Part 1: The cell-surface membrane and nucleus

Eukaryotic cells are organised into compartments. That matters because different reactions can happen in different places at the same time. Two structures underpin that organisation straight away: the cell-surface membrane and the nucleus.

The cell-surface membrane forms the outer boundary of the cell. It is a phospholipid bilayer with embedded proteins. Its key jobs are to:

• control what enters and leaves the cell
• allow cell recognition and cell signalling
• separate the internal conditions of the cell from the external environment

In a light microscope the cell-surface membrane is too thin to be seen clearly as a separate structure, so what you usually notice is simply the boundary between neighbouring cells.

The nucleus is usually the most obvious organelle in a eukaryotic cell. It is surrounded by a double membrane called the nuclear envelope, and this envelope contains many nuclear pores that control exchange between nucleus and cytoplasm.

Inside the nucleus:

• chromosomes consist of protein-bound, linear DNA
• nucleoplasm is the granular material filling the nucleus
• one or more nucleoli make ribosomal RNA and assemble ribosomal subunits

The nucleus acts as a control centre because genes in the DNA are used to make mRNA, which then directs protein synthesis in the cytoplasm. Different cells switch on different genes, which is why eukaryotic cells can become specialised.

Part 2: Organelles for energy transfer

Two membrane-bound organelles are especially important for energy transformations: mitochondria and chloroplasts.

Mitochondria are the sites of the aerobic stages of respiration and ATP production. Each mitochondrion has:

• a double membrane
• an inner membrane folded into cristae, which provide a large surface area
• a matrix containing enzymes, DNA and ribosomes

Cells with high ATP demand usually contain many mitochondria, and mitochondria in very active cells often have densely packed cristae. That is why muscle cells and cells involved in active transport are rich in mitochondria.

Chloroplasts are found in the photosynthetic cells of plants and algae. Not all plant cells have them: root cells, for example, do not. A chloroplast has:

• a double membrane envelope
• stacks of thylakoids called grana
• a stroma containing enzymes, DNA and ribosomes

The thylakoid membranes contain chlorophyll and provide a large surface area for the light-dependent stage of photosynthesis. The stroma contains the enzymes needed for sugar production in the later stage. Chloroplasts can also make some of their own proteins because they contain DNA and ribosomes.

Part 3: Ribosomes, ER, Golgi apparatus and lysosomes

Cells also need a transport-and-processing system for proteins and lipids.

Ribosomes are small particles made of ribosomal RNA and protein. They are the site of protein synthesis. In the cytoplasm of eukaryotic cells they are 80S. Smaller 70S ribosomes are found inside mitochondria and chloroplasts.

The rough endoplasmic reticulum (RER) is a network of flattened sacs called cisternae. Ribosomes are attached to its outer surface. The RER:

• provides a large surface area for protein synthesis
• folds and transports proteins made on its ribosomes
• is especially abundant in cells that secrete proteins

The smooth endoplasmic reticulum (SER) has no ribosomes. It synthesises, stores and transports lipids and also handles some carbohydrate processing.

The Golgi apparatus is a more compact stack of flattened membranes. It receives proteins and lipids from the ER, then:

• modifies them
• labels and sorts them
• packages them into Golgi vesicles
• forms lysosomes

A useful way to picture the pathway is:

1. ribosome makes a polypeptide
2. RER folds and transports it
3. Golgi apparatus modifies and sorts it
4. Golgi vesicles carry it to its destination

Lysosomes are single-membrane organelles formed by the Golgi apparatus. They contain hydrolytic enzymes. These enzymes can digest worn-out organelles, material taken in by phagocytosis, and unwanted cell components.

Part 4: Cell walls and vacuoles

Many eukaryotic cells also have structures outside or alongside the endomembrane system that help with support and storage.

A cell wall is found outside the cell-surface membrane in plants, algae and fungi. The composition differs:

• plant cell walls contain cellulose
• algal cell walls contain cellulose or glycoproteins, or both
• fungal cell walls contain chitin, glycan and glycoproteins

Cell walls:

• provide mechanical strength
• help prevent osmotic lysis
• support the organism as a whole
• in plants, allow water to move along cell walls

Plant cells have both a cell-surface membrane and a cell wall. The wall does not replace the membrane.

A mature plant cell usually has one large central vacuole surrounded by the tonoplast. The vacuole contains cell sap, which is a solution of mineral salts, sugars, amino acids, wastes and sometimes pigments.

The vacuole helps by:

• keeping the cell turgid
• storing solutes
• contributing to petal colour when pigments are present

Part 5: Specialisation, tissues, organs and systems

In complex multicellular organisms, eukaryotic cells become specialised. All cells come from earlier cell divisions and contain the same genes, but different genes are expressed in different cell types. That gives rise to different structures and different combinations of organelles.

Examples:

• palisade mesophyll cells contain many chloroplasts because they photosynthesise
• muscle cells contain many mitochondria because they need ATP for contraction
• phagocytes contain many lysosomes because they digest engulfed pathogens
• secretory cells have abundant RER and Golgi apparatus because they make and export proteins

This specialisation builds up into higher levels of organisation:

• a tissue is a group of similar cells working together
• an organ is a group of tissues working together
• an organ system is a group of organs working together

For example, in a leaf, palisade mesophyll, spongy mesophyll, xylem and phloem are tissues. Together they form the organ called the leaf. In animals, different tissues combine to form organs such as the stomach or the heart, and organs combine into systems such as the digestive or circulatory system.