2.5: Structures Outside the Cell Wall - Biology

2.5: Structures Outside the Cell Wall - Biology

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Learning Objectives

  • The overall purpose of this Learning Object is to list the various cellular components that are often found external to the bacterial cell wall.

In this section on Prokaryotic Cell Anatomy we are looking at the various anatomical parts that make up a bacterium. We will now look at the following structures located outside the cell wall of many bacteria: (1) glycocalyx (capsule) and S-layer, (2) flagella, and (3) pili.

Cell Wall - What's it for?

Cell membranes surround every cell you will study. Cell walls made of cellulose are only found around plant cells and a few other organisms. Cellulose is a specialized sugar that is classified as a structural carbohydrate and not used for energy. If a plant cell is like a water balloon, the cell wall is like a cardboard box that protects the balloon. The balloon is protected from the outside world by a structure that provides protection and support.

While many sugars, such as glucose, can dissolve in water (H20), cellulose will not dissolve in water and can form long chains to support plants. When you eat plant material, you can’t even digest and break down cellulose for energy. Cows and other herbivores have special bacteria in their stomachs to digest the cellulose polymers.

While cell walls protect the cells, they also allow plants to grow to great heights. You have a skeleton to hold you up. A 100-foot tall redwood tree does not. It uses the strong cell walls to maintain its shape. For overall support, dense cells in the core of the trunk can let a tree grow very high. Cell walls are slightly elastic for smaller plants, leaves, and thin branches. Winds can push them from side to side and they bounce back. Big redwoods need strength in high winds and sway very little (except at the top).

Organs in Plants?

Your body includes organ systems, such as the digestive system, made of individual organs, such as the stomach, liver, and pancreas, which work together to carry out a certain function (in this case, breaking down and absorbing food). These organs, in turn, are made of different kinds of tissues, which are groups of cells which work together to perform a specific job. For example, your stomach is made of muscle tissue to facilitate movement and glandular tissue to secrete enzymes for chemical breakdown of food molecules. These tissues, in turn, are made of cells specialized in shape, size, and component organelles, such as mitochondria for energy and microtubules for movement.

Plants, too, are made of organs, which in turn are made of tissues. Plant tissues, like ours, are constructed of specialized cells, which in turn contain specific organelles. It is these cells, tissues, and organs that carry out the dramatic lives of plants.

Structure of Plant Cell (Explained With Diagram)

The protoplasm is the living part of the cell. It is externally bounded by cell membrane or plasma membrane. The cytoplasm contains several cell organelles namely mitochondria, plastids, ribosomes, endoplasmic reticulum, lysosomes etc. (Fig. 2.1).

Cell wall is the non-living protective layer outside the plasma membrane in the plant cells, bacteria, fungi and algae. The synthesis of cell wall in controlled by Golgi bodies. In bacteria the cell wall is composed of protein and non-cellulosic carbohydrates while in most algae, fungi and all plant cells, the cell-wall is formed of cellulose. Cell wall provides mechanical support and gives a definite shape to the cell. It protects plasma membrane and helps in imbibition’s of water and movement of solutes towards protoplasm.

Protoplasm is the living, colourless, elastic, colloidal semi fluid substance present in the cell. Protoplasm with non-living inclusions is called protoplast. Water is the chief constituent of an active protoplast and normally constitutes 90% of the system. The remaining parts are organic and inorganic materials.

Each protoplast keeps itself in communication with neighbouring protoplasts through small openings in the cell wall known as plasmodesmata. Protoplasm consists of cytoplasm and nucleus and is externally bounded by the cell membrane or plasmalemma.

(iii) Cell membrane:

It is a thin film like pliable membrane, and serves as protective covering of the cell. Cell membrane mainly consists of proteins and lipids but in certain cases, polysachharides have also been found. It facilitates the entrance of nutrients into the cells and allows exit of nitrogenous wastes, regulates the passage of materials into and out of the cells. It controls and maintains differential distribution of ions inside and outside the cell.

