chapter 4. living things are composed of cells. cells are the basic units of structure and...
TRANSCRIPT
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.