Cell Biology: Cell Membrane Structure

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Questions and Answers

What role does cholesterol play in the fluidity of the cell membrane?

It restricts movement of phospholipids at warm temperatures. (correct)

It increases fluidity at all temperatures.

It has no effect on membrane fluidity.

It decreases fluidity only at lower temperatures.

Which statement correctly describes the structure of the cell membrane?

It is rigid and does not allow for any movement.

It is a fluid mosaic of lipids and proteins. (correct)

It consists solely of proteins embedded in a lipid layer.

It is a stable structure that does not change.

What explains why saturated fatty acids make the membrane less fluid?

They are always found in liquid form.

They are only hydrophobic.

They contain multiple double bonds.

Their straight chains allow maximum interaction. (correct)

What happens to the fluidity of the membrane when unsaturated fatty acids are present?

It increases due to bent chain structure. Signup and view all the answers

Which of the following correctly describes amphipathic molecules?

They contain both hydrophilic and hydrophobic properties. Signup and view all the answers

Which component of the cell membrane acts as an identification tag?

Carbohydrates Signup and view all the answers

Why are saturated fats often referred to as 'bad fats'?

They are solid at room temperature and can clog arteries. Signup and view all the answers

What is the typical consistency of cell membranes at room temperature?

Fluid with properties similar to salad oil. Signup and view all the answers

What is the main role of membrane proteins?

To determine most of the membrane's specific functions Signup and view all the answers

Which type of protein is entirely located on the surface of the membrane?

Peripheral proteins Signup and view all the answers

What characteristic allows hydrophobic molecules to pass through the lipid bilayer easily?

Their nonpolar nature Signup and view all the answers

What is the main function of carbohydrates found on the exterior surface of the cell membrane?

To determine cell-cell or cell-protein interactions Signup and view all the answers

What defines the selective permeability of a cell membrane?

The permeability of the lipid bilayer and transport proteins Signup and view all the answers

Which of the following substances would be expected to pass through the lipid bilayer most easily?

Carbon dioxide (CO2) Signup and view all the answers

Which type of transport protein specifically binds to substances and facilitates their movement across the membrane?

Transporters Signup and view all the answers

What role do glycolipids play in cell membranes?

Determine blood types and cell signaling Signup and view all the answers

What is the role of enzymes in cellular reactions?

Enzymes act as biological catalysts by lowering the activation energy. Signup and view all the answers

What does the term 'activation energy' refer to?

The energy barrier that reactants must overcome to initiate a reaction. Signup and view all the answers

Why do enzymes not catalyze non-spontaneous reactions?

Enzymes only catalyze reactions that occur naturally. Signup and view all the answers

What determines the specificity of an enzyme's function?

The size and shape of the enzyme's active site. Signup and view all the answers

How does the optimal functioning of an enzyme depend on environmental conditions?

Each enzyme performs optimally at specific temperatures, pH, and salt concentrations. Signup and view all the answers

What is the primary characteristic of passive transport?

Does not require energy or protein channels Signup and view all the answers

In a hypertonic solution, what effect does osmosis have on animal cells?

Cells shrink Signup and view all the answers

How does osmosis differ from regular diffusion?

Osmosis is the diffusion of water across a membrane, not solutes Signup and view all the answers

What is the role of the contractile vacuole in the protist Paramecium?

To pump out excess water Signup and view all the answers

Which statement correctly describes isotonic environments for marine animals?

Their internal solute concentration matches that of sea water Signup and view all the answers

Which type of molecules typically utilize facilitated diffusion to cross membranes?

Large polar molecules like glucose Signup and view all the answers

Why are plant cells considered happiest in hypotonic solutions?

It keeps cells from bursting due to their cell walls Signup and view all the answers

In active transport, what is primarily required for molecules to move across the membrane?

Cellular energy in the form of ATP Signup and view all the answers

What type of transport requires energy to move solutes against a concentration gradient?

Active transport Signup and view all the answers

What is the primary source of energy used in active transport mechanisms like the Na+/K+ pump?

ATP Signup and view all the answers

What process describes the movement of large molecules into the cell?

Endocytosis Signup and view all the answers

Which type of energy is defined as the energy stored due to an object's position?

Potential energy Signup and view all the answers

Which of the following statements accurately describes the first law of thermodynamics?

Energy can only be transformed from one form to another. Signup and view all the answers

What type of system allows for energy and matter to be exchanged with its surroundings?

Open system Signup and view all the answers

What is the term for the energy associated with the motion of molecules?

Thermal energy Signup and view all the answers

Which sequence correctly describes the transformation of energy in living cells?

Light energy → Chemical energy → Work Signup and view all the answers

Study Notes

The Cell Membrane

The cell membrane acts as the boundary between the cell's interior and the external environment.
Controls what enters and exits the cell.
Organizes the cell's chemical reactions.
Contains groups of enzymes that work in metabolism.

The Lipid Bilayer

Phospholipids form a stable bilayer in water.
The heads of the phospholipids face outwards towards the water.
The tails face inwards away from the water.
Membrane components are amphipathic, meaning they have both hydrophilic and hydrophobic properties.

Cell Membranes Structure

The cell membrane is a fluid mosaic of lipids and proteins.
Lipid molecules form a flexible bilayer.
Protein molecules are embedded in the plasma membrane.
Carbohydrates act as cell identification tags on the surface.

Membrane Fluidity

Cell membranes have the consistency of salad oil at room temperature.
Lipids have rapid lateral movement.
Lipids rarely flip-flop between layers.
Membranes must be fluid to function properly.
Fluidity depends on the composition of lipids.

Saturated Fatty Acids

All C-C bonds are single bonds.
Straight chains allow for maximum interaction between fatty acid tails.
Makes the membrane less fluid (more viscous).
Solid at room temperature.
Example: "Bad Fats" like animal fats that clog arteries.

