Take a tour inside the cell

Diversity of cells

• Eukarya – Plants, animals, fungi, and protists
• Bacteria – Single-celled microorganisms
• Archaea – Single-celled microorganisms

The four eukaryotic kingdoms are fungi, protists, Animalia, and Plantae. Kingdom Plantae includes plants that manufacture their own food. Kingdom Animalia is for animals that ingest and digest food from external sources rather than making their own. Kingdom fungi are decomposers who decompose organic wastes to obtain nutrients. Kingdom protists are for all other organisms which do not fit in either of the other three kingdoms.

Two main classes of cells:

• Eukaryotic cells – These have a central nucleus, cell membrane, the cells are complex, membrane-bound organelles, and much larger
• Prokaryotic cells – These have no central nucleus, rather, genetic information is located within the nucleoid, no membrane-bound organelles, the cell is simple and much smaller than a eukaryotic cell.

Prokaryotic cells include archaea and bacteria and lack a central nucleus and other membrane-bound organelles. Eukaryotes include plant and animal cells that have a nucleus and membrane-bound organelles. The mitochondria and chloroplast were free-living and were engulfed by prokaryotic cells. This is explained by the endosymbiotic theory.

Differences between eukaryotic and prokaryotic cell

1. Eukaryotic cells are larger than the prokaryotic cells.
2. Eukaryotic cells are more complex than prokaryotic cells.
3. Eukaryotic cells have a true nucleus while prokaryotic cells do not.
4. Eukaryotic cells have specialized structures in the cytoplasm called organelles and are membrane-bound, while the prokaryotic cell has no membrane-bound organelles.
5. Not all eukaryotic cells have a cell wall, while all prokaryotic cells have a cell wall.

Common grounds for all cells

• All cells have a cytoplasm
• All cells have a genetic material
• All cells make proteins for their functions

Customs: Plasma membrane

• This restricts the movement of substances in and out of the cell
• It is made up of two lipid layers (bilayer), with hydrophobic and hydrophilic ends

The plasma membrane is made up of two layers of phospholipids with many proteins embedded in it. The role of the plasma membrane is to regulate entry and exit of molecules through osmosis, and diffusion among other transport processes. Proteins present in the cell membrane act as receptors, enzymes, structural components, and pumps. The cell membrane is described as being amphipathic because it contains hydrophilic heads as well as the hydrophobic tails. The phosphate end is the polar side and the fatty-acid tails are the non-polar.

Constituents

• Phospholipids – Have polar and non-polar ends
• Proteins – Both integral and peripheral
• Sterols – Embedded in the plasma membrane and which controls its fluidity
• Carbohydrates – Acting as receptors and junctions

Within the cell membrane, glycoproteins are used for recognition and acting as receptors and antigens. Glycolipids are attached to the phospholipids together with sugar molecules. Cholesterol helps maintain the fluidity of the cell by preventing its melting in high temperatures and freezing in low temperatures.

Other cellular components

• The cytoplasm – Fluid-like interior of the cell where organelles found floating
• DNA-containing region – Maybe a central nucleus or another region of the cell (nucleoid)

Ribosomes

• The site for protein synthesis made up of ribosomal RNA and proteins.
• Each ribosome has large ribosomal subunits and small ribosomal subunits. Prokaryotic ribosomes are smaller than eukaryotic ribosomes.
• There are the membrane-bound ribosomes, those found on the surface of rough endoplasmic reticulum and the free ribosomes in the cytoplasm

Ribosomes are the site for protein synthesis and are the most numerous organelles in the cell. They can be found floating freely in the cytoplasm or embedded on the rough endoplasmic reticulum. If the protein is to be used within the cell, it is produced by the free-floating ribosomes, and if the protein is to be used outside the cell, it is produced by the ribosomes attached to the rough endoplasmic reticulum.

The endoplasmic reticulum

The endoplasmic reticulum is an extensive network of membranes composed of both rough and smooth regions. It forms the internal transport system of the cell and helps modify proteins and synthesis of lipids. Rough endoplasmic reticulum has no ribosomes, synthesizes lipids, steroid sex hormones, and helps in detoxification of drugs by adding hydroxyl groups to make them soluble and be flushed out of the body. Rough endoplasmic reticulum has embedded ribosomes and is involved in the synthesis of proteins.

Eukaryotic cells

• Cell walls – mechanical strength, not present in animal cells
• Chloroplast – Needed for photosynthesis, and not present in animal cells
• Large central vacuoles – Needed for storage of nutrients, not present in animal cells
• Centrioles –  Needed for cell division, not present in plants

Nucleus

• This contains the genetic material (DNA) in the form of chromosomes or chromatic.
• There are the nucleoli within the nucleus – This is where ribosomal subunits are located and which directs the synthesis of large and small ribosomal subunits
• Surrounded by the nuclear envelope

The endomembrane system

• This is the post office of the cell
• The endomembrane system consists of rough endoplasmic reticulum which synthesizes proteins, smooth endoplasmic reticulum that synthesizes lipids.
• Golgi apparatus is a stack of flattened membrane sacs that package substances, peroxisomes which helps break down lipids such as fatty acid and other molecules, and vesicles that carry molecules around the cell.

