- In plants (including bacteria) a cell is always surrounded by a cell wall lined throughout with plasma lemma.
- The cell wall is found in plants and is absent in animals.
- In case of animal cells, the outermost layer of cell is plasma lemma, which is also occasionally called ‘cell membrane’ or ‘plasma membrane’.
- Cell wall is the outermost part of the cell and is always non-living, though produced and maintained by living protoplasm.
- It is a rigid structure and protects the inner parts of a cell.
- It maintains the shape of the cell and provides mechanical support to the tissues.
- It originates from the phragmoplast (phragma = fence, separation).
- Endoplasmic reticulum, golgi complex, mitochondria and microtubules play an important role in the formation of the cell wall.
- It is mainly composed of cellulose. However it may also contain hemicellulose, pectin, chitin, cutin and lignins.
- The composition of these substances varies from cell to cell.
- The cell wall is complex in nature and is differentiated into middle lamella, primary cell wall and secondary cell wall.
1. Middle lamella:
- It is the outmost layer of plant cell wall and connects the two adjacent cells.
- It is composed of calcium and magnesium pectate and does not contain any cellulose.
- Some consider middle lamella as intercellular substance or intercellular matrix.
2. Primary cell wall:
- It is thin, elastic and lies between middle lamella and secondary cell wall.
- It is mainly composed of cellulose.
- It develops after middle lamella by deposition of hemicellulose, cellulose and pectin substances.
3. Secondary cell wall:
- It is the inner most layer of cell wall and lies between primary cell wall and plasma membrane.
- It is relatively thick and is primarily composed of microfibrils of cellulose.
- In some tissues, besides cellulose, lignin and suberin are also found in the secondary cell wall.
- The cell wall has minute apertures through which the cells of a tissue are interconnected.
- These apertures of cell wall are known as plasmadesmata.
- They are also referred to as canals of the cell wall.
Functions of cell wall
- 1. It determines the shape and size of a cell
- 2. It provides protection to the inner parts of a cell from the attack by pathogens.
- 3. It provides mechanical support to the tissues and act as a skeletal framework of plants.
- 4. It helps in transport of substances between two cells.
- Every kind of animal cell is bounded by a living, extremely thin and delicate membrane called plasmalemma, cell membrane or plasma membrane.
- In plant cells, plasma membrane occurs just inner to cell wall, bounding the cytoplasm.
- The plasma membrane exhibits a tri-laminar (i.e., three-layered) structure with a translucent layer sandwiched between two dark layers.
- At molecular level, it consists of a continuous bilayer of lipid molecule (i.e., phospholipids and cholesterol) with protein molecules embedded in it or adherent to its both surfaces.
- Some carbohydrate molecules may also be attached to the external surface of the plasma membrane, they remain attached either to protein molecules to form glycoproteins or to lipids to form glycolipids.
- The term was coined by J.Q. Plower in 1931.
- This membrane is present just beneath the cell wall in plant cells, while it is the outer membrane in animal cell.
- In plants, it lies between the cytoplasm and the cell wall.
- It is a living, ultra thin, elastic, porous, semi-permeable membrane covering of cell.
- The plasma membrane is about 75-100 angstroms thick.
- In most of the cells, it is trilaminar (three layered) and made up of protein and lipids.
- The outer protein layer is 25 angstroms thick, the middle lipid layer is 25 to 30 angstroms thick and the inner protein layer is 25 to 30 angstroms thick.
- The three –layered protein-lipid-protein membrane is called a unit membrane.
- The outer and inner layers are made up of proteins and the middle layer is made up of lipids.
- Structure can be best explained by fluid mosaic theory.
- It is found to contain many pores through which exchange of molecules may occur.
Functions of the plasma membrane
- The plasma membrane is a selectively permeable membrane; its main function is to control selectively the entrance and exit of materials.
- This allows the cell to maintain a constant internal environment (homeostasis).
- Transport of small molecules such as water, oxygen, carbon dioxide, ethanol, ions, glucose, etc., across the plasma membrane takes place by various means such as osmosis, diffusion and active transport.
- The process of active transport is performed by special type of protein molecules of plasma membrane called transport proteins or pumps, consuming energy in the form of ATP molecules.
- For bulk transport of large-sized molecules, plasma membrane performs endocytosis (i.e., endocytosis, pinocytosis, receptor-mediated endocytosis and phagocytosis) and exocytosis both of these processes also utilise energy in the form of ATP molecules.
- Various cell organelles such as chloroplasts, mitochondria, endoplasmic reticulum and lysosomes are also bounded by membranes similar to the plasma membrane.
- The plasma membrane is followed by cytoplasm which is distinguished into
–(a) Cytoplasmic matrix / hyaloplasm and
–(b) Cytoplasmic structures
a) Cytoplasmic matrix:
- The space between the plasma membrane and the nucleus is filled by amorphous, translucent, homogeneous colloidal liquid known as hyaloplasm or cytoplasmic matrix.
- The portion of cytoplasm other than cell organelles is known as hyaloplasm. When the cell is active, the cytoplasm is in fluid state.
- The cytoplasm is in gel condition, when the cell is dormant.
- The cytoplasmic matrix consists of various inorganic molecules such as water, salts of sodium and other metals and various organic compounds viz., carbohybrates, lipids, nucleoproteins, nucleic acids (RNA and DNA) and variety of enzymes.
- The peripheral layer of cytoplasmic matrix is relatively nongranular, viscous, clear and rigid and is known as ectoplasm.
- The inner portion of cytoplasmic matrix is granular, less viscous and is known as endoplasm.
b) Cytoplasmic structures:
- In the cytoplasmic matrix certain non-living and living structures remain suspended.
- The living structures or cytoplasmic organoids are membrane bound and are called organelles or organoids.
- These living structures include plastids, mitochondria, endoplasmic reticulum, golgi complex, lysosomes, ribosomes, microtubules, microfilaments, centrosome, basal granules, sphaerosomes, microbodies, cilia and flagella etc.
- The non living structures or cytoplasmic inclusions called paraplasm or deutoplasm include ergastic substances, crystals, fats, oil droplets, starch granules glycogen granules, vacuole etc.