Cell Membrane
Assessment statements
• Draw and label a diagram to show the structure of a membrane.
• Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain the structure of cell membranes.
• List the functions of membrane proteins.
• Defi ne ‘diffusion’ and ‘osmosis’.
• Explain passive transport across membranes by simple diffusion and facilitated diffusion.
• Explain the role of protein pumps and ATP in active transport across membranes.
• Explain how vesicles are used to transport materials within a cell between the rough endoplasmic reticulum, Golgi apparatus and plasma membrane.
• Describe how the fluidity of the membrane allows it to change shape, break and re-form during endocytosis and exocytosis.
It might be thought that membranes are present primarily to provide
shape for a cell. Whilst this is certainly important, there is also considerable
activity at membrane surfaces, especially at the plasma membrane in
contact with the extracellular space. The current model for membrane
structure was proposed by Singer and Nicolson in 1972. Their uid
mosaic model, illustrated in Figure 2.13, was based on the knowledge
available at the time but has been supported by more recent research, with
only minor modifi cations.
The structure of membranes
All membranes, wherever they occur in cells, have the same basic
structure. Membranes are usually between 7 and 10 nm thick, and
are composed of two layers of phospholipid, which form a bilayer.
Phospholipids are made up of a polar, hydrophilic area containing a
phosphate group bonded to glycerol, and a non-polar, hydrophobic area
containing fatty acids. In the bilayer, the hydrophobic (water-hating)
parts all point towards each other, and the hydrophilic (water-loving)
areas point outwards, as Figure 2.14 shows.
It is the diff erent properties of each end of the molecule that cause the
phospholipids to arrange themselves in this way. The hydrophilic ‘heads’
of the molecules always appear on the outside of the membrane where
water is present, while the hydrophobic ‘tails’ orientate inside the double
layer, away from water. The whole structure is fl exible or ‘fl uid’ because
the phospholipids can fl oat into a position anywhere in the membrane.
There is much evidence to support the plasma membrane as a ‘fl uid
mosaic’. Further evidence has come from using radioactively labelled
phospholipids. Research shows that these molecules move not only within
their own layer, but also between the two layers of the membrane.
Embedded in the bilayer are diff erent molecules that contribute to the
functions of membranes. Cholesterol is often present in animal cells and
is most commonly found in the plasma membrane. Cholesterol molecules
align themselves with the phospholipids making the membrane more
rigid, and less permeable to water-soluble molecules.
There are also diff erent types of protein in the bilayer. Integral
proteins are embedded in the bilayer, whereas peripheral proteins are
attached to the surface. Many of the proteins on the outer surface are
glycoproteins – that is, they have carbohydrate groups attached to them.
Some of these serve as hormone binding sites and have special shapes to
recognise the specifi c hormones to which the cell will respond. Others
are important in cell-to-cell communication and adhesion. Some integral
proteins are enzymes immobilised within the membrane structure and
perfectly placed to carry out sequences of metabolic reactions. Finally,
there are proteins that span the bilayer acting as channels for ions and
molecules to pass by passive transport, or forming pumps that use active
transport to move molecules into or out of the cell.
Transport across membranes
Diffusion, facilitated diffusion and osmosis
Many molecules pass across the plasma membrane. Water, oxygen,
carbon dioxide, excretory products, nutrients and ions are continuously
exchanged and many cells also secrete products such as hormones and
enzymes through the membrane.
The simplest way in which a molecule could move into or out of a
cell is by di usion. No energy is required, and movement occurs by
way of a simple concentration gradient. For example, as carbon dioxide
concentration builds up in cells because of respiratory activity, it begins to
diff use through the plasma membrane to an area where the concentration
is lower. Diff usion occurs where the membrane is fully permeable to the
substance or where protein channels in the membrane are large enough
for it to pass through.
In cases where molecules are large, or where charged particles such
as chloride ions (Cl–) must pass, simple diff usion is impossible. These
substances are often transported across membranes by facilitated
di usion. Here an integral protein in the membrane forms a channel
so that the substance particles can pass through them into or out of the
cell (Figure 2.15). Some of these channels are permanently open whereas
others can open and close to control the movement of the substance.
Furthermore, they are specifi c – that is, they only allow a particular
substance to pass through. As in simple diff usion, no energy is used by
the cell. In both cases, the transport relies on the kinetic energy of the
particles moving down their concentration gradient.
A special case of diff usion is osmosis (Figure 2.16). This is the passive
movement of water across a partially permeable membrane from an area
of lower solute concentration to an area of higher solute concentration.
