The Structure and Function of Plants

The Structure and Function of Plants

We are going to learn about plants.  We will learn about how their structure and functions compare to humans.  Lets begin by comparing the taxonomy, the classification of plants, to the classification of humans.  

TAXONOMY OF PLANTS

Taxonomy-the branch of science concerned with classification of organisms.   It begins with the largest groups of related organisms, called domains,  and goes down to the most specific and closest relationships, called species.  There are three domains: Bacteria, Archaea, and Eukarya.  Remember Eukaryote cells are cells with a nucleus and organelles.  Bacteria were prokaryotes....meaning their cells don't have a nucleus. Archaea have some of the properties of Bacteria, like no nucleus,  and some of the properties of Eukaryotes, like enzymes helping out reactions. Archaea can live in very harsh environments where other organisms can't live.  They are called "extremophiles" and can live in ammonia, salt lakes, and hot springs. There are SIX kingdoms, four of which are in the domain Eukarya.  Plants and animals are both in the same domain.....but that's as close as our relationship gets.  At the next level of taxonomy, Kingdom, plants are in the Kingdom Plantae and we are in the Kingdom Animalia.  So, we are much more closely related to the frogs we learned about than the plants we are going to learn about. 

Here is the taxonomy of humans again:

Kingdom Plantae is divided into four phylum: Angiospermorphyta (anthophyta) FLOWERING PLANTS, Coniferophyta  CONIFERS, filicinophyta (pteridophyta) FERNS, and Bryophyta- MOSSES.

THE PLANT BODY

There are more than 250,000 species of plants.  They range from microscopic algae to giant redwood trees.  They live in every corner of the Earth.  We think of roots stems and leaves when we think of plants, full of vascular pipelines that feed and water the plant.  But not all plants have vascular tissues.  Green algae and mosses are examples of this.  

Shoots are the above ground parts of a plant. This includes the stem, its branching's, leaves and flowers.  Stems help display leaves to the sun and flowers to pollinators.

Roots are the descending part of the plant. Roots absorb water, minerals and ions and anchor the plant.

The word "vascular" in plants means the vessels, or you might think of them as pipelines, where water, sap and nutrients move up or down the plant.

Plants have two "vascular" tissues....two sets of pipelines.  

XYLEM- distributes water and ions throughout the plant

PHLOEM- distributes sugars from photosynthesis

Apical Meristems- located in the dome shaped tip of stems or roots with self-perpetuating embryonic stem cells that can differentiate into any kind of cell.  From our cell differentiation module, remember that meristems are totipotent, meaning they can differentiate into any specialized cell.  Primary growth- lengthens a plants stems and roots.  Secondary growth-  thickens a plant.  In trees we see secondary growth as tree rings, and comes from lateral meristems. 

Three main plant tissues:

1. Ground Tissue- make up most of the plant body

     Ground tissue is made up of :     Parenchyma cells- think walled, flexible, heal wounded areas.

                                                               Collenchyma cells-make ribs  or strings in leaves, flexible, 

                                                                Sclerenchyma- protects seeds- thick walls made with lignan in them- strong and waterproof

2. Vascular Tissue- two kinds of conducting tissues to distribute water and sugar

     Xylem- conducts water, minerals, and ions- mechanically supports the plant- conducting cells are not alive- walls of the dead cells connect to form tubes though pits in the side walls. 

     Phloem- conducts sugars and solutes- its conducting cells are alive.  They connect at perforated end walls. 

3. Dermal Tissue- covers and protects plants surfaces- it is called the EPIDERMIS- single layer of cells with a wax on the outside cell walls.  Cuticle- is the wax.  Its helps prevent water loss and resists attacks.  

Stomata- Stem and leaves have openings surrounded by guard cells .  Guard cells can change shape and open and close the pore in the leaf.  Water vapor, carbon dioxide, and oxygen move through the Stomata.

 

THE TWO TYPES  OF FLOWERING PLANTS: MONOCOTS AND DICOTS

Monocots- includes grasses, corn, lilies, irises, cattails and palms- they have one Cotyledon- a leaflike structure in the seed-it withers when other leaves grow

Dicots- magnolias, roses, geraniums, and hollyhocks- has two Cotyledon

see the chart for the differences between monocots and dicots:

 

LEAF SHAPE

Leaves are metabolic factories that have food-producing photosynthetic cells.

Monocots- mostly have flat, knife blade shaped leaves. The base of the blade encircles the stem.

