Chapter 4-5 Prokaryotic Cells and

Membranes

Four Important People

Anton Von Leeuwenheok

Inventing microscopes and first observing

unicellular living organisms

Robert Hooke

Discovered and coined the term “cells”

Matthias Schleiden

Established all plants are composed of cells

Theodor Schwann

Established all animals are composed of cells

Compound Light Microscopes

Out of the trillions of different cells only a small handful have ever been visible to the naked eye.

Thus, we need a tool to help study them

Anton Von Leeuwenhoek did not invent the first microscope, but he and his colleagues first used the object as a scientific tool.

He, among others, experimented with grinding glass into lenses in order to bend light and make smaller objects appear larger.

Compound Light Microscopes Compound light

microscopes work

by running beams

of light through a

series of lenses.

These lenses

continuously zoom

in on a specific

target.

The object can be

magnified up to

1500 times

Electron Microscope Electron microscopes fire beams of electrons at

whatever you want magnified.

The electrons then interact with the structures of the object. These interactions are calculated and displayed by a computer.

The result is a magnification of 500,000 to 1 million times larger than normal.

Electron microscopes have allowed us to study the internal structures and functions of the cell.

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Cell Theory

The cell is the simplest level of organization in biology

that meets the definitions of “living”

All functions of an organ/organ system/organism are

collective functions of the cells that make them up

In other words, the only reason you can communicate,

remove waste, move, etc is because your cells can

Cells have to find a unique balance between

personal, cellular survival and survival of the

organism as a whole

The importance of cells is clearly outlined in the cell

theory

Cell Theory

#1: All organisms are composed of one or more

cells

#2: The cell is the basic unit of structure and

function in an organism

#3: All cells come from preexisting cells

Cell Size

One of the keys to life is the fact that cells are so small

Although coordinating 10 trillion tiny cells is more

difficult than coordination 1000 large cells, the benefits

of multiple smaller cells far outweigh the difficulties.

1: Surface area/volume ratio

Cells take in nutrients and remove waste through their

membrane.

The more surface area the membrane has, the more

nutrients/waste can pass through

If the cell is too big though, it takes too long for nutrients to

reach the center of the cell

Smaller cells can take up the same amount of space

(volume) as larger cells, but with 4 times as much surface

area

Cell Size 2: Redundancy

If you have multiple cells making up an organism, the

death of one cell has less of an impact

Instead of one cell performing millions of functions, millions

of cells perform one function each

You can have lots of back-ups in case a cell is tired,

damaged, or is dividing into a new cell

Prokaryotic Cells Prokaryotic cells are distinguished by their lack of

membrane-bound organelles

Prokaryote is Greek for “before nucleus”

Prokaryotic cells are much simpler in size and structure

than eukaryotic cells

Eukaryotic cells are on average 50x larger than

prokaryotic cells

Prokaryotic cells have to be small enough to infect a

Eukaryotic cell by inserting themselves inside the Eukaryotic

cells

The two prokaryotic groups are archaea and bacteria

Bacteria are simpler in design and live in more neutral

environments

Archaea are highly diverse and complex and live in harsher

climates

Prokaryotic Cell Envelope

Prokaryotic cells are bound by different layers of

membranes

Plasma Membrane: two layers of phospholipids that

regulate what goes in and out of the cell

Very flexible

Cell Wall: rigid outer layer that maintains the shape of the

cell and prevents collapsing or swelling

Very rigid

(Also found in plant cells)

Cytoplasm

Inside cells are numerous different objects swimming in

a semi-liquid solution

This solution is called cytoplasm.

The cytoplasm has three functions:

Provide a medium to move against within the cell (imagine

trying to swim in an empty pool)

Internal support of the cell

In Eukaryotic cells, stabilize the organelles in place

Other prokaryotic structures

Nucleoid

› A general region of the cell where the DNA is stored

Plasmid

› A circular section of DNA

Fimbriae

› Fibers on the surface of the cell that allow cells to

attach to surfaces

Sex pili

› Tubular structures used to pass DNA from cell to cell

Membranes

The reason you are able to build immunities, use your

senses, keep your temperature regulated, stay

hydrated, and hundreds of other functions is because

of your membrane.

The membrane is one of the most-studied organelles

because it’s easy to see with microscopes

Multiple different models of the structure of the

membrane have been proposed as early as 1900.

In 1972, the model recognized as the most accurate

depiction of the cell membrane was introduced: the

fluid-mosaic model

Cell Membrane Structure The majority of the cell membrane is a molecule

called a phospholipid

A phospholipid is a long chain of lipids attached to a

phosphate molecule

The phosphate molecule is hydrophilic, or “water-

loving”. It can safely and freely be in contact with

water and water-soluble molecules

“Polar”

The lipid chain is highly hydrophobic. Near the

presence of water or water-soluble molecules it will

repel like a magnet

“Nonpolar”

Cell Membrane Structure

The membrane is actually two layers of phospholipids.

The phosphate heads face outside toward the

environment and inside toward the cytoplasm.

The lipid chains are sandwiched in between the two

phosphate heads

This is called the “phospholipid bilayer”

Cell Membrane Structure

Attached to phospholipids are various other chemicals

that enhance the stability of the membrane

Cholesterol: a steroid; stiffens and strengthens the

membrane

Glycolipid: Carbohydrate attached to the lipid, important

for cell recognition and immunity

Glycoprotein: Protein attached to the lipid, important for

multiple different functions (performing tasks, entry/exit

from the cell, connection to other cells…)

Cell Membrane Structure

The glycolipids and glycoproteins are located randomly

within the cell membrane

Think “chocolate chips” in a cookie

What is now known is that the molecules that make up

the membrane are not stationary

Proteins and lipids are able to move back and forth

within the membrane like bumper cars on a track

This gives the membrane a “fluid” appearance. Hence,

the fluid-mosaic model

Glycolipids

When we get to cell immunity, we’ll cover this in more

detail.

Every cell in our body has specific structures of

glycolipids unique to us (or unique to the human

species)

When a foreign substance makes contact with a

glycolipid, the cell immediately recognizes what it is

based on the structure of the foreign substance

Glycolipid

If the cell does not recognize the structure on the

foreign substance, it will make a copy of this glycolipid

Other cells will then be given a copy of this foreign

glycolipid so they can recognize it as a dangerous

entity BEFORE it can penetrate the cell

This is an important role for the immune system,

particularly white blood cells

Impermeability The membrane is almost 100% effective at

preventing leaks

Hydrophobic substances can’t get past the phosphate

heads

Hydrophilic substances can’t get past the lipid chains

If the cell is going to take in nutrients, get rid of

waste, or send or receive messages, it needs gaps in

the membrane

Glycoproteins are large enough to span the entire

distance of the membrane and perform these

functions

Because the cell prevents leaks, but still allows

substances through, it is called “semi-permeable”