1. Cells are the structural and functional units of life. The smallest
organisms are composed of only a single cell while the largest are made
up of billions of cells.
Even when comparing the most diverse and complex organisms, at the
cellular level they are remarkably similar. Even though the human body
has over 100 different cell types, they all share certain features and
they even have many characteristics in common with plants.
I. Cells - Hooke (1663) - described cork "cells" and "nutritive juices"
A. Development of cell theory
1. Schleiden (1838) - cells are fundamental living unit of all
plants
2. Schwann (1839) - cells are fundamental living unit of all
animals
3. Virchow (1858) - all cells come from preexisting cells
B. Cell Theory:
1. all living organisms are composed of cells
2. cells are the fundamental units of all organisms and the
chemical reactions of life take place within cells
3c. all cells come from preexisting cells
II. Two types of cells distinguish two fundamentally distinct groups of
organisms
A. Prokaryotes (before, nucleus) - no nucleus, i.e. DNA not
membrane bound
B. Eukaryotes (true, nucleus) - have a nucleus, i.e. DNA membrane
bound
C. Major differences between prokaryotes and eukaryotes:
Feature
Prokaryotes
Eukaryotes
Nucleus
no
yes

2. Membrane bound organelles
no
yes
DNA (hereditary material)
loop (no proteins)
complex chromosomes
(DNA + protein)
C. Prokaryotes perform most of the metabolic functions that
Eukaryotes do but the reactions do not take place in distinct
compartments called organelles. Prokaryotes have an outer plasma
membrane and some also have a cell wall. Only a relatively small number
of organisms are prokaryotes (3,000 species). The majority of organisms
are Eukaryotes.
TYPICAL PLANT CELL
I. Cell Wall - plant cells have cell walls made of cellulose, beta
glucose polymers.
A. Cell wall prevents the cell from being ruptured due to
enlargement from water intake by vacuole. Helps keep plants erect.
B. Cell plays an important role in absorption, transport and
secretion of many substances
C. Cell walls may have two or more layers and may vary in thickness
depending on their role in the plant. All have at least two layers:
1. Primary wall - deposited before and during growth of the
cell. In addition to cellulose may contain hemicellulose
(polysaccharide) pectin and glycoproteins. Also can become lignified.
a. Actively dividing cells as well as most mature cells
carrying on metabolic processes have only a primary wall. The cell wall
is not of uniform thickness and may contain thin areas called primary
pit fields.
b. Plasmodesmata, thin threads of cytoplasm which connect
adjacent cells are aggregated in primary pit fields.
2. Middle lamella - occurs between the primary walls of
adjacent cells and made up of pectic substances and galacturonic acids
(polysaccharides). Difficult to distinguish from primary wall,
especially after lignification.
3. Secondary wall - if present is laid down by the protoplast
of the cell on the inner surface of the primary wall. Usually occurs

3. only after cell has stopped growing and the primary wall is no longer
enlarging.
a. Very important for adding strength since different
composition than primary wall. Lacks pectins and glycoproteins but has
more hemicellulose and the cellulose is laid down in a much denser
pattern.
b. Secondary wall also has three different layers called
S1, S2, S3 in which the orientation of the fibers is different such
that it provides more strength. These multiple layers plus lignin are
found in wood and give it its strength. When secondary wall laid down
it does not cover the primary pit fields of primary wall. Therefore
there are depressions or pits in the secondary walls.
D. Growth of cell wall is poorly understood but requires loosening
of wall structure which is regulated by hormones, and an increase in
protein synthesis, respiration and water uptake. The new microfibrils
are placed on top of older, layer upon layer.
II. Protoplast - the living contents of the cell. Contains the
cytoplasm and the nucleus:
A. Cytoplasm - semifluid ground substance of the cell, forms most
of the cell mass. About 70% water. Contains organelles, several
membrane systems, cytoskeleton and ribosomes.
B. The cytoplasm is delimited from the cell wall by the plasma
membrane - a semifluid, selectively permeable lipid bilayer embedded
with proteins, carbohydrates and other chemicals. The proteins regulate
the flow of materials in and out of the cell.
1. Called fluid mosaic model because phospholipids move about
freely in the plane of the membrane and the proteins scattered about
like a mosaic.
2. Plasma membrane - keeps cells distinct from the environment:
a. mediates transport of substances into and out of
protoplast.
b. coordinates synthesis and assembly of cellulose
microfibrils that make up cell wall.
c. translates hormonal and environmental signals involved
in the control of cell growth and differentiation.
d. Plasma membrane is very sensitive and loses its
integrity with excessive heat or cold, and some chemicals.
II. Nucleus - the cell control center. Usually most prominent structure
in the cell.
A. Two major functions:
1. stores the genetic information in chromosomes and passes it
on to daughter cells during cell division or replication.

