchid's native habitat.

Anyone who tries to cultivate these beautiful plants learns before long that the pink lady-slipper (Cypripedium acaule) is much harder to grow than the yellow lady-slipper (Cypripedium calceolus). This is because of the fungus. Yellow lady-slippers grow in slightly acidic, rich soils. Their associated mycorrhizal fungus thrives under the same conditions as those in woodland and shade gardens.

The pink lady-slipper, on the other hand, grows in sterile, acid soil, not the typical garden variety. Plant the pink lady-slipper in rich garden soil, and its associated fungus cannot survive. As a result, the pink lady-slipper slowly languishes and eventually dies. Most lady-slipper orchids are still collected from the wild, harming native populations. Buy them only from nurseries that propagate their plants.

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Plant Cell Structure

Plant cell structure is not included in all health science subjects (e.g. all courses in anatomy & physiology) but is is an important part of general biology. Basic cell biology is included in UK A-Level Biology and equivalent courses. It is also useful general knowledge for anyone working in life sciences. This page helps with the task: With the help of a diagram describe the structure of a plant cell.

Diagram of a plant cell

Above: Diagram of the structure of a plant cell

Note: The diagram above is a general plant cell - so not a particular part of any specific plant. The labels in pink are links to pages of further information about the part of the plant cell indicated. The structures are not necessarily drawn to scale but in enough detail to aid recognition and to help students re-draw this diagram by hand to include in study notes or homework.

The structure of plant cells has similarities and differences compared with the structure of animal cells. The following table lists the parts of plant cells shown in the diagram above with brief notes about each of the structures types of organelles in plant cells.

Organelles in Plant Cells and other parts of plant cells incl. e.g. the cell wall, plasma membrane, and cytoplasm:

Part of Plant Cell:

Notes:

Outer-layer of cell:

1.

Cell Wall

Plant cells have cell walls - as compared with animal cells which do not have cell walls, and prokaryotic cells (bacteria) which do have cell walls but they are of a different construction than those of plant cells.

Function(s) of plant cell walls:

The main functions of plant cell walls are mechanical. Plant cell walls form part of a transport system called the apoplast system via which water and some solutes can pass through plant tissue via apoplastic pathways (along / through cell walls) and symplastic pathways (i.e. through the cytoplasm of a series of adjacent cells).

Structure of plant cell walls:

The most important chemical composition of a plant cell wall is cellulose. Long cellulose molecules grouped into bundles called microfibrils are twisted into rope-like macrofibrils. Sometimes there is also another layer component of cell wall e.g.

  • lignin - which gives strength, e.g. strengthens wood (xylem cells) in trees.
  • suberin - which helps prevent water from penetrating, e.g. suberin in mangroves minimize salt intake from their habitat.

2.

Plasmamembrane
(also called the plasmalemma and/or the cell surface membrane)

Functions of the plasma membrane:

  • As a differentially-permeable surface, the plasma membrane controls movement of solutes in and out of the cell.
  • Synthesis and assembly of cell wall components

Structure of the plasma membrane:

The plasma membrane is flexible enough to move closer to or away from the cell wall - according to changes in the water content of the cytoplasm within the cell.

3.

Plasmodesmata

Plasmodesmata are tiny strands of cytoplasm that pass through pores in plant cell walls, forming "connections" or "pathways" between adjacent cells.

Specifically, plasmodesmata form the symplast pathway for the movement of water and solutes through plant structures (see diagram, above-left). These cell-cell connections are especially important for the survival of plant cells during conditions of drought.

Inside the Cell:

4.

Cytoplasm

The cytoplasm is the part of a plant cell that includes all the contents of the cell within the cell membrane but outside of the nucleus of the cell. It therefore includes the cytosol (i.e. the semi-fluid part of a cell's cytoplasm - as shown shaded pale green in the diagram above) as well as the plant cell organelles incl. mitochondria, chloroplasts, etc.. Also located within the cytoplasm is the cytoskeleton, which is a network of fibres whose function is to provide mechanical support to the cell, including helping to maintain the cell's shape.
In short. cytoplasm consists mainly of water and contains enzymes, salts, organelles, and various organicmolecules. It has a clear appearance (i.e. in colour) and a gel-like texture. The cytoplasm helps to move materials around the cell and also dissolves cellular waste.

5.

Vacuole

A cell's vacuole can occupy a large proportion of the total volume of the cell - e.g. 90% of the volume of some mature plant cells. Each vacuole is enclosed by a vacuolar membrane called the tonoplast.

Contents of the vacuole:

  • Cell sap, which is a solution of salts, sugars and organic acids.
  • Enzymes needed for recycling components of cells, e.g. chloroplasts.
  • Anthocyanins are sometimes present in cell vacuoles. These are chemical pigments responsible for some of the (non-green) colours of glowers, e.g. reds, blues, purples.

Functions of the vacuole:

  • Helps maintain turgor pressure pressure (turgidity) inside the cell. This pressure pushes the plasma membrane against the cell wall. Plants need turgidity to maintain rigidity.

6.

Cell Nucleus

The nucleus is the "control center" of a eukaryotic cell (i.e. plant cells and animal cells - but notbacterial cells, which do not have a membrane-bound cell nucleus).

Functions of the cell nucleus:

The cell nucleus controls the activity of the cell by regulating protein synthesis within the cell.

