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Cells and Membranes Unit 2
Learning Goals
Quiz #3:
What are the properties and structures of an enzyme?
Compare and contrast diffusion and osmosis
Analyze scenarios for diffusion and osmosis
Quiz #4:
Identify the structures that make up the plasma membrane
Describe the processes of active and passive transport
Determine when the cell will use either type of transport
Quiz #5:
Identify each of the 12 organelles in the cell by name, picture, and function
(These will be the last quizzes before the midterm)
Human Beings
Human beings are made up of close to 10 trillion cells.
Your brain has close to 1 trillion, 100 billion in nerve cells alone.
Cells have different life spans
Skin cells: 3-7 days
Blood cells: 100-120 days
Nerve cells: lifetime
Cells are specialized
Red Blood Cells do nothing but carry oxygen.
Stem Cells have the ability to develop into any part of the body.
Cells have different sizes
Smallest: granule cells, in the cerebellum
Around 10 billion in an area the size of a golf ball
Largest: Central Nervous System cells which run from the base of your spine to the bottom of your feet (the ones that tickle).
Human Beings
Despite all this variety, cells are amazingly similar.
They all have similar internal structures called organelles
These organelles all perform the same function in every cell
Therefore, cells that make up slugs are nearly identical to the cells that make up pine trees, mushrooms, and humans.
Because of this, it’s important to study cell anatomy (appearance) and physiology (function)
Compound Light Microscopes
Out of the trillions of different cells only a small handful have ever been visible to the naked eye.
Thus, we need a tool to help study them
Anton Von Leeuwenhoek did not invent the first microscope, but he and his colleagues first used the object as a scientific tool.
He, among others, experimented with grinding glass into lenses in order to bend light and make smaller objects appear larger.
Compound light microscopes work by running beams of light through a series of lenses.
These lenses continuously zoom in on a specific target.
The object can be magnified up to 1500 times
Hooke’s Observations
One of Leeuwenhoek’s colleagues was Robert Hooke.
Hooke was looking at a piece of cork under a light microscope when he noticed it was covered in small geometric shapes.
The shapes reminded Hooke of a monastery, and the small rooms that the monks lived in.
These rooms were called “cells.”
Over the next 200 years, scientists took the concept of “cells” as a building block for larger organisms and expanded it.
Schleiden and Schwann, German scientists in the 1830’s, studied different living plants and animals and discovered that cells in living things were also “alive”.
Electron Microscopes
Light microscopes help us to visualize cells and the environment they live in.
Interactions among other cells
Reactions to changes in environments.
They aren’t powerful enough, however, to see what’s inside the cell.
For this, we use an electron microscope
Electron microscopes fire beams of electrons at whatever you want magnified.
The electrons then interact with the structures of the object. These interactions are calculated and displayed by a computer.
The result is a magnification of 500,000 to 1 million times larger than normal.
Electron microscopes have allowed us to study the internal structures and functions of the cell.
Passive Transport
o “Passive” means having no energy or desire.
o Passive transport is when nutrients pass through channel proteins in the plasma membrane without any help or direction from the cell.
o The process of passive transport is called Facilitated Diffusion
o It’s “facilitated” because the particles can’t go through the membrane without proteins (just like a door facilitates your ability to walk through a wall)
o The channel proteins are the exact size and shape for each molecule
o They aren’t just big holes in the membrane
o The cell can move waste or nutrients in and out of the cell based on the concentration gradient
o The benefit of passive transport is it requires no energy.
Active Transport
o “Active” means requiring energy or a desire to perform an action
o In active transport, the cell uses carrier proteins to pass nutrients or waste through the plasma membrane of the cell
o Active transport occurs when the cell needs to move against the concentration gradient, and thus requires energy
o The energy is needed because the molecule would naturally go with the concentration gradient; we’re going against it with active transport
o The plasma membrane has a uniquely-shaped carrier protein for every type of molecule (glucose, amino acids, etc).
