Cell Membranes: Structure, Function, and Transport Mechanisms

What is the primary function of cell membranes?

To create an enclosed space separating the internal cell environment from the external environment.

What are intracellular membranes?

Membranes that form compartments within the cell, such as organelles and vacuoles.

How do membranes control the exchange of materials?

They are partially permeable, allowing selective passage of substances.

What processes allow substances to cross membranes?

Diffusion, facilitated diffusion, osmosis, and active transport.

What role do membranes play in cell signaling?

They act as interfaces for communication between cells.

What is the fluid mosaic model of membranes?

A model that describes the arrangement of biological molecules in cell membranes, highlighting their fluidity and mosaic-like structure.

What are the main components of the fluid mosaic model?

Phospholipids, cholesterol, glycoproteins, and glycolipids.

What is the structure of a phospholipid bilayer?

Two layers of phospholipids with hydrophobic tails facing inwards and hydrophilic heads facing outwards.

How does cholesterol affect membrane fluidity?

It increases fluidity at low temperatures and stabilizes the membrane at high temperatures.

What is the function of glycolipids and glycoproteins in membranes?

They act as receptor molecules and can function in cell recognition and signaling.

What types of receptors do glycolipids and glycoproteins include?

Signaling receptors for hormones and neurotransmitters, receptors for endocytosis, and receptors for cell adhesion.

What are transport proteins and their function?

Proteins that create hydrophilic channels allowing ions and polar molecules to pass through the membrane.

What are the two types of transport proteins?

Channel (pore) proteins and carrier proteins.

How do carrier proteins function?

They change shape to transport substances across the membrane.

What is the significance of the distribution of proteins in the membrane?

It gives the membrane a mosaic appearance and determines the position of proteins.

What happens if hydrolytic enzymes in lysosomes are not compartmentalized?

They would break down most of the cellular components.

What is the hydrophobic core of the phospholipid bilayer composed of?

The tails of phospholipids.

Why are phospholipids important for cell membranes?

They act as a barrier to most water-soluble substances, preventing unwanted molecules from entering the cell.

What is the role of cholesterol in membrane stability?

It increases mechanical strength and stability, preventing breakdown of membranes.

What is the appearance of the membrane when viewed from above?

It looks like a mosaic due to the scattered pattern of proteins within the phospholipid bilayer.

What is the impact of temperature on membrane fluidity?

At low temperatures, cholesterol prevents rigidity; at high temperatures, it prevents excessive fluidity.

What is the role of carbohydrate chains in glycolipids and glycoproteins?

They enable these molecules to act as receptors and participate in cell recognition.

What factors affect the permeability of cell membranes?

Temperature and solvent concentration.

How does temperature affect the fluidity of lipids in cell membranes?

As temperature increases, lipids become more fluid, reducing the membrane's effectiveness as a barrier to polar molecules.

What happens to proteins in cell membranes at high temperatures?

Proteins begin to denature, disrupting the membrane structure and allowing substances to pass freely.

Is the change in membrane fluidity due to temperature reversible?

Yes, if temperatures decrease, lipids return to their normal levels of fluidity.

What effect do organic solvents have on cell membranes?

Organic solvents can increase membrane permeability by dissolving lipids, causing the membrane to lose its structure.

What is the purpose of using beetroot in experiments on membrane permeability?

Beetroot cells contain a dark purple-red pigment, and the higher the permeability, the more pigment leaks out.

What general pattern is expected regarding temperature and membrane permeability?

As temperature increases, membrane permeability also increases.

How does increased temperature affect the movement of phospholipids in the membrane?

Increased temperature gives phospholipids more energy, causing them to move more and become less tightly packed.

What happens to the phospholipid bilayer at high temperatures?

It may start to melt and break down, further increasing permeability.

What effect does temperature have on the conformation of proteins in the membrane?

High temperatures can break intermolecular forces between amino acids, affecting protein specificity and function.

What can cause increased membrane permeability at temperatures below 0°C?

