BIOL 202 - Lesson 5: Cells (Part II)
Structure of the Plasma Membrane
Plasma Membrane
Plasma membrane is a membrane that separates cell contents from its environment, and is present in both prokaryotes and eukaryotes (e.g., it is a component of all cells).
The main component of the plasma membrane is the phospholipid bilayer, but along the phospholipid bilayer there are membrane proteins. However, the plasma membrane has more components than the phospholipid bilayer:
Carbohydrates
Sugar chains sticking out of the plasma membrane
Tend to be hydrophilic, which is why they’re on the outside
Tags on cell surface provide molecular fingerprints for different cells (useful for identifying cells for immune system protection)
Blood types
Cancer vs. non-cancer cells
Cholesterol
Embedded inside the phospholipid bilayer
Hydrophobic, which is why it’s on the inside of the plasma membrane
Helps to stabilize plasma membrane in animals
Up to 25% of plasma membranes can be cholesterol
Absent from bacteria, plants, or fungi
Receptor Proteins
Detects chemical or physical signals in the cellular environment
Embedded in the plasma membrane
Signal molecules bind to the receptor protein, and then the protein responds
Examples of signals are flavour chemicals, hormones, and light
When exposed to a stressful situation, the adrenal gland produces adrenaline. Then, adrenaline molecules flow through the bloodstream and attach to adrenaline receptor proteins related to stress mechanisms (e.g., heartbeat or sweat). Once the adrenaline receptor proteins receive the adrenaline molecules in its cavity, it produces the stress response.
When you eat something sweet, sugar enters the sweet receptor proteins on your tongue, and then that protein produces the sweet sensation and makes you happy. Though many animals (like cats) don’t have DNA that codes for sweet receptors.
Recognition Proteins
Some proteins on membrane surface serve as molecular fingerprints (similar to how carbohydrates work)
Recognition proteins are used to distinguish self vs. non-self cells
If immune system recognizes foreign cells, they will be destroyed
Some recognition proteins are used to distinguish different cell types
Important in distinguishing self vs. non self, but can be problematic for organ transplants
Unless you have an identical twin, you have to rely on a partial match for organ donation
Partial matches usually lead to organ rejection (because it is considered a foreign substance), but immunosuppressants can prevent organ rejection
After death, kidney and liver donation while alive, least invasive organ donation process is stem cell donation
Leukemia patients often require stem cell transplant, many die waiting for a suitable match
Register your stem cells to save a life
Transport Proteins
Important to provide a passage for molecules to enter in the cell
Example: Glucose transporter helps cells to incorporate glucose molecules. If you don’t have a glucose transporter, you’ll be unable to bring glucose into the cell. The cell will die, and then you’ll die.
Enzymes
Many enzymes float within the cytoplasm, but some enzymes are embedded within the plasma membrane
Perform chemical reactions either inside or outside the cell (orientation of enzyme depends on what it is making)
Example: plants have a cellulose (in cell wall of plant); in order for plants to synthesize cellulose, outside of the cell membrane, then the enzyme cellulose synthase has to be oriented outwards and also embedded in the plasma membrane
Movement of Molecules: Diffusion
Transport of Molecules Across the Plasma Membrane
The plasma membrane is the gatekeeper of the cell, deciding what molecules enter the cell. Some molecules enter easily, whereas others are blocked. Some molecules enter only through transport proteins.
The transport of molecules can be divided into two ways, depending on whether vesicles (small membrane-bound organelles in the cell) are required for transportation:
Diffusion Example, Before Diving Into Transportation
An important concept to consider is concentration gradient.
Different areas of a solution have different concentrations of the solute
Example: in a glass of water, there are different concentrations of red food dye
A concentration gradient refers to the difference in concentration of a substance between two areas
The larger the concentration difference, the larger the concentration gradient
Diffusion occurs when the solute moves randomly from high concentration areas into low concentration areas until there is no more concentration gradient (e.g., all parts of the solvent are equally concentrated with the solute)
Example: in a glass of water, the red food dye move from high concentration areas (e.g., dark red spots) to lower concentration areas (e.g., just water, or lightly affected areas) until the glass of water is pink
Simple Diffusion in Small Molecules
Simple diffusion is relevant in biology because you can predict molecular movement by using this concept.
Example: Red blood cells in the lungs try to capture oxygen in order to carry it to the other parts of the body. In the lungs, there are more oxygen molecules surrounding the red blood cells than there are within the red blood cells. Due to the process of simple diffusion, oxygen cells will move into the cell.
