Chapter 4

CELLS: UNITS OF LIFE

Cells- basic units of life.

•Cell membrane separates living matter from the environment.

•Complex cells house specialized structures, called organelles.

•Interior of cells consists of cytoplasm.

•Cell biologists use a variety of microscopes to magnify different types of images of cell contents.

The study of cells (cell biology) – began in 1660, when Robert Hooke found out remnants of cells that were once alive in abark from a type of oak tree.

Called these units “cells” as they looked like cubicles.

In 1673, Antonie van Leeuwenhoek- used lens.“To my great surprise, I found that it contained many very small animalcules, the motions of which were very pleasing to behold. The motion of these little creatures, one among another, may be likened to that of a great number of flies disporting in the air”.

Lens reveal the world of cell

A. The Early Years• Robert Hooke (1660)

• 1st person to see the outlines of cells

• Antonie van Leeuwenhoek (1673)• developed high magnification lenses• 1st record of microorganisms

• Matthias Schleiden & Theodore Schwann (1839) & Rudolph Virchow (1855)• contributed to the cell theory

Cell Theory1. All living things are composed of

cells. [Schleiden & Schwann].

Cells “elementary particles of organisms, the unit of structure and function”

2. All cells come from preexisting cells. [Virchow]

Range of Light, Electron and Scanning Probe Microscopes

B. Cell Size• Most are 1-100m in diameter• Smallest?• Largest?• Why can’t cells grow to be as large

as a car?• - surface area/volume ratio• - as cell grows, its volume

increases more rapidly than its surface area

C. Types of Cells3 basic types:• Bacterial• Archaean Prokaryotic

• Eukaryotic

Cells of the Three Domains of Life

1. Bacterial cells • 1-10 m in diameter • NO membrane-bound organelles• 1 circular DNA molecule located in

nucleoid region• plasma membrane, cytoplasm &

ribosomes• most have a cell wall

(peptidoglycan)• may have a polysaccharide

capsuleEx. bacteria & cyanobacteria

Examples

A. E. coli- inhabits the intestines of animal species, including humans.

B. Streptococcus pyogenes

C. Cyanobacteria

Bacteria – classified based on

•Cell wall structure

•Biochemical characteristics (metabolic pathways and shape)

Bacterial cells may be

Round (cocci)

Rod shaped (bacilli)

Spiral (spirilla)

Comm-shaped (vibrious)

Spindle-shaped (fusiform)

2. Archaean cells• 1-10 m in diameter• NO membrane-bound organelles• cell walls lack peptidoglycan• have characteristics of both

bacteria & eukaryotic cells

Ex. methanogens, extreme halophiles & extreme thermophiles

Archaea

transport ions within their cells like bacteria

Like eukaryotes histone proteins are associated with their genetic material.

Identified from environments that have extremes of temperature, pH or salinity.

3. Eukaryotic cells• 10-100 m in diameter • nucleus & other membrane-bound

organelles• 2 or more linear DNA molecules

located in nucleus• plasma membrane, cytoplasm &

ribosomes• some have a cell wall (cellulose or

chitin) Ex. plants, animals, fungi, protista

Three basic types of cells

Bacteria do not contain organelles.

Archaea – share features of both bacteria and eukaryotes and have unique biochemical featues.

Eukaryotic cells use membranes to increase surface area to allow bigger cells

Provide compartmentalization for unique chemical processes.

Generalized Generalized Animal Cell Plant Cell

D. Organelles of Eukaryotic CellsOrganelles:• compartmentalize a cell’s activities

• keep reactions isolated from one another

• increase efficiency in the cell

Organelles synthesize and process proteins

Genes- (instructions for building each protein) stored in the nucleus.

Connected to nucleus- endomembrane system (series of compartments formed from highly folded membranes.

In Rough endoplasmic reticulum (ER)- cell’s proteins are manufactured.

Other compartments have unique enzymes which process the synthesized proteins.