Deformation of channel or carrier proteins and ice crystals piercing the membrane can increase permeability.

What are some limitations of using colorimetry in measuring membrane permeability?

Cuvettes may differ in thickness, and beetroot pieces may not be identical in size, affecting absorbance readings.

What is diffusion?

The net movement of molecules from a region of higher concentration to a region of lower concentration due to random motion.

What is facilitated diffusion?

the passive transport of substances across a cell membrane down their concentration gradient with the help of transport proteins

What is the role of kinetic energy in diffusion?

Random movement of molecules or ions is caused by their natural kinetic energy.

What is the expected outcome of diffusion across a cell membrane over time?

Molecules or ions tend to reach an equilibrium situation, where they are evenly spread within a given volume.

What apparatus is used to investigate the effect of temperature on membrane permeability?

Scalpel, cork borer, cutting board, ruler, test tubes, water baths, stopwatch, and colorimeter.

What is the significance of using equal-sized beetroot cubes in experiments?

Equal surface areas and volumes ensure that the rate of pigment leakage is consistent across samples.

What types of molecules can only cross the phospholipid bilayer with the help of proteins?

Large polar molecules such as glucose and amino acids, and ions such as sodium (Na+) and chloride (Cl-).

What is the process called that allows certain substances to cross the phospholipid bilayer with the help of proteins?

Facilitated diffusion.

What are the two types of proteins that enable facilitated diffusion?

Channel proteins and carrier proteins.

What is the primary function of channel proteins?

To allow charged substances, such as ions, to diffuse through the cell membrane.

How do gated channel proteins control the exchange of ions?

They can open or close their pores by moving part of the protein on the inside surface of the membrane.

What distinguishes carrier proteins from channel proteins?

Carrier proteins can switch between two shapes, allowing them to transport molecules across the membrane.

What determines the direction of movement of molecules during diffusion?

The relative concentration of the molecules on each side of the membrane.

What is the definition of diffusion?

The movement of molecules from an area of high concentration to an area of low concentration.

What is the role of ATP in active transport?

ATP provides the energy required for carrier proteins to change shape and transport molecules or ions against their concentration gradient.

What is the main difference between facilitated diffusion and active transport?

Facilitated diffusion does not require energy, while active transport does.

What is Visking tubing commonly used for in experiments?

To investigate the rate of diffusion through a partially permeable membrane.

What happens to starch and glucose when placed in Visking tubing filled with a mixture of both?

Only glucose diffuses out of the tubing, while starch remains inside.

How can the concentration gradient affect the rate of diffusion?

A steeper concentration gradient increases the rate of diffusion.

What is the significance of surface area to volume ratio in diffusion?

A higher surface area to volume ratio allows for faster rates of diffusion.

What happens to the rate of diffusion when an agar cube increases in size?

The diffusion rate decreases because the volume increases faster than the surface area.

What is the purpose of using a colorimeter in diffusion experiments?

To quantitatively measure the concentration of glucose that has diffused into the surrounding solution.

What is the effect of using a higher molarity acid in diffusion experiments with agar?

It allows for monitoring the diffusion of acid into the agar by observing a color change.

What is the role of carrier proteins in active transport?

They are specific for particular molecules or ions and facilitate their movement against a concentration gradient.

What is the energy source for active transport?

ATP (adenosine triphosphate) produced during respiration.

What is the primary function of ATP in cellular processes?

ATP is hydrolyzed to release energy.

What type of protein undergoes a conformational change during active transport?

Carrier proteins.

What is active transport important for in the kidneys?

The reabsorption of useful molecules and ions into the blood after filtration.

How does active transport assist in digestion?

It aids in the absorption of some products of digestion from the digestive tract.

What role does active transport play in plants?

It facilitates the loading of sugar from photosynthesizing cells into phloem tissue for transport.

What is the process of loading inorganic ions from soil into root hairs called?

Active transport.

What is the difference between carrier proteins and channel proteins in active transport?