Enzymes o An enzyme is a protein that functions as a natural, organic
catalyst o Catalysts speed up chemical reactions without adding any new
elements or chemicals to the reaction
o The reactants in the reaction that the enzyme helps to function are called substrates
o Enzymes allow reactions to occur millions of times faster than would otherwise
o Enzyme equation: o E + S ES E + P
o Enzymes don’t become part of the reaction; they just facilitate the reaction
o The activation energy is the amount of energy required for a reaction to occur
o Enzymes lower the activation energy without changing the reaction
o Enzymes control reactions (they only occur in the presence of enzymes)
o Enzymes reduce the total amount of energy cells use
o Enzymes speed up the metabolic processes of the cell
Induced fit model
A substrate reacts with an enzyme because the substrate is a perfect fit
The location of the enzyme where the substrate fits is called the activation (active) site
When the substrate reacts with the active site, it alters the shape of the enzyme temporarily
This alteration allows the enzyme to act as a catalyst, and only when the substrate is present
Factors Affecting Enzymes
o Substrate Concentration
o In order for an enzyme to cause a reaction, the enzyme and substrate must come into contact
o The more substrates in the cell, the more reactions can occur
o If the active sites of the enzymes are constantly filled, there is no way for the rate to increase
o Temperature
o Warmer temperatures cause molecules to move faster, which means the likelihood of substrate/enzyme contact increases
o If the temperature raises too high, the bonds holding the protein together fail.
o This is called “denaturing”
o pH
o Each enzyme has a narrow pH range where it can function
o Denaturing can also occur if the pH of a cell becomes too low or too high
Osmosis
o Osmosis is the diffusion of water across a selectively permeable membrane
o Unlike other substances in an organism, the diffusion of water is not based on equal numbers of molecules on either side of a membrane
o Instead, water attempts to reach an equal concentration on either side of the membrane
o Concentration is a measurement of the amount of solutes (dissolved solids) within a solvent (liquid).
o If there are more solutes on one side of the membrane, then there needs to be more water on that side of the membrane to maintain the same concentration
o The number of water molecules on each side of the membrane might not be balanced, but the concentration of the water is.
Osmosis
Water will attempt to form an isotonic solution
Isotonic means the concentration is the same either side of a membrane.
Because water will naturally flow toward high solute concentrations, cells can draw water across a membrane by moving solutes in or out of the cell
Cells don’t always necessarily want to be isotonic
Hypertonic is when the solute concentration is higher OUTSIDE the cell, causing water to flow out
This is dangerous. The cell can become dehydrated, or collapse within itself
Meat is preserved with salts using this method
Hypotonic is when the solute concentration is higher INSIDE the cell
If a plant needs water, it can take in solutes to draw water inside the cell
If an cell takes in too much water it risks bursting, or “lysing”
The cell walls of plants are rigid, and they withstand the pressure from the water and prevent it from lysing.
Turgor pressure is the ability for plants to remain erect by bringing in water
Hypotonic
Cell Size
Why does the size of the cell matter? In fact, why are cells so small?
1) Diffusion is slow. If the cell was too big, it would take forever for nutrients to reach organelles at the center of the cell
2) Redundancy. More, smaller cells mean more cells can get damaged/destroyed without harming the entire organism
Organelles
Let’s review the order of life so far.
The simplest, smallest particles on earth are the atoms.
A group of atoms make up a molecule
A group of biomolecules make up an organelle.
Organelles are specialized structures inside the cell that perform a specific function.
Not all cells have the same organelles, or organelles at all.
Cells that contain no membrane-bound organelles are called prokaryotes (pro-carry-oats).
Eukaryotes (you-carry-oats) contain some or all of the organelles we will talk about in this unit.
All multicellular organisms—and a few single-celled organisms—are eukaryotes.
Plasma Membrane
Just like our internal organs need protection from the environment, the cell needs to provide it’s organelles with a safe environment to do their jobs.
The plasma membrane (or cell membrane) is a flexible boundary surrounding the cell.
The membrane prevents harmful substances from entering the cell.
The membrane, although flexible, provides a sturdy surrounding for the cell
The ability to pick and choose which items enter and exit the cell is called selective permeability.
Phospholipids Membranes are made up of molecules called phospholipids.
Phospholipids are chains of lipids attached to a phosphate.
