Cell Biology PDF - Introduction to Cells

## 2.1 - Introduction to Cells ### Cell Theory 1. All living things are composed of cells. 2. The cell is the smallest unit of life. 3. All cells arise from pre-existing cells. ### Atypical Examples of Cells: Cells typically function as autonomous units. * **Striated Muscle Cells** are an excep...

## 2.1 - Introduction to Cells ### Cell Theory 1. All living things are composed of cells. 2. The cell is the smallest unit of life. 3. All cells arise from pre-existing cells. ### Atypical Examples of Cells: Cells typically function as autonomous units. * **Striated Muscle Cells** are an exception to this. They often fuse together to form long fibers. * **Cells** are typically microscopic. * **Giant Algae Cells** are an exception to this. Acetabularia cells may exceed 7 cm in length. ### Characteristics of Life: Even unicellular organisms carry out all of the characteristics of life! 1. **Nutrition:** acquires food and materials necessary for growth and survival. 2. **Metabolism:** chemical reactions inside the cell to release energy. 3. **Growth:** an irreversible increase in size. 4. **Excretion:** the ability to react to changes in the environment. 5. **Homeostasis:** getting rid of the waste products associated with metabolism. 6. **Response:** keeping conditions within the organism within tolerable limits. 7. **Reproduction:** produces offspring. **The cell is the essential unit of life.** ## Why Is This a Problem? The surface area (SA) determines the rate of material exchange. The larger the surface area, the more materials can come in and out of a cell at once. The volume (V) determines how many materials are needed. The larger the volume, the more materials the cell needs to function. If a cell gets too big, it won’t be able to take in enough materials to sustain the even larger requirements. **So the cell will die.** ### What Is Happening to the Surface Area to Volume Ratio (SA:V Ratio) as the Cell Gets Larger? As the radius increases, the SA:V ratio decreases. One way to increase the SA:V ratio without making the cell larger is to change the shape. For example, neurons, red blood cells, and muscle fibers all have strange shapes to increase the amount of surface area they have relative to their volume. Certain cells are specialized for material exchange (and therefore have a very high SA:V ratio). For example, cells in your small intestine take in the nutrients from food you eat. **Surface area to volume ratio determines cell size and shape.** ## 2.2 - Cell Size Did you notice that the unit for size in the activity was all meters? This is why the metric system is so useful! No more memorizing 12 inches in a foot! Gross. ### Metric System Prefixes | Prefix | Symbol | Size Compared to Standard Unit | |---|---|---| | deci- | d | 10^-1 1/10th | 0.1... of a meter/gram/etc. | | centi- | c | 10^-2 1/100th | 0.01... of a meter/gram/etc. | | milli- | m | 10^-3 1/1,000th | 0.001... of a meter/gram/etc. | | micro- | µ | 10^-6 1/1,000,000th | 0.000001... of a meter/gram/etc. | | nano- | n | 10^-9 1/1,000,000,000th | 0.00000001... of a meter/gram/etc. | This is called **notation**. Some fields of science use very very large or very very small numbers and writing a ton of zeros is too much work. **Example 1:** A scientist measures a red blood cell at 5 micrometers in diameter. How many centimeters is that? How many nanometers is that? * 0.000005 m = 0.0005 cm = 5000 nm. ### Why Are Cells So Small? Cells are the size they are because it’s the perfect balance of surface area and volume. **As the cell radius increases:** * The **surface area** increases by 10². * But the **volume** increases by 10³. ## 2.3 - Magnification * The human eye can only see things as small as 0.1 mm (100 µm). * Light microscopes use light and magnifying lenses to see objects as small as 0.000002 mm (2 µm). ### To Magnify Means To: * Make something appear larger than it is. * Bigger size / Actual size. ### Magnification The actual size of something in an image can be calculated using the following equation: * **Magnification = Image size / Actual size** **Example 1:** What is the magnification of this image? * **The size of the object in the micrograph (like this one provided) is 14 mm.** * **For example, a red blood cell is typically 6-8 µm in diameter.** * **Therefore the magnification is 14 / .006 = 2333.33x** **Example 2:** Given that the magnification is 12/1, what is the actual length of this creature? * **The image length of the creature is 52 mm.** * **The magnification of the image is 12:1.** * **Actual length = Image length / magnification = 52 mm / 12 = 4.33 mm** **Example 3:** Given that the magnification is 200, how should the scale bar be labeled? * **The scale bar should be labeled 150 nm / 200 = 0.75 **mm.