1. Cell wall: The cell wall of algae has two layers. The outer layer is composed of pectose and the inner layer is composed of cellulose, pectin or mucilage. Flagellated algae do not have cell walls. The cell walls of diatoms are composed of two layers of siliceous membranes. The siliceous cell wall of an intact diatom is called a frustule. Euglenophyta have membranes called periblasts instead of cell walls.
2. Plasma membrane: The fine and thin membrane inside the cell wall is called plasma membrane. It is composed of lipids and proteins. The plasma membrane of flagellated algae is rigid in nature. This plasma membrane is called periplast.
3. Protoplasm: All the living parts of the cell inside the cell wall are collectively called protoplasm. It consists of two parts. Cytoplasm and nucleus.
(i) Cytoplasm: Different parts of cytoplasm are-
Plasted: Plasted is a dye. Green colored plastids are called chloroplasts. Algae contain blue phycocyanin, red phycoerythrin, brown pheophanin and yellow-brown phycoxanthin along with chloroplasts. They carry out photosynthesis.
Pyrinoid: Pyrinoid is a protein aggregate. Chloroplasts contain one or more pyrenoids. It stores carbohydrates.
Cell vacuole: Each cell has a small cell cavity. It is filled with tissue.
Food: Sugar or starch or white matter is stored as food in green algae. Laminarin and mannitol in brown algae, volutin and fat in golden-yellow algae, and floridian starch in red algae are stored as stored food. Some algae contain leucosin and paramylum as stored food.
(ii) Nucleus : Each cell of algae contains one or more well-formed nuclei. It controls all biological functions of cells.
The body of sinocytic algae is cylindrical, branched, wallless and multinucleated. The body of acinocytic algae is walled and the cells contain one or more nuclei.

The body of some algae is made up of three parts. Hold fast, stipe and frond. Fronds are flat like leaves. Such a structure is called a complex body. Such as- Fucus, Laminaria, Sargassum.

Their body consists of Parva and Parvamadhya. Lateral branches arise from the pars. Like- Chara, Nitella etc.

The body of polysiphon algae consists of many siphons or hollow cylindrical parts. Its central sinusoid is called central siphon and surrounding sinusoid is called pericentral siphon. Eg- Codium, Polysiphonia.

Algae with hollow cylindrical body are called siphon algae. These hollow tubes are called sinusites. Such as Vaucheria, Botrydium, Protosiphonia etc.

The thallus of algae consists of two parts. Recumbent part and aerial part. A thallus with such features is called heterotrichous. Such as Coleochaete, Fritschiella, Stigeoclonium, Trentepohlia etc.

If the cells of algae divide and form a thin and flat leaf-like thallus, it is called thalloid. For example, Enteromorpha, Porphyra, Ulva etc.

Dendroid means tree-like. The underside of each algal cell is covered with mucilage and joins together to form colonies. Such structures are called dendroids. For example, Prasinocladus.

If the cells of algae are joined together to form a narrow, linear, unbranched structure, it is called filamentous. Eg- Oedogonium, Ulothrix, Scytonema, Tolypothrix, Cladophora, Pithophora, Spirogyra etc.

Motile algae lose their flagella and become encased in a mucilage coat and survive the hostile environment. This condition is called Pamela Dasha. Such as Chlamydomonas, Ulothrix, Sargassum, Fucus, Tetraspora, Aphanothece, Gonium etc.