Nucleus

The Nucleus

• These are found in all eukaryote cells except

red blood cells, phloem sieve tubes and xylem vessels

• Some cells are multinucleate

• The nucleus is about 10-20µm in diameter

• It is surrounded by a double membrane or an envelope

• The outer layer of the membrane is continuous with the endoplasmic reticulum

Nucleus

It is surrounded by a double membrane or an envelope

The outer layer of the membrane is continuous with the endoplasmic reticulum

Nucleus

There are pores in the envelope, about 80-100nm in diameter, which allow exchange of materials

The nucleoplasm is a gel-like matrix which contains chromosomes

NucleusChromosomes are seen as

diffuse chromatin unless the cell is dividing and they have been stained

Chromatin is coils of DNA and histone proteins bound to the DNA

Heterochromatin is seen under EM as darker staining because it is more tightly coiled

Euchromatin is lighter staining and less tightly coiled

NucleusThere is one or more

nucleolus (plural = nucleoli)

where ribosomal RNA (rRNA) is manufactured

It is seen as a dense, dark staining region

Endoplasmic Reticulum (ER)

This is a system of flattened sacs or cisternae

throughout the cytoplasm, continuous with the outer membrane of the nucleus

Rough ER- this is associated with ribosomes

Rough ER is involved in the synthesis of proteins and then the transport of protein around the cell

Smooth ER –has no ribosomes

It is involved in steroid and lipid production

In an electron micrograph ER is seen as a series of parallel linesSometimes it is possible to see the ribosomes on rough ER as small dots

Ribosomes

these are 20nm in diameter

They are free in the cytoplasm or bound to ER

Ribosomes are made of RNA and proteins

The RNA is ribosomal RNA (rRNA),made in the nucleolus

Ribosomes are the site of protein synthesis

Ribosomes

They "read" the RNA code

Free ribosomes make cell proteins

Ribosomes on the ER make proteins that may be secreted from the cell

Chloroplasts, mitochondria and Prokaryotes have 70S ribosomes (smaller)

Eukaryotes have 80S (larger ) ribosomes

The ribosome is made of two sub-

units:• A smaller and a lager subunit• A polysome is several ribosomes

simultaneously reading the same strand of mRNA to increase the efficiency of protein synthesis

Model of a ribosome – the blue area is the smaller subunit

Golgi apparatus

These are found in all cells and are abundant in metabolically active secreting cells

They are a system of flattened sacs called cisternae only visible under EM

Golgi apparatusNote the curved parallel lines

These are

vesicles containing substances being made by the Golgi apparatus

1. . Proteins or lipids made in the ER are transported, in vesicles, through

the cytoplasm

2. The proteins/lipids are modified through the Golgi apparatus, often by

having a carbohydrate attached

3. These vesicles fuse with the outer forming face of the Golgi apparatus

4. The modified product buds off in Golgi vesicles from the inner maturing

face

5. The vesicles move to the cell surface membrane and fuse with it to release

the contents outside (e.g. digestive enzymes)

If the product is not to be secreted the vesicle will transfer it to where it is needed within the cell

Lysosomes

• about 0.1 – 0.5µm in diameter

• only seen under using EM

• made by rough ER and Golgi apparatus

• spherical membrane bound structures containing digestive enzymes

• These enzymes may be

– used to digest other organelles or the entire cell (autolysis)

– Released form the cell by exocytosis (such as when sperm digest the ovum barrier)

– Digest foreign matter taken in to the cell by endocytosis. They fuse with the food vacuole to form a secondary lysosome

In plant cells the vacuole may act as a lysosome

Lysosomes digesting a food particle in a phagocyte

This is a food particle

This is a simplified Amoeba

Golgi apparatus

Lysosomes digesting a food particle in a phagocyte

1. endocytosis – the food particle is taken into the cell

It is in a food vacuole made from the cell surface membrane

Lysosomes digesting a food particle in a phagocyte

2. The Golgi apparatus has modified proteins to form hydrolytic (digestive) enzymes

These are budded off in lysosomes

Lysosomes digesting a food particle in a phagocyte

3. The lysosome and food vacuole fuse to form a secondary vacuole

Lysosomes digesting a food particle in a phagocyte

4. The hydrolytic enzymes digest the food particle and the digested nutrients are absorbed into the cytoplasm

Lysosomes digesting a food particle in a phagocyte

5. Undigested matter is removed by exocytosis

Note – in white blood cells this step does not occur. The entire cell will be destroyed eventually