Active transport
Many of the substances a cell needs occur in low concentrations in the
surroundings outside the plasma membrane. Plants must take in nitrate
ions from very dilute solutions in the soil to build their proteins, and
muscle cells actively take in calcium ions to enable them to contract. To
move these substances into the cell against a concentration gradient, the
cell must use metabolic energy released from the breakdown of ATP. This
is called active transport (Figure 2.17). Specifi c proteins in the plasma
membrane act as transporters or ‘carriers’ to move substances through.
Many of the carrier proteins are specifi c to particular molecules or ions so
that these can be selected for transport into the cell.
The sodium–potassium pump maintains the concentration of sodium
and potassium ions in the cells and extracellular fl uid. Cells are able to
exchange sodium ions for potassium ions against concentration gradients
using energy provided by ATP. Sodium ions are pumped out of the cell
and potassium ions are pumped into the cell.
Exocytosis and endocytosis
Cells often have to transport large chemical molecules or material in bulk
across the plasma membrane. Neither diff usion nor active transport will
work here. Instead, cells can release or take in such materials in vesicles,
as shown in Figure 2.19. Uptake is called endocytosis and export is
exocytosis. Both require energy from ATP.
During endocytosis, part of the plasma membrane is pulled inward and
surrounds the liquid or solid that is to be moved from the extracellular
space into the cell. The material becomes enclosed in a vesicle, which
pinches off from the plasma membrane and is drawn into the cell. This is
how white blood cells take in bacteria
Materials for export, such as digestive enzymes, are made in the rER and
then transported to the Golgi apparatus to be processed. From here they
are enclosed within a membrane-bound package known as a vesicle, and
moved to the plasma membrane along microtubules. The vesicles fuse with
the plasma membrane and in doing so release their contents to the outside.
The flexibility and fluidity of the plasma membrane allow this to happen.
11 Outline the difference between simple diffusion and facilitated diffusion.
12 Suggest why the term ‘fluid mosaic’ is used to describe membrane structure.
13 Suggest why the fatty acid ‘tails’ of the phospholipid molecules always align themselves in the middle of the membrane.
14 Outline the difference between integral membrane proteins and peripheral membrane proteins.
15 List the six ways that substances move from one side of a membrane to the other.
16 State which of these transport mechanisms require energy from ATP.
17 List the functions of proteins that are found in a membrane.
>Passive transport the movement of substances down a concentration gradient from an area of high concentration to an area of lower concentration without the need for energy to be used
> Diffusion one example of passive transport; many molecules pass into and out of cells by diffusion e.g.oxygen, carbon dioxide and glucose
> Osmosis another example of passive transport but the term is only used in the context of water molecules; osmosis is the movement of water molecules across a partially permeable membrane from a region of lower solute concentration, where there is a high concentration of water molecules, to a region of higher solute concentration, where the concentration of water molecules is lower
>Active transport the movement of substances against the concentration gradient, which always involves the expenditure of energy in the form of ATP
1. Diffusion
2. Facilitated Diffusion
3. Active Transport
4. Endocytosis and Exocytosis
Conclusion
Movement across the cell membrane occurs through passive and active processes, including diffusion, osmosis, facilitated diffusion, active transport, and bulk transport (endocytosis/exocytosis). These mechanisms ensure cells maintain homeostasis by regulating the entry and exit of nutrients, gases, ions, and waste.
🔑 Key Modes of Transport
1. Passive Transport (No Energy Required)
Simple Diffusion
- Molecules move from high to low concentration directly through the lipid bilayer.
- Examples: Oxygen (O₂), Carbon dioxide (CO₂).
Osmosis
- Movement of water across a selectively permeable membrane.
- Driven by differences in solute concentration.
Facilitated Diffusion
- Uses protein channels or carriers to move molecules down their concentration gradient.
- Examples: Glucose transport via GLUT proteins, ion channels for Na⁺, K⁺, Cl⁻.
2. Active Transport (Energy Required – ATP)
Primary Active Transport
- Direct use of ATP to pump molecules against concentration gradient.
- Example: Sodium-Potassium Pump (Na⁺/K⁺ ATPase).
Secondary Active Transport (Co-transport)
- Uses energy stored in ion gradients created by primary transport.
- Example: Glucose uptake coupled with Na⁺ ions.
3. Bulk Transport (Vesicular Transport)
Endocytosis
- Cell engulfs material into vesicles.
