Plant Cell Diagram — Class 9 Organelles and Their Functions

A plant cell has all the basic eukaryotic organelles plus structures unique to plants:

Structures UNIQUE to plant cells (absent in animal cells): • Cell wall — rigid outer covering made of cellulose • Chloroplasts — carry out photosynthesis • Large central vacuole — stores water and maintains turgidity • Plastids (leucoplasts, chromoplasts) • Glyoxysomes

Structures ABSENT in plant cells (present in animal cells): • Centrioles/centrosome (absent in higher plants) • Lysosomes (generally absent or rare)

Structures COMMON to both plant and animal cells: • Cell membrane (plasma membrane) • Nucleus and nucleolus • Mitochondria • Endoplasmic reticulum (rough and smooth) • Golgi apparatus (Golgi body) • Ribosomes • Cytoplasm • Peroxisomes

Cell Wall: • Location: Outermost covering of the plant cell • Composition: Primarily cellulose (a polysaccharide made of glucose units); also contains hemicellulose and pectin • Function: (1) Provides rigidity and structural support. (2) Maintains the shape of the cell. (3) Allows the cell to withstand high turgor pressure without bursting. (4) Protects the cell from mechanical damage and pathogens. (5) Allows exchange of substances (it is permeable). • The cell wall has pores called plasmodesmata — channels that connect adjacent plant cells and allow communication and transport.

Cell Membrane (Plasma Membrane): • Location: Just inside the cell wall • Composition: Phospholipid bilayer with embedded proteins (fluid mosaic model) • Function: Selective permeability — controls what enters and exits the cell. Acts as a barrier between the cell interior and the outside environment.

Nucleus: • The control centre of the cell • Contains DNA (genetic information) wound around histone proteins to form chromosomes • Surrounded by a double nuclear membrane with pores (nuclear pores) • Contains the nucleolus — where rRNA is synthesised and ribosomes are assembled • Function: Directs all cell activities; contains genes that encode proteins; controls cell division • In plant cells, the large vacuole often pushes the nucleus to the periphery (side) of the cell

Chloroplasts: • Found only in plant cells and algae • Contain the green pigment chlorophyll • Double-membrane organelle; inner membrane folded into thylakoids (stacked into grana) • Fluid inside = stroma (where dark reactions/Calvin cycle occur) • Function: Carry out photosynthesis — convert sunlight + CO₂ + water → glucose + oxygen • Chloroplasts have their own DNA and ribosomes (semi-autonomous organelles)

Plastids (types): • Chloroplasts — green; photosynthesis • Chromoplasts — yellow, orange, red; store pigments (carotenoids); give colour to fruits and flowers • Leucoplasts — colourless; store food – Amyloplasts: store starch (in potatoes, seeds) – Elaioplasts: store oils/fats – Proteinoplasts: store proteins

Central Vacuole: • Most distinctive feature of mature plant cells • Can occupy up to 90% of the cell volume • Surrounded by a membrane called the tonoplast • Filled with cell sap (water, dissolved salts, sugars, pigments, waste products) • Functions: (1) Maintains turgidity (turgor pressure) — keeps plant cells firm and the plant upright. (2) Stores nutrients, water, and waste products. (3) Pigments in vacuoles give colour to flowers (anthocyanins). (4) Pushes the cytoplasm and nucleus to the edge of the cell.

Mitochondria: • Double-membrane organelle; inner membrane folded into cristae • Site of aerobic cellular respiration • Function: Break down glucose to produce ATP (energy currency of the cell) — glucose + O₂ → CO₂ + H₂O + ATP • Called the 'powerhouse of the cell' • Have their own DNA and ribosomes (like chloroplasts, semi-autonomous)

Endoplasmic Reticulum (ER): • Network of membrane-bound tubes and sacs connected to the nuclear membrane • Rough ER (RER): has ribosomes on its surface; synthesises and processes proteins • Smooth ER (SER): no ribosomes; synthesises lipids and detoxifies substances • Function: Transport of materials within the cell; protein and lipid synthesis

Golgi Apparatus (Golgi Body / Golgi Complex): • Stack of flattened membrane sacs (cisternae) • Function: Receives proteins from the RER, modifies them (adds sugars, etc.), packages them, and dispatches them to their destinations (cell membrane, outside cell, or lysosomes) • Called the 'post office' or 'packaging and dispatching centre' of the cell • In plant cells, Golgi bodies also produce polysaccharides for the cell wall

Ribosomes: • Tiny organelles made of rRNA and proteins; NOT membrane-bound • Found on the surface of RER or free in the cytoplasm • Function: Site of protein synthesis (translation of mRNA into proteins) • Plant cells also have smaller ribosomes inside chloroplasts and mitochondria (70S type)

Peroxisomes: • Small membrane-bound vesicles • Contain enzymes like catalase • Break down hydrogen peroxide (H₂O₂) — a toxic byproduct of metabolism — into water and oxygen • In plant cells, also involved in photorespiration

Cytoplasm: • Jelly-like fluid (cytosol) that fills the cell • Medium for all chemical reactions • Contains all organelles (except the nucleus) • Contains the cytoskeleton — network of protein fibres that gives the cell shape and assists in movement of organelles