Cells - plant cytoplasmic activities

How are chloroplasts motile organelles?

• Movement avoids photo-damage - process of photorelocation

• Requires three steps: photoperception, signal transduction, chloroplast movement

What is the ‘sliding theory of cytoplasmic streaming’?

• Motile endoplasm moves against a stationary exoplasm

• Force is generated by myosin motors that move along actin cables (bundles of F-actin)

• Cytoplasmic streaming is mediated by plant-specific myosins - XI, XIII, and VIII

Where does cell plate formation begin in a plant cell?

• Starts in the middle of the cell

• Plasmodesmata begin to form - endoplasmic reticulum crosses the cell plate during cytokinesis

How do plant cells divide?

• Cortical interphase microtubule array concentrates in the middle of the cell

• Nuclear envelope breaks down and a spindle is formed

• Cytokinesis is accompanied by de novo synthesis of the plasma membrane and cell wall

• A microtubular array (phragmoplast) guides and supports the formation of the cell plate

• The microtubules guide secretory vesicles to the forming cell plate

• Vesicles of the cell plate form the cell wall in their interior - fuse to give rise to the primary cell wall

• Plasmodesmata are kept in the newly forming primary cell wall

• The cell plate initiates in the cell centre and expands outwards until it fuses with the existing plasma membrane at the cortical division zone

What are the stomata and what is their structure?

• Openings in the epidermis of plant leaves - epidermal cells have no chloroplasts

• Enclosed by a pair of guard cells

• Mediate the exchange of water and gas between the leaf tissue and the environment

• Contain chloroplasts

• Open and close in response to environmental stimuli

What stimuli cause stomata to open/close?

• Open: light, low CO2, high temperature

• Close: low light/darkness, high CO2, high water content gradient

What are the guard cells?

• Responsible for the dynamic behaviour of stomata

• Stomata open due to a swelling of the guard cells

• Chloroplasts provide the required ATP

• Turgor pressure is generated by an increase of potassium in vacuoles

What are trichomes?

• ‘Hair-like’ extensions of plant leaves

• Outgrowths of the epidermis (leaf, flower, stem)

• Found in ~30% of all vascular land plants

• Important sites of secondary metabolite production

• Develop from epidermis cells

What are the two types of trichomes and their functions?

• Glandular: biosynthesis, secrete and store secondary metabolites; defend against microbes, insects and animals

• Non-glandular: UV protection; plant defence against herbivores and mechanosensing

What do the phloem and xylem transport respectively?

• Phloem: sucrose from ‘sources’ to ‘sinks’

• Xylem: water

What is the structure of the xylem?

• Consists of trachea of dead and empty cells

• Cell wall is enforced by lignin deposits

What is the structure of the phloem?

• Sieve cells form a sieve tube

• Sieve cells have lost their ribosomes and most organelles so rely on the companion cells for survival

• Cells are separated by ‘sieve plates’

What are the main differences between the xylem and phloem?

• Xylem: one-way transport; transports water and minerals; dead cells; no end walls (with exceptions); lignified outer walls

• Phloem: two-way transport; transports sucrose and amino acids; living cells - require support; contain sieve plates

What is the pressure flow hypothesis for phloem?

• Flow of assimilates in sieve tubes can be explained by the pressure flow hypothesis

• Hypothesis describes a mechanism by which osmotically generated intracellular pressure (turgor pressure) generates a bulk flow of sugar, water and other assimilate

How does the pressure flow hypothesis explain the transport of dissolved assimilates in the phloem?

• Photosynthetic source cells produce sucrose

• Efflux of sucrose from the source causes an influx of protons and sucrose into the companion cell

• Sucrose molecules migrate into the phloem cells via symplastic transport through the plasmodesmata

• Osmotic water influx from the xylem increases the turgor pressyre

• Turgor pressure drives the flow of sucrose and other assimilates

• Sucrose travels into the sink cells and water travels back into the xylem

• Sucrose is stores into the vacuole of the sink cell - moved in via active transport