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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, sospecimens 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 µm in diameter, and a microscope produces an image of it which

is 1 mm (1000 µm) 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 asrevealed 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 proteinmicrotubules 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 µm 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 thevesicles 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 µm 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 µm long and 2 – 3 µm 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 µm wide, 3 – 10 µm 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 aproduct 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 Animal cells

cellulose cell wall present no cellulose cell wall

many cells contain chloroplasts, site of

photosynthesis

no chloroplasts, animal cells cannot carry

out photosynthesis



carbohydrates stored as starch often

present

carbohydrates stored as glycogen often

present

large, permanent vacuole present no large permanent vacuoles

no centriole centriole present outside the nucleus

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