It is a jelly like fluid mass of protoplasm excluding the nucleus and surrounded by plasma membrane on the outside. It is semi permeable in nature. The cytoplasm is composed of matrix the membrane bound organelles and non-living inclusions like vacuoles and granules. The living cytoplasmic organelles are the site of various important metabolic activities such as photosynthesis, respiration, protein synthesis etc.

Plastids are the largest cytoplasmic organelles bounded by double membranes.

There are three types of plastids:

These are colourless plastids found in storage organs where light is not available e.g. underground stem, roots and deeper tissues. Plastids without pigments are called leucoplasts. They store starch, fats or proteins in meristematic, embryonic and germ cells.

Coloured plastids are called chromoplasts. These may contain red, yellow, brown, purple, blue or green pigments. These are mostly found in petals of flowers and fruits.

In a plant cell, chloroplasts are the most prominent forms of plastids that contain chlorophyll, the green pigment. The chlorophyll enables the chloroplast to harness kinetic solar energy and trap it in the form of potential energy. All living organisms directly or indirectly depend on them for energy. Chloroplast in enclosed in two smooth membranes separated by a distinct periplastidial space. The interior of chloroplast is differentiated into two parts— The Stroma and the Grana.

Stroma is the colourless ground substance that fills the chloroplast. It contains cholorophyll bearing double membranes lamellae that form flattened sac like structures called thylakoides collectively called Grana. Quantasomes are the smallest units present on the inner surface of thylakoides capable of carrying out photochemical reactions (Fig. 2.2).

Ribosomes are the submicroscopic organelles. These are site of protein synthesis in the cell. These are found in all the cells either attached to the membranes of endoplasmic reticulum or scattered in the cytoplasm. The ribosomes are spheroidal bodies. In prokaryotic cells (Bacteria) the ribosomes are abour 15 nm and in eukaryotic cells about 25 nm.

Ribosomes from prokaryotes exist as 70S units and ribosomes in eukaryotes exust as 80S units. A ribosome is formed of two subunits – a large subunit and a small subunit. The small subunit forms a sort of cap on the flat surface of large subunit. The two units of bacterial ribosome (70S) are represented by 50S and 30S subunits, and eukaryotic ribosomes (80S) are represented by 60S and 40S subunits.

The two subunits of ribosomes usually exist free in the cytoplasm and join only during protein synthesis when a number of ribosomes get attached to mRNA in a linear fashion. These groups or clusters of ribosomes are known as Polyribosomes. The larger subunits (i.e. 60S and 50S) are attached to the membrane of endoplasmic reticulum and the smaller subunits are then bound to larger subunits. The cleft separating the two subunits lies parallel to the membrane. The messenger RNA is held by the smaller subunit, while tRNA molecule is bound to the larger subunit.

(vii) Mitochondria:

Mitochondria are sausage- shaped spherical or thread like organelles present in the cytoplasm. They break down the complex carbohydrates and sugars into usable forms and supply energy for the cell, they are also called as the powerhouse of the cell.

The mitochondria are surrounded by a double walled membrane known as outer and inner membranes. The spaces between these two membranes are known as perimitochondrial space. The outer membrane is smooth but the inner membrane is variously folded into thin cristae. Inner membrane is covered with special particles called Oxysomes, these are the sites of aerobic respiration. (Fig. 2.2).

The nucleus is the most important part of the cell which regulates all metabolic and hereditary activities within the cell It is more or less spherical, lying in the cytoplasm and occupying about two-thirds of the cell space. A typical nucleus is composed of the following structures (Fig. 2.2).

It is a selectively permeable envelope-like structure surrounding the nucleolus and nucleoplasm. It is formed of two layers separated by a fluid-filled perinuclear space. The nuclear membrane disappears during prophase stage of nuclear division and reorganizes during telophase. It regulates the passage of ions, small molecules and macromolecules of ribosomal subunits, mRNA, t RNA etc.

Nucleoplasm is the transparent semifluid ground substance formed of a mixture of proteins, phosphorus and some nucleic acids. Chromatin fibres or the chromonemata remain suspended in the nucleoplasm.

Chromatin fibres form a network in the nucleoplasm called chromatin net work or nuclear reticulum. Chromatin fibres are the sites of main genetic material which controls all the activities of the cell, metabolism and heredity. During cell division, the chromatin threads condense and form thick chromosomes.

