#cell structure

Electrolysis of dyes

Cell Structure through a microscope (plant cell and onion cell)

Photosynthesis/Celluar Respiration with a cabomba plant

Fermentation of yeast

Which implies the rough er is the box factory

The ribosomes are the cardboard

02.05.2020 

hello! yesterday has been a productive day for me since I was able to finish my assignment in our biology class and my output turned out so well, I really did not expect that it would turned out like this but yeah was really happy tho. 

for today our class was cancelled since our professor was out of the country and I’m really planning to work on my thesis since i’ve been procrastinating for almost 4 days now lol. 

anyways, hope you’re all having a great day ahead xx

 (The first person to discover and document a living cell was named Anton van Leeuwenhoek, in Holland. Matthias Schleiden in 1838 expanded upon the idea of cells, by concluding all plants are made of cells. In 1839, Theodor Schwann concluded that all animals are made of cells. Then, Rudolf Virchow in 1855 came to the conclusion that “where a cell exists, there must be a pre-existing cell. These conclusions lead to the formation of cell theory. As we understand it now, cell theory states that:

All living things are made of cells

Cells are the basic unit of all organisms

All cells arise from preexisting cells

All cells are enclosed by a membrane responsible for regulating what goes in and out of the cell. They also contain nucleic acid, which contains genetic information that directs the cells activities and controls inheritance. (More on genetics coming soon!) There are two kinds of cells: Prokaryotes and Eukaryotes. 

Prokaryotes have no nucleus or membrane-bound organelles. For example, bacteria are unicellular prokaryotes. 

Eukaryotes have a nucleus and membrane-bound organelles. Most complex life, including us, the pets we keep as companions, the flowers we grow in our gardens, and the worms who keep our gardens alive.

Here is an example of a prokaryotic cell:

Differences between eukaryotic and prokaryotic cells:

Prokaryotic cells do not have membrane-bound organelles, like a nucleus, while eukaryotes have organelles surrounded by a membrane, like mitochondria.

Prokaryotic cells contain a single, circular chromosome, while in eukaryotes, chromosomes are linear. Human body cells can contain up to 46 chromosomes in each nucleus.

Prokaryotic cells can contain plasmids. Eukaryotic cells do not. Plasmids are small DNA molecules within the cell that are able to replicate independently of the chromosomes.

In eukaryotic cells, ribosomes are much larger than in prokaryotic cells.

In prokaryotic cells, respiration is typically aerobic or anaerobic, while in eukaryotic cells, respiration is mostly aerobic.

Cytoskeletal elements like microfilaments and microtubules which are present in eukaryotic cells are absent in prokaryotic cells.

Most prokaryotic cells are unicellular. While some eukaryotic cells, like euglena and paramecium, are unicellular, many are multicellular and specialised.

Eukaryotic cells are much larger than prokaryotic cells. (The mitochondria used to be its own prokaryotic cell before it combined with other prokaryotes to form a eukaryotic cell, to give an idea for scale.)

Most prokaryotes have tough external cell walls. While there are some notable exceptions, most only have a cell membrane.

Moving back to the development of eukaryotes, the theory of endosymbiosis states just that. Prokaryotes came together and formed eukaryotic cells.

Complex organisms have cells specialised to perform certain functions. For example, humans have all kinds of different cell types, each with shapes and structures that help them do their job. Nerve cells have elongated axons, wrapped in a myelin sheath to help transfer electrical signals. Smooth muscle cells contain fibres of actin and myosin that help them move. Adipose cells have massive collections of triglycerides that allow them to insulate, and store energy. Columnar epithelial cells can contain microvilli that help increase the surface area within the intestines, improving digestion. Finally, white blood cells contain numerous lysosomes, allowing them to break down intruders, and dead or corrupted cells. 

(No I did not mean to make the white blood cell look like it has a moustache. He, however, does look very dapper.)

While plant and animal cells look incredibly similar, there are some extremely notable differences that make them distinct. Plant cells have cell walls and chloroplasts. 

While animal cells have centrioles and lysosomes. (In plants, the vacuole does what the lysosome does in the animal cell.)

(Yes, the animal cell drawing because we all know that animals are superior and their cells look way cooler.)

Let’s talk about some organelles. It’s nice to think about organelles as a factory, each with their own job to keep the factory running. I won’t use all organelles in this example, as it may get a little confusing with the organelles needed for replication (although, it’s awesome to imagine a factory going through mitosis. Imagine how convenient that would be?)

Nucleus and nucleolus

The nucleus contains chromosomes made of DNA, wrapped with special proteins called histones, in a chromatin network. Chromosomes contain genes, which are bits of DNA that code for polypeptides. It is surrounded by a selectively permeable membrane, allowing RNA in and out. The nucleolus is a nondividing segment of the nucleus, where ribosomes are made. The nucleoli are not membrane-bound but are tangles of chromatin and ribosomes.

In the factory, the nucleus is the boss. It gives the instructions that messenger RNA bring to the ribosomes.