Smooth endoplasmic reticulum- lipids are synthesized and modified, toxins – neutralized.

Golgi apparatus- complete the processing and sorts proteis for export out of cell or into lysosomes (digestive enzymes).

Other compartments within the cell provide energy, contain special enzymes for oxidation reactions, or store useful molecules.

1. Nucleus (exports RNA instructions)• surrounded by a double

membrane (nuclear envelope), perforated with nuclear pores

• contains DNA & nucleolus (stores RNA nucleotides)

• functions to separate DNA from rest of cell

The Endomembrane System a series of compartments made

from folded membrane and connected via vesicles

• endoplasmic reticulum• rough• smooth

• Golgi apparatus• lysosomes

2. Endoplasmic reticulum (ER)• interconnected network of membranes extending from nucleus to plasma membrane

Rough ER - studded with ribosomes • site of protein synthesis (most will

be exported out of the cell)Free ribosomes in the cytoplasm synthesize

proteins that remain in cell.

Smooth ER - lacks ribosomes• site of lipid synthesis• contains enzymes that detoxify

drugs & poisons

3. Golgi apparatus • stacks of membrane-enclosed

sacs

Functions:• links simple carbohydrates

together to form starch• links simple carbohydrates to

proteins (glycoprotein) or lipids (glycolipid)

• completes folding of proteins• temporarily stores secretions (milk)

Organelle interaction in a mammary gland cell.

4. Lysosomes (suicide sacs)• vesicles containing > 40 types of

digestive enzymes

• function to recycle damaged organelles, break down cellular byproducts & destroy invading microbes

5. Peroxisomes (oxidative reactions)

• vesicles containing several types of enzymes (produced in cytoplasm)

• found in all eukaryotic cells• function to help cell use oxygen

& metabolize potentially toxic compounds

• hydrogen peroxide is produced as a by-product of peroxisome activity

• the enzyme catalase converts hydrogen peroxide to water

• #/cell varies• contain DNA• inherited from female parent• site of cellular respiration

(production of ATP)

6. Mitochondria• double-membrane

• outer is smooth• inner is highly

folded (cristae)

7. Chloroplasts• possess 3 membranes

• outer/inner membranes surround stroma• 3rd membrane system folded into flattened

sacs (thylakoids)

• #/cell varies • contain DNA• found in plants & protists• function in photosynthesis

E. The Endosymbiont TheoryProposes that chloroplasts and

mitochondria evolved from once free-living bacteria engulfed by larger archaea.

Based on fact that mitochondria & chloroplasts resemble certain bacteria (size, shape, membrane structure & method of making proteins).

Summary

Cell theory

Types of cells

Bacteria do not contain organelles

Archaea – unique cells

Eukaryotic cells- compartmentalization for unique chemical processes.

Organelles- synthesize and process proteins

Nucleus- exports RNA instructions

Nuclear pores- movement of RNA from the nucleus to the cytoplasm

Ribosomes- manufactured in the nucleolus and transported to the cytoplasm.

The endomembrane system- site for protein synthesis and provides mechanisms for protein processing and packaging.

Rough ER- involved in synthesis

Smooth ER and Golgi apparatus- process proteins

Vesicles- used to package and transport proteins

Lysosomes- packages of digestive enzymes- used to recycle cellular components and release nutrient molecules from ingested products.

Peroxisomes store oxidative enzymes – breakdown molecules and protect cell.

Mitochondria and chloroplasts- unique membrane structure

Manipulate energy by manufacturing nutrients or extracting energy from them.

Contain unique DNA

1.DNA is found in the ---------- of bacterial cells

2.The interior of the cell minus the nucleus is called ----

3.Name the organelle essential for protein synthesis.

4.The site of synthesis of ribosomes in the nucleus is called ------

5.Water storing organelle in plant cell is called ---

6.Which organelle contains the digestive enzymes?

7.Name the enzyme present in peroxisomes.

Questions