CELL AS THE UNIT OF STRUCTURE AND FUNCTION IN PLANTS
Cell components
Cell Wall
A cell wall is a rigid, semi-permeable protective layer in some cell types. This outer covering is
positioned next to the cell membrane (plasma membrane) in most plant
cells, fungi, bacteria, algae, and some archaea.
Structure
Cell walls are a strong collection of laminated fibers. Each fiber contains hundreds of strands of
cellulose. Cellulose, as you should recall, is a macromolecule composed of glucose monomers
(subunits).
Cell wall composition varies depending on the organism. In plants, the cell wall is composed
mainly of strong fibers of the carbohydrate polymer cellulose. Cellulose is the major component of
cotton fiber and wood, and it is used in paper production. Bacterial cell walls are composed of a
sugar and amino acid polymer called peptidoglycan. The main components of fungal cell walls
are chitin, glucans, and proteins.
The plant cell wall is multi-layered and consists of up to three sections. From the outermost layer
of the cell wall, these layers are identified as the middle lamella, primary cell wall, and secondary
cell wall. While all plant cells have a middle lamella and primary cell wall, not all have a secondary
cell wall.
• Middle lamella: This outer cell wall layer contains polysaccharides called pectins. Pectins
aid in cell adhesion by helping the cell walls of adjacent cells to bind to one another.
• Primary cell wall: This layer is formed between the middle lamella and plasma
membrane in growing plant cells. It is primarily composed of cellulose microfibrils
contained within a gel-like matrix of hemicellulose fibers and pectin polysaccharides. The
primary cell wall provides the strength and flexibility needed to allow for cell growth.
• Secondary cell wall: This layer is formed between the primary cell wall and plasma
membrane in some plant cells. Once the primary cell wall has stopped dividing and
growing, it may thicken to form a secondary cell wall. This rigid layer strengthens and
supports the cell. In addition to cellulose and hemicellulose, some secondary cell walls
contain lignin. Lignin strengthens the cell wall and aids in water conductivity in plant
vascular tissue cells.
Function
,A major role of the cell wall is to form a framework for the cell to prevent over expansion. Cellulose
fibers, structural proteins, and other polysaccharides help to maintain the shape and form of the
cell. Additional functions of the cell wall include:
• Support: The cell wall provides mechanical strength and support. It also controls the
direction of cell growth.
• Withstand turgor pressure: Turgor pressure is the force exerted against the cell wall as
the contents of the cell push the plasma membrane against the cell wall. This pressure
helps a plant to remain rigid and erect, but can also cause a cell to rupture.
• Regulate growth: The cell wall sends signals for the cell to enter the cell cycle in order to
divide and grow.
• Regulate diffusion: The cell wall is porous allowing some substances, including proteins,
to pass into the cell while keeping other substances out.
• Communication: Cells communicate with one another via plasmodesmata (pores or
channels between plant cell walls that allow molecules and communication signals to
pass between individual plant cells).
• Protection: The cell wall provides a barrier to protect against plant viruses and other
pathogens. It also helps to prevent water loss.
• Storage: The cell wall stores carbohydrates for use in plant growth, especially in seeds.
Protoplasmic components
• Cytoplasma
• Endoplasmic reticulum
• Ribosomes
• Mitochondrion
• Lysosomes
• Golgi apparatus
• Plastids
• Spherosomes
• Peroxisomes
• Glyoxysomes
• Protein bodies
Non protoplasma
• Starch grain
• Crystal
• Vacuoles
Nucleus
Function
,The nucleus is the control center of a cell as such it is the most important part of the cell. The
control arises from the genetic information stored in the nucleus. Genes are made of the nucleic
acid DNA. Hundreds of genes are linked together into enormous molecules called chromosomes
(chromatin). Genes can be switched on or off and are indirectly responsible for making proteins
which do the work of the cell.
Structure
The nucleus is a large roundish organelle. It is bounded by a double membrane which has
numerous pores. Inside the nucleus are chromosomes and a dark region called a nucleolus which
makes ribosomes. (Ribosomes are necessary for protein synthesis.)
Plasma Membrane
Functio]n
The plasma membrane is also a vital component of cellular control. Its responsibility is to restrict
what can enter or leave the cell. This process is called homeostasis.
Structure
The plasma membrane is made up of a bilayer of phospholipids. Many specialized proteins are
found floating in the bilayer and are necessary for homeostasis. Because the lipid bilayer is fluid
numerous pores or openings are created as the phospholipids swirl about. This makes the
membrane selectively permeable to small molecules. Large molecules rarely make it through
these pores.
Endoplasmic Reticulum
Function
, This folded membrane forms sacs to store proteins or other substances. It creates a vast surface
area where the manufacture of proteins and new membranes can take place.
Structure
Endoplasmic reticulum is a folded mass of membranes made of the same phospholipids found in
the plasma membrane. There are two types of ER smooth (without ribosomes) and rough (with
ribosomes)
Ribosomes
Function
Ribosomes are responsible for protein synthesis
Structure: Cells normally have millions of ribosomes; each ribosome has two parts which come
together during protein synthesis. A ribosome is made of numerous proteins and RNA.
Golgi complex
Function
The Golgi complex takes proteins made by the endoplasmic reticulum and exports them out of the
cell as needed. It is able to keep dangerous proteins from damaging the cell which made them. It
is like a packaging department ready to deliver the protein products made in the cell.
Structure
The Golgi complex looks like a stack of pancakes made of membranes with smaller vacuoles or
vesicles on either side of the main structure.
