The Cell Structure, Lecture notes of Biology

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CELL STRUCTURE

The cell structure

• Cell is composed of a protoplasmic substance surrounded by cell membrane Protoplasm consists of cytoplasm and nucleus. Cells are divided into: 1 - Animal cells 2- Plant cells

Cell wall

• The cells of some bacteria, fungi, plants and algae are surrounded by a wall of Cell wall.
• Cell walls have holes and made up of cellulose fibers , which makes substances and water pass through them easily. Function of cell walls: They support and protect plant cells , and give them their definite shapes.

Plasma membrane

• A thin membrane which surrounds the cell, it separates between the cell components and its external medium.

• Carrier proteins bind to specific molecules, which are then transferred across the membrane after the carrier protein undergoes a change of shape. The passage of glucose into a cell is by a carrier protein.
• Transport proteins use energy (ATP) to transport materials across the membrane. When energy is used for this purpose, the materials are said to be actively transported, and the process is called active transport. The Na+ - K+ pump, for example, uses ATP to maintain higher concentrations of Na+ and K+ on opposite sides of the plasma membrane.
• Recognition proteins give each cell type a unique identification. This provides for a distinction between cell types, between self-cells and foreign cells, and between normal cells and cells infected with viruses. o Recognition proteins are glycoproteins because they have short polysaccharide chains (oligosaccharides) attached. The oligosaccharide part of the glycoprotein extends away from the surface of the membrane.
• Adhesion proteins attach cells to neighboring cells or provide anchors for the internal filaments and tubules that give stability to the cell.
• Receptor proteins provide binding sites for hormones or other trigger molecules. In response to the hormone or trigger molecule, a specific cell response is activated.
• Plasma membranes often contain specific proteins embedded within the lipid bilayer. These proteins are called integral proteins.
• Some integral proteins, such as cell surface markers , emerge from only one side of the membrane.
• Others, such as receptor proteins and transport proteins , extend across the plasma membrane and are exposed to both the cell’s interior and exterior environments. Proteins that extend across the plasma membrane are able to detect environmental signals and transmit them to the inside of the cell.
• Peripheral proteins , lie on only one side of the membrane and are not embedded in it.
• Integral proteins exposed to the cell’s external environment often have carbohydrates attached. These carbohydrates can act as labels on cell surfaces. Some labels help cells recognize each other and stick together.
• Viruses can use these labels as docks for entering and infecting cells.
• Integral proteins play important roles in actively transporting molecules into the cell. Some act as channels or pores that allow certain substances to pass.

• Other integral proteins bind to a molecule on the outside of the cell and then transport it through the membrane. Still others act as sites where chemical messengers such as hormones can attach.

Nucleus

Definition: The largest and most obvious organelle in cell, it has a spherical or ellipsoidal shape. Nucleus consists of: 1 - Nuclear membrane A double layer which separates between the components of the nucleus and cytoplasm. It has nuclear pores through which substances pass between the nucleus and cytoplasm 2 - Nucleoplasm A jelly-like transparent fluid which contains some filaments coiled around each other called chromatins. 3 - Nucleolus There might be more than one nucleolus in cells (especially those which are responsible for secreting proteins like enzymes and hormones). The nucleolus (plural, nucleoli) is the site where DNA is concentrated when it is in the process of making ribosomal RNA. Ribosomes are organelles made of protein and RNA that direct protein synthesis in the cytoplasm.

• Microtubules radiate outward from a central point called the centrosome near the nucleus.
  • Microtubules hold organelles in place, maintain a cell’s shape, and act as tracks that guide organelles and molecules as they move within the cell. Microfilaments
• Finer than microtubules , microfilaments are long threads of the beadlike protein actin and are linked end to end and wrapped around each other like two strands of a rope. - Microfilaments contribute to cell movement, including the crawling of white blood cells and the contraction of muscle cells. Intermediate Filaments
• Intermediate filaments are rods that anchor the nucleus and some other organelles to their places in the cell.
  • They maintain the internal shape of the nucleus.
• Hair-follicle cells produce large quantities of intermediate filament proteins. These proteins make up most of the hair shaft.

Ribosomes

• They are non-membranous round organelles inside the cell which synthesize proteins. They are made of protein and RNA molecules. Ribosome assembly begins in the nucleolus and is completed in the cytoplasm. One large and one small subunit come together to make a functioning ribosome

They exist in two regions of the cell: 1 - In Cytoplasm: They synthesize protein and send it immediately to the cytoplasm to be used in growth and renewal processes. 2 - Attached to the external surface of endoplasmic reticulum: They are more than those in cytoplasm, they synthesize proteins which are send outside the cell through the inner endoplasmic reticulum after modifying them.

Centrosome

• A structure which exists in animal cells (except nerve cell). It is composed of two particles called centrioles
  • There aren't any centrosomes in plant, algae and most fungi cells, but instead, there are parts of cytoplasm that carry out the functions of centrosome.
• Centrioles consist of two short cylinders of microtubules at right angles to each other and are situated in the cytoplasm near the nuclear envelope.
• Centrioles occur in animal cells, where they organize the microtubules of the cytoskeleton during cell division. Function: 1 - It plays an important role in cell division process because spindle fibers between the two centrioles extend between the poles of cell, which helps the cell split into two cells. 2 - It forms cilia and flagella.

