Unit 2: Cell processes: transport, energy and division 1. The cell in its environment. Cellular transport All cells are surrounded by a cell membrane that separates the cell from the outside environment. The cell membrane is selectively permeable, which means that some substances can pass through the membrane while others cannot. Cells must let needed materials (oxygen and food molecules) enter. In contrast, waste materials must move out of cells. 1.1 Diffusion Diffusion is the main method by which small molecules move across the cell membrane. Is the process by which molecules move from an area of higher concentration to an area of lower concentration. Diffusion of oxygen Have you ever used a microscope to observe one-celled organisms in pond water? These organisms obtain the oxygen they need to survive from the water around them. Luckily for them, there are many more molecules of oxygen in the water outside the cell than there are inside the cell. The cell membrane is permeable to oxygen molecules. The oxygen molecules diffuse from the area of higher concentration (the pond water) through the cell membrane to the area of lower concentration (the inside of the cell). Substances that can move into and out of a cell do so by one of three methods: diffusion, osmosis, or active transport. Before Diffusion There is a higher concentration of oxygen molecules outside the cellthan inside the cell. After Diffusion The concentration of oxygen molecules is the same outside and inside the cell.
Effects of osmosis on cells In osmosis, water diffuses through a selectively permeable membrane Water molecules Cell membrane 1. Normal Red Blood Cell Concentration of water inside the cell is the same as outside. 3. High Water Concentration Outside Cell During osmosis water moves into the cell 2. Low Water Concentration Outside Cell Water moves out of the cell during osmosis 1.2. Osmosis Osmosis is the diffusion of water molecules through a selectively permeable membrane. Because cells cannot function properly without adequate water, many cellular processes depend on osmosis. Water molecules move by diffusion from an area where they are highly concentrated through the cell membrane to an area where they are less concentrated.
1.3. Active Transport For a cell moving materials through the cell membrane by diffusion and osmosis do not require the cell to use its own energy. The movement of dissolved materials through a cell membrane without using cellular energy is called passive transport. What if a cell needs to take in a substance that is present in a higher concentration inside the cell than outside? The cell would have to move the molecules in the opposite direction than they naturally move by diffusion. Cells can do this, but they have to use energy. Active transport is the movement of materials through a cell membrane using cellular energy. Active transport requires the cell to use its own energy, while passive trnasport does it. Cells have several ways of moving materials by active transport: Transport Proteins Transport proteins in the cell membrane pick up molecules outside the cell and carry them in, using energy. These proteins also carry molecules out of cells in a similar way. Transport by Engulfing Another method of active transport. 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. The cell must use energy in this process. Amoeba engulfing food This single-celled amoeba is surrounding a smaller organism. The amoeba will engulf the organism and use it for food. Engulfing is a form of active transport.
2. Cellular Energy Every living thing needs energy. All cells need energy to carry out their functions, such as making proteins and transporting substances into and out of the cell. Based on the way by which organisms and their cells obtain food and energy they can be divided into two groups: Autotrophs: organisms that makes its own food. Plants and certain other organisms, such as algae and some bacteria, use the energy in sunlight to make their own food through the process of photosynthesis. Heterotrophs: organisms that cannot make its own food. Many heterotrophs obtain food by eating other organisms. Autotrophs and Heterotrophs Grass, which makes its own food during photosynthesis, is an autotroph. Zebras and lions are heterotrophs, because they cannot make their own food.
2.1. Photosynthesis It is a complex process. During photosynthesis, plants and some other organisms use energy from sun to convert carbon dioxide (CO 2 ) and water (H 2 O) into oxygen (O 2 ) and sugars. The photosynthesis takes place in two stages: sunlight Stage 1: Capturing the Sun s Energy This process occurs mostly in the leaves. Chloroplasts in plant cells capture energy from sunlight. These green organelles contain pigments that absorb light. The main photosynthetic pigment in chloroplasts is chlorophyll. Cells containing chloroplasts. Stage 2: Using Energy to Make Food Cell use the captured energy to produce food. In this stage cell needs two raw materials: carbon dioxide (CO 2 ) and water (H 2 O). In plants the roots absorb water from the soil. This water then moves up through the plant s stem to the leaves. Carbon dioxide is one of the gases in the air. Carbon dioxide enters the plant through small openings on the undersides of the leaves calles stomata. Once in the leaves, the water and carbon dioxide move into the chloroplasts. Inside the chloroplasts, water and carbon dioxide combine in chemical reactions powered with energy obtained during the first stage. These reactions produce chemicals as products: Sugars (glucose C 6 H 12 O 6 ). Cells use energy in glucose to carry out important cell functions. The other product is oxygen (O 2 ) which exits the leaf through the stomata. In fact, almost all oxygen in Earth s atmosphere was produced by living things through the process of photosynthesis. Carbon dioxide Stoma Water Oxygen
The Photosynthesis Equation: The events of photosynthesis can be summed up by the following chemical equation: Light energy 6 CO 2 + 6 H 2 O C 6 H 12 O 6 + 6 O 2 carbon dioxide water a sugar oxygen Notice that on the left the raw materilas are side of the equation and the products are on the right side of the equation. Light energy, which is necessary for the chemical reaction to occur, is writen above the arrow. What happens to the sugar (glucose) produced in photosynthesis? Plant cells use some of the sugar for food. The cells break down the sugar molecules to release the energy they contain. These energy can be used to carry out the plant s functions. Some sugar molecules are converted into other compounds, such as cellulose. Other sugar molecules may be stored in the palnt s cells for later use. When you eat food from plants, such as potatoes or carrots, you are eating the plant s stored energy.
