Cells and Seeds
Attributes of Plants
Chlorophyll - most plants possess this pigment
Photosynthesis - most plants can produce their own food
Continuous growth throughout life
Special zones of growth (meristems)
Production of cellulose (for cell walls)
Fixed position throughout life
Complex structure involving specialized cells organized into tissues
First primitive microscopes invented by Dutch spectacle makers in 1590
Advanced by Anton van Leeuwenhoek through 1600's. Saw the first living microscopic organisms, and called them animalcules.
The first cell discovery
Robert Hooke, English physicist. Examined cork slices, used the term "cell" (1665) to describe compartments seen in cork tissue
"These pores or cells, were not very deep, but consisted of a great many little boxes, separated out of one continued long pore, by certain diaphragms.
Cell Theory: All living things made of cells
Matthias Schleiden and Theodor Schwann both published separate conclusions that expressed the principle of cell theory, 1838-1839.
All living things are composed of one or more cells.
Biogenic Law: Cells come from cells
Rudolf Virchow, 1858.
Every living cell comes from a preexisting cell
No spontaneous generation of cells.
Many still did not believe this, thought animals could arise from dust.
Evidence for Biogenic Law
1860 contest sponsored by Paris Academy of Sciences to prove or disprove spontaneous generation.
Louis Pasteur's experiments showed that boiled substance remains sterilized if isolated from microorganisms in air.
compound and dissecting microscope
maximum magnification about 1000x
Useful magnifications of 200,000x
Transmission (TEM) and scanning electron microscopes (SEM)
Eukaryotic versus Prokaryotic Cells
lack membrane-bound nuclei and other organelles
have nuclei and other organelles
Protists, Plants, Fungi, and Animals
Cell Size and Structure
Plant cells are surrounded by a non-living cell wall.
Inside cell wall is the living protoplasm, which contains the nucleus and cytoplasm.
Cytoplasm = cytosol, a jelly-like fluid, and organelles, small bodies for specific functions.
a typical plant cell
not all structures are visible with light microscope
Size and Number
In higher plants cells are usually 10-100 ΅m long
(1000 ΅m = 1 mm)
Pear tree leaf contains ~ 50 million cells
Full grown pear tree contains ~ 15 trillion cells
the cytoplasmic membrane (also called cell or plasma membrane) surrounds the cell
composed of double layer of phospholipid molecules
other molecules present, such as proteins
semi-permeable, can regulate what passes in and out of cell
Most conspicuous organelle
Control center of the cell
Contains the chromosomes
chromosomes made of DNA and protein
genes are located along DNA strands
chromosomes found in pairs
organelles containing green pigment (chlorophyll)
site of photosynthesis
2-10 ΅m in length (visible in light microscope)
outer and inner membranes present
"Power-houses" of the cell
Extract energy from food in process called respiration
Small organelles, 1-3 ΅m in length (too small to see in light microscope)
Function in protein synthesis by linking amino acids together
Tiny! Approximately 17-25 nm in diameter (1 ΅m = 1000 nm), much too small for light microscope
membrane bound sacs filled with a watery fluid called cell sap
cell sap contains salts, sugars, organic acids, and may contain water-soluble pigments
small and numerous in size in new cells but increase in size as cell matures
often transparent: presence is detected by the space it takes up, not by seeing it directly
found outside cytoplasmic membrane
consists of mostly cellulose
cellulose molecules grouped into bundles known as microfibrils
pectins form the middle lamella, act as cement
plasmodesmata interconnect living plant cells
primary walls formed first
then secondary walls may be formed
in woody plants, lignin may be added to give mechanical strength to wall
cytoplasmic streaming (rotation of cytoplasm around inside of cell) exhibited by all living cells
facilitates exchange and movement of materials within a cell
microfilaments and microtubules generate the movement of the cytoplasm
Plants grow through a combination of two processes, cell division and cell elongation
Cell division occurs only in regions of the plant called meristems.
The part of cell division in which two exact copies of all the chromosomes and the genes they contain are divided into two new nuclei is called mitosis.
Cytokinesis = division of the cytoplasm following mitosis.
Regions of cell divisions in plants
Apical meristems are located at the tips of both roots and shoots (in buds).
responsible for growth in length, called primary growth.
Lateral meristems, also called cambium, are located along the length of roots and shoots.
responsible for growth in width, called secondary growth.
Phases of Mitosis
State that most cells within meristems are in, between mitosis events.
Cells increase in size, replicate chromosomes, and prepare for next mitosis event.
Chromosomes become shorter, thicker and appear double (two identical chromatids)
nucleolus and nuclear membrane begin to disappear
Chromosomes become aligned in a plane in the center of the cell (the equator)
two chromatids of each chromosome separate and move to opposite poles
Each group of daughter chromosomes becomes surrounded by a new nuclear envelope
Daughter chromosomes become longer and thinner
New nucleoli appear, cell plate forms
Seeds: alive or dead?
Seeds have very low metabolic activity
Considered dormant, not dead
Viable seed can germinate, grow new plant
What kinds of plants produce seeds?
Benefits of seeds:
reproduction with genetic recombination
survival through harsh conditions
Cotyledons = seed leaves
may be used for food storage
Epicotyl (stem axis above cotyledon attachment)
Hypocotyl (stem axis below cotyledon attachment)
Radicle (embryonic root)
Plumule (embryonic shoot with immature leaves)
Stored food in seeds
as endosperm or in cotyledons
embryo and seedling rely on stored food until they can produce their own
heterotroph (embryo) vs. autotroph (green plant)
To germinate, dormancy must be broken
Seeds easily absorb water = imbibition
Seed swells, seed coat ruptures
If other conditions met, embryo will grow
stored food breaks down
energy released through cellular respiration
mitosis and cell expansion
Triggers to break dormancy
nicking or breaking seed coat
embryo needs further development
cold temperature treatment
for desert plants
Inhibiting chemicals on seeds
Triggers to break dormancy (cont)
Allelopathy (mutual suffering) - a plant releases chemicals to prevent germination of other seeds nearby (examples: walnut and madrone trees)
Presence of red light
absorbed by overhead canopy
if received by seed, less competition
like scarification, especially in chaparral vegetation
Longevity of Seeds
Viability = seeds capable of germinating
Certain seeds retain capacity to germinate for many years, sometimes hundreds or thousands
arctic lupine frozen in permafrost for 10,000 yrs
Others viable only briefly
seeds of cottonwood tree viable for 24 hours once released