Cell Structure and Function

First Glimpse of The Cell

• 1662 – Robert Hooke– English Scientist– One of the first microscopists.– Looked at thin slices of cork with a compound

microscope and called the outside walls “cells”– b/c they looked like rooms that monks live in.

• 1668 – Anton Van Leeuwenhoek– Dutch Draper– Received no higher education or university degrees– First to view living cells – “animalcules”, sperm,

blood, etc.– He also examined bacteria from the scrapings of his

own teeth!

Birth of Cell Theory• 1831 – Robert Brown

– Discovered nucleus in plant cells

• 1838 - Matthias Schleiden– All plants are composed of cells

• 1839 – Theodor Schwann– All animal tissue is composed of cells

• 1855 – Robert Remak and Rudolf Virchow– Cells come from other living cells

Cell Theory

1) All organisms are composed of one or more cells.

2) Cells are the smallest living things, the basic units of organization for all organisms.

3) Cells arise only by the division of a previously existing cell.

BASIC CHARACTERISTICS OF CELLS

1. Maintain a homeostatic condition2. Take up nutrients, digest them, and excrete

waste products3. Take up O2/CO2 and release CO2/O2

4. Maintain water and salt content5. Grow, reproduce, and move6. Respond to external stimulation7. Expend energy to carry out activities8. Inherit genetic programs from parent and

pass onto offspring9. Die

This picture is a human white blood cell is trapping bacterial cells. This type of cell defends the body against pathogens by engulfing them, delivering them to the lysosome of the cells, and destroying them with the help of the lysosomal enzymes

Prokaryotic Cells- before nucleus

• SIMPLE- capable of much less complex activities• CONTAIN MUCH LESS GENETIC INFO• NO MEMBRANE BOUND NUCLEUS- houses genetic info

in the nucleoid• LACK ORGANELLES (aside from ribosomes)• Represent the vestiges of an EARLY STAGE IN

EVOLUTION• SMALL- seldom reach diameters greater than a few

μm (micrometers)• Up to 700 million could fit on the head of a

thumbtack.• EXAMPLE- Bacteria

Eukaryotic Cell– true nucleus

• More COMPLEX• TRUE NUCLEUS- where genetic information is housed

and surrounded by a complex membranous envelope• MANY ORGANELLES- cytoplasm is filled with

organelles that are specialized for various activities• LARGE- cells range in diameter from 10-100 μm  

• EXAMPLE: yeast, ameoba,, red blood , and liver

Just How Small Are We Talking?• The smallest objects that the unaided human eye

can see are about 0.1 mm long.– you might be able to see an ameoba or a human egg,

without using magnification. • Smaller cells are easily visible under a light microscope.• To see anything smaller than 500 nm, you will need an

electron microscope.• Most cells < 50 m

• Micrometer or micron (m) = 1000 mm• Nanometer (nm)= 1000 m• Angstrom – Å• 1 Å = 0.1 nm = 1.0 x 10-4 m = 1.0 x 10-8 cm

Cell scale demo

Limits to Cell Size• Communication• Diffusion/Transportation• Surface Area to Volume ratio

– Smaller cells have more surface area per unit volume

– Larger cells must import/export more materials through the cell membrane

– Volume increases at a cubic rate, surface area at a squared rate

The Bigger They Come The Harder to They are to

MaintainCell

Radius

(mm)

Surface

Area(mm2)

Volume

(mm3)

S.A.:Vol

1 6 1 6:1

2 24 8 3:1

3 54 27 2:1

5 150 125 1.2:1

Cell Radiu

s(mm)

Surface

Area(mm2)

Volume

(mm3)

S.A.:Vol

1 12.56 4.18 3:1

2 50.24 33.49 1.5:1

3 113 113 1:1

5 314 523 0.6:1

Cubic Cell

Spherical Cell

Some cells are much larger than others.

• Given the constraints imposed by the S.A. to

volume ratio, how would you expect the level of activity in large cells to

compare with that in small cells?