- Types:
- Phagocytosis: "Cell eating" (e.g., macrophages engulf bacteria).
- Pinocytosis: "Cell drinking" (uptake of fluids).
- Receptor-mediated endocytosis: Specific uptake using receptors.
Exocytosis
- Vesicles fuse with the membrane to release contents outside the cell.
- Example: Secretion of hormones, neurotransmitters.
📊 Comparison Table
| Transport Type | Energy Use | Direction | Examples |
|---|---|---|---|
| Simple Diffusion | No | High → Low | O₂, CO₂ |
| Osmosis | No | High → Low (water) | Water |
| Facilitated Diffusion | No | High → Low | Glucose, Ions |
| Active Transport | Yes (ATP) | Low → High | Na⁺/K⁺ Pump |
| Endocytosis | Yes | Into cell | Bacteria, fluids |
| Exocytosis | Yes | Out of cell | Hormones, waste |
⚠️ Key Considerations
- Selective Permeability: The cell membrane allows only certain molecules to pass, maintaining internal balance.
- Energy Trade-off: Passive transport is efficient but limited; active transport consumes ATP but enables precise control.
- Medical Relevance: Defects in transport proteins can cause diseases (e.g., cystic fibrosis due to faulty chloride channels).
In short, cell membranes regulate movement through a mix of passive diffusion, active pumping, and vesicular transport, ensuring cells survive and function properly. Would you like me to create a diagrammatic summary of these processes for easier visualization?
Human cell membranes regulate movement of substances through passive transport (diffusion, osmosis, facilitated diffusion), active transport (ATP-driven pumps, co-transport), and bulk transport (endocytosis and exocytosis). These mechanisms maintain homeostasis by controlling nutrient uptake, waste removal, and signal transmission.
🔑 Major Modes of Transport Across Human Cell Membranes
1. Passive Transport (No Energy Required)
Simple Diffusion
- Molecules move from high to low concentration directly through the lipid bilayer.
- Examples: Oxygen (O₂), Carbon dioxide (CO₂).
Osmosis
- Movement of water across a selectively permeable membrane.
- Driven by solute concentration differences.
Facilitated Diffusion
- Uses protein channels or carriers to move molecules down their gradient.
- Examples: Glucose transport via GLUT proteins, ion channels for Na⁺, K⁺, Cl⁻.
- Important in neurons and muscle cells.
2. Active Transport (Energy Required – ATP)
Primary Active Transport
- Direct use of ATP to pump molecules against their gradient.
- Example: Sodium-Potassium Pump (Na⁺/K⁺ ATPase), vital for nerve impulse transmission.
Secondary Active Transport (Co-transport)
- Uses energy stored in ion gradients created by primary transport.
- Example: Glucose uptake coupled with Na⁺ ions in intestinal cells.
3. Bulk Transport (Vesicular Transport)
Endocytosis
- Cell engulfs material into vesicles.
- Types:
- Phagocytosis: "Cell eating" (macrophages engulf pathogens).
- Pinocytosis: Uptake of extracellular fluid.
- Receptor-mediated endocytosis: Specific uptake using receptors (e.g., LDL cholesterol uptake).
Exocytosis
- Vesicles fuse with the membrane to release contents outside the cell.
- Example: Secretion of hormones, neurotransmitters, and digestive enzymes.
📊 Comparison Table
| Transport Type | Energy Use | Direction | Examples |
|---|---|---|---|
| Simple Diffusion | No | High → Low | O₂, CO₂ |
| Osmosis | No | High → Low (water) | Water |
| Facilitated Diffusion | No | High → Low | Glucose, Ions |
| Active Transport | Yes (ATP) | Low → High | Na⁺/K⁺ Pump |
| Endocytosis | Yes | Into cell | Bacteria, LDL |
| Exocytosis | Yes | Out of cell | Hormones, Waste |
⚠️ Key Considerations
- Selective Permeability: Human cell membranes allow only certain molecules to pass, ensuring internal stability.
- Energy Trade-off: Passive transport is efficient but limited; active transport consumes ATP but enables precise regulation.
- Medical Relevance: Defects in transport proteins can cause diseases (e.g., cystic fibrosis due to faulty chloride channels, familial hypercholesterolemia due to defective LDL receptor-mediated endocytosis).
✅ In summary, human cell membranes use a combination of passive, active, and vesicular transport to regulate movement of molecules, ensuring proper cellular function and survival. Would you like me to create a visual diagram showing these processes for easier understanding?