Small molecules move across the plasma membrane by simple diffusion. This reaction doesn’t require energy, because it follows the natural law (e.g., molecules are moving down the concentration gradient).
Examples of small molecules that can be transported using simple diffusion include oxygen, carbon dioxide, and water
Not all molecules can move across the plasma membrane:
Oxygen, carbon dioxide, and water can pass through the plasma membrane
Although water is a polar molecule with a charge, the molecules of water are small enough that they can still pass through the plasma membrane
Sugars such as glucose (polar molecule), sodium ions (small but has a strong charge), and amino acids (polar molecule) cannot pass the plasma membrane; this is because the plasma membrane has a tendency to repel hydrophilic or charged molecules, because the plasma membrane has a hydrophobic layer in the middle
Glucose is a very important resource for the cell, because that is how we obtain energy. However, glucose is repelled by the plasma membrane because of its polarity (it is charged). So, how is it transported into the cell?
Luckily, the cell has evolved a solution to this problem: transport proteins. Each type of protein transporter transports a certain type of molecule… and that includes glucose (via glucose transporters).
This technically counts as facilitated diffusion because the glucose molecules are moving the concentration gradient down. It is facilitated
It is important to consider the different categories of molecular transportation. Those categories are…
Passive Transport
Molecules follow the law of nature and move down the concentration gradient
No energy is required for this process
So far, simple diffusion and facilitated diffusion are considered to be forms of passive transport
Active Transport
Molecules move against the concentration gradient
This type of transport requires an energy output
Movement of Molecules: Osmosis
Overview
Osmosis is a third type of molecular passive transport that requires no energy. Osmosis can happen when the cell is exposed to molecules that do not pass the membrane, such as glucose or ions. In this scenario, we discount the presence of a transport protein. Just imagine that there’s a cell with the plasma membrane and no (or not enough) transport proteins.
The concentration of the solute exists, but the solute cannot move. Instead of the solute moving across the plasma membrane, water moves. Water will move to reduce the concentration gradient, and will have a major effect on the volume of the cell (e.g., shrinking or enlarging effects).
Solute Terminology
Hypertonic Solution
If the solute concentration in the environment is higher than in the cell, then this solution is said to be hypertonic relative to the cell.
Example: I put a red blood cell in a saltwater solution. The saltwater solution has a higher concentration of salt (solute) than inside the cell, so the solution is hypertonic relative to the cell. The solute (salt) cannot cross the plasma membrane, but water molecules move out of the cell (by osmosis) to try and balance the concentration of water and solute between the inside and outside of the cell. This causes the cell to lose water, and it may shrink as a result.
Example: I place a cell in a concentrated sugar solution (like high-fructose syrup), the concentration of sugar outside the cell will be higher than inside the cell. As a result, water will move out of the cell to try to dilute the external sugar solution, leading to dehydration and shrinkage of the cell.
Example: In individuals with diabetes, especially when poorly, controlled, blood glucose levels can become excessively high (hyperglycemia). As the concentration of the solute (glucose) rises in the bloodstream, the plasma in the blood becomes hypertonic. This causes water to leave the cells and move into the bloodstream to try and dilute elevated glucose levels. The cells in the bloodstream therefore become dehydrated.
This process (osmosis) is trying to achieve a state of equilibrium, but because the outside solution has a higher concentration of solute, the water moves out of the cell to dilute the concentration of the outside solution. The cell loses water and shrinks, and becomes crenated.
Plant Cell | Animal Cell | |
Hypertonic Solution |
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Isotonic Solution
If the solute concentration is the same between the environment and the cell, then this solution is said to be isotonic.
Example: If you place a red blood cell in a solution where the solute concentration is equal to the solute concentration inside the cell, the solution is isotonic relative to the cell.
Example: Intravenous (IV) drips use an isotonic solution of 0.9% saline, which is the same salt concentration as the fluid in our cells, so it doesn’t cause our cells to gain or lose water (e.g., swell or shrink).
In this case, there is no net movement of water into or out of the cell because the concentrations of solute and water are balanced both inside and outside the cell. The water molecules still move in and out of the cell, but at an equal rate, so the cell maintains its shape and size.
Plant Cell | Animal Cell | |
Isotonic Solution |
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Hypotonic Solution
If the solute concentration in the environment is lower than in the cell, then this solution is said to be hypotonic.
Example: If you place a red blood cell in a solution where the solute concentration outside of the cell is lower than the solute concentration inside of the cell, the solution us hypotonic relative to the cell. Water molecules move into the cell via osmosis in an attempt to dilute the solute concentration inside the cell, so water enters the cell and it swells.
