Cambridge AS Level Biology - Cell Structures and Organelles

Golgi Apparatus

A critical member of the biochemical manufacturing and supply chain inside a cell. It receives biochemicals in a 'bulk flow' from the rough endoplasmic reticulum (rER). It is the only organelle in the cell that receives, sorts, modifies, concentrates, packs and dispatches biochemicals for use inside and outside the cell.

Nucleolus

A part of the nucleus which makes ribosomes, using the information in its own DNA.

Deoxyribonucleic Acid (DNA)

A polymeric molecule made up of one or more deoxyribonucleotides. A strand of it can be thought of as a chain with a deoxyribonucleotide at each chain link. Each nucleotide is made up of a base (adenine, cytosine, guanine, and thymine, typically abbreviated as A, C, G and T), a deoxyribose sugar, and a phosphate.

Ribonucleic Acid (RNA)

A polymeric molecule made up of one or more nucleotides. A strand of it can be thought of as a chain with a nucleotide at each chain link. Each nucleotide is made up of a base (adenine, cytosine, guanine, and uracil, typically abbreviated as A, C, G and U), a ribose sugar, and a phosphate. Its ribose sugar backbone has a hydroxyl (-OH) group.

Electron Microscope

A type of optical instrument which uses a beam of electrons to create an image of the specimen. It is capable of much higher magnifications and has a greater resolving power than a light microscope, allowing it to see much smaller objects in finer detail.

Rough ER

A variation of the Endoplasmic Reticulum. Earns its name from many tiny organelles called ribosomes which cover its surface. Forms an extensive system of flattened sacs spreading in sheets throughout the cell.

Smooth ER

A variation of the Endoplasmic Reticulum. It makes lipids and steroids, such as cholesterol and the reproductive hormones oestrogen and testosterone.

Endoplasmic Reticulum (ER)

An extensive system of membranes running through the cytoplasm. Has two variations of structure: rough and smooth. Aids in the folding of protein molecules in sacs called cisternae and the transport of synthesized proteins in vesicles to the Golgi apparatus.

Chromatin

Chromosomes which are in a loosely coiled state within the nucleus.

Genes

Chromosomes which have been organised into functional units. Controls the activities of the cell and inheritance.

Lysosomes

Spherical sacs, surrounded by a single membrane and having no internal structure. They contain digestive (hydrolytic) enzymes which must be kept separate from the rest of the cell to prevent damage. They are responsible for the breakdown (digestion) of unwanted structures such as old organelles or even whole cells =, as in mammary glands after lactation (breast feeding). In white blood cells, they are used to digest bacteria. Enzymes are sometimes released outside the cell - for example, in the replacement of cartilage with bone during development. The heads of the sperm contains a special type of these sacs, the acrosome, for digesting a path to the ovum.

Ultrastructure

The 'fine', or detailed, structure of a cell as revealed by the electron microscope.

Nucleus

The largest cell organelle. Contains DNA and other genetic materials of a cell. When the cell is ready to divide, this organelle divides first so that each new cell will have its own copy of this crucial organelle.

Nuclear Pores

The many pores possessed by the nuclear envelope, which allow and control exchange between the nucleus and cytoplasm. Examples of substances leaving these pores are mRNA and ribosomes for protein synthesis. Examples of substances entering through these pores are proteins to help make ribosomes, nucleotides, ATP (adenosine triphosphate), and some hormones such as thyroid hormone T3.

Nuclear Envelope

The two membranes which surround a nucleus. Its outer membrane is continuous with the endoplasmic reticulum.

Chromosomes

Thread-like structures located inside the nucleus of animal and plant cells, which contains the DNA.

Ribosomes

Tiny organelles located in the cytoplasm and on the surface of the rough ER. At very high magnifications, they can be seen to consist of two subunits: a large and a small subunit. They are the sites of protein synthesis. They are made of RNA (ribonucleic acid) and protein.