Carrier proteins undergo conformational changes, while channel proteins do not.

What is endocytosis?

The process by which the cell surface membrane engulfs material, forming a small sac around it.

What is phagocytosis?

The bulk intake of solid material by a cell, with specialized cells called phagocytes.

What are phagocytic vacuoles?

Vacuoles formed during phagocytosis that contain engulfed solid material.

What is pinocytosis?

The bulk intake of liquids by a cell.

What is micropinocytosis?

A form of pinocytosis where the vacuole formed is extremely small.

What is exocytosis?

The process by which materials are removed from or transported out of cells.

How are substances packaged for exocytosis?

They are packaged into secretory vesicles formed from the Golgi body.

What happens to secretory vesicles during exocytosis?

They travel to the cell surface membrane, fuse with it, and release their contents outside the cell.

What type of transport is required for endocytosis and exocytosis?

Both processes require energy and are forms of active transport.

What is osmosis?

The diffusion of water molecules from a dilute solution to a more concentrated solution across a partially permeable membrane.

What does a partially permeable membrane allow?

It allows small molecules like water to pass through but not larger molecules like solute molecules.

What is water potential?

The tendency of water to move out of a solution, with dilute solutions having high water potential and concentrated solutions having low water potential.

What is the water potential of pure water at atmospheric pressure?

0 kPa.

How does the concentration of solutes affect water potential?

Any solution with solutes will have a water potential lower than 0 kPa, resulting in negative values.

Why can water pass through the phospholipid bilayer?

Because water molecules are small enough to pass between phospholipids in the cell membrane.

What is the significance of a more negative water potential?

It indicates a lower water potential, further away from pure water.

How do animal cells respond to a hypertonic environment?

Animal cells lose water, shrink, and may undergo crenation, which is usually fatal.

What happens to animal cells in a hypotonic environment?

Animal cells gain water, swell, and may burst in a process called cytolysis.

What is the outcome for animal cells in an isotonic environment?

There is no net movement of water, and the cells remain unchanged.

How do plant cells respond to a hypertonic environment?

Plant cells lose water, causing the protoplast to shrink away from the cell wall, a process known as plasmolysis.

What occurs in plant cells when placed in a hypotonic environment?

Water enters the plant cell, causing it to swell and become turgid, which is important for plant support.

What is turgidity in plant cells?

The state of being swollen and firm due to water entering the vacuole, which pushes against the cell wall.

What is plasmolysis?

The process where the protoplast of a plant cell shrinks away from the cell wall due to loss of water.

Why is maintaining constant water potential important for animal cells?

To prevent cell shrinkage or bursting due to osmotic pressure changes.

What is the difference between the effects of osmosis on animal and plant cells?

Animal cells can undergo crenation or cytolysis, while plant cells experience plasmolysis or turgidity due to their cell wall.

What is the role of the cell wall in plant cells during osmosis?

The cell wall provides structural support and prevents the cell from bursting when water enters.

What is the significance of a plant being turgid?

Turgidity helps the plant maintain its upright structure and allows leaves to capture sunlight effectively.

What happens to plant cells in an isotonic environment?

The movement of water into and out of the cell occurs at the same rate, resulting in no change to the cells.

What is the consequence of a plant not receiving enough water?

The plant cells cannot remain turgid, leading to wilting.

What is the difference between a partially permeable membrane and a freely permeable cell wall?

A partially permeable membrane allows only certain molecules to cross, while a freely permeable cell wall allows all substances to pass through.

What is the main reason animal cells do not undergo plasmolysis?

Animal cells do not have a cell wall; thus, they cannot plasmolyse.

What is the effect of a concentrated sucrose solution on animal cells?

It causes water to leave the cells, leading to crenation.

What is cytolysis?

The bursting of an animal cell due to excessive water intake in a hypotonic environment.

What occurs during the process of osmosis in plant cells when placed in pure water?

Water enters the cell, causing the protoplast to expand and exert pressure against the cell wall.