Movement across plant cell membranes occurs through passive transport (diffusion, osmosis, facilitated diffusion), active transport (ATP-driven pumps and co-transport), and bulk transport. In plants, ion regulation (K⁺, Na⁺, Ca²⁺, Cl⁻) and water movement are especially critical for maintaining turgor pressure, nutrient uptake, and signaling.
🌱 Key Transport Mechanisms in Plant Cells
1. Passive Transport (No Energy Required)
Simple Diffusion
- Molecules move down their concentration gradient through the lipid bilayer.
- Examples: Oxygen (O₂), Carbon dioxide (CO₂).
Osmosis
- Water moves across the membrane to balance solute concentrations.
- Essential for maintaining turgor pressure, which keeps plant cells rigid.
Facilitated Diffusion
- Transport proteins help ions and molecules move down their gradient.
- Example: Potassium (K⁺) channels in guard cells regulate stomatal opening.
2. Active Transport (Energy Required – ATP)
Ion Pumps
- ATP-driven pumps move ions against their gradient.
- Example: Proton (H⁺) pumps create electrochemical gradients used for nutrient uptake.
Secondary Active Transport (Co-transport)
- Uses ion gradients to drive uptake of other molecules.
- Example: Sucrose-H⁺ symporters transport sugars into phloem cells.
3. Bulk Transport (Vesicular Transport)
Endocytosis
- Plant cells internalize molecules via vesicles.
- Plays a role in signaling and nutrient regulation.
Exocytosis
- Vesicles release materials outside the cell.
- Important for cell wall formation and secretion of enzymes.
📊 Comparison Table
| Transport Type | Energy Use | Direction | Plant Examples |
|---|---|---|---|
| Simple Diffusion | No | High → Low | O₂, CO₂ |
| Osmosis | No | High → Low (water) | Water into vacuole |
| Facilitated Diffusion | No | High → Low | K⁺ channels in guard cells |
| Active Transport | Yes (ATP) | Low → High | H⁺ pumps, nutrient uptake |
| Endocytosis | Yes | Into cell | Hormone signaling |
| Exocytosis | Yes | Out of cell | Cell wall materials |
🌍 Plant-Specific Considerations
- Cell Wall + Membrane Interaction: The cell wall provides structural support, while the membrane controls selective permeability.
- Ion Regulation: Potassium (K⁺), sodium (Na⁺), calcium (Ca²⁺), and chloride (Cl⁻) are vital for osmotic balance, enzyme activation, and signaling.
- Turgor Pressure: Driven by osmosis, it maintains rigidity and supports growth.
- Nutrient Uptake: Active transport ensures absorption of minerals like nitrate (NO₃⁻) and phosphate (PO₄³⁻) from soil.
✅ In summary, plant cell membranes regulate movement through a mix of passive, active, and vesicular transport, with ion control and water balance being central to plant survival and growth. Would you like me to create a diagram showing these processes in a plant cell for easier visualization?
Is there any extra ordinary movements through cell membrane?
Yes — beyond the “ordinary” modes like diffusion, osmosis, and active transport, cells sometimes use specialized or extraordinary mechanisms to move substances across their membranes. These are less common but crucial in specific contexts:
🌟 Extraordinary Movements Across Cell Membranes
1. Electrogenic Pumps
- Unlike standard pumps, these create a net charge difference across the membrane.
- Example: Proton (H⁺) pumps in plant cells generate a strong electrochemical gradient that powers nutrient uptake and drives processes like phloem loading.
2. Endocytosis of Large Complexes
- Cells can engulf entire particles or macromolecules that are far too large for normal transport.
- Example: White blood cells performing phagocytosis to ingest bacteria.
- This is extraordinary because it bypasses the size limits of typical transport proteins.
3. Exocytosis of Specialized Vesicles
- Neurons release neurotransmitters in a highly regulated, rapid burst — a process called synaptic vesicle exocytosis.
- This is extraordinary because it’s precisely timed and essential for communication between nerve cells.
4. Aquaporins (Water Channels)
- While osmosis is common, aquaporins make water movement extraordinarily fast and selective, allowing cells to rapidly adjust to osmotic stress.
- Example: Kidney cells in humans, guard cells in plants.
5. Gap Junctions & Plasmodesmata
- Instead of moving substances across a single membrane, cells sometimes connect directly to neighboring cells.
- Gap junctions (animals) and plasmodesmata (plants) allow ions, signaling molecules, and even RNA to pass directly between cells.