Unsaturated Fatty Acids

Some C-C bonds are double bonds.
Bent chains space tails apart.
Makes the membrane more fluid.
Liquid at room temperature.
Example: "Good Fats" like vegetable fats that do not clog arteries.

Cholesterol

Cholesterol acts as a fluidity buffer.
It has different effects on membrane fluidity at different temperatures.
At warm temperatures, it restrains movement of phospholipids.
At cool temperatures, it maintains fluidity by preventing tight packing of phospholipids.

Membrane Proteins

Membranes are a mosaic of different proteins embedded in the lipid bilayer.

Peripheral Proteins

Entirely on the membrane surface.
Ionic and H-bond interactions with hydrophilic lipid and protein groups.

Integral Proteins

Possess hydrophobic domains that are anchored to hydrophobic lipids.
Span the membrane.
Contain both hydrophobic and hydrophilic regions.

Membrane Carbohydrates

Carbohydrates are found on the exterior surface of the cell membrane.
Provide specificity for cell-cell or cell-protein interactions.

Glycolipids

Sugars attached to a lipid.
Example: Blood antigens that determine blood type.

Glycoproteins

Sugars attached to a protein.
Example: Protein receptors.

Membranes are Selectively Permeable

The cell membrane controls traffic in and out of the cell by being selectively permeable.
It allows some substances to cross but not others.

Permeability of the Lipid Bilayer

Hydrophobic (nonpolar) molecules dissolve in the lipid bilayer and pass through the membrane rapidly.
Example: CO2, O2, steroid hormones.
Polar molecules and ions do not cross the membrane easily.

Transport Proteins

Span the membrane.
Each transport protein is specific to one solute.
Some are channels and others are transporters.
Two ways molecules can move through the membrane: passive transport and active transport.

Passive Transport

In passive transport, substances diffuse through membranes without work by the cell.
Substances spread from areas of high concentration to areas of lower concentration.
No energy is required.
No protein channel or carrier is required.

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane.
Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration.
This can also be explained as moving from high water concentration to low water concentration.

Cell Behaviour in Different Environments

Hypertonic: Higher solute concentration compared to the inside of the cell.
Hypotonic: Lower solute concentration relative to the inside of the cell.
Isotonic: Equal concentration of solutes.

Organisms without Cell Walls

Example: Animal cells.
Osmosis causes cells to shrink in a hypertonic solution and swell in a hypotonic solution.

Organisms with Cell Walls

Plant cells are happiest in hypotonic solutions because the cell wall prevents the membrane from rupturing, making them turgid.

Facilitated Diffusion

Only small, non-polar molecules (e.g., O2, CO2) can readily diffuse through the lipid bilayer (simple diffusion).
Other molecules pass through selective protein pores using facilitated diffusion.
Facilitated diffusion: Substances move down concentration gradients through transport proteins.
Type of passive transport – does not require energy.

Active Transport

Transport proteins can move solutes across a membrane against a concentration gradient.
Active transport requires energy (in the form of ATP).
The Na+/K+ pump is a very important example of active transport.

Active Transport: Bulk Transport

Exocytosis: The movement of large molecules or particles to the outside of the cell.
A membrane-bound vesicle fuses with the membrane and expels its contents.

Endocytosis

The movement of large molecules or particles to the inside of the cell.
The membrane folds inward, trapping material from the outside.

Energy and the Cell

Cell membranes are sites where chemical reactions can occur in an orderly manner.
Living cells transform energy by means of enzyme-controlled chemical reactions.
Energy is the capacity to perform work.

Two Forms of Energy

Kinetic Energy: energy that is actually doing work.
Potential Energy: stored energy.

Organisms Transform Energy

Light Energy  Chemical Energy  Work.
KE  PE  KE.

Important Definitions:

Thermodynamics

The study of energy transformations.
System: the collection of matter under study.
A system can be “open” or “closed”.
A closed system is isolated from its surroundings.
In an open system, energy and matter can be transferred between the system and its surroundings.
Organisms are considered open systems.

Two Laws Govern Energy Transformations

First law of thermodynamics

Energy can be changed from one form to another.
However, energy cannot be created or destroyed.

Cellular Reactions require Enzymes

Enzymes are proteins that act as biological catalysts.
Catalyst = chemical that speeds up the rate of a reaction without itself being changed/consumed.
Enzymes are used over and over again!
Without enzymes, life’s reactions would take too long.

Enzymes Lower Activation Energy

Activation energy (EA): the amount of energy reactants must absorb to start a chemical reaction.
It's essentially an energy barrier.
Without this barrier, most biological molecules would spontaneously break down.
Activation energy is often supplied in the form of heat from the surroundings.
Instead of heat, organisms use enzymes to speed up reactions.
Enzymes speed up the cell’s chemical reactions by lowering energy barriers.
Enzymes do not add energy or catalyze non-spontaneous reactions.

Enzyme Specificity

A specific enzyme catalyzes each cellular reaction.
Enzymes are selective.
This selectivity determines which chemical reactions occur in a cell.
Specificity is due to enzyme shape (think lock and key).
Substrate: The specific reactant on which the enzyme acts
Active site: The site on a enzyme to which the substrate binds

Every Cell Has Many Enzymes

Each cell has thousands of different enzymes, each performing a specific chemical reaction.
Each enzyme functions optimally at a specific temperature, pH, and salt concentration.

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Description

Explore the fascinating world of the cell membrane with this quiz, focused on its structure, lipid bilayer, and membrane fluidity. Understand how the cell membrane regulates what enters and exits the cell, and learn about the important roles of proteins and carbohydrates. Test your knowledge on the essential functions that make up this vital cellular component.