Lecture Notes on Cell Membranes and Transport Mechanisms
Overview of Membrane Proteins and Phospholipids
The outer layer of a cell, called the plasma membrane, is a flexible boundary made of lipids (fats) and proteins. These proteins are vital for many cell jobs, like moving things in and out, communicating, and holding the cell's shape.
Major Types of Membrane Proteins
Transport Channels and Carriers
What they do: These proteins help chosen ions and small molecules cross the fatty membrane. They also create and keep electric differences across the membrane, which are important for nerve signals and other cell activities.
Types:
Channels: These act like tunnels, letting things pass through quickly. Some are always open, while others only open when given a specific signal (like a voltage change or a chemical binding).
Carrier Proteins: These bind to specific molecules, change their shape, and then move the molecules across. This process is slower than using channels.
Example: Ion channels (e.g., potassium, sodium channels) for nerve signals; glucose transporters for sugar entry.
Receptors for Signal Transduction
What they do: Receptor proteins have special spots where signaling molecules (like hormones or signals from other cells) can attach.
Process: When a signal molecule attaches, the receptor changes shape. This starts a chain of events inside the cell, turning the outside signal into a cell action, like changing cell behavior or gene activity.
Cytoskeleton Attachment
What they do: Membrane proteins act as anchors, connecting the cell's outer membrane to its internal skeleton (cytoskeleton). This helps the cell keep its shape, gives it strength, and helps with movement, division, and organelle placement.
Example: Integrins link the cell's outside environment to its inside skeleton, affecting how cells stick together and move.
Enzymatic Functions
What they do: Some membrane proteins are enzymes, which means they speed up specific chemical reactions either within or on the surface of the membrane. This focused activity helps with cell metabolism or strengthening signals near the membrane.
Location: They can be fully stuck in the membrane (integral) or loosely attached to it (peripheral).
Example: Adenylyl cyclase, a membrane enzyme, turns ATP into cyclic AMP (cAMP), an important messenger in many cell communication paths.
Intercellular Attachments (Cellular Adhesion Molecules - CAMs)
What they do: These proteins help cells stick together, forming special connections that are key for keeping tissues intact, communicating, and cell development.
Types of Connections:
Tight Junctions: Create a waterproof seal between cells, stopping liquids from leaking between them (found in gut lining cells).
Desmosomes: Act like strong rivets, holding cells tightly together and giving tissues strength (found in skin and muscle cells).
Gap Junctions: Form small tunnels between animal cells, allowing ions and small molecules to pass directly for quick cell-to-cell communication.
Cell Recognition (Identity Markers)
What they do: Membrane proteins, often with sugars attached (glycoproteins), act like unique ID tags on the cell surface. These tags help cells tell friends from foes, recognize other cells of the same type to form tissues, and are crucial for the immune system to spot healthy cells versus infected or cancerous ones.
Example: MHC (Major Histocompatibility Complex) proteins are vital for the immune system to recognize cells.
Phospholipids
Structure: Each phospholipid has two parts: a head that loves water and two tails that hate water. It's made of:
A water-loving "head": Contains a choline, a phosphate, and a glycerol group. It's polar and easily mixes with water.
Two water-hating "tails": Long chains of fatty acids (16-22 carbons) that are nonpolar and avoid water.
Arrangement in Bilayer: In water, phospholipids naturally form a double layer (bilayer), which is the basic structure of all cell membranes. This arrangement works best because:
The water-loving heads face outward, towards the water outside and inside the cell.
The water-hating tails face inward, forming a dry center that keeps them away from water.
Amphipathic Nature: This dual nature (part water-loving, part water-hating) is key to how the membrane forms and stays stable, creating a barrier that controls what goes in and out.
Plasma Membrane Characteristics
The plasma membrane is selectively permeable, meaning it chooses what substances can enter or leave the cell, helping the cell maintain balance.
How Molecules Cross: How well a molecule crosses the membrane depends on its size, if it's attracted to water (polarity), and its electrical charge:
Small, Nonpolar (Fat-loving) Molecules: Like oxygen (O2), carbon dioxide (CO2), and many fats, can easily pass through the lipid part of the membrane quickly, moving from an area of high concentration to low.
Small, Polar Molecules: Like water (H_2O) and urea, can also squeeze between the lipids. However, special channel proteins called aquaporins greatly speed up water's movement.
Large Polar Molecules (e.g., glucose) and Ions: Because they are big, charged, or surrounded by water, these molecules struggle to pass the fatty middle of the membrane. They need specific transport proteins (channels or carriers) to cross.
Fluid Mosaic Model of the Plasma Membrane
Description: This model, proposed in 1972, says the plasma membrane is like a constantly moving fluid, not a stiff wall.
"Fluid": Means the lipid molecules can easily move sideways, spin, and wiggle their tails. This movement is important for cell growth, movement, and releasing substances.
"Mosaic": Means the membrane has many different proteins (some fully embedded, some attached) scattered throughout the lipid layer, like a mosaic art piece. These proteins also move around within the fluid.
Experimental Proof: An experiment merging mouse and human cells showed this fluidity. The distinct proteins from each cell quickly mixed together over the entire combined cell surface, proving that proteins can move sideways in the membrane.
Fluidity of the Membrane
How fluid the membrane is is vital for cell function and depends on several things:
Saturated vs. Unsaturated Fatty Acids:
Saturated fatty acids: Have straight tails that pack tightly, making the membrane less fluid.