- Extraordinary because they bypass the usual “in/out” barrier.
6. Endocytosis of Signaling Molecules
- Cells internalize receptors and ligands together to regulate signaling pathways.
- Example: Receptor-mediated endocytosis of LDL cholesterol in human cells.
- Extraordinary because it couples transport with signal regulation.
⚡ Why These Are “Extraordinary”
- They go beyond simple concentration gradients.
- They often involve large molecules, rapid responses, or direct cell-to-cell communication.
- They are critical for specialized functions like immunity, nerve signaling, and plant nutrient transport.
So, while diffusion and active transport are the everyday workhorses, these extraordinary movements highlight the adaptability and complexity of living cells. Would you like me to illustrate these unusual processes in a visual diagram to make them easier to grasp?
Great question! The biomolecules that move through the cell membrane can be grouped into several categories depending on their size, polarity, and transport mechanism:
🧬 Biomolecules Crossing the Cell Membrane
1. Small Nonpolar Molecules (via Simple Diffusion)
- Oxygen (O₂)
- Carbon dioxide (CO₂)
- Nitrogen (N₂)
- Lipid-soluble molecules (e.g., steroid hormones)
2. Small Polar Molecules (via Facilitated Diffusion or Channels)
- Water (through aquaporins)
- Glycerol
- Urea
3. Ions (via Channels or Pumps)
- Sodium (Na⁺)
- Potassium (K⁺)
- Calcium (Ca²⁺)
- Chloride (Cl⁻)
- Hydrogen (H⁺, protons)
4. Carbohydrates
- Monosaccharides (e.g., glucose, fructose) via GLUT transporters
- Disaccharides (e.g., lactose, sucrose) after breakdown into monosaccharides
5. Amino Acids & Small Peptides
- Transported by specific amino acid transporters
- Important for protein synthesis and metabolism
6. Lipids
- Fatty acids (can diffuse or use transport proteins)
- Cholesterol (via receptor-mediated endocytosis, e.g., LDL uptake)
7. Macromolecules (via Endocytosis/Exocytosis)
- Proteins (e.g., enzymes, antibodies)
- Polysaccharides
- Nucleic acids (DNA/RNA fragments, often via vesicular transport)
- Hormones (peptide hormones secreted by exocytosis)
📊 Summary Table
| Biomolecule Type | Examples | Transport Method |
|---|---|---|
| Small nonpolar | O₂, CO₂, steroids | Simple diffusion |
| Small polar | Water, urea | Aquaporins, facilitated diffusion |
| Ions | Na⁺, K⁺, Ca²⁺, Cl⁻ | Ion channels, pumps |
| Carbohydrates | Glucose, fructose | Facilitated diffusion (GLUT) |
| Amino acids | Glycine, lysine | Specific transporters |
| Lipids | Fatty acids, cholesterol | Diffusion, receptor-mediated endocytosis |
| Macromolecules | Proteins, nucleic acids | Endocytosis/exocytosis |
✨ In short, everything from gases to ions, sugars, amino acids, lipids, and even large macromolecules can move through the cell membrane — but each requires a specific mechanism depending on its properties.
Would you like me to highlight which biomolecules move passively vs. actively across the membrane, so you can see the energy requirements clearly?