Lysosome
Function
Lysosomes are only needed by animal cells. They help break down and remove old worn out cell
parts. They are also involved in the digestion of food particles brought into the cell through
infoldings in the plasma membrane
Structure
Cell components
Cell Wall
A cell wall is a rigid, semi-permeable protective layer in some cell types. This outer covering is
positioned next to the cell membrane (plasma membrane) in most plant
cells, fungi, bacteria, algae, and some archaea.
Structure
Cell walls are a strong collection of laminated fibers. Each fiber contains hundreds of strands of
cellulose. Cellulose, as you should recall, is a macromolecule composed of glucose monomers
(subunits).
Cell wall composition varies depending on the organism. In plants, the cell wall is composed
mainly of strong fibers of the carbohydrate polymer cellulose. Cellulose is the major component of
cotton fiber and wood, and it is used in paper production. Bacterial cell walls are composed of a
sugar and amino acid polymer called peptidoglycan. The main components of fungal cell walls
are chitin, glucans, and proteins.
The plant cell wall is multi-layered and consists of up to three sections. From the outermost layer
of the cell wall, these layers are identified as the middle lamella, primary cell wall, and secondary
cell wall. While all plant cells have a middle lamella and primary cell wall, not all have a secondary
cell wall.
• Middle lamella: This outer cell wall layer contains polysaccharides called pectins. Pectins
aid in cell adhesion by helping the cell walls of adjacent cells to bind to one another.
• Primary cell wall: This layer is formed between the middle lamella and plasma
membrane in growing plant cells. It is primarily composed of cellulose microfibrils
contained within a gel-like matrix of hemicellulose fibers and pectin polysaccharides. The
primary cell wall provides the strength and flexibility needed to allow for cell growth.
• Secondary cell wall: This layer is formed between the primary cell wall and plasma
membrane in some plant cells. Once the primary cell wall has stopped dividing and
growing, it may thicken to form a secondary cell wall. This rigid layer strengthens and
supports the cell. In addition to cellulose and hemicellulose, some secondary cell walls
contain lignin. Lignin strengthens the cell wall and aids in water conductivity in plant
vascular tissue cells.
Function
,A major role of the cell wall is to form a framework for the cell to prevent over expansion. Cellulose
fibers, structural proteins, and other polysaccharides help to maintain the shape and form of the
cell. Additional functions of the cell wall include:
• Support: The cell wall provides mechanical strength and support. It also controls the
direction of cell growth.
• Withstand turgor pressure: Turgor pressure is the force exerted against the cell wall as
the contents of the cell push the plasma membrane against the cell wall. This pressure
helps a plant to remain rigid and erect, but can also cause a cell to rupture.
• Regulate growth: The cell wall sends signals for the cell to enter the cell cycle in order to
divide and grow.
• Regulate diffusion: The cell wall is porous allowing some substances, including proteins,
to pass into the cell while keeping other substances out.
• Communication: Cells communicate with one another via plasmodesmata (pores or
channels between plant cell walls that allow molecules and communication signals to
pass between individual plant cells).
• Protection: The cell wall provides a barrier to protect against plant viruses and other
pathogens. It also helps to prevent water loss.
• Storage: The cell wall stores carbohydrates for use in plant growth, especially in seeds.
Protoplasmic components
• Cytoplasma
• Endoplasmic reticulum
• Ribosomes
• Mitochondrion
• Lysosomes
• Golgi apparatus
• Plastids
• Spherosomes
• Peroxisomes
• Glyoxysomes
• Protein bodies
Non protoplasma
• Starch grain
• Crystal
• Vacuoles
Nucleus
Function
,The nucleus is the control center of a cell as such it is the most important part of the cell. The
control arises from the genetic information stored in the nucleus. Genes are made of the nucleic
acid DNA. Hundreds of genes are linked together into enormous molecules called chromosomes
(chromatin). Genes can be switched on or off and are indirectly responsible for making proteins
which do the work of the cell.
Structure
The nucleus is a large roundish organelle. It is bounded by a double membrane which has
numerous pores. Inside the nucleus are chromosomes and a dark region called a nucleolus which
makes ribosomes. (Ribosomes are necessary for protein synthesis.)
Plasma Membrane
Functio]n
The plasma membrane is also a vital component of cellular control. Its responsibility is to restrict
what can enter or leave the cell. This process is called homeostasis.
Structure
The plasma membrane is made up of a bilayer of phospholipids. Many specialized proteins are
found floating in the bilayer and are necessary for homeostasis. Because the lipid bilayer is fluid
numerous pores or openings are created as the phospholipids swirl about. This makes the
membrane selectively permeable to small molecules. Large molecules rarely make it through
these pores.
Endoplasmic Reticulum
Function
, This folded membrane forms sacs to store proteins or other substances. It creates a vast surface
area where the manufacture of proteins and new membranes can take place.
Structure
Endoplasmic reticulum is a folded mass of membranes made of the same phospholipids found in
the plasma membrane. There are two types of ER smooth (without ribosomes) and rough (with
ribosomes)
Ribosomes
Function
Ribosomes are responsible for protein synthesis
Structure: Cells normally have millions of ribosomes; each ribosome has two parts which come
together during protein synthesis. A ribosome is made of numerous proteins and RNA.
Golgi complex
Function
The Golgi complex takes proteins made by the endoplasmic reticulum and exports them out of the
cell as needed. It is able to keep dangerous proteins from damaging the cell which made them. It
is like a packaging department ready to deliver the protein products made in the cell.
Structure
The Golgi complex looks like a stack of pancakes made of membranes with smaller vacuoles or
vesicles on either side of the main structure.
Lysosome
Function
Lysosomes are only needed by animal cells. They help break down and remove old worn out cell
parts. They are also involved in the digestion of food particles brought into the cell through
infoldings in the plasma membrane
Structure
