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Transport Across Cell Membrane

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Transport Across Cell Membrane

Cell membranes surround each cell and some organelles, such as the nucleus and the Golgi body. They are comprised of a phospholipid bilayer and this acts as a semipermeable barrier that regulates what enters and exits the cell or organelle. This is highly important as this is how cells can obtain molecules such as oxygen and remove waste products like urea.

There are two main ways in which molecules are transported across the cell membrane:

  • Passive transport

  • Active transport

The main difference between these modes of transport is that active transport requires energy in the form of ATP, but passive transport does not.

What are the passive transport methods?

Passive transport refers to transport across the cell membrane that does not require energy from metabolic processes. Instead, this form of transport relies on the natural kinetic energy of molecules and their random movement . There are three modes of passive transport:

  • Simple diffusion
  • Facilitated diffusion
  • Osmosis

Simple diffusion

Simple diffusion is the movement of molecules from a region of high concentration to a region of low concentration until an equilibrium is reached.

Oxygen can freely diffuse through the cell membrane using this form of passive transport.

Transport across cell membrane, simple diffusion, StudySmarterSimple diffusion

Facilitated diffusion

Facilitated diffusion is the same as simple diffusion but this form of transport uses membrane proteins, such as channel proteins and carrier proteins. Channel proteins provide a hydrophilic channel for the passage of charged and polar molecules, like ions. Meanwhile, carrier proteins change their conformational shape for the transport of molecules.

Glucose is an example of a molecule that is transported across the cell membrane through facilitated diffusion.

Transport across cell membrane, facilitated diffusion, StudySmarterFacilitated diffusion

Osmosis

Osmosis is the movement of water molecules from a region of high water potential to a region of lower water potential through a semipermeable membrane. This form of transport looks very different between plant and animal cells as plant cells have cell walls surrounding each cell. Cell walls allow plant cells to withstand water flowing in. However, animal cells are more prone to cell bursting as they do not have this added support.

The rate of osmosis can be increased if aquaporins are present in the cell membrane. Aquaporins are membrane proteins that selectively transport water molecules.

The uptake of water by plant root hair cells is dependent on this form of passive transport.

Transport across cell membrane, osmosis, StudySmarterThe diagram shows the movement of molecules through the cell membrane during osmosis

What are the active transport methods?

Active transport is the transport of molecules across the cell membrane using carrier proteins and energy from metabolic processes in the form of ATP. Carrier proteins are membrane proteins that allow the passage of specific molecules across the cell membrane. They are used in both facilitated diffusion and active transport. Carrier proteins use ATP to change their conformational shape in active transport, allowing a bound molecule to pass through the membrane. In facilitated diffusion, however, ATP is not needed to change the shape of the carrier protein.

Transport across cell membrane, active transport, StudySmarterThe diagram shows the movement of molecules in active transport

A process that relies on active transport is the uptake of mineral ions in plant root hair cells. The type of carrier proteins involved is specific for mineral ions.

There are different modes of active transport, such as cotransport and bulk transport.

Transport Across Cell Membrane - Key takeaways

  • The cell membrane is a phospholipid bilayer that surrounds each cell and some organelles. It regulates what enters and exits the cell and organelles.
  • Passive transport does not require energy in the form of ATP. Passive transport relies on the natural kinetic energy and random movement of molecules.
  • Simple diffusion, facilitated diffusion, and osmosis are the forms of passive transport.
  • Active transport across the cell membrane requires carrier proteins and energy in the form of ATP.
  • There are different types of active transport, such as cotransport and bulk transport.

Frequently Asked Questions about Transport Across Cell Membrane

There are two ways in which molecules are transported across the cell membrane: passive transport and active transport. The passive transport methods are simple diffusion, facilitated diffusion osmosis - these rely on the natural kinetic energy of molecules. Active transport requires energy in the form of ATP.

Amino acids are transported across the cell membrane via facilitated diffusion. Facilitated diffusion uses membrane proteins to transport molecules. Amino acids are charged molecules and therefore need membrane proteins, specifically channel proteins, to cross the cell membrane.

Membrane proteins such as channel proteins and carrier proteins facilitate transport across membranes. This type of transport is called facilitated diffusion.

Water molecules are transported across the cell membrane via osmosis which is defined as the movement of water from a region of high water potential to a region of lower water potential through a semipermeable membrane. The rate of osmosis is increased if aquaporins are present in the cell membrane.

Final Transport Across Cell Membrane Quiz

Question

What form of transport relies on the natural kinetic energy of molecules?

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Answer

Passive transport. This includes simple diffusion, facilitated diffusion and osmosis.

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What form of transport requires carrier proteins and energy in the form of ATP?


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Answer

Active transport.

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Define simple diffusion.


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Simple diffusion is the movement of molecules from a region of high concentration to a region of low concentration.

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What is the difference between simple diffusion and facilitated diffusion?


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Answer

Facilitated diffusion requires membrane proteins, such as channel proteins and carrier proteins. Simple diffusion does not require membrane proteins.

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Question

Osmosis is the movement of water molecules from a region of high water potential to a region of low water potential, through a __________ ________.


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Answer

Semipermeable membrane.

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What can increase the rate of osmosis?


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Answer

If aquaporins are present in the cell membrane.

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Question

What form of transport does the uptake of mineral ions through the plant root hair cell rely on?


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Answer

Active transport.

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What form of transport does the uptake of glucose molecules rely on?


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Answer

Facilitated diffusion.

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What form of transport does the uptake of oxygen rely on?


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Answer

Simple diffusion.

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Identify 2 types of active transport methods.

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Answer

Cotransport and bulk transport.