** ### Besides Magnification, Microscopists Must Consider Resolution. * **Resolution** is the shortest distance between two points where the points can still be seen as separate entities. * The maximum resolution of a light microscope is 0.2 µm or 200 nm. This is limited by the wavelength of visible light (400-700 nm). This is why the maximum magnification for a light microscope is usually 400x. ### Electron Microscopes * Electrons have much shorter wavelengths than light, so images from an electron microscope have a resolution about 1000 times higher than a light microscope. **Scanning Electron Microscope (SEM):** * Bounce electrons off a sample and get an image of the surface. **Transmission Electron Microscope (TEM):** * Send electrons through a thin slice of a sample and get an image of the inside (this image is always a cross-section). **Most life on earth is too small to see without microscopes.** ## 2.4 - Prokaryotic Cells * Prokaryotes were the first organisms to evolve and still the simplest are prokaryotes. * All bacteria and archaea are prokaryotes. ### What Is a Prokaryote? By definition, prokaryotes are organisms that * **Lack** a membrane-bound nucleus. * **Instead** of a nucleus, prokaryotes have a nucleoid, a region containing naked DNA in a single uncmebrane-bound chamber filled with cytoplasm. **Most prokaryotes are between 1 µm and 10 µm.** ### Prokaryotic Structures: * **Nucleoid:** Area of the cytoplasm that contains the single DNA molecule. * **Ribosome:** Cell structures responsible for protein production. * **Cytoplasm:** A gel-like substances composed mainly of water that also contains enzymes, salts, and various organic molecules. * **Plasma membrane:** Surrounds the cytoplasm and regulates the flow of substances in and out of the cell. * **Cell wall:** Outer covering that protects and provides structure. * **Capsule:** Polysaccharide layer outside of the cell wall. * **Pili:** Hair-like projection used for attachment and conjugation. * **Flagellum:** Whip-like protrusion that aids in movement. ## Label the Structural Components on the Following Diagrams: * **Diagram 1:** * Pili * Ribosomes * Nucleoid * Capsule * Cell Wall * Plasma Membrane * Flagellum * **Diagram 2:** * Circles * Rods * Spirals. **What Structures Can You Identify From These Electron Micrographs?** * **Micrograph 1:** Nucleoid * **Micrograph 2:** Cell wall, Pili, Flagellum **What Structures Are Labeled?** * **Diagram 1:** * 1. Cell wall * 2. Plasma membrane * 3. Nucleoid * 4. Cytoplasm * 5. Ribosomes ## Make a Simplified, Labeled Drawing of a Prokaryote. * **Nucleoid** * **Cell / Plasma Membrane** * **Cytoplasm** * **Ribosomes** * **Cell wall** * **Pili / Capsule** * **Flagellum.** **Prokaryotic cells are the oldest and simplest form of life.** ## 2.5 - Eukaryotic Cells * Eukaryotic cells have a more complicated internal cell structure than prokaryotic cells. ### What Is a Eukaryote? Eukaryotic cells have **organelles**, which are specialized subunits within a cell that perform specific functions. * The organelles in a eukaryotic cell are all **compartmentalized**, meaning they are separated by single or double membranes. ### Advantages of Compartmentalization * **Substances that could damage the cell can be held safely inside the membrane of an organelle.** * **Conditions such as pH can be maintained at a specific level for a particular process.** * **Organelles can be moved around a cell.** ## Eukaryotic Organelles and Cellular Components ### Nucleus **Primary Function:** Store and protect genetic material. * **One of the few organelles surrounded by a double membrane called the nuclear envelope.** * **The nuclear envelope has holes in it called nuclear pores used to transfer molecules in and out of the nucleus.** * **The nucleolus is a small, dense area inside the nucleus where ribosomes are made.** ### Mitochondria **Primary Function:** Make energy ("the powerhouse of the cell") * **One of the few organelles surrounded by a double membrane is folded to form structures called cristae.** * **The fluid inside the mitochondria is called the matrix.** * **Has its own genetic material (DNA) - called mitochondrial DNA (mtDNA) which in humans is inherited from only the mother.** ### Free Ribosomes **Primary Function:** Make proteins that work in the cytoplasm. * **The only organelle not surrounded by its own membrane.** * **Float freely in the cytoplasm.** * **In electron micrographs, they appear as dark granules about 20 nm in diameter.** ### Rough Endoplasmic Reticulum (RER) **Primary Function:** Make proteins! that work outside the cell or on the cell membrane. * **Made of flattened membrane sacs called cisternae.** * **Called 'rough' because ribosomes are attached to the outside.** * **They are similar to free ribosomes.** * **Ribosomes make proteins, pass them into the cisternae where they are carried in the cisternae to the Golgi.** ### Golgi Apparatus **Primary Function:** Process and package proteins made in the RER. * **Like the ER, made of flattened sacs called cisternae.** * **Unlike the ER, the cisternae are shorter and curved.** * **Send the processed proteins to the cell membrane, or to be either embedded or secreted.** ### Lysosome **Primary Function:** Break down food, worn-out substances, organelles, or even the entire cell! * **Spherical.** * **Formed from Golgi vesicles.** * **Contain digestive enzymes (a type of protein) that perform the primary function.