Cell Wall and Cell Membrane

A cell wall is an external layer covering certain cells that is external to the cell membrane. All cells contain cell membranes, but normally only plants, fungi, most bacteria, algae, and archaea have cells with cell walls. The cell wall provides strength and structural support to the cell and can regulate to some amount, what kind and concentration of molecules or particles enter and leave the cell. The resources that make up the cell wall vary depending on the type of organism. The cell wall has changed many different times among different sets of organisms.

Cell Wall Functions

The cell wall has some different functions. It is elastic but delivers strength to the cell, which adds protection to the cell against bodily damage. The cell wall can also deliver protection from pathogens such as bacteria that try to raid the cell. The structure of the cell wall permits many minor molecules to go through it, but not bigger molecules that could harm the cell.

Cell Wall Structure

A cell wall is a specialized form of the extracellular matrix that surrounds every cell of a plant. The cell wall is mainly responsible for many of the characteristics that distinguish plant cells from animal cells. Though it is an inactive product serving mainly mechanical and structural purposes. A cell wall is also defined as the non-living component, covering the outermost layer of a cell. Cell wall composition varies according to the organism and is permeable and also it separates the interior contents of the cell from the exterior environment. The cell wall also provides shape, support, and protection to the cell and its organelles. The cell wall’s cellular component is present exclusively in eukaryotic plants, fungi, and a few prokaryotic organisms

Generally, the cell wall is the outer covering of a cell, present adjacent to the cell membrane, which is also called the plasma membrane. The cell wall is normally present in all plant cells, fungi, bacteria, algae, and some archaea. Animal cells are not regular in their shape and this is mainly due to the lack of cell walls in their cells. Cell wall composition usually varies along with organisms.

Plant Cell Walls

The key constituent of the plant cell wall is cellulose, a carbohydrate that produces long fibers and gives the cell wall its stiffness. Cellulose fibers group together to produce bundles, which are known as microfibrils. Other vital carbohydrates contain hemicellulose, pectin, and lignin. These carbohydrates form a network between structural proteins to create the cell wall. Plant cells that are in the progression of growing have primary cell walls, which are tiny. Once the cells are fully grown-up, they grow secondary cell walls. The secondary cell wall is a dense layer that is formed on the interior of the primary cell wall. There is also an additional layer among plant cells named the middle lamella; it is pectin-rich and benefits plant cells to stick together.

The cell walls of plant cells support them to maintain turgor pressure, which is the pressure of the cell membrane persuasive against the cell wall. Plants cells must have lots of water inside them, leading to great turgidity. Whereas a cell lacking a cell wall, such as an animal cell, can swell and burst with too much water to spread into it, plants must be in hypotonic solutions to uphold turgor pressure and their basic shape. The cell wall is capable of holding water so that the cell does not burst. When turgor pressure is gone, the plant will begin to wither. Turgor pressure is what gives plant cells their typical square shape; the cells are filled with water, so they fill up the space accessible and press against each other.

The composition of cell walls: In the plant cell wall, the main carbohydrates are hemicellulose, cellulose, and pectin. The cellulose microfibrils are connected via hemicellulose chains to form the cellulose-hemicellulose network, which is surrounded by the pectin matrix. The most common hemicellulose in the primary cell wall is called xyloglucan. In grass cell walls, xyloglucan and pectin are reduced in great quantity and partially substituted by glucuronoarabinoxylan, one more type of hemicellulose. 10Cell walls usually grow by a procedure known as acid growth, mediated by expansions, extracellular proteins triggered by acidic conditions that adjust the hydrogen bonds between pectin and cellulose. The purposes are to increase cell wall extensibility. The external part of the primary cell wall of the plant’s epidermis is generally impregnated with cutin and wax, developing a permeability wall known as the plant cuticle.

Secondary cell walls have a wide variety of additional compounds that change their mechanical properties and permeability. The main polymers that makeup wood (mainly secondary cell walls) include

• cellulose, 35-50%

• xylan, 20-35%, a type of hemicelluloses

• lignin, 10-25%, 

A complex phenolic polymer that enters the spaces in the cell wall in between hemicellulose, cellulose, and pectin components, pouring out water and firming up the wall.