All structures of the endomembrane system are Nuclear envelope, plasma membrane, endoplasmic reticulum, lysosomes, vacuoles, and reticulum. The function of the endomembrane system is to synthesize proteins and transport them within and without the cell. Metabolism and movement of lipids and lastly, detoxifying poisons, and drugs.

The mitochondria as the fireplace

• The cell uses mitochondria to make energy through a process called cellular respiration
• The space between the two membranes of mitochondria is called intermembrane space
• The matrix is the inside of the matrix

Mitochondria are called the powerhouse of the cell because it produces most ATP molecules in cellular respiration. Both TCA cycle and Oxidative phosphorylation occurs inside the mitochondria. The number of mitochondria in a cell depends on the type of cell and amount of energy needed by the cell. For instance, the muscle cell needs a large amount of energy and hence more mitochondria than in a cell whose purpose is to transmit nerve impulse only. Mitochondria have two membranes: the outer membrane which covers the organelle and the inner membrane which is folded to form cristae that increase the surface area for cellular respiration.

Chloroplast

• Plant cells and other photosynthetic cells use light to make glucose that is the main source of energy
• Thylakoid is fluid-filled space which has chlorophyll to trap light energy

Fluid-filled space inside the chloroplast is called the stroma, and the stacks of membranes in the stroma form what is known as grana. The stroma contains enzymes that control the process of carbon fixation. Light reactions of photosynthesis provide energy, NADPH, and ATP for light-independent reactions. The general function of the chloroplast is to absorb and convert light energy into sugar molecules that produce ATP and NADPH.

Cytoskeleton

• Microfilaments – Made of the protein actin, helps cells contract, allow cells to crawl and helps in animal cell division
• Microtubules – Made up of protein tubulin, mostly found in cilia and flagella and helps in vesicle movement within the cell
• Intermediate filaments – composed of various proteins and reinforces other proteins.
• Cilia and flagella – Cilia are shorter and more numerous. Cilia beat to move materials and fluid along surfaces.

Cytoskeleton refers to the network of fibers made of protein, which supports the cell and anchors the organelles within the cytoplasm. The three types of cytoskeleton are actin filaments, microtubules, and intermediate filaments. The microtubules are composed of a and b tubulin protein subunits which separate the chromosomes during cell division. The intermediate filaments have overlapping tetramers of protein. Out of the three types, the microtubules are the largest. The four components required so as to move material along the microtubules are a vesicle, motor protein to provide energy-driven motion, microtubules, and the connector molecule.

The cell wall

• Made up of cellulose and for woody plants, lignin
• Fungal cells have chitin
• The bacterial cell wall has peptidoglycan among other components

The three functions of the cell wall are protecting the internal contents of the cell, acting as the skeleton, and regulating the growth of cells. The cell wall is made up of cellulose and pelcin, forming a thick, stiff, and rigid inner primary and secondary wall.

Prokaryotic cells

• Smaller than eukaryotic cells
• Capsule – helps bacteria cells avoid capture by cells of the human immune system and helps attach to surfaces and other cells.
• Pili and fimbriae are used for attachment on surfaces
• Bacteria flagella help the bacterial cell to move around

A prokaryote is defined as a single-celled organism that lacks membrane-bound organelles and whose DNA is found freely floating in the cytoplasm. The oldest prokaryote was formed around 3.5 billion years ago. All prokaryotes have a 70S ribosome, cytoplasm, cell membrane, and peptidoglycan cell wall.

