A cell wall is present in organisms in the kingdoms Archaebacteria, Eubacteria, Protista, Fungi, and Plantae. Animals are the only organisms that do not have a cell wall.
The cell wall is one of the features of plant cells that distinguishes them from animal cells. The wall protects the plant cell and maintains its shape. The cell wall consists mainly of cellulose microfibrils embedded in a matrix of protein and sugar polymers.
A primary cell wall is laid down during cell division, and is relatively thin and flexible in order to accommodate cell enlargement and elongation. It is strengthened when the cell matures and stops growing. A secondary cell wall is present between the plasma membrane and primary cell wall in some cells. The secondary wall is often deposited in several laminated layers. It is strong and durable, and provides both cell protection and support. Wood consists mainly of secondary cell walls.
Cell walls in prokaryotes (e.g., bacteria) and plants define the cell’s shape and give rigidity to the cell. Unlike plant cell walls, bacterial cell walls consist mainly of peptidoglycans and not cellulose. Peptidoglycans are polysaccharide chains (amino sugars) cross-linked by small peptides.
Cell membranes define and compartmentalize space, regulate the flow of materials, detect external signals, and mediate interactions between cells.
The plasma membrane is a thin membrane that surrounds and defines the boundaries all living cells. It consists of a double layer (bilayer) of phospholipids with various pro- teins attached to or embedded in it.
As early as the 1890s Charles Overton (1865–1933) was aware that cells seemed to be enveloped by a selectively permeable layer that allowed different substances to enter and leave cells at significantly different rates. He recognized that lipid-soluble substances penetrated readily into cells, whereas water-soluble substances did not. He concluded that lipids were present on the cell surface as some sort of a “coat.” Irving Langmuir (1881–1957) proposed that that cells were covered by a lipid monolayer.
The plasma membrane is only about 8 nm (nanometers) thick. It would take over 8,000 plasma membranes to equal the thickness of an average piece of paper.
His work became the basis for further investigation into the membrane structure. Langmuir was awarded the Nobel Prize for Chemistry in 1932.
In 1925 Evert Gorter (1881–1954) and F. Grendel, two Dutch physiologists, hypothesized that there is a bilayer structure of lipids on the cell surface. Their work was significant because it was the first attempt to understand membranes at the molecular level.
Following the earlier work of Gorter and Grendel on cell membranes, Hugh Davson (1909–1996) and James F. Danielli (1911–1984) proposed a sandwich model for the structure of cell membranes in 1935. This model was a phospholipid bilayer between two layers of globular proteins. Since cell membranes are so fragile in vivo, one can only propose theoretical models for their structure. Current techniques still do not permit direct observation of the functioning of plasma membranes.
The current model, frequently referred to as the fluid mosaic model, is based on work completed by Seymour J. Singer (1924–) and Garth L. Nicholson (1943–) in 1972. Their research revealed that the plasma membrane is a mosaic of integral proteins bobbing in a fluid bilayer of phospholipids. This pattern is not static because the positions of the proteins are constantly changing, moving about like icebergs in a sea of lipids. Peripheral proteins are not embedded in the lipid bilyaer but are appendages loosely bound to the membrane surface. Membrane carbohydrates on the surface function as cell markers to distinguish one cell from another. This model has been tested repeatedly and has been shown to accurately predict the properties of many kinds of cellular membranes; this structure has also been confirmed using a technique known as freeze-fracture electron microscopy.
Cell junctions are the specialized connections between the plasma membranes of adjoining cells. The three general types of cell junctions are tight junctions, anchoring junctions, and communicating junctions. Tight junctions bind cells together, forming a barrier that is leak-proof. For example, tight junctions form the lining of the digestive tract, preventing the contents of the intestine from entering the body. Anchoring (or adhering) junctions link cells together, enabling them to function as a unit and forming tissue, such as heart muscle or the epithelium that comprises skin. Communicating (or gap) junctions allow rapid chemical and electrical communication between cells. They consist of channels that connect the cytoplasm of adjacent cells.
What are the main components of the cell wall in plants?
The cell wall is one of the features of plant cells that distinguishes them from animal cells. The wall protects the plant cell and maintains its shape. The cell wall consists mainly of cellulose microfibrils embedded in a matrix of protein and sugar polymers.
How does a primary cell wall differ from a secondary cell wall?
A primary cell wall is laid down during cell division, and is relatively thin and flexible in order to accommodate cell enlargement and elongation. It is strengthened when the cell matures and stops growing. A secondary cell wall is present between the plasma membrane and primary cell wall in some cells. The secondary wall is often deposited in several laminated layers. It is strong and durable, and provides both cell protection and support. Wood consists mainly of secondary cell walls.