Most of the plant cell walls are categorized as arabinogalactan proteins (AGP), hydroxyproline-rich glycoproteins (HRGP), glycine-rich proteins (GRPs), and proline-rich proteins (PRPs). Every class of glycoprotein is defined by a typical, highly monotonous protein sequence. Most are glycosylated, comprise hydroxyproline (Hyp), and develop cross-linked in the cell wall. These proteins are frequently concentrated in specific cells and cell corners. Cell walls of the skin may contain cutin. The Casparian band in the endodermis roots and cork cells of plant bark have suberin. Both cutin and suberin are polyesters that act as permeability barricades to the movement of water. The relative arrangement of carbohydrates, secondary compounds, and proteins differs among plants and between the cell type and time. Plant cell walls also have several enzymes, like peroxidases, hydrolases, esterases, and transglycosylases that cut, shapely, and cross-link wall polymers.

Secondary Walls - particularly in grasses - can also contain microscopic silica crystals, which can strengthen the wall and guard it against herbivores.

Cell walls in certain plant tissues also act as storage deposits for carbohydrates that can be ruined and resorbed to source the metabolic and growth required for the plant. 

Structure and Composition of the Cell Membrane

The cell membrane is a tremendously flexible structure made primarily of back-to-back phospholipids (a “bilayer”). Cholesterol is also present in a cell membrane, which donates to the fluidity of the membrane, and there are many proteins surrounded within the membrane that have a wide range of functions.

A single phospholipid molecule or particle has a phosphate group on one end, known as the “head,” and two side-by-side chains or series of fatty acids that make up the lipid tails. The phosphate group is negatively charged, creating the head polar and hydrophilic (water-loving) molecule. A hydrophilic molecule is one that has an affinity toward the water. The phosphate heads are therefore attracted to the water molecules of both the extracellular and intracellular surroundings. The lipid ends or tails, on the other hand, are uncharged, or nonpolar, and are hydrophobic (water-hating). A water-hating molecule is repelled by water. Some lipid tails comprise saturated fatty acids and certain have unsaturated fatty acids. This mixture adds to the fluidity of the tails that are continuously in motion. Phospholipids are hence amphipathic molecules. An amphipathic molecule has both a hydrophilic and a hydrophobic area. Soap works to eliminate oil and grease marks because it has amphipathic properties. The hydrophilic portion can melt in the water while the hydrophobic portion can hold the grease in micelles that then can be washed away.

The cell membrane is made up of two neighboring layers of phospholipids. The lipid end or tail of one layer faces the lipid tails of the other layer, meeting at the line of the two layers. The phospholipid heads face the external, one layer wide-open to the interior of the cell and one layer exposed to the external. Due to the condition that phosphate groups are polar and hydrophilic (Water-loving); they are attracted to the water in the intracellular fluid. Intracellular fluid (ICF) is the fluid present on the inner side of the cell. The phosphate groups are also attracted to the outside fluid. Extracellular fluid (ECF) is the fluid outside the attachment of the cell membrane. Interstitial fluid (IF) is the name given to extracellular fluid, which is not present within the blood vessels. Since the lipid tails are hydrophobic, (water-hating) they meet in the inner region of the membrane, without watery intracellular and extracellular fluid from this area. The cell membrane has several proteins, as well as other lipids (like cholesterol), that are related to the phospholipid bilayer. A vital feature of the membrane is that it remains fluid; the lipids and proteins in the cell membrane are not severely locked in place.

Transport Across the Cell Membrane

One of the great phenomena of the cell membrane is to control the concentration of materials inside the cell. These materials include ions such as Ca⁺, K⁺, Na⁺, and Cl^-; nutrients containing fatty acids, sugars, and amino acids; and waste yields, mainly carbon dioxide (CO₂), which need to leave the cell.

The membrane’s lipid bilayer assembly delivers the first level of control. The phospholipids are strongly packed together, and the membrane has a hydrophobic center. This assembly causes the membrane to be selectively permeable. A membrane that contains selective permeability permits only materials meeting certain standards to travel through it unaided. In the case of the cell membrane, only comparatively small, non-polar resources can move through the lipid bilayer. Some cases of these are other lipids, oxygen, carbon dioxide, and alcohol. Hydrophilic materials like amino acids, glucose, and electrolytes need some help to cross the membrane because they are prevented by the hydrophobic tails of the phospholipid bilayer. All materials that move along the membrane do so by one of two general techniques, which are classified on basis of whether energy is required or not. Passive transport is the movement of materials across the membrane without spending cellular energy. The second technique is active transport which is the movement of materials across the membrane with the help of using energy from adenosine triphosphate (ATP).

