Chapter 4Chapter 4

Living things are composed of cells. Cells are the basic units of structure and

function in living things. All cells come from other cells. Important scientists: Schleiden, Schwann,

Virchow Similarities between cells support the theory

of evolution – which includes an explanation of the original cell’s origin.

The Cell Theory

Plasma membrane:

Separates the contents of the cell from the external environment

Allows a difference in composition between inside and out

Serves as a selective barrier so cells can exchange materials with external environment and accumulate necessary molecules

Organelles: Internal structures that are specialized to perform

particular functions: metabolic activities, synthesis of important compounds, reproduction

DNA: Genetic instructions

All cells share the same basic organization

Materials used or produced inside the cell must go

in or out of the cell through the plasma membrane. As a cell increases in size the surface area to

volume ratio decreases compared to the volume of the cell, the surface area is smaller

This limits the cell size – large cells would not be efficient at transporting materials across the plasma membrane

Exceptions: the shape of a cell (long and thin) can influence the SA/V ratio ex: some human nerve cells

Limits to Cell Size

Magnification – how large the image appears

compared to its actual size Resolution – the ability of the eye to

distinguish between 2 distinct points (clarity of image)

Light microscopes – up to 1,000 x magnification

Electron microscopes – up to 250,000 x magnification

Microscopes

More than one lens – multiply the objective

lens by the ocular lens to determine the total magnification

Bright-field microscopy – light is transmitted through a cell. Little to no contrast makes internal structures difficult to see

Dark-field microscopy – the cell is visible as a bright object against a dark background

There are many types of stains and dyes in use that allow us to see internal cell structures

Compound light microscopes

Image is formed by electrons striking the specimen. The image cannot be viewed directly – it must be

focused on a photographic plate or fluorescent computer screen.

Live cells cannot be viewed this way. Transmission electron microscopy (TEM) –

extremely thin slices are made of the specimen Scanning electron microscopy (SEM) –

specimen is coated with a thin film of gold and the surface is scanned

Electron microscopes

Cell Fractionation

Used to study cell organelles – allows researchers to purify different cell parts

Uses a centrifuge – which spins rapidly and forms: The pellet – heavier materials, such as nuclei,

form at the bottom of the tube And the supernatant – the liquid above the

pellet which contains lighter particles as well as dissolved materials

Organelles purified in this way can be studied to determine their chemical makeup and reactions

Basic Cell Types: Prokaryotes

“Before the nucleus” Differences from eukaryotes:

On average, smaller DNA not located in a membrane-bound

nucleus DNA is located in a nucleoid May have folded plasma membranes for

cellular reactions Most have cell walls Many have flagella Have ribosomes for protein synthesis

Basic Cell Types: Eukaryotes

“True nucleus” Highly organized and complex with a

membrane-bound nucleus and organelles Cytoplasm – outside of the nucleus; contains

the organelles and the fluid they are suspended in – which is the cytosol

Nucleoplasm – within the nucleus Organelles are specialized for different

functions

Cell Membranes

Allow different parts of the cell to do different jobs: Reactants are more likely to contact each other Reactive compounds are isolated from other cell

parts Many different activities can occur simultaneously

Allow cells to store energy: A difference in concentration across the

membrane represents potential energy Provide work surfaces for the cells:

Chemical reactions are carried out by enzymes embedded in the membrane

Endomembrane system

The internal membrane system of cells. Mitochondria and chloroplasts are not part of

this system – they function independently. Some organelles have direct contact with each

other. Others transport materials throughout the

cells by using vesicles.

The Cell Nucleus

Usually the largest organelle, located in the cell center. Most cells only have one.

Surrounded by the nuclear envelope – a double membrane which contains nuclear pores.

DNA is contained in the nucleus, with associated proteins which make up chromatin. During cell division the chromatin becomes compacted and forms chromosomes.

Many also contain at least one nucleolus which is responsible for making ribosomes.