BIO B2.2-2.3 - Cell Specialization and Stem Cells

Animal Cell Structure

• Plasma/cell membrane - controls what enters and exits the cell, made of phospholipids, encloses a compartmentalized cytoplasm

• Cytoplasm - most of the metabolism that occurs in the cell happens in the cytoplasm, composed mostly of water

• Nucleus - contains chromosomes made of DNA bound to histone proteins, contained in a double membrane with pores

• Mitochondria - responsible for aerobic respiration/ATP production, has a double membrane

• Smooth Endoplasmic Reticulum - involved in lipid synthesis and detoxification, single membrane without ribosomes attached

• Rough Endoplasmic Reticulum - site of protein synthesis, involved in transporting proteins to Golgi apparatus, made of phospholipid membrane, studded with ribosomes

• Golgi Apparatus - modifies and packages proteins for export out of cell, made of phospholipid membrane

• Secretory Vesicles - specialized vesicles that move molecules outside o the cell through exocytosis.

• 80S Ribosomes - needed for protein synthesis

What are organelles?

They are specialized structures that compartmentalize different processes, allowing for greater efficiency.

What do organelles provide?

They provide a protective environment for specialized activities in the cell.

What do organelles do?

They are able to store and use concentrations of metabolites and enzymes which are not compatible with metabolism occurring within the cytoplasm of a cell.

Are the cell wall, cytoskeleton, and cytoplasm considered organelles?

No

• Cell wall is an extracellular structure.

• Cytoplasm is the gel-like fluid spread throughout the cell and it doesn’t have a compartmentalized structure.

• Cytoskeleton is found throughout the cell, but doesn’t have a compartmentalized structure.

Are the nuclei, vesicles, ribsomes, and the plasma membrane considered organelles?

Yes

What is cell fractionation?

It is a process which separates cell organelles while preserving their functions and prepare cells for ultracentrifugation.

What are the steps of cell fractionation?

1. Homogenization: Tissue, containing cells is broken up in a blender. The cells are blended in a cold, buffered solution which is isotonic to the cytoplasm of the cells.

2. The blended solution is filtered to remove large cell debris and you are left with the organelles.

What is ultracentrifugation?

Ultracentrifugation uses a fast centrifuge to separate the cell organelles according to density.

What are the steps of ultracentrifugation?

1. The filtered solution containing the cell organelles is spun at low speed by an ultracentrifuge.

2. The densest organelles (nucleus) form a pellet at the bottom of the centrifuge tube.

3. The pellet is removed, and the process is repeated at faster speeds, producing a series of pellets containing one type of organelle each time.

What is the plasma membrane?

It is a phospholipid bilayer which surrounds and encloses the cell. It controls what enters and exits the cell. It is considered an organelle even though it is not a subcellular structure.

NOS: Students should recognize that progress in science often follows the development of new techniques. What is an example of this?

The study of the function of individual organelles became possible when ultracentrifuges had been invented and methods of using them for cell fractionation had been developed.

What is stored in the nucleus?

Eukaryotic cells store their chromosomes in the nucleus., the chromosomes contain the genetic information in the form of DNA, for the growth and development of the cell.

What occurs in the nucleus?

Genes in chromosomes are transcribed into mRNA within the nucleus. The mRNA leaves the nucleus to be translated into polypeptides by ribosomes. However, post-transcriptional modification of mRNA can happen before the mRNA meets ribosomes in the cytoplasm.

What are the advantages of the separation of the nucleus and cytoplasm?

• DNA is protected in the nucleus from potentially harmful reactions in the cytoplasm.

• Transcription of DNA to mRNA occurs in the nucleus and is modified before entering the cytoplasm.

• The nucleus separates the process of transcription from metabolism occurring in the cytoplasm of the cell.

What does transcription and translation look like in prokaryotes?

Prokaryotes don’t have a nucleus, which means there is no separation of the processes of transcription and translation, so they happen at the same time. As mRNA is being transcribed, ribosomes attach to the mRNA and translation can begin.

What are lysosomes?

They are membrane bound organelles which contain digestive enzymes.

