Eukaryotic cells
All eukaryotic cells have the same basic structure, although many cells are adapted to
carry out different functions.
Eukaryotic cells are surrounded by a cell surface membrane, containing cytoplasm and
various organelles.
See page 49 Jones and Jones for the structure of an animal cell seen under a light
microscope, and page 50 for a plant cell seen under a light microscope.
Light microscopes
Light rays are focused onto a transparent specimen by a condenser lens. They then
pass through the specimen and are focused again by two more lenses, the objective
lens and the eyepiece lens, which produce a magnified image.
Stains are often used to make different parts of the cell show up clearly.
include picture of light microscope
Electron microscopes
The principle is the same as light microscopes, except that beams of electrons are
used instead of beams of light, and they are focused by electromagnets instead of
lenses. The specimens have to be extremely thin, and viewed in a vacuum, so
specimens must be dead. They can be stained by heavy metals such as lead.
There are 2 types of electron microscope:The T.E.M. or transmission electron microscope which is used to look at the structure
of thin sections.
The S.E.M. or scanning electron microscope which obtains pictures of the surface of
structures as electrons are reflected off them.
include picture of electron microscope
Magnification and resolution
Magnification is the number of times larger an image is than the specimen. For
example if a cell is 10 m in diameter, and a microscope produces an image of it which
is 1 mm (1000 m) in diameter, then the microscope has magnified the specimen 100
times.
magnification
=
size of image
size of specimen
There is no limit to the amount a light microscope can magnify, but the resolution is
limited.
Resolution is the amount of detail which can be seen. The limit of resolution depends
on the wavelength of light, and for light microscopes is about 200 nm. Any objects
smaller than this, or points less than this distance apart, will appear as blurs.
Electron microscopes have a much shorter wavelength and so have a maximum
resolution of 0.5 nm.
See Jones and Jones page 51-55
Answer question 1 page 52
Cell ultrastructure
The ultrastructure of a cell is the fine detail and structure of the organelles as
revealed by the electron microscope.
Cytoplasm





“The bulk of the substance of the cell”.
Fluid that remains when all organelles are removed is cytosol.
Contains an aqueous solution of various essential mineral ions and soluble organic
compounds e.g. sugars and amino acids, soluble proteins, many of which are
enzymes, cell organelles and a network of fine strands of globular protein
microtubules and microfilaments collectively referred to as cytoskeleton.
90% water.
Site of certain metabolic pathways e.g. glycolysis.
Nucleus
Controls biochemistry and growth of cell and the inheritance of the cell






Largest organelle in eukaryotic cell
Spherical or ovoid 10 – 20 m in diameter
All cells have one except:red blood cell which has none
phloem sieve cell which has none
ciliated protozoan Paramecium and cells of club fungi which have 2
some white blood cells have a much lobed single nucleus
Surrounded by double membrane
Perforated by pores which are 80 – 100 nm but cover 1/3 of membrane area
When not dividing chromosomes are dispersed as a diffuse network called
chromatin (coloured material) = coils of DNA bound to histones
Nucleolus
Synthesis of ribosomes

Visible as a round dark stain within the nucleus
Endoplasmic reticulum
Synthesis and movement of substances in the cytoplasm




Network of folded membranes forming sheets, tubes and flattened sacs called
cisternae
Originates from outer membrane of nuclear membrane and is often still attached
Rough endoplasmic reticulum (RER) has ribosomes attached. Vesicles are formed
from swellings at the margins that become pinched off. RER is the site of
synthesis of proteins such as digestive enzymes which are packaged in the
vesicles before being discharged from the cell.
Smooth endoplasmic reticulum (SER) has no ribosomes. SER is the site of
synthesis of substances needed by cells, such as lipids.
Golgi apparatus
Site of synthesis of cell secretions





Stack of membranous sacs called cisternae.
In all cells but most prominent in metabolically active.
Site of synthesis of biochemicals such as hormones and enzymes which are
packaged into swellings at margins which are pinched off as vesicles.
Collects proteins and lipids made in ER, adds additional substances and
repackages.
Involved in formation of cell wall and lysosomes.
Lysosomes
Contain hydrolytic enzymes