Ribosomes

The ribosomes are where proteins are made. Inside are rRNA, which puts together instructions from the DNA, and creates amino acids, that are connected and joined into a polypeptide chain.

In the factory, the ribosomes are the workers, following instructions from their boss to create the product.

Endoplasmic Reticulum

The ER is a system of membrane channels that live within the cytoplasm. There are 2 distinct types, with their own jobs. 

The Rough ER

The Rough ER is covered in ribosomes and is the site of protein synthesis and transport. 

The Rough ER is the assembly line, where the workers make the product, and moves the product through the factory.

The Smooth ER

The Smooth ER is responsible for a lot. It synthesises steroid hormones, and other important lipids connect the Rough ER to the Golgi Apparatus, detoxifies the cell, and is the site of carbohydrate (glycogen) metabolism.

The Smooth ER is the overachiever that wants a promotion so bad it starts doing everything. No promotion for you Smooth ER, because no one gets fired, retires or quits in a cell.

Golgi Apparatus

The Golgi Apparatus is a flattened sac of membranes, surrounded by vesicles. They modify, package and store what the Rough ER makes. It also moves these substances to other parts of the cell, and to the membrane for transport outside of the cell.

In the factory, the Golgi Apparatus puts products into boxes, preparing them to be shipped. 

Lysosomes

A lysosome contains hydrolytic enzymes and is enclosed by a single membrane. It is the site of intracellular digestion and helps perform apoptosis, which is programmed cell death. This is essential in embryonic development.

In the factory, the lysosomes are the vat of acid, where the employees who do a bad job are pushed into, in order to keep the factory running. (Or, a trash can.)

Mitochondrion

The mitochondrion is the powerhouse of the cell. (Aka cellular respiration). Cells can have thousands of mitochondria. They are made of an outer double membrane, and a folded inner membrane called cristae. Enzymes used during cellular respiration are embedded in the cristae membrane. They can self-replicate

In the factory, the mitochondria are the electricity, essential in keeping the factory alive and running.

Vacuoles

Vacuoles are membrane-bound structures that store substances for the cell. Some freshwater protista, like amoeba and paramecium, have contractile vacuoles that pump excess water out. Other cells like adipose cells have vacuoles that are designed for storage. 

In the factory, vacuoles are the cupboards, where the excess product is kept. (I don’t think there’s a good example for contractile vacuoles that I can think of.)

Plastids

Plastids are only found in plants and algae. There are 3 types:

Chloroplasts are green because chlorophyll is green. They perform photosynthesis. They have a double outer membrane and an inner one that forms a series of structures called grana, which lie in the stroma. They, like mitochondria, can self replicate. 

Leucoplasts are necessary for storing starch. They do not have colour, and are in roots, like turnips, or tubers, like potatoes.

Chromoplasts store carotenoid pigments that lead to the red-orange-yellow colouring of many plants. They are found in flower petals, which help attract pollinators.

Cytoskeleton

The cytoskeleton is a complex network of protein filaments that extend through the cytoplasm, giving the cell shape and letting it move. It has two structures

Microtubules are thick, hollow tubes that make up the cilia, flagella, and spindle fibres. They are formed from a protein called tubulin.

Microfilaments are made of the protein actin and support the shape of the cell. They are used to form the cleavage furrow when animal cells replicate, to move amoeba by sending out pseudopods, and to allow skeletal muscles to contract by sliding along myosin filaments.

Centrioles and Centrosomes

These organelles are unique to animal cells. They are outside the nuclear membrane and help organise the spindle fibres which are used during mitosis and meiosis. Plant cells have microtubule organising regions which perform similar functions. Two centrioles make up 1 centrosome. Centrioles and spindle fibres have the same structure, 9 triplets of microtubules arranged in a circle.

Cilia and Flagella

Cilia and flagella also have microtubules, arranged in a different way. Cilia are much shorter than flagella, however, both are used for movement.

Cell Wall

Cell walls are not found in animal cells. In fungi, they are made of chitin, and in plants and algae, they are made of cellulose. In plant cells, the primary cell wall is outside the plasma membrane. In some cells, there is a secondary cell wall. In order to replicate, a middle lamella between the 2 cell walls is formed, keeping the daughter cells attached.

Cytoplasm and Cytosol

The cytoplasm is the area between the nucleus and cell membrane. The cytosol is the semiliquid portion of the cytoplasm. In eukaryotic cells, organelles are moved through the cytosol as the cytoplasm cycles. This is a process called cyclosis.

Cell Membrane

The cell membrane is selectively permeable. This means that it only allows certain molecules to pass through. It is called a fluid mosaic, as it is made of many small particles that move around that allow the membrane to be permeable. The membrane consists of a phospholipid bilayer, with proteins dispersed throughout it, and embedded cholesterol giving it stability. Phospholipid molecules have a hydrophilic, polar head, and a hydrophobic, non-polar tail made of fatty acids. Carbohydrate chains on the surface are necessary for cell-to-cell recognition.

Normally, a cell membrane consists of around 60% protein. These do many different things, depending on the protein. For example, ATP synthetase is an enzyme. Some are involved in the sodium-potassium pump bringing ions into and out of the cell. (Remember the salty banana, Sodium ions are normally on the outside of the cell, while potassium ions are normally on the inside of the cell.)