If Shaq, the 7-foot tall, 300-pound, basketball player for the Lakers were twice as tall, would he be twice as good a ball player?

•The same surface/volume ratio principle illustrated with the cubes applies to Shaq. •In general, if his height was doubled and his proportions remained geometrically similar,

• then his surface area would quadruple,• However, his volume and mass would octuple! • He would weigh roughly 2400 pounds!

•Not only would Shaq no longer be able to rebound, but, like the landlubber blue whale he would be crushed under his own weight. His bones would no longer be able to support him.

From: http://invsee.asu.edu/Modules/size&scale/unit4/unit4.htm#cells

Applying S.A: V ratio to LIfe:

Form Follows Function• Nerve cells are long and skinny to

transmit messages

• Red Blood Cells are close to spherical to maximize S.A. to volume• Skin cells fit together tightly

• Sperm cells lack almost all organelles and have a streamline structure and flagella for motility.

A Brief Tour of a Eukaryotic Cell

• Cool Cell Animation

Plasma MembraneConsistent from Bacteria to

Mammals

1) Forms a protective outer barrier for the cell

2) Helps maintain a constant internal environment

3) Regulates exchange of substances in and out of the cell

Fluid-Mosaic Model1972 – Singer and Nicolson

•The membrane is made of a phospholipid bilayer that is viscous and free to move.

•Globular proteins are embedded in the bilayer and move about.

•The hydrophobic ends of the lipids create a non-polar region within the membrane.

•This region impedes the passage of all water soluble molecules.

•Hydrophilic heads exist at the inner and outer surfaces and allow specific chemical interactions to take place.

Membrane Structure

• Lipid bilayer• Transmembrane proteins• Network of supporting fibers

– Shape and structure– scaffolding

• Exterior proteins and glycolipids– “sugar coating” acts as cell identity

markers– Glycoproteins – self recognition– Glycolipids – tissue recognition

CytoplasmThe material within a cell

excluding the nucleus

The cytoplasm of most eukaryotic cells is filled with membranous structures that extend to every nook and cranny of the cell’s interior.

The NucleusRoger, Headquarters

• Genetic headquarters• Largest and most easily seen organelle• Repository of genetic information• Discovered by Robert Brown – 1831• Fungi and other groups may have >1

nucleus• Red blood cells do not have a nucleus

– This maximizes the space available for hemoglobin– They do, however, develop from bone marrow cells that

DO have a nucleus. They lose it once they mature.

Nuclear Structure• Nuclear envelope

– A phospholipid bilayers– Nuclear pores

• Membranes pinch together, filled w/ proteins that restrict movement

• Proteins moving into the nucleus• RNA and RNA complexes to be exported into the

cytoplasm

• Nucleolus– Site of intensive rRNA synthesis

• Nucleoli– Tiny granules that are precursors to ribosomes

• Nucleoplasm– Semifluid area that organizes the contents and

provides sites of attachment for enzymes in DNA duplication

Nuclear Shots

Liver Cell NucleusNucleus Diagram

Nuclear PoreNucleus with Pores

ChromosomesPackaging DNA

• Stored as thin strands (chromatin) except for cell division

• During cell division DNA coils around histones in a condensed forms called chromosomes

• After cell division chromosomes uncoil and can’t be seen with a light microscope

Endoplasmic Reticulum

• Highway of the cell– System of passageways that allow materials to

be channeled to different locations within the cell

• Lipid bilayer with embedded proteins• Site of membrane phospholipid synthesis• Highly developed in pancreas and salivary

glandsRough ER

Smooth ER

Rough or Smooth?

