Cytoskeleton Plastids, Vacuoles

Cytoskeleton

• present in all eukaryotic cells.

• 3-dimensional, interconnected network of fibrous protein.

• Components:

1. Microtubules – long rods (~24 nm in diam.), assembled from subunits of a globular protein called tubulin, which is made up of 2 polypeptides (α-tubulin and β-tubulin).

The microtubules are formed when tubulin subunits spontaneously self-assemble into long chains called protofilaments. Protofilaments line up laterally to form the microtubule wall.

2. Microfilaments – solid threads composed of actin, also a globular protein. A microfilament is composed of 2 parallel chains of actin twisted to form a helix.

3. Intermediate Microfilaments (little is known in plant cells; more common in animal cells).

(A) Microtubule (B) Microfilament

Cell Plate (Phragmoplast) Formation

in Cytokinesis

• Cytoplasmic streaming – continuous flow of

cytoplasmic particles and organelles around the

periphery of the cell involves the formation of

microfilament bundles parallel to the direction of

cytoplasmic flow.

• Tip growth – example: pollen tube develops a tubular

extension that grows down the stigma of the flower and

serves to deliver the male nucleus to the egg.

Microtubules guide the cell wall precursors through the

cytoplasm to the growing tip.

Organelles Unique to Plants –

Plastids & Vacuoles

Plastids

• double membrane-bound organelles in plants.

• contain their own DNA (in nucleoid region) and ribosomes.

• semi-autonomous and reproduce by fission similar to the division process in prokaryotes.

If plastids only arise from other plastids and can’t be built "from scratch", then where do they come from?

The egg. Plastids are inherited cytoplasmically, primarily through the female. (However, there are examples of paternal inheritance of plastids.) The plastid DNA carries several genes (e.g., large subunit of rubisco, and genes for resistance to some herbicides.

• Chemistry of the membranes differs from the plasma membrane -plastid membranes are comprised of glycosylglycerides rather than phospholipids (phosphate in the polar head group in glycosylglycerides is replaced with galactose or a related sugar).

Types of plastids

1. Proplastids - small, precursors to the other plastid types, found in young cells, actively growing tissues;

2. Chloroplasts - sites of photosynthesis (energy capture). They contain photosynthetic pigments including chlorophyll, carotenes and xanthophylls. The chloroplast is packed with membranes, called thylakoids. The thylakoids may be stacked into pancake- like piles called grana (granum, singular). The "liquid" material in the chloroplast is the stroma. A chloroplast is from 5-20 μm in diameter and there are usually 50-200 per cell. The chloroplast genome has about 145 kbase pairs, it is smaller than that of the mitochondria (200 kbases). About 1/3 of the total cell DNA is extranuclear (in the chloroplasts and mitochondria).

3. Chromoplasts - non-photosynthetic, colored plastids; give some fruits (tomatoes, carrots) and flowers their color;

4. Amyloplasts - colorless, starch-storing plastids.(leucoplast - another term for amyloplast)

6. Etioplasts - plastids whose development into chloroplasts has been arrested (stopped). These contain a dark crystalline body, prolamellar body, which is essentially a cluster of thylakoids in a somewhat tubular form.

Plastids can dedifferentiate and convert

from one form into another. For example,

think about the ripening processing in

tomato. Initially, green tomatoes have

chloroplasts which then begin to

accumulate lycopene (red) and become

chromoplasts.

Usually you find only chromoplasts or

chloroplasts in a cell, but not both.

Vacuoles

• This is the large, central cavity containing fluid,

called cell sap, found in plant cells. The

vacuole is surrounded by a membrane

(tonoplast).

• The vacuole is penetrated by strands of

cytoplasm - transvacuolar strands.

• The tonoplast and plasma membrane have

different properties, such as thickness (tonoplast

thicker).

• The vacuole makes up to 90% or more of the cell volume. Meristematic and embryonic cells are exceptions. Young tissues have many small vacuoles (provacuoles). As the cell grows the vacuoles expand and eventually coalesce. These small vacuoles appear to be derived from the Golgi.

• central vacuole -- contains water, ions, organic acids, sugars, enzymes, and a variety of secondary metabolites.

• lytic vacuoles -- contain digestive or hydrolytic enzymes, e.g., proteases (digest protein), ribonucleases (digest RNA) and glycosidases (break links between monosaccharides). These enzymes are typically not used for recycling cellular components

but rather leak out on cell senescence.

• protein bodies -- store proteins.

Lytic Vacuoles and Protein Bodies

Why do plants cells have a

large central vacuole?

Important roles of the vacuoles:

• A. Energetically efficient means to increase surface to volume ratio in the dendritic growth form

Since 90% of the cell volume is vacuole, therefore ~90% of the cell is water, which is relatively cheap in metabolic terms. Thus, it allows plants to get big with a minimal energy investment.

Plants are particularly 'smart,' since, the cell wall, which comprises much of the remaining 10% or so of the cell is a polymer of glucose.

Cellulose is a great bargain! It is stronger and

cheaper than comparable polymers.

polymer

compound/production

value

(weight product/weight glucose to make)

tensile strength (newton m2 x 109)

cellulose 0.83 30

collagen 0.40 2

silk 0.40 10

• B. Water storage

Probably a minor role; mostly in succulents

• C. Waste disposal

- the vacuole is analogous to the lysosome.

• D. pH regulation

The vacuole is a pool to dump excess

protons. There is an active proton pump in the

tonoplast. The cell sap has a pH of 2-5.7,

whereas the cytosol is ca. 7.0.

• E. Storage of essential ionsIons are pumped into the vacuole for water

balance. Potassium and calcium in particular, are stored in the vacuole.

• F. Cell enlargementCell growth requires some force to allow for

the cell to increase in size. Water pressure provides the force and it moves into the vacuole. For example, root hair enlargement is due entirely to vacuolar enlargement.

• G. Facilitates diffusionThe cytosol essentially forms a thin coating around the

large vacuole which in effect, increases the surface-to-volume ratio of the cytoplasm. It provides easier access and shorter diffusion distances between any part of the cytoplasm and the external environment of the cell. This can be particularly important for chloroplasts.

• H. Structural supportThe vacuole helps to maintain turgor pressure in plant

cells due to the opposing forces of tensile strength of the wall vs. compression strength of water.

Plasmodesmata Interconnect Living Plant Cells

• membrane-lined channels traversing the cell wall and connect the cytoplasms of adjacent cells forming a continuum (symplast).

cell-cell communication

• contain cytoplasm, plasma membrane, and desmotubule.