Cells and Seeds

Chapter 1

 

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

Early microscopes

•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.

Modern Microscopes

•Light microscopes

–compound and dissecting microscope

–maximum magnification about 1000x

•Electron microscopes

–Useful magnifications of 200,000x

–Transmission (TEM) and scanning electron microscopes (SEM)

 

Eukaryotic versus Prokaryotic Cells

•Prokaryotic cells

–lack membrane-bound nuclei and other organelles

–Bacteria only

•Eukaryotic cells

–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

Cytoplasmic membrane

•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

 

Nucleus

•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

 

Chloroplasts

•organelles containing green pigment (chlorophyll)

•site of photosynthesis

•2-10 ΅m in length (visible in light microscope)

•outer and inner membranes present

 

Mitochondria

•"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)

Ribosomes

•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

Vacuoles

•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

Cell Wall

•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

Cell Wall

•primary walls formed first

•then secondary walls may be formed

•in woody plants, lignin may be added to give mechanical strength to wall

 

Cyclosis

•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

•video

 

Growth Process

•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.

Meristems

•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

Interphase

•State that most cells within meristems are in, between mitosis events.

•Cells increase in size, replicate chromosomes, and prepare for next mitosis event. 

 

Prophase

•Chromosomes become shorter, thicker and appear double (two identical chromatids)

•nucleolus and nuclear membrane begin to disappear

Metaphase

•Chromosomes become aligned in a plane in the center of the cell (the equator)

Anaphase

•two chromatids of each chromosome separate and move to opposite poles

Telophase

•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

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

–dispersal

–survival through harsh conditions

Seed structure

•Hilum

•Micropyle

•Seed Coat

•Embryo

–Cotyledons = seed leaves

•may be used for food storage

–Embryo axis

•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)

 

Germination

•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

•Scarification

–nicking or breaking seed coat

•After-ripening

–embryo needs further development

•Stratification

–cold temperature treatment

•Heat treatment

–for desert plants

•Inhibiting chemicals on seeds

Triggers to break dormancy (con’t)

•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

•Fire

–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