Structure of the cell nucleus:

Each cell nucleus is surrounded (one could equally say "enclosed") by a nuclear membrane that is also known as the "nuclear envelope". The contents of the nucleus - so, inside the nuclear membrane - includes DNA (genetic material) in the form of genes and a nucleolus.

7.

Nucleolus
(inside the nucleus)

The nucleolus is located within the nucleus and is the site of synthesis of:

  • transfer RNA
  • ribosomal RNA
  • ribosomal subunits

8.

Nuclear Membrane
(enclosing the nucleus)

The nuclear membrane is also known as the nuclear envelope and encloses the contents of the nucleus of the cell - separating the contents of the nucleus from the rest of the cell. Nuclear pores in the nuclear membrane enable various substances, such as nutrients and waste products, to pass into and out of the nucleus.

9.

Rough Endoplasmic Reticulum (RER)

Rough endoplasmic reticulum is the site of protein synthesis (which takes place within the ribosomes attached to the surface of the RER) as well as storage of proteins and preparation for secretion of those proteins.

10.

Smooth Endoplasmic Reticulum (SER)

Smooth endoplasmic reticulum is the site of lipid synthesis and secretion within cells.

11.

Mitochondrion (pl.)
the singular form is "mitochondria"

Mitochondria are structures found in both plant and animal cells. They are bounded by double membranes, the inner of which is folded inwards, forming projections (called cristae), hence the representation of mitochondria in diagrams e.g. as above.

Their function of mitochondria is energy production. Mitochondria contain enzyme systems needed to synthesize adenosine triphosphate (ATP) by oxidative phosphorylation.
The quantity of mitochondria within cells varies with the type of cell. In the case of plant cells, mitochondria may be particularly abundant in sieve tube cells (also called sieve tube members), root epidermal cells and dividing meristematic cells.

Read more about mitochondria.

12.

Chloroplasts

Chloroplasts are the sites of photosynthesis within plant cells.

Chloroplasts are very important parts of plant cells. Some cells include up to 50 chloroplasts. The number of chloroplasts per cell varies according to the type of cell and its function. They are plentiful in leaf cells that receive sunlight - as opposed to root cells that do not receive light.

Chloroplasts are a type of plastid. There are also other types of plastids (not all of which are present in all plant cells but all of which are derived from proplastids).

See the diagram of plastids on the right.

13.

Golgi Body (also called the Golgi Complex and/or the Golgi Apparatus)

The Golgi apparatus of a cell is sometimes called the "post office" of the cell or is more generally described as a "packaging organelle" because it plays a role in transporting proteins. It's structure and appearance takes the form of a stack of tiny pancake-like shapes, each of which is enclosed by a single membrane and contains fluid and biochemicals such as proteins, sugars and enzymes.

Functions of the Golgi Apparatus:

  • Modifies some newly-synthesized biomolecules before storing them in granules, sometimes called vesicles - ready for transport later.
  • Forms lysosomes - which are tiny sacs filled with enzymes that enable the cell to utilize its nutrients, so are sometimes described as "cell digestion machines". Lysosomes also destroy the cell after it has died.
  • Transports the proteins produced in the ER: After a protein has been synthesized in the ER, a transition vesicle (or "sac") is formed then floats through the cytoplasm to the Golgi apparatus, into which it is absorbed. After processing the molecules inside the sac, a secretory vesicle is formed and released into the cytoplasm, moves to the cell membrane, then releases the molecules from the cell.

14.

Microtubules

Functions of microtubules:

  • facilitate addition of cellulose to cell wall
  • form the spindles and cell plates of dividing cells
  • play a role in cytoplasmic streaming (i.e. moving the fluid cytoplasm within the cell) e.g. to/from chloroplasts.

Structure of microtubules:

Microtubules are hollow rope-like structures composed of the protein tubulin.
They can be as long* as 25μm (25 micrometres = 25 x 10-6 m) and have a diameter of approx 25 nm (25 nanometres = 25 x 10-9 m). See scientific numbers for more about these units.
* some sources state up to several mm (millimetres) long.

15.

Ribosomes

Ribosomes take part in the synthesis of some proteins by catalyzing the formation of those proteins from individual amino acids (using messenger RNA as a template). Examples of proteins catalyzed by ribosomes include glycoproteins, lysosome proteins, membrane proteins and some organelle proteins.

In general, ribosomes can be either "free" (in the cytoplasm of the cell, but not within the nucleus or membrane-bound organelles) or "membrane-bound", which in the cases of plant and animal cells means attached to the rough endoplasmic reticulum (RER) - hence the black dots shown in the diagram above, distinguishing the RER from the smooth endoplasmic reticulum (SER). An individual ribosome could be "membrane-bound" while it is making one protein, then "free" while making a different protein. Some diagrams of plant cells include "free" ribosomes while others only show ribosomes attached to endoplasmic reticulum, forming RER.

Notes:

  1. The information on this page is intended for approx. UK A-Level Biology (as studied by some 16-18 yr olds) and international equivalents. Students of more or less advanced courses may be expected to describe these parts of plant cells differently - e.g. in more or less detail.
  2. The numbers above-left are just for ease of reference to this table. They do not indicate numbers of chloroplasts, ribosomes, etc..

Some of the organelles mentioned above have structures and functions that cannot be described in much detail in this short table but are described more fully on other pages - see links. It is useful to know about the differences between plant and animal cells and other related information:

See also the general structure of an animal cell and a comparison of plant, animal and bacterial cells
and cell functions.

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