tags:
- bio
topic: Cells
date: 2023-11-28Introduction
key concepts
- All living things exhibit the following processes, known through the acronym MRSGREN - this distinguishes them from nonliving creatures.
- MRSGREN:: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition
- DORA: DNA, Organelles, Reproduction, Average Size (an acronym to remember the differences between prokaryotes and eukaryotes)
Cells
- There are two cell types: prokaryotic and eukaryotic cells.
- Both contain a cell membrane and ribosomes along with nucleic acids (DNA) and proteins.
- However, prokaryotic cells are rather simplistic in comparison - lacking a nucleus and membrane-bound organelles.
Prokaryotic Cells
- Note the existence of the external cell wall, as well as the DNA located as a single circular chromosome (rather than a nucleus)
- Plasmids:: Lone, circular DNA molecules separate from the main chromosome.
- Also note the ribosomes - these are simple subcellular components and lack membranes (unlike other membrane-bound organelles, such as mitochondria and chloroplasts)
- Binary fission:: The process in which prokaryotic cells, such as bacteria, replicate.
Eukaryotic Cells
Left (animal cell). Right (plant cell)
- DNA in eukaryotic cells exist as a set of linear chromosomes, bound to proteins.
- Multiple subcellular components (also known as organelles) exist within eukaryotic cells, including mitochondria, vacuoles, endoplasmic reticulum etc.
- Typically multicellular, but can be unicellular in protists.
- Mitosis:: The process in which standard eukaryotic cells replicate,
- Meiosis:: The process in which eukaryotic gametes replicate.
Plant Cells
- Similar to prokaryotes, plant cells contain an external cell wall that provides 'turgidity' (structure) to allow them to grow against gravity.
- Chloroplasts:: An green organelle found in plant cells containing chlorophyll, allowing it to facilitate aerobic respiration.
Organelles
| Organelle | Membrane Bound? | Function |
|---|---|---|
| Cell wall:: | Not membrane bound; | Strengthens cell and maintains shape |
| Cell membrane:: | Not membrane bound; | Controls material exchange and intra-cell communication |
| Cytoplasm:: | Not membrane bound; | A medium suitable for biochemical reactions to occur |
| Ribosome:: | Not membrane bound; | Synthesises proteins for the cell by translating RNA. |
| Nucleus:: | Membrane bound; | Stores cell genetic information (DNA) and controls cell. |
| Rough endoplasmic reticulum:: | Membrane bound; | Dotted with ribosomes; packages proteins |
| Smooth endoplasmic reticulum:: | Membrane bound; | Synthesises hormones, lipids and phospholipids (includes cell membrane) |
| Golgi body:: | Membrane bound; | Packages proteins delivered by both ERs to be secreted from the cell |
| Mitochondria:: | Membrane bound; | Carries out aerobic respiration to form ATP |
| Vacuole:: | Membrane bound; | Maintain cell shape and stores water, nutrients ions and waste. |
| Chloroplasts:: | Membrane bound; | Carries out photosynthesis in plants to produce organic molecules. |
The Cell Membrane
- Often described using the "fluid mosaic model", where embedded molecules act as the 'mosaic'.
- Controls nutrient exchange between the internal (intercellular) and external (extracellular) environments of the cell.
- Primarily consists of a phospholipid bilayer
- Phospholipid heads:: Polar hydrophilic heads.
- Phospholipid tails:: Nonpolar hydrophobic tails
- Transmembrane proteins:: Proteins which span the entire membrane.
- Peripheral proteins:: Proteins which are attached to the surface.
- Integral proteins:: Proteins which are integrated into the lipid bilayer in some way.
- Channel proteins:: Subset of integral proteins that enable facilitated diffusion through, essentially, a tunnel.
- Carrier proteins:: Subset of integral proteins also used in facilitated diffusion
- Glycoproteins:: Membrane proteins with carbohydrate chains.
- Glycolipids:: Phospholipids with carbohydrate chains.
- Cholesterol:: Maintains structural integrity and regulate cell fluidity.
Methods of Transport
Active Transport
- Movement against the concentration gradient with the aid of ATP (energy).
Passive Transport
Diffusion
- Small molecules can simply diffuse straight through the cell membrane, provided that the concentration gradient is in their favour.
Facilitated Diffusion
- Channel proteins form tunnels across the cell membrane allowing specific molecules (such as polar/ions) to pass through while avoiding the hydrophobic + nonpolar core of the plasma membrane.
- Carrier proteins are selective and shift shape to accommodate the target molecule.
Osmosis
- Aquaporins:: Channel proteins specifically for water.
- Osmosis is the process in which water moves from areas of low solute concentration to high solute concentration across a semi-permeable membrane.