Unit 2: Cell Structure and Function

What do ribosomes do?

At the site of protein synthesis, ribosomes bind to messenger RNA (mRNA) and synthesise proteins according to mRNA sequence. Ribosomes are free at cytoplasm, and bound to endoplasmic reticulum to form rough endoplasmic reticulum

What is the endomembrane system and what is its function?

Consists of endoplasmic reticulum, golgi complex, lyosomes, vacuoles, transport vesicles to work together to modify, package and transport polysaccharides, lipids, and proteins within the cell 

What is the endoplasmic reticulum and what does it do?

series of interconnected, membrane-bound sacs in the cytoplasm of eukaryotic cells, made of largely phospholipid bilayers that act like structural support. Provides mechanical support by helping cells maintain shape and helps in intracellular transport (Cytoskeleton also helps in this function) and transports proteins out of the ER in vesicles, transports lipids and ions throughout the cell, helping vesicles move through the cell

What is the rough endoplasmic reticulum?

studded with ribosomes and is formed from folds of membrane that are continuous with the nuclear envelope, helping to compartmentalise the cell and is involved in protein synthesis

What is the smooth endoplasmic reticulum?

Smooth ER does not have ribosomes on the surface, and is involved in the detoxification of cells and lipid synthesis

What is the golgi complex and what does it do?

membrane-bound structure that consists of a series of flattened membrane sacs known as cisternae. Correctly folding and chemically modifying newly synthesised cellular products transferred from ER and packaging proteins for trafficking into vesicles which transport them around the cell

What are lyosomes and what do they do?

membrane-encolsed sacs which contain hydrolytic enzymes. Breaking down cellular waste materials (intracellular digestion), destroy pathogens, and programmed cell death (apoptosis), recycling of the cell’s organic materials (proteins being hydrolysed back to amino acids for reuse)

What are vacuoles and what to do they do in both plants and animals?

membrane-bound sac with contents that are chemically different to those in cytoplasm 

• In Plants: permanent vacuoles store water, helping to maintain water balance and keeping the cell turgid by exerting pressure on cell walls 

• In Animals: temporary vacuoles may store metabolites or transport substances

What are mitochondria and what do they do?

membrane-bound organelles that provide compartments for the metabolic reactions of aerobic (oxygen needed) respiration within eukaryotic cells.

• Double membrane of mitochondrion (plural) contains a smooth outer membrane and highly folded inner membrane that forms cristae 

  • The folds of inner membrane provide large surface area that enable adenosine triphosphate (ATP) to be synthesised during respiration

• Krebs cycle (citric acid cycle) reactions occur in the matrix of the mitochondria, while the reactions of electron transport and ATP synthesis take place on the inner mitochondrial membrane

What kind of cells have more mitochondrion?

More metabolically active cells (muscle cells) tend to have more mitochondrion

What are chloroplasts?

found in green plants and photosynthetic algae, and is a site for photosynthesis, where it is surrounded by a double membrane

• Harness light energy in photosynthesis, converting it into stored chemical energy in the form of food 

What do thylakoids in chloroplast do?

Membrane-bound compartments called thylakoids stack to form structures called grana, where thylakoid membranes contain chlorophyll, which absorbs light energy for the reactions of photosynthesis

• Light-dependent reactions occur in the grana 

• Membrane contain chlorophyll pigments and electron transport proteins that make up photosystems

What do the stroma in the chloroplasts do?

Stroma is the fluid within the inner chloroplast membrane and outside the thylakoid 

• Carbon fixation (Calvin-Benson cycle) reactions of photosynthesis occur in the stroma

Draw the animal cell.

Draw the plant cell

What does having a high surface area to volume ratio do?

increases the ability of biological system to obtain necessary resources (oxygen, glucose, amino acids), eliminate waste products (carbon dioxide, urea), acquire or dissipate thermal energy, exchange chemicals with surorunding (secreting or binding to hormones at cell surface)

What happens when surface area and volume both increase?

As surface area and volume increases, the ratio decreases as volume increases faster than surface area increases

Explain the feature of root hair cells that increase SA:V ratio

• Root hair have single-celled extensions found on epidermis cells in plant roots, which aid in absorption of water and minerals from the soil 

  • Root hairs increase in SA of plant roots, so increasing rate of absorption

Explain the feature of guard cells that increase SA:V ratio

• Guard cells surround the stomata on the lower surface of a leaf, where their role is to open and close the stomata, allowing plants to carry out gas exchange while controlling water loss 

  • Guard cells can become turgid, expanding and curving outwards to allow stomata to open, allowing gases to diffuse into the leaf where they come into contact with the large surface area of the spongy mesophyll (loosely packed layer of cells located beneath the palisade mesophylll layer, containing photosynthetic cells with larger air spaces that facilitate gas exchange)

Explain the feature of gut epithelial cells that increase SA:V ratio

• Gut epithelial cells are located on the inside surface of small intestine, absorbing nutrients for distribution around rest of the body 

  • Surface area of food-contacting surface increases by folded structures of microvilli

Explain the feature of cillia that increase SA:V ratio

• Cillia are hair-like projections on cell surfaces that can beat to move fluid over the surface of cells (unicellular eukaryotes living in watery environments have many cillia) 

  • Keeps the fluid moving next to the surface of the cells, maintaining steep concentration gradient that increases rate of diffusion

How does the SA:V ratio affect heat exchange?