Before I get to cell transport, it’s good to define some important vocabulary.

Selectively permeable: The substances that are able to pass change depending on the needs of the cell. For example, the axons of a neuron contain gated channels that open or close depending on the presence of a stimulus.

Solvent: What a solute is dissolved into. For example, water

Solute: What is dissolved into the solvent. For example, salt.

Hypertonic: Having a greater concentration of solute than another solution

Hypotonic: Having a lower concentration of solute than another solution

Isotonic: Having an equal concentration of solute

Passive Transport

Passive transport is when molecules move along a concentration gradient from an area of high concentration to a region of low concentration. As the name suggests, it uses no energy and is the lazy bum of cell transport. It is done either by diffusion or osmosis. There are 2 different kinds of diffusion.

Simple Diffusion

Simple diffusion is the movement of particles from an area of high concentration to an area of low concentration, through the cell membrane. This is how earthworms breathe, as oxygen diffuses through their skin. Humans breathe a similar way, with alveoli, as oxygen diffuses across them.

Facilitated Diffusion

Facilitated diffusion uses protein channels to help move substances that cannot permeate the membrane. For example, in a neuron, calcium ions cannot cross the membrane, and so require facilitated diffusion.

Osmosis

Osmosis is specifically the diffusion of water across a membrane. Water flows down a gradient towards a region with a high solute concentration. 

Here, cell A is hypertonic to cell B, and cell B is hypotonic to cell A. Placing cells in hypertonic and hypotonic solutions yields some very interesting results.

When a cell is placed in a hypertonic solution, water flows out of the cell, towards the area of higher solute concentration, causing plasmolysis, where the cell shrinks. When a cell is placed in a hypotonic solution, 1 of 2 things can happen depending on the type of cell. Animal cells lyse, which is a fancy way of saying that they burst. Plant cells, because of their wall only swell, or become turgid. Turgor pressure is what keeps a stem standing tall. As the plant loses water, it deflates, loses turgor pressure, and wilts.

Freshwater protista use their contractile vacuole to pump out excess water, as they live in a hypotonic environment.

When a cell is in an isotonic solution, nothing happens, as water flows in and out of the cell at a normal rate.

Active Transport

Active transport moves molecules against their gradient, which means it needs energy. This energy is normally utilised in the form of ATP. 

Exocytosis

Exocytosis is when molecules are actively released from the cell. For example, in neurons, vesicles containing neurotransmitters use exocytosis to move the neurotransmitters across the synaptic cleft to pass an impulse on to the dendrites of the next cell.  

Endocytosis

Endocytosis is the process by which cells take in various molecules. There are 3 different types, pinocytosis, phagocytosis, and receptor-mediated endocytosis

Pinocytosis

Pinocytosis is also given the name cell drinking. The cell takes in large, dissolved molecules. The plasma membrane invaginates small particles and traps them in a vesicle.

Phagocytosis

Phagocytosis is when large molecules or small organisms are engulfed by pseudopods. The cell membrane wraps around the molecule or organism and forms a vacuole. This is how phagocytic white blood cells, like macrophages, engulf pathogens. It is also how amoeba eat. 

Receptor-Mediated Endocytosis

Receptor-mediated endocytosis is important for allowing cells to take up large quantities of a specific substance. Specific extracellular substances bind to receptors on the cell membrane and are then brought into vesicles. This is how cells take cholesterol from the blood.

The sodium-potassium pump is another good example.

All cells carry out specific life processes. These are:

Ingestion: the intake of nutrients

Egestion: Enzymatic breakdown, and hydrolysis of food making it small enough to be assimilated by the body.

Respiration: The process that produces ATP

Transport: The distribution of molecules from one part of the cell to another, or to another cell

Regulation: Homeostasis

Synthesis: The ability to combine small molecules or substances into larger, more complicated ones.

Excretion: Removal of metabolic wastes

Egestion: Removal of undigested waste

Irritability: Ability to respond to stimuli

Locomotion: The ability to move from place to place (not all cells, for example, plant cells, lack this ability)

Metabolism: All processes needed to maintain life.

There are many ways to see and study cells and cell structure. The compound microscope is the most commonly used one to study cell structure. However, phase-contrast microscopes, transmission electron microscopes, and scanning electron microscopes are great for different purposes.

Phase-contrast microscopes are light microscopes used to enhance contrast. They are good for studying living, unstained cells.

Transmission Electron Microscopes are used to study the interior of cells. However, processing kills the cell, and it is extremely expensive, extremely complicated, takes a lot of time.

Scanning electron microscopes are used to study the surface of the cell. The process for preparing these cells also kills the tissue.

Ultracentrifuge is also helpful, as it causes cell fractionation, isolating specific components of the cell, depending on their density. The more dense organelles, like the nuclei, land on the bottom, while less dense organelles, like the ribosomes, stay on top.

Freeze fracture is used to study the membrane structure.

Paul Lorenz