** * **Because they contain high concentrations of enzymes, they always stain very dark in electron micrographs.** * **In practically all animal cells, but basically no plant cells.** ### Smooth Endoplasmic Reticulum (SER) **Primary Function:** Make lipids and hormones! * **Made of flattened membrane sacs called cisternae (like the RER and Golgi).** * **Called 'smooth' because there are no ribosomes attached.** * **Try not to think of it as its own organelle - think of it as a naked RER.** * **Very surface; most common in liver and gonad cells.** ## 2.6 - Membrane Structure * **Phospholipids** a major component of **all** cell membranes. * **Phospholipids** are special because they are **amphipathic**. Part of the molecule is **hydrophilic** and part is **hydrophobic.** * **Hydrophilic** substances are attracted to water. * **Hydrophobic** substances are not attracted to water. ### Phospholipid Bilayer #### Draw a simplified version of a phospholipid: * **Hydrophilic head** * **Hydrophobic tail** #### Draw a simplified version of a bilayer: * **Hydrophilic head** * **Hydrophobic tails** **The primary function of a membrane is to be a semi-permeable barrier.** * **Semipermeable means only certain molecules can go through it.** ### Complete the diagram: * **Nothing large or charged!** * **Small nonpolar molecules.** * **Small uncharged polar molecules.** * **Ions.** **All other functions of the cell membrane are carried out by proteins.** ## Membrane Proteins * Membrane proteins are diverse in **structure** and **position.** ### Big Idea: **Structure Determines Function.** * As proteins perform a variety of functions, it follows that they would have a variety of different structures. * **Integral Membrane Proteins:** * **Permanently attached to the cell membrane.** * Transmembrane proteins have at least one section on each side of the membrane. * **Peripheral Membrane Proteins:** * **Temporarily attached to the cell membrane.** * Either attached directly to a phospholipid or to an integral membrane protein. * Largely hydrophilic. ## Label the Hydrophilic and Hydrophobic Sections of the Proteins Below: * **Protein 1:** * **Hydrophilic** * **Hydrophobic** ## Proteins in the Cell Membrane Perform Important Functions. We’re Going To Discuss Five: * **1. Adhesion:** Membrane proteins allow cells to stick together and stick to the extracellular matrix. Think of the extracellular matrix like scaffolding for the body, giving multicellular organisms structure. * **2. Recognition:** Cells can distinguish one type of neighboring cell from another using glycoproteins. **Glycoproteins** are proteins that have saccharides attached to them. For example, T-cells are a key component of the immune system. They use glycoproteins on their cell membrane to recognize foreign antigens. * **3. Communication:** Membrane proteins can send and receive signals from one cell to another. The signaling molecule is called a **ligand**. The receiving protein is called a **receptor**. For example, neurons communicate by passing neurotransmitters like dopamine to neighboring neurons. Communication can happen between neighboring cells or cells that are far away. For example, insulin (a ligand) is made in the pancreas but insulin receptors are all over the body. Ligands that travel to their receptors through tissue fluids (like blood or sap) are called **hormones.** * **4. Enzymes:** Enzymes are proteins that help chemical reactions occur. Enzymes are responsible for almost all chemical reactions in your body. Their importance cannot be overstated. Enzymes can be free-floating, or embedded in the membrane. * **5. Transport:** Allows molecules to pass through the membrane. **Passive transport** does not require energy - it always moves from high to low concentration. **Active transport** requires energy in the form of ATP. ## Two Kinds: * **Channel Proteins:** Open on both ends. * **Carrier Proteins:** Open at one end at a time. **In addition to phospholipids and proteins, cell membranes also contain cholesterol.** ### Cholesterol * Cholesterol is a kind of lipid known as a steroid. * Cholesterol is amphipathic (just like phospholipids). This makes it fit easily into the cell membrane. * **Cholesterol disrupts packing of phospholipids which prevents the membrane from becoming too rigid.** * **Cholesterol restricts molecular motion, which prevents the membrane from becoming fluid (too loose).** * **Cholesterol reduces membrane permeability to small, hydrophilic particles (like sodium or hydrogen ions).** ### Draw and Label a Simplified Diagram of the Cell Membrane: **Include:** * phospholipids * at least one cholesterol * glycoprotein * channel protein * carrier protein * transmembrane protein * peripheral protein. **This is called the Fluid Mosaic Model and was theorized in 1972.** ### Why Is It Called the Fluid Mosaic Model? * **Fluid:** The bilayer is held together by weak hydrophobic interactions between the tails of the phospholipids. **Individual phospholipids and membrane proteins can move around within the layer but overall the membrane retains a consistent shape.