• As the cell mass increases, the SA:V ratio decreases, rate of heat exchange with environment decreases 

  • Small animals with higher SA:V ratio will lose more heat to surroundings, increasing their metabolic rate to maintain body temperature 

  • Large animals with lower SA:V ratio, will lose less heat, decreasing metabolic rate

How do phospholipids function in the plasma membrane structure?

• contain a phosphate head (polar, hydrophilic) and two fatty acid tails (non-polar, hydrophobic) 

  • When placed in aqueous environment, phospholipids can arrange themselves so hydrophilic heads are toward water and hydrophobic tails are away from water, forming phospholipid bilayers, which are the basic structures of membranes

How do proteins function in the plasma membrane structure?

• Proteins can be embedded within phospholipid bilayers, for cell recognition/signalling, membrane transport, or membrane-bound enzymes 

  • Can be hydrophilic (polar side groups), or hydrophobic (non-polar side groups), or both (hydrophilic regions are folded within interior of protein or exposed to aqueous cytosol, and hydrophobic regions are on any surface that interacts with fatty acids)

What are glycoproteins?

proteins with attached carbohydrate chains, still involved in cell recognition and signalling

What are glycolipids?

lipids with attached carbohydrate chains, still involved in cell recognition and signalling

What are steroids?

 like cholesterol, are present within cell membranes to regulate membrane fluidity

What is the fluid mosaic model?

Plasma membrane consists of a structural framework of phospholipids that contains embedded components (proteins, steroids, glycoproteins, and glycolipids) which can move around the surface of the cell within their membrane layer

How does the selective permeability of the plasma membrane work?

• Selective permeability occurs with the presence of the hydrophobic interior of the membrane, allowing cell surface membranes to control which substances can cross between external and internal environment 

  • Smaller, nonpolar molecules (N₂, O₂, and CO₂) freely pass across the membrane, as these molecules can interact with the nonpolar fatty acid tails of the phospholipids and are small enough to pass through 

  • Large, polar molecules (glucose and ions) cannot interact with hydrophobic interior of the membrane and the tightly packed phospholipids block the passage of these larger molecules 

    • Move across membrane through embedded channels and transport proteins 

  • Small, polar molecules (H₂O and NH₃) do not interact with the hydrophobic interior, but are small enough to pass between phospholipids in small volumes

How do cell walls affect permeability?

• have a structural role of providing plants with rigidity, where they are freely permeable for small molecules, but also provide a permeability barrier for some substances (like larger molecules) 

  • Limits the volume of water that can be taken up by a cell, preventing osmotic lysis (bursting due to water intake during osmosis)

How are solute concentration gradients formed?

• A membrane may prevent a substance from passing through, resulting in a higher concentration on the supply side of membrane 

• A membrane contains proteins that actively pump substances from one side to other, allowing a high concentration to be built up only on one side

What is passive transport and what types of processes are passive transport?

 net movement of molecules from high concentration to low concentration without the direct input of metabolic energy

simple diffusion, facilitated diffusion, osmosis

What is simple diffusion?

movement of molecules, down a concentration gradient, directly across the phospholipid bilayer 

What is facilitated diffusion?

movement of large, polar molecules and ions across membranes via transport proteins, occurring down a concentration gradient and requires no energy input

What are channel proteins?

pores that allow the passage of substances, through opening and closing

What are carrier proteins?

• proteins with a binding site on one side of a membrane that change shape to release the transported substance on the other side 

  • Passage of water via transport proteins known as aquaporins 

  • Transport of ions, including Na⁺ and K⁺  

What is osmosis?

movement of water, down a water potential gradient, either between phospholipids or via aquaporins

What is active transport, and what are some examples?

• direct input of energy to move molecules from regions of low concentration to regions of high concentration

  • Energy from respiration (adenosine triphosphate, ATP) is required for active transport

  • Always occurs across membrane, involving use of carrier proteins 

  • Establishment and maintenance of concentration gradients; Na⁺/K⁺ ATPase is a transport protein that has this role 

    • Na⁺/K⁺ ATPase uses energy from ATP to pump sodium and potassium ions across the membrane of nerve cells, contributing to the maintenance of membrane potential (the difference in electrical charge across the membrane) in neurones

endocytosis, exocytosis

Explain how nerve impulses work

Na+/K+ ATPase Pumps sodium ions out of nerve cell axons (elongated part of nerve cell along which nerve impulses are conducted toward other cells) and potassium ions via active transport

• 3 sodium ions from inside of axon bind to pump, ATP attaches to pump and transfer phosphate to the pump, changing its shape and resulting in the pump opening to the outside of the axon 

• 3 sodium ions are released out of the axon, where 2 potassium ions from outside the axon bind to binding sites

• Attached phosphate is released, altering shape of pump

What is exocytosis and what are some examples?