The nucleolus is a spherical body lying in the nucleoplasm closely associated to the nucleolar organizer region of the chromosome. It was first described by Schleiden in 1838. It contains large amount of RNA though DNA is also present. Its main function is the synthesis of ribosomal RNA (rRNA), which helps in synthesis of ribosomes.

Golgi body is also referred to as golgi complex or golgi apparatus. It plays a major role in transporting chemical substances in and out of the cell. It has three distinct components flattened sac or cisternae, clusters of transition tubules and vesicles and large vesicles or vacuoles. Golgi is mainly associated with secretory activity of the cell. It is also associated with the concentration, storage, condensation and packaging of materials for export from the cell across plasmalemma.

(x) Endoplasmic Reticulum:

Endoplasmic reticulum (ER) is the connecting link between the nucleus and cytoplasm of the plant cell. Basically, it is a network of interconnected and convoluted sacs that are located in the cytoplasm. Based on the presence or absence of ribosomes, ER can be of smooth or rough types. The former type lacks ribosomes, while the latter is covered with ribosomes. Overall, endoplasmic reticulum serves as a manufacturing, storing and transporting structure for glycogen, proteins, steroids and other compounds.

Lysosomes are tiny membrane-bound, vesicular structure of cytoplasm which enclose hydrolytic enzymes and perform intracellular digestion. These are also known as suicidal bags. These are found in all animal cells but only in few plant cells.

Types of Lysosomes:

Based on function or stage of digestion lysosomes are of following four types:

(i) Primary lysosomes are newly formed lysosomes with hydrolytic enzymes.

(ii) Secondary lysosomes are newly formed by the fusion of phagosomes and primary lysosomes. Here contents of phagosome are digested or hydrolysed.

(iii) Residual bodies are exhausted secondary lysosomes. These contain undigested remains.

(iv) Autophagic vacuoles are formed by the fusion of primary lysosomes with cell organelles from cell’s own cytoplasm. This brings about autodigestion of cell or its organelles.

Lysosomes bring about digestion of extracellular and intracellular materials. Because of this basic character lysosomes perform following functions:

(i) Lysosomes of granular leucocytes or macrophages devour foreign substances and microbes which enter the cell and guard our body against infection.

(ii) Lysosomes remove worn out cell organelles, dead cell and provide energy during starvation by controlled breakdown of stored food substances.

(iii) During dedifferentiation of tissues and cells, the lysosomes dissolve the specialized parts of the cells. This helps in regeneration of damaged tissue or damaged part and formation of bone from cartilage.

(iv) The lytic enzymes of sperm acrosome help in the penetration of sperm into the ovum.

(v) During metamorphosis, reabsorption of various larval structures like external gills of tadpole, the tail of tadpole in frog or the larval organs in the pupa of various insects is brought about by autolytic action of lysosomes.

(vi) Lysosomes bring about cellular breakdown associated with ageing.

(vii) Lysosomes may cause cancer by breaking down chromosomes.

(xii) Peroxisomes:

Peroxisomes occur widely both in plant and animal cells. They are spherical or ovoid bodies surrounded by a single membrane. It contains certain oxidative enzymes, used for the metabolic breakdown of fatty acids into simple sugar forms. In green plants, peroxisomes help in undergoing photorespiration.

(xiii) Vacuoles:

Vacuoles are sap- filled vesicles in the cytoplasm. These are surrounded by a membrane called tonoplast. In a plant cell, there can be more than one vacuole however, the centrally located vacuole is larger than others.

Tonoplast is a semi permeable membrane it enables the vacuoles to concentrate and store nutrients and waste products. It facilitates the rapid exchange of solutes aid gases between the cytoplasm and adjoining fluids.

(xiv) Cilia and Flagella:

Cilia and flagella are motile hair -like appendages on the free surfaces of the cells. These are cytoplasmic processes and create water currents, food currents, act as sensory organs and perform several other functions of the cell. Cilia and flagella can be differentiated on the basis of their size, however, other physiological and morphological characteristics are almost the same.