The phospholipids line up head-to-toe in a special formation called a “phospholipid bilayer.”
Bi=2. In other words, a layer of two phospholipids.
Phospholipids resemble a tennis ball (phosphate) with two tails hanging off of one end (lipids)
The lipids are hydrophobic. They do not like contact with water.
The phosphates are hydrophilic. They are able to react and coexist with water.
Cells can survive because the phosphate edges of their plasma membrane can interact with the watery external and internal environments.
Whether a substance is hydrophobic or hydrophilic, it cannot sneak through the entire phospholipid bilayer.
Organelle #1: Nucleus
The plasma membrane is only one small part of the organelles that make up a cell.
For the following organelles you need to know the name, function, appearance of each organelle, and any differences between plant and animal cells.
The nucleus holds the cell’s DNA
Proteins enter the nucleus, read the DNA, then leave the nucleus to perform whatever the DNA coded for.
Inside of the nucleus are three structures:
The chromatin is the actual strands of DNA, spread out into what looks like a giant mesh
The nucleolus is a structure that builds ribosomes.
The nuclear envelope is a small phospholipid bilayer membrane that protects the DNA from the rest of the cell
Nucleus: manages cell functions and contains the DNA blueprint of the cell.
Organelle #2: Ribosomes
Ribosomes are the sites where cells make proteins.
Ribosomes are built inside the nucleolus (which is inside the nucleus).
They then pick up a copy of RNA and leave the nucleus.
Once outside of the nucleus, they can read the RNA sequence and make a protein.
Organelle #3: Cytoplasm
It’s hard to see the cytoplasm. Under a microscope, it looks like the gaps between the other organelles.
Actually, the cytoplasm is a clear, gelatinous fluid.
The cytoplasm (or cytosol) braces every organelle in place
The cytoplasm also allows for movement within the cell
Organelle #4: The Endoplasmic Reticulum
There are two endoplasmic reticulum: The smooth ER and the rough ER
The rough ER is the site where the ribosomes get the supplies for the proteins and modify them
The smooth ER is the site where lipids are made and where they are sent in the cell
Organelle #5: The Golgi Apparatus
The golgi (gole-jee) apparatus is a small bundle of flattened, curved tubes.
The golgi puts proteins into tiny cellular packages and sends them to their destination
Organelle #6: Vacuole
Vacuoles are bubble-like structures located all around the cytoplasm
Vacuoles are used for storing water until needed by the cell.
Animal cells have thousands of small vacuoles. Plant cells typically have one enormous vacuole.
Organelle #7: Lysosome
Lysosomes break apart damaged or old cell parts in a process called “hydrolyzing.”
Lysosomes then carry these to the golgi apparatus, where they will either:
Be carried to the plasma membrane and discharged them from the cell, or
Be recycled to the endoplasmic reticulum
Organelle #8: Chloroplast
Chloroplasts are bright green structures found in plant cells
The green color of plants comes from the millions of chloroplasts in their leaves, stems, etc.
Chloroplasts capture sunlight for photosynthesis to provide the plants with energy
Energy + 6 CO2 + 6 H20 C6H12O6 + 6 O2
Organelle #9: Mitochondria
Mitochondria are found in both plant and animal cells.
Mitochondria are very easy to spot in a cell. They look like kidney or lima beans.
Mitochondria break down carbohydrates and produce energy for the cell.
► 6 O2 + C6H12O6 6 CO2 + 6 H2O + energy
Organelle #10: The Cytoskeleton
We used to think that ribosomes, vacuoles, and all the other organelles in the cell just floated around on their own.
Then we discovered a system of tubes and filaments that the cell uses called the cytoskeleton
The cytoskeleton does two things:
Attaches organelles to the cytoplasm to keep them from moving
Provides a “highway” for transporting organelles throughout the cell
Organelle #11: Cilia
The final two organelles are used to help the cell move in its environment.
Cilia are tiny, numerous extensions that look like hair on the plasma membrane.
The cilia all move together in one swift motion, like a rowboat
Organelle #12: Flagella
Flagella are long extensions of proteins sticking out of the plasma membrane
Flagella help the cell move by whipping back and forth in a fast motion, like a propeller.