What is Active Transport?

Active transport is the movement of molecules like water oxygen and other important molecules across the membrane against the concentration channel with the help of enzymes and usage of cellular energy. It is required for the gathering of molecules like amino acids, glucose, and ions inside the cell in high concentrations.

Active Transports are of Two Types

1. Primary Active Transport: In the primary active transport, transporting the molecules, it uses chemical energy to push the molecule.

2. Secondary Active Transport:  In the secondary active transport, proteins present in the cell membrane use the electromagnetic gradient to move across the membrane.

Primary Active Transport

During primary active transport, the existence of molecules in the extracellular fluid that is necessary for the cell is recognized by the specific transmembrane proteins on the cell membrane, which act as pumps for transferring the molecules. 

Secondary Active Transport

Secondary active transport is governed by an electrochemical gradient. Here, channels are made by pore-forming proteins (Pores are small holes). A simultaneous movement of another molecule against the concentration gradient can be seen with the secondary active transport.

What is Passive Transport?

Passive transport is the transport of molecules across the membrane through a concentration gradient without the use of cellular energy by movement. It uses natural entropy to transport molecules from a higher concentration to a lower concentration until the concentration becomes balanced. Then, there will be no net transport of molecules at the equilibrium. Four main kinds of passive transport are found: osmosis, simple diffusion, facilitated diffusion, and filtration.

Osmosis

In the process of osmosis, the water, and other molecules or substance are transported through the selectively permeable cell membrane. Many aspects affect this transport. One of the main factors is the cell having less negative water potential and other factors are the solute potential of a molecule and the pressure potential of a cell membrane.

Simple Diffusion

In the process of simple diffusion, the transportation of molecules or solute across a permeable membrane is known as simple diffusion. Mainly non-polar molecules use simple diffusion, to maintain a better flow of molecules.

Facilitated Diffusion

Facilitated diffusion is the natural passive transportation of molecules or ions across the cell membrane through the specific-trans membrane of integral proteins. The molecules, which are big and insoluble need a carrier molecule for their transportation through the plasma membrane. No cellular energy or external energy is required for this process.

Filtration

The cardiovascular system (CVS) in the human body produces hydrostatic pressure, which helps water and other soluble biochemical molecules or substance to travel across the cell membrane. This process is named filtration because the cell membrane permits only substances that are soluble and could freely pass through the membrane’s pore.

The Function of the Cell Wall

There are various functions of cell wall like :

• The cell wall of the plants provides definite shape, strength, and rigidity.

• The cell wall also protects against mechanical stress and physical shocks.

• Cell wall helps to control cell expansion due to the intake of water.

• The cell wall also helps in preventing water loss from the cell.

• The cell wall is responsible for transporting substances between and across the cell.

• The cell wall acts as a barrier between the interior cellular components and the external environment.

Cell Membrane

The cell membrane is also called the plasma membrane, Cell membrane is found in all cells and separates the interior of the cell from the outside environment. The cell membrane also consists of a lipid bilayer that is semipermeable. The cell membrane is also responsible for the transportation of materials entering and exiting the cell. The thin membrane surrounds every living cell, fixing the cell from the environment around it. The cell membranes are the cell’s constituents, often large, water-soluble, highly charged molecules such as proteins, nucleic acid, carbohydrates, and substances involved in cellular metabolism. The surrounding of the cell membrane is a water-based environment, containing ions, acids, and alkalis that are toxic to the cell, as well as nutrients that the cell must absorb to live and grow. 

The cell membrane is mainly composed of a mix of proteins and lipids. Depending on the cell membrane’s location and role in the body, lipids can make up anywhere from 20% to 80% of the membrane, with the remainder being proteins. Though lipids help to give membranes their flexibility, proteins monitor and maintain the cell's chemical climate and assist in the transfer of molecules across the membrane.

Functions of the Cell Membrane

There are various functions of cell membrane-like :

• The cell membrane protects the integrity of the interior cell.

• The cell membrane provides support and maintains the shape of the cell.

• The cell membrane helps in regulating cell growth through the balance of endocytosis and exocytosis.

• It also plays an important role in cell signaling and communication.

• The cell membrane acts as a selectively permeable membrane by allowing the entry of only selected substances into the cell.