The main difference between cilia and flagella are as follows:

Cilia and flagella are cylindrical processes projecting from the free surface of the cell. These originate from their basal bodies embedded in the cytoplasm. The basal bodies form their kinetic centres. A ciliam or flagellum consists of a longitudinal axoneme enclosed in a spiral sheath of cytoplasm and a plasma membrane continuous with the cell membrane.

Biology UNIT 2

a.) is a protective structure made of cellulose fibrils.

b.) is found just inside the plasma membrane.

c.) is very similar to the animal cell wall.
regulates the composition of the cytoplasm.

a.) Storing compounds produced by the cell

b.) Working with mRNA to synthesize proteins

c.) Converting light energy to chemical energy

d.) Separating the cell from its surroundings

a.) Chloroplasts central vacuoles

b.) Cell walls chloroplasts

c.) Lysosomes plasma membranes

d.) Central vacuoles ribosomes

a.) The chloroplast serves as a protein manufacturing facility.

b.) The chloroplast creates internal pressure for a cell.

c.) The chloroplast converts light energy to chemical energy.

d.) The chloroplast stores compounds produced by the cell.

a.) smooth endoplasmic reticulum (ER)

b.) rough endoplasmic reticulum (ER)

a.) smooth endoplasmic reticulum (ER)

d.) rough endoplasmic reticulum (ER)

1 The framework of a membrane is a bilayer of phospholipids with their hydrophilic heads facing the aqueous environment inside and outside of the cell and their hydrophobic tails clustered in the center.

2 The diverse proteins found in and attached to membranes perform many important functions.

3 The kinky tails of some proteins help keep the membrane fluid by preventing the component molecules from packing solidly together.

4 Membranes include a mosaic, or mix, of carbohydrates embedded in a phospholipid bilayer.

b.) a protein involved in enzymatic activity

c.) a glycoprotein that is involved in cell-cell recognition

d.) an active transport protein that moves molecules across a membrane against their concentration gradient

A) blood or tissue type of the patient

B) non-polarity of the drug molecule

C) lack of charge on the drug molecule

D) similarity of the drug molecule to other molecules transported by the target cells

a.) A . the diffusion gradient there is shallower

b.) A . its membrane transport proteins will not be saturated

c.) B . the diffusion gradient there is steeper

a.) The cell does not expend ATP.

b.) Sodium and potassium ions are transported against their concentration gradients.

c.) Potassium ions are transported down their concentration gradient.

d.) The cell is not expending energy.

a.) Active transport requires the expenditure of cellular energy, and facilitated diffusion does not.

b.) Facilitated diffusion can move solutes against a concentration gradient, and active transport cannot.

c.) Active transport involves transport proteins, and facilitated diffusion does not.

a.) a rock on a mountain ledge

b.) the high-energy phosphate bonds of a molecule of ATP

c.) a person sitting on a couch while watching TV

d.) an archer with a flexed bow

a.) the net amount of disorder is always increasing

b.) no chemical reaction is 100 percent efficient

c.) energy cannot be created or destroyed but can be converted from one form to another


Reproduction in prokaryotes is asexual and usually takes place by binary fission. Recall that the DNA of a prokaryote exists as a single, circular chromosome. Prokaryotes do not undergo mitosis. Rather the chromosome is replicated and the two resulting copies separate from one another, due to the growth of the cell. The prokaryote, now enlarged, is pinched inward at its equator and the two resulting cells, which are clones, separate. Binary fission does not provide an opportunity for genetic recombination or genetic diversity, but prokaryotes can share genes by three other mechanisms.

In transformation, the prokaryote takes in DNA found in its environment that is shed by other prokaryotes. If a nonpathogenic bacterium takes up DNA for a toxin gene from a pathogen and incorporates the new DNA into its own chromosome, it too may become pathogenic. In transduction, bacteriophages, the viruses that infect bacteria, sometimes also move short pieces of chromosomal DNA from one bacterium to another. Transduction results in a recombinant organism. Archaea are not affected by bacteriophages but instead have their own viruses that translocate genetic material from one individual to another. In conjugation, DNA is transferred from one prokaryote to another by means of a pilus, which brings the organisms into contact with one another. The DNA transferred can be in the form of a plasmid or as a hybrid, containing both plasmid and chromosomal DNA. These three processes of DNA exchange are shown in Figure 9.

