Book : Botany
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Posted by: CHELSEA
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Writer CHELSEA

Plant Cell

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Cell Wall

A fundamental difference between plant and animal cells is that the plant cell is surrounded by a rigid cell wall, mostly made of polysaccharides (cellulose, hemicellulose, pectin) and lignin. Plants have two types of cell walls, primary and secondary. Primary cell walls are thin and characteristic of young, growing cells. Secondary cell walls are thicker and stronger, and they are deposited when most cell enlargement has ended. Secondary cell walls have their strength and toughness due to lignin; a glue like material. The lignified secondary walls provide the plants the structural reinforcement necessary to grow vertically above the soil.

Plasma Membrane

All cells are enclosed in a membrane that serves as their outer boundary, separating the cytoplasm from the external environment. This plasma membrane allows the cells to take up and retain certain substances while excluding others. Thus, plasmalemma accounts for selective traffic of solutes across membrane. All biological membranes consist of a double layer (bilayer) of phospholipids in which proteins are embedded. The membrane is not a static structure, but it is a dynamic structure. Both lipid and protein molecules are free to move and are usually in a constant motion. However, these molecules readily move in the plane of membrane, a process known as lateral diffusion. Phospholipids are a class of lipids in which 2 fatty acids are linked to glycerol, which is linked to a phosphate group.

Mitochondria

Mitochondria are cytoplasmic organelles. Mitochondria are the sites of oxidative phosphorylation. Mitochondria are surrounded by two membranes. The outer membrane is smooth and the inner membrane is highly convoluted. The folds of inner membrane are called ‘cristae’. The components of respiratory electron transport chain are found in inner membrane. The inner membrane is also characterized by the presence of stalked particles with spherical heads containing ATPase. ATPase catalyses the synthesis of ATP. The inner membrane is highly impermeable to the passage of protons (H+), which allows the formation of electrochemical gradient necessary for ATP synthesis. The compartment enclosed by inner membrane is called ‘matrix’. Mitochondrial ribosomes are smaller (70 S) than those found in cytosol (80 S). Mitochondria contain circular, histone-free DNA molecule, similar to those of bacteria. Mitochondrial genome of plants consists of about 200, which is much larger than animal mitochondria.

Plastids

Plastids are the organelles which are peculiar to plant cells.Plastids that contain high concentration of carotenoid pigments are called ‘chromoplasts’. They give yellow, orange and red colors to many fruits (tomato), roots (carrot) and flower petals. Nonpigmented plastids are called ‘leucoplasts’. An important type of leucoplast is ‘amyoplast’ which is a starch-storing plastid.Chloroplasts are the plastids that contain green pigment, chlorophyll. They are found in green tissues of plant, especially leaf. They are absent in roots. The chloroplast is surrounded by the inner and outer membranes. Chloroplasts also contain third system of membrane called thylakoid. All the chlorophyll is contained within this membrane, which is the site of light reactions of photosynthesis. Thylakoid membranes are highly folded and appear like stacked coins.These stacked membranes are known as grana lamellae (or grana thylakoid). The membranes without stacking are known as stroma lamellae (or stroma thylakoid). Each stack is called granum.The inner space within a thylakoid is known as lumen. The region of the chloroplast that is inside the inner membrane and surrounds thylakoids is known as stroma.

Endoplasmic Reticulum

Cells have an elaborate network of internal membranes called endoplasmic reticulum (ER). ER is continuous with the outer membrane of nuclear envelope (but not plasmalemma). The ER lumen of one cell is connected to adjacent cell via plasmodesmata. There are 2 types of ER, smooth and rough, which are interconnected. Rough ER is covered with ribosomes which synthesize proteins to be delivered to lumen of ER. Smooth ER lacks ribosomes.Smooth ER is the site of lipid synthesis and membrane assembly.Both types of ER are involved in secretion.

Golgi Bodies

Golgi apparatus (or Golgi complex) is made of one or more dictysomes (or Golgi bodies) which are stacks of 3- 10 flattened sacs (cisternae) and vesicles. Plant cells contain up to several hundred Golgi bodies dispersed in cytoplasm.The cisternae close to plasmalemma are called transface, and the cisternae close to center of cell are called cisface. The medial cisternae are between transand ciscisternae. Golgi body is a dynamic structure, new cisternae are continuously produced from endoplasmic reticulum at cisface while old cisternae are lost in the form of vesicle at transface. Golgi apparatus has intermediary position between ER and extracellular space.It is involved in the transport and processing of many substances that are produced in ERand eventually discharged outside the cell via Golgi. It plays a key role in synthesis and secretion of complex polysaccharides and in processing of glycoproteins. Various proteins (including enzymes) are first synthesized in rough ER then they reach to Golgi via vesicles that bud off from ER and fuse with Golgi.

Nucleus

The nucleus is surrounded by a double membrane called the nuclear envelope. The space between these two membranes is called the perinuclear space. The joining sites of the two nuclear membranes are called the nuclear pores.The material filled in the nucleus is called nucleoplasm (or nuclear sap). About 8% of the surface area of the nuclear membrane is occupied by pores. These poresallow the transport of substances between cytosol and nucleus. Nucleus is the site of storage and replication of chromosomes, composed of DNA and its associated proteins (histones). The DNA-protein complex is known as chromatin. Nucleus contains a densely granular region called the nucleolus, which is the site of ribosome (ribosomal RNA) synthesis. Ribosomal proteins are synthesized in cytosol and transported into nucleus via nuclear pores, where they bind with rRNA to form 40S and 60S subunits. These subunits pass into cytosol and aggregate to form 80S ribosomes.The genes are transcribed in nucleus to form mRNA, tRNA and rRNA. mRNA and tRNA pass from nucleus to cytosol where they are used for protein synthesis. The nucleotide sequence of mRNA is translated into amino acid sequence of proteins by ribosomes. tRNA assists by transferring amino acids to mRNA codons.


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