CELLS

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

3. 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:

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 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.

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, 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.

OTHER CELLULAR STRUCTURES

I. Cytoskeleton - 3-D network of protein fibers which provides structural framework for the cell and suspends the organelles.

II. Ergastic substances - passive products, i.e. not directly involved in metabolism of cell. May be storage compounds or waste products.

A. May appear in the cytoplasm, cell wall or in organelles. E.g. starch grains, crystals (silica), pigments, oil droplets, resins, gums, tannins.

III. Flagella and cilia - hairlike structures which extend from the surface of some eukaryotic cells. 

A. Mostly function in locomotion (movement).

B. Structurally flagella and cilia are the same but flagella are longer than cilia.

C. All cilia/flagella have 9 + 2 pattern, i.e. an outer ring of nine pairs of microtubules which surrounds two central microtubules. This supports the theory of evolution.

DIFFERENCES BETWEEN PLANT AND ANIMAL CELLS

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