Reproduction can be very rapid: a few minutes for some species. This short generation time coupled with mechanisms of genetic recombination and high rates of mutation result in the rapid evolution of prokaryotes, allowing them to respond to environmental changes (such as the introduction of an antibiotic) very quickly.

Figure 9. Besides binary fission, there are three other mechanisms by which prokaryotes can exchange DNA. In (a) transformation, the cell takes up prokaryotic DNA directly from the environment. The DNA may remain separate as plasmid DNA or be incorporated into the host genome. In (b) transduction, a bacteriophage injects DNA into the cell that contains a small fragment of DNA from a different prokaryote. In (c) conjugation, DNA is transferred from one cell to another via a mating bridge that connects the two cells after the sex pilus draws the two bacteria close enough to form the bridge.

Evolution Connection

The Evolution of Prokaryotes

How do scientists answer questions about the evolution of prokaryotes? Unlike with animals, artifacts in the fossil record of prokaryotes offer very little information. Fossils of ancient prokaryotes look like tiny bubbles in rock. Some scientists turn to genetics and to the principle of the molecular clock, which holds that the more recently two species have diverged, the more similar their genes (and thus proteins) will be. Conversely, species that diverged long ago will have more genes that are dissimilar.

Scientists at the NASA Astrobiology Institute and at the European Molecular Biology Laboratory collaborated to analyze the molecular evolution of 32 specific proteins common to 72 species of prokaryotes. 1 The model they derived from their data indicates that three important groups of bacteria—Actinobacteria, Deinococcus, and Cyanobacteria (which the authors call Terrabacteria)—were the first to colonize land. (Recall that Deinococcus is a genus of prokaryote—a bacterium—that is highly resistant to ionizing radiation.) Cyanobacteria are photosynthesizers, while Actinobacteria are a group of very common bacteria that include species important in decomposition of organic wastes.

The timelines of divergence suggest that bacteria (members of the domain Bacteria) diverged from common ancestral species between 2.5 and 3.2 billion years ago, whereas archaea diverged earlier: between 3.1 and 4.1 billion years ago. Eukarya later diverged off the Archaean line. The work further suggests that stromatolites that formed prior to the advent of cyanobacteria (about 2.6 billion years ago) photosynthesized in an anoxic environment and that because of the modifications of the Terrabacteria for land (resistance to drying and the possession of compounds that protect the organism from excess light), photosynthesis using oxygen may be closely linked to adaptations to survive on land.

2.5: Structures Outside the Cell Wall - Biology

Different Types of Cells

There are lots of different types of cells. Each type of cell is different and performs a different function. In the human body, we have nerve cells which can be as long as from our feet to our spinal cord. Nerve cells help to transport messages around the body. We also have billions of tiny little brain cells which help us think and muscle cells which help us move around. There are many more cells in our body that help us to function and stay alive.

Although there are lots of different kinds of cells, they are often divided into two main categories: prokaryotic and eukaryotic.

Prokaryotic Cells - The prokaryotic cell is a simple, small cell with no nucleus. Organisms made from prokaryotic cells are very small, such as bacteria. There are three main regions of the prokaryotic cell:

1) The outside protection or "envelope" of the cell. This is made up of the cell wall, membrane, and capsule.
2) The flagella, which are a whip-like appendages that can help the cell to move. Note: not all prokaryotic cells have flagella.
3) The inside of the cell called the cytoplasmic region. This region includes the nucleoid, cytoplasm, and ribosomes.

Eukaryotic Cells - These cells are typically a lot bigger and more complex than prokaryotic cells. They have a defined cell nucleus which houses the cell's DNA. These are the types of cells we find in plants and animals.

Watch the video: Το κύτταρο- Βιολογία Λυκείου- ΕΛΛΗΝΙΚΟΙ ΥΠΟΤΙΤΛΟΙ (October 2022).