2.2. Respiration Before food can provide your body with energy, it must pass through ypur digestive system. There, the food is broken down into small molecules. These small molecules can then pass out of the digestive system and into your bloodstream. Next, the molecules travel through the bloodstream to the cells of your body. Inside the cells, the energy in the molecules is released. Now, you ll learn how your body cells obtain energy from the food you eat. After you eat a meal, your body converts some of the food into glucose, a type of sugar. Respiration is the process by wich cells obtain energy from glucose. During respiration, cells break down simple food molecules such as sugar and release the energy they contain. Plant cells, as well as animal cells, respire. Energy from Respiration All organisms need energy to live. The guepard uses energy to sprint for food. Although the mushrooms don t move, they still need energy to grow and reproduce.
Stage 1 In In the cytoplasm, glucose is is broken down into smaller molecules. Oxygen is is not involved and a small amount of ofenergy is isreleased. Storing and Releasing Energy During photosynthesis, plants capture the energy from sunlight and save it in the form of carbohydrates, including sugars and starches. Similarly, when you eat a meal, you add to your body s energy savings account. When cells need energy, they withdraw it by breaking down the carbohydrates in the process of respiration. Breathing and Respiration Breathing: moving air in and out of your lungs. Respiration: this process takes place inside the cells and is sometimes called cellular respiration. Stage 2 In In the mitochondria, the smaller molecules combine with oxygen to to produce water and carbon dioxide. This reaction releases a large amount of of energy. The two terms are connected: Breathing brings oxygen, which is usually necessary for cellular respiration. Like photosynthesis, respiration takes place in two stages.
The Respiration Equation: The events of respiration can be summed up by the following chemical equation: C 6 H 12 O 6 + 6 O 2 6 CO 2 + 6 H 2 O + energy sugar oxygen carbon dioxide water Notice that the raw materials for respiration are sugar an oxygen. Plants and other organisms that undergo photosynthesis make their own sugar. The glucose in the cells of animals and other organisms comes from the food they consume. The oxygen used in respiration comes from the air or water surrounding the organism. Photosynthesis and Respiration Photosynthesis and respiration can be thought of as opposite pracesses. Together these two peocesses form a cycle that keeps the levels of oxygen and carbon dioxide fairly constant in the Earth s atmosphere. As you can see in this figure, living things use both gases over and over again.
3. Cell Division All cells are derived from preexisting cells. Cell division is the process by wich cells produce new cells. 3.1. Reasons for cell division Cell growth Repair and replacement of damaged cell parts: some tissues must be repaired often such as the lining of gut, white blood cells, skin cells with a short lifespan. Other cells do not divide at all after birth such as muscle and nerve. Reproduction of the species. 3.2. Cell Cycle During a cell s life cycle there are various different phases. The Cell Cycle includes two main parts: Interphase: is the longest part of a cell s life cycle and is called the resting stage because the cell isn t dividing. during interphase. During interphase cell grows, develops, makes a copy of its DNA, prepares to divide into two cells and carry on all their normal metabolic functions. Cell division: includes Mitosis (nuclear division) and Cytokinesis (division of the cytoplasm).
3.3 Mitosis Mitosis is the process by which somatic cells divide and multiply. It results in the production of two daughter cells from a single parent cell. The two daughter cells are identical to one another and to the original parent cell. In a typical animal cell, mitosis can be divided into four principal stages: Prophase: The chromatin, diffuse in interphase, condenses to form double-rod structures called chromosomes. Each chromosome has duplicated and now consists of two sister chromatids (the two rods). Each chromatid in a chromosome is an exact copy of the other. The two chromatids are held together by a structure called centromere. At the end of the prophase, the nuclear envelope breaks down. Metaphase: The chromosomes align at the equatorial plate and are held in place by microtubules attached to the mitotic spindle and to part of the centromere. Anaphase: The centromere divide. Sister chromatids separate and move toward the corresponding poles. Telophase: Daughter chromosomes arrive at the poles and the microtubules dissapear. The condensed chromatin expands and the nuclear envelope reappears. Cytokinesis: The cytoplasm divides, the cell membrane pinches inward ultimately producing two daughter cells. Centromere Sister Chromatids