What is the main function of lysosomes?

The digestion of macromolecules and other toxins within the cell.

What do the digestive enzymes in lysosomes do?

The digestive enzymes within a lysosome are capable of digesting other cell components, and are kept separate from the cytoplasm and other cell structures by the membrane surrounding lysosomes.

What happens when a cell is damaged?

The enzymes are released from lysosomes, resulting in the digestion of the cell.

What is phagocytosis?

A process by which solid materials, such as bacteria, are taken into a cell by endocytosis.

How does phagocytosis work?

1. A bacterium is taken into the cell by endocytosis, forming a phagocytic vacuole.

2. Lysosomes fuse with the phagocytic vacuole to form a phagolysosome.

3. The lysosome’s enzymes digest the bacterium and it fuses with the plasma membrane.

4. Then it leaves through exocytosis.

What are the specialized cells that have varying sizes?

Sperm, Ova (egg cells), Nuerons, Red blood cells, White blood cells, and Striated muscle fibers.

What size adaptation do sperms have?

They are the smallest cells in the body with a length of 50-70 micrometers, a width of 2-3 micrometers, and a very small volume; helps it swim to the egg.

What size adaptations do egg cells (Ova) have?

They are the cells with the largest volume, a diameter of ~100 micrometers, and can store large amounts of food reserves; once the egg is fertilized it can grow and develop.

What size adaptations do neurons have?

They have ranging widths from 4-100 micrometers, and can have a length over 1m; effectively transmits nerve impulse.

What size adaptions do red blood cells have?

Have a diameter of 6-8 micrometers, indented on both sides, only 1 micrometer in the middle, and a very narrow middle; allowing them to fit and move through capillaries, transporting CO2 and O2.

What size adaptions do white blood cells have?

Range from 6-20 micrometers depending on the type and function of white blood cell.

What size adaptations do striated muscle fibers have?

They are formed from the fusion of several cells; range in length from a few millimeters to several centimeters, diameter of 10-100 micrometers, contract with great force in unison they need a lot of ATP, so they are multi-nucleated.

NOS: What are scientific models?

They are simplified versions of complex systems.

NOS: What can be used to model cells to identify the relationship between surface are of a cell and volume of a cell as the cell grows?

Cubes can be used to model the surface area to volume ration. If you divide a large cube, it should be clear that the volume of the playdough remains the same, but total surface area increases.

What happens to the surface area to volume ratio as the cell grows?

As the cell grows, the volume of the cell increases at a faster rate than the surface area because it is a function of cubes. The surface area to volume ratio decreases as the cell grows.

Specialized cells involved in the exchange of materials have adaptations to…

increase surface area.

What adaptations do erthrocytes (red blood cells) have?

They are thin and flat with a biconcave shape, which increases their surface area for the exchange of gases.

What adaptation does the proximal convoluted tubule have?

The cells lining the proximal convoluted tubule include extension of the cytoplasm known as microvilli. These increase the rate of absorption of nutrients and water as filtrate passes through the nephrons of the kidney.

What type of tissue is alveolar epithelium?

It is an example of a tissue where more than one cell type is present, because different adaptations are required for the overall function of the tissue.

Why does alveolar tissue have adaptaitons?

It consists of two types of cells to adapt for the rapid exchange of gases.

What adaptations do type I pneumocytes have?

They are long and extremely flat cells adapted for gas exchange.

What adaptations do type II pneumocytes have?

• They are cuboid-shaped cells which secrete a surfactant that reduces surface tension in the alveoli and provides a liquid for rapid diffusion and gases.

• They contain many many secretory vesicles (lamellar bodies), which release surfactant by exocytosis.

What do muscles do?

They contract are involved in movement.

What are the three types of muscle cells?

• Cardiac muscle

• Striated muscle (skeletal muscle)

• Smooth muscle

What do striated muscles do?

They are attached to the skeleton and are involved in the movement of bones.

What do cardiac muscles do?

They are located in the heart and are responsible for the continued rhythmic beating of the heart.

What do both the striated and cardiac muscles contain?