Dicots- have diverse leaves- they can have one broad blade, attached by a stalk call a petiole.  Often the blade has lobes on it. Dicots also have "compound" leaves with several stalked leaflets.  They also may have specializations like hairs, scales, spines and hooks. Deciduous plants, like a birch tree, have their leaves drop off as winter approaches.  Camellias, or evergreens, also drop, but not all at once.  

 

PLANT NUTRITION AND TRANSPORT

Nutrients

There are 16 elements important to the survival of a plant.  Three of those elements: Oxygen, Carbon, and Hydrogen, make up carbohydrates, lipids, proteins, and nucleic acids. They come from carbon dioxide and oxygen in the air and Hydrogen in water.  The rest of the 16 elements dissolve in water and are mineral ions from the soil.  Macronutrients are usually in large amounts in plant tissues...these are elements such as nitrogen and potassium.  Micronutrients are just in trace amounts, and these are elements like iron and manganese.  Root growth expands where there is plentiful water and nutrients. 

Absorption

Plants have symbiotic relationships that help them absorb some of these nutrients.  Symbiosis- a mutually beneficial relationship.  Nitrogen is one of those macronutrients which the plant uses a lot of.  There is lots of Nitrogen in the air, in a triple covalent bond that is very hard to break.  We learned in ecology class about the nitrogen cycle, where nitrogen fixing bacteria help break that bond and make the nitrogen available to plants.  They live in root nodules- swellings in the plant root. Mycorrhizae are fungus that cover roots and have a larger surface area for absorbing more nutrients.  They help the plant and get sugars from the plant. The plant also can have "root hairs" which are tiny projections that increase surface area. 

Controlling how much nutrient is absorbed

The plant cells have control over how much water and nutrient gets into the roots.  The inside of the root has a central column of vascular tissue. Around this is a layer of cells called the endodermis.  A waxy band around these cells assures that water can only enter by going THROUGH the cytoplasm of these cells.  Remember cells have a semipermeable membrane that helps them control what gets in and out.  If the plant has too great a concentration of a mineral inside the endodermis, that mineral will not be able to get through the membrane. Once the water and nutrient are in the vascular cylinder they are transported throughout the plant. 

Transporting and conserving water

Transpiration-the evaporation of water vapor through the stomata.  Water travels through pipelines called Xylem in one direction....up.  The cells that make the pipeline are dead, but the remaining cell walls make the tube. Water is pulled up through the plant by the drying of water by air, on the plant leaves.  As that water evaporates, more water molecules are pulled up from behind, by capillary action-using the cohesion and adhesion of hydrogen bonds.  Hydrogen bonds are strong enough to hold water together in the xylem, but not strong enough to hold them together when exposed to the air.  Ninety percent of the water in a plant is lost to transpiration.  When evaporation exceeds uptake of water, the plant can wilt and die. 

Things that prevent water loss:

The cuticle- the waxy covering over all surface cells

Guard Cells of stomata- Two cells line the stomata openings.  These cells can change shape and open or close the stomata. Most evaporation takes place there. The guard cells swell with water uptake, and change shape, opening the stomata. When they lose water, the guard cells collapse in, closing the stomata.  Photosynthesis begins as the sun comes up, and carbon dioxide levels in the cells drop as it is used to make sugar.  This triggers potassium ions transported into the cell, and water follows potassium by osmosis.  When the sun does down, CO2 levels rise in the cell, potassium is transported out, and water follows the potassium out of the cell by osmosis.  Some plants like Cactus open their stomata at night.  

Transport of Organic Substances

Carbohydrates are stored as starch in plant cells. Starch is a carbohydrate consisting of numerous glucose units joined together in long chains. They are too long to transport to areas of the plant that need the food, and not soluble.  Plants use hydrolysis ( basically adding water) to cut up starch into little units of sucrose- a sugar.  The sugar is moved by a process called translocation- moving the sugars through the vascular system called the phloem.  These tubes are made of living cells with large pores connecting them. Companion cells use energy (ATP) to load the sugars into the Phloem cell, creating a high pressure.  Companion cells help unload the sugar at the sight it is needed.

PLANT REPRODUCTION

Plants have a variety of reproductive strategies.  Plants can produce asexually and sexually.  Asexual reproduction can be new plants sending runner out above ground, like a strawberry plant, or they can be grown from cuttings or buds.  Asexual reproduction means the offspring are clones of the parent. Groves of Navel Oranges from Southern California are all clones from a single tree in Riverside California.

