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- No energy + down concentration gradient
Example:
- Diffusion -> Substances (e.g oxygen across alveoli)
- Facillitated diffusion -> through a channel protein = specific (diameter and chemical protein) and faster.
- Osmosis -> water through a partially permeable membrane
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- Use ATP (energy) to move against the concentration gradient.
- Use protein pumps.
e.g mineral uptake in root hair cells
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a) Temperature:
- High temperatures increase fluidity, more permeable.
- Low temperatures reduce fluidity, more rigid and less permeable.
b)pH:
- Extreme pH levels can denature membrane proteins and disrupt lipid interactions, increasing permeability.
c) Chloresterol
- Cholesterol is interspersed between fatty acid tails of the phospholipid bilayer
- Prevents fatty acid chains from packing too closely together in cold temperatures (maintain fluidity)
- Restrains movement of fatty acid chains in high temperatures (prevent excessive fluidity)
d) Fluidity allows formation of vesicle
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- specialised channel proteins that facilitate rapid water transport across cell membranes (facilitated diffusion)
- Provide a hydrophilic pathway, allowing water to bypass the membrane's hydrophobic core.
- Essential for water balance regulation, e.g in kidneys.
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Hypotonic - water enter cell, cell enlarge
Hypertonic - water leaves cell, cell shrink
isotonic - no change
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Proteins that transport lipids in the bloodstreams, e.g Chloresterol.
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a) Function:
- Maintains resting membrane potential in neurons.
- Active transport of Na⁺ and K⁺ ions.
- Essential for nerve impulse transmission and action potential propagation.
b) Mechanism:
1. 3 Na⁺ bind to pump from inside the cell.
2. ATP hydrolysis → Phosphate attaches to pump → Conformational change.
3. 3 Na⁺ released to extracellular space.
4. 2 K⁺ bind from outside the cell.
5. Phosphate detaches → Pump returns to original shape.
6. 2 K⁺ is released into the cytoplasm.
c) Key Points:
- Moves 3 Na⁺ out, 2 K⁺ in per ATP hydrolyzed → Net negative charge inside cell.
- Establishes electrochemical gradient necessary for action potential.
- Maintains polarity of the neuron, enabling nerve impulses.
Present in all cells but crucial in neurons and muscle cells.
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- Integral/Intrinsic -> Inside membrane (include transmembrane)
- Peripheral/Extrinsic -> outside membrane (can be attached to an integral protein)
- Functions ->
a. hormone binding sites/receptors;
b. (immobilized) enzymes;
c. cell adhesion;
d. cell (to cell) communication;
e. passive transport/channels;
f. active transport/pumps;
g. facilitate diffusion;
h. carry electrons;
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Creates specialized environments, optimizing conditions for biochemical reactions and reducing interference between incompatible processes.
- Enhances efficiency by concentrating enzymes and substrates, increasing reaction rates.
- Organelle content can be moved around.
- Protects the cell by isolating harmful substances and separating damaging reactions from the cytoplasm.
- More membrane area available for processes that happen within or across the membrane.
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- Thylakoid membranes provide a large surface area for light absorption and the electron transport chain.
- Grana (stacked thylakoids) enhance light capture efficiency.
- Stroma contains enzymes for the Calvin cycle/carbon fixation (concentrated so speed up reaction) + photophosphorylation
- Chlorophyll and pigments in thylakoid membranes capture light energy.
-The small thylakoid volume allows a rapid proton gradient buildup, enabling efficient ATP synthesis after minimal light absorption.
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- The outer mitochondrial membrane separates the mitochondrion from the rest of the cell, creating a specialized compartment for optimal aerobic respiration.
- The inner membrane contains electron transport chains and ATP synthesis.
- Cristae increase surface area, enhancing ATP production efficiency (oxidative phosphorylation)
- Intermembrane space allows rapid proton accumulation due to its small volume, aiding in ATP generation.
- Matrix contains enzymes for the Krebs cycle.
- Ribosomes and DNA for the expression of genes.
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Structure: membrane system with ribosomes attached to the outer surface
Function of ribosomes on rER: synthesis of polypeptide
Process: Polypeptide enters rER lumen then is transported via vesicle.
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- rNA and protein assemeblers
- Two subunits: one large (3 binding site for tRNA ->catalyse peptide bond formation & exit tunnel) + one small (binding site for mRNA)
- Not attached to membranes
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Structure: A stack of flattened membrane-bound cisternae.
