The Plant Cell

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THE PLANT CELLDagmar Stengel / Mike Guiry

Dept. of BotanyNUI, Galway

Lecture Overview• Common components: plant and animal cells

• Differences

• Details of plant cell components

• Cellular responses to the environment

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Figure 7.8 Overview of a plant cell

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Plant cell TEM

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– Nucleus– Mitochondria– Endoplasmatic Reticulum– Golgi apparatus– Ribosomes– Lysosomes and microbodies (peroxisomes and glyoxisomes)– Plasma membrane– Cytosol– Microtubules and microfilaments (cytoskeleton)

(Campbell pp.105-127)

Plant and animal cells:Common cell components

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So what is the difference?

1. Vacuole

2. Chloroplast

3. Cell wall

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Vacuole= Large membrane-bounded vesicle

• may occupy up to 90% of cell volume

• surrounded by membrane = tonoplast

• Contains solutes dissolved in water

• Intracellular digestion

• Storage and osmotic regulation• Storage: carbohydrates, organic acids, secondary

products

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Chloroplast• = membrane-bounded plastid• contains chlorophyll• location of photosynthesis

Cell wall • rigid multi-layered surrounding of plant cell• usually made up of polysaccharides

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Vacuoles• Vacuole and vesicle: membrane-bound sacs

– Large: vacuole– Small: vesicle

Different types:Food vacuoles: engulfing or phagocytosis of food

particles (not in plants)

Contractile vacuoles: mainly in freshwater algae; pump excess water out of cell

Central vacuole: surrounded by membrane called tonoplast

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Functions of Plant Vacuoles• Storage: organic compounds incl. Proteins

• K+ and Cl-

• hydrolytic enzymes that digest macromolecules

• Recycle molecular components from organelles

• metabolic by-products

• pigments in petals to attract pollinators

• substances to deter herbivores, poisons

• Involved in extension growth of plants

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Figure 7.15

The plant cell vacuole 

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The Tonoplast

= single membrane surrounding the vacuole

• Transmembrane electrochemical gradient achieved by proton-pumping ATPase

• special transport proteins facilitate transport across Tonoplast

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• Usually lens-shaped • Surrounded by double membrane • internal membrane system arranged into flattened sacs

(=thylakoids)

• thylakoids stacked forming grana (1 granum)

• 2 compartments: thylakoid space and stroma

• needed for photosynthesis

• depending on light conditions, chloropasts can move within the cells

• e.g. to surface to catch more light in low light conditions

The Chloroplast

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Figure 7.18 The chloroplast, site of photosynthesis

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Chloroplast Function

- Photosynthesis -

• contains green pigment chlorophyll• pigments of other colours (red, blue,

yellow/brown) • pigments used in photosynthesis to

produce sugars (carbohydrates)

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Chloroplasts: Origin

– not related to endomembrane system

• contain small amounts of DNA that program synthesis of own proteins

= semi-autonomous organelles

• belong to family of “plastids”

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Plastids= family of organelles bounded by 2 external

membranes

– Chloroplasts - photosynthetic; green due to chlorophyll content

– Chromoplasts: containing pigments other than chlorophyll(in fruits, leaves, flowers)

– Leucoplasts: involved in lipid biosynthesis

– Amyloplasts: store starch (colourless)

– Etioplasts: intermediate state in production of chloroplasts, in tissue exposed to light for the first time

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• in unspecialised cells

• fate depends on location in the cell and the environment

• E.g. proplastid becomes a chloroplast if exposed to light!

Proplastids

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Endosymbiont Theory:Chloroplasts arose as primitive

photosynthetic organisms that colonized a non-photosynthetic cell

Presence of Chloroplast genome andDouble membrane has lead to

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The Cell Wall - Structure• almost all plant cells have a ‘Primary Cell Wall’

• thicker than plasmamembrane

• consists of long-chain polysaccharides (cellulose)

• composition varies between different species

• aggregated into bundles to form fibres and microfibrils

• 10-25 nm in diameter• fibres in layers • great tensile strength

Cell walls TEM

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The Cell Wall - Functions

• thicker, stronger and more rigid than similar components around animal cells

• Responsible for:– nutrition– osmoregulation– growth– intercellular communication

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Cell Wall - Functions• Cell adhesion, protection and support• barrier against pathogens and deters

herbivores• dynamic nature of cell wall allows expansion

and plant growth• determines morphology• communication between cells through

plasmodesmata• exchange of selected molecules and fluids• Secondary cell wall may contain lignin for

greater support• unique cell wall adaptations: function and

environment

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Formation• Young cells: formation of thin, flexible cell wall (=

primary cell wall)between primary cell walls of adjacent cells is middle lamella:- sticky polysaccharides (pectins)

• Mature cells: thickening of cell wall occurseither

– by hardening of substance of primary cell wall or

– by adding secondary cell wall between plasma membranes and primary wall

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Plasmodesmata (one Plasmodesma)

• structures in which the membranes from adjacent cells connect through a pore in the cell wall

• link adjacent plasma membranes and cytoplasm

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Microbodies

• single membrane-bound compartments for specific metabolic pathways

Glyoxysomes=special type of peroxisome in plant cells• Common in fat-storing tissue of germinating

seeds• Contain enzymes that initiate conversion of

fat to sugar• Seeds contain oils - sugar means energy!

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How the environment affects a plant cell

– Which environmental factors?

• Water stress: high or low• Temperature stress: high or low• Light stress: high or low

• Cellular responses to environmental stressCampbell 6th ed. Chapter 39

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Reponses to light– Light needed in photosynthesis

Response to low light:• Chloroplast movement• Synthesis of more pigments

Response to high light:• Chloroplast movement• Reduction in pigment content• Production of protective ‘sun-screen’ pigments

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Why are plant cells important?

• Oxygen budget: Animal (human!) Respiration

• Biomass: Food chain: EcologyAgriculture: animal and human nutrition

• Useful substances: AgricultureMedicine Industry

Cell - Tissue - Organisms - Ecosystem

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Responses to water deficit • Effect: more water lost through

transpiration than replaced from soil• Response: - Closing stomata by changing turgor- Water deficit stimulates synthesis and

release of plant hormone:Helps keep stomata closed

- Change in leaf position: e.g. grasses

- decrease in photosynthesis

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Responses to salt stress

• e.g. excess of sodium chloride in the soil

• Effect:– lowers water potential, leading to water

deficit in plant cell:– root may loose water rather than

absorb it– salts in high concentrations may be

toxic to plants

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Salt stress (contd)

• Response:Production of compatible solutes in cell:

= Organic compounds that keep water potential lower than that of soil

• Decreases water loss

• But only specialised plants (halophytes) can tolerate this for long

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Responses to Heat Stress

Effect:• Denaturation of enzymes• Damage to metabolism• increase in transpiration: causes further

stress due to high water loss Response:

• cells begin to synthesise special proteins in large quantities:

• heat shock proteins• protect enzymes from denaturation

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Responses to Cold Stress • Effect: change of fluidity of cell membranes

• Usually proteins and lipids move laterally in membrane

• Lipids become locked into crystalline structure

• Affects functions of membranes

• Alters transport across membranes

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Cold stress (contd)

• Response: change lipid composition in membrane

• e.g. increase the proportion of unsaturated fatty acids

• keeps membranes fluid at lower temperatures

• takes several days to do this:sharp changes in temperatures are harmful!

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Animal-plant interactions

• Production and accumulation of pigments (colour!) to attract or deter

• Production of chemical defences, harmful substances

• e.g. production of aminoacid canavanine which is taken up by insect

• replaces arginine: disrupts functioning of insect metabolism

• results in death of herbivores