1.2 cell structure and organisation

Cell theory key concepts

• All living organisms are composed of one or more cells

• The cell is the basic unit of life.

• Cells can only arise from pre-existing cells.

Organelle definition

 A specialised structure with a specific function inside a cell.

Useful conversions

1000 micrometres = 1mm (mm → um = x1000)

Nucleus

• Spherical, around 10-20um

• Contains DNA which, with protein, comprises the chromosomes.

• Chromosomes direct protein synthesis as they are the site of transcription.

• DNA provides a template for DNA replication.

• It is bounded by two membranes, the nuclear envelope with pores which allow the passage of large molecules such as mRNA, and ribosomes out of the nucleus.

• The granular material is the nucleoplasm, containing chromatin, made of coils of DNA bound to protein. During cell division, chromatin condenses into chromosomes.

• Spherical bodies called nucleolus are the sites of formation of rRNA, a constituent of ribosomes.

Mitochondria

• Cylindrical 1-10um in length.

• Two membranes separated by the inter membrane space. The inner membrane is folded inwards to form cristae.

• An organic matrix, a solution containing compounds such as proteins and lipids.

• A small circle of DNA to replicate and code for some of its proteins and RNA.

• 70s ribosomes which allow protein synthesis.

• The function of mitochondria is to produce ATP in aerobic respiration. The cristae provide a large surface area for the attachment of enzymes involved in respiration.

• Metabolically active cells such as muscle, need a plentiful supply of ATP, so contain many mitochondria.

• Being cylindrical, mitochondria have a larger surface area than a sphere of the same volume, their surface area to volume ratio is bigger. It reduces the diffusion distance between the edge and the centre, making aerobic respiration more efficient.

Chloroplasts

• Occur in the cells of photosynthesising tissue. In many plants the highest concentration is in palisade mesophyll.

• The stroma is fluid filled and contains some of the products of photosynthesis, including lipid droplets and starch grains.

• Contain 70s ribosomes and circular DNA which enable them to make their own proteins and self replicate.

• Within the stroma are closed flattened sacs called thylakoids, of which a stack is a granum. The photosynthetic pigments are found in the thylakoids. This arrangement provides a large surface area, efficient for trapping light energy.

Endoplasmic reticulum

• An elaborate system of parallel double membranes forming flattened sacs with interconnected, fluid filled spaces between them, called cisternae. The ER is connected with the nuclear envelope, allowing the transport of materials through the cell.

• Rough ER has ribosomes on the outer surface and transports the proteins made there. RER is present in large amounts in cells that make a lot of protein.

• Smooth ER comprises membranes that lack ribosomes. It is associated with the synthesis and transport of lipids.

Ribosomes

• Smaller in prokaryotic (70s) than eukaryotic cells (80s in cytoplasm).

• Have one large and one small subunit.

• Assembled in the nucleolus from rRNA and protein.

• Important in protein synthesis as they are the site of translation, where mRNA and tRNA are used to assemble the polypeptide chain.

• Large subunit has 2 tRNA attachment sites, small subunit has 1 mRNA attachment site.

Golgi apparatus

• More compact than ER

• Vesicles containing polypeptides pinch off from the RER and fuse with the stack of membranes which constitute the Golgi body.

• Proteins are modified and packaged in the Golgi body.

• At the other end, vesicles containing these modified proteins are pinched off and carry proteins elsewhere in the cell or move to and fuse with the cell membrane, secreting the modified proteins by exocytosis.

• Functions of the golgi body include: 

• Producing secretory enzymes, packaged into secretory vesicles.

• Secreting carbohydrates e.g. for formation of plant cell walls

• Producing glycoprotein

• Transporting and storing lipids

• Forming lysosomes, containing digestive enzymes.