Summary of Section 4

• Eukaryotic cells are surrounded by a cell membrane, which is composed of a phospholipid bilayer in which proteins and glycoproteins are embedded. The membrane forms a barrier between the cell and its external environment; membrane proteins and glycoproteins mediate selective transport of substances into and out of the cell, and interactions with the extracellular environment, including neighbouring cells.
• The eukaryotic cytoskeleton is composed of three types of filament-like structures assembled from cytoskeletal proteins: microfilaments (also known as actin filaments), microtubules and intermediate filaments. The cytoskeleton collectively provides shape, support and an internal transport system, and is responsible for cell movements.
• The nucleus, which has a double membrane (or nuclear envelope), contains most, but not all, of a cell's DNA, which is packaged by binding to histone proteins to form chromatin. DNA replication and also transcription to synthesise RNA take place in the nucleus, as does the assembly of ribosomes. RNA and assembled ribosome components leave the nucleus, and structural and ribosomal proteins and the enzymes needed for DNA replication and transcription enter it, through pores in the nuclear envelope.
• Messenger RNA is translated by ribosomes to synthesise proteins. Cytosolic proteins and proteins destined for the nucleus, mitochondria, chloroplasts and peroxisomes are synthesised by ribosomes that are free in the cytosol. Proteins destined for these organelles carry amino acid signal sequences that target them for recognition by special proteins that transport them to the appropriate cell compartments.
• Proteins destined for lysosomes, for secretion from the cell, or for embedding in the cell membrane have signal sequences that relocate the ribosome on which they are being translated to the rough ER, where their translation is completed. Glycosylation of proteins also starts in the RER.
• The smooth ER is the site of phospholipid assembly and also detoxification (for example of drugs or toxins).
• Proteins and lipids pass from the RER to the Golgi apparatus for further processing (e.g. glycosylation), and sorting and packaging into vesicles, which deliver proteins and lipids to lysosomes or to the cell membrane (either for embedding in the cell membrane, or secretion from the cell).
• Vesicles move short distances within the cell by diffusion, but are transported longer distances by movement along microtubules. Movement of vesicles to specific sites in the cell is achieved because the vesicles have special coats and surface signals that identify their contents and ensure they fuse with the appropriate membrane.
• Substances are secreted from cells by exocytosis in which vesicles fuse with the cell membrane. One way in which substances are imported into cells is by engulfment into a cell membrane-bound vesicle. This process is known as endocytosis.
• Ingested materials and 'old' organelles are digested within lysosomes. Some fatty acids and amino acids are broken down in peroxisomes.
• Mitochondria are the site of the majority of ATP production. They have a double membrane, the inner membrane of which is folded; and they also have their own DNA and ribosomes which synthesise some of the proteins required for ATP synthesis. Most mitochondrial proteins are, however, encoded by the nuclear DNA and are imported into the mitochondria.
• Plant cells have a cell wall and two organelles not found in animal cells: chloroplasts, which are the site of photosynthesis, and the vacuole. Chloroplasts have three membrane systems: outer, inner and internal thylakoid membranes. The complex internal membrane system is where photosynthesis takes place. Many other activities including synthetic reactions take place in the chloroplast stroma. The vacuole is the site where water, ions and small organic molecules are stored. It provides turgidity to the cell and may contain enzymes that digest large organic molecules.
• New organelles form by growth of pre-existing organelles, followed by division. These processes are independent of nuclear division.
• Organelles have a number of other functions, including roles in programmed cell death and calcium signalling.