Cell organelles and their functions (SACE Stage 2 Biology)

What this dot point is asking

You need to describe the structure and function of the main organelles and explain how several of them work together, especially in producing and exporting proteins.

The major organelles

Nucleus

The nucleus is surrounded by a nuclear membrane with pores. It contains the cell's DNA as chromosomes. It controls the cell's activities by controlling gene expression and is the site of transcription. The nucleolus inside it makes ribosomes.

Mitochondria

Mitochondria are the site of aerobic cellular respiration, releasing energy as ATP from glucose. They have a double membrane; the inner membrane is folded into cristae, increasing surface area for the reactions. Cells with high energy demands, such as muscle cells, contain many mitochondria.

Ribosomes

Ribosomes are the site of protein synthesis (translation). They may be free in the cytoplasm or attached to the rough endoplasmic reticulum. They are not membrane-bound.

Endoplasmic reticulum

The endoplasmic reticulum (ER) is a network of membranes. Rough ER is studded with ribosomes and processes and transports proteins. Smooth ER has no ribosomes and makes lipids and helps detoxify substances.

Golgi apparatus

The Golgi apparatus is a stack of flattened membrane sacs. It modifies, sorts and packages proteins and other molecules into vesicles for secretion or delivery elsewhere in the cell.

Chloroplasts

Chloroplasts are found in plant cells and some protists. They contain the green pigment chlorophyll and are the site of photosynthesis. Their internal membranes (thylakoids stacked into grana) provide a large surface area for capturing light.

Vacuole

In plant cells a large central vacuole stores water, sugars and waste and maintains turgor pressure, keeping the cell firm and supporting the plant.

Lysosomes

Lysosomes contain digestive enzymes that break down worn-out organelles, food particles and invading material.

Cell membrane and cell wall

The cell membrane controls movement of substances in and out. Plant cells also have a rigid cellulose cell wall for support and shape.

How organelles cooperate

SACE rarely asks for an isolated list; the higher marks come from explaining cooperation. The protein export pathway above is one example, but several others recur:

• Energy supply. Mitochondria release the ATP that ribosomes, the Golgi and secretory vesicles all need - protein synthesis and export are energy-expensive, so a secretory cell is rich in both mitochondria and rough ER.
• Membrane recycling. Vesicles continually bud from the ER and Golgi and fuse with the cell membrane, so the membrane system is dynamic rather than fixed.
• Quality control and disposal. Misfolded proteins and worn-out organelles are delivered to lysosomes, whose enzymes break them down so their components can be reused.

Reading organelle abundance

Examiners often give a cell description and ask you to infer its role from which organelles are abundant. A cell packed with mitochondria is doing a lot of active work (for example a muscle or kidney tubule cell driving active transport). A cell rich in rough ER and Golgi is specialised for synthesising and exporting protein (for example a pancreatic or antibody-secreting cell). A plant cell with many chloroplasts is a photosynthetic mesophyll cell. Being able to reason backwards from organelle abundance to function is a frequently tested skill, so practise justifying the link rather than just naming the organelle.

Plant versus animal cells

A clean comparison earns easy marks. Both plant and animal cells have a nucleus, mitochondria, ribosomes, ER, Golgi and a cell membrane. Plant cells additionally have a cellulose cell wall, chloroplasts and a large permanent central vacuole; animal cells lack these but more commonly show prominent lysosomes and centrioles. The presence of a cell wall and chloroplasts is the quickest way to identify a plant cell in a micrograph.