Example: If you put a red blood cell into pure water (e.g., salt concentration is higher in the red blood cell), then water will swell the cell.
In this case, water molecules will move into the cell via osmosis, since the concentration of water is higher outside the cell than inside. This influx of water can cause the cell to swell as it tries to balance out the solute concentration. In extreme cases, the cell could burst (a process known as lysis) if too much water enters.
We have looked at animal cells, but what happens to plant cells when placed in a hypotonic environment? Well, the plant cell is surrounded by a rigid cell wall, providing mechanical support to the cell.
When exposed to a hypotonic environment, water will flow into the cell, but the cell will not burst because the cell wall will maintain the integrity of the cell’s structure. The water within the plant cell will exert pressure on the cell wall, and the cell will become turgid.
Plant cells’ turgidity (firmness) is necessary for the plant’s support and growth, and indicate the plant is healthy. However, a dehydrated plant will lose its turgidity and wilt (no longer firm).
Plant Cell | Animal Cell | |
Hypotonic Solution |
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How Much Water Should We Drink?
Your body senses water balance, and adjusts the amount of urine based on water balance. The salt concentration is deducted from your brain and influences the output of urine. Usually your water intake is not an issue, but in intense situations where you don’t think of your body’s needs or don’t have access to water or salt, it can be dangerous.
Not enough water - Matthew Hall died while quad-biking in hot conditions.
Too much water - Zyrees Oliver, a football player died of over-hydration.
Solutions include drinking when you’re thirsty, not forgetting to include salts in your diet, avoiding excess amounts of salt, and getting advice from experts
Movement of Molecules: Active Transport
Overview and Gastric Juice Example
Active transport moves molecules against the concentration gradient, but this kind of transport requires an energy input. Let’s take the example of gastric juice:
Gastric juice is found in the stomach and is very acidic; this acidity is important to digest food and kill harmful bacteria that may have been ingested.
Cells in the lining of the stomach produce H+ ions, which are sent outside of the cell and into the stomach via H+ pumps (proton pumps) in the cells, which is a form of active transport.
The cell uses energy to push the H+ ions it produced into the stomach (which has many more protons than the cell does) via proton pumps, which goes against the concentration gradient.
Adenosine triphosphate (ATP) is used as an energy source for the proton pump to push H+ ions against the concentration gradient.
ATP is a type of energy-rich molecule that can be hydrolyzed and changed into a low-energy molecule known as adenosine diphosphate (ADP). This conversion releases energy, which can be allotted to perform other tasks.
Eventually, the ADP is recycled back into ATP… think of ATP and ADP as working like rechargeable batteries.
Movement of Molecules: Vesicle Transport
Vesicle-Mediated Transport
Remember that for small molecules, no vesicles are used for transportation. However, for large molecules, vesicles are required for transportation. Vesicles are used to either import or export large molecules from the cell.
Endocytosis
If the vesicles are used for importing molecules inside the cell, the process is known as endocytosis. There are three types of endocytosis:
Phagocytosis
Process of cell engulfing other organisms, usually for the purpose of defending against pathogens (viruses or bacteria) or clearing cellular debris (dead or dying cells)
It is worth noting that cells coming from the same individual have a cellular fingerprint that let white blood cells (or other bodily defense cells) know that they shouldn’t attack it
Pinocytosis
Process of cell intaking/drinking large amounts of liquid and simultaneously taking in large molecules
Receptor-Mediated Endocytosis
Process of endocytosis of specific molecules in which the molecules bind to specialized receptors on the cell membrane, allowing for selective uptake of only certain substances from the extracellular environment (like a lock and key mechanism)
Example: Liver cells carry LDL receptors, the LDL molecules bind to the LDL receptors of the liver cell, the plasma membrane forms vesicles and engulfs LDL, and the LDL cholesterol is removed from the bloodstream
Familial Hypercholesterolemia: this is an inherited disorder with a high incidence rate among French Canadians (1/100), in which individuals have a reduced number of LDL receptors and so LDL cholesterol accumulates in the blood and increases the risk of cardiovascular diseases
Exocytosis
If the vesicles are used for exporting molecules outside of the cell, the process is known as exocytosis. Here are some examples:
Secretion by exocytosis
Vesicles fuse with plasma membrane and discharge or secrete materials into the surrounding cellular environment
Example: The digestive enzymes produced by the pancreas are secreted into the small intestine, where the digestion process really happens
Exocytosis as a form of communication