Concept or Term	Definition or Description
Diversity of cells	The classification of cells into different groups based on their characteristics and evolutionary relationships.
Eukarya	One of the three domains of life, including organisms such as plants, animals, fungi, and protists.
Bacteria	Single-celled microorganisms belonging to the domain Bacteria.
Archaea	Single-celled microorganisms belonging to the domain Archaea.
Four eukaryotic kingdoms	Fungi, protists, Animalia, and Plantae; these are the four main groups into which eukaryotic organisms are classified based on their characteristics and evolutionary relationships.
Kingdom Plantae	The kingdom that includes plants, which are multicellular organisms capable of photosynthesis and manufacturing their own food.
Kingdom Animalia	The kingdom that includes animals, which are multicellular organisms that ingest and digest food from external sources rather than making their own.
Kingdom Fungi	The kingdom that includes fungi, which are decomposers that obtain nutrients by decomposing organic wastes.
Kingdom Protists	The kingdom that includes all other eukaryotic organisms that do not fit into the other three kingdoms (Fungi, Animalia, and Plantae).
Eukaryotic cells	Cells that have a central nucleus, cell membrane, complex internal structures called organelles, and are generally larger than prokaryotic cells.
Prokaryotic cells	Cells that lack a central nucleus and membrane-bound organelles, and are generally smaller and simpler than eukaryotic cells.
Differences between eukaryotic and prokaryotic cells	Eukaryotic cells are larger, more complex, have a true nucleus, contain membrane-bound organelles, not all have a cell wall, while prokaryotic cells are smaller, simpler, lack a nucleus and membrane-bound organelles, and all have a cell wall.
Common grounds for all cells	All cells have a cytoplasm, genetic material (DNA or RNA), and produce proteins for their functions.
Plasma membrane	The outer boundary of a cell that regulates the movement of substances in and out of the cell. It is composed of a double layer of phospholipids with embedded proteins.
Constituents	Phospholipids: Lipids with polar (hydrophilic) and non-polar (hydrophobic) ends. Proteins: Integral proteins that span the entire membrane and peripheral proteins that are attached to the surface of the membrane. Sterols: Embedded in the plasma membrane and control its fluidity. Carbohydrates: Act as receptors and junctions.
Cytoplasm	The fluid-like interior of the cell where organelles are found floating.
DNA-containing region	The region of the cell where the genetic material (DNA) is located, either in a central nucleus (eukaryotic cells) or in a nucleoid (prokaryotic cells).
Ribosomes	Organelles responsible for protein synthesis, made up of ribosomal RNA (rRNA) and proteins. They can be membrane-bound or free-floating in the cytoplasm.
Endoplasmic reticulum	An extensive network of membranes involved in protein synthesis, modification, and lipid synthesis. The rough endoplasmic reticulum has embedded ribosomes and is involved in protein synthesis, while the smooth endoplasmic reticulum lacks ribosomes and synthesizes lipids.
Mitochondria	Organelles known as the “powerhouse of the cell” that produce energy (ATP) through cellular respiration. They have two membranes, an outer membrane, and an inner membrane with folded structures called cristae.
Chloroplast	Organelles found in plant cells and some photosynthetic organisms that carry out photosynthesis to convert light energy into glucose. They contain thylakoids, grana, and stroma.
Cytoskeleton	A network of protein fibers (microfilaments, microtubules, and intermediate filaments) that provide structural support, help in cell movement, and facilitate cell division.
Cell wall	A rigid outer layer that provides support and protection to the cell. It is composed of different substances depending on the organism, such as cellulose in plants, chitin in fungi, and peptidoglycan in bacterial cells.
Capsule	A protective layer outside the cell wall of some prokaryotic cells, which helps them evade the immune system and attach to surfaces.
Pili and fimbriae	Hair-like appendages on the surface of some prokaryotic cells that aid in attachment to surfaces.
Bacterial flagella	Long whip-like structures on the surface of some prokaryotic cells that allow them to move.

Two types of cells	Eukaryotic (more complex, with nucleus and membrane-bound organelles); Prokaryotic (simpler, smaller, and without a nucleus)
Plasma membrane	– All cells contain it. <br> – It is a selective barrier for small, nonpolar, and uncharged substances. <br> – As the cell increases in size, volume grows more proportionately than area, so cells split before becoming too large.
Why many cells instead of smaller cells?	Cells increase in volume more than area, so cells split before becoming too large.
Fluid inside a cell	Cytosol, cytoplasm
Chromosomes	Carry genes in the form of DNA
Ribosomes	Organelles that make proteins
Differences between prokaryotic and eukaryotic cells	1) Location of DNA: Prokaryotes lack a nucleus. <br> 2) Other membrane-bound organelles: Prokaryotes lack them. <br> 3) Complexity: Eukaryotes are more complex. <br> 4) Size: Eukaryotes tend to be larger. <br> 5) Presence of flagella/fimbriae: Prokaryotes have them, eukaryotes don’t.
Prokaryotes have	Fimbriae, nucleoid, ribosomes, plasma membrane, cell wall/capsule, flagella
Nucleoid	In prokaryotes, the region where the cell’s DNA is stored, but it is not an enclosed organelle.
Differences between animal and plant cells	Animal cells have centrioles, lysosomes, and extracellular matrix (ECM). <br> Plant cells have a central vacuole, chloroplasts, amyloplast (starch grain), cell wall, and chlorophyll; they perform photosynthesis.
Cell membrane	Also known as the plasma membrane
Composition of membranes	A double layer of phospholipids, along with other lipids (like cholesterol) and proteins
Primary purpose of cell membrane	To control movement in and out of the cell and define the cell from the outside space
The nucleus	– Part of the endomembrane system. <br> – Contains most of the genes. <br> – Has a double membrane. <br> – Has pores that allow mRNA to enter and exit the nucleus. <br> – Pore complex lines each pore and regulates the entrance and exit of proteins.
DNA location	Chromosomes, made up of a material called chromatin (proteins and DNA)
Nucleolus	Produces ribosomes
Ribosome	Notice that there is a space between the small subunit and large subunit. This is where the ribosome assembles proteins.
Transport vesicles	Budd off the endoplasmic reticulum (ER) and other endomembrane organelles. They fuse with their respective membranes and move along microtubule tracks with the help of motor proteins (such as kinesin).
Golgi Apparatus	A stack of flattened sacs
Golgi Function	Packing, sorting, and modification of phospholipids and proteins. Many polysaccharides are made here. Also, storage of lysosome enzymes.
Lysosomes	Membranous sacs containing digestive enzymes. They merge with food vacuoles and digest their contents.