The Plant Cell

EARLY STUDIES OF CELLS

English scientist Robert Hooke was the first person to describe cells in 1665.

Hooke examined the microscopic structure of cork and found that it was organized into small units which resembled the cubicles in monasteries where monks slept. These rooms were called "cells."

He gave that name to the little compartments in cork and the term was eventually applied to mean the basic unit of life.

Although the cork cells were not living, Hooke later looked at living plants and described cells there also.

Cell Theory

In the mid-nineteenth century, scientists firmly established the Cell Theory which is one of the major principles in biology. Cell Theory recognizes The cell as the basic unit of life. All organisms are composed of cells and All cells arise from pre-existing cells.

THE PLANT CELL

All plants are composed of cells. Angiosperms are complex multicellular

organisms composed of many different types of cells.

Cell Wall

Found surrounding plants cells not animal cells

Surrounds all the other parts of the cell (other parts called the protoplast)

Plant cell walls may consist of one or two layers called primary and secondary wall

Middle Lamella is a layer of sticky material between walls of adjacent cells

Primary Wall

The first wall layer is the primary wall It is formed early in the life of a plant cell Composed of cellulose and other

polysaccharides

Wall Strucuture

The cellulose is in the form of fibrils, extremely fine fibers, which can only be seen with an electron microscope.

The fibrils are embedded in a matrix of other polysaccharides.

Secondary Wall

The secondary wall develops internal to the primary wall in some cells.

Only cells that are specialized for support, protection, or water conduction have secondary walls.

Lignin is a major component of the secondary wall in addition to the cellulose and other polysaccharides.

Lignin

Complex organic molecule that is very difficult to break down

Provides support to plant cells and gives wood its characteristic strength

Provides protection against attack by pathogens and consumption by herbivores.

Only one group of wood rotting fungi are able to decompose lignin

Cell Wall Components

Important component of worldwide biomass

Because of all the plant material in the world Cellulose is the most

abundant organic compound on Earth

Lignin is a close second

Wooden artifacts from ancient Egypt•Throne from tomb of Tutankamen ~ 3300 yrs old•Funeary barge for Khufu at Giza ~ 4600 yrs old

Wall Openings

Cell wall is not a solid structure. Minute pores exist called pits Some pits are large enough to be seen

with the light microscope. Pits allow for the transfer of materials

from cell to cell through cytoplasmic connections called plasmodesmata.

The Protoplast

All of the plant cell enclosed by the cell wall.

It consists of the nucleus plus the cytoplasm

Cytoplasm contains a variety of organelles - each with a specific function

The Plant Cell

Nucleolus

Cell Wall

Ribosomes

Cell Structures and Functions

Cell Wall -- Support and protection Plasma Membrane -- Regulates passage of

materials into and out of cell Nucleus -- Control center of cell, directs

protein synthesis and cell reproduction Nucleolus -- Ribosome formation Ribosomes -- Protein synthesis Endoplasmic Reticulum -- Transport and

protein synthesis (rough ER)

Cell Structures and Functions Golgi apparatus -- Processing and packaging of

proteins, secretion Mitochondrion -- Cellular respiration Chloroplast -- Photosynthesis Leucoplast -- Storage, especially starch Chromoplast -- Imparts color Central Vacuole -- Storage of various substances Cytoskeleton -- Cell support and shape Plasmadesma -- Movement of materials between

cells

Plasma membrane

The outermost layer of the protoplast Composed of lipids (phospholipids) and

proteins. Acts as a permeability barrier allowing

some molecules to pass but not others

Fluid Mosaic Model

ProteinsLipid bilayer

Diffusion and OsmosisHow things move in and out of cells

Cells constantly exchange materials with their environment. One way this occurs is by diffusion.

Diffusion is the movement of particles or molecules from areas of higher concentration to lower concentration.

Diffusion

Diffusion also occurs within living organisms, but the membranes present barriers to this movement of molecules.

Membranes are differentially permeable. They permit the diffusion of some molecules, but present a barrier to the passage of other molecules.

Osmosis

The diffusion of water across cell membranes is called osmosis.

Water can move freely through membranes. The direction the water molecules move is

dependent upon the relative concentrations of substances on either side of the membrane, moving from high concentration to low

Osmosis

If you place a cell in a highly concentrated solution of salt or sugar, water will leave the cell.

The water is actually diffusing from an area of high concentration to an area of lower concentration.

Hypertonic solution

Osmosis

If a cell is left in a hypertonic solution, for any length of time, so much water will leave that the protoplast actually shrinks away from the cell wall.

When this happens the cell is said to be plasmolyzed. In a wilted leaf many of the cells would be plasmolyzed.

Osmosis

On the other hand, if you place a cell in distilled water, water will enter the cell. Again the water is moving from higher (outside the cell) to lower concentration

Hypotonic Solution

Osmosis

When a plant cell is in a hypotonic, solution, water will enter until the vacuole is fully extended pushing the cytoplasm up against the cell wall. Such cells look plump, or turgid; this is the normal appearance of cells in a well-watered plant.

Osmosis

When the cell is placed in a solution of the same concentration, isotonic, there is no net movement of water and the cell is not turgid.

Membrane Transport

Diffusion take place when molecules move along a concentration gradient.

Cells can also move substances against a concentration gradient Called active transport Requires energy by the cell Membrane proteins transport these

substances across the membrane

Summary I - Cells

All living organisms are composed of cells - cells are the basic unit of life.

Plant cells are eukaryotic having an organized nucleus and membrane bound organelles.

Substances can move in and out of cells by diffusion and osmosis or be carried in or out by active transport.

CELL DIVISION The Cell Cycle

The life of an actively dividing cell can be described in terms of a cycle which is the time from the beginning of one division to the beginning of the next

Cell Division

Interphase The time between successive divisions is known

as interphase and consists of 3 phases:• The G1 Phase or first gap phase. The cell is

actively growing• The S or synthesis phase when DNA is

duplicated; other chromosomal components are also synthesized

• The G2 or second gap phase where the final preparations for cell division take place

Appearance of anInterphase Nucleus Chromatin consists of

DNA and protein, Prominent in the

nucleus of interphase cell

Chromatin appears thread-like

Duplicated during the S phase prior to mitosis.

Chromatin

Nucleolus

Cell Division Mitosis - two exact copies of the

nucleus result from a process known as mitosis.

Cytokinesis, the division of the cytoplasm, usually occurs during the later stages of mitosis.

Mitosis Consists of Four Intergrading stages

Prophase Metaphase Anaphase Telophase

Prophase Nucleus changes dramatically. Chromatin begins to condense and thicken, coiling up

into bodies referred to as chromosomes. Each chromosome is composed of two chromatids. By the end of prophase the chromosomes are fully

formed. Nuclear membrane and the nucleoli disperse into the

cytoplasm and are no longer visible. This leaves the chromosomes free in the cytoplasm.

Mid to Late Prophase

Chromosomes Each chromosome

is composed of two identical chromatids.

The chromatids are joined at the centromere

Chromatids

Metaphase The chromosomes arrange themselves across

the center of the cell The spindle, composed of microtubles is

completed Spindle fibers stretch from each pole of the

cell to the centromeres of the chromosomes Other spindle fibers stretch from pole-to-pole

Late Metaphase to Early Anaphase

Anaphase Chromatids of each chromosome separate,

pulled by the spindle fibers to opposite ends of the cell.

This divides the genetic material into two identical sets each with the same number of chromosomes.

At the end of anaphase the spindle is no longer visible.

Telophase During telophase the chromatin reappears as

the chromosomes, at each end of the cell, begin to unwind and lengthen.

At each pole a nuclear membrane reappears around the chromatin.

Two distinct nuclei become evident. Within each nucleus, nucleoli become

visible.

Early Telophase

Cytokinesis The division of the cytoplasm separates the

two identical nuclei into two cells. Cytokinesis begins during the latter part of

anaphase and is completed by the end of telophase.

The cell plate forms and becomes the cell walls separating the newly formed daughter cells

Summary

Cell Division and Asexual Reproduction

The production of new cells through cell division enables plants to grow, repair wounds, and regenerate lost cells.

Cell division can lead to the production of new genetically identical individuals or clones. This type of reproduction is known as asexual reproduction.

Many crops are propagated asexually and are gentically identical

Summary II - Mitosis Mitosis followed by cytokinesis, results

in two genetically identical daughter cells

Growth, replacement of cells, and asexual reproduction all depend on the process of cell division