Cell Structure and Signalling

Bacterial vs Animal Cells

• Bacterial cells are prokaryotic and do not contain a nucleus or membrane bound organelles

• Animal and plant cells are eukaryotic and contain a nucleus and many membrane bound organelles

Plasma Membrane

• All cells are enclosed by a plasma membrane, which is a phospholipid bilayer that contains proteins and other molecules (like cholesterol and vit E) embedded or anchored to the membrane. 

• These proteins allow cellular communication and transport to happen between the inside and outside of the cell

• The phospholipids that make up the phospholipid bilayer are amphipathic, meaning they consist of hydrophilic heads made up of a phosphate and glycerol and hydrophobic tail made up of saturated and unsaturated fatty acids. This makes the cell membrane hydrophilic on the outside and hydrophobic on the inside.

Integral Protiens

Integral

• Transmembrane proteins

• Embedded in the membrane with hydrophobic regions in the membrane and hydrophilic ends on either side. These proteins connect the inside and outside of a cell.

• Can function as channels or transporters as well as receptors for hormones or NT’s.

Peripheral Protiens

Peripheral Proteins

• Attached to integral membrane proteins or to the edges of the lipid bilayer temporarily to perform various functions

2 types of Passive Transport and How do They Work

Passive Transport

• Simple Diffusion: Movements of molecules with their conc gradients (high to low). No energy is required. Small non-polar molecules can quickly diffuse across the membrane while small polar (uncharged) molecules slowly diffuse.

• Facilitated Diffusion: Movement of molecules with their conc gradients (high to low). No energy is required. This method transports larger polar molecules (like glucose) across the membrane using transport proteins which transport the molecule once it binds to the protein. The rate of transport is limited by the number of transport proteins. If there is a high conc on one side of the membrane, all proteins will be occupied and therefore transportation will only be as fast as the number of proteins.

2 Types of Active Transport and How do They Work

Active Transport

• Primary Transport: Energy is used to move a molecule against its conc gradient. For example, the sodium potassium pump uses 20-30% of a person’s daily energy to maintain an electrochemical gradient in nerve cells by transporting sodium to the outside (where there is already lots of sodium) and potassium to the inside (where there is already a lot of potassium).

• Secondary Active Transport: In this method, molecules are still moved across their conc gradient, however, they are moved via the energy stored in a concentration gradient that was established by primary active transport. For example, the NaK pump moves sodium out of the cell actively. However, there are passive transport proteins that can move the sodium back in. As sodium goes back in with its concentration gradient, glucose hitches a ride with sodium into the cell against its conc gradient.

Nucleus (5 points)

• The library and command center

• Largest organelle in the cell

• Has a double membrane that is connected to the ER

• Most of the cellular DNA is in the nucleus (some is in mitochondria)

• The nucleus is responsible for DNA replication and transcription of mRNA.

• Proteins, RNA and nucleotides all move in and out of the nucleus through nuclear pores. Nuclear localization sequences (NLS) are responsible for “tagging” molecules that need to enter the nucleus and bringing them into the nucleus. Nuclear export sequences (NES) are responsible for “tagging” proteins that need to exit the nucleus. 

Endoplasmic Reticulum (3 points)

• Protein factory

• Made up of smooth and rough ER (rough ER has ribosomes (which make proteins) attached to it)

• The rough ER is where proteins get translated

• The smooth ER is where enzymes are stored for things like lipid synthesis, drug metabolism, and glycogen storage and breakdown. 

Golgi Complex (2 Points)

• Amazon warehouse

• Modifies, sorts and distributes proteins around the cell

• Proteins from the rough ER get transported to the golgi apparatus. Over here, the proteins receive post translational modifications, and are transported via vesicles to where they need to go.

Mitochondria (5 points)

• Powerhouse of the cell

• Sites of energy production

• Contains a double membrane

• The inner membrane is where cellular respiration happens

• The krebs cycle and other oxidative pathways occur in the matrix.

• Mitochondria has its own DNA separate from the matrix.

Lysosomes (4 points)

• Recycling depot

• Membrane bound organelle that contain enzymes to breakdown molecules that can be reused

• Anything the cell doesn’t need, or foreign invaders are broken down in the lysosome. 

• The pH inside the lysosome is about 5.5 (different from rest of cell) so that the breakdown enzymes in the organelle can function

• The pH is maintained by pumps that actively transport H+ into the lysosome.

4 Steps of Cellular Signalling

• Cells need to communicate with each other to maintain homeostasis.

• Signalling follows 4 general steps

1. Secretion of a messenger molecule from a secretory cell which gets activated by a secretion stimulus

2. Binding of the messenger to a receptor inside or outside the cell

3. A change occurring in the cell

4. Termination of the signal/degradation of the messenger

Endocrine Hormones

• Endocrine hormones and messengers are secreted into the blood and travel to target cells (insulin tells muscle cells to uptake glucose)

Paracrine Messengers

• Paracrine messengers act on cells that are nearby (eg: neurotransmitters to carry a signal)

Autocrine Messengers

• Autocrine messengers bind to the same cell they were released from (cytokines and T cells)

Juxtacrine Messengers

Juxtacrine are direct cell to cell interactions

Intercellular Receptors

• Intercellular receptors bind to hydrophobic messengers that can diffuse across the plasma membrane (ex: steroid/thyroid hormone superfamily)

Extracellular Receptors

• Extracellular receptors are often large and hydrophilic and bind to receptor proteins embedded in the plasma membrane which causes a conformational change in the cell and begins a signalling cascade