Small spherical vesicles 0.2 – 0.5 m or larger (particularly in plant cells)
Bound by single membrane
Contain a concentrated mixture of hydrolytic digestive enzymes which are
produced in the Golgi or ER
Fuse with food vacuole and digest contents, e.g. bacteria that have been engulfed
by cells of immune system
Digestion of broken-down organelles in cytoplasm
When an organism dies the hydrolytic enzymes in the lysosomes escape into the
cytoplasm and cause self-digestion (autolysis)

Contents are acidic and enzymes have a low optimum pH
Chloroplasts
Site of photosynthesis








Members of groups of organelles known as plastids
Only found in plant cells
Chloroplasts are biconvex discs, 4 – 10 m long and 2 – 3 m wide
In land plants usually found in mesophyll cells of leaves, and cells of outer cortex
of herbaceous stems
Bound by double membrane – outer membrane is smooth continuous boundary
Inner membrane has strands of branching membranes called lamellae or
thylakoids. Stacks of thylakoids form grana – site of light reaction of
photosynthesis.
Grana are surrounded by stroma, where thylakoids are arranged loosely – site of
dark reaction of photosynthesis
Other plastids = leucoplasts (in storage organs such as roots and seeds), and
chromoplasts (coloured plastids in fruits and flowers)
Mitochondria
Site of part of respiration








In EM appear as rod or cylinders.
Occur in large numbers, may be more than 1000 in metabolically active cells.
Size varies 0.5 – 1.5 m wide, 3 – 10 m long.
Found in all cells, metabolically active cells contain thousands (e.g. muscle fibres
and hormone secreting cells).
Double membrane, outer smooth, inner is folded to form cristae.
Interior contains an aqueous solution of metabolites and enzymes called the
matrix.
Mitochondria are the sites of the aerobic stages of respiration
Contain DNA
Ribosomes
Site of protein synthesis





Two types – 70S (prokaryote) and 80S (eukaryote)
20 nm in diameter, consisting of two sub-units
Many thousand per cell
Made of protein and RNA
Ribosomes free in cytoplasm are site of synthesis of proteins which are retained
in the cell

Ribosomes of RER are site of synthesis of proteins which are subsequently
secreted outside the cell
Centrioles




Centrosome occurs in animal cells but not in plant cells
Consists of two hollow cylindrical bodies called centrioles
Each centriole is made of nine triplets of microtubules and is identical to the
basal body that lies at the base of a flagellum or cilium
Centrioles separate and move towards opposite ends of nucleus before cell
division
Microtubules






Occur in most plant and animal cells
Straight unbranched hollow cylinders 25 nm wide
Made of tubulin – a protein
Constantly made and broken down
Move cytoplasmic components within cell
Occur in centrioles, spindle, cilia / flagella
Cell wall









Only found in plant cells.
Cell wall is external to the cell and is therefore not an organelle, although it is a
product of various organelles.
Plant cell walls consist of cellulose together with other substances (mainly other
polysaccharides).
Between cell walls is a gel-like layer of calcium pectate called the middle lamella.
This is the first layer to be deposited in the formation of a new cell wall.
The primary cell wall layers of cellulose are laid down on to the middle lamella.
More layers are added to form the secondary cell wall, with cellulose fibres lying
in different directions.
Some cell walls are strengthened with a chemical called lignin.
The outermost layer of cells may become coated with wax to protect the surface.
The cytoplasm of one plant cell is joined to that of the next through gaps in the
cell wall called plasmodesmata. The plasmodesmata are 25 nm wide and filled with
cytoplasm and endoplasmic reticulum.
Differences between plant and animal cells
Plant cells
cellulose cell wall present
many cells contain chloroplasts, site of
photosynthesis
Animal cells
no cellulose cell wall
no chloroplasts, animal cells cannot carry
out photosynthesis
carbohydrates stored as starch often
present
large, permanent vacuole present
no centriole
carbohydrates stored as glycogen often
present
no large permanent vacuoles
centriole present outside the nucleus