Cell TheoryChapter 6

IB Biology

Most cells are too small to see with the naked eyeLet’s look at some early cell scientist first.

Historical Overview

• 1665- Hooke studies cork and names the structures “cells”.

• 1675- Leeuwenhoek discovers unicellular organisms

• 1838- Schleiden: Plants are made of cells

• 1839- Schwann: Animals are made of cells

• Which all led to the…

Cell Theory• Rudolf Virchow: 1855

• 1) cells are the basic unit of life

• 2) all organisms are made from one or more cells

• 3) cells only arise from pre-existing cells.

Debatable exceptions to the rule:

• Skeletal muscle and fungal hyphae have multinucleated cytoplasm

• Viruses are non-cellular structures of DNA or RNA that are surrounded by a protein coat

• Some biologists consider unicellular organisms to be acellular.

What units are used to measure cells?• 1 mm = 1000 micrometers (um)

• 1 mm = 1,000,000 nanometers (nm)

• Or…A micrometer is 1 x 10-6 metersA nanometer is 1 x 10-9 meters

Today’s Microscopes

• Two Main types of scopes1) light microscope

2) electron microscope

Light Microscopy Advantage:

• Inexpensive and easy to use

• Used to study stained or living cells, in color

• Objects can be magnified up to 2000XOurs at school only magnify 1000X

• Can resolve objects 200 nm apart (500 times better than the human eye)

Light Microscopy Disadvantage

•Resolving power is not great

•Limited by focal length of lens

The Structure of a Compound Light Microscope

Ocular (eye piece)

Nose barrel

Coarse focus

Fine focus

Base

Light source

Condenser

Revolving nose-piece

Objective lens

Stage clip Stage

Arm

Electron Microscopes

• Two typesScanning Electron (SEM)

• uses electron beams that bounce off the specimen

• Sees surface details only.

Transmission Electrion (TEM)• Used electron beams that

pass through specimen• Views cross-sections

Electron Microscopy Advantages

• Can magnify up to 250,000x (125x that of a light microscope.

• Can see more detail at higher resolution (0.2 nm apart),1000 times the resolving power of light microscopes.

• Details are easily visible

• Depending on type of EM, can see surface details or interior details of specimens

HIV, magnified 24,000x

Electron Microscopy Disadvantages

• Expensive

• Requires cells to be killed and chemically treated before viewing.

• No color can be seen.

What is a cell?

•Cells are the basic unit of living things.

•Smallest unit that can carry on functions of living things

Cell Size

• Surface area to volume ratio is crucial

• Is it more advantageous to be a single cell that is large or to be broken down into several small cells ?

(Explain your answer)

A small cell has a greater ratio of surface area to volume than a large cell of the same shape

Surface areaof one large cube= 5,400 µm2

Total surface areaof 27 small cubes= 16,200 µm2

Cell size - (surface area:volume)• As cell size increases, the surface area to volume ratio decreases

(sa/vol)

• Rates of chemical exchange may then be inadequate for cell size

• Cell size, therefore, remains small

Natural laws limit cell size

• At minimum, a cell must be large enough to house the parts it needs to survive and reproduce

• The maximum size of a cell is limited by the amount of surface needed to obtain nutrients from the environment and dispose of wastes

Multicellular Organisms have the same DNA in every cell, so why are cells so different?

• Cells differentiate in multicellular organisms because some genes are expressed and some are not, depending on the type of cell.

Related Terms to Know for IB:

• Virus

• Organelle

• Tissue

• Organ

• Organ system

Viruses

Viruses are non-cellular

Contain only DNA or RNA and a protein coat

Must live in a host to reproduce and survive

Organelle

•Organelle- a discrete structure within a cell that has a specific function.

Tissue

•Tissue- a group of similar cells performing a particular task.

Organ

• Organ- a structural unit made up of a group of tissues which work together to perform a function.

Organism

•Organ system- several organs working together to perform a job.

•Ex: heart/lungs