# 20 Summary of cells
Here is a brief summary of previous topics.
Movement in and out of cells
- Cells are the smallest units of living things. They are too small to be seen with the naked eye, so we need to use microscopes to see their structures.
- Cells have a cell membrane, cytoplasm and a nucleus. Plant cells also have a cell wall, and oft en have chloroplasts and a large vacuole containing cell sap.
- The cell membrane is partially permeable, and it controls what enters and leaves the cell.
- The cytoplasm is a jelly-like solution of many different substances in water. It is the site of many different metabolic reactions.
- The nucleus contains the chromosomes, which are made of DNA. This is the genetic information and it controls the activities of the cell.
- The cell wall of a plant cell is made of criss-crossing fibres of cellulose. It is fully permeable. It helps to support the cell, and prevents the cell bursting if it absorbs a lot of water.
- The vacuole of a plant cell contains cell sap, which is a solution of sugars and other substances in water.
- Chloroplasts contain the green pigment chlorophyll, which absorbs sunlight for photosynthesis. There may be starch grains inside the chloroplasts, which are the form in which plants store the food that they make in photosynthesis.
- A tissue is a group of similar cells which work together to carry out a particular function. Tissues are grouped into organs, and organs are grouped into organ systems.
Movement in and out of cells
- Particles in gases, liquids and solutions are in constant random motion. As a result of this, there is a net movement from where they are in a high concentration to where they are in a low concentration. This is diffusion.
- Diffusion is important to cells. For example, oxygen enters a respiring cell by diffusion, and carbon dioxide diffuses out of it.
- Water molecules are small and can diffuse through a partially permeable membrane. Larger molecules dissolved in the water cannot do this. The diffusion of water through a partially permeable membrane is called osmosis.
- Osmosis is important to cells. In a dilute solution, water passes into a cell through its partially permeable cell membrane. The cell gets bigger. Animal cells may burst, but plant cells do not because of their strong cell wall.
- In a concentrated solution, water passes out of a cell by osmosis through its partially permeable membrane. The cell shrinks. Plant cells may become plasmolysed – that is, the cell membrane pulls away from the cell wall.
- A solution containing a lot of water is said to have a high water potential. A solution containing only a little water has a low water potential. Water moves by osmosis down a water potential gradient, from a high water potential to a low water potential.
- Cells can use energy to move substances up their concentration gradient, from a low concentration to a high concentration. This is called active transport. It uses energy that the cells release by respiration.
Video: Cell Structure and Function