A level Biology exam revision resources written by A level Examiners





 

Figure 1 below shows a diagrammatic representation of the structures within an animal cell that are visible with the electron microscope. It is worth noting that not every animal cell necessarily contains all the organelles shown (though many do).

Figure 1 : Structures within an Animal Cell
Eukaryotic Cells



Cytoplasm

The solution within the cell membrane (cytosol) which contains many enzymes that catalyse metabolic reactions. Also included non-soluble granules such as glycogen granules which act as an energy store.


Cytoskeleton

A network of protein fibres (microtubules and microfilaments) which extends throughout the cell. The cytoskeleton attaches to the cell membrane and gives the cell its shape, as well as playing a role in movement of certain cells e.g. amoeba


Nucleus

This is the largest organelle. Surrounded by a double membrane with nuclear pores which is called the nuclear envelope. The pores allow entry and exit of substances (such as mRNA required for the synthesis of proteins). The fluid in the nucleus is called the nucleoplasm, which contains nucleotides in solution. Floating in the nucleoplasm is the chromatin. Chromatin is visible as diffuse threadlike fibres. It is the chromosomes present in their fully unwound state. Chromosomes are linear strands of DNA with attached proteins. The nucleolus is a dark region of chromatin that is involved in ribosome production. The main role of the nucleus is to house the genetic material, which is essential for protein synthesis and cell division.


Mitochondria

Mitochondria are the site of most of the reactions of aerobic respiration. A double membrane surrounds mitochondria. The inner membrane is highly folded into cristae, which increase its surface area. The fluid within the mitochondria is called the matrix, it contains small circlet of DNA and many of the enzymes involved in respiration. Mitochondria contain small (70s) ribosomes.


Chloroplasts

These are found in plant cells, they are larger than mitochondria. Chloroplasts are the site of photosynthesis. Therefore they are found in photosynthetic organisms (plants and algae). Like mitochondria they have a double membrane. Within chloroplasts are many membrane bound discs called thylakoids. It is these that contain chlorophyll molecules. The thylakoids form stacks called grana. The fluid between grana is called the stroma, it contains a circlet of DNA and enzymes required for photosynthesis. Chloroplasts contain small (70s) ribosomes. Chloroplasts can also contain starch grains.


Ribosomes

These are the sites of protein synthesis. They are composed of protein and RNA, and are manufactured in the nucleolus of the nucleus. Ribosomes are either found free in the cytoplasm or they are found attached to the rough endoplasmic reticulum. They are often found in 'whorls' called polysomes. All eukaryotic ribosomes are large ribosomes (80s). However ribosome are still the smallest organelle.


Rough endoplasmic reticulum (RER)

This consists of a series of interconnected flattened tubules called cisternae. It is encrusted with ribosomes, which give it a rough appearance. The ribosomes synthesise proteins, which are isolated and transported by the RER. From the RER the proteins are transported to the Golgi apparatus.


Smooth endoplasmic reticulum (SER)

This is similar to the RER in structure but it is distinctly more tubular. It is involved in synthesising and transporting materials, mainly lipids and in calcium ion storage. The calcium ions are especially important in muscle cells where the SER is called the sarcoplasmic reticulum.


Golgi Apparatus

This organelle is also formed by a series of flattened membrane vesicles. It is formed from the endoplasmic reticulum. Its function is the modification and transport of proteins from the RER.


Vacuoles

These are membrane-bound sacs containing water or dilute solutions of salts and other solutes. Most cells can have small vacuoles that are formed as required, but plant cells usually have a large permanent vacuole. The membrane surrounding the vacuole in plant cells is called the tonoplast.


Lysosomes

These are small membrane-bound vesicles formed from the Golgi apparatus. They contain a mixture of hydrolytic enzymes. Their role is to break down unwanted organelles (or old worn out cells) so that the materials may be recycled.


Centriole

This is a pair of short protein microtubules oriented at right angles to each other. They are involved in cell division as the spindle forms from them.


Flagellum

This is a long tail present constructed of protein microtubules. It is only present in certain cells e.g. sperm and is used for motility.


Cilia

These are usually present in large numbers. Structurally they are similar to flagella. Their function is to transport materials over the cell surface. This is achieved by all the cilia on a cell beating rhythmically and creating a current across the surface of the cell.


Microvilli

These are small finger-like folding of the cell membrane, which increase the surface area of the cell hugely. They are therefore found in cells that are involved in exchange e.g. gut epithelial cell of the small intestine in mammals.


Plasma Membrane

This forms a thin layer round the outside of the cell. It is constructed from phospholipids and globular proteins. It separates the contents of the cell from the outside environment. The membranes that surround organelles are very similar in structure to the one surrounding the cell. One of the advantages of having membrane bound organelles is that they compartmentalise cells i.e. they allow chemically different environments.


Cell Wall

This is a thick layer outside the cell membrane used to give a cell strength and rigidity. Animal cells have no cell wall. Plant cells have a cell wall composed of cellulose, fungi have a cell wall composed of chitin. Plant cell walls also contain other polysaccharides compounds including lignin (this is the woody substance).




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