Endoplasmic reticulum

• A group of membranous microtubes which permeate all parts of cytoplasm, it is connected with both plasma and nuclear membranes. Function: It transports substances across all parts of cell and form nucleus to cytoplasm.

Then it directs the modified proteins to:

• Parts of cells which may use them.
• Secretary vesicles which expel them from cell in the form of secretions.
• The no. of Golgi bodies inside cell differs according to the secretory activity of cell, The cells of glands have great no. Golgi bodies
• Golgi bodies in plants and algae are called Dictyosomes.

Mitochondria

• A sac-like membranous structure that exists in all cells, it is composed of: 1 - Outer membrane 2 - Inner membrane: It includes folds in it called Cristae 3 - Cristae: They are folds occurring in the inner membrane, they increase the surface area of the medium of chemical reactions occurring inside mitochondria to speed them up Function : 1 - It stores respiratory enzymes in the cell 2 - It stores substances which are formed as a result of oxidation of nutrients such as glucose. 3 - It stores the energy resulted from respiration process in the form of a compound called Adenosine Triphosphate (ATP) from which the cell can release energy again. Mitochondrial DNA
• Mitochondria have their own DNA and can reproduce only by the division of preexisting mitochondria.
• Scientists think that mitochondria originated from prokaryotic cells that were incorporated into ancient eukaryotic cells. This symbiotic relationship provided the prokaryotic invaders with a protected place to live and provided the eukaryotic cell with an increased supply of ATP.

Vacuoles

They are small membranous sacs which exist in cells, they exist with great no. in animal cells, they can accumulate in one or more bigger vacuoles in plant cells.

Function: They store water, wastes and food till the cell use or get rid of them.

Lysosomes

- They are small spherical membranous vesicles formed by Golgi bodies , they contain digestive enzymes within them. Function: 1 - Digestion of nutrients being swallowed by cells and breaking them to simple substances. 2 - Responsible for breaking down cells when it is time for the cells to die. The digestion of damaged or extra cells by the enzymes of their own lysosomes is called autolysis

• White blood cells use Lysosomes in killing microbes.
• Cell is not affected by the enzymes of Lysosomes because their enzymes separated from the other components of cell by a membrane.

Peroxisomes

• Peroxisomes are similar to lysosomes but contain different enzymes and are NOT produced by the Golgi apparatus. - Peroxisomes are abundant in liver and kidney cells, where they neutralize free radicals (oxygen ions that can damage cells) and detoxify alcohol and other drugs.
• Peroxisomes are named for the hydrogen peroxide, H2O2, they produce when breaking down alcohol and killing bacteria. Peroxisomes also break down fatty acids , which the mitochondria can then use as an energy source.

Plastids

• They are membranous organelles which have different shapes and exist in plant cells, there are three kinds of plastids which are classified according to the kind of pigments in them, they are:
  • Leucoplasts: They are colorless plastids which don't contain any pigments.

Viruses

• A virus is a nonliving particle made up of nucleic acid and a protein coat or nucleic acid and a lipid-protein (lipoprotein) coat.
• Even though viruses are not living organisms, they are of interest to biologists because they cause many diseases in living organisms and they are useful tools for genetic research.

Characteristics of viruses

• Viruses are not alive because they lack some of the key characteristics of living organisms.
  • Viruses do not have cytoplasm or organelles.
  • They cannot carry out cellular functions such as metabolism and homeostasis.
  • They do not grow as cells do by dividing in two.
  • Even though viruses do have genetic material, or a genome—either DNA or RNA—they cannot reproduce outside their host cell. They must enter a living cell and use the host cell’s ribosomes, ATP, enzymes, and other molecules to reproduce.

Viral Size and Structure

• Viruses are some of the smallest particles that are able to cause disease. But they vary in size and shape. The shape of a virus is the result of its genome and the protein coat that covers the genome.
• A protein coat, or capsid , is the only layer surrounding some viruses.
  • The capsid of some viruses, such as TMV, forms a helix. The rabies and measles viruses are also helical viruses.
  • The adenovirus capsid has the shape of an icosahedron, a shape with 20 triangular faces and 12 corners. Other viruses with this shape include those that cause herpes simplex, chickenpox, and polio.
  • The influenza virus is spherical in shape.
• Some viruses have a bilipid membrane called an envelope that surrounds the capsid. The envelope is formed from either the nuclear membrane or the cell membrane of the host cell as the viral capsid buds from the host cell.
• Proteins in the envelope help new viruses recognize host cells. Enveloped viruses include the chickenpox virus (varicola virus) and human immunodeficiency virus (HIV), which causes acquired immunodeficiency syndrome (AIDS).

• Reverse transcriptase uses RNA as a template to make DNA , which then inserts into the host cell’s genome. Reverse transcriptase reverses the normal process of transcription, in which DNA serves as a template for producing RNA.
• The host cell’s enzymes transcribe the virus DNA, and cell ribosomes translate the RNA into proteins that become part of the new viruses. Human immunodeficiency virus (HIV) is a retrovirus.