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Question

Where can you find cell membranes?

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Answer

Cell surface membranes surround each cell. Membranes also surround some organelles, such as the nucleus.

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How do cell membranes allow cell communication?


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Answer

Glycolipids and glycoproteins act as receptors and antigens. Signaling molecules can bind to these receptors and antigens. This will elicit chemical reactions within the cell.

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What is compartmentalisation and why is it important?


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Compartmentalization is the separation of each cell and each organelle so that incompatible metabolic reactions are kept separate. This is important so that the optimal conditions for each metabolic reaction are maintained without interfering with other reactions.

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What kind of molecules are cell membranes highly permeable to?


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Small, uncharged polar molecules.

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What kind of molecules are cell membranes impermeable to?


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Large, charged nonpolar molecules.

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What is the model that is widely used to describe the cell membrane structure?


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Answer

Fluid mosaic model.

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What are the two distinct regions of a phospholipid? Describe the phospholipid bilayer.


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Answer

The hydrophilic phospholipid heads face the aqueous environment (extracellular and intracellular) while the hydrophobic phospholipid tails form a core away from the aqueous environment.

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Phospholipid fatty acid tails that have kinks are ________. This is because they have at least one carbon _____ ____.


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Answer

Unsaturated. Double bond.

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What are the two types of membrane protein?


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Integral proteins and peripheral proteins.

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What are the two types of integral protein and what is their main function?


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Channel proteins and carrier proteins. Their main function is to transport molecules across the cell membrane.

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What are the main functions of glycoproteins?


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Answer

Cell adhesion and cell communication.

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What are the main functions of glycolipids?


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Cell adhesion and cell recognition.

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What are the 2 main functions of cholesterol?


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Preventing water and ions from leaking out of the cell and regulating membrane fluidity.

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At colder temperatures, cholesterol will prevent the _________ of phospholipids. 


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Answer

Crystallization.

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What happens to cell membranes when placed in a solvent that is less polar than water, such as ethanol?


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Answer

The cell membrane structure breaks down and is dissolved. The cell membrane becomes highly permeable and the cell contents leak out.

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Why do cell membranes become more permeable at higher temperatures?


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Answer

At higher temperatures, phospholipids have more kinetic energy and move more. This enables small molecules to pass through the cell membrane.

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What is the pigment that is responsible for the color of beetroot?


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Answer

Betalain pigment.

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When investigating how temperature affects cell membrane permeability in beetroot cells, what does a higher absorbance reading indicate?


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Answer

A higher absorbance reading indicates there is more pigment present in the sample solution. This means that the cell membrane structure is more permeable than the betalain pigment has leaked out of the cell and into the solution.

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What process does the gaseous exchange of oxygen and carbon dioxide rely on?

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Answer

Simple diffusion.

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What is the waste product of the breakdown of amino acids? How does it enter the blood?


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Urea. There is a higher concentration of urea in liver cells than in the blood. This concentration gradient means urea diffuses into the blood via simple diffusion.

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Define facilitated diffusion. 


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Answer

The movement of molecules down their concentration gradient, using membrane proteins.

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What are channel proteins?


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Transmembrane proteins which provide a hydrophilic channel for the passage of charged molecules, like ions.

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What are the different types of stimuli that trigger the opening or closing of channel proteins?


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Voltage, mechanical pressure and ligand binding.

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What are carrier proteins?


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Transmembrane proteins which undergo a reversible conformational change for the passage of molecules.

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What processes require the presence of carrier proteins?


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Answer

Both passive and active transport across the cell membrane.

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What kind of process allows the nerve impulse to travel along axons? Identify a protein that is required for this process.


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Answer

Facilitated diffusion. Voltage-gated sodium ion channels are needed for the passage of ions.

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Question

Describe the molecular properties of glucose and what it means for its transport across cell membranes. 

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Answer

Glucose is a large and highly polar molecule. This means it needs membrane proteins for its transport across cell membranes.

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Identify the membrane protein needed in glucose transport. 

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Answer

Glucose transporter proteins (GLUTs).

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What are the main factors that affect the rate of diffusion?


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Answer

Concentration gradient, distance, temperature, surface area and molecular properties.

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Question

How is a steep concentration gradient maintained for gaseous exchange?


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Answer

Continuous ventilation and blood flow maintain the steep concentration gradient.

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Describe the diffusion distance in gaseous exchange?


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Answer

The diffusion distance is kept very small. 


The capillary endothelium and alveoli walls are only one cell thick. 


The capillaries are so wrapped tightly around the alveoli.

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How is a large surface area provided for in gaseous exchange?


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Answer

Many alveoli are present in each lung.

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Question

When the membrane potential of neurones become less negative, what process occurs?

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Answer

Depolarisation.

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Question

Define osmosis in terms of water potential.

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Answer

Osmosis is the movement of water molecules down a water potential gradient, through a semipermeable membrane.

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Question

What is the water potential of pure water?


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Answer

0kPa.

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Water molecules will move from a more dilute solution to a more ______ solution.


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Answer

Concentrated.

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Describe the water potential of a solution as more solutes are dissolved in it.


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Answer

The water potential becomes more negative as more solutes are dissolved in a solution.

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What kind of membrane is needed for osmosis?


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Answer

A semipermeable membrane.

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Question

What are the three types of tonicity?


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Answer

Hypotonic, isotonic and hypertonic.

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Question

Compare plant cells placed in a hypertonic solution to plant cells placed in a hypotonic solution.


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Answer

In a hypertonic solution, plant cells will undergo plasmolysis and will become flaccid. 


But in a hypotonic solution, planning cells will become turgid and firm.

Show question

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