Plant Cell

What is a Plant Cell?

Plant cells are eukaryotic cells, or cells with a membrane-bound nucleus.  Both animal cell and plant cells are eukaryotic and having similar organelles.

They are eukaryotic cells having a true nucleus along with specialized structures known as organelles which perform specific functions

Plant cells are generally larger than animal cells while animal cells have various sizes and tend to have irregular shapes, in contrast plant cells are similar in size and are basically rectangular or cube shaped.

Additionally, a plant cell has features that an animal cell does not. A cell wall, a large vacuole, and plastids are a few of these. Chloroplasts and other plastids help plants store and harvest the materials they require. 
Additionally, animal cells have components such lysosomes, centrioles, cilia and flagella that plants do not.

Plant Cell Diagram

Plant cells are usually rectangular and bigger than animal cells. Even though both types of cells have some similarities, plant cells are unique when compared to their cell organelles that perform different functions. When we look at these cells closely under a special microscope, we can see some of the key differences.

Plant Cell Structure

Just like our body has different organs, a plant cell has various structures called organelles that work together to keep the cell alive. 

Plant cell organelles:

Cell Wall

• The cell wall is a tough outer layer that protects the cell. 
• It's made of complex sugars like cellulose, pectin, and hemicellulose, along with some proteins and other polymers
• Rigid, non-living layer present on outer side of cell membrane.
• Provide Shape, protection from mechanical damage & infections
• It is outermost layer, it helps in cell-cell interaction it is quite permeable but prevents the entry of very large macromolecules & let pass very small monomers.
• It is a non-living rigid structure found outer to the plasma membrane of fungi and plants.
• Cell wall of a young plant cell (primary wall) is capable of growth. It gradually diminishes as the cell matures and the secondary wall is formed on the inner side (towards membrane).
• The middle lamella is a layer containing calcium pectate which glues the neighbouring cells together. Cell wall and middle lamellae may be traversed by plasmodesmata. It connects the cytoplasm of neighbouring cells.

Composition of cell wall:

• Matrix contains 60% water, 5 -15% hemicellulose, 2 - 8% Pectic substance,1-2 % proteins.
• Cellulose 10 to 15%
• other ingredients lignin, cutin, suberin, silica, minerals, waxes, tannins ,resins ,gums

Functions:

 

 Cell wall performs number of functions:

• Protects the protoplasm against mechanical injury
• protects the cell from attack of pathogens
• provides rigidity and shape to the cell.
• Cell wall helps in sell expansion.
• Cutin and suberin of the cell wall reduce the loss of water.

Cell membrane

• The cell membrane, also known as the plasma membrane, is a thin, semi-permeable layer that surrounds every cell. 
• It is an important component of the cell, serving as a protective shield that regulates the movement of substances in and out of the cell.
• Chemical studies on human RBCs show that cell membrane is composed of a lipid bilayer, protein & carbohydrate.
• Term was originally used by Nageli and Cramer in 1855.

Composition of the Cell Membrane

The cell membrane is composed of :

1. Lipids: These are fatty molecules that provide the structural framework of the membrane. The most common types of lipids found in the cell membrane are phospholipids, cholesterol, and glycolipids.

2. Proteins: These are complex molecules that perform a wide range of functions, including transporting substances across the membrane, recognizing and responding to signals, and maintaining the structure of the membrane.

3. Carbohydrates: These are simple sugars that are attached to lipids or proteins, forming glycolipids or glycoproteins. They play a crucial role in cell-cell recognition and signaling.

4. Water: This is the most abundant molecule in the cell membrane, making up about 20% of its composition.

Functions of the Cell Membrane

The cell membrane performs a wide range of functions, including:

• Cell membrane separates the cell from the external environment they allow the cell to maintain their identity internal environment and functionality

• cell membrane provides organic connection between adjacent cells.

• Membrane has selective permeability that allow only selected molecules to pass inwardly.

• Cell membrane contains enzyme for performing certain reactions.

• Cell membrane regulates movement of ions nutrients waste and signalling molecules.

• Cell membrane receive and transmit signals through receptors and signalling pathways

• it also helping cell to cell interaction it mediates attention recognition and communication between cells.

• Cell membrane has receptors for certain hormones, it has carrier protein for active transport.

• It provides mechanical support and maintain cell structure.

