Topic 1: Cell Biology

1.1 Introduction to cells

Cell Theory

The Cell Theory defines the key characteristics of cells. While it generalizes well for most cells, there are some exceptions.

Rules:

  • Living organisms are composed of cells

  • Cells are the smallest units of life

  • Cells come from pre-existing cells

Exceptions:

  • Aseptate fungal hyphae

    • challenges that cell is a single unit, since it is a long structure with many nuclei and a shared cytoplasm

  • Giant algae

    • too large, challenging that cells must be simple and small structures

  • Striated muscle cell

    • has many nuclei, challenging that each cell should have a single nucleus

Functions of life and Unicellular organisms

Something is said to be alive when it conducts all these 7 functions:

  1. Nutrition: obtain food and materials for growth

  2. Metabolism: chemical reactions in the cell (e.g.: cell respiration)

  3. Excretion: getting rid of waste products from metabolism

  4. Growth: irreversible size increase

  5. Response: react to changes in environment

  6. Homeostasis: keep conditions tolerable

  7. Reproduction: produce offspring (a)sexually

Unicellular organisms (those consisting of only one cell) must carry out all these functions in that one cell.

Below there are examples of two such organisms:

Surface Area to Volume ratio (SA:V)

Cells sizes range from 1 to 100µm. The reason why they can't grow further is related to the surface area to volume ratio.

Those two quantities are significant because of their roles and different growth rates:

- Surface area - rate of exchange of materials; grows to the power of two (4πrˆ2)

- Volume - rate of metabolism; grows to the power of three (4/3πrˆ3)

Since the denominator grows faster than the numerator in the SA:V ratio, the surface area to volume ratio becomes smaller as cell size increases.

That is a problem because the cell won't have enough surface area to absorb the necessary substrates or eliminate the waste products from the excessive reactions within the cell's volume, causing the cell to malfunction.

Multicellular organisms: emergent properties and cell differentiation

Due to the SA:V limitation, in order for an organism to be larger, it has to be composed by multiple cells.

Multiple cells also allow for certain groups of cells (tissues) to specialize in different functions, by the process of differentiation, at which only some genes are expressed in each tissue.

The interactions of different tissues with different functions give rise to what we call emergent properties. For instance, the retina can sense light and the muscles can perform movement, but when multiple tissues interact they can hunt.

Stem cells

Stem cells are those that haven't yet completely differentiated and specialized, thus, can be used therapeutically to develop into specific tissues, curing diseases and disabilities.

There are different sources of stem cells that have different abilities:

  • Totipotent: first cells following fertilisation, can give rise to any type of cell and to a complete organism

  • Pluripotent: embryonic cells, can give rise to any type of cell, but not to a complete organism

  • Multipotent: umbilical cord or other, can give rise to only a few closely related types of cell

  • Unipotent: specific (adult) organs, can give rise to only directly associated types of cell (e.g.: skin, liver and bone marrow)

Ethical issues:

  • ​Toti- and Pluripotent: embryonic origin

    • Problem: killing of a potential life​

    • But:

      • isn't human yet​ - doesn't have any essential human features

      • has no nervous system

      • IVF (in vitro fertilization) already produces many embryos

      • If produced deliberately, no individual that would otherwise live is having that chance denied

  • Multipotent: umbilical cord​​ or other sources

    • Problem: no consent if donor is too young (e.g.: newborn)

    • But: parents can lawfully consent to a procedure that does no harm

  • Unipotent: person's organs​

    • No ethical issues​

Benefits:​​

  • Transplants obtained without requiring death

  • Treatment of diseases and disabilities

    • Example 1: Stargardt's macular dystrophy (or Stargardt's disease)​

      • Disease: causes progressive loss of central vision due to a genetic mutation that causes malfunction of an active transport protein ​on photoreceptor cells

      • Treatment: embryonic stem cells are attached to the retina. Research has shown this to be a successful method of recovering loss of vision

    • Example 2: Leukemia

      • Disease: type of cancer of the blood or bone marrow, reducing the number of produced white blood cells, causing those affected to have higher risk of infections, anemia and bleeding​

      • Treatment: very common and successful, harvesting multipotent stem cells from the patient or a donor (either from bone marrow, umbilical cord or peripheral blood) and using radio- and chemotherapy to kill of cancerous cells, replacing them by the harvested stem cells.

Magnification

Since biology at times deals with very small images from microscopes or electron micrograph, those images need a "scale", called magnification.

Magnification: (Drawing size)/(Real size)

Scale bars can also be used to determine magnification.

Prokaryotes

Prokaryotes are characterized by a lack of compartmentalization of their simple structure.

