Plant Plant Cells & Cells & TissuesTissues

Plant CellsPlant Cells Plant Cells have

a few unique organelles: Vacuoles Plastids (e.g.,

chloroplasts) Cell wall

VacuolesVacuoles Definition: membranous

sacs filled with water that serve a variety of functions: Storage of “food”, water, oil,

pigments, toxins, & waste). Helps to maintain turgor

pressure. Young plant cells often

contain many small vacuoles, but as the cells mature, these unite to form a large central vacuole.

PlasmolysisPlasmolysis DefinitionDefinition: : the contraction of

cells within plants due to the loss of water through osmosis. It is when the cell membrane

“shrinks away” from the cell wall and the vacuole collapses.

Occurs when water is drawn out of the cell (high water concentration low water concentration).

Causes plants to wilt.

PlastidsPlastids Definition: large organelles that

are associated with pigments and other storage products (e.g., starch).

A typical plant cell will usually have 20-40 plastids.

Several Types: Chloroplasts Amyloplasts/Leucoplasts Chromoplasts

ChloroplastsChloroplasts Definition: plastids that

capture light energy from the sun to make “food”; contain chlorophyll (Elodea)

AmyloplastsAmyloplasts Definition: colorless

plastids that store starch

Leucoplasts are pretty similar.

Found in things like potatoes & onions

ChromoplastsChromoplasts Definition: plastids that make

& store carotenoid pigments Found in red & yellow

peppers, tomatoes, flower petals …

When leaves change color in autumn, it is the result of the conversion of chloroplasts into chromoplasts.

Cell WallCell Wall Definition: a flexible outer

barrier made of cellulose fibrils that surrounds plant cells.

Functions: Protection & Support Determines plant shape

& size Maintains turgor pressure Regulates movement in

and out Cell-cell communication

PlasmodesmataPlasmodesmata Definition: small tubes that

connect plant cells to each other, establishing living bridges between cells. Allow certain molecules to pass

directly from one cell to another. These channels, penetrate the

cell wall and are lined with plasma membrane, uniting all connected cells with essentially one continuous cell membrane.

StomataStomata Definition: pores found in the

underside of plant leaves in the epidermis that allow for the exchange of gases (CO2) and H20.

The stomata are flanked by two half moon shaped guard cells, which contains chloroplasts and regulates the opening and closing of the stoma.

Normally stomata open when the light strikes the leaf in the morning and close during the night.

Pea leaf stoma (SEM x 3, 520)

TrichomesTrichomes Definition: epidermal hairs

or scales found on nearly all plants.

Functions: Glandular secretions Absorb nutrients Protect against predators Reduce evaporation Reflect solar radiation Collect water

Cell DivisionCell Division The mechanism that

allows the nuclei of cells to split and provide each daughter cell with a complete set of chromosomes during cellular division.

Permits plants to grow & develop.

Garlic Root TipGarlic Root Tip In plants cell

division occurs in tissues called meristems, which are often found in root tips.

Root cap

Apical meristem

Phases of Cell DivisionPhases of Cell Division Interphase

G1 phase S phase G2 phase

Mitosis: Prophase Metaphase Anaphase Telophase

Cytokinesis

InterphaseInterphase Cellular growth Preparation for mitosis

MitosisMitosis

ProphaseProphase Appearance of string-

like chromosomes due to condensing of chromatin.

MetaphaseMetaphase Chromosomes line up

in the middle of the cell (equator)

AnaphaseAnaphase Chromosomes

separate and move to opposite ends of the cell (poles).

Telophase & CytokinesisTelophase & Cytokinesis Two new nuclei are

formed and the cytoplasm is beginning to separate.

TissuesTissues

Plant TissuesPlant Tissues Definition: a group of cells that forms a structural and

functional unit. Three tissue systems in plants:

Ground tissue: variety of functions including support and photosynthesis

Vascular tissue: “Plumbing system” throughout plants; conducts water, food (CHO), and dissolved minerals; also strengthens and supports the plant.

Dermal tissue: Provides covering for the plant body.

SIMPLE SIMPLE TISSUESTISSUES

The Ground The Ground Tissue SystemTissue System

Parenchyma cellsParenchyma cells Definition: a simple tissue

composed of spherical-shaped cells; most common type of tissue (most of apples & potatoes); found throughout plant

Have thin primary walls Living, metabolizing tissue Functions:

Photosynthesis & Respiration Storage (starch, oil droplets,

water, and salts) Wound healing & regeneration

CollenchymaCollenchyma Definition: a simple tissue found

beneath the epidermis in young stems and in large veins of leaves (“strings” in celery).

Provides a flexible support system (like our skeletal system).

Found uniformly throughout the plant.

Elongated cells that have unevenly thickened cell walls (especially in the corners)

Alive at maturity

SclerenchymaSclerenchyma Definition: a simple tissue

specialized for structural support; occurs in many areas

Two types: Fibers Sclereids

Have both primary and secondary cell walls.

At functional maturity cells are often dead.

FibersFibers Long, tapered cells that

often occur in groups or clumps (abundant in the wood and bark of flowering plants)

Found in leaves & stems.

Primary function is support.

These sclerenchyma fibers clearly show the thick cell

walls.

Notice the pits (identified by the arrows) which allow

materials to pass from one fiber to another

SclereidsSclereids Short, cuboidal cells

common in shells of nuts and pits of fruits (peaches, cherries)

Found in all plant parts, usually clustered.

Primary function is for protection.

The red cells in this picture are sclereids from a pear.

Notice the irregular shape.

The surrounding cells are parenchyma cells.

COMPLEX COMPLEX TISSUESTISSUES

The Vascular The Vascular Tissue SystemTissue System

Two Complex TissuesTwo Complex Tissues Xylem Phloem Both are continuous

throughout the plant body.