Replication in Viruses That Infect Prokaryotes

• Scientists have gained a better understanding of virus replication by studying bacteriophages , viruses that infect bacteria.
• Bacteriophages, or phages, have complex capsids.
  • Phage capsids are made up of a hexagonal head filled with DNA.
  • Attached to the head is a protein tail with one or more tail fibers.
  • The tail fibers attach the virus to a cell. o The tail helps the virus inject its genome into the host cell.
• The most commonly studied bacteriophages, T phages, infect Escherichia coli, a bacterium found in the digestive tract of many animals and humans.
• Research led to the discovery that many phages and other viruses can reproduce by one or both of two different processes: the lytic cycle or the lysogenic cycle.
• Because viruses are lifeless particles, their spread depends on other agents. A vector is an intermediate host that transfers a pathogen or a parasite to another organism.
  • Vectors of viral diseases include humans, animals, mosquitoes, ticks, and fleas.
• The West Nile virus, a virus that causes fever and headache and, in very rare cases, coma and paralysis, infects mainly birds, such as crows and jays. If a mosquito bites a bird infected with West Nile virus and then bites a human, the virus can be spread. Mosquitoes can transmit several other viruses, such as the yellow fever virus.

Protists

• Protists are a diverse collection of eukaryotic organisms , such as protozoa, algae, slime molds, and water molds. Protists are sometimes described as animal-like, plantlike, or funguslike. However, these organisms lack the cellular differentiation found in animals, plants, and fungi.
• Single-celled or simple multicellular eukaryotic organisms that generally do not fit in any other kingdom are called protists.
• Most protists are microscopic, but a few protists, such as some algae, are several meters in length.
• Protists are defined by exclusion— most protists are eukaryotic organisms that cannot be classified as fungi, plants, or animals.
• As a result, protists are the most diverse group of eukaryotes.

Nutrition

• Protists obtain energy in a number of ways.
  • Many protists are autotrophs , organisms that can make their own food molecules. o These protists make food in much the same way that plants do. The protists absorb energy from sunlight with the aid of specialized light-absorbing pigments. Protists often utilize chlorophyll, as plants do, but they may use additional pigments. The protists use the captured light energy, water molecules, and carbon dioxide molecules to make carbohydrates.
  • Some protists are heterotrophs , organisms that must get their food by eating other organisms or their byproducts and remains.
  • Some heterotrophic protists engulf smaller protists and digest them.
  • Other heterotrophic protists obtain energy in the same way that fungi do. These protists secrete digestive enzymes into the environment. The enzymes break down cells or bits of food into small molecules that the protists can absorb and use.

Motility

• Most protists are able to move at some time during their life cycles.
  • Some protists move with the aid of long, whiplike structures called flagella (singular, flagellum).
  • Other protists move with the aid of cilia (singular, cilium), which are shorter than flagella and often form rows. Finally, some protists, such as amoebas, move by temporarily extending structures called pseudopodia (singular, pseudopodium).

Reproduction

• Many protists reproduce asexually. During binary fission , a single protist cell divides into two cells. Some protists reproduce by multiple fission , a form of cell division that produces more than two offspring. Both types of fission produce offspring that are genetically identical to the parent cell.
• One way by which many protists reproduce sexually is conjugation. During conjugation, two individuals join and exchange genetic material stored in a small second nucleus. Then, the cells divide to produce four offspring. The offspring are genetically different from the parent cells. Many protists can reproduce both asexually and sexually.

Fungi

Fungi are in their own kingdom—kingdom Fungi. They differ from other organisms in many ways, including structure, method of reproduction, and methods of obtaining nutrients.

• Fungi are eukaryotic, non-photosynthetic organisms, and most are multicellular heterotrophs. Most fungi are microscopic molds or yeasts.
  • Molds , such as the fungi that grow on bread and oranges, are tangled masses of filaments of cells.
  • Yeasts are unicellular fungi whose colonies resemble those of bacteria. Yeasts are best known as the microorganisms that make bread rise.
• The study of fungi is called mycology.

Obtaining Nutrients

• Fungi get their nutrients by absorbing organic molecules from their environment.
• Fungi also secrete enzymes into their food and then absorb the digested nutrients through their cell walls.
• Like animals, fungi store energy in the form of glycogen.
• Most fungi are saprophytic —that is, they live on organic compounds that they absorb from dead organisms in the environment. This characteristic makes fungi a very important recycler of organic material in nature.

Structure of Fungi

• Filaments of fungi are called hyphae.
• The cell walls of hyphae contain chitin , a polysaccharide that also makes up the exoskeleton of insects, crustaceans, and other arthropods. The presence of chitin distinguishes cell walls of fungi from those of plants, which have cellulose but no chitin. - A mat of hyphae that forms the body of a fungus is a mycelium.
• In some species, the cells that make up hyphae are divided by cross sections called septa. Hyphae whose cells are divided by septa are called septate hyphae.