Cell Structure and Function

Cell Wall

Location: Outside plasma membrane, mainly in plant cells.

Shape , Size and Structure: Variable

Structure: A rigid cell wall surrounding the cell, consisting of cellulose microfibrils running through a matrix of the other complex polysaccharide.

Function: Support, protection and rigidity

Plasma membrane (Cell Membrane)

Location: Outer surface of all cells.

Shape, Size and Structure: Lipo protein unit membrane 8-10 nm thick.

Structure: Phospholipid bilayer (Fluid mosaic Model)

Function: Limiting membrane controls flow of materials into and out of the cell.

Endoplasm Reticulum (ER)

Location :Throughout cytoplasm.

Shape , Size and Structure:Lipoprotein membrane
a) Rough ER with ribosomes
b) Smooth ER with out ribosomes

Structure: A system of flattened membrane bound sac called cisternae, forming tubes and sheets.

Function: Cytoskeleton, formation of vacuoles and vesicles. Acts as conducting and compartmenting system.
Rough ER associated with protein synthesis, Smooth ER involved in steroid synthesis.

Golgi apparatus

Location :  In cytoplasm

Shape , Size and Structure: Stack of membraneous sacs, variable size and shape.

Structure: A stack of flattened membrane bounded sacs called cisternae. Continuously formed at one end of the stack and budded off as vesicles at the other.

Function: Manufacture of complex polysaccharides, concentration and packaging of these proteins, secretion, wall synthesis, lysosome formation.

Mitochondria

Location : Few to 1000 in cytoplasm, more in active cells.

Shape , Size and Structure: Sausage shaped approximately 500x 200 nm.

Structure: Surrounded by an envelope of two membranes. The inner being folded to form cristae contains matrix with a few ribosomes, a circular DNA molecule and phosphate granules.

Function: Power house of cell, breaking down carbohydrates from carbon dioxide and water.

Chloroplasts

Location : In cytoplasm of green plant cells.

Shape , Size and Structure:Disc shaped about 5,000 x 10,000 nm in diameter.

Structure: Large plastid containing chlorophyll and carrying out photosynthesis. It is surrounded by an envelope of two membranes and contains gel like stroma through which runs a system of membranes that are stacked in place to form grana. It may store starch. The stroma also contains ribosomes as and circular DNA molecules and lipid droplets.

Function: Site of photosynthesis, light energy trapped by chlorophyll to make carbohydrates from carbon dioxide and water.

Ribosomes

Location : Free in cytoplasm or associated with rough ER.

Shape , Size and Structure: Spherical, about 20-25 nm diameter.

Structure: Very small organelles consisting of a large and small subunit. They are made up of roughly equal parts of protein and RNA. Slightly smaller ribosomes are found in mitochondria and chloroplasts in plants.

Function: Site of protein synthesis. They are either bound to the ER or lie free in the cytoplasm. They may form polysomes (Polyribosomes) collection of ribosomes along with messenger RNA.

Lysosomes

Location :In cytoplasm of many cells.

Shape , Size and Structure: Spherical about 500 nm diameter.

Structure: A simple spherical sac bounded by a single membrane and containing digestive enzymes. No internal structural visible.

Function: Cell disposal units containing digestive enzymes, autophagy.

Vacuoles

Location : Prominent in cytoplasm of plant cells

Shape , Size and Structure: Variable

Structure: A sack bounded by a single membrane called the tonoplast. It contains cell sap, a concentrated solution of various substances such as mineral salts, sugars, and pigments, organic acids and enzymes.

Function:

Maintain turgidity.

Storage of various substances including waste products.

Nucleus

Shape , Size and Structure: Largest cell organelle enclosed by an envelope of two membranes that is perforated by nuclear pores. It contains chromatin which is the extended form taken by chromosomes during interphase. It also contain nucleolus.

Function: Chromosomes contain DNA the molecule of inheritance. DNA is organised into genes which control all the activities of the cell. Nuclear division is the basis of cell division and hence reproduction the nucleolus manufacture ribosomes.

Nuclear membrane

Location : Surrounds nucleus 

Shape , Size and Structure: Two unit membranes with pores.

Function: Governs traffic of materials into and out of the nucleus and separates cytoplasmic components from nuclear components.

Nucleolus

Location : One to four in each nucleus

Shape , Size and Structure: Spherical and dense, not enclosed by membrane.

Function: Site of synthesis of ribosomal RNA.

Chromosomal material (Chromatin)

Location : In nucleus

Shape , Size and Structure: Dispersed as fine DNA/ protein strands in active cells coiling to form chromosomes during division.

Function: Chromosomes govern cell activities by their hereditary instructions for protein synthesis which are coded into DNA molecule.

Cytoskeleton : Microfilaments and microtubules

Location : In cytoplasm

Shape , Size and Structure: Rods and tubes 5-20 nm diameter.

Function: It maintains the shape of the cell. It contains protein filaments

and regulates distribution and orientation of organelles.

Centrioles

Location : Members of centriole pair lie at right angles to each other near nuclear membrane in cytoplasm of animal cells.

Shape , Size and Structure: Complex, rod-like structures 200 nm diameter x 400 nm long.

Function: Each member of the pair moves to form a pole of the nuclear spindle during cell division.

Granules

Location : In cytoplasm

Shape , Size and Structure:Variable

Function: Variable, mainly storage and excretory.

Microbodies

Shape, Size and Structure: A roughly spherical organelle bounded by a single membrane. It contents appear finely granular except for occasional filamentous deposits.

Function: All contains catalase an enzyme that breaks down hydrogen peroxidase. All are associated with oxidation reaction. In plants are the site of glyoxalate cycle.