1. It modifies polypeptides from the rER, transported in vesicles.
- Processing occurs sequentially, moving proteins from the cis side (near ER) to the trans side. Two models:
a) Vesicle transport model - cisternae don't move, vesicle transfer proteins between them.
b) Cisternal maturation model -Vesicles from the rough ER fuse to form new cisternae at the cis side, which gradually move through the Golgi. At the trans side, cisternae break into vesicles for transport.
2. Packages and sorts proteins into vesicles for lysosomes, the plasma membrane, or secretion.
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Undifferentiated cells -> can divide and differentiate into various specialized cell types.
1. Totipotent - can differentiate into any cell type.
2. Pluripotent - can differentiate into any range of cell types, not every cell type.
3. Mutipotent (Adult stem cells) - can differentiate into a range of cell types. E.g haematopoietic stem cells in bone marrow for blood cells only.
Advantages:
- Can divide indefinitely and differentiate into specific cell types.
- Used for tissue repair and regeneration.
- Fewer ethical concerns compared to embryonic stem cells.
- Donors can give informed consent for their use.
- No embryo destruction, avoiding ethical objections.
- Lower risk of immune rejection when using patient’s own cells.
- Reduced cancer risk compared to embryonic stem cells.
Disadvantages:
- Difficult to obtain due to limited availability in the body.
- Limited differentiation potential compared to embryonic stem cells.
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- Streamlined shape, small volume and narrow, 50µm -> to minimise resistance against movement.
- Long flagellum with a 9+2 microtubule arrangement generates force for movement.
- Mitochondria in the midpiece provide ATP for flagellar propulsion.
- A compact nucleus in the head contains haploid DNA.
- Acrosome contains enzymes to digest the egg’s zona pellucida.
- Receptors in the plasma membrane recognize ZP3 glycoproteins for egg binding.
- Acrosomal exocytosis exposes binding proteins that facilitate sperm-egg membrane fusion and nucleus entry.
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- Generate force and motion through contraction.
- Striated (Skeletal) Muscle:
a) Composed of long, unbranched cylindrical fibers (myofibrils).
b) Multinucleated with nuclei located at the periphery.
c) Striations due to the arrangement of actin and myosin in sarcomeres (contractile units).
d) Size: 20–100 µm in diameter, up to 100 mm long → enables more significant force generation and contraction over long distances.
- Sarcoplasmic reticulum (SR): Specialized endoplasmic reticulum around myofibrils.
- Sarcomere (Basic Contractile Unit): Have light and dark bands giving it a stirated look
- Cardiac Muscle:
a) Shorter, branched fibers with usually one nucleus per cell.
b) Intercalated discs: Specialized junctions allowing rapid electrical signal propagation for synchronized contractions.
- Sacrolema : plasma membrane
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- Dendrites receive signals and transmit them to the cell body.
- Axon conducts electrical signals over long distances.
- Myelin sheath insulates the axon and speeds up transmission.
- Saltatory conduction allows impulses to jump between nodes of Ranvier, increasing speed.
- Synaptic terminals store neurotransmitters, which bridge the synaptic gap to continue signal transmission.
- Cerebellar granule cells -> 4µm in diameter, but twin axons (~3mm) = small size allowing the cerebellum to pack many for high processing capacity.
- Motor neurons -> 20µm in diameter = large cell body to synthesize enough proteins to maintain a long axon for transmitting signals from the CNS to distant muscles
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- A narrow, coiled tube near the kidney's outer surface, receiving a large volume of filtrate from the blood.
- One cell thick, with:
a) Apical membrane in contact with filtrate.
b) Basal membrane close to blood capillaries.
- For reabsorption, molecules/ions must cross both apical and basal membranes.
- Large surface area for reabsorption is provided by:
a) Microvilli on the apical membrane.
b)Infoldings (invaginations) on the basal membrane.
- Both membranes have ample space for transport proteins, facilitating selective reabsorption (ensure only necessary substances are reabsorbed).
- Many mitochondria to provide ATP for active transport.
- Basement membrane - a glycoprotein layer to strengthen the single layer of cells.
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- Metabolism is proportional to the volume.
- Rate of substances crossing a membrane proportional to the surface area.
- Too small, slow diffusion, waste accumulate more rapidly.
- Low SA: V cell overheat due to metabolism faster than heat loss.
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Vesicles transport material to the plasma membrane.
The vesicle fuses with the membrane.
Phospholipid bilayers merge due to membrane fluidity.
Contents are expelled from the cell.
The membrane returns to its original shape.
Example: Neurotransmitter, Hormone, digestive enzymes.
