Biology hurdles

What is meant by an organism's genome?
An organism's entire set of genes. In a multicellular organisms each cell has the full genome, so it has the instructions to develop into any type of cell.
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What happens during differentiation?
A cell only uses the genes it needs to follow its pathway of development. Other genes are unused. Once a pathway of development has begun in a cell, its usually fixed + the cell cannot change to a different pathway + is said to be 'committed'.
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What are stem cells?
Cells that have the capacity to divide + differentiate along different pathways. Human embryos consist of stem cells in their early stages, but gradually commit themselves to a pattern of differentiation.
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What can happen once a cell is committed?
Once a cell is committed, it may still divide several more times, but all of the cells formed will differentiate in the same way + so are no longer stem cells.
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What are the advantages of larger organisms being multicellular?
If a cell becomes too large then it may not be able to excrete or take in essential materials as quickly as the SA:V ratio becomes smaller. It also allows division of labour- different tissues become specialised for different functions (through diffe
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Where can stem cells be found in the human body?
Bone marrow, skin and liver. However these stem cells do not have as great a capacity to differentiate as embryonic stem cells.
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How do you calculate magnification?
Size of image/magnification
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Describe the differences between prokaryotic and eukaryotic cells.
Eukaryotic cells are divided up by membranes into separate compartments such as the nucleus + mitochondria whereas prokaryotic aren’t compartmentalized + don’t have a nucleus, mitochondria or any other membrane bound organelles.
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Prokaryotic cells divide by binary fission. Describe binary fission (splitting in two)
The bacteria chromosome is replicated so there are 2 identical copies that are moved to opposite side of the cell + the cell + the wall + plasma membrane are then pulled inwards so the cell pinches apart to form 2 identical cells.
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How else can some prokaryotes divide?
They can double in volume and divide by binary fission every 30 minutes.
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Describe the ultrastructure of Eukaryotic cells.
There are different types of organelle that form compartments in eukaryotic cells, each bounded by one or two membranes.
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What do organelles with one membrane have?
Organelles with a single membrane: a rough and a smooth endoplasmic reticulum, a Golgi apparatus, lysosomes and vesicles + vacuole. Organelles with a double membrane: Nucleus, Mitochondrion + Chloroplast.
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Describe the advantage of compartmentalisation in Eukaryotic cells.
Enzymes and substrates used in a process can be concentrated in a small area, with pH and other conditions at optimum levels + with no other enzymes that might disrupt the process.
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What is diffusion?
The passive movement of particles from a region of higher concentration to a region of lower concentration, as a result of the random motion of particles. It can occur if there is a concentration gradient + the membrane is permeable to the particle.
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What are partially permeable membranes?
Membranes that allow some substances to pass through but not others. Some of these substances move between the phospholipid molecules in the membrane by simple diffusion.
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What is facilitated diffusion?
The use of channel proteins to allow certain substances to diffuse through membranes. Cells can control whether substances pass through their plasma membranes by the types of channel protein that are inserted into the membrane.
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Describe the direction of facilitated diffusion?
From high conc. to lower conc. Cells cannot control the direction of movement and both simple + facilitated diffusion are passive processes (no energy used by the cell to make them occur).
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How are channel proteins used in the membranes of neurons?
Sodium and potassium channel proteins in the membranes of neurons that open and close, depending on the voltage across the membrane. They're voltage gated channels + are used to transmit nerve impulses.
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Describe the structure and function of potassium channels in axons.
The axons of neurons contain potassium channels that are used during an action potential. They close when the axon is polarized but open in response to depolarization of the axon membrane, allowing K+ ions through, which repolarizes the axon.
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What happens after the K+ ions are allowed through the potassium channels in axons?
They only remain briefly before a globular sub-unit blocks the pore. The channel then returns to its original closed conformation.
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What is osmosis?
The passive movement of water molecules from a region of lower solute concentration to a region of higher solute concentration across a ppm.
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How is osmosis different to diffusion?
As the direction in which water moves is due to concentration of solutes rather than the concentration of water molecules. Also because water is a solvent (a liquid in which particles dissolve).
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What is active transport?
The movement of substances across membranes using energy from ATP as it can move substances against the concentration gradient -a region of lower conc. to higher conc.
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How are protein pumps used in the membrane used for active transport?
Each pump only transports particular substances, so cells can control what is absorbed and what is expelled. Pumps work in a specific direction - the substance can only enter the pump on one side + can only exit on the other side.
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Describe endocytosis.
The fluidity of membranes allows them to move and change shape. 1.Part of the plasma membrane is pulled inwards, 2.A droplet of fluid becomes enclosed when a vesicle is pinched off, 3.Vesicles can then move through the cytoplasm carrying their conten
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How are vesicles used to move proteins from the rough ER to the Golgi apparatus?
1. Proteins are synthesized by ribosomes + enter the rough endoplasmic reticulum, 2.Vesicles bud off from rER + carry the proteins to the Golgi apparatus which modifies the proteins. 3.Vesicles bud off Golgi + carry the modp to the plasma membrane.
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Describe exocytosis.
Where vesicles move to the plasma membrane and fuse with it, releasing the contents of the vesicle outside the cell. The membrane then flattens out again.
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Card 2


What happens during differentiation?


A cell only uses the genes it needs to follow its pathway of development. Other genes are unused. Once a pathway of development has begun in a cell, its usually fixed + the cell cannot change to a different pathway + is said to be 'committed'.

Card 3


What are stem cells?


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Card 4


What can happen once a cell is committed?


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Card 5


What are the advantages of larger organisms being multicellular?


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