• process by which materials are transported out of cells, where substance to be released in packaged into internal vesicles, which travel to plasma membrane, fusing with the membrane and secreting their contents outside of the cell 

  • Release of neurotransmitters during nerve impulse transmission 

  • Secretion of insulin during blood glucose regulation 

  • Enzyme secretion in the digestive system 

What is endocytosis and what are some examples?

• process by which cells take in large molecules and particulate matter, where the plasma membrane folds in on itself to form new vesicles and the material from the external environment is engulfed by the vesicle, and the vesicle then carries the material into the cell

  • Phagocytosis, where cells of immune system engulf and destroy pathogens 

  • Ingestion of cholesterol in order to build new membranes 

  • Taking in water

Describe a hypotonic solution

surrounding solution has lower solute concentration

Describe a hypertonic solution

surrounding solution has a higher solute concentration

Describe an isotonic solution

surrounding solution and cell contents have equal solute concentration

What is water potential?

tendency of water molecules to move from one place to another

What is osmosis?

the movement of water molecules from regions of high water potential to regions of low water potential (a hypotonic solution to hypertonic solution)

What is solute potential?

• the effect that solutes in a solution have on water potential 

  • Pure water with no dissolved solute has Ψ_s = 0 

  • As solutes are added to a solution, Ψ_s decreases, becoming more negative as pressure potential remains constant

    • Solute molecules bind to water molecules via H-bonding as they dissolve in water, reducing Ψ

How do you calculate solute potential?

Ψ_s = -iCRT

• i = number of molecules that a solute dissociates into when it dissolves in a solution (NaCl dissociates into 2 ions, i = 2) 

• C = molar concentration of solute 

• R = pressure constant (8.31) 

• T = temperature (in Kelvin)

What is pressure potential?

• hydrostatic pressure to which water is subjected 

  • Most biological systems are at the same pressure as surorunding atmosphere, Ψ_p = 0 

  • Pressure potential in plant cells are positive as cytoplasm exerts pressure on the inside of cell wall 

How do you calculate water potential?

Ψ = Ψ_s + Ψ_p

What is osmoregulation

maintains the internal water balance between red blood cells and blood plasma 

How do contractile vacuoles in paramecium osmoregulate?

• Paramecium are protists (single-celled eukaryotes), living in freshwater environments, meaning that their surroundings are hypotonic to cytoplasm, so they take in water through osmosis 

• The water taken in is stored inside specialised vacuole known as contractile vacuoles, that pump excess water out of the cell

How do central vacuoles in plant cells osmoregulate?

• A full vacuole maintains pressure potential and provides structural support to the cell 

• Ion channels in the vacuole membrane can open and close to allow movement of ions into or out of cytoplasm

How does having a membrane benefit eukaryotic cells?

• Allow reaction of glycolysis to kept separate (in the cytosol) from those of the Krebs cycle and oxidative phosphorylation (inside mitochondria) 

• Minimises competing interactions between the enzymes and intermediates of the different stages 

• Minimises risk of conflicting side reactions occurring (no metabolic benefit) and risk of enzyme reactions being inhibited by substrates and products 

• Increase the internal surface area for reactions 

• Membrane-bound nucleus encloses genetic material, providing distinct compartment for transcription of DNA during protein synthesis

What are some examples of how compartmentalisation of eukaryotic cells benefit the cell processes?

• Cells that have high need for ATP (muscle cells) can produce more mitochondria to provide greater number of reactions within electron transport chains to produce ATP 

• Cells that have an active role in secretion of proteins (insulin-producing cells) in the pancreas, can increase the number of ribosomes/rough ER/Golgi to meet the demand for secreted insulin 

• Cells with a role in removal of waste, such as neutrophils and monocytes, can produce more lysosomes to fulfil their role more effectively

How does compartmentalisation work in prokaryotes?

• Single-celled organisms that lack a nucleus and other membrane-bound organelles (like bacteria)

• Genetic material in prokaryotes are not packed within the nucleus, but is found in the nucleoid

• Metabolic processes like photosynthesis or nitrogen fixation occur in specialised areas 

  • Lack of mitochondria causes energy release to occur across plasma membrane 

• Molecules such as glycogen are stored within storage granules

What is endosymbiosis?

process that caused the evolution of compartmentalised cells with distinct, membrane-bound organelles, where one organism lives within another in a mutually beneficial relationship and the engulfed organism is not digested

What does the endosymbiotic theory state?

one organism must have engulfed the other by the process of endocytosis

What is an example that supports endosymbiosis theory?

• Mitochondria and chloroplasts 

  • Both replicated by binary fission (asexual reproduction where single parent cell copies DNA and splits into 2 identical daughter cells), contain circular, non-membrane-boudn DNA, transcribe mRNA from DNA, 70S ribosomes to synthesise proteins, double membranes, similar size