XylemXylem Conducts water and dissolved

minerals from the roots and the stems and leaves & provides support.

Composed of four types of cells: Tracheids* Vessel elements* Parenchyma cells Fiber (*Both of these conduct water!)

TracheidsTracheids Tracheids are

typically found in gymnosperms and seedless vascular plants and aid in transporting water. The hollow cells

that are stained red are tracheids from a fern.

Vessel ElementsVessel Elements Vessel elements are

typically found in angiosperms and aid in transporting water. Upper right is a long section

of vessel elements showing the perforation plate

Lower right is a cross section of vessel elements.

PhloemPhloem Conducts food materials,

carbohydrates, in solution and provides support.

Composed of four different cells: Sieve-tube members (carry food) Companion cells (aid STM) Schlerenchyma fibers (support) Parenchyma cells (storage)

Sieve-tube Sieve-tube members (STM)members (STM) Long, thin cells that are stacked end-

to-end to form extended sieve tubes. Some of the most-specialized cells

in nature that are alive, but lose their organelles at maturity.

The end of the cells are called sieve plates.

Adjacent to STM are companion cells, which assist in the STM by moving food into the sieve-tube for transport.

Sieve-tube Members Sieve-tube Members (STM)(STM)

This is a sieve tube member from

milkweed.

Notice how STM’s are connected by sieve plates (identified by

the arrows).

Sieve-tube Members Sieve-tube Members (STM)(STM)

This picture show several sieve tube member in cross

section.

Notice the perforated sieve plate near the

top that would connect two sieve

tube members.

Sieve-tube Members Sieve-tube Members (STM)(STM)Sieve tube members can secrete a protein called callose to plug the seive plates and stop the flow of food if there is damage in

the phloem.

This picture shows an example.

Companion CellsCompanion Cells Companion Cells are

alive and regulate the movement of material through sieve tubes. The arrows point out

several companion cells adjacent to the sieve tube members.

Notice the sieve plate as well.

Dermal Tissue Dermal Tissue SystemSystem

DermisDermis In herbaceous plants the

dermis is a single layer of cells called the epidermis.

Epidermis

EpidermisEpidermis Comprised of two types

of cells: Parenchyma cells

(storage) Guard cells (together

create stoma) Epidermal parenchyma

contain no chloroplasts, so they appear transparent allowing light to penetrate into interior photosynthetic tissues.

This picture shows the single layer of parenchyma

cells that makes up the epidermis and also a very

thick cuticle.

EpidermisEpidermis Functions:

Absorption of water Secretion of cuticle Protection Control of gas

exchange

Epidermis--CuticleEpidermis--Cuticle This picture shows

the single layer of parenchyma cells that makes up the epidermis and also a very thick cuticle.

Cuticle Parenchyma cells

CuticleCuticle A waxy layer that is

secreted by epidermal cells of stems and leaves. Functions to prevent

water loss. Also prevents important

gases like CO2 from diffusing into leaf/stem.

Epidermis—Guard CellsEpidermis—Guard Cells This picture shows

the single layer of parenchyma cells that makes up the epidermis along with the cuticle and a pair of guard cells creating a stoma.

Epidermis—Guard CellsEpidermis—Guard Cells This picture

shows the parenchyma cells that makes up the epidermis and a couple pairs of guard cells (and stomata) from the top.

Stomata & Guard CellsStomata & Guard Cells Stomata are tiny pores in the

epidermis between two cells called guard cells. Many gases, including CO2,

O2 and H2O vapor, pass through stomata by diffusion.

Stomata are generally open during the day during photosythesis. The water lost when stoma are open elps to cool the leaf!

Usually closed at night to conserve water.

Woody plants initially produce an epidermis that splits as the plant grows. These plants also

have periderm, which is several cells thick and forms under the epidermis.

Periderm composes the outer bark.

PeridermisPeridermis

Composed of two types of cells: Cork Cells that

secrete suberin to help waterproof the plant.

Cork Parenchyma that function for storage.

PeridermisPeridermis

Peridermis from an ivy stem.

Plants Exhibit Plants Exhibit Localized Localized Growth at Growth at MeristemsMeristems

MeristemsMeristems Definition: specific areas of plant growth, which

are composed of cells that do not differentiate. These cells retain the ability to divide, which

other plant cells lose. Meristems allow plants to grow their entire

lives. Types:

Apical Lateral Intercalary

Types of GrowthTypes of Growth Primary growth: an increase in the length

of a plant. All plants have primary growth.

Secondary growth: an increase in girth of a plant. Only select plants (gymnosperms and woody

dicots) have extensive secondary growth. Wood and bark are produced by secondary

growth.

Types of Types of Meristematic Meristematic

TissuesTissues

Apical MeristemsApical Meristems Found at or near tips of roots and

the buds of stems. Cells are small and “boxy” in shape

due to parenchyma cells. Responsible for the increase in

length of roots and stems. Root tips have a root cap at the tip

of the apical meristem, which is followed by an area of cell elongation and an area of cell differentiation.

Stem buds have a dome-like apical meristem in addition to leaf primordia and bud primordia, which protect the apical meristem.

Lateral meristemsLateral meristems Definition: areas that extend the

entire lengths of stems and roots except at tips, which are responsible for secondary growth in trees & shrubs.

Two types: Vascular cambium —a layer of

cells that forms a thin cylinder around the stem and root trunk, between the wood and bark.

Cork cambium —a thin cylnder of cells in the outer bark region.

Intercalary meristemsIntercalary meristems Growth regions that

occur at the base of nodes.

Found in many plants that don’t have a vascular cambium (grasses).