• Rough ER– Studded with ribosomes– Site of protein synthesis and segregation– Proteins can be used within the cell or

exported outside of the cell

• Smooth ER– Found in lesser quantities– May be responsible for synthesis of

steroids– Break down lipids and toxins in the liver

Golgi Apparatus (Bodies)

Delivery System of the Cell• Discovered in 1898 by Camillo Golgi

• Flattened stacks of membranes thought to form from vesicles produced by the RER

• Abundant in glandular cells – secretions• Collection, packaging, and distribution • Proteins from ER are modified

Cis

Trans

exocytosis

Cisternae

Ribosomes

• Site of protein synthesis• They are not membrane bound

– Eukaryotic ribosomes slightly larger than prokaryotic

• Consists of small and larger subunits

• Cluster on the ER to make protein for export– Free ribosomes make proteins for use

within the cell• Ribosomal subunits are

manufactured in the nucleolus

Lysosomes and Vesicles• Vesicles transport materials in and out of the

cell– Exocytosis– Endocytosis – phago- (solid) and pino- (liquid)

• Lysosomes – membrane bound digestive vesicles that arise from the golgi bodies

• Lysosomes contain a concentrated mix of digestive enzymes– Catalyze breakdown of protein, NA’s, lipids, carbo’s– Recycle old organelles – mitochondria replaced

every 10 days

• Lysosomes in metabolically inactive eukaryotic cells dissolve cells from the inside out

Vacuoles• Large fluid containing sacs• In plants they may occupy more than 90

percent of the cell’s volume• Bounded by a single membrane• In addition to water the vacuole may

contain gases (O2, N2, and/or CO2), acids, salts, sugars, pigments

• In plants the vacuole keeps toxins separate from the rest of the cell and maintain internal pressure which aids in the support of the plant

MitochondriaPowerhouse of the Cell

• Site of aerobic respiration• Energy released and ATP produced• Inner membrane (cristae) houses

the electron transport system• Mitochondria have their own DNA

(mDNA)

Mitochondria- cont.• Could have been a bacteria-like

organism incorporated into another cell 1.5 bya

• Mitochondria are particularly numerous in muscle cells

• All mitochondria of offspring is maternal– Mitochondria of sperm remain outside

fertilized egg– mDNA is inherited maternally

Mitochondria Structure

Electron Microscope View

CentriolesMicrotubule Assembly Centers

• Centrioles – help to assemble microtubules

• Help assemble spindle fibers which move and align chromosomes during cell division

• Found only in animal cells

Cytoskeleton• Three main types of components

1) Microtubules – composed of the protein tubulin

2) Microfilaments – contractile protein actin3) Intermediate filaments – variety of

proteins• Carry out many functions for the cell

1) Maintain cell shape2) Anchor organelles within the cytoplasm3) Help in cell movement4) Help to organize the internal contents of

the cell

Cytoskeleton Fibers• Actin filaments

– About 7 nm in diameter– 2 protein chains loosely twined together– Contraction, “pinching”, and cellular extension

• Microtubules– About 25 nm in diameter– Cell movement, transport of materials witin

the cell

• Intermediate filaments– About 8-10 nm in diameter– The most durable element of the cytoskeleton– Structural stability

Cell Movement

• Cell motion is tied to the movement of actin filaments, microtubules or both

• Actin filaments can form and dissolve very rapidly allowing cells to change shape quickly

• In cells treated with drugs that make microfilaments dissolve all cell locomotion stops

Some Crawl, Some Swim

• Some cells use a pseudopod (false foot)– Cytoplasmic oozing forces a “foot” out in a

certain direction, the cell then drags itself

• Some cells use cilia or flagella to swim– Whip-like flagella and shorter cilia both

have a 9 + 2 structure of microtubules in eukaryotes

– The beating or turning of these structures propels the cell

Endosymbiosis

• Proposes that today’s eukaryotic cells evolved by a symbiosis in which one species of prokaryote was engulfed by and lived inside another species of prokaryote

• Mitochondria and chloroplasts are thought to be two prime examples of this theory– Double membranes– Both contain circular DNA similar to

bacteria– Mitochondria divide by simple fission

Sources

Brum, Gilbert D., L. McKane, and G. Karp. 1994. Biology: Exploring Life, 2d ed. New York: Wiley.

Raven, Peter H. and G.B. Johnson. 1999. Biology, 5th ed. New York: McGraw-Hill.

http://cellsalive.com/

http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookTOC.html

http://www.pbrc.hawaii.edu/~kunkel/gallery