Understand membrane transport, osmosis, ATP, enzymes, and how cells couple chemical reactions.
Part of the free Knowledge Mountain biology library. Read an explanation, try a question, and use a memory cue to check what you can recall.
The plasma membrane is mainly a phospholipid bilayer with embedded proteins. Many components move sideways. Different proteins transport materials, receive signals, or help cells recognize and attach to one another.
The hydrophobic interior blocks many ions and large polar molecules. Small nonpolar molecules, such as oxygen, cross much more readily.
True or false: after diffusion reaches equilibrium, molecules stop moving.
False. They keep moving randomly, but there is no net movement from one side to the other.
Before equilibrium, more particles move from the high-concentration side simply because more are there. At equilibrium, movements in opposite directions balance.
Simple diffusion and facilitated diffusion move substances down their gradients without direct energy input. Facilitated diffusion uses a channel or carrier protein. Active transport uses energy to move a substance against a gradient.
Using a protein does not automatically make transport active. Ask about the direction relative to the gradient and whether energy is required.
An animal cell is placed in a solution with a higher concentration of nonpenetrating solutes than its cytoplasm. Predict what happens.
Water moves out of the cell, so the cell shrinks.
The surrounding solution is hypertonic. Water crosses the selectively permeable membrane toward the side with more effective solute, reducing the water imbalance.
Why can a plant cell become firm in a hypotonic solution without usually bursting like an animal cell?
Water enters, but the cell wall resists further expansion and turgor pressure develops.
The wall limits expansion; it does not stop water from entering. An animal cell lacks this rigid wall and can lyse if too much water enters.
Exocytosis releases cargo when a vesicle fuses with the plasma membrane. Endocytosis brings material in by forming a vesicle from the membrane.
Phagocytosis takes in large particles. Receptor-mediated endocytosis concentrates specific cargo using receptors. These processes require cellular energy.
ATP couples energy-releasing processes to energy-requiring cell work. Hydrolysis to ADP and phosphate can drive transport, movement, or chemical reactions when the processes are properly coupled.
ATP is continually regenerated. It is a short-term energy-transfer molecule, while fats and carbohydrates can serve as longer-term fuel stores. Breaking a bond alone requires energy; the overall hydrolysis reaction releases usable free energy.
A person uses food energy to run. Is all of that energy converted into useful movement?
No. Some becomes movement and other work, and a substantial amount is dispersed as heat.
Energy is conserved, but transformations increase the total entropy of the surroundings and system. Cells need a continuing energy supply to maintain organization.
An enzyme lowers a reaction’s activation energy. It speeds the approach to equilibrium without changing the reaction’s overall free-energy change or equilibrium position.
The substrate binds at the active site, which may adjust shape during induced fit. The enzyme is not used up by one catalytic cycle.
An enzyme works faster as temperature rises, then its rate suddenly drops. What is a likely explanation?
The higher temperature disrupted its shape, causing denaturation and loss of active-site function.
Moderate warming often increases molecular collisions. Excessive heat damages the protein’s functional structure, so collision frequency alone no longer predicts activity.
An inhibitor resembles a substrate and competes for the active site. What type of inhibition is this?
Competitive inhibition.
The inhibitor occupies the place where substrate would bind. In a simple competitive case, adding more substrate can reduce the inhibitor’s effect.
In feedback inhibition, a pathway’s end product inhibits an earlier enzyme. This slows production when the cell already has enough of that product.
The inhibitor often binds away from the active site and changes enzyme function. It helps conserve resources and keeps supplies balanced.