International A Level Biology / Cell structure

What each organelle does revision notes

Edexcel International A Level Biology (YBI11), chapter 7: Cell structure, specification point 3.3. Free lesson.

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Organelles have structures that suit their jobs, from making proteins to releasing energy in respiration. Knowing how they work and using precise wording helps you explain cell processes clearly.

How the nucleus controls the cell

The nucleus holds the DNA instructions for the proteins a cell can make. Chromatin is DNA wound around proteins called histones. The genes in the DNA are copied or used to make messenger RNA (mRNA), which carries a copy of a gene's instructions. The nuclear envelope is a double membrane that keeps DNA apart from the cytoplasm. Nuclear pores let mRNA and ribosome subunits pass out of the nucleus. The nucleolus makes ribosomes by joining ribosomal RNA with proteins to form two subunits.

How a mitochondrion makes ATP

A mitochondrion is the site of aerobic respiration, which uses oxygen and releases energy. The energy released is used to make ATP, which the cell can use. Its structure supports this process. The double membrane keeps respiration apart from the cytoplasm. Cristae are folds of the inner membrane. They provide a large surface area for enzymes involved in making ATP. More folds allow more enzymes and can support more ATP production. The matrix is the fluid inside the mitochondrion. It contains enzymes for other stages of respiration. Mitochondria have circular DNA and their own…

How proteins and lipids are made and moved

For proteins that are transported through the cell, the route is from the rough ER to the Golgi in vesicles. Each structure has a different role, so take care not to mix up making a protein with carrying it. Ribosomes are the site of protein synthesis. They join amino acids in the order set by mRNA. Each ribosome has two subunits and no membrane. Rough ER has ribosomes on flattened sacs called cisternae. Proteins made by the ribosomes enter the ER, where they are folded and transported. A vesicle is a small membrane sac. It buds off the rough ER and carries proteins to the Golgi.…

What lysosomes and centrioles do

A lysosome is a sac with a single membrane. It contains hydrolytic enzymes, which break down substances. In a white blood cell, lysosomes can fuse with a vesicle containing an engulfed bacterium and digest it. They also digest worn-out organelles. A lysosome is an organelle, not an enzyme. The hydrolytic enzymes are molecules inside it. Centrioles are two hollow cylinders made of microtubules. In a dividing animal cell, a pair at each pole helps form the spindle, whose fibres pull chromosomes apart.

Linking organelle structure to a cell's needs

A cell that makes and secretes many proteins needs the parts that support protein production. For example, a pancreas cell makes and secretes large amounts of digestive enzymes, which are proteins. A large nucleolus makes more ribosomes. Many nuclear pores let more mRNA and ribosome subunits leave the nucleus. Together, these features allow more protein to be made at ribosomes in the cytoplasm.