The cell membrane has several special features that enable it to perform its functions, including:

1. Microvilli: These are small, finger-like projections that extend from the surface of the cell membrane, increasing its surface area and allowing it to absorb more nutrients.

2. Receptors: These are specialized proteins that are embedded in the cell membrane, allowing cells to recognize and respond to specific signals.

3. Carrier Proteins: These are proteins that are embedded in the cell membrane, helping to transport substances across the membrane.

4. Enzymes: These are proteins that are embedded in the cell membrane, catalyzing specific chemical reactions that are necessary for cellular function.

Nucleus

The nucleus is a membrane-bound organelle found in eukaryotic cells, including plant cells. It's often referred to as the "control center" of the cell because it contains most of the cell's genetic material in the form of DNA.

Structure of the Nucleus

The nucleus is surrounded by a double membrane called the nuclear envelope. The nuclear envelope has pores that allow molecules to pass through and communicate with the rest of the cell.

It composed of:

1. Chromatin: This is the complex of DNA and proteins that make up the chromosomes. Chromatin is found in the nucleus and is organized into visible chromosomes during cell division.

2. Nucleolus: This is a region within the nucleus where ribosome synthesis occurs. Ribosomes are essential for protein synthesis in the cell.

3. Nuclear matrix: This is a network of proteins and other molecules that provide structural support to the nucleus and help organize the chromatin.

Functions of the Nucleus

The nucleus performs several functions in plant cells:

1. Genetic information storage: The nucleus contains most of the cell's genetic material in the form of DNA.

2. Gene expression: The nucleus regulates gene expression by controlling the transcription of DNA into RNA.

3. Cell division: The nucleus plays a critical role in cell division, including mitosis and meiosis.

4. Cell growth and differentiation: The nucleus helps regulate cell growth and differentiation by controlling the expression of specific genes.

Plastids

• One of the main distinguishing characteristics of most plant cells is the presence of plastids.

• The credit for the discovery of these organelles is generally given to Schimper (1880).

• The term 'plastid' was coined by E. Haeckel (1866).

• Plastids generally contain pigments and may synthesise and accumulate various substances.

• However, some plastids are devoid of pigments, and they serve primarily for the storage of various substances.

The plastids are of three types:

1. Chloroplasts: Contain chlorophyll and carotenoid pigments. They trap light energy for photosynthesis.

2. Chromoplasts: Contain fat soluble carotenoid pigments like carotene, xanthophylls etc. This gives a yellow, orange or red colour.

3. Leucoplasts: These are colourless plastids of varied shapes and sizes with stored nutrients. They include:

 Amyloplasts: Store starch. E.g. potato.

 Elaioplasts: Store oils and fats.

 Aleuroplasts: Store proteins.

Chloroplasts:

• They are greenish plastids which possess photosynthetic pigments chlorophyll and carotenoid.

• Chloroplast is double membrane bound cell organelles which is in the mesophyll cells of leaf.

• These are lens-shaped, oval, spherical, discoid or ribbon-like organelles.

• Their number varies from 1 to 20-40 per cell in the mesophyll.

• A chloroplast has 3 parts - envelope matrix and thylakoids

• Envelope is made up of 2 smooth membranes and it is the outer covering of chloroplast.

• Matrix is the ground substance of chloroplast it is also known as stroma it contains 50% soluble protein and remaining DNA, RNA, ribosomes and enzymes.

• Thylakoids are membrane lined flattened sacs which run throughout the matrix of the chloroplast it takes part in photosynthesis, Chlorophyll pigments are present in the thylakoids. Thylakoids are arranged in stacks called grana.

• The ribosomes of the chloroplasts are smaller (70S) than the cytoplasmic ribosomes (80S).

Vacuole

• They are the membrane-bound space found in the cytoplasm. 
• It contains water, sap, excretory product and other materials not useful for the cell.
• Vacuoles are liquid-filled cavities, can occupy up to 90% of the volume of the cell. The presence of which is characteristic features of plant cells.
• In young dividing cells, vacuoles are small and numerous and remain scattered throughout the cytoplasm while in mature cells there is a single large central vacuole in each cell, and it is surrounded by a thin membrane called tonoplast.
• It is filled with a fluid called cell sap which is the solution of sugars, salts, organic acids, glucosides and alkaloids.
• Many vacuoles contain a pigment called anthocyanin which gives red colour in very acidic sap and blue to purple colour in alkaline medium.
• Due to the change of the colour of anthocyanin with the change in pH, anthocyanin has been called 'vegetable chameleon'.
• Functions of vacuoles-
• it maintains turgidity of the cell which is very useful for herbaceous plants.
• Vacuoles stores water, salts, minerals, sugars and other nutrients.
• They contain waste products such as toxic compounds and help remove them from the cell.