Drawings of prokaryotes should include:

  • pili

  • flagellum

  • (peptidoglycan) cell wall

  • plasma membrane

  • 70s ribosomes in the cytoplasm

  • nucleoid (containing naked circular DNA)

They might also include:

  • slime capsule around the cell wall

  • mesosomes (infoldings of the plasma membrane)

  • plasmids (exchangeable small sections of DNA)

Prokaryotes divide by binary fission, process in which the DNA replicates and the cell divides.

Eukaryotes

Eukaryotes are more complex than prokaryotes and they can also be larger, because the compartmentalization of their organelles increases surface area (avoiding the usual SA:V threshold).

Drawings of eukaryotes should include:

  • cell wall (only if plant or fungi cell, made of cellulose and chitin respectively)

  • plasma membrane

  • 80s ribosomes in the cytoplasm

  • nucleus

    • surrounded by nuclear envelope with nuclear pores​

  • mitochondria

  • other membrane bound organelles, such as:

    • chloroplasts​ (only if plant cell)

    • large central vacuole (only if plant cell)

    • lysosomes

    • centrioles

    • Golgi Apparatus

    • vesicles

    • Rough endoplasmatic reticulum (RER) - packages proteins

    • Smooth endoplasmatic reticulum (SER) - packages carbohydrates and lipids

We should know how to identify organelles from specific tissues:

  • Exocrine gland cells of the pancreas:

    • big dark circles: vesicles​

    • darker structure with foldings: mitochondrion

    • lighter structure with foldings: golgi apparatus

    • large dotted structure: nucleus

    • long dark lines: RER - abundance suggests main function of synthesizing and secreting proteins

  • Palisade mesophyll cells of the leaf:​

    • dotted and traced large structure: chloroplast​ - abundance suggests photosynthesis as main function

    • straight, thick darker structure: cell wall

    • wavy line: plasma membrane

    • dark dots on white: free ribosomes

    • wavy line separating two sections of different tones: nuclear membrane

Electron microscopes

Electron microscopes are important for the study of sub cellular structures. They provide bigger resolution (ability to distinguish between two close objects) and magnification than the light microscopes.

 

Phospholipid bilayer

Phospholipids have amphipathic properties (meaning that one side is hydrophobic and the other hydrophilic).

Therefore, they can form a bilayer in water, which is what constitutes membranes.

Fluid Mosaic Model

Within this fluid membrane, there are different components:

  • Membrane proteins: diverse in structure, position and function

    • examples: integral and peripheral proteins; pump or channel proteins​

    • glycoprotein (cited in Syllabus)

  • Cholesterol​: component of animal cell membranes; reduces membrane fluidity and permeability to some solutes (in mammalian membranes)

Models: Davson-Danielli to Singer-Nicolson

Initially, there was the Davson-Danielli model, based on evidence from electron microscopy, such as the image below:

  • dark lines - usually proteins

  • lighter lines - phospholipids

       ⇾ seemed like phospholipids were in the middle of 2 protein layers (Davson-Danielli model)

However, this model had issues:

  • assumed all membranes were identical (which fails to explain how membranes carry ≠ functions)

  • proteins in contact with water (but most of them are hydrophobic)

Additional evidence came along:

  • Freeze-fracture electron micrographs, showing proteins in the middle of the bilayer

  • Hydrophobic proteins should pass through membrane

  • The fusion of colored membranes led to mixed colors (showing there was movement)

Which led to the creation of the Singer-Nicolson model also known as Fluid mosaic model

Passive and Active Transport

A few details beyond the table above are required by the syllabus:

  • Since active transport goes against the concentration gradient, it requires energy (ATP)

  • Channel proteins are specific (only allow one type of protein to pass)

  • "Sodium-potassium pumps"

Referred above as "3Na+ and 2K+ pumps", they serve to maintain a resting potential, in which there are many Na+ molecules outside the axon and many K+ inside. The mechanism is as follows:

  1. 3 Na+ attach to the binding sites inside the axon

  2. ATP donates phosphate group, causing protein to change shape and expel Na+

  3. 2 K+ attach to the binding sites outside the axon

  4. Leads to phosphate detachment, causing protein to change shape and release K+ inside

And it repeats.

  • Potassium channels for facilitated diffusion in axons

Such channels open in response to depolarization of the axon, allowing K+ to diffuse out of the axon.

1.2 Ultrastructure of Cells

1.3 Membrane Structure

PhospholipidBilayer.png
MembraneMicrograph.jpg

1.4 Membrane Transport

 
 
 
 

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