In this article, we will learn about the Cellular Structure of Algae with diagrams. Algae are photosynthetic plants that vary in size and shape. Their size ranges from microscopic to over 50 meters in length. Based on the structural variations there are present different phyla or divisions of algae.

Cell Wall

• The cell wall of algae is made up of cellulose, hemicellulose, mucilage, pectin and other substances like alginic acid, fucoidin, fucin, calcium carbonate, silica etc. 
• The electron microscopic view of the cell wall reveals that the cellulosic cell wall is made up of cellulose microfibrils which remain variously oriented in a granular matrix.
• The cell wall of the diatom is silicified and shows characteristic secondary structures.
• The cell wall of Cyanophyceae is made of mucopeptide. Which is composed of peptide of amino acid, covalently linked with amino- sugars, glucosamine and muramic acid. 
• Gymnodinium and Pyramimonas lack a true cell wall, instead they contain pellicle, which is a boundary membrane.

Plasma membrane

• It is located beneath the cell wall. The structure of the algal plasma membrane is similar to the other eukaryotic cells.
• Some algae contain a stout and strong plasma membrane which is known as periplast.

The Protoplast

• The protoplast is referred to as the protoplasmic content of the cell.
• The protoplast of eukaryotic algal is covered by a lipoproteinaceous external boun­dary known as the cell membrane, and made up of one or more usually spherical or ellipsoidal nucleus and cytoplasm.
• The cell membrane is very thin and elastic and selectively permeable, helps in the passage of materials in and out of the cells. It is composed of lipid and protein and is fluid mosaic in nature similar to other biological membrane systems. 
• The nucleus of eukaryotic algae is well organised. The nucleus id surrounded by a  double-layered nuclear membrane. The inner side of the membrane is occupied by a chromatin reticulum embedded in a matrix called karyolymph. The outer side of the nuclear membrane is conti­nuous with the endoplasmic reticulum.
• The nucleus contains one, two or more nucleoli or endosomes, the number varies in different algae.
• The chromosome number changes from species to species and may contain a localized or diffused centro­mere. The lowest number of chromosomes is found in Porphyra linearis, which is  n=2. The highest number of chromosomes is found in Netrium digitali which is around n=592.
• The eukaryotic algae contain membrane-bound organelles such as chloroplasts, mito­chondriai, golgi apparatus, endoplasmic reticu­lum and, in some cases, eye spot or stigma.
• The nucleus of prokaryotic algal lacks membrane, instead the protoplast is divided into the outer peripheral chromoplasm and inner colourless centroplasm. The photosynthetic pigments are present at the outer peripheral chromoplasm.
• In inner colourless centroplasm where the genetic mate­rial is not found within the membrane-bound nucleus and the DNA strands do not combine with histones to form chromosomes.
• In Cyanophyceae the centroplasm represents the incipient nucleus.
• In Dinophyceae the nucleus is membrane-bound but lacks chromosomes and mitotic apparatus. 

Chloroplast

• Chloroplast is a double-membrane structure containing photosynthetic pig­ments.
• There are mainly eight types of chloroplast in algae such as;
  • Cup shaped (e.g., Chlamydomonas and Volvox)
  • Discoid (e.g., Chara, Vaucheria and centric diatoms)
  • Parietal (e.g., Chaetophorales, Phaeophyceae, Rhodo- phyceae, many Chrysophyceae and pinnate diatoms)
  • Girdle shaped or C-shaped (e.g., Ulothrix)
  • Spiral (e.g., Spirogyra)
  • Reticulate (e.g., Oedogonium, Hydrodictyon and Cladophora)
  • Stellate (e.g., Zygnema)
  • Ribbed (e.g., Volvocales).
• Chloroplast has three major structural regions such as;
  •  An envelope composed of two mem­branes with an enclosed space.
  •  Solu­ble enzymes containing mobile stroma.
  • Organized internal lamellar membranes containing pigments and involved in energy capture and trans­duction.
• A disc like structure is formed from the internal lamellar system which are stacked together to form grana. This disc is a sac or vesicle and known as thylakoid, which are encloses an interthylakoid space.
• The thylakoid membrane separates the thylakoid from stroma.
• In Cyanophyceae the thylakoids are lie free in the cyto­plasm and they are not enclosed in membrane bound groups.
• Chlorophyll a, and other accessory pigments occur on the surface of thylakoid in the form of small vesicles known as the phycobilisomes.

a. Pigmentation

The pigments are responsible for the variation in color of thallus. Pigments are the chemical compound that reflects certain wavelengths of visible light, which makes them colorful. Pigments are responsible for the color of flower, corals, and even animal skin. Except reflection pigment also absorbs a certain amount of wavelength.

There are present different types of pigment such as.

Chlorophylls

• Five types of chlorophyll have been identified in algae such as Chi a, b, c, d, and e.
• Chlorophyll a is considered as the universal type of chlorophyll, it found in almost all type of algae.
• Chloro­phyceae contain Chlorophyll b.
• Phaeophyceae Cryptophyceae, Bacillariophyceae and Chryso­phyceae contain Chlorophyll c.
• The red algae contain Chlorophyll d.
• Xanthophyceae contains chlorophyll e.

Caroteinoids

Caroteinoids is made up of Carotenes and xanthophylls. There are five types of Caroteinoids that are identified in algae such as α-carotene in Chlorophyceae, Cryptophyceae and Rhodophyceae; β-carotene in all algal groups, except Cryptophyceae; c-carotene in Chlorophyceae; e- carotene in Bacillariophyceae, Cryptophyceae, Phaeophyceae and Cyanophyceae and flavacene in. members of Cyanophyceae.