Golgi Apparatus

• In plant cells they are found as smaller vesicles termed as dictyosomes. 
• Golgi apparatus is a stack of flat membrane enclosed sacs. 
• It consist of cisternae, tubules, vesicles and golgi vacuoles.
• In plants, the cisternae are 10-20 in number placed in piles separated from each other by a thin layer of inter cisternal cytoplasm often flat or curved. 
• Golgi apparatus compartmentalises a series of steps leading to the production of functional protein.

Ribosomes

• Ribosomes were first observed by George Palade (1953) as dense particles or granules in the electron microscope. 
• Ribosomes are composed of two rounded sub units, united together to form a complete unit.
• Each ribosome is made up of one small and one large sub-unit
• Ribosomes are the sites of protein synthesis in the cell. 
• Ribosome is not a membrane bound organelle
• Ribosome consists of RNA and protein: RNA 60 % and protein 40%.
• During protein synthesis, many ribosomes are attached to

the single mRNA and is called polysomes or polyribosomes.

Mitochondria

• It  was first observed by Kolliker (1880) in the muscle cells.

• Structurally, a mitochondrion consists of two membranes, an outer membrane and an inner membrane,

• The inner membrane is most frequently convoluted to form a large number of infoldings called cristae or mitochondrial crests.

• The membranes of the mitochondrion separate it into two distinct compartments.

• An outer compartment sometimes called intermembranal space is formed between the outer and inner membrane.

• This mitochondrial chamber is filled with dense proteinaceous material called the matrix,

• The mitochondrial matrix contains most of the enzymes of Kreb's cycle.

• Matrix possesses a circular DNA, a few RNA molecules, ribosomes (70S) and components for protein synthesis.

Function: 

• Mitochondria are the sites of aerobic respiration.

• They produce energy in the form of ATP. So they are called ‘power houses’ of the cell.

• Mitochondria plays an important role in regulating cell growth and division by controlling the availability of ATP molecule.

Lysosome

• Lysosomes were discovered by Christian de Duve (1953), They are known as suicidal bags. 

• These are membrane bound vesicular structures formed when small pieces of golgi body are pinched off from its tubules by the process of packaging in it.

• It is found in eukaryotic cells.

• Lysosomal vesicles contain almost all types of hydrolytic enzymes (hydrolases– lipases, proteases, carbohydrases). 

• They are active at acidic pH ( 4-5).They digest carbohydrates, proteins, lipids and nucleic acids.

• On the basis of their morphology and function , there are 4 types of lysosomes: Primary , Secondary , Residual bodies and Autophagic vacuoles.

In plants, the role of lysosomes is undertaken by the vacuoles.

Plant Cell Functions

Plant cells are the basic structural and functional units of plants. They are eukaryotic cells, meaning they have a true nucleus and other membrane-bound organelles. Plant cells are specialized to perform various functions, including photosynthesis, growth, and development.

Photosynthesis: Photosynthesis is the process by which plant cells convert light energy from the sun into chemical energy in the form of glucose. This process occurs in specialized organelles called chloroplasts, which are present in plant cells.

Chloroplasts: Chloroplasts are organelles found in plant cells that are responsible for photosynthesis. They contain pigments such as chlorophyll, which absorbs light energy and transfers it to other molecules. Chloroplasts also contain enzymes and other molecules necessary for photosynthesis.

Transport of Water and Nutrients: Some plant cells, such as xylem and phloem cells, are specialized to transport water and nutrients from the roots and leaves to different parts of the plant. Xylem cells transport water and minerals from the roots to the leaves, while phloem cells transport sugars and other organic compounds produced by photosynthesis from the leaves to the rest of the plant.

In addition to photosynthesis and transport, plant cells perform various other functions, including:

- Growth and development: Plant cells grow and divide to form new tissues and organs.

- Storage: Plant cells can store nutrients and other substances, such as starch and lipids.

- Defense: Plant cells can produce defense compounds to protect against pathogens and herbivores.