LECTURE 3

CELL STRUCTURE

HISTORY• The cell was first discovered by Robert Hooke in 1665

examining very thin slices of cork and saw a multitude of

tiny pores that remarked looked like the walled

compartments of a honeycomb which was then called

cells

• The description of these cells (which • The description of these cells (which

were actually non-living cell walls),

without structure or function

understanding, was published in

Micrographia. His cell observations

gave no indication of the nucleus and

other organelles found in most living

cells.

CELL THEORY• Cell theoryCell theoryCell theoryCell theory refers to the idea that cells are the basic

unit of structure in every living thing. • Cell Theory (the study of cells is cytology)Cell Theory (the study of cells is cytology)Cell Theory (the study of cells is cytology)Cell Theory (the study of cells is cytology)

– All organisms are composed of one or more cells.

– The cell is the basic living unit of organization for all living

things

–– All cells arise from preexisting cells

– Cells contain all of the hereditary information

• Basic structures of all cell types:Basic structures of all cell types:Basic structures of all cell types:Basic structures of all cell types:– plasma membrane plasma membrane plasma membrane plasma membrane ---- a physical boundary that separates a physical boundary that separates a physical boundary that separates a physical boundary that separates

the cell from the outside environmentthe cell from the outside environmentthe cell from the outside environmentthe cell from the outside environment

– organellesorganellesorganellesorganelles---- internal structures that are suspended I the internal structures that are suspended I the internal structures that are suspended I the internal structures that are suspended I the

cytoplasmcytoplasmcytoplasmcytoplasm

– cytoplasmcytoplasmcytoplasmcytoplasm –––– sort of jellysort of jellysort of jellysort of jelly----like substance, mostly water.like substance, mostly water.like substance, mostly water.like substance, mostly water.

Types of cellsTypes of cellsTypes of cellsTypes of cells

• Cells can be subdivided into the following

subcategories:

– Prokaryotes: Prokaryotes lack a nucleus (though they do

have circular DNA) and other membrane-bound organelles

(though they do contain ribosomes). Bacteria and Archaea

are two divisions of prokaryotes.are two divisions of prokaryotes.

– Eukaryotes: Eukaryotes, on the other hand, have distinct

nuclei and membrane-bound organelles (mitochondria,

chloroplasts, lysosomes, rough and smooth endoplasmic

reticulum, vacuoles). In addition, they possess organized

chromosomes which store genetic material.

Prokaryotes vs Eukaryotes

Prokaryotes

– Includes cyanobacteria and

bacteria

– lack most of the membrane

bound organelles,

Eukaryotes

– Protists, fungi, plants

and animals…

– specialized membrane

bound organelles

(nucleus), nuclear – have DNA in a concentrated

region,

– tend to grow rapidly and

divide often.

– prokaryotes (0.2 – 5 µm)

smaller than eukaryotes…

(nucleus), nuclear

membrane

– eukaryotes (10 – 100

µm)

Prokaryotic Cell

• Nucleoid: DNA concentration

• No organelles with membranes

• Ribosomes: protein synthesis

• Plasma membrane (all • Plasma membrane (all cells); semi-permeable

• Cytoplasm/cytosol (all cells)

In a prokaryotic cell, most of the functions of organelles, such as

mitochondria, chloroplasts, and the Golgi apparatus, are taken

over by the prokaryotic cell membrane.

Structure of Eukaryotic Cell

• nucleus

• cytoplasm

• cell (plasma) membrane

• mitochondria

• endoplasmic reticulum

All eukaryotic cells have a number of features in common.

• endoplasmic reticulum

• Golgi apparatus ribosomes

• Cytoskeleton

• cenriole

• However, there are also some important differences between

animal cells and plant cells.

Plant Cell

Comparison of animal and plant cellsComparison of animal and plant cellsComparison of animal and plant cellsComparison of animal and plant cells

• Both animal (A) and plant (B) cells have – a cell membrane (1),

– nucleus (2), and

– cytoplasm (3).

• Plant cells have also –– a cell wall (4) and

– certain organelles, notably chloroplasts,

which are unique to plant cells

Cell Size and ShapeCell Size• Surface to volume ratio limits size of cell.