Are bacterial cell walls different from plant cell walls?
Cell walls in prokaryotes (e.g., bacteria) and plants define the cell’s shape and give rigidity to the cell. Unlike plant cell walls, bacterial cell walls consist mainly of peptidoglycans and not cellulose. Peptidoglycans are polysaccharide chains (amino sugars) cross-linked by small peptides.
What are the functions of the cell membranes?
Cell membranes define and compartmentalize space, regulate the flow of materials, detect external signals, and mediate interactions between cells.
What is the structure of the plasma membrane?
The plasma membrane is a thin membrane that surrounds and defines the boundaries all living cells. It consists of a double layer (bilayer) of phospholipids with various pro- teins attached to or embedded in it.
What contributions did Overton and Langmuir make to the study of membranes?
As early as the 1890s Charles Overton (1865–1933) was aware that cells seemed to be enveloped by a selectively permeable layer that allowed different substances to enter and leave cells at significantly different rates. He recognized that lipid-soluble substances penetrated readily into cells, whereas water-soluble substances did not. He concluded that lipids were present on the cell surface as some sort of a “coat.” Irving Langmuir (1881–1957) proposed that that cells were covered by a lipid monolayer.
How thick is the plasma membrane?
The plasma membrane is only about 8 nm (nanometers) thick. It would take over 8,000 plasma membranes to equal the thickness of an average piece of paper.
His work became the basis for further investigation into the membrane structure. Langmuir was awarded the Nobel Prize for Chemistry in 1932.
Who first proposed that the cell membrane is composed of a bilayer structure of lipids?
In 1925 Evert Gorter (1881–1954) and F. Grendel, two Dutch physiologists, hypothesized that there is a bilayer structure of lipids on the cell surface. Their work was significant because it was the first attempt to understand membranes at the molecular level.
Who first proposed a model for the plasma membrane?
Following the earlier work of Gorter and Grendel on cell membranes, Hugh Davson (1909–1996) and James F. Danielli (1911–1984) proposed a sandwich model for the structure of cell membranes in 1935. This model was a phospholipid bilayer between two layers of globular proteins. Since cell membranes are so fragile in vivo, one can only propose theoretical models for their structure. Current techniques still do not permit direct observation of the functioning of plasma membranes.
What is the current model of the plasma membrane?
The current model, frequently referred to as the fluid mosaic model, is based on work completed by Seymour J. Singer (1924–) and Garth L. Nicholson (1943–) in 1972. Their research revealed that the plasma membrane is a mosaic of integral proteins bobbing in a fluid bilayer of phospholipids. This pattern is not static because the positions of the proteins are constantly changing, moving about like icebergs in a sea of lipids. Peripheral proteins are not embedded in the lipid bilyaer but are appendages loosely bound to the membrane surface. Membrane carbohydrates on the surface function as cell markers to distinguish one cell from another. This model has been tested repeatedly and has been shown to accurately predict the properties of many kinds of cellular membranes; this structure has also been confirmed using a technique known as freeze-fracture electron microscopy.
What are the main functions of the plasma membrane?
The main purpose of the plasma membrane is to provide a barrier that keeps cellular components inside the cell while simultaneously keeping unwanted substances from entering the cell. The membrane allows essential nutrients to be transported into the cell and aids in the removal of waste products from the cell. The specific functions of a membrane depend on the kinds of phospholipids and proteins present in the plasma membrane.What are the main components of the plasma membrane?
|
Component |
Function |
|
Cell
surface markers |
"Self"-recognition;
tissue recognition |
|
Interior
protein network |
Determines shape of
cell |
|
Phospholipid
molecules |
Provide
permeability barrier, matrix for proteins |
|
Transmembrane
proteins |
Transport molecules
across membrane and against gradient. |
What are cell junctions?
Cell junctions are the specialized connections between the plasma membranes of adjoining cells. The three general types of cell junctions are tight junctions, anchoring junctions, and communicating junctions. Tight junctions bind cells together, forming a barrier that is leak-proof. For example, tight junctions form the lining of the digestive tract, preventing the contents of the intestine from entering the body. Anchoring (or adhering) junctions link cells together, enabling them to function as a unit and forming tissue, such as heart muscle or the epithelium that comprises skin. Communicating (or gap) junctions allow rapid chemical and electrical communication between cells. They consist of channels that connect the cytoplasm of adjacent cells.