Xanthophylls

• There are present different types of xanthophylls such as lutein, violaxanthin and neoxanthin which are found in the members of Chlorophyceae and Phaeophyceae.
• Phaeophyceae and Bacillariophyceae contains Fucoxanthin, which is considered as the main xanthophyll pigment.
• Myxoxanthophyll, myxoxanthin and oscilloxan- thin pigments are found in Cyanophyceae.

Phycobilins

• These are the water- soluble linear tetr’apyrroles. They absorb and transfer the light energy to the reaction center.
• These are biliproteins of either red (phycoerythrin) or blue (phycocyanin) in colour.
• Phycobilins are mainly found in Rhodophyceae and Cyanophyceae.

b. Pyrenoids

• These are the proteinaceous bodies found in chloroplasts or chromatophores.
• They help in  the synthesis and storage of starch.
• In Bacillariophyceae they accumulate lipids.
• Their number varies from species to species for example, Chlamydomonas contains one pyrenoid whereas Oedogonium contains more than one per chromatophore.

Mitochondria

• Except Cyanophyceae all algal cells contain Mitochondria.
• The mitochondria is covered by a double membrane envelope.
• In plant mitochondria the inner membrane encloses an aqueous matrix of solutes, soluble enzymes and the mitochondrial glucose.
• The inner membrane of mitochondria is larger than the outer membrane. The inner membrane produces sac-like cristae of variable shape and number by the process invagination.
• An intermembrane space is located between the inner and outer membrane which is continuous with the intercristal space.
• It has a highly proteinaceous and granu­lar matrix. The organelle contains a circu­lar DNA and ribosomes, which help in the synthesis of proteins. Which means the organelle is  semiautonomous in nature.
• The Micromonas (Chlorophyceae) contain a single mitochondria per cell.
• The mitochon­dria is absent in  cells of blue green.

Endoplasmic Reticulum (ER)

• Electron microscopic studies show that algae contains endoplasmic reticulum which is an extensive membrane network of interconnecting tubules and cisternae (flattened sac).
• The membrane of the Endoplasmic Reticulum traverses the entire cytoplasm.
• The ER is made up of interconnected parallel cisternae associated with the ribosome, attached to the cytoplasmic face of the membrane. These are referred to as the rough endo­plasmic reticulum, where protein is synthesised.
• Those ER mem­branes do not bear ribosomes are known as the smooth endoplasmic reticulum (SER).

Dictyosomes or Golgi Apparatus

• Except blue-green algae all algal cells contain Dictyosomes or Golgi Apparatus.
• Golgi Apparatus is an intermediate between the endoplasmic reticulum and plasma membrane and is a part of the cell’s endomembrane system.
• Golgi bodies can be found in neclue regions for example in Chlamydomonas, or may be near plastids such as in diatom and Bulbochaete.
• It is made up of 2-20 flat vesicles which are arranged in stacks.
• This stack is known as the dictyosome, all these dictyosomes form the Golgi apparatus. It helps in packaging of materials, formation of new plasma membranes.

Eye-Spot or Stigma

• The eye-spot or stigma is a motile vegetative and reproductive cell and contains pigmented spots in the anterior, middle or posterior part of the cell.
• It is located in thylakoids, helping to sense the light intensity and direction.
• Visit our Previous article for more details “Eyespot apparatus Definition, Function, Types, Structure, Proteins”

Vacuoles

Except Cyanophyceae, all algal members contain one or more vacuoles. Each vacuole is surrounded by a distinct membrane known as tonoplast. 

Algal cell contains three types of vacuoles such as;

1. Simple vacuole

• Simple vacuole small in size and exhibits periodic contraction and expan­sion.
• Simple vacuole also known as contractile vacu­oles.
• They help to throw out the metabolic wastes of the cells and also regulate the water con­tent of the cell by discharging the excess amount at short intervals.

2. Complex Vacuole

• Complex Vacuole mainly found in Dinophyceae and Euglenophyceae.
• It is made up of a tube-like cytopharynx, a large reservoir and a group of vacuoles of varying sizes. 
• It helps in osmoregulation inside the cell.
• It also stores reserve food material including laminarin and chrysolaminarin.

3. Gas Vacuoles

• It is found in Cyanophyceae as a gas containing cavities occurring as stacks of small transparent cylinders of uniform diameter. 
• Gas can easily pass through their walls.
• It helps by providing buoyancy to the planktonic forms and also protects from the incident bright light.

Flagella

Motile vegetative or reproductive algal cells perform their locomotion or movement by using thread-like protoplasmic appendages known as the flagella. Mainly two types of flagella have been identified in algae such as;

1. Whiplash or Acronematic is a hairless and smooth surfaced-.flagella

2. Tinsel or Pleuronematic contain one or more rows of lateral fine filamentous hairs called mastigonemes or flimmers.

There are also other types of flagella such as;

• Pantonematic: masti­gonemes are arranged in two opposite rows.
• Pantoacronematic: When Pantonematic flagella contain a terminal fibril is known as pantoacronematic.
• Stichonematic: contain one-sided masti­gonemes.

Structure of Algal Flagella

• These are extremely fine, hyaline emergence of cytoplasm.
• Each flagellum contains a single granule at the base.
• In a firm wall containing algal cells the flagellum emerges through a pore.
• Each flagella contains an axoneme, which is a central or axial thin filament.
• A cytoplasmic membrane or sheath is protecting the axoneme. This sheath is made of an extension of the cell or plasma membrane. 
• The axoneme contains an apical naked portion known as the end-piece.
• The transverse section of flagella shows two central singlet fibrils surroun­ded by nine peripheral doublet fibrils.
• The fibril is surrounded by a membrane whereas the two central ones are further protected with an additional membrane.