CELL ULTRASTRUCTURE: EUKARYOTIC ANIMAL CELLS
Eukaryotic cells are characterised by their possession of many membrane bound organelles. Some of the details of these are described below:
The Nucleus
Cytoplasm
Endoplasmic Reticulum
Ribosomes
Golgi Apparatus/Body
Lysosomes
- Found in all eukaryote cells except mature phloem sieve elements and red blood cells.
- Largest cell organelle, approx 10 mm (diameter) x 20 mm (length).
- Controls cell’s activities, contains genetic information (DNA) which is capable of replication ensuring it is all passed on to next generation.
- Surrounded by nuclear envelope (two membranes, outer is continuous with endoplasmic reticulum), with nuclear pores to let substances (e.g. mRNA and ribosomal proteins) in and out.
- Contains gel-like nucloplasm which contains chromatin and one or more nucleoli, plus a variety of chemicals e.g. ions, proteins and nucleotides. Chromatin is composed of coils of DNA bound to basic proteins called histones.
- During nuclear division chromatin condenses tightly to become chromosomes. During interphase, some continues to stain intensely and is called heterochromatin (the rest, loosely coiled is called euchromatin - thought to contain the DNA which is genetically active during interphase (therefore in cells which have many genes constantly being expressed e.g. liver cells there is more euchromatin and less heterochromatin).
- Nucleolus is a rounded structure, which makes ribosomal RNA. They are not visible during prophase and reorganise during telophase of nuclear division.
Cytoplasm
- An aqueous substance containing a variety of cell organelles.
- Contains 90% water, ions, salts, sugars, amino acids, fatty acids, nucleotides, vitamins and dissolved gases.
- It is the site of many metabolic pathways e.g. glycolysis, the first stage of aerobic respiration. Synthesis of fatty acids, amino acids and nucleotides also occurs.
- It contains many microfilaments, which make up the cytoskeleton (gives the cytoplasm 3D structure and allows the controlled movement of substances within it). The mass movement of cytoplasm sometimes occurs and is called ‘cytoplasmic streaming’.
Endoplasmic Reticulum
- Complex system of membranes running through the cytoplasm. Cannot be seen with light microscope.
- Contains flattened membrane-bound sacs called cisternae. These may be covered with ribosomes forming rough ER. If ribosomes are absent it is smooth ER.
- Concerned with the synthesis and transport of substances. Rough ER with proteins made by the ribosomes. The growing polypeptide is bound to the ribosome until it is complete. The signal sequence at the start fits a receptor in the ER membrane and binds to the ER allowing it to pass through into the ER cisternae. Once inside the signal sequence is removed and the protein folds up into its tertiary structure trapping it in the ER. It is now often sent to the Golgi apparatus.
- Polypeptides with no signal sequence are made by ribosomes free in the cytoplasm and remain in the cytoplasm for use in the cell, (i.e. the signal sequence is specific and causes a protein to be targeted to appropriate places (if the sequences are switched between proteins the protein is sent to the wrong place in the cell).
- ER is involved in lipid synthesis (e.g. epithelium of intestine make lipids from fatty acids and glycerol in the smooth ER and passes them to the Golgi for export. Adrenal cortex cells and interstitial cells of testis which both secrete steroid hormones (steroids are a type of lipid) contain a lot of smooth ER.
- In liver, rough and smooth ER are involved in detoxification.
Ribosomes
- 20nm in diameter.
- Found in large numbers in prokaryote (70s type) and eukaryote (80s type) cells. A typical bacterial cell may have 10 000 ribosomes.
- They are the site of protein synthesis.
- Each has two sub units (large and small) made of roughly equal mass of ribosomal RNA and protein (they are ribonucleoproteins).
- They form complex 3D structures and are capable of self-assembly.
- All have an identical structure. Some are bound to ER (making proteins which are usually secreted) and some are not (making e.g. haemoglobin in young red blood cells).
- Mitochondria and chloroplasts have their own ribosomes, which are the 70s type.
Golgi Apparatus/Body
- Discovered by Golgi in 1898, can just be seen with a light microscope but not in detail.
- Consists of stack of flattened, membrane bound sacs called cisternae together with a system of associated Golgi vesicles. It is possible a complex system of interconnected tubules is formed around the central stack.
- New cisternae are constantly formed at one end (probably from buds of ER) and at the other end the cisternae are constantly breaking up into vesicles.
- Golgi transports and chemically modifies the substances within it. Important in secretary cells e.g. acinar cells of the pancreas, which secrete digestive enzymes into duodenum. Pathways of secretary substances can be found using radioactively labelled amino acids and subsequently breaking up the tissue at different time slots and looking to see which parts of the cell are radioactive.
- Generally proteins received by the Golgi from the ER have short carbohydrate chains added to become glycoproteins, these can be further modified in the Golgi and may become markers to direct the proteins to the correct destination.
- The Golgi is sometimes involved in the secretion of carbohydrates e.g.1 synthesis of new cell walls by plants (polysaccharides of the cell wall matrix not cellulose). e.g.2 mucin (glycoprotein forming mucus in solution) is secreted by goblet cells in respiratory tract and intestine. The Golgi also secretes slime wax and gum.
- The Golgi may be involved in lipid transport, e.g. lipids absorbed as FAs and glycerol in the small intestine are resynthesised in smooth ER, coated in protein and transported through the Golgi to the cell membrane where they enter the lymphatic system.
- Golgi also forms lysosomes.
Lysosomes
- Abundant in cells exhibiting phagocytic activity.
- Single membrane organelle.
- Contain hydrolytic enzymes such as proteases, nucleases, lipases and carbohydrases. They are involved in destroying foreign bodies in the cell or old organelles (process called autophagy, where organelle is enclosed in a single membrane from the smooth ER and fuses with a lysosome. It is very prominent in cells which are undergoing reorganisation during differentiation), molecules which are no longer needed etc. They are synthesised on rough ER and transported to the Golgi. Golgi vesicles later budding off, are called primary lysosomes.
- Inside is a low pH so the enzymes have a low pH optimum. They are kept apart from the rest of the cell because otherwise they would destroy it.
- In animal cells they are spherical, 0.2-0.5 mm diameter.
- In plants the large central vacuole may act as lysosomes.
- Sometimes the whole cell must be destroyed and does so in a process called autolysis when lysosomes release their contents within it. A normal event, prominent in differentiation e.g. tadpole tail being reabsorbed during metamorphosis, or programmed cell death in tissues (when programmed cell death no longer works cancer can result).