Unsaturated fatty acids: Have bends (kinks) in their tails due to double bonds. These kinks prevent tight packing, making more space between lipids and increasing membrane fluidity.
So, membranes with more unsaturated fatty acids are generally more fluid.
Cholesterol: Acts like a "fluidity balancer" in animal cell membranes.
At warm temperatures (e.g., 37^{\circ}C), cholesterol limits the movement of phospholipids, making the membrane less fluid.
At cold temperatures, it stops phospholipids from packing too closely, helping the membrane stay fluid and not become too stiff or breakable.
Temperature: Higher temperatures increase movement, making the membrane more fluid; lower temperatures make it more rigid.
Active vs. Passive Transport
Substances move across the plasma membrane in two main ways:
Active Transport:
Energy Needed: This process requires energy (ATP) to move substances uphill, against their natural flow, from an area of low concentration to high concentration.
Types:
Primary Active Transport: Directly uses energy from ATP to pump substances across the membrane.
Example: The Sodium-Potassium Pump (Na^+/K^+ ATPase) uses ATP to pump three Na^+ ions out and two K^+ ions into the cell. This creates electric differences important for nerves and cell volume.
Secondary Active Transport (Co-transport): Doesn't directly use ATP. Instead, it uses the energy of one substance moving downhill (e.g., Na^+ or H^+), which was set up by primary active transport, to pull another substance uphill.
Symport: Both substances move in the same direction (e.g., Na^+ and glucose entering gut cells).
Antiport: Substances move in opposite directions (e.g., Na^+ leaving as H^+ enters).
Vesicular Transport: Involves forming and fusing small sacs (vesicles) to move larger molecules or bulk amounts across the membrane. This needs a lot of energy.
Exocytosis: The cell releases large molecules by merging a vesicle with the plasma membrane, sending its contents outside (e.g., hormone release).
Endocytosis: The cell takes in large molecules or particles by forming new vesicles from the plasma membrane.
Phagocytosis ("Cellular Eating"): The cell engulfs large particles or even other cells by reaching out and forming a big vesicle called a phagosome. Common in immune cells.
Pinocytosis ("Cellular Drinking"): The cell takes in surrounding fluid and its dissolved contents by forming small vesicles. This is a general intake process.
Receptor-mediated Endocytosis: A very specific way cells take in certain molecules (ligands) that bind to special receptors on the membrane. This causes the membrane to pinch off into a vesicle with the specific cargo (e.g., cells taking in cholesterol).
Passive Transport:
Energy Needed: No cell energy is required.
Mechanism: Substances move naturally, from an area where they are more concentrated to where they are less concentrated.
Simple Diffusion: Small, fat-soluble molecules (e.g., O2, CO2) directly cross the lipid membrane from high to low concentration until balanced. Speed depends on temperature, concentration difference, surface area, and molecule size.
Osmosis: The movement of water across a membrane from where there's more water (less solute) to where there's less water (more solute). This aims to balance solute concentrations. Aquaporins help water move faster.
Facilitated Diffusion: Polar molecules or ions cross the membrane with help from specific membrane proteins (channels or carriers), still moving from high to low concentration.
Channel Proteins: Provide watery paths for specific ions or small polar molecules, often very quickly.
Carrier Proteins: Bind to specific substances, change shape, and move them. They are specific and can get "full" if too many substances try to bind.
Key Terms: Osmolarity and Tonicity
Osmolarity: A measure of the total amount of solute particles in a solution.
Tonicity: Describes how a solution affects a cell's volume. It depends on solutes that cannot cross the cell membrane.
Hypertonic Solution: Has more non-crossing solutes than inside the cell. Water will leave the cell.
Isotonic Solution: Has the same amount of non-crossing solutes as inside the cell. No net water movement; cell volume stays the same.
Hypotonic Solution: Has fewer non-crossing solutes than inside the cell. Water will enter the cell.
Effects of Tonicity on Cells
How a cell reacts depends heavily on the tonicity of its surroundings:
In Hypertonic Solutions (more external solutes): Water moves out of the cell.
Animal Cells: Shrink and become wrinkled (crenated).
Plant Cells: The cell membrane pulls away from the cell wall (plasmolysis), but the rigid cell wall prevents complete shriveling.
In Isotonic Solutions (equal external solutes): Water moves in and out equally; no net change in cell volume.
Animal Cells: Maintain their normal shape and size, which is ideal.
Plant Cells: Become limp (flaccid); they work but lack full stiffness.
In Hypotonic Solutions (fewer external solutes): Water moves into the cell.
Animal Cells: Swell up and may burst (lyse) because they don't have a rigid cell wall to hold them back.
Plant Cells: Become firm (turgid) as water fills the central vacuole and pushes the membrane against the cell wall. This stiffness is crucial for plants to stand upright.
Application to Different Organisms
Red Blood Cells (Animal Cells): Are very sensitive to tonicity because they lack a cell wall. They work best in isotonic environments (like a 0.9\% salt solution) where they keep their normal shape, preventing shrinking or bursting.
Plant Cells: Thrive in hypotonic environments. Water entering creates turgor pressure, pushing the membrane against the cell wall, giving the plant structure and preventing wilting. In isotonic or hypertonic conditions, plant cells become limp or shrink, respectively.
Protists (e.g., Paramecium): Living in freshwater (a hypotonic environment), they use a specialized organelle called a contractile vacuole to actively pump out extra water and prevent bursting.