In plant cells, the biomolecules that move through the cell membrane are similar to those in animal cells, but with some plant-specific emphasis on ions, water, and nutrients needed for growth and photosynthesis. Here’s a clear breakdown:
🌱 Biomolecules Crossing Plant Cell Membranes
1. Small Nonpolar Molecules (via Simple Diffusion)
- Oxygen (O₂) – needed for respiration
- Carbon dioxide (CO₂) – needed for photosynthesis
- Nitrogen (N₂) – limited diffusion, but important in root uptake
2. Water & Small Polar Molecules
- Water (H₂O) – moves rapidly via aquaporins (osmosis)
- Glycerol, urea – small polar molecules that can diffuse or use channels
3. Ions (Critical for Plant Physiology)
- Potassium (K⁺) – regulates stomatal opening/closing
- Calcium (Ca²⁺) – signaling molecule in stress responses
- Sodium (Na⁺) – regulated to avoid toxicity
- Chloride (Cl⁻) – osmotic balance
- Magnesium (Mg²⁺) – essential for chlorophyll
- Phosphate (PO₄³⁻), Nitrate (NO₃⁻), Sulfate (SO₄²⁻) – nutrient ions absorbed from soil
4. Carbohydrates
- Glucose, sucrose – transported via sucrose-H⁺ symporters into phloem for distribution
- Fructose – taken up by specific transporters
5. Amino Acids & Small Peptides
- Amino acids (e.g., glutamine, lysine) – transported for protein synthesis
- Small peptides – involved in signaling and defense
6. Lipids
- Fatty acids – for membrane synthesis and signaling
- Sterols – regulate membrane fluidity
7. Macromolecules (via Vesicular Transport)
- Proteins (enzymes, signaling molecules) – secreted by exocytosis into the cell wall
- Polysaccharides (e.g., cellulose precursors) – exported for cell wall construction
- Hormones (auxins, gibberellins, cytokinins) – transported actively or via vesicles
📊 Summary Table
| Biomolecule Type | Examples | Transport Method |
|---|---|---|
| Small nonpolar | O₂, CO₂ | Simple diffusion |
| Water/polar | H₂O, glycerol | Osmosis, aquaporins |
| Ions | K⁺, Ca²⁺, NO₃⁻, PO₄³⁻ | Ion channels, pumps |
| Carbohydrates | Glucose, sucrose | Facilitated diffusion, symporters |
| Amino acids | Glutamine, lysine | Specific transporters |
| Lipids | Fatty acids, sterols | Diffusion, transport proteins |
| Macromolecules | Proteins, polysaccharides | Endocytosis/exocytosis |
✨ In short, plant cell membranes regulate the movement of gases, water, ions, sugars, amino acids, lipids, and macromolecules, with special emphasis on nutrient ions and water balance to support photosynthesis, growth, and cell wall formation.
Would you like me to highlight which of these biomolecules are most critical for photosynthesis and plant growth so you can see their role more clearly?
In animal cells, a wide variety of biomolecules move through the cell membrane, each using specific transport mechanisms depending on their size, polarity, and function. Here’s a clear breakdown:
🧬 Biomolecules Crossing Animal Cell Membranes
1. Small Nonpolar Molecules (Simple Diffusion)
- Oxygen (O₂) – for cellular respiration
- Carbon dioxide (CO₂) – waste product of respiration
- Nitrogen (N₂) – limited diffusion
2. Small Polar Molecules
- Water (H₂O) – via osmosis and aquaporins
- Urea – diffuses slowly across the membrane
- Glycerol – small enough to pass through channels
3. Ions (via Channels and Pumps)
- Sodium (Na⁺) – crucial for nerve impulses
- Potassium (K⁺) – maintains resting membrane potential
- Calcium (Ca²⁺) – signaling, muscle contraction
- Chloride (Cl⁻) – osmotic balance
- Hydrogen (H⁺, protons) – pH regulation
4. Carbohydrates
- Glucose – transported by GLUT transporters
- Fructose – via specific transporters
- Galactose – absorbed in intestinal cells
5. Amino Acids & Small Peptides
- Essential and non-essential amino acids (e.g., lysine, glycine, glutamine)
- Transported by amino acid transporters for protein synthesis
6. Lipids
- Fatty acids – diffuse or use transport proteins
- Cholesterol – via receptor-mediated endocytosis (LDL uptake)
- Steroid hormones – diffuse directly due to lipid solubility
7. Macromolecules (Vesicular Transport)
- Proteins – secreted by exocytosis (e.g., insulin, antibodies)
- Polysaccharides – exported for extracellular matrix functions
- Nucleic acids (DNA/RNA fragments) – occasionally transported via endocytosis
- Peptide hormones – released by exocytosis (e.g., growth hormone)
📊 Summary Table
| Biomolecule Type | Examples | Transport Method |
|---|---|---|
| Small nonpolar | O₂, CO₂ | Simple diffusion |
| Small polar | Water, urea | Osmosis, aquaporins |
| Ions | Na⁺, K⁺, Ca²⁺, Cl⁻ | Ion channels, pumps |
| Carbohydrates | Glucose, fructose | Facilitated diffusion (GLUT) |
| Amino acids | Lysine, glycine | Specific transporters |
| Lipids | Fatty acids, cholesterol | Diffusion, receptor-mediated endocytosis |
| Macromolecules | Proteins, hormones | Endocytosis/exocytosis |
✨ In short, animal cell membranes regulate the passage of gases, water, ions, sugars, amino acids, lipids, and macromolecules, ensuring proper metabolism, signaling, and homeostasis.
Would you like me to highlight which of these biomolecules move passively vs. actively so you can see the energy requirements clearly?
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