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Membrane encloses the target particle.
Membrane invaginates to form a vesicle. Edges fuse, sealing the vesicle.
The inner membrane layer becomes the vesicle’s outer layer and vice versa.
The vesicle detaches and moves into the cytoplasm.
Requires energy for membrane shape changes.
Examples:
Phagocytosis – Engulfing large particles (e.g., white blood cells ingesting bacteria).
Pinocytosis – Uptake of fluids and dissolved substances.
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- A ligand-gated ion channel (opens when ACh binds).
- Conformational change occurs upon binding → channel opens, allowing cation influx (Na⁺, K⁺, and Ca²⁺).
- Action of Acetylcholine at Synapse:
1. ACh binds to receptor → channel opens.
2. Na⁺ influx into the postsynaptic neuron → depolarization. If the threshold potential is reached, voltage-gated Na⁺ channels open → action potential is initiated.
- Binding is reversible: ACh eventually dissociates from the receptor.
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- Membrane proteins that open/close in response to changes in membrane potential.
- Essential for action potential generation and propagation in neurons.
- Facillitated diffusion.
- Na⁺ Channels:
a) above -50mV -> opens = Na⁺ diffuse in.
- K⁺ Channels:
a) +40mV -> open = K⁺ diffuse out of neuron.
b) The pore is too big for Na⁺ to shed off water = only K⁺ can with series of amino acids at narrowest part of pore.
- Both channels close below -50mV
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- Facilitates glucose absorption in the small intestine and kidneys.
- Uses secondary active transport → glucose uptake depends on Na⁺ concentration gradient.
- Process:
1. Na⁺ gradient established:
a) Na⁺/K⁺ pump actively pumps Na⁺ out of the epithelial cell into the blood → creates a low Na⁺ concentration inside the cell.
b) Active transport.
2. Na⁺-glucose co-transport transfers glucose and Na⁺ together as energy is released by the movement of Na⁺ is greater than glucose's.
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- Proteins embedded in the phospholipid bilayer, with extracellular domains extending outward.
- Bind adjacent cells -> forming junctions that connect cells.
- Same CAM type on both cells → binds to form tissues of the same cell type.
- Different CAMs → asymmetric junctions link different cell types.
- Essential for forming complex structures by linking various cell types.
- Prevent/facilitate extracellular moment of substances in tissue.
- Prevent malignancy spreading in tumours.
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- Small, dynamic sacs made of a single membrane layer enclosing internal material.
- Formed via membrane invagination, allowing transport of substances within cells.
- Clathrin-coated vesicles:
Clathrin (three-legged protein) binds to the inner plasma membrane. Causes membrane indentation and budding, forming a spherical vesicle with a clathrin cage.
- Used in endocytosis, exocytosis, and intracellular transport.
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- Large spherical cell (≈110 µm in diameter) → Maximizes food reserves for early development.
- Zona pellucida: Glycoprotein layer containing ZP3, which binds sperm and regulates fertilization; hardens after fertilization to prevent more penetrating.
- Binding proteins in the plasma membrane aid in sperm fusion, allowing sperm nucleus entry.
- Cortical granules: Vesicles near the plasma membrane release enzymes into the zona pellucida post-fertilization, making it impenetrable to additional sperm.
- Yolk: Stores lipids and nutrients for embryo development.
- Mitochondria: Provide ATP for energy; replicate to supply mitochondria for all cells in the adult body.
- Centrioles for mitosis.
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- Size: 6–8 µm in diameter, ~1 µm thick in the center.
- Biconcave shape → Increases surface area to volume ratio for efficient oxygen diffusion.
- Small and flexible → Can travel through narrow capillaries.
- Minimal diffusion distance → Ensures rapid gas exchange between cytoplasm and plasma membrane.
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- Specialized for gas exchange → Facilitate diffusion of O₂ and CO₂.
- Thin structure (0.15 µm thick) → Minimizes diffusion distance for gases.
- Passive process → Relies on concentration gradients of gases.
- Nucleus causes slight thickening in some areas.
- Adjacent capillaries have a single layer of thin cells → Enhances efficient gas exchange.
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More abundant than Type 1 pneumocytes.
Approximately 10 µm in diameter with dense cytoplasm.
Contain mitochondria, rough ER, and lysosomes → Support surfactant production.
Synthesize and store phospholipids in lamellar bodies (vesicles with multiple layers of phospholipids and proteins).
Secrete surfactant via exocytosis → Reduces surface tension, preventing alveolar collapse.