** This makes the membrane the right amount of flexible and fluid. * **Mosaic** A mosaic is a piece of art made out of lots of different embedded pieces. **The cell membrane is made out of lots of different embedded parts as well.** **The structure of biological membranes makes them fluid and dynamic.** ## 2.7 - Membrane Transport ### Chemistry Background * A **solution** is a homogeneous mixture comprised of a **solute** dissolved in a **solvent.** **For example, a solution of sugar water is sugar (the solute) dissolved in water (the solvent).** **Concentration** is the amount of solute per unit volume. ### Complete the diagram below: * **Low concentration** * **High concentration** * **Diluted** * **Concentrated** * **Individual molecules move randomly and unpredictably.** * **However the overall movement of many molecules is certain.** ### Diffusion * **Diffusion** is the overall (net) movement of molecules from areas of high concentration to areas of low concentration. * Careful! Some students think that molecules only move in this direction. This is wrong. Molecules move in both directions but molecules move from high to low concentration. ### What If There's a Membrane? * **Some students think that at equilibrium, no molecules move. This is wrong. At equilibrium, molecules are still moving but an equal number of them are moving to each side. Therefore at equilibrium, there is no net movement.** ### What If There Are Multiple Solutes? * **Each solute diffuses independently of one another until they reach equilibrium** ### There Are Three Types of Diffusion: Simple Diffusion, Facilitated Diffusion, and Osmosis. * **Simple Diffusion:** Certain small hydrophobic molecules such as oxygen can diffuse into the cell directly through the cell membrane. * **Facilitated Diffusion:** Large, hydrophilic molecules and ions cannot diffuse directly across the membrane. They need help from a transmembrane protein. * **Osmosis:** Osmosis is the diffusion of water through a semipermeable membrane. Water molecules can pass through the membrane without help but many molecules (like sugar) cannot. This means that the solute cannot move from low concentration to high concentration to reach equilibrium. The water moves instead. ### Tonicity * **Tonicity** allows us to compare the solute concentration of two solutions. * **Hypotonic:** Lower concentration than the other solution. * **Isotonic:** Same concentration as the other solution. * **Hypertonic:** Higher concentration than the other solution. #### Label the parts of the diagram that are each of the three tonicities: * **Hypotonic** * **Isotonic** * **Hypertonic** * **Remember** the solute generally cannot diffuse across the membrane by itself. So more water will flow from the hypotonic side (lower solute) to the hypertonic side (higher solute) until the solutions become isotonic (same concentration of solute). **Why Does This Matter?** * Cells live in specific environments. If tonicity changes, it can have serious consequences. This is why medicine often uses an isotonic solution instead of pure water. Normal human blood has an osmolarity of about 300mOsm (milliosmoles) and most saline solutions match this. ### All Kinds of Diffusion (Simple, Facilitated, and Osmosis!) * Are **passive transport** because they do not require **ATP.** ## Active Transport * **Active transport** does require energy! Why? Active transport is the movement of solutes against their concentration gradient. **Active transport** moves solutes from low concentration to high concentration with the use of ATP (adenosine triphosphate). **For example, the sodium-potassium pump is an active transporter that uses ATP to pump 3 sodium ions out of the cell for every 2 potassium ions it pumps into the cell. Why do you think it's called a "pump"?** ### In Summary: * **Annotate the diagram with the following labels:** passive transport, active transport, diffusion, facilitated diffusion, and ATP. | Type of Transport | Direction of Movement | Needs a Protein? | Needs ATP? | |---|---|---|---| | Simple Diffusion | high » low | no | no | | Osmosis | high » low | no, but can be made more efficient with proteins | no | | Facilitated Diffusion | high » low | yes | no | | Active Transport | low » high | yes | yes | ## Another Way Things Can Move Across the Membrane Is Inside Vesicles. ### Vesicle Transport * Because the plasma membrane is so fluid and malleable, vesicles can easily be formed simply by pinching off part of a plasma membrane. ### Endocytosis * **Vesicle formation can be used to take materials into the cell. It's often used to intake materials that are too large to pass through the membrane by themselves.** ### Exocytosis * **Vesicles can also be deconstructed by melding with other plasma membranes. This can be used by a cell to release materials** ### Endomembrane System * **Cells are constantly using vesicles to move materials around the cell, and to expel them into the environment. All the organelles that are continuously trading bits of membrane make up the endomembrane system.** * **Vesicles are transported along the cytoskeleton by motor proteins.** **Membranes control the composition of cells by active and passive transport.**