Both muscles types contain contractile myofibrils.

What adaptation does cardiac muscle have?

Cardiac muscle cells are branched and connected by intercalated discs to other muscle cells. This allows for the rapid transmission of electrical impulses through heart muscle tissue.

What triggers the contraction of the cardiac muscle cells?

Electrical impulses

What are intercalated discs?

They are gap junctions between cells and allow ions to flow between two cells.

What adaptation does striated muscle have?

Striated muscles are composed of long multinucleated fibers that are formed by the fusion of cells. Since they are formed by the fusion of cells, it is debatable if they should be considered cells.

What are gametes?

They are reproductive cells. Male gametes are sperm and female gametes are ova (eggs). They have haploid nuclei.

What happens when gametes fuse during fertilization?

A diploid zygote is formed.

What are sperms?

They are small cells with a streamlined shape, adapted to swimming; they are composed of three sections, a head, a midpiece, and a flagellum (tail).

What adaptations do sperms have?

The head of the sperm contains a haploid nucleus and an acrosome.

• The haploid nucleus contains the paternal genetic information which combines with the maternal genetic information at fertilization.

• The acrosome contains hydrolytic enzymes, which help the sperm to fertilize the egg.

The midpiece contains many mitochondria that provide the sperm with ATP energy to swim.

• The flagellum allows the sperm to swim through the female reproductive system to reach the egg.

What are eggs (ova)?

They are large cells that contain a haploid nucleus with the maternal genetic information which is combined with the male genetic information at fertilization.

What adaptations do eggs have?

• The ovum is surrounded by a glycoprotein matrix, known as the zona pellucida, which prevents polyspermy after fertilization.

• The ovum contains many vesicles known as cortical granules which make the zona pellucida impenetrable to sperm after fertilization.

• The cytoplasm of the egg contains many lipid droplets which provide the developing embryo with energy after fertilization.

What are stem cells?

They are undifferentiated cells which have an unlimited capacity to divide, and can differentiate into specialized cells.

What is a stem cell niche?

Stem cell niches are locations in the body where stem cells can be maintained or promoted to proliferate and differentiate.

What are examples of stem cell niches?

• Bone marrow is a niche for hematopoeitic stem cells which can differentiate into all types of blood cells.

• Hair follicles contain various pools of stem cells, such as epithelial, melanocyte, and mesenchymal stem cells that self renew, differentiate, regulate hair growth, and maintain skin homeostasis.

What are totipotent stem cells?

They can differentiate into all types of cells or develop into an embryo. Ex. zygotes are totipotent.

What are pluripotent stem cells?

They can differentiate into all types of cells, but not develop into an embryo. Ex. embryonic stem cells are pluripotent.

What are multipotent stem cells?

They can differentiate into a limited range of cells. Ex. Adult stem cells are multipotent and stomach lining or intestinal cells.

What do adult stem cells do?

They replenish dying cells and repair damaged tissue.

What is fertilization?

It is the fusion of gametes, which leads to the development of a zygote.

What does the zygote develop into?

It develops into a blastocyst, 32 cells, through cell division over five days. The blastocyst contains embryonic stem cells.

Why can’t embryonic stem cells differentiate into an embryo?

They are pluripotent; there is no placenta at this time which is how food and oxygen transfers to embryo, but pluripotent cells can’t make placenta, so there is no human yet.

What does the blastocyst develop into?

A fetus, as the embryonic stem cells differentiate into specialized cells and tissues in a controlled manner through a group of gene regulating chemicals known as morphogens.

What are morphogens?

They are group of gene regulating chemicals which determine the specialized cell that develops according to their concentration. They are produced and released from embryo cells, and diffuse through tissues.

What happens to the morphogens?

They spread, creating a concentration gradient across tissue, with the highest concentration near the source cells. Morphogens bind to receptors on the cells resulting in the activation or repression of genes. The concentration of the morphogen determines which genes will be expressed determining the type of specialized cell.

What is cell differentiation?

All cells from a multicellular organism have the same genome. Stem cells differentiate into specialized cells by expressing some genes and not expressing others.