When neurons communicate with each other, they secrete chemicals at the synapse using exocytosis
Tour of the Eukaryotic Cell
Overview and Different Eukaryotic Structure Categories
Eukaryotic cells have different components with different functions, and are more complex compared to prokaryotic cells. Specifically, the membrane-bound organelles of the eukaryotic cell can be divided into three categories:
Endomembrane System (Originate from Membrane Invagination)
The endomembrane system is composed of eukaryotic organelles that resulted from membrane invagination. Here are its major components:
Nucleus
The nucleus is the genetic control centre of the cell. The nucleus is surrounded by nuclear membrane, which provides an additional barrier of defense against foreign molecules
The nuclear membrane houses chromatin, which is the tangled/spread out form of DNA
Within the nucleus lie chromosomes (which look like messy worm structures), which contain DNA and protective proteins
The nucleus has one or more nucleoli within its center, which are necessary structures for ribosome synthesis
Rough Endoplasmic Reticulum
Looks like granular, dotted wall structure close to the nucleus
Electromicroscope image: looks like long lines with dots
Consist of ribosomes to produce proteins, some of which are injected into the rough ER, and then modifying enzymes within the rough ER tweak the injected proteins
Ribosomes on the surface of the rough ER are specialized in producing proteins that will be secreted
Some cells that produce antibodies (proteins that attack invaders) will be packed with rough ER, because it specializes in antibody secretion
Smooth Endoplasmic Reticulum
Looks like coral tube-like structure without any dots
Electromicroscope image: looks like messy lines without dots
Does not contain ribosomes
Synthesize molecules such as lipids
The smooth ER also detoxifies chemicals
The liver specializes in detoxification of chemicals, such as alcohol and drugs… liver cells therefore have a lot of smooth ER, in addition to mitochondria (responsible for producing energy) because of the energy requirements for the detoxification process
Lysosome
These are organelles specialized for garbage disposal and recycling of materials in the cell. For instance, they can recycle old mitochondria to use the material again for other purposes
Lysosomes contain digestive enzymes to destroy cellular materials if necessary; in the case of the phagocytosis of an invading cell, the invader will merge with the lysosomes and the digestive enzymes of the lysosomes will destroy the invader
Golgi Apparatus
Look like bag-like structures stacked together,
Molecules synthesized in endoplasmic reticulum is sent to the Golgi apparatus
Packing center of the cell, inside molecules are modified and ready to be shipped outside of the cell
Proteins are customized into forms that the cell can use, can fold proteins into usable shapes or add other materials onto them like lipids or carbohydrates
The Golgi apparatus produces vesicles in order to transport transformed molecules outside of the cell, bringing them towards the cell membrane and beginning the process of exocytosis
Organelles Originating from Endosymbiosis
These organelles originate from endosymbiosis:
Mitochondria
Look like oval-shaped red beans with dark red lines in it
Bacteria-like organelle that harvests energy from food and generate ATPs (adenosine triphosphates), which is the process of cellular respiration
Has a double-membrane structure (outer membrane serving as a wall, and an inner membrane like inner walls)
The lines inside of the cell are due tot he folding of the inner membranes, which is important for the cellular respiration process
Has its own DNA (circular shape within the inner membrane)
Cytoskeleton
This is the inner scaffold of the cell
Provides structural support and scaffold for cell shape
Control movement of organelles & transport vesicles in the cell, like a railway providing a track for organelles
The cytoskeleton produces movement in certain organisms
Whipping movement of sperm is generated by cytoskeleton
Contraction of muscles is mediated by cytoskeleton
Additional Structures for Plant Cells
Plant cells have all the eukaryotic cell organelles and structures, but have additional structures unique to plants.
Cell wall
Pretty obvious-looking wall structure
Plant cells are rigid because they have the extra structural support of the cell wall, located outside of the plasma membrane
Provide mechanical support and resistance to water stress
Provide protection from insects and other pests
Plant cell walls mainly consist of cellulose, which why eating plants means eating a lot of dietary fibres
Vacuoles
Looks like a big empty cartridge in the cell
Membrane-bound storage sacs to maintain water levels
Created by membrane invagination
Plant cells are often packed with vacuoles, and some animal cells have vacuoles
Vacuoles can be used to store water, food, waste, pigments, or defense molecules
Chloroplasts
Looks like green pea oval-like beans
Bacteria-like organelles derived from endosymbiosis
Harvest energy from light and generate food molecules (photosynthesis)
Have double membrane structure (outer and inner membranes)
Extensive stack of internal membrane system (thylakoids)
Have their own DNA
Contain chlorophyll - green pigment needed for photosynthesis