Plants can also produce sexually, meaning that a sperm fertilizes an egg and creates unique offspring. We will learn about the reproductive strategies of three pants...a Peach and a flower, which are similar, and a fern which is quite different.

Flowering Plants

Flowering plants have plant sex. Their reproductive systems are designed to produce and protect sperm and eggs.  They protect embryos in their early development. They use pollinators, wind, and water to help the sperm and egg meet. 

A Flower is made of the same three tissues as the rest of the plant- ground tissue, vascular tissue, and epidermis.  Epidermal cells of petals contain fragrant oils or are brightly colored to attract pollinators.  By fragrant...I don't mean they smell good.  You may have noticed that pear trees in the spring smell like rotten meat.  This attracts flies to help with pollination. The petals and the sepals, leaves or petals beneath the flower, protect the reproductive parts.  Stamen- are the male reproductive parts.  They are closest to the petals.  A Stamen is an Anther on top of a stalk. An Anther is the pollen bearing structure of the flower, and is made of pollen sacs, where pollen grains develop.  Pollen grains are sperm-producing bodies.  The female reproductive part is in the middle and is called a Carpel or a pistil.  The bottom of the Carpel is an ovary, where eggs develop, are fertilized, and seeds mature.  The stigma is the top of the Carpel, and is sticky to capture pollen. the stalk it sits on is called the style. 

Flowers with both male and female parts are called "perfect" flowers.  Those that have just male or just female parts are called "imperfect" flowers.

Pollen

1. Spore producing cells grow by mitosis (diploid) cell division. 

2. Then these spores divide by meiosis (haploid) cell division, ending in cytokinesis ( cytoplasmic division which creates 4 individual cells with half the parent DNA)

3. A wall develops around the spores.. the wall will protect the spore from the environment 

4. Spores divide twice more by mitosis.....now they are pollen grains

Megaspores

1. cell masses form in the ovary.  They are Ovules.

2. two protective layers form around the ovule. 

3. Inside the ovule a cell divides by meiosis, forming four haploid spores called MEGASPORES. Just like with humans 3 disintegrate and one develops. 

4. The remaining megaspore goes through mitosis three times WITHOUT Cytokinesis ( without cytoplasmic division)- so its a cell with 8 nuclei

5. After each nucleus migrates to the right position- it becomes a 7 celled embryo sac, with two nuclei in the same cell becoming the endosperm- nutrition for the new seed.  One of the other cells becomes the egg. 

Getting Together

1. Pollen grains stick to the Stigma- this is called pollination.

2. It germinates- meaning it starts to grow a tube down the style to the ovary.  It has two nuclei.

3. The sperm nucleus fuses with the egg nucleus, making a zygote (baby plant) and a sperm nucleus fuses with both nuclei of the endosperm (3n).

4. The new zygote will form one or two seed leaves, cotyledons,  inside the Capsella.  The outside will harden. The parent plant will feed that fertilized seed/embryo.  The endosperm ( nutrient portion) accumulates. It is now ready to grow a new plant.

PEACH

A peach is a flowering plant called a dicot ( two seed leaves).  While the zygote is going through changes, so are other parts of the flower.  In the case of a fruit bearing plant, The fruit is the ovary of the flower.  Fruits are a method of spreading seeds.  Animals eat them, poop out the seeds in a new place, and a new plant can grow. Some fruits are not what we think of like a peach.....the winged fruit of maple trees spins like a helicopter and uses the wind to find a new home.  Some fruit have spines or hooks or hairs so they stick to passing animals.  Peach trees are self-pollinators, meaning they have "perfect" flowers- male and female parts. 

FERNS

Ferns are not flowering plants, so they have a different reproductive strategy.  The life cycle of ferns is different from other land plants as both the gametophyte and the sporophyte phases are free living. 

Reproduction by Spores

https://www.sciencelearn.org.nz/image_maps/57-fern-life-cycle

Co-evolution of Pollinators and the Pollinated:

Coevolution- two or more species jointly evolving as an outcome of their close ecological interactions.  When one species evolves , the change effects the selection pressures oepraturing on the other.  Think about it....why is a flower white?  Does it stand out in the dark and thus attract moths.  Why does the flower of a pear tree stink....does it attract flies.  Why do certain flowers have a shape that makes bees work to brush up against both the anthers and the stigma?  A snap dragon certainly does!