– Volume increases at the cube of the diameter (V=d3)– Surface area increases at the square of the diameter

(A=d2)– As a cell grows its volume increases at a rate faster than its

surface areasurface area– Rates of chemical exchange may then be inadequate for cell

size– Cell size, therefore, remains small

Cell Shape• Form follows function

– similar functions in different organisms have similar shapes

Cell size

Cell StructureCell StructureCell StructureCell Structure

1.1.1.1. CytoplasmCytoplasmCytoplasmCytoplasm– material outside of nucleus,

consists of fluid and particles and membranes

2.2.2.2. NucleoplasmNucleoplasmNucleoplasmNucleoplasm– material inside nucleus

3.3.3.3. Nucleus Nucleus Nucleus Nucleus –––– largest largest largest largest organelleorganelleorganelleorganelle– Nuclear envelope– Nuclear envelope

– Double Membrane envelope with many nuclear pores

4.4.4.4. NucleolusNucleolusNucleolusNucleolus (nuceioli plural)– Site of ribosome assembly

– Ribosomes leave through pores and are found free in the cytoplasm or associated with Endoplasmic reticulum

Genetic material in the Nucleus

• Chromatin– Seen in cells that

are not dividing

– Decondensed DNA associated with RNA and proteins

• Chromosomes• Chromosomes– Seen in cells that

are dividing

– Condensed DNA in a highly organized and compact form

– nucleolus: rRNA; ribosome synthesis

5.5.5.5. RibosomesRibosomesRibosomesRibosomes

– Protein manufacture, organelles that are part of the protien synthesis

machinery

– Free : cytosol; protein function in cell

– Bound : endoplasmic reticulum; membranes, organelles, and

export

– sets of membranes continuous with the nuclear and plasma membranes

– Membranes act to divide up the cytoplasm into compartments and channels

– Smooth ER• no ribosomes;

6.6.6.6. Endoplasmic reticulum (ER)Endoplasmic reticulum (ER)Endoplasmic reticulum (ER)Endoplasmic reticulum (ER)

no ribosomes;

• Functions: synthesis of lipids,

phosolipid & steroid ; metabolism of

carbohydrates; detoxification of drugs

and poisons

– Rough ER• with ribosomes;

• Functions: synthesis of secretory

proteins (glycoproteins) & membrane

production

7.7.7.7. Golgi Golgi Golgi Golgi ComplexComplexComplexComplex– Cisternae: flattened membranous sacs (look like a stack of

pita bread)

– sets of smooth membranes derived from the ER– Functions: sorting and modifying proteins, ultimately

transports products to the plasma membrane or are stored in the cytoplasm, also produces the lysosomes

8.8.8.8. LysosomesLysosomesLysosomesLysosomes (in animals)

– sac of hydrolytic enzymes

(powerful digestive

enzymes); digestion of

macromolecules

– Function as the cell’s

recycling center by digesting

worn-out organelles or

materials ingested by cell: materials ingested by cell:

Phagocytosis

• Autophagy: recycle cell’s own

organic material

• Tay-Sachs disease~lipid-

digestion disorder

– Rheumatoid arthritis is due to damage in joints due to leaky white

blood cell lysosomes

9.9.9.9. VacuolesVacuolesVacuolesVacuoles– membrane-bound sacs

(larger than vesicles)

– Food (phagocytosis)

– Contractile (pump excess

water)

– Central (storage in plants)

tonoplast membranetonoplast membrane

10.Mitochondria– quantity in cell

correlated with

metabolic activity;

– cellular respiration;

– double membranous – double membranous

(phospholipid);

– cristae/matrix;

– intermembrane space;

– contain own DNA

11. Chloroplasts– Type of plastid; Site of photosynthesis & Also double

membrane bound– thylakoids (flattened disks); grana (stacked thylakoids);

stroma; Pigments (chorophyl and others)– Also have their own DNA molecules

12.Other plastids

– leukoplasts

• store starch

and are

colorless, and

– chromoplasts– chromoplasts

• store colored

pigments

13. Microbodies– Various organelles which

regulate different metabolic reactions such as

– peroxisomes , single membrane

• break down hydrogen peroxide then converted to water, Metabolism of fatty acids; detoxification of alcohol acids; detoxification of alcohol (liver)

– glyoxysomes are important in germiniating seeds

• Organelles involved with Energy Production and Utilization

14. The Cytoskeleton• Fibrous network in cytoplasm

• Support, cell motility,

biochemical regulation

• Microtubules:– thickest;

– tubulin protein;

– shape, support, transport,

chromosome separationchromosome separation

• Microfilaments :– Thinnest;

– actin protein filaments;

– motility, cell division, shape

• Intermediate filaments:

– middle diameter;

– keratin; shape, nucleus anchorage

15.Cell Wall

15. Membrane

• The membrane that surrounds a cell is made up of proteins and lipids.

• Depending on the membrane’s location and role in the body, lipids can make up anywhere from 20 to 80 percent of the to 80 percent of the membrane, with the remainder being proteins.

• Cholesterol, which is not found in plant cells, is a type of lipid that helps stiffen the membrane.

THANK YOU

MATURNUWN