Cell Biology Basics: From Membrane to Molecules
Cell Membrane
The cell membrane protects the contents of the cell. It’s made up mainly of proteins and phospholipids.
Selectively Permeable
The cell membrane also helps control the materials that enter and leave. It’s selectively permeable, which means that some substances can pass through it while others cannot. The cell membrane must let in needed materials, such as oxygen and food molecules. And waste materials must move out of cells.
Diffusion
Diffusion is the main method by which small molecules move across the cell membrane from an area of higher concentration to an area of lower concentration. Molecules are always moving. As they move, the molecules bump into one another. The more molecules there are in an area, the more collisions there will be. Collisions cause molecules to push away from one another. Over time, the molecules of a substance will continue to spread out. Eventually, they will spread evenly throughout the entire area.
Osmosis
Osmosis is a special case of diffusion of water molecules through a selectively permeable membrane. In osmosis, water molecules move by diffusion through the cell membrane from an area of high concentration of water (low solute concentration) to an area where the water concentration is lower (high solute concentration). When all areas are at the same level of concentration, this is called equilibrium.
Passive Transport & Active Transport
The movement of substances through the cell membrane without using energy is called Passive Transport. Diffusion and osmosis are both types of passive transport. When the cell needs to take in materials that are in higher concentration inside the cell than outside the cell, the cells has to use its own energy. This is known as Active Transport.
Active transport can:
- Transport Proteins: One way of moving materials by active transport is when transport proteins in the cell membrane pick up molecules outside the cell and carry them in, using energy! Some of these molecules include Ca, K and Na.
- Transport by Engulfing: First the cell membrane surrounds and engulfs, or encloses, a particle. Once the particle is engulfed, the cell membrane wraps around the particle and forms a vacuole within the cell. Energy is used in the process.
Concentration
The concentration of a substance is the amount of the substance in a given volume, and it can be expressed as a ratio. A ratio compares two numbers. It tells you how much you have of one item in comparison to another.
Elements & Compounds
All organisms are made of cells. Cells are the basic building blocks for living things. All cells are made of elements and compounds. An element is any pure substance made of only one type of atom that cannot be broken down into simpler substances, such as carbon, hydrogen, oxygen, nitrogen, phosphorous, and sulphur. They are listed on the Periodic Table of Elements. A compound is a substance made of two or more different elements combined chemically. The smallest unit of any compound is a molecule. An example, water, sugar, salt or carbon dioxide. Organic Compounds, are those found in living things. Many contain carbon, combined with other elements.
Carbohydrates
An energy-rich organic compound made of carbon and water (Carbon, Hydrogen, Oxygen), such as sugars and starches. It’s energy for immediate use. Sugar is produced during the photosynthesis process that takes place in plants. Sugars can combine to form larger molecules called starches, or complex carbohydrates. Plant cells store excess energy as starch molecules. Foods that contain starch are potatoes, pasta, rice and bread. Your body breaks down starch into glucose which your cells use to produce energy! Carbohydrates are also found in the form of cellulose in the cell walls of plants, providing strength and structure to plants.
Lipids
Such as fats, oils and waxes. Energy-rich organic compounds made of carbon and water. They contain more energy than carbohydrates. Stored in cells for later use. Cell membranes are made mainly of lipids.
Proteins
Large organic molecules made of carbon, hydrogen, oxygen, nitrogen and sometimes sulphur. They have a structural function in the cell, like the bricks in a house.
Amino Acids
Protein molecules are made of amino acid molecules. There are about 20 common amino acids, but can combine to form 1000s of different proteins.
Enzymes
They are found in the structure of cells, particularly in the membrane. Enzymes are proteins that speed up the chemical reactions that take place in cells. Without them, many essential reactions would take too long or not occur at all. For example, Amylase in your saliva speeds up the digestion of food by breaking down starches into sugars in your mouth. Maltase, Sucrase, and Lactase are secreted by the small intestine to break down the sugars maltose, sucrose, and lactose.
Nucleic Acids
Very long organic molecules of Carbon, Hydrogen, Oxygen, Nitrogen and Phosphorus. They contain the instructions that cells need to carry out all functions of life. There are 2 types: DNA and RNA.
DNA
DNA is the genetic material that carries information about an organism, directs all of the cell’s functions, and is passed from parent to offspring.
RNA
RNA plays an important role in the production of proteins, and it’s found in the cytoplasm as well as in the nucleus.