4. 2. controls ongoing activities of the cell by determining which
protein molecules (enzymes) are produced and when they are produced,
i.e. when genes are turned on and off.
B. Nucleus is enclosed by nuclear envelope made of two lipid
bilayers and perforated by tiny pores which regulate passage of
substances.
C. Contents of nucleus:
1. nucleoplasm - inner matrix of the nucleus.
2. chromatin (threadlike material) - the genetic material (DNA)
which is combined with proteins called histones. When tightly coiled
called chromosomes.
3. nucleolus (nucleoli) - one or two small bodies which are the
sites of formation of ribosomal RNA.
CONTENTS OF THE CYTOPLASM
I. Mitochondrion (Mitochondria) - powerhouses of the cell - sites of
respiration, i.e. where energy is produced by breaking down organic
molecules.
A. May be 1 to 1,000's per cell,
B. Mitochondria have two lipid bilayer membranes:
1. smooth outer membrane.
2. inner membrane folded into pleats or projections called
crista (cristae). Greatly increases surface area available for chemical
reactions to take place.
C. Mitochondria have their own ribosomes, and DNA. And their DNA
is arranged like that in prokaryotes, i.e. not associated with histones
and arranged in a loop. Supports endosymbiont theory.
II. Plastids - organelles which contain pigments and produce or store
food. Found only in plants.
A. Chloroplasts are the most common plastid.
1. contain chlorophyll in flattened sacs called thylakoids
which are arranged in stacks called grana.
B. A single leaf cell may contain 40 -5 0 chloroplasts and a square
millimeter of leaf may contain 500,000 chloroplasts.
C. Chlorophyll makes plants appear green because it absorbs reds
and blues and reflects green. Chlorophyll uses the sun's energy to make
food and structural materials.
D. Like mitochondria plastids have two membranes, their own
ribosomes, and DNA. And their DNA is arranged like that in prokaryotes,

5. i.e. not associated with histones and arranged in a loop. Supports
endosymbiont theory.
III. Vacuole - large membrane bound sac found only in plants.
A. Tonoplast = vacuole membrane
B. Central vacuole may occupy up to 90% of the volume of a mature
plant cell. It contains mostly water but often also contains sugars,
salts, proteins, citric acid, and many pigments. The contents of the
vacuole are often called the cell sap.
C. Central vacuoles help keep plants erect because when full of
water they push against rigid cell walls.
IV. Ribosomes - small beadlike structures scattered throughout the
cytoplasm or attached to the endoplasmic reticulum.
A. Made up primarily of protein and RNA.
B. Sites of protein (polypeptide) synthesis.
V. Endoplasmic reticulum (ER) - a complex 3-dimensional network of
membranes which extends from the nuclear envelope to the plasma
membrane.
A. May be smooth or rough. Rough ER has ribosomes attached for
protein synthesis and transport.
B. The ER appears to function as a communication system within the
cell as well as a system for channeling materials such as proteins and
lipids throughout cell.
C. ER of adjacent cells is interconnected by threads called
plasmodesmata.
VI. Golgi Complex - collective term for all the golgi bodies
(dictyosomes) of a cell.
A. Golgi bodies are groups of flat, disk-shaped sacs, each of which
is called a cisterna (pl. cisternae).
B. Involved in secretion, processing and packaging materials for
storage or transport to other areas, e.g. in plants they help form cell
wall.