The Cell
Eukaryotic Cells & Prokaryotic




Most common cells are eukaryotic cells, for example onion cells and human cells.
The word eukaryotic means ‘true nucleus’, therefore eukaryotic cells have a true
nucleus which is surrounded by a membrane. DNA is located inside the nucleus.
Eukaryotic cells also contain tiny structures surrounded by membranes called
organelles, for example chloroplasts.
There is another type of cell, simpler than eukaryotic cells, called prokaryotic cells.
These cells have no real nucleus. They have DNA, but it is not surrounded by a
membrane. The region in the cell that contains the DNA is called the nucleoid.
Definition Eukaryotic Cell: a cell which has a true nucleus surrounded by a membrane and
organelles surrounded membranes
Definition Prokaryotic Cell: a cell with no true nucleus and no organelles surrounded by a
membrane
Similarities between prokaryotic and eukaryotic cells
•
•
•
•
Both
Both
Both
Both
cell types contain a plasma membrane
cells contain DNA, but in eukaryotic cells the DNA is found inside a membrane
cells contain ribosomes (that make protein)
cells contain cytoplasm
Differences between Prokaryotic and Eukaryotic Cells
•
•
•
•
Eukaryotic cells are generally larger in size than prokaryotic cells
Eukaryotic cells are more complex and organelles, which are surrounded by a membrane
Most single-celled organisms, such as bacteria, are prokaryotic cells.
Most multi-cellular organisms, such as animals, are made from eukaryotic cells
Ultrastructure of the Cell



If a very high powered microscope, called an electron microscope, is used to view a cell,
we can see the cell in much more detail.
Apart from the nucleus, animal and plant cells also contain tiny structures called
organelles.
The structure of a cell which can be seen under an electron microscope is called the
ultrastructure of the cell.
Ultrastructure of Animal and Plant Cells
The Cytoplasm
Cytoplasm is a watery-gel found inside the cell. The organelles are embedded into this
cytoplasm.
Structures and Organelles in Both Animal and Plant Cells
Plasma membrane



The cell/plasma membrane is made of many fats (lipids)
and proteins.
The fats form two layers, known as the lipid bilayer
and proteins are embedded into this bilayer.
The cell membrane is fluid, meaning that the particles
in it are free to move around and are not in a fixed
position.
Functions:
 To control what enters and leaves the cell: The cell
membrane is semi-permeable, which means some small
particles can pass through it, but larger particles
cannot get through it. The membrane acts like a sort of
sieve, allowing only small substances to pass through.
 Gives shape and support to the cell.
Nucleus
Surrounded by a double membrane which contains
pores
(substances move in and out of the nucleus through the pores).
Function: Houses the chromatin*, which are made of DNA and
protein. The DNA contains the genetic code for making proteins.
*When the cell is not diving the DNA and protein structures
inside the nucleus are known as chromatin.
When the cell is about to divide the chromatin coil up and
become visible under a microscope. They are then known as
chromosomes.
Nucleolus
A small spherical body found inside the nucleus
Function: The nucleolus makes a special type of acid called
RNA. Some of this RNA is used to make other structures in
the cell called ribosomes.
Mitochondrion
•
•
•
Surrounded by a double-membrane
The inner membrane has many foldings called
cristae
The area inside the inner membrane is known as the
matrix or lumen
Function: to make energy for the cell
Ribosomes
Some are found in the cytoplasm and some are attached
to endoplasmic reticulum
Function: to make proteins.
Endoplasmic Reticulum
•
•
•
The endoplasmic reticulum (ER) is a network of membranes that branch out from the
nucleus
The cisternae are the foldings in the membrane and the space inside the endoplasmic
reticulum is called the lumen.
There are 2 types endoplasmic reticulum, smooth ER and rough ER
Smooth Endoplasmic Reticulum
Smooth ER does not contain any ribosomes attached to it.
Functions:
• Makes lipids/fats
• Stores calcium ions
• Breaks down drugs and poisons
Rough ER
The rough ER has ribosomes attached to
its membrane.
Functions:
• Assists in making and folding proteins and releasing them from the ribosomes
• Place where carbohydrates can be added to proteins
• Forms a little membrane around proteins to help transport proteins to other parts of
the cell
Golgi Apparatus
Consists of many membrane-bound spaces piled on
top of each other.
The space inside the membrane is called the lumen.
Function:
•
Acts like a mini ‘warehouse’ for receiving,
sorting and shipping molecules in the cell, such
as proteins.
(Some proteins need to be brought to the cell
membrane, others to other membranes of
organelles and some need to be released from
the cell.)
Lysosome
A spherical organelle which contains enzymes. The
Enzymes help to break down materials in the cell.
Functions:
•
•
Breaks down particles either made in the cell
or taken into the cell
Breaks down damaged cell organelles
Vacuole


A fluid filled space surrounded by a membrane. The
fluid inside the vacuole is called sap and it contains
salts, sugars and other substances dissolved in water.
Plant cells contain large vacuoles whereas animal cells
only contain small vacuoles, if any.
Functions:
 Stores water and salts
 Removes excess water from the cell
Plant Cells Only – Chloroplasts and the Cell Wall
Chloroplasts





Chloroplasts are green structures found inside plant
cells, where photosynthesis takes place.
Inside the chloroplast there is a watery substance
called stroma.
In the stroma there are a number of grana.
Each granum (singular) contains a stack of
thylakoids.
The chlorophyll is found inside the lamellae.
Function:
To make food for the plant during photosynthesis
Cell Wall
 Plant cells contain a primary cell wall, which is made mainly from a
substance called cellulose.
 Some plant cells also develop a secondary cell wall.
 The area between two cells walls is celled the middle lamella and it
contains a sticky substance called